The People's Common Sense Medical Adviser in Plain English by Ray Vaughn Pierce

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Title: The People's Common Sense Medical Adviser in Plain English
or, Medicine Simplified, 54th ed., One Million, Six Hundred
and Fifty Thousand

Author: R. V. Pierce

Release Date: May 28, 2006 [EBook #18467]

Language: English

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[Illustration]

* * * * *

THE PEOPLE'S

COMMON SENSE
MEDICAL ADVISER
IN PLAIN ENGLISH:
OR,
MEDICINE SIMPLIFIED.


BY
R.V. PIERCE, M.D.


ONE OF THE STAFF OF CONSULTING PHYSICIANS AND SURGEONS
AT THE INVALIDS' HOTEL AND SURGICAL INSTITUTE, AND
PRESIDENT OF THE WORLD'S DISPENSARY
MEDICAL ASSOCIATION.


FIFTY-FOURTH EDITION.

ONE MILLION, SIX HUNDRED AND FIFTY THOUSAND.

_Carefully Revised by the Author, assisted by his full Staff of
Associate Specialists in Medicine and Surgery, the Faculty of the
Invalids' Hotel and Surgical Institute._

* * * * *

Entered according to Act of Congress, in the year 1895, by the WORLD'S
DISPENSARY MEDICAL ASSOCIATION, In the office of the Librarian of
Congress, at Washington, D.C.

* * * * *

TO
MY PATIENTS,
WHO HAVE SOLICITED MY PROFESSIONAL SERVICES,
FROM THEIR HOMES
IN EVERY STATE, CITY, TOWN, AND ALMOST EVERY HAMLET,
WITHIN THE AMERICAN UNION;
ALSO TO THOSE DWELLING IN EUROPE, MEXICO, SOUTH AMERICA,
THE EAST AND WEST INDIES, AND OTHER
FOREIGN LANDS,
I RESPECTFULLY DEDICATE
THIS WORK.

* * * * *




TABLE OF CONTENTS

PREFACE TO THE PRESENT EDITION
PREFACE_TO_THE_FIRST_EDITION
INTRODUCTORY WORDS

PART I

CHAPTER I. BIOLOGY
CHAPTER II. PHYSIOLOGICAL ANATOMY. THE BONES.
CHAPTER III. PHYSIOLOGICAL ANATOMY. THE MUSCLES.
CHAPTER IV. PHYSIOLOGICAL ANATOMY. THE DIGESTIVE ORGANS.
CHAPTER V. PHYSIOLOGICAL ANATOMY. ABSORPTION.
CHAPTER VI. PHYSICAL AND VITAL PROPERTIES OF THE BLOOD.
CHAPTER VII. PHYSIOLOGICAL ANATOMY. CIRCULATORY ORGANS.
CHAPTER VIII. PHYSIOLOGICAL ANATOMY. THE ORGANS OF RESPIRATION.
CHAPTER IX. PHYSIOLOGICAL ANATOMY. THE SKIN.
CHAPTER X. PHYSIOLOGICAL ANATOMY. SECRETION.
CHAPTER XI. PHYSIOLOGICAL ANATOMY. EXCRETION.
CHAPTER XII. PHYSIOLOGICAL ANATOMY. THE NERVOUS SYSTEM.
CHAPTER XIII. THE SPECIAL SENSES. SIGHT.
CHAPTER XIV. CEREBRAL PHYSIOLOGY.
CHAPTER XV. THE HUMAN TEMPERAMENTS.
CHAPTER XVI. MARRIAGE. LOVE.
CHAPTER XVII. REPRODUCTION.

PART II. HYGIENE.

CHAPTER I. HYGIENE DEFINED.--PURE AIR.
CHAPTER II. FOOD. BEVERAGES. ALCOHOLIC LIQUORS. CLOTHING.
CHAPTER III. PHYSICAL EXERCISE. MENTAL CULTURE. SLEEP. CLEANLINESS.
CHAPTER IV. HYGIENE OF THE REPRODUCTIVE ORGANS.
CHAPTER V. PRACTICAL SUMMARY OF HYGIENE.

PART III. RATIONAL MEDICINE.

CHAPTER I. THE PROGRESS OF MEDICINE.
CHAPTER II. REMEDIES FOR DISEASE.
CHAPTER III. BATHS AND MOTION AS REMEDIAL AGENTS.
CHAPTER IV. HYGIENIC TREATMENT OF THE SICK.

PART IV. DISEASES AND THEIR REMEDIAL TREATMENT.

INDEX

FOOTNOTES

* * * * *




PREFACE TO THE PRESENT EDITION

The popular favor with which former editions of this work have been
received has required the production of such a vast number of copies,
that the original electrotype plates from which it has heretofore been
printed, have been completely worn out.

The book has been re-produced in London, England, where six editions
have already been necessary to supply the demand for it.

In order to continue its publication to meet the demand which is still
active in this country, it has been necessary, inasmuch as the original
electrotype plates have become worn and useless, to re-set the work
throughout. This has afforded the Author an opportunity to carefully
revise the book and re-write many portions, that it may embody the
latest discoveries and improvements in medicine and surgery. In
performing this labor he has been greatly assisted by contributions and
valuable aid kindly supplied by his staff of associate specialists in
medicine and surgery who constitute the Faculty of the Invalids' Hotel
and Surgical Institute.

That part of the book treating of Diseases and Their Remedies will be
found to be thoroughly reliable; the prescriptions recommended therein
having all received the sanction and endorsement of medical gentlemen of
rare professional attainments and mature experience.

THE AUTHOR.

BUFFALO, N.Y., January, 1895.

* * * * *




PREFACE TO THE FIRST EDITION.

Every family needs a COMMON SENSE MEDICAL ADVISER. The frequent
inquiries from his numerous patients throughout the land, suggested to
the Author the importance and popular demand for a reliable work of this
kind. Consequently, he has been induced to prepare and publish an
extensive dissertation on Physiology, Hygiene, Temperaments, Diseases
and Domestic Remedies. It is for the interest and welfare of _every_
person, not only to understand the means for the preservation of health,
but also to know what remedies should be employed for the alleviation of
the common ailments of life.

The frequency of accidents of all kinds, injuries sustained by
machinery, contusions, drowning, poisoning, fainting, etc., and also of
sudden attacks of painful diseases, such as headache, affections of the
heart and nerves, inflammation of the eye, ear and other organs, renders
it necessary that non-professionals should possess sufficient knowledge
to enable them to employ the proper means for speedy relief. To impart
this important information is the aim of the author.

Moreover, this volume treats of Human Temperaments, not only of their
influence upon mental characteristics and bodily susceptibilities, but
also of their vital and non-vital combinations, which transmit to the
offspring either health, hardihood, and longevity, or feebleness,
disease, and death. It clearly points out those temperaments which are
compatible with each other and harmoniously blend, and also those which,
when united in marriage, result in barrenness, or produce in the
offspring imbecility, deformity, and idiocy. These matters are freely
discussed from original investigations and clinical observations, thus
rendering the work a true and scientific guide to marriage.

While instruction is imparted for the care of the body, those diseases
(alas how prevalent!) are investigated which are sure to follow as a
consequence of certain abuses, usually committed through ignorance. That
these ills do exist is evident from the fact that the Author is
consulted by multitudes of unfortunate young men and women, who are
desirous of procuring relief from the weaknesses and derangements
incurred by having unwittingly violated physiological laws.

Although some of these subjects may seem out of place in a work designed
for _every_ member of the family, yet they are presented in a style
which cannot offend the most fastidious, and with a studied avoidance of
all language that can possibly displease the chaste, or disturb the
delicate susceptibilities of persons of either sex.

This book should not be excluded from the young, for it is eminently
adapted to their wants, and imparts information without which millions
will suffer untold misery. It is a _false_ modesty which debars the
youth of our land from obtaining such information.

As its title indicates, the Author aims to make this book a useful and
practical Medical Adviser. He proposes to express himself in plain and
simple language, and, so far as possible, to avoid the employment of
technical words, so that all his readers may readily comprehend the
work, and profit by its perusal. Written as it is amid the many cares
attendant upon a practice embracing the treatment of thousands of cases
annually, and therefore containing the fruits of a rich and varied
experience, some excuse exists for any literary imperfections which the
critical reader may observe.

THE AUTHOR.

BUFFALO, N.Y., July, 1875.

* * * * *




INTRODUCTORY WORDS.

Health and disease are physical conditions upon which pleasure and pain,
success and failure, depend. Every _individual_ gain increases public
gain. Upon the health of its people is based the prosperity of a nation;
by it every value is increased, every joy enhanced. Life is incomplete
without the enjoyment of healthy organs and faculties, for these give
rise to the delightful sensations of existence. Health is essential to
the accomplishment of every purpose; while sickness thwarts the best
intentions and loftiest aims. We are continually deciding upon those
conditions which are either the source of joy and happiness or which
occasion pain and disease. Prudence requires that we should meet the
foes and obviate the dangers which threaten us, by turning all our
philosophy, science, and art, into practical _common sense_.

The profession of medicine is no _sinecure_; its labors are constant,
its toils unremitting, its cares unceasing. The physician is expected to
meet the grim monster, "break the jaws of death, and pluck the spoil out
of his teeth." _His_ ear is ever attentive to entreaty, and within his
faithful breast are concealed the disclosures of the suffering. Success
may elate him, as conquest flushes the victor. Honors are lavished upon
the brave soldiers who, in the struggle with the foe, have covered
themselves with glory, and returned victorious from the field of battle;
but how much more brilliant is the achievement of those who overwhelm
disease, that common enemy of mankind, whose victims are numbered by
millions! Is it meritorious in the physician to modestly veil his
discoveries, regardless of their importance? If he have light, why hide
it from the world? Truth should be made as universal and health-giving
as sunlight. We say, give light to all who are in darkness, and a remedy
to the afflicted everywhere.

We, as a people, are becoming idle, living in luxury and ease, and in
the gratification of artificial wants. Some indulge in the use of food
rendered unwholesome by bad cookery, and think more of gratifying a
morbid appetite than of supplying the body with proper nourishment.
Others devote unnecessary attention to the display of dress and a
genteel figure, yielding themselves completely to the sway of fashion.
Such intemperance in diet and dress manifests itself in the general
appearance of the unfortunate transgressor, and exposes his folly to the
world, with little less precision than certain vices signify their
presence by a tobacco-tainted breath, beer-bloated body, rum-emblazoned
nose, and kindred manifestations. They coddle themselves instead of
practicing self-denial, and appear to think that the chief end of life
is gratification, rather than useful endeavor.

I purpose to express myself candidly and earnestly on all topics
relating to health, and appeal to the common sense of the reader for
justification. Although it is my aim to simplify the work, and render it
a practical common-sense guide to the farmer, mechanic, mariner, and
day-laborer, yet I trust that it may not prove less acceptable to the
scholar, in its discussion of the problems of Life. Not only does the
method adopted in this volume of treating of the Functions of the Brain
and Nervous System present many new suggestions, in its application to
hygiene, the management of disease, generation and the development and
improvement of man, but the conclusions correspond with the results of
the latest investigations of the world's most distinguished _savants_.
My object is to inculcate the facts of science rather than the theories
of philosophy.

Unto us are committed important health trusts, which we hold, not merely
in our own behalf, but for the benefit of others. If we discharge the
obligations of our trusteeship, we shall enjoy present strength,
usefulness, and length of days; but if we fail in their performance,
then inefficiency, incapacity, and sickness, will follow, the sequel of
which is pain and death. Let us, then, prove worthy of this generous
commission, that we may enjoy the sweetest of all pleasures, the
delicious fruitage of honest toil and faithful obedience.

* * * * *




PART I.

PHYSIOLOGY.




CHAPTER I.

BIOLOGY.


In this chapter we propose to consider Life in its primitive
manifestations. _Biology_ is the science of living bodies, or the
science of life. Every organ of a living body has a function to perform,
and _Physiology_ treats of these functions.

_Function_ means the peculiar action of some particular organ or part.
There can be no vital action without change, and no change without
organs. Every living thing has a structure, and _Anatomy_ treats of the
structures of organized bodies. Several chapters of this work are
devoted to _Physiological Anatomy_, which treats of the human organism
and its functions.

The beginning of life is called _generation_; its perpetuation,
_reproduction_. By the former function, individual life is insured; by
the latter, it is maintained. Since nutrition sustains life, it has been
pertinently termed _perpetual reproduction_.

LATENT LIFE is contained in a small globule, a mere atom of matter, in
the sperm-cell. This element is something which, under certain
conditions, develops into a living organism. The entire realm of nature
teems with these interesting phenomena, thus manifesting that admirable
adjustment of internal to external relations, which claims our profound
attention. We are simply humble scholars, waiting on the threshold of
nature's glorious sanctuary, to receive the interpretation of her divine
mysteries.

Some have conjectured that chemical and physical forces account for all
the phenomena of life, and that organization is not the result of vital
forces. Physical science cannot inform us what the beginning was, or how
vitality is the result of chemical forces; nor can it tell us what
transmutations will occur at the end of organized existence. This
mysterious life-principle eludes the grasp of the profoundest
scientists, and its presence in the world will ever continue to be an
astonishing and indubitable testimony of Divine Power.

The physical act of generation is accomplished by the union of two
cells; and as this conjugation is known to be so generally indispensable
to the organization of life, we may fairly infer that it is a universal
necessity. Investigations with the microscope have destroyed the
hypothesis of "spontaneous generation." These show us that even the
minutest living forms are derived from a parent organization.

GENERATION. So long as the vital principle remains in the sperm-cell, it
lies dormant. That part of the cell which contains this principle is
called the _spermatozoön_, which consists of a flattened body, having a
long appendage tapering to the finest point. If it be remembered that a
line is the one-twelfth part of an inch in length, some idea may be
formed of the extreme minuteness of the body of a human spermatozoön,
when we state that it is from 1/800 to 1/600 part of a line, and the
filiform tail 1/50 of a line, in length. This life-atom, which can be
discerned only with a powerful magnifying glass, is perfectly
transparent, and moves about by executing a vibratile motion with its
long appendage. Within this speck of matter are hidden the multifarious
forces which, under certain favorable conditions, result in
organization. Magnify this infinitesimal atom a thousand times, and no
congeries of formative powers is perceived wherewith to work out the
wonders of its existence. Yet it contains the principle, which is the
contribution on the part of the male toward the generation of a new
being.

The _ovum_ or germ-cell, is the special contribution on the part of the
female for the production of another being. The human ovum, though
larger than the spermatozoön, is also extremely small, measuring not
more than from 1/20 to 1/10 of a line, or from 1/240 to 1/120 of an
inch, in diameter.

[Illustration: Fig. 1.

_A_. Human Spermatozoön magnified about 3,800 diameters.
_B_. Vertical and lateral views of spermatozoa of man.
_C, D, E, F._ Development of spermatozoa within the vesicles of evolution.
_G_. Cell of the sponge resembling a spermatozoön.
_H_. Vesicles of evolution from the seminal fluid of the dog in the parent cell
_I_. Single vesicles of different sizes.
_J_. Human spermatozoön forming in its cell.
_K_. Rupture of the cell and escape of the spermatozoön.
]

The sperm and the germ-cells contain the primary elements of all organic
structures, and both possess the special qualities and conditions by
which they may evolve organic beings. Every cell is composed of minute
grains, within which vital action takes place. The interior of a cell
consists of growing matter; the exterior, of matter which has assumed
its form and is less active.

When the vital principle is communicated to it, the cell undergoes a
rapid transformation. While this alteration takes place within the cell,
deteriorating changes occur in the cell-wall. Although vital operations
build up these structures, yet the animal and nervous functions are
continually disintegrating, or wasting, them.

Throughout the animal kingdom, germ-cells present the same external
aspect when carefully examined with the microscope. No difference can be
observed between the cells of the flowers of the oak and those of the
apple, but the cells of the one always produce oak trees, while those of
the other always produce apple trees. The same is true of the germs of
animals, there being not the slightest apparent difference. We are
unable to perceive how one cell should give origin to a dog, while
another exactly like it becomes a man. For aught we know, the ultimate
atoms of these cells are identical in physical character; at least we
have no means of detecting any difference.

SPECIES. The term species is generally used merely as a convenient name
to designate certain assemblages of individuals having various striking
points of resemblance. Scientific writers, as a rule, no longer hold
that what are usually called _species_ are constantly unvarying and
unchangeable quantities. Recent researches point to the conclusion that
_all species vary more or less_, and, in some instances, that the
variation is so great that the limits of general specific distinctness
are sometimes exceeded.

Our space will not permit us to do more than merely indicate the two
great fundamental ideas upon which the leading theories of the time
respecting the origin of species are based. These are usually termed the
doctrine of _Special Creation_ and the doctrine of _Evolution_.
According to the doctrine of Special Creation, it is thought that
species are practically immutable productions, each species having a
_specific centre_ where it was originally created, and from which it
spread over a certain area until its further progress was obstructed by
unfavorable conditions. The advocates of the doctrine of Evolution hold,
on the contrary, that species are not permanent and immutable, but that
they are subject to modification, and that "the existing forms of life
are descendants by true generation of pre-existing forms."[1] Most
naturalists are now inclined to admit the general truth of the theory of
evolution, but they differ widely respecting the mode in which it
occurred.


THE PROCESS OF GENERATION.


The vital _principle_, represented in the _sperm_-cell by a
spermatozoön, must be imparted to a _germ_-cell in order to effect
impregnation. After touching each other, separate them immediately, and
observe the result. If, with the aid of a powerful lens, we directly
examine the spermatozoön, it will be perceived that, for a short time,
it preserves its dimensions and retains all its material aspects. But it
does not long withstand the siege of decay, and, having fulfilled its
destiny, loses its organic characteristics, and begins to shrink.

If we examine the fertilized germ, we discover unusual activity, the
result of impregnation. Organic processes succeed one another with
wonderful regularity, as if wrought out by inexplicable intelligence.
Here begin the functions which constitute human physiology.

Generation requires that a spermatozoön be brought into actual contact
with a germ that fecundation may follow. If a spermatic cell, or
spermatozoön, together with several unimpregnated ova, no matter how
near to one another, if not actually touching, be placed on the concave
surface of a watch-crystal, and covered with another crystal, keeping
them warm, and even though the vapor of the ova envelops it, no
impregnation will occur. Place the spermatozoön in contact with an ovum,
and impregnation is instantly and perfectly accomplished. Should this
vitalizing power be termed nerve-force, electricity, heat, or motion? It
is known that these forces may be metamorphosed; for instance, nervous
force may be converted into electricity, electricity into heat, and heat
into motion, thus illustrating their affiliation and capability of
transformation. But nothing is explained respecting the real nature of
the vital principle, if we assert its identity with any of these forces;
for who can reveal the true nature of any of these, or even of matter?


ALTERNATE GENERATION.


In several insect families, the species is not wholly represented in the
adult individuals of both sexes, or in their development, but, to
complete this series, supplementary individuals, as it were, of one or
of several preceding generations, are required. The son may not resemble
the father, but the grandfather, and in some instances, the likeness
re-appears only in latter generations. Agassiz states: "Alternate
generation was first observed among the Salpae. These are marine
mollusks, without shells, belonging to the family Tunicata. They are
distinguished by the curious peculiarity of being united together in
considerable numbers so as to form long chains, which float in the sea,
the mouth(_m_) however being free in each.

[Illustration: Fig. 2. ]

[Illustration: Fig. 3. ]

"Fig. 2. The individuals thus joined in floating colonies produce eggs;
but in each animal there is generally but one egg formed, which is
developed in the body of the parent, and from which is hatched a little
mollusk.

"Fig. 3, which remains solitary, and differs in many respects from the
parent. This little animal, on the other hand, does not produce eggs,
but propagates, by a kind of budding, which gives rise to chains already
seen in the body of their parent(a), and these again bring forth
solitary individuals, etc."

It therefore follows that generation in some animals require? two
different bodies with intermediate ones, by means of which and their
different modes of reproduction, a return to the original stock is
effected.

UNIVERSALITY OF ANIMALCULAR LIFE.--Living organisms are universally
diffused over every part of the globe. The gentle zephyr wafts from
flower to flower invisible, fructifying atoms, which quicken beauty and
fragrance, giving the promise of a golden fruitage, to gladden and
nourish a dependent world. Nature's own sweet cunning invests all living
things constraining into her service chemical affinities, arranging the
elements and disposing them for her own benefit, in such numberless ways
that we involuntarily exclaim,

"The course of Nature is the art of God."

The microscope reveals the fact that matter measuring only 1/120000 of
an inch diameter may be endowed with vitality, and that countless
numbers of animalcules often inhabit a single drop of stagnant water.
These monads do not vary in form, whether in motion or at rest. The life
of one, even, is an inexplicable mystery to the philosopher. Ehrenberg
writes: "Not only in the polar regions is there an uninterrupted
development of active microscopic life, where larger animals cannot
exist, but we find that those minute beings collected in the Antarctic
expedition of Captain James Ross exhibit a remarkable abundance of
unknown, and often most beautiful forms."

Even the interior of animal bodies is inhabited by animalcules. They
have been found in the blood of the frog and the salmon, and in the
optic fluid of fishes. Organic beings are found in the interior of the
earth, into which the industry of the miner has made extensive
excavations, sunk deep shafts, and thus revealed their forms; likewise,
the smallest fossil organisms form subterranean strata many fathoms
deep. Not only do lakes and inland seas abound with life, but also, from
unknown depths, in volcanic districts, arise thermal springs which
contain living insects. Were we endowed with a microscopic eye, we might
see myriads of ethereal voyagers wafted by on every breeze, as we now
behold drifting clouds of aqueous vapor. While the continents of earth
furnishes evidences of the universality of organic beings, recent
observations prove that "animal life predominates amid the eternal night
of the depths of the liquid ocean."


THE ORIGIN OF LIFE.


The ancients, rude in many of their ideas, referred the origin of life
to divine determination. The thought was crudely expressed, but well
represented, in the following verse:

"Then God smites his hands together,
And strikes out a soul as a spark,
Into the organized glory of things.
From the deeps of the dark."

According to a Greek myth, Prometheus formed a human image from the dust
of the ground, and then, by fire stolen from heaven, animated it with a
living soul. Spontaneous generation once held its sway, and now the idea
of natural evolution is popular. Some believe that the inpenetrable
mystery of life is evolved from the endowments of nature, and build
their imperfect theory on observations of her concrete forms and their
manifestations, to which all our investigations are restricted. But
every function indicates purpose, every organism evinces intelligent
design, and _all_ proclaim a Divine Power. Something cannot come out of
nothing. With reason and philosophy, _chance_ is an impossibility. We,
therefore, accept the display of wisdom in nature as indicative of the
designs of God. Thus "has He written His claims for our profoundest
admiration and homage all over every object that He has made." If you
ask: Is there any advantage in considering the phenomena of nature as
the result of DIVINE VOLITION? we answer, that this belief corresponds
with the universally acknowledged ideas of accountability; for, with a
wise, and efficient Cause, we infer there is an intelligent creation,
and the desire to communicate, guide and bless, is responded to by man,
who loves, obeys, and enjoys. Nothing is gained by attributing to nature
vicegerent forces. Is it not preferable to say that she responds to
intelligent, loving Omnipotence? Our finiteness is illustrated by our
initiation into organized being. Emerging from a rayless atom, too
diminutive for the sight, we gradually develop and advance to the
maturity of those _conscious powers_, the exercise of which furnishes
indubitable evidence of our immortality. We are pervaded with invisible
influences, which, like the needle of the compass trembling on its
pivot, point us to immortality as our ultimate goal, where in the sunny
clime of Love, even in a spiritual realm of joy and happiness, we may
eternally reign with Him who is all in all.

* * * * *




CHAPTER II.

PHYSIOLOGICAL ANATOMY.

THE BONES.


All living bodies are made up of tissues. There is no part, no organ,
however soft and yielding, or hard and resisting, which has not this
peculiarity of structure. The _bones_ of animals, as well as their flesh
and fat, are composed of tissues, and all alike made up of cells. When
viewed under a microscope, each cell is seen to consist of three
distinct parts, a _nucleolus_, or dark spot, in the center of the cell,
around which lies a mass of granules, called the _nucleus;_ and this, in
turn, is surrounded with a delicate, transparent membrane, termed the
_envelope_. Each of the granules composing the nucleus assimilates
nourishment, thereby growing into an independent cell, which possesses a
triple organization similar to that of its parent, and in like manner
reproduces other cells.

[Illustration: Fig. 4.
Nucleated cell.
From Goeber.
1. Periphery of the
cell, or cell-wall.
2. Nucleus. 3. Nucleolus
in the center.]

A variety of tissues enters into the composition of an animal structure,
yet their differences are not always distinctly marked, since the
characteristics of some are not unlike those of others. We shall notice,
however, only the more important of the tissues.

The _Areolar_, or _Connective Tissue_, is a complete network of delicate
fibers, spread over the body, and serves to bind the various organs and
parts together. The fibrous and serous tissues are modifications of the
areolar.

The _Nervous Tissue_ is of two kinds: The gray, which is pulpy and
granulated, and the white fibrous tissue. The _Adipose Tissue_ is an
extremely thin membrane, composed of closed cells which contain fat. It
is found principally just beneath the skin, giving it a smooth, plump
appearance.

[Illustration: Fig. 5.
Arrangement of fibers in the
Areolar Tissue. Magnified 135 diameters.]

The _Cartilaginous Tissue_ consists of nucleated cells, and, with the
exception of bone, is the hardest part of the animal frame. The _Osseous
Tissue_, or bone, is more compact and solid than the cartilaginous, for
it contains a greater quantity of lime. The _Muscular Tissue_ is
composed of bundles of fibers, which are enclosed in a cellular
membrane.

[Illustration: Fig. 6.
Human Adipose Tissue.]

Various opinions have been entertained in regard to the formation, or
growth, of bone. Some anatomists have supposed that all bone is formed
in cartilage. But this is not true, for there is an _intra-membranous_,
as well as an _intra-cartilaginous_, formation of bone, as may be seen
in the development of the cranial bones, where the gradual calcification
takes place upon the inner layers of the fibrous coverings.
Intra-cartilaginous deposit is found in the vicinity of the
blood-vessels, within the cartilaginous canals; also, there are certain
points first observed in the shafts of long bones, called _centers of
ossification_. These points are no sooner formed than the cartilage
corpuscles arrange themselves in concentric zones, and, lying in contact
with one another, become very compact. As ossification proceeds, the
cup-shaped cavities are converted into closed interstices of bone, with
extremely thin lamellæ, or layers. These, however, soon increase in
density, and no blood-vessels can be observed within them.

[Illustration: Fig. 7.
Vertical section of cartilage near the surface of
ossification. _1_. Ordinary appearance of the temporary
cartilage. _1_'. Portion of the same more
highly magnified. _2_. The cells beginning to form
into concentric zones. _2_'. Portion more magnified.
_3_. The ossification is extending in the inter-cellular
spaces, and the rows of cells are seen
resting in the cavities so formed, the nuclei being
more separated than above. _3_'. Portion of the
same more highly magnified.]

[Illustration: Fig. 8.
Thigh-bone,
sawn open
lengthwise.]

[Illustration: Fig. 9.
Lower end of the thigh-bone
sawn across, showing its central
cavity.]

The bony plates form the boundaries of the _Haversian_, or nutritive
canals of the bones. In the _second stage of ossification_, the
cartilage corpuscles are converted into bone. Becoming flattened against
the osseous lamellæ already formed, they crowd upon one another so as to
entirely obliterate the lines that distinguish them; and, simultaneously
with these changes, a calcareous deposit takes place upon their
interior. Bones grow by additions to their ends and surfaces. In the
child, their extremities are separated from the body of the bone by
layer of cartilage, and the cancellated, or cellular structure, which
remains for a time in the interior, represents the early condition of
the ossifying substances.

The bones contain more earthy matter in their composition than any other
part of the human body, being firm, hard, and of a lime color. They
compose the skeleton or frame work, and, when united by natural
ligaments, form what is known as the _natural_ skeleton; when they are
wired together, they are called an _artificial_ skeleton. The number of
bones in the human body is variously estimated; for those regarded as
single by some anatomists are considered by others to consist of several
distinct pieces. There are two hundred distinct bones in the human
skeleton besides the teeth. These may be divided into those of the Head,
Trunk, Upper Extremities, and Lower Extremities.

[Illustration: Fig. 10.
The bones of the skull separated. _1_. Frontal,
only half is seen. _2_. Parietal. _3_. Occipital, only
half is seen. _4_. Temporal. _5_. Nasal. _6_. Malar.
_7_. Superior maxillary (upper jaw). _8_. Lachrymal.
_9_. Inferior maxillary (lower jaw). Between
_4_ and _6_ a part of the sphenoid or wedge-shaped
bone, is seen. Another bone assisting to form
the skull, but not here seen, is called the _ethmoid_
(sieve-like, from being full of holes), and is situated
between the sockets of the eyes, forming the
roof of the nose.]

THE BONES OF THE HEAD are classed as follows: eight belonging to the
Cranium, and fourteen to the Face. The bones of the Cranium are the
_occipital_, two _parietal_, two _temporal, frontal, sphenoid_, and
_ethmoid_. Those composing the face are, the two _nasal_, two _superior
maxillary,_ two _lachrymal_, two _malar_ two _palate_, two _inferior
turbinated, vomer_, and _inferior maxillary_. The cranial bones are
composed of two dense plates, between which there is, in most places a
cancellated or cellular tissue. The external plate is fibrous, the
internal, compact and vitreous. The skull is nearly oval in form, convex
externally, the bone being much thicker at the base than elsewhere, and
it is, in every respect admirably adapted to resist any injury to which
it may be exposed, thus affording ample protection to the brain
substance which it envelops. The internal surface of the cranium
presents eminences and depressions for lodging the convolutions of the
brain, and numerous furrows for the ramifications of the blood-vessels.
The bones of the cranium are united to one another by ragged edges
called _sutures_, which are quite distinct in the child but which in old
age are nearly effaced. Some authorities suppose that by this
arrangement the cranium is less liable to be fractured by blows; others
think that the sutures allow the growth of these bones, which takes
place by a gradual osseous enlargement at the margins. The bones of the
_Face_ are joined at the lower part and in front of the cranium, and
serve for the attachment of powerful muscles which assist in the process
of mastication. Although the soft parts of the face cover the bony
structure, yet they do not conceal its principal features, or materially
change its proportions. The form of the head and face presents some
remarkable dissimilarities in different races.

[Illustration: Fig. 11.
_1_. The first bone of the sternum (breast-bone).
_2_. The second bone of the sternum.
_3_. The cartilage of the sternum. _4_. The
first dorsal vertebra (a bone of the spinal
column). _5_. The last dorsal vertebra. _6_.
The first rib. _7_. Its head. _8_. Its neck. _9_.
Its tubercle. _10_. The seventh or last true
rib. _11_. The cartilage of the third rib. _12._
The floating ribs.]

[Illustration: Fig. 12.
A vertebra of the neck. _1_. The
body of the vertebra. _2_. The spinal
canal. _4_. The spinous process
cleft at its extremity. _5_. The
transverse process. _7_. The interior
articular process. _8_. The
superior articular process.]

THE TRUNK has fifty-four bones, which are as follows: The _Os Hyoides_,
the _Sternum_, twenty-four Ribs, twenty-four _vertebræ_ or bones of the
Spinal Column, the _Sacrum_, the _Coccyx_, and two _Ossa Innominata_.
The _Os Hyoides_, situated at the base of the tongue, is the most
isolated bone of the skeleton, and serves for the attachment of muscles.
The _Sternum_, or breast-bone, in a child is composed of six pieces, in
the adult of three, which in old age are consolidated into one bone. The
_Ribs_ are thin, curved bones, being convex externally. There are twelve
on each side, and all are attached to the spinal column. The seven upper
ribs, which are united in front of the sternum, are termed _true_ ribs;
the next three, which are not attached to the sternum, but to one
another are called _false_ ribs; and the last two, which are joined only
to the vertebræ, are designated as _floating_ ribs. The first rib is the
shortest, and they increase in length as far as the eighth, after which
this order is reversed.

[Illustration: Fig. 13.
_1_. The cartilaginous substance
which connects the bodies of
the vertebræ. _2_. The body of the
vertebra. _3_. The spinous process.
_4,4_. The transverse processes.
_5,5_. The articular processes.
_6,6_. A portion of the bony bridge
which assists in forming the spinal
canal (7).]

[Illustration: Fig. 14.
Backbone, spinal
column, or vertebral
column. All
animals possessing
such a row of bones
are called _vertebrates_.
Above _b_ are
the cervical (neck)
vertebræ; _b_ to _c_,
dorsal (back) or
chest vertebræ; _c_
to _d_, lumbar (loins)
vertebræ; _d_ to _e_, sacrum;
_e_ to _f_, coccyx.]

The _Spinal Column_ or backbone, when viewed from the front presents a
perpendicular appearance, but a side view shows four distinct curves.
The bones composing it are called _vertebræ_. The body part of a
vertebra is light and spongy in texture, having seven projections called
_processes_, four of which are the _articular_ processes, which furnish
surfaces to join the different vertebræ of the spinal column. Two are
called _transverse_, and the remaining one is termed the _spinous_. The
transverse and spinous processes serve for the attachment of the muscles
belonging to the back. All these processes are more compact than the
body of the vertebra, and, when naturally connected, are so arranged as
to form a tube which contains the _medulla spinalis_, or spinal cord.
Between the vertebræ is a highly-elastic, cartilaginous and cushion-like
substance, which freely admits of motion, and allows the spine to bend
as occasion requires. The natural curvatures of the spinal column
diminish the shock produced by falling, running or leaping, which would
otherwise be more directly transmitted to the brain. The ribs at the
sides, the sternum in front, and the twelve dorsal bones of the spinal
column behind, bound the thoracic cavity, which contains the lungs,
heart, and large blood-vessels.

[Illustration: Fig. 15.
A representation of the pelvic bones. _e_. The
lumbo-sacral joint. 2. The sacrum. _3_. Coccyx. _1,1_.
The innominata. _4,4_. Acetabula.]

The _Pelvis_ is an open bony structure, consisting of the Os Innominata,
one on either side, and the Sacrum and Coccyx behind. The _Sacrum_,
during childhood, consists of five bones, which in later years unite to
form one bone. It is light and spongy in texture, and the upper surface
articulates with the lowest vertebra, while it is united at its inferior
margin to the coccyx. The _Coccyx_ is the terminal bone of the spinal
column. In infancy it is cartilaginous and composed of several pieces,
but in the adult these unite and form one bone. The _Innominata_, or
nameless bones, during youth, consist of three separate pieces on each
side; but as age advances they coalesce and form one bone. A deep
socket, called the _acetabulum_, is found near their junction, which
serves for the reception of the head of the thigh-bone.

[Illustration: Fig. 16.
1. Portions of the backbone. 2. Cranial
bones. _4_. Breast-bone. _5_. Ribs. _7_. Collar-bone.
_8_. Arm-bone (humerus). _9_. Shoulder-joint.
_10, 11_. Bones of the fore-arm (ulna and
radius). _12_. Elbow-joint. _13_. Wrist-joint. _14_.
Bones of the hand. _15, 16_. Pelvic bones. _17_.
Hip-joint. _18_. Femur. _19, 20_. Bones of the
knee-joint. _21, 22_. Fibula and tibia. _23_. Ankle
bone. _24_. Bones of the foot.]

THE BONES OF THE UPPER EXTREMITIES are sixty-four in number, and are
classified as follows: The Scapula, Clavicle, Humerus, Ulna, Radius,
Carpus, Metacarpus, and Phalanges. The _Scapula_, or shoulder-blade, is
an irregular, thin, triangular bone, situated at the posterior part of
the shoulder, and attached to the upper and back part of the chest. The
_Clavicle_, or collar-bone, is located at the upper part of the chest,
between the sternum and scapula, and connects with both. Its form
resembles that of the italic letter _f_, and it prevents the arms from
sliding forward. The _Humerus_, the first bone of the arm, is long,
cylindrical, and situated between the scapula and fore-arm. The _Ulna_
is nearly parallel with the radius, and situated on the inner side of
the fore-arm. It is the longer and larger of the two bones, and in its
articulation with the humerus, forms a perfect hinge-joint. The
_Radius_, so called from its resemblance to a spoke, is on the outer
side of the fore-arm, and articulates with the bones of the wrist,
forming a joint. The ulna and radius also articulate with each other at
their extremities. The _Carpus_, or wrist, consists of eight bones,
arranged in two rows. The _Metacarpus_, or palm of the hand, is composed
of five bones situated between the carpus and fingers. The _Phalanges_,
fourteen in number, are the bones of the fingers and thumb, the fingers
each having three and the thumb two.

THE BONES OF THE LOWER EXTREMITIES, sixty in number, are classed as
follows: The Femur, Patella, Tibia, Fibula, Tarsus, Metatarsus, and
Phalanges. The _Femur_, or thigh-bone, is the longest bone in the body.
It has a large round head, which is received into the acetabulum, thus
affording a good illustration of a ball and socket joint. The _Patella,_
or knee-pan, is the most complicated articulation of the body. It is of
a round form, connects with the tibia by means of a strong ligament, and
serves to protect the front of the joint, and to increase the leverage
of the muscles attached to it, by causing them to act at a greater
angle. The _Tibia_, or shin bone, is enlarged at each extremity and
articulates with the femur above and the astragalus, the upper bone of
the tarsus, below. The _Fibula_, the small bone of the leg, is situated
on the outer side of the tibia, and is firmly bound to it at each
extremity. The _Tarsus_, or instep, is composed of seven bones, and
corresponds to the carpus of the upper extremities. The _Metatarsus_,
the middle of the foot, bears a dose resemblance to the metacarpus, and
consists of five bones situated between the tarsus and the phalanges.
The tarsal and the metatarsal bones are so united as to give an arched
appearance to the foot, thus imparting elasticity. The _Phalanges_, the
toes, consist of fourteen bones, arranged in a manner similar to that of
the fingers.

We are not less interested in tracing the formation of bone through its
several stages, than in considering other parts of the human system. The
formation of the Haversian canals for the passage of blood-vessels to
nourish the bones, the earlier construction of bony tissue by a
metamorphosis of cartilaginous substance, and also the commencement of
ossification at distinct points, called _centers of ossification_, are
all important subjects, requiring the student's careful attention. The
bones are protected by an external membranous envelope, which, from its
situation is called the _periosteum_. The bones are divided into four
classes, _long, short, flat_ and _irregular_, being thus adapted to
subserve a variety of purposes.

The Long Bones are found in the limbs, where they act as levers to
sustain the body and aid in locomotion. Each_long_ bone is composed of a
cylinder, known as the _shaft_, and two _extremities_. The shaft is
hollow, its wails being _thickest_ in THE middle and growing thinner
toward the extremities. The _extremities_ are usually considerably
enlarged, for convenience of connection with other bones, and to afford
a broad surface for the attachment of muscles. The clavical, humerus,
radius, ulna, femur, tibia, fibula, the bones of the metacarpus,
metatarsus and the phalanges, are classed as long bones.

Where the principal object to be attained is strength, and the motion of
the skeleton is limited, the individual bones are short and compressed,
as the bones of the carpus and tarsus. The structure of these bones is
spongy, except at the surface, where there is a thin crust of compact
matter.

[Illustration: Fig. 17.
Anatomy of a joint, _1, 1_.
Bones of a joint. _2, 2_. Cartilage.
_3, 3, 3, 3_. Synovial
membrane.]

[Illustration: Fig. 18.
Anatomy of knee joint.
_1._ Lower end of thigh-bone.
_3._ Knee-pan. _2, 4_ Ligaments
of the knee-pan. _5_. Upper
end of the tibia, or shin-bone.
_6, 12_. Cartilages.]

When protection is required for the organs of the body, or a broad flat
surface for the attachment of the muscles, the bones are expanded into
plates, as in the cranium and shoulder-blades.

The _irregular_ or _mixed_ bones are those which, from their peculiar
shape, cannot be classed among any of the foregoing divisions. Their
structure is similar to the others, consisting of cancellar tissue,
surrounded by a crust of compact matter.

The vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, malar, two
maxillary, palate, inferior turbinated, and hyoid are known as irregular
bones.

The formation of the joints requires not only bones, but also
cartilages, ligaments, and the synovial membrane, to complete the
articulation. _Cartilage_ is a smooth, elastic substance, softer than
bone, and invested with a thin membrane, called _perichondrium_. When
cartilage is placed upon convex surfaces, the reverse is true. The
_Ligaments_ are white, inelastic, tendinous substances, softer than
cartilage, but harder than membrane. Their function is to bind together
the bones. The _Synovial Membrane_ covers the cartilages, and is then
reflected upon the ligaments, thus forming a thin, closed sac, called
the _synovial capsule._

All the synovial membranes secrete a lubricating fluid, termed
_synovia_, which enables the surfaces of the bones and ligaments to move
freely upon one another. When this fluid is secreted in excessive
quantities, it produces a disease known as "dropsy of the joints." There
are numerous smaller sacs besides the synovial, called _bursæ mucosæ_,
which in structure are analogous to them, and secrete a similar fluid.
Some joints permit motion in every direction, as the shoulders, some in
two directions only, as the elbows, while others do not admit of any
movement. The bones, ligaments, cartilages, and synovial membrane, are
supplied with nerves, arteries, and veins.

When an animal is provided with an internal bony structure, it indicates
a high rank in the scale of organization. An elaborate texture of bone
is found in no class below the vertebrates. Even in the lower order of
this sub-kingdom, which is the highest of animals, bone does not exist,
as is the case in some tribes of fishes, such as sharks, etc., and in
all classes below that of the cartilaginous fishes, the inflexible
substance which sustains the soft parts is either shell or some
modification of bone, and is usually found on the outside of the body.
True bone, on the contrary, is found in the interior, and, therefore, in
higher animals, the skeleton is always internal, while the soft parts
are placed external to the bony frame. While many animals of the lowest
species, being composed of soft gelatinous matter, are buoyant in water,
the highest type of animals requires not only a bony skeleton, but also
a flexible, muscular system, for locomotion in the water or upon the
land. Each species of the animal kingdom is thus organically adapted to
its condition and sphere of life.

* * * * *




CHAPTER III.

PHYSIOLOGICAL ANATOMY.

THE MUSCLES.


[Illustration: Fig. 19.
Muscular fillers highly
magnified.]

The _Muscles_ are those organs of the body by which motion is produced,
and are commonly known as _flesh_. A muscle is composed of _fascieuli_,
or bundles of fibers, parallel to one another. They are soft, varying in
size, of a reddish color, and inclosed in a cellular, membranous sheath.
Each _fasciculus_ contains a number of small fibers, which, when
subjected to a microscopic examination, are found to consist of
_fibrillae_, or little fibers; each of these fibrillae in turn being
invested with a delicate sheath. The fibers terminate in a glistening,
white _tendon_, or hard cord, which is attached to the bone. So firmly
are they united, that the bone will break before the tendon can be
released. When the tendon is spread out, so as to resemble a membrane,
it is called _fascia_. Being of various extent and thickness, it is
distributed over the body, as a covering and protection for the more
delicate parts, and aids also in motion, by firmly uniting the muscular
fibers. The spaces between the muscles are frequently filled with fat,
which gives roundness and beauty to the limbs. The muscles are of
various forms; some are longitudinal, each extremity terminating in a
tendon, which gives them a _fusiform_ or spindle-shaped appearance;
others are either fan-shaped, flat, or cylindrical.

[Illustration: Fig. 20.
1. A spindle-shaped muscle, with tendinous
terminations. 2. Fan-shaped muscle.
3. Penniform muscle. 4. Bipenniform
muscle.]

[Illustration: Fig. 21.
Striped muscular fibre showing cleavage in
opposite directions. 1. Longitudinal cleavage.
2. Transverse cleavage. 3. Transverse section of
disc. 4. Disc nearly detached. 5. Detached disc,
showing the sarcous elements. 6. Fibrillæ. 7,8.
Separated fibrillae highly magnified.]

Every muscle has an _origin_ and an _insertion_. The term _origin_ is
applied to the more fixed or central attachment of a muscle, and the
term _insertion_ to the movable point to which the force of the muscle
is directed; but the origin is not absolutely fixed, except in a small
number of muscles, as those of the face, which are attached at one
extremity to the bone, and at the other to the movable integument, or
skin. In most instances, the muscles may act from either extremity. The
muscles are divided into the Voluntary, or muscles of animal life, and
the Involuntary, or muscles of organic life. There are, however, some
muscles which cannot properly be classified with either, termed
Intermediate. The _Voluntary Muscles_ are chiefly controlled by the
will, relaxing and contracting at its pleasure, as in the motion of the
eyes, mouth, and limbs. The fibers are of a dark red color, and possess
great strength. These fibers are parallel, seldom interlacing, but
presenting a striped or striated appearance; and a microscopic
examination of them shows that even the most minute consist of parallel
filaments marked by longitudinal and transverse _striae_, or minute
channels. The fibers are nearly the same length as the muscles to which
they belong. Each muscular fiber is capable of contraction; it may act
singly, though usually it acts in unison with others. By a close
inspection, it has been found that fibers may be drawn apart
longitudinally, in which case they are termed _fibrillae_, or they may
be separated transversely, forming a series of discs. The _Sarcolemma_,
or investing sheath of the muscles, appears to be formed even before
there are any visible traces of the muscle itself. It is a transparent
and delicate membrane, but very elastic. The _Involuntary Muscles_ are
influenced by the sympathetic nervous system, and their action pertains
to the nutritive functions of the body. They differ from the voluntary
muscles in not being striated, having no tendons, and in the net-work
arrangements of their fibers. The _Intermediate Muscles_ are composed of
striated and unstriated fibers; they are, therefore, both voluntary and
involuntary in their functions. The muscles employed in respiration are
of this class, for we can breathe rapidly or slowly, and, for a short
time, even suspend their action; but soon, however, the organic muscles
assert their instinctive control, and respiration is resumed.

[Illustration: Fig. 22.
Unstriated muscular fiber; at _b_, in its natural
state; at _a_, showing the nuclei after the action of
acetic acid. ]

[Illustration: Fig. 23.
A view of the under side of the diaphragm.]

THE DIAPHRAGM, or midriff, is the muscular division between the thorax
and the abdomen. It has been compared to an inverted basin, the
concavity of which is directed toward the abdomen. The muscles receive
their nourishment from the numerous blood-vessels which penetrate their
tissues. The voluntary muscles are abundantly supplied with nerves,
while the involuntary are not so numerously furnished. The color of the
muscles is chiefly due to the blood which they contain. They vary in
size according to their respective functions. For example, the functions
of the heart require large and powerful muscles, and those of the eye,
small and delicate ones. There are between four hundred and sixty and
five hundred muscles in the human body.

[Illustration: Fig. 24.
A representation of the superficial layer of muscles on the anterior
portion of the body.]

[Illustration: Fig. 25.
A representation of the superficial layer of muscles on the posterior
portion of the body.]

Very rarely is motion produced by the action of a single muscle, but by
the harmonious action of several. There is infinite variety in the
arrangement of the muscles, each being adapted to its purpose, in
strength, tenacity, or elasticity. While some involuntarily respond to
the wants of organic life, others obey, with mechanical precision, the
edicts of the will. The peculiar characteristic of the muscles is their
contractility; for example, when the tip of the finger is placed in the
ear, an incessant vibration, due to the contraction of the muscles of
the ear, can be heard. When the muscles contract, they become shorter;
but what is lost in length is gained in breadth and thickness, so that
their actual volume remains the same. Muscles alternately contract and
relax, and thus act upon the bones. The economy of muscular power thus
displayed is truly remarkable. In easy and graceful walking, the forward
motion of the limbs is not altogether due to the exercise of muscular
power, but partly to the force of gravity, and only a slight assistance
of the muscles is required to elevate the leg sufficiently to allow it
to oscillate.

Motion is a characteristic of living bodies. This is true, not only in
animals, but also in plants. The oyster, although not possessing the
power of locomotion, opens and closes its shell at pleasure. The coral
insect appears at the door of its cell, and retreats at will. All the
varied motions of animals are due to a peculiar property of the muscles,
termed _contractility_. Although plants are influenced by external
agents, as light, heat, electricity, etc., yet it is supposed that they
may move in response to inward impulses. The sensitive stamens of the
barberry, when touched at their base on the inner side, resent the
intrusion, by making a sudden jerk forward. Venus's fly-trap, a plant
found in North Carolina, is remarkable for the sensitiveness of its
leaves; which close suddenly and capture insects which chance to alight
upon them. The muscles of the articulates are situated within the solid
framework, unlike the vertebrates, whose muscles are external to the
bony skeleton. All animals have the power of motion, from the lowest
radiate to the highest vertebrate, from the most repulsive polyp to that
type of organized life made in the very image of God.

The muscles, then, subserve an endless variety of purposes. By their aid
the farmer employs his implements of husbandry, the mechanic deftly
wields his tools, the artist plies his brush, while the fervid orator
gives utterance to thoughts glowing with heavenly emotions. It is by
their agency that the sublimest spiritual conceptions can be brought to
the sphere of the senses, and the noblest, loftiest aims of to-day can
be made glorious realizations of the future.

* * * * *




CHAPTER IV.

PHYSIOLOGICAL ANATOMY.

THE DIGESTIVE ORGANS.


_Digestion_ signifies the act of separating or distributing, hence its
application to the process by which food is made available for nutritive
purposes. The organs of digestion are the Mouth, Teeth, Tongue, Salivary
Glands, Pharynx, Esophagus, the Stomach and the Intestines, with their
glands, the Liver, Pancreas, Lacteals, and the Thoracic Duct.

[Illustration: Fig. 26.
A view of the lower jaw. _1_. The body.
_2, 2_. Rami, or branches. _3, 3_. Processes of
the lower jaw. _m_. Molar teeth. _b_. Bicuspids,
_c_. Cuspids. _i_. Incisors.]

The _Mouth_ is an irregular cavity, situated between the upper and the
lower jaw, and contains the organs of mastication. It is bounded by the
lips in front, by the cheeks at the sides, by the roof of the mouth and
teeth of the upper jaw above, and behind and beneath by the teeth of the
lower jaw, soft parts, and palate. The soft palate is a sort of pendulum
attached only at one of its extremities, while the other involuntarily
opens and closes the passage from the mouth to the pharynx. The interior
of the mouth, as well as other portions of the alimentary canal, is
lined with a delicate tissue, called _mucous membrane_.

The _Teeth_ are firmly inserted in the alveoli or sockets, of the upper
and the lower jaw. The first set, twenty in number, are temporary, and
appear during infancy. They are replaced by permanent teeth, of which
there are sixteen in each jaw; four incisors, or front teeth, four
cuspids, or eye teeth, four bicuspids, or grinders, and four molars, or
large grinders. Each tooth is divided into the crown, body, and root.
The _crown_ is the grinding surface; the _body_, the part projecting
from the jaw, is the seat of sensation and nutrition; the _root_ is that
portion of the tooth which is inserted in the alveolus. The teeth are
composed of dentine, or ivory, and enamel. The ivory forms the greater
portion of the body and root, while the enamel covers the exposed
surface. The small white cords communicating with the teeth are the
nerves.

The _Tongue_ is a flat oval organ, the base of which is attached to the
os hyoides, while the apex, the most sensitive part of the body, is
free. Its surface is covered with a membrane, which, at the sides and
lower part, is continuous with the lining of the mouth. On the lower
surface of the tongue, this membrane is thin and smooth, but on the
upper side it is covered with numerous papillae, which, in structure,
are similar to the sensitive papillae of the skin.

[Illustration: Fig. 27.
The salivary glands. The largest one, near the ear, is the
parotid gland. The next below it is the submaxillary gland.
The one under the tongue is the sublingual gland.]

The _Salivary Glands_ are six in number, three on each side of the
mouth. Their function is to secrete a fluid called _saliva_, which aids
in mastication. The largest of these glands, the _Parotid_, is situated
in front and below the ear; its structure, like that of all the salivary
glands, is cellular. The _Submaxillary_ gland is circular in form, and
situated midway between the angle of the lower jaw and the middle of the
chin. The _Sublingual_ is a long flattened gland, and, as its name
indicates, is located below the tongue, which when elevated, discloses
the saliva issuing from its porous openings.

The _Pharynx_ is nearly four inches in length, formed of muscular and
membranous cells, and situated between the base of the cranium and the
esophagus, in front of the spinal column. It is narrow at the upper
part, distended in the middle, contracting again at its junction with
the esophagus. The pharynx communicates with the nose, mouth, larynx,
and esophagus.

The _Esophagus_, a cylindrical organ, is a continuation of the pharynx,
and extends through the diaphragm to the stomach. It has three coats:
first, the muscular, consisting of an exterior layer of fibers running
longitudinally, and an interior layer of transverse fibers; second, the
cellular, which is interposed between the muscular and the mucous coat;
third, the mucous membrane, or internal coat, which is continuous with
the mucous lining of the pharynx.

[Illustration: Fig. 28.
A representation of the interior of the stomach.
_1_. The esophagus. _2_. Cardiac orifice opening into
the stomach. _6_. The middle or muscular coat.
_7_. The interior or mucous coat. _10_. The beginning
of the duodenum. _11_. The pyloric orifice.]

The _Stomach_ is a musculo-membranous, conoidal sac, communicating with
the esophagus by means of the cardiac orifice (see Fig. 28). It is
situated obliquely with reference to the body, its base lying at the
left side, while the apex is directed toward the right side. The stomach
is between the liver and spleen, subjacent to the diaphragm, and
communicates with the intestinal canal by the pyloric orifice. It has
three coats. The peritoneal, or external coat is composed of compact,
cellular tissue, woven into a thin, serous membrane, and assists in
keeping the stomach in place. The middle coat is formed of three layers
of muscular fibers: in the first, the fibres run longitudinally; in the
second, in a circular direction; and in the third, they are placed
obliquely to the others. The interior, or mucous coat, lines this organ.
The stomach has a soft, spongy appearance, and, when not distended, lies
in folds. During life, it is ordinarily of a pinkish color. It is
provided with numerous small glands, which secrete the gastric fluid
necessary for the digestion of food. The lining membrane, when divested
of mucus, has a wrinkled appearance. The arteries, veins, and
lymphatics, of the stomach are numerous.

[Illustration: Fig. 29.
Small and large intestines. _1, 1, 2, 2_.
Small intestine. _3_. Its termination in the
large intestine. _4_. Appendix vermiformis.
_5_. Caecum. _6_. Ascending colon.
_7_. Transverse colon. _8_. Descending colon.
_9_. Sigmoid flexure of colon. _10_. Rectum.]

The _Intestines_ are those convoluted portions of the alimentary canal
into which the food is received after being partially digested, and in
which the separation and absorption of the nutritive materials and the
removal of the residue take place. The coats of the intestines are
analogous to those of the stomach, and are, in fact, only extensions of
them. For convenience of description, the intestines may be divided into
the _small_ and the _large_. The small intestine is from twenty to
twenty-five feet in length, and consists of the Duodenum, Jejunum, and
Ileum. The _Duodenum_, so called because its length is equal to the
breadth of twelve fingers, is the first division of the small intestine.
If the mucous membrane of the duodenum be examined, it will be found
thrown into numerous folds, which are called _valvulæ conniventes_, the
chief function of which appears to be to retard the course of the
alimentary matter, and afford a larger surface for the accommodation of
the absorbent vessels. Numerous _villi_, minute thread-like projections,
will be found scattered over the surface of these folds, set side by
side, like the pile of velvet. Each _villus_ contains a net-work of
blood-vessels, and a lacteal tube, into which the ducts from the liver
and pancreas open, and pour their secretions to assist in the conversion
of the chyme into chyle. The _Jejunum_, so named because it is usually
found empty after death, is a continuation of the duodenum, and is that
portion of the alimentary canal in which the absorption of nutritive
matter is chiefly effected. The _Ileum_, which signifies something
rolled up, is the longest division of the small intestine. Although
somewhat thinner in texture than the jejunum, yet the difference is
scarcely perceptible. The large intestine is about five feet in length,
and is divided into the Caecum, Colon, and Rectum. The _Caecum_ is about
three inches in length. Between the large and the small intestine is a
valve, which prevents the return of excrementitious matter that has
passed into the large intestine. There is attached to the cæcum an
appendage about the size of a goose-quill, and three inches in length,
termed the _appendix vermiformis_. The _Colon_ is that part of the large
intestine which extends from the cæcum to the rectum, and which is
divided into three parts, distinguished as the ascending, the
transverse, and the descending.

[Illustration: Fig. 30.
Villi of the small intestine greatly
magnified.]

[Illustration: Fig. 31.
A section of the Ileum, turned inside out,
so as to show the appearance and arrangement
of the villi on an extended surface.]

The _Rectum_ is the terminus of the large intestine. The intestines are
abundantly supplied with blood-vessels. The arteries of the small
intestine are from fifteen to twenty in number. The large intestine is
furnished with three arteries, called the _colic arteries_. The
_ileo-colic artery_ sends branches to the lower part of the ileum, the
head of the colon, and the appendix vermiformis. The _right colic
artery_ forms arches, from which branches are distributed to the
ascending colon. The _colica media_ separates into two branches, one of
which is sent to the right portion of the transverse colon, the other to
the left. In its course, the _superior hemorrhoidal artery_ divides into
two branches, which enter the intestine from behind, and embrace it on
all sides, almost to the anus.

The _Thoracic Duct_ is the principal trunk of the absorbent system, and
the canal through which much of the chyle and lymph is conveyed to the
blood. It begins by a convergence and union of the lymphatics on the
lumbar vertebræ, in front of the spinal column, then passes upward
through the diaphragm to the lower part of the neck, thence curves
forward and downward, opening into the subclavian vein near its junction
with the left jugular vein, which leads to the heart.

[Illustration: Fig. 32.
_c, c_. Right and left subclavian veins. _b_.
Inferior vena cava. _a_. Intestines. _d_. Entrance
of the thoracic duct into the left
subclavian vein. _4_. Mesenteric glands,
through which the lacteals pass to the
thoracic duct.]

[Illustration: Fig. 33.
The inferior surface of the liver. 1. Right lobe.
2. Left lobe. 3. Gall-bladder.]

The _Liver_, which is the largest gland in the body, weighs about four
pounds in the adult, and is located chiefly on the right side,
immediately below the diaphragm. It is a single organ, of a dark red
color, its upper surface being convex, while the lower is concave. It
has two large lobes, the right being nearly four times as large as the
left. The liver has two coats, the _serous_, which is a complete
investment, with the exception of the diaphragmatic border, and the
depression for the gall-bladder, and which helps to suspend and retain
the organ in position; and the _fibrous_, which is the inner coat of the
liver, and forms sheaths for the blood-vessels and excretory ducts. The
liver is abundantly supplied with arteries, veins, nerves, and
lymphatics. Unlike the other glands of the human body, it receives two
kinds of blood; the arterial for its nourishment, and the venous, from
which it secretes the bile. In the lower surface of the liver is lodged
the gall-bladder, a membranous sac, or reservoir, for the bile. This
fluid is not absolutely necessary to the digestion of food, since this
process is effected by other secretions, nor does bile exert any special
action upon, starchy or oleaginous substances, when mixed with them at a
temperature of 100° F. Experiments also show that in some animals there
is a constant flow of bile, even when no food has been taken, and there
is consequently no digestion to be performed. Since the bile is formed
from the venous blood, and taken from the waste and disintegration of
animal tissue, it would appear that it is chiefly an excrementitious
fluid. It does not seem to have accomplished its function when
discharged from the liver and poured into the intestine, for there it
undergoes various alterations previous to re-absorption, produced by its
contact with the intestinal juices. Thus the bile, after being
transformed in the intestines, re-enters the blood under a new form, and
is carried to some other part of the system to perform its mission.

The _Spleen_ is oval, smooth, convex on its external, and irregularly
concave on its internal, surface. It is situated on the left side, in
contact with the diaphragm and stomach. It is of a dark red color,
slightly tinged with blue at its edges. Some physiologists affirm that
no organ receives a greater quantity of blood, according to its size,
than the spleen. The structure of the spleen and that of the mesenteric
glands are similar, although the former is provided with a scanty supply
of lymphatic vessels, and the chyle does not pass through it, as through
the mesenteric glands. The _Pancreas_ lies behind the stomach, and
extends transversely across the spinal column to the right of the
spleen. It is of a pale, pinkish color, and its secretion is analogous
to that of the salivary glands; hence it has been called the _Abdominal
Salivary Gland_.

[Illustration: Fig. 34.
Digestive organs. _3_. The tongue. _7_. Parotid
gland. _8_. Sublingual gland. _5_. Esophagus. _9_.
Stomach. _10_. Liver. _11_. Gall-bladder, _14_. Pancreas.
_13, 13_. The duodenum. The small and large intestines
are represented below the stomach.]

Digestion is effected in those cavities which we have described as parts
of the alimentary canal. The food is first received into the mouth,
where it is masticated by the teeth, and, after being mixed with mucus
and saliva, is reduced to a mere pulp; it is then collected by the
tongue, which, aided by the voluntary muscles of the throat, carries the
food backward into the pharynx, and, by the action of the involuntary
muscles of the pharynx and esophagus, is conveyed to the stomach. Here
the food is subjected to a peculiar, churning movement, by the alternate
relaxation and contraction of the fibers which compose the muscular wall
of the stomach. As soon as the food comes in contact with the stomach,
its pinkish color changes to a bright red; and from the numerous tubes
upon its inner surface is discharged a colorless fluid, called the
_gastric juice_, which mingles with the food and dissolves it. When the
food is reduced to a liquid condition, it accumulates in the pyloric
portion of the stomach. Some distinguished physiologists believe that
the food is kept in a gentle, unceasing, but peculiar motion, called
_peristaltic_, since the stomach contracts in successive circles. In the
stomach the food is arranged in a methodical manner. The undigested
portion is detained in the upper, or cardiac extremity, near the
entrance of the esophagus, by contraction of the circular fibers of the
muscular coat. Here it is gradually dissolved, and then carried into the
pyloric portion of the stomach. From this, then, it appears, that the
dissolved and undissolved portions of food occupy different parts of the
stomach. After the food has been dissolved by the gastric fluid, it is
converted into a homogeneous, semi-fluid mass, called _chyme_. This
substance passes from the stomach through the pyloric orifice into the
duodenum, in which, by mixing with the bile and pancreatic fluid, its
chemical properties are again modified, and it is then termed _chyle_,
which has been found to be composed of three distinct parts, a
reddish-brown sediment at the bottom, a whey-colored fluid in the
middle, and a creamy film at the top. Chyle is different from chyme in
two respects: First, the alkali of the digestive fluids, poured into the
duodenum, or upper part of the small intestine, neutralizes the acid of
the chyme; secondly, both the bile and the pancreatic fluid seem to
exert an influence over the fatty substances contained in the chyme,
which assists the subdivision of these fats into minute particles. While
the chyle is propelled along the small intestine by the peristaltic
action, the matter which it contains in solution is absorbed in the
usual manner into the vessels of the villi by the process called
_osmosis_. The fatty matters being subdivided into very minute
particles, but not dissolved, and consequently incapable of being thus
absorbed by osmosis, pass bodily through the epithelial lining of the
intestine into the commencement of the lacteal tubes in the villi. The
digested substances, as they are thrust along the small intestines,
gradually lose their albuminoid, fatty, and soluble starchy and
saccharine matters, and pass through the ileo-caecal valve into the
cæcum and large intestine. An acid reaction takes place here, and they
acquire the usual fæcal smell and color, which increases as they
approach the rectum. Some physiologists have supposed that a second
digestion takes place in the upper portion of the large intestine. The
lacteals, filled with chyle, pass into the mesenteric glands with which
they freely unite, and afterward enter the _receptaculum chyli_, which
is the commencement of the thoracic duct, a tube of the size of a
goose-quill, which lies in front of the backbone. The lymphatics, the
function of which is to secrete and elaborate lymph, also terminate in
the _receptaculum chyli_, or receptacle for the chyle. From this
reservoir the chyle and lymph flow into the thoracic duct, through which
they are conveyed to the left subclavian vein, there to be mingled with
venous blood. The blood, chyle, and lymph, are then transmitted directly
to the lungs.

The process of nutrition aids in the development and growth of the body;
hence it has been aptly designated a "perpetual reproduction." It is the
process by which every part of the body assimilates portions of the
blood distributed to it. In return, the tissues yield a portion of the
material which was once a component part of their organization. The body
is constantly undergoing waste as well as repair. One of the most
interesting facts in regard to the process of nutrition in animals and
plants is, that all tissues originate in cells. In the higher types of
animals, the blood is the source from which the cells derive their
constituents. Although the alimentary canal is more or less complicated
in different classes of animals, yet there is no species, however low in
the scale of organization, which does not possess it in some form.[2]
The little polyp has only one digestive cavity, which is a pouch in the
interior of the body. In some animals circulation is not distinct from
digestion, in others respiration and digestion are performed by the same
organs; but as we rise in the scale of animal life, digestion and
circulation are accomplished in separate cavities, and the functions of
nutrition become more complex and distinct.


* * * * *




CHAPTER V.

PHYSIOLOGICAL ANATOMY.

ABSORPTION.


[Illustration: Fig. 35.
Villi of the small intestine greatly magnified.]

_Absorption_ is the vital function by which nutritive materials are
selected and imbibed for the sustenance of the body. Absorption, like
all other functional processes, employs agents to effect its purposes,
and the _villi_ of the small intestine, with their numberless projecting
organs, are specially employed to imbibe fluid substances; this they do
with a celerity commensurate to the importance and extent of their
duties. They are little vascular prominences of the mucous membrane,
arising from the interior surface of the small intestine. Each villus
has two sets of vessels. (1.) The blood-vessels, which, by their
frequent blending, form a complete net-work beneath the external
epithelium; they unite at the base of the villus, forming a minute vein,
which is one of the sources of the portal vein. (2.) In the center of
the villus is another vessel, with thinner and more transparent walls,
which is the commencement of a lacteal.

The _Lacteals_ originate in the walls of the alimentary canal, are very
numerous in the small intestine, and, passing between the laminae of the
mesentery, they terminate in the _receptaculum chyli_, or reservoir for
the chyle. The mesentery consists of a double layer of cellular and
adipose tissue. It incloses the blood-vessels, lacteals, and nerves of
the small intestine, together with its accessory glands. It is joined to
the posterior abdominal wall by a narrow _root_; anteriorly, it is
attached to the whole length of the small intestine. The lacteals are
known as the absorbents of the intestinal walls, and after digestion is
accomplished, are found to contain a white, milky fluid, called _chyle_.
The chyle does not represent the entire product of digestion, but only
the fatty substances suspended in a serous fluid.

Formerly, it was supposed that the lacteals were the only agents
employed in absorption, but more recent investigations have shown that
the blood-vessels participate equally in the process, and are frequently
the more active and important of the two. Experiments upon living
animals have proved that absorption of poisonous substances occurs, even
when all communication by way of the lacteals and lymphatics is
obstructed, the passage by the blood-vessels alone remaining. The
absorbent power which the blood-vessels of the alimentary canal possess,
is not limited to alimentary substances, but through them, soluble
matters of almost every description are received into the circulation.

The _Lymphatics_ are not less important organs in the process of
absorption. Nearly every part of the body is permeated by a second
series of capillaries, closely interlaced with the blood-vessels,
collectively termed the _Lymphatic System_. Their origin is not known,
but they appear to form a _plexus_ in the tissues, from which their
converging trunks arise. They are composed of minute tubes of delicate
membrane, and from their net-work arrangement they successively unite
and finally terminate in two main trunks, called the _great lymphatic
veins_. The lymphatics, instead of commencing on the intestinal walls,
as do the lacteals, are distributed through most of the vascular tissues
as well as the skin. The lymphatic circulation is not unlike that of the
blood; its circulatory apparatus is, however, more delicate, and its
functions are not so well understood.

[Illustration: Fig. 36.
A general view of the Lymphatic System.]

The _lymph_ which circulates through the lymphatic vessels is an
alkaline fluid composed of a plasma and corpuscles. It may be considered
as blood deprived of its red corpuscles and, diluted with water. Nothing
very definite is known respecting the functions of this fluid. A large
proportion of its constituents is derived from the blood, and the exact
connection of these substances to nutrition is not properly understood.
Some excrementitious matters are supposed to be taken from the tissues
by the lymph and discharged into the blood, to be ultimately removed
from the system. The lymph accordingly exerts an important function by
removing a portion of the decayed tissues from the body.

[Illustration: Fig. 37.
1. A representation of a lymphatic
vessel highly magnified. 2. Lymphatic
valves. 3. A lymphatic gland and its vessels.]

In all animals which possess a lacteal system there is also a lymphatic
system, the one being the complement of the other. The fact that lymph
and chyle are both conveyed into the general current of circulation,
leads to the inference that the lymph, as well as the chyle, aids in the
process of nutrition. The body is continually undergoing change, and
vital action implies waste of tissues, as well as their growth. Those
organs which are the instruments of motion, as the muscles, cannot be
employed without wear and waste of their component parts. Renovated
tissues must replace those which are worn out, and it is a part of the
function of the absorbents to convey nutritive material into the general
circulation. Researches in microscopical anatomy have shown that the
skin contains multitudes of lymphatic vessels and that it is a powerful
absorbent.

Absorption is one of the earliest and most essential functions of animal
and vegetables tissues. The simpler plants consist of only a few cells,
all of which are employed in absorption; but in the flowering plants
this function is performed by the roots. It is accomplished on the same
general principles in animals, yet it presents more modifications and a
greater number of organs than in vegetables. While animals receive their
food into a sac, or bag called the _stomach_, and are provided with
absorbent vessels such as nowhere exist in vegetables, plants plunge
their absorbent organs into the earth, whence they derive nourishing
substances. In the lower order of animals, as in sponges, this function
is performed by contiguous cells, in a manner almost as elementary as in
plants. In none of the invertebrate animals is there any _special_
absorbent system. Internal absorption is classified by some authors as
follows: _interstitial_, _recrementitial_, and _excrementitial_; by
others as _accidental_, _venous_, and _cutaneous_. The general cutaneous
and mucous surfaces exhale, as well as absorb; thus the skin, by means
of its sudoriferous glands, exhales moisture, and is at the same time as
before stated, a powerful absorbent. The mucous surface of the lungs is
continually throwing off carbonic acid and absorbing oxygen; and through
their surface poisons are sometimes taken into the blood. The continual
wear and waste to which living tissues are subject, makes necessary the
provision of such a system of vessels for conveying away the worn-out
materials and supplying the body with new.

* * * * *




CHAPTER VI.

PHYSICAL AND VITAL PROPERTIES OF THE BLOOD.


[Illustration: Fig. 38.
Red corpuscles of human blood, represented
at _a_, as they are seen when
rather _beyond_ the focus of the microscope;
and at _b_ as they appear when,
_within_ the focus. Magnified 400 diameters.]

[Illustration: Fig. 39.
Development of human lymph and chyle-corpuscles
into red corpuscles of blood. _A_. A lymph, or white
blood-corpuscle. _B_. The same in process of conversion
into a red corpuscle. _C_. A lymph-corpuscle with the
cell-wall raised up around it by the action of water. _D_.
A lymph-corpuscle, from which the granules have
almost disappeared. _E_. A lymph-corpuscle, acquiring
color; a single granule, like a nucleus, remains. _F_. A
red corpuscle fully developed.]

_Blood_ is the animal fluid by which the tissues of the body are
nourished. This pre-eminently vital fluid permeates every organ,
distributes nutritive material to every texture, is essentially modified
by respiration, and, finally, is the source of every secretion and
excretion. Blood has four constituents: Fibrin, Albumen, Salts (which
elements, in solution, form the _liquor sanguinis_), and the Corpuscles.
Microscopical examination shows that the corpuscles are of two kinds,
known as the _red_ and the _white_, the former being by far the more
abundant. They are circular in form and have a smooth exterior, and are
on an average 1/3200 part of an inch in diameter, and are about
one-fourth of that in thickness. Hence more than ten millions of them
may lie on a space an inch square. If spread out in thin layers and
subjected to transmitted light, they present a slightly yellowish color,
but when crowded together and viewed by refracted light, exhibit a deep
red color. These blood-corpuscles have been termed _discs_, and are not,
as some have supposed, solid material, but are very nearly fluid. The
red corpuscles although subjected to continual movement, have a tendency
to approach one another, and when their flattened surfaces come in
contact, so firmly do they adhere that they change their shape rather
than submit to a separation. If separated, however, they return to their
usual form. The colorless corpuscles are larger than the red and differ
from them in being extremely irregular in their shape, and in their
tendency to adhere to a smooth surface, while the red corpuscles float
about and tumble over one another. They are chiefly remarkable for their
continual variation in form. The shape of the red corpuscles is only
altered by external influences, but the white are constantly undergoing
alterations, the result of changes taking place within their own
substance. When diluted with water and placed under the microscope they
are found to consist of a spheroidal sac, containing a clear or granular
fluid and a spheroidal vesicle, which is termed the _nucleus_. They have
been regarded by some physiologists as identical with those of the lymph
and chyle. Dr. Carpenter believes that the function of these cells is to
convert albumen into fibrin, by the simple process of cell-growth. It is
generally believed that the red corpuscles are derived in some way from
the colorless. It is supposed that the red corpuscle is merely the
nucleus of a colorless corpuscle enlarged, flattened, colored and
liberated by the bursting of the wall of its cell. When blood is taken
from an artery and allowed to remain at rest, it separates into two
parts: a solid mass, called the clot, largely composed of fibrin; and a
fluid known as the _serum_, in which the clot is suspended. This process
is termed _coagulation_. The serum, mostly composed of _albumen_, is a
transparent, straw-colored fluid, having the odor and taste of blood.
The whole quantity of blood in the body is estimated on an average to be
about one-ninth of its entire weight. The distinctions between the
arterial and the venous blood are marked, since in the arterial system
the blood is uniformly bright red, and in the venous of a very dark red
color The blood-corpuscles contain both oxygen and carbonic acid in
solution. When carbonic acid predominates, the blood is dark red; when
oxygen, scarlet. In the lungs, the corpuscles give up carbonic acid, and
absorb a fresh supply of oxygen, while in the general circulation the
oxygen disappears in the process of tissue transformation, and is
replaced, in the venous blood, by carbonic acid. The nutritive portions
of food are converted into a homogeneous fluid, which pervades every
part of the body, is the basis of every tissue, and which is termed the
_blood_. This varies in color and composition in different animals. In
the polyp the nutritive fluid is known as _chyme_, in many mollusks, as
well as articulates, it is called _chyle_, but in vertebrates, it is
more highly organized and is called blood. In all the higher animal
types it is of a red color, although redness is not one of its essential
qualities. Some tribes of animals possess true blood, which is not red;
thus the blood of the insect is colorless and transparent; that of the
reptile yellowish; in the fish the principle part is without color, but
the blood of the bird is deep red. The blood of the mammalia is of a
bright scarlet hue. The temperature of the blood varies in different
species, as well as in animals of the same species under different
physiological conditions; for this reason, some animals are called
_cold-blooded._ Disease also modifies the temperature of the blood; thus
in fevers it is generally increased, but in cholera greatly diminished.
THE blood has been aptly termed the "vital fluid," since there is a
constant flow from the heart to the tissues and organs of the body, and
a continual return after it has circulated through these parts. Its
presence in every part of the body is one of the essential conditions of
animal life, and is effected by a special set of organs, called the
_circulatory organs_.

* * * * *




CHAPTER VII.

PHYSIOLOGICAL ANATOMY.

CIRCULATORY ORGANS.


Having considered the formation of chyle, traced it through the
digestive process, seen its transmission into the _vena cava_, and,
finally, its conversion into blood, we shall now describe how it is
distributed to every part of the system. This is accomplished through
organs which, from the round of duties they perform, are called
_circulatory_. These are the Heart, Arteries, Veins, and Capillaries,
which constitute the _vascular system_.

Within the thorax or chest of the human body, and enclosed within a
membranous sac, called the _pericardium_, is the great force-pump of the
system, the heart. This organ, to which all the arteries and veins of
the body may be either directly or indirectly traced, is roughly
estimated to be equal in size to the closed fist of the individual to
whom it belongs.

It has a broad end turned upwards, and a little to the right side,
termed its _base_; and a pointed end called its _apex_, turned
downwards, forwards, and to the left side, and lying beneath a point
about an inch to the right of, and below, the left nipple, or just below
the fifth rib. Attached to the rest of the body only by the great
blood-vessels which issue from and enter it at its base, the heart is
the most mobile organ in the economy, being free to move in different
directions.

The heart is divided into two great cavities by a fixed partition, which
extends from the base to the apex of the organ, and which prevents any
direct communication between them. Each of these great cavities is
further subdivided transversely by a movable partition, the cavity above
each transverse partition being called the _auricle_, and the cavity
below, the _ventricle_, right or left, as the case may be.

[Illustration: Fig. 40.
General view of the heart and lungs, _t_. Trachea, or
windpipe, _a_. Aorta, _p_. Pulmonary artery, 1, 2.
Branches of the pulmonary artery, one going to the
right, the other to the left lung. _h._ The heart.]

The walls of the auricles are much thinner than those of the ventricles,
and the wall of the right ventricle is much thinner than that of the
left, from the fact that the ventricles have more work to perform than
the auricles, and the left ventricle more than the right.

In structure, the heart is composed almost entirely of muscular fibers,
which are arranged in a very complex and wonderful manner. The outer
surface of the heart is covered with the pericardium, which closely
adheres to the muscular substance. Inside, the cavities are lined with a
thin membrane, called the _endocardium_. At the junction between the
auricles and ventricles, the apertures of communication between their
cavities are strengthened by _fibrous rings_. Attached to these fibrous
rings are the movable partitions or valves, between the auricles and the
ventricles, the one on the right side of the heart being called the
_tricuspid valve_, and the one on the left side the _mitral valve._ A
number of fine, but strong, tendinous chords, called _chordæ
tendineæ_, connect the edges and apices of these valves with
column-like elevations of the fleshy substance of the walls of the
ventricles, called _columnæ carneæ_.

[Illustration: Fig. 41.
1. The descending vena cava. 2.
The ascending vena cava. 3. The
right auricle. 4. The opening between
the right auricle and the right
ventricle. 5. The right ventricle. 6.
The tricuspid valves. 7. The pulmonary
artery. 8, 8. The branches
of the pulmonary artery which pass
to the right and the left lung. 9. The
semilunar valves of the pulmonary
artery. 10. The septum between the
two ventricles of the heart. 11, 11.
The pulmonary veins. 12. The left
auricle. 13. The opening between
the left auricle and ventricle. 14.
The left ventricle. 15. The mitral
valves. 16, 16. The aorta. 17. The
semilunar valves of the aorta.]

The valves are so arranged that they present no obstacle to the free
flow of blood from the auricles into the ventricles, but if any is
forced the other way, it gets between the valve and the wall of the
heart, and drives the valve backwards and upwards, thus forming a
transverse partition between the auricle and ventricle, through which no
fluid can pass.

At the base of the heart are given off two large arteries, one on the
right side, which conveys the blood to the lungs, called the _pulmonary
artery_, and one on the left side, which conveys the blood to the system
in general, called the _aorta_. At the junction of each of these great
vessels with its corresponding ventricle, is another valvular apparatus,
consisting of three pouch-like valves, called the _semilunar valves_,
from their resemblance, in shape, to a half-moon. Being placed on a
level and meeting in the middle line, they entirely prevent the passage
of any fluid which may be forced along the artery towards the heart,
but, flapping back, they offer no obstruction to the free flow of blood
from the ventricles into the arteries.

[Illustration: Fig. 42.
A representation of the venous and arterial
circulation of the blood.]

The _Arteries_, being always found empty after death, were supposed by
the ancients, who were ignorant of the circulation of the blood, to be
tubes containing air; hence their name, which is derived from a Greek
word and signifies an _air-tube._ Arteries are the cylindrical tubes
which carry blood to every part of the system. All the arteries, except
the coronary which supply the substance of the heart, arise from the two
main trunks, the pulmonary artery and the aorta. They are of a
yellowish-white color, and their inner surface is smooth. The arteries
have three coats. (1.) The external coat, which is destitute of fat, and
composed chiefly of cellular tissue, is very firm and elastic, and can
readily be dissected from the middle coat. (2.) The middle, or fibrous
coat, is thicker than the external, and composed of yellowish fibers,
its chief property is contractility. (3.) The internal coat consists of
a colorless, thin, transparent membrane, yet so strong that it can, it
is thought, better resist a powerful pressure than either of the others.
Arteries are very elastic as well as extensible, and their chief
extensibility is in length. If an artery of a dead body be divided,
although empty, its cylindrical form will be preserved.

The _Veins_ are the vessels through which the venous blood returns to
the auricles of the heart. They are more numerous than the arteries, and
originate from numerous capillary tubes, while the arteries are given
off from main trunks. In some parts of the body, the veins correspond in
number to the arteries; while in others, there are two veins to every
artery. The veins commence by minute roots in the capillaries, which are
everywhere distributed through the body, and gradually increase in size,
until they unite and become large trunks, conveying the dark blood to
the heart. The veins, like the arteries, have three coats. The external,
or cellular coat, resembles that of the arteries; the middle is fibrous,
but thinner than the corresponding one of the arteries; and the internal
coat is serous, and analogous to that of those vessels. The veins belong
to the three following classes: (1.) The systemic veins, which bring the
blood from different parts of the body and discharge it into the vena
cava, by means of which it is conveyed to the heart; (2), the pulmonary
veins, which bring the arterial, or bright red blood from the lungs and
carry it to the left auricle; (3), the veins of the portal system, which
originate in the capillaries of the abdominal organs, then converge into
trunks and enter the liver, to branch off again into divisions and
subdivisions of the minutest character.

The _Capillaries_ form an extremely fine net-work, and are distributed
to every part of the body. They vary in diameter from 1/3500 to 1/2000
of an inch. They are so universally prevalent throughout the skin, that
the puncture of a needle would wound a large number of them. These
vessels receive the blood and bring it into intimate contact with the
tissues, which take from it the principal part of its oxygen and other
elements, and give up to it carbonic acid and the other waste products
resulting from the transformation of the tissues, which are transmitted
through the veins to the heart, and thence by the arteries to the lungs
and various excretory organs.

The blood from the system in general, except the lungs, is poured into
the right auricle by two large veins, called the superior and the
inferior _vena cava_,' and that returning from the lungs is poured into
the left auricle by the _pulmonary veins._

During life the heart contracts rhythmically, the contractions
commencing at the base, in each auricle, and extending towards the apex.

Now it follows, from the anatomical arrangement of this organ, that when
the auricles contract, the blood contained in them is forced through the
auriculo-ventricular openings into the ventricles; the contractions then
extending to the ventricles, in a wave-like manner, the great proportion
of the blood, being prevented from re-entering the auricles by the
tricuspid and mitral valves, is forced onward into the pulmonary artery
from the right ventricle, and into the aorta from the left ventricle.

When the contents of the ventricles are suddenly forced into these great
blood-vessels, a shock is given to the entire mass of fluid which they
contain, and this shock is speedily propagated along their branches,
being known at the wrist as the _pulse_.

On inspection, between the fifth and sixth ribs on the left side of the
chest, a movement is perceptible, and, if the hand be applied, the
impulse may be felt. This is known as the throbbing, or beating of the
heart.

If the ear is placed over the region of the heart, certain sounds are
heard, which recur with great regularity. First is heard a comparatively
long, dull sound, then a short, sharp sound, then a pause, and then the
long, dull sound again. The first sound is caused mainly by the
tricuspid and mitral valves, and the second is the result of sudden
closure of the semilunar valves.

No language can adequately describe the beauty of the circulatory
system. The constant vital flow through the larger vessels, and the
incessant activity of those so minute that they are almost
imperceptible, fully illustrate the perfectness of the mechanism of the
human body, and the wisdom and goodness of Him who is its author.

Experiments have shown that the small arteries may be directly
influenced through the nervous system, which regulates their caliber by
controlling the state of contraction of their muscular walls. The effect
of this influence of the nervous system enables it to control the
circulation over certain areas; and, notwithstanding the force of the
heart and the state of the blood-vessels in general, to materially
modify the circulation in different spots. Blushing, which is simply a
local modification of the circulation, is effected in this way. Some
emotion takes possession of the mind, and the action of the nerves,
which ordinarily keep up a moderate contraction of the muscular coats of
the arteries, is lost, and the vessels relax and become distended with
arterial blood, which is a warm and bright red fluid; thereupon a
burning sensation is felt, and the skin grows red, the degree of the
blush depending upon the intensity of the emotion.

The pallor produced by fright and by extreme anxiety, is purely the
result of a local modification of the circulation, brought about by an
over-stimulation of the nerves which supply the small arteries, causing
them to contract, and to thus cut off more or less completely the supply
of blood.

* * * * *




CHAPTER VIII.

PHYSIOLOGICAL ANATOMY.

THE ORGANS OF RESPIRATION.


THE ORGANS OF RESPIRATION are the Trachea, or windpipe, the Bronchia,
formed by the subdivision of the trachea, and the Lungs, with their
air-cells. The _Trachea_ is a vertical tube situated between the lungs
below, and a short quadrangular cavity above, called the _larynx_, which
is part of the windpipe, and used for the purpose of modulating the
voice in speaking or singing. In the adult, the trachea, in its
unextended state, is from four and one-half to five inches in length,
about one inch in diameter, and, like the larynx, is more fully
developed in the male than in the female. It is a fibro-cartilaginous
structure, and is composed of flattened rings, or segments of circles.
It permits the free passage of air to and from the lungs.

The _Bronchia_ are two tubes, or branches, one proceeding from the
windpipe to each lung. Upon entering the lungs, they divide and
subdivide until, finally, they terminate in small cells, called the
_bronchial or air-cells,_ which are of a membranous character.

[Illustration: Fig. 43.
An ideal representation of the respiratory organs. _3._ The
larynx. _4._ The trachea. _5, 6._ The bronchia. _9, 9, 9, 9._ Air-cells.
_1, 1, 1, 2, 2, 2._ Outlines of the lungs.]

The _Lungs_ are irregular conical organs rounded at the apex, situated
within the chest, and filling the greater part of it, since the heart is
the only other organ which occupies much space in the thoracic cavity.
The lungs are convex externally, and conform to the cavity of the chest,
while the internal surface is concave for the accommodation of the
heart. The size of the lungs depends upon the capacity of the chest.
Their color varies, being of a pinkish hue in childhood but of a gray,
mottled appearance in the adult. They are termed the _right_ and _left_
lung. Each lung resembles a cone with its base resting upon the
diaphragm, and its apex behind the collar-bone. The right lung is larger
though shorter, than the left, not extending so low, and has three
_lobes_, formed by deep fissures, or longitudinal divisions, while the
left has but two lobes. Each lobe is also made up of numerous _lobules_,
or small lobes, connected by cellular tissue, and these contain great
numbers of cells. The lungs are abundantly supplied with blood-vessels,
lymphatics, and nerves. The density of a lung depends upon the amount of
air which it contains. Thus, experiment has shown that in a _foetus_
which has never breathed, the lungs are compact and will sink in water;
but as soon as they become inflated with air, they spread over a larger
surface, and are therefore more buoyant. Each lung is invested, as far
as its root, with a membrane, called the _pleura_, which is then
continuously extended to the cavity of the chest, thus performing the
double office of lining it, and constituting a partition between the
lungs. The part of the membrane which forms this partition is termed the
_mediastinum_. Inflammation of this membrane is called _pleurisy_. The
lungs are held in position by the root, which is formed by the pulmonary
arteries, veins, nerves, and the bronchial tubes. Respiration is the
function by which the venous blood, conveyed to the lungs by the
pulmonary artery, is converted into arterial blood. This is effected by
the elimination of carbonic acid, which is expired or exhaled from the
lungs, and by the absorption of oxygen from the air which is taken into
the lungs, by the act of inspiration or inhalation. The act of
expiration is performed chiefly by the elevation of the diaphragm and
the descent of the ribs, and inspiration is principally effected by the
descent of the diaphragm and the elevation of the ribs.

[Illustration: Fig. 44.
A representation of the heart and lungs. 4. The
heart. 5. The pulmonary artery. 8. Aorta. 9, 11.
Upper lobes of the lungs. 10, 13. Lower lobes. 12.
Middle lobe of the right lung. 2. Superior vena
cava. 3. Inferior vena cava.]

When the muscles of some portions of the air-passages are relaxed, a
peculiar vibration follows, known as snoring. Coughing and sneezing are
sudden and spasmodic expiratory efforts, and generally involuntary.
Sighing is a prolonged deep inspiration, followed by a rapid, and
generally audible expiration. It is remarkable that laughing and
sobbing, although indicating opposite states of the mind, are produced
in very nearly the same manner. In hiccough, the contraction is more
sudden and spasmodic than in laughing or sobbing. The quantity of oxygen
consumed during sleep is estimated to be considerably less than that
consumed during wakefulness.

[Illustration: Fig. 45.
View of the pulmonary circulation.]

It is difficult to estimate the amount of air taken into the lungs at
each inspiration, as the quantity varies according to the condition,
size, and expansibility of the chest, but in ordinary breathing it is
supposed to be from twenty to thirty cubic inches. The consumption of
oxygen is greater when the temperature is low, and during digestion. All
the respiratory movements, so far as they are independent of the will of
the individual, are controlled by that part of the brain called the
_medulla oblongata_. The respiratory, or breathing process, is not
instituted for the benefit of man alone, for we find it both in the
lower order of animals and in plant life. Nature is very economical in
the arrangement of her plans, since the carbonic acid, which is useless
to man, is indispensable to the existence of plants, and the oxygen,
rejected by them, is appropriated to his use. In the lower order of
animals, the respiratory act is similar to that of the higher types,
though not so complex; for there are no organs of respiration, as the
lungs and gills are called. Thus, the higher the animal type, the more
complex its organism. The effect of air upon the color of the blood is
very noticeable. If a quantity be drawn from the body, thus being
brought into contact with the air, its color gradually changes to a
brighter hue. There is a marked difference between the properties of the
venous and the arterial blood.

The venous blood is carried, as we have previously described, to the
right side of the heart and to the lungs, where it is converted into
arterial blood. It is now of uniform quality, ready to be distributed
throughout the body, and capable of sustaining life and nourishing the
tissues. Man breathes by means of lungs; but who can understand their
wonderful mechanism, so perfect in all its parts? Though every organ is
subservient to another, yet each has its own office to perform. The
minute air-cells are for the aeration of the blood; the larger bronchial
tubes ramify the lungs, and suffuse them with air; the trachea serves as
a passage for the air to and from the lungs, while at its upper
extremity is the larynx, which has been fitly called the organ of the
human voice. At its extremity we find a sort of shield, called the
_epiglottis_, the office of which is supposed to be to prevent the
intrusion of foreign bodies.

* * * * *




CHAPTER IX.

PHYSIOLOGICAL ANATOMY

THE SKIN.


Through digestion and respiration, the blood is continually supplied
with material for its renewal; and, while the nutritive constituents of
the food are retained to promote the growth of the body, those which are
useless or injurious are in various ways expelled. There are, perhaps,
few parts of the body more actively concerned in this removal than the
skin.

[Illustration: Fig. 46: An ideal
view of the papillae. 1, 1. Cutis vera.
2.2. Papillary layer. 3, 3. Arteries of the papillae.
4, 4. Nerves of the papillae. 5, 5. Veins of the papillae.]

The skin is a membranous envelope covering the entire body. It consists
of two layers, termed the Cutis Vera, or true skin, and the Epidermis,
or cuticle. The _Cutis Vera_ is composed of fibers similar to those of
the cellular tissue. It consists of white and yellow fibers, which are
more densely woven near the surface than deeper in the structure; the
white give strength, the yellow strength and elasticity combined. The
true skin may be divided into two layers, differing in their
characteristics, and termed respectively the superficial or papillary
layer, and the deep or fibrous layer. Upon the external surface, are
little conical prominences, known as _papillae_. The papillae are
irregularly distributed over the body, in some parts being smaller and
more numerous than in others, as on the finger-ends, where their summits
are so intimately connected as to form a tolerably smooth surface. It is
owing to their perfect development, that the finger-tips are adapted to
receive the most delicate impressions of touch. Although every part of
the skin is sensitive, yet the papillae are extremely so, for they are
the principal means through which the impressions of objects are
communicated. Each papilla not only contains a minute vein and artery,
but it also incloses a loop of sensitive nerves. When the body is
exposed to cold, these papillae can be more distinctly seen in the form
of prominences, commonly known as "goose-pimples."

[Illustration: Fig. 47.
A section of the skin, showing its arteries and
veins. A, A. Arterial branches. B, B. Capillaries
in which the branches terminate. C. The venous
trunk into which the blood from the capillaries
flows.]

The internal, or fibrous layer of the skin, contains numerous
depressions, each of which furnishes a receptacle for fat. While the
skin is supplied with a complete net-work of arteries, veins, and
nerves, which make it sensitive to the slightest touch, it also contains
numerous lymphatic vessels, so minute that they are invisible to the
naked eye.

Among the agents adapted for expelling the excretions from the system,
few surpass the _Sudoriferous Glands_. These are minute organs which
wind in and out over the whole extent of the true skin, and secrete the
perspiration. Though much of it passes off as insensible transpiration,
yet it often accumulates in drops of sweat, during long-continued
exercise or exposure to a high temperature. The office of the
perspiration is two-fold. It removes noxious matter from the system, and
diminishes animal heat, and thereby equalizes the temperature of the
body. It also renders the skin soft and pliable, thus better adapting it
to the movements of the muscles. The _Sebaceous Glands_, which are
placed in the true skin, are less abundant where the sudoriferous glands
are most numerous, and _vice versa_. Here, as elsewhere, nature acts
with systematic and intelligent design. The perspiratory glands are
distributed where they are most needed,--in the eyelids, serving as
lubricators; in the ear passages, to produce the _cerumen_, or wax,
which prevents the intrusion of small insects; and in the scalp, to
supply the hair with its natural pomatum.

[Illustration: Fig. 48.
A perspiratory gland, highly
magnified. 1, 1. The gland. 2, 2.
Excretory ducts uniting to form
a tube which tortuously perforates
the cuticle at 3, and opens
obliquely on its surface at 4.]

[Illustration: Fig. 49.
A representation of oil-tubes from the scalp
and nose.]

[Illustration: Fig. 50.
Anatomy of the skin. 5, 5. Cutis vera (true skin).
4, 4. Nervous tissue. 3, 3. Sensitive layer in which are
seen the nerves. 2, 2. The layer containing pigment
cells. 1, 1. Epidermis (cuticle).]

The _Epidermis_, or _Cuticle_, so called because it is _placed upon the
skin,_ is the outer layer of the skin. Since it is entirely destitute of
nerves and blood-vessels, it is not sensitive. Like the cutis vera, it
has two surfaces composed of layers. The internal, or _Rete Mucosum,_
which is made up chiefly of pigment cells, is adapted to the
irregularities of the cutis vera, and sends prolongations into all its
glandular follicles. The external surface, or epidermis proper, is
elastic, destitute of coloring matter, and consists of mere horny
scales. As soon as dry, they are removed in the form of scurf, and
replaced by new ones from the cutis vera. These scales may be removed by
a wet-sheet pack, or by friction. The cuticle is constantly undergoing
renewal. This layer serves to cover and protect the nervous tissue of
the true skin beneath. We may here observe that the cuticle contains the
pigment for coloring the skin. In dark races, as the negro, the cuticle
is very thick and filled with black pigment. The radiation of animal
heat is dependent upon the thickness and color of this cuticle. Thus, in
the dark races, the pigment cells are most numerous, and in proportion
as the skin is dark or fair do we find these cells in greater or lesser
abundance. The skin of the Albino is of pearly whiteness, devoid even of
the pink or brown tint which that of the European always possesses. This
peculiarity must be attributed to the absence of pigment cells which,
when present, always present a more or less dark color. The theory that
_climate_ alone is capable of producing all these diversities is simply
absurd. The Esquimaux, who live in Greenland and the arctic regions of
America, are remarkable for the darkness of their complexion. Humboldt
remarks that the American tribes of the tropical regions have no darker
skin than the mountaineers of the temperate zone. Climate may _modify_
the complexion, but it cannot _make_ it.

[Illustration: Fig. 51.
Structure of the human hair. _A_. External surface of the shaft, showing the
transverse striae and jagged boundary, caused by the imbrications of the scaly
cortex. _B_. Longitudinal section of the shaft, showing the fibrous character of
the medullary substance, and the arrangement of the pigmentary matter. _C_.
Transverse sections, showing the distinction between the cortical and medullary
substances, and the central collection of pigmentary matter, sometimes found in
the latter. Magnified 310 diameters.]

_Hairs_ are horny appendages of the skin, and, with the exception of the
hands, the soles of the feet, the backs of the fingers and toes, between
the last joint and the nail, and the upper eyelids, are distributed more
or less abundantly over every part of the surface of the body. Over the
greater part of the surface the hairs are very minute, and in some
places are not actually apparent above the level of the skin; but the
hair of the head, when permitted to reach its full growth, attains a
length of from twenty inches to a yard, and, in rare instances, even six
feet. A hair may be divided into a middle portion, or _shaft_, and two
extremities; a peripheral extremity, called the _point;_ and a central
extremity, inclosed within the hair sac, or follicle, termed the _root_.
The root is somewhat greater in diameter than the shaft, and cylindrical
in form, while its lower part expands into an oval mass, called the
_bulb_. The shaft of the hair is not often perfectly cylindrical, but is
more or less flattened, which circumstance gives rise to waving and
curling hair; and, when the flattening is spiral in direction, the
curling will be very great. A hair is composed of three different layers
of cell-tissues: a loose, cellulated substance, which occupies its
center, and constitutes the _medulla_, or pith; the fibrous tissue,
which incloses the medulla, and forms the chief bulk of the hair; and a
thin layer, which envelops this fibrous structure, and forms the smooth
surface of the hair. The medulla is absent in the downy hairs, but in
the coarser class it is always present, especially in white hair. The
color of hair is due partly to the granules and partly to an
inter-granular substance, which occupies the interstices of the granules
and the fibers. The quantity of hair varies according to the proximity
and condition of the follicles. The average number of hairs of the head
may be stated at 1,000 in a superficial square inch; and, as the surface
of the scalp has an area of about one hundred and twenty superficial
square inches, the average number of hairs on the entire head is
120,000. The hair possesses great durability, as is evinced by its
endurance of chemical processes, and by its discovery, in the tombs of
mummies more than two thousand years old. The hair is remarkable for its
elasticity and strength. Hair is found to differ materially from horn in
its chemical composition. According to Vauquelin, its constituents are
animal matter, a greenish-black oil, a white, concrete oil, phosphate of
lime, a trace of carbonate of lime, oxide of manganese, iron, sulphur,
and silex. Red hair contains a reddish oil, a large proportion of
sulphur, and a small quantity of iron. White hair contains a white oil,
and phosphate of magnesia. It has been supposed that hair grows after
death, but this theory was probably due to the lengthening of the hair
by the absorption of moisture from the body or atmosphere.

The _nails_ constitute another class of appendages of the skin. They
consist of thin plates of horny tissue, having a root, a body, and a
free extremity. The root, as well as the lateral portion, is implanted
in the skin, and has a thin margin which is received into a groove of
the true skin. The under surface is furrowed, while the upper is
comparatively smooth. The nails grow in the same manner as the cuticle.

* * * * *




CHAPTER X.

PHYSIOLOGICAL ANATOMY.

SECRETION.


The term _Secretion_, in its broadest sense, is applied to that process
by which substances are separated from the blood, either for the
reparation of the tissues or for excretion. In the animal kingdom this
process is less complicated than in vegetables. In the former it is
really a _separation_ of nutritive material from the blood. The process,
when effected for the removal of effete matter, is, in a measure,
chemical, and accordingly the change is greater.

Three elementary constituents are observed in secretory organs: the
cells, a basement membrane, and the blood-vessels. Obviously, the most
_essential_ part is the _cell_.

The physical condition necessary for the healthy action of the secretory
organs is a copious supply of blood, in which the nutritive materials
are abundant. The nervous system also influences the process of
secretion to a great extent. Intense emotion will produce tears, and the
sight of some favorite fruit will generally increase the flow of saliva.

The process of secretion depends upon the anatomical and chemical
constitution of the cell-tissues. The principal secretions are (1),
Perspiration; (2), Tears; (3), Sebaceous matter; (4), Mucus; (5),
Saliva; (6), Gastric juice; (7), Intestinal juice; (8), Pancreatic
juice; (9), Bile; (10), Milk.

PERSPIRATION is a watery fluid secreted in minute glands, which are
situated in every part of the skin, but are more numerous on the
anterior surfaces of the body. Long thread-like tubes, only 1/100th of
an inch in diameter, lined with epithelium, penetrate the skin, and
terminate in rounded coils, enveloped by a net-work of capillaries,
which supply the secretory glands with blood. It is estimated by Krause
that the entire number of perspiratory glands is two million three
hundred and eighty-one thousand two hundred and forty-eight, and the
length of each glandular coil being 1/16 of an inch, we may estimate the
length of tubing to be not less than two miles and a third. This
secretion has a specific gravity of 1003.5, and, according to Dr.
Dalton, is composed of

Water, 995.50
Chloride of Sodium, 2.23
Chloride of Potassium, 0.24
Sulphate of Soda and Potassa, 0.01
Salts of organic acids, with Soda and Potassa, 2.02
-------
1000.00

Traces of organic matter, mingled with a free volatile acid, are also
found in the perspiration. It is the acid which imparts to this
secretion its peculiar odor, and acid reaction. The process of its
secretion is continuous, but, like all bodily functions, it is subject
to influences which augment or retard its activity. If, as is usually
the case when the body is in a state of repose, evaporation prevents its
appearance in the _liquid_ form, it is called _invisible_ or _insensible
perspiration_. When there is unusual muscular activity, it collects upon
the skin, and is known as _sensible perspiration_. This secretion
performs an important office in the animal economy, by maintaining the
internal temperature at about 100° Fahr. Even in the Arctic regions,
where the explorer has to adapt himself to a temperature of 40° to 80°
below zero, the generation of heat in the body prevents the internal
temperature from falling below this standard. On the contrary, if the
circulation is quickened by muscular exertion, the warmer blood flowing
from the internal organs into the capillaries, raises the temperature of
the skin, secretion is augmented, the moisture exudes from the pores,
and perceptible evaporation begins. A large portion of the animal heat
is thrown off in this process, and the temperature of the skin is
reduced. A very warm, dry atmosphere can be borne with impunity but if
moisture is introduced, evaporation ceases, and the life of the animal
is endangered. Persons have been known to remain in a temperature of
about 300° Fahr. for some minutes without unpleasant effects. Three
conditions may be assigned as effective causes in retarding or
augmenting this cutaneous secretion, variations in the temperature of
the atmosphere, muscular activity, and influences which affect the
nerves. The emotions exert a remarkable influence upon the action of the
perspiratory glands. Intense fear causes great drops of perspiration to
accumulate on the skin, while the salivary glands remain inactive.

TEARS. The lachrymal glands are small lobular organs, situated at the
outer and upper orbit of the eye, and have from six to eight ducts,
which open upon the conjunctiva, between the eyelid and its inner fold.
This secretion is an alkaline, watery fluid. According to Dr. Dalton,
its composition is as follows:

Water, 882.0
Albuminous matter, 5.0
Chloride of Sodium, 13.0
Mineral Salts, a trace,
------
1000.0

The function of this secretion is to preserve the brilliancy of the eye.
The tears are spread over this organ by the reflex movement of the
eyelid, called winking, and then collected in the _puncta lachrymalia_
and discharged into the nasal passage. This process is constant during
life. The effect of its repression is seen in the dim appearance of the
eye after death. Grief or excessive laughter usually excite these glands
until there is an overflow.

SEBACEOUS MATTER. Three varieties of this secretion are found in the
body. A product of the sebaceous glands of the skin is found in those
parts of the body which are covered with hairs; also, on the face and
the external surface of the organs of generation. The _sebaceous glands_
consist of a group of flask-shaped cavities, opening into a common
excretory duct. Their secretion serves to lubricate the hair and soften
the skin. The _ceruminous glands_ of the _external auditory meatus_, or
outer opening of the ear, are long tubes terminating in a glandular
coil, within which is secreted the glutinous matter of the ear. This
secretion serves the double purpose of moistening the outer surface of
the membrana tympani, or ear-drum, and, by its strong odor, of
preventing the intrusion of insects. The _Meibomian glands_ are arranged
in the form of clusters along the excretory duct, which opens just
behind the roots of the eyelashes. The oily nature of this secretion
prevents the tears, when not stimulated by emotion, from overflowing the
lachrymal canal.

MUCUS. The mucous membranes are provided with minute glands which
secrete a viscid, gelatinous matter, called _mucus_. The peculiar animal
matter which it contains is termed _mucosin_. These glands are most
numerous in the Pharynx, Esophagus, Trachea, Bronchia, Vagina and
Urethra. They consist of a group of secreting sacs, terminating at one
extremity in a closed tube, while the other opens into a common duct.
The mucus varies in composition in different parts of the body; but in
all, it contains a small portion of insoluble animal matter. Its
functions are threefold. It lubricates the membranes, prevents their
injury, and facilitates the passage of food through the alimentary
canal.

SALIVA. This term is given to the first of the digestive fluids, which
is secreted in the glands of the mouth. It is a viscid, alkaline liquid,
with a specific gravity of about 1005. If allowed to stand, a whitish
precipitate is formed. Examinations with the microscope show it to be
composed of minute, granular cells and oil globules, mingled with
numerous scales of epithelium. According to Bidder and Schmidt, the
composition of saliva is as follows:

Water, 995.16
Organic matter, 1.34
Sulpho-cyanide of Potassium, 0.06
Phosphates of Sodium, Calcium and Magnesium, .98
Chlorides of Sodium and Potassium, .84
Mixture of Epithelium, 1.62
-------
1000.00

Two kinds of organic matter are present in the saliva; one, termed
_ptyalin_, imparts to the saliva its viscidity, and it obtained from the
secretions of the parotid, submaxillary and sublingual glands; another,
which is not glutinous, is distinguished by the property of coagulating
when subjected to heat. The saliva is composed of four elementary
secretions, derived respectively, from the mucous follicles of the
mouth, and the parotid, the submaxillary, and the sublingual glands. The
process of its secretion is constant, but is greatly augmented by the
contact of food with the lining membrane. The saliva serves to moisten
the triturated food, facilitate its passage, and has the property of
converting starch into sugar; but the latter quality is counteracted by
the action of the gastric juice of the stomach.

GASTRIC JUICE. The minute tubes, or follicles, situated in the mucous
membrane of the stomach, secrete a colorless, acid liquid, termed the
gastric juice. This fluid appears to consist of little more than water,
containing a few saline matters in solution, and a small quantity of
free hydrochloric acid, which gives it an acid reaction. In addition to
these, however, it contains a small quantity of a peculiar organic
substance, termed _pepsin_, which in chemical composition, is very
similar to ptyalin, although it is very different in its effects. When
food is introduced into the stomach, the peristaltic contractions of
that organ roll it about, and mingle it with the gastric juice, which
disintegrates the connective tissue, and converts the albuminous
portions into the substance called chyme, which is about the consistency
of pea-soup, and which is readily absorbed through the animal membranes
into the blood of the delicate and numerous vessels of the stomach,
whence it is conveyed to the portal vein and to the liver. The secretion
of the gastric juice is influenced by nervous conditions. Excess of joy
or grief effectually retard or even arrest its flow.

INTESTINAL JUICE. In the small intestine, a secretion is found which is
termed the _intestinal juice_. It is the product of two classes of
glands situated in the mucous membrane, and termed respectively, the
_follicles of Lieberkuhn_ and the _glands of Brunner_. The former
consist of numerous small tubes, lined with epithelium, which secrete by
far the greater portion of this fluid. The latter are clusters of round
follicles opening into a common excretory duct. These sacs are composed
of delicate, membranous tissue, having numerous nuclei on their walls.
The difficulty of obtaining this juice for experiment is obvious, and
therefore its chemical composition and physical properties are not
known. The intestinal juice resembles the secretion of the mucous
follicles of the mouth, being colorless, vitreous in appearance, and
having an alkaline reaction.

PANCREATIC JUICE. This is a colorless fluid, secreted in a lobular gland
which is situated behind the stomach, and runs transversely from the
spleen across the vertebral column to the duodenum. The most important
constituent of the pancreatic juice is an organic substance, termed
_pancreatin_.

THE BILE. The blood which is collected by the veins of the stomach,
pancreas, spleen, and intestines, is discharged into a large trunk
called the portal vein, which enters the liver. This organ also receives
arterial blood from a vessel called the _hepatic artery_, which is given
off from the aorta below the diaphragm. If the branches of the portal
vein and hepatic artery be traced into the substance of the liver, they
will be found to accompany one another, and to subdivide, becoming
smaller and smaller. Finally, the portal vein and hepatic artery will be
found to terminate in capillaries which permeate the smallest
perceptible subdivisions of the liver substance, which are polygonal
masses of not more than one-tenth of an inch in diameter, called the
_lobules_. Every lobule rests upon one of the ramifications of a great
vessel termed the _hepatic vein_, which empties into the inferior vena
cava. There is also a vessel termed the _hepatic duct_ leading from the
liver, the minute subdivisions of which penetrate every portion of the
substance of that organ. Connected with the hepatic duct, is the duct of
a large oval sac, called the _gall-bladder_.

Each lobule of the liver is composed of minute cellular bodies known as
the _hepatic cells_. It is supposed that in these cells the blood is
deprived of certain materials which are converted into bile. This
secretion is a glutinous fluid, varying in color from a dark golden
brown to a bright yellow, has a specific gravity ranging from 1018 to
1036, and a slightly alkaline reaction. When agitated, it has a frothy
appearance. Physiologists have experienced much difficulty in studying
the character of this secretion from the instability of its constituents
when subjected to chemical examination.

[Illustration: Fig. 52.
Section of the Liver, showing the
ramifications of the portal vein. 1. Twig
of portal vein. 2, 2', 2", 2"'. Interlobular
vein. 3, 3', 3", Lobules.]

_Biliverdin_ is an organic substance peculiar to the bile, which imparts
to that secretion its color. When this constituent is re-absorbed by the
blood and circulates through the tissues, the skin assumes a bright
yellow hue, causing what is known as the jaundice. _Cholesterin_ is an
inflammable crystallizable substance soluble in alcohol or ether. It is
found in the spleen and all the nervous tissues. It is highly probable
that it exists in the blood, in some state or combination, and assumes a
crystalline form only when acted upon by other substances or elements.
Two other constituents, more important than either of the above, are
collectively termed _biliary salts_. These elements were discovered in
1848, by Strecker, who termed them _glycocholate_ and _taurocholate of
soda_. Both are crystalline, resinous substances, and, although
resembling each other in many respects, the chemist may distinguish them
by their reaction, for both yield a precipitate if treated with
subacetate of lead, but only the glycocholate will give a precipitate
with acetate of lead. In testing for biliary substances, the most
satisfactory method is the one proposed by Pettenkoffer. A solution of
cane-sugar, one part of sugar to four parts of water, is mixed with the
suspected substance. Dilute sulphuric acid is then added until a white
precipitate falls, which is re-dissolved in an excess of the acid. On
the addition of more sulphuric acid, it becomes opalescent, and passes
through the successive hues of scarlet, lake, and a rich purple. Careful
experiments have proved that it is a _constant_ secretion; but its flow
is mere abundant during digestion. During the passage through the
intestines it disappears. It is not eliminated, and Pettenkoffer's test
has failed to detect its existence in the portal vein. These facts lead
physiologists to the conclusion, that it undergoes some transformation
in the intestines and is re-absorbed.

After digestion has been going on in the stomach for some time, the
semi-digested food, in the form of chyme, begins to pass through the
_pyloric orifice_ of the stomach into the duodenum, or upper portion of
the small intestine. Here it encounters the intestinal juice, pancreatic
juice, and the bile, the secretion of all of which is stimulated by the
presence of food in the alimentary tract. These fluids, mingling with
the chyme, give it an alkaline reaction, and convert it into chyle. The
transformation of starch into sugar, which is almost, if not entirely,
suspended while the food remains in the stomach, owing to the acidity of
the chyme, is resumed in the duodenum, the acid of the chyme, being
neutralized by the alkaline secretions there encountered.

Late researches have demonstrated that the pancreatic juice exerts a
powerful effect on albuminous matters, not unlike that of the gastric
juice.

Thus, it seems that while in the mouth only starchy, and while in the
stomach only albuminous substances are digested, in the small intestine
all kinds of food materials, starchy, albuminoid, fatty and mineral, are
either completely dissolved, or minutely subdivided, and so prepared
that they may be readily absorbed through the animal membranes into the
vessels.

MILK. The milk is a white, opaque fluid, secreted in the lacteal glands
of the female, in the mammalia. These glands consist of numerous
follicles, grouped around an excretory duct, which unites with similar
ducts coming from other lobules. By successive unions, they form large
branches, termed the _lactiferous ducts_, which open by ten to fourteen
minute orifices on the extremity of the nipple. The most important
constituent of milk is _casein_; it also contains oily and saccharine
substances. This secretion, more than any other, as influenced by
nervous conditions. A mother's bosom will fill with milk at the thought
of her infant child. Milk is sometimes poisoned by a fit of ill-temper,
and the infant made sick and occasionally thrown into convulsions, which
in some instances prove fatal. Sir Astley Cooper mentions two cases in
which terror instantaneously and permanently arrested this secretion. It
is also affected by the food and drink. Malt liquors and other mild
alcoholic beverages temporarily increase the amount of the secretion,
and may, in rare instances, have a beneficial effect upon the mother.
They sometimes affect the child, however, and their use is not to be
recommended unless the mother is extremely debilitated, and there is a
deficiency of milk.

* * * * *




CHAPTER XI.

PHYSIOLOGICAL ANATOMY.

EXCRETION.


The products resulting from the waste of the tissues are constantly
being poured into the blood, and, as we have seen, the blood being
everywhere full of corpuscles, which, like all living things, die and
decay, the products of their decomposition accumulate in every part of
the circulatory system. Hence, if the blood is to be kept pure, the
waste materials incessantly poured into this fluid, or generated in it,
must be as continually removed, or excreted. The principal sets of
organs concerned in effecting the separation of excrementitious
substances from the blood are the lungs, the skin, and the kidneys.

The elimination of carbonic acid through the lungs has already been
described on page 66, and the excretory function of the skin on page 70.

[Illustration: Fig. 53.
View of the kidneys, ureters, and bladder. ]

The kidneys are two bean-shaped organs, placed at the back of the
abdominal cavity, in the region of the loins, one on each side of the
spine. The convex side of each kidney is directed outwards, and the
concave side is turned inwards towards the spine. From the middle of the
concave side, which is termed the _hilus_, a long tube of small caliber,
called the _ureter_, proceeds to the bladder. The latter organ is an
oval bag, situated in the pelvic cavity. It is composed principally of
elastic muscular fibers, and is lined internally with mucous membrane,
and coated externally with a layer of the _peritoneum_, the serous
membrane which lines the abdominal and pelvic cavities. The ureters
enter the bladder through its posterior and lower wall, at some little
distance from each other. The openings through which the ureters enter
the bladder are oblique, hence it is much easier for the secretion of
the kidneys to pass from the ureters into the bladder than for it to get
the other way. Leading from the bladder to the exterior of the body is a
tube, called the _urethra_, through which the urine is voided.

The excretion of the kidneys, termed the _urine_, is an amber-colored or
straw-colored fluid, naturally having a slightly acid reaction, and a
specific gravity ranging from 1,015 to 1,025. Its principal constituents
are _urea_ and _uric acid_, together with various other animal matters
of less importance, and saline substances, held in solution in a
proportionately large amount of water. The composition of the urine and
the quantity excreted vary considerably, being influenced by the
moisture and temperature of the atmosphere, by the character of the food
consumed, and by the empty or replete condition of the alimentary tract.
On an average a healthy man secretes about fifty ounces of urine in the
twenty-four hours. This quantity usually holds in solution about one
ounce of urea, and ten or twelve grains of uric acid. In the amount of
other animal matters, and saline substances, there is great variation,
the quantity of these ranging from a quarter of an ounce to an ounce.
The principal saline substances are common salt, the sulphates and
phosphates of potassium, sodium, calcium, and magnesium. In addition to
the animal and the saline matters, the urine also contains a small
quantity of carbonic acid, oxygen and nitrogen.

* * * * *




CHAPTER XII.

PHYSIOLOGICAL ANATOMY.

THE NERVOUS SYSTEM.


Hitherto, we have only considered the anatomy and functions of the
organs employed in Digestion, Absorption, Circulation, Respiration,
Secretion and Excretion. We have found the vital process of nutrition to
be, in all its essential features, a result of physical and chemical
forces; in each instance we have presupposed the existence and activity
of the nerves. There is not an inch of bodily tissue into which their
delicate filaments do not penetrate, and form a multitude of conductors,
over which are sent the impulses of motion and sensation.

[Illustration: Fig. 54.
The Nervous System.]

Two elements, _nerve-fibers_ and _ganglionic corpuscles_, enter into the
composition of nervous tissue. Ordinary nerve-fibers in the living
subject, or when fresh, are cylindrical-shaped filaments of a clear, but
somewhat oily appearance. But soon after death the matter contained in
the fiber coagulates, and then the fiber is seen to consist of an
extremely delicate, structureless, outer membrane, which forms a tube
through the center of which runs the _axis-cylinder_. Interposed between
the axis-cylinder and this tube, there is a fluid, containing a
considerable quantity of fatty matter, from which is deposited a highly
refracting substance which lines the tube. There are two sets of
nerve-fibers, those which transmit sensory impulses, called _afferent_
or _sensory_ nerves, and those which transmit motor impulses, called
_efferent_ or _motor_ nerves. The fibers when collected in bundles are
termed nerve trunks. All the larger nerve-fibers lie side by side in the
nerve-trunks, and are bound together by delicate connective tissue,
enclosed in a sheath of the same material, termed the _neurilemma_. The
nerve-fibers in the trunks of the nerves remain perfectly distinct and
disconnected from one another, and seldom, or never, divide throughout
their entire length. However, where the nerves enter the nerve-centers,
and near their outer terminations, the nerve-fibres often divide into
branches, or at least gradually diminish in size, until, finally, the
axis-cylinder, and the sheath with its fluid contents, are no longer
distinguishable. The investing membrane is continuous from the origin to
the termination of the nerve-trunk.

[Illustration: Fig. 55.
Division of a
nerve, showing a
portion of a nervous
trunk (_a_)
and separation of
its filaments (_b, c, d, e_.)]

In the brain and spinal cord the nerve-fibers often terminate in minute
masses of a gray or ash-colored granular substance, termed _ganglia_, or
_ganglionic corpuscles_.

The ganglia are cellular corpuscles of irregular form, and possess
fibrous appendages, which serve to connect them with one another. These
ganglia form the cortical covering of the brain, and are also found in
the interior of the spinal cord. According to Kölliker, the larger of
these nerve-cells measure only 1/200 of an inch in diameter. The brain
is chiefly composed of nervous ganglia.

Nerves are classified with reference to their origin, as
_cerebral_--those originating in the brain, and _spinal_--those
originating in the spinal cord.

There are two sets of nerves and nerve-centers, which are intimately
connected, but which can be more conveniently studied apart. These are
the _cerebro-spinal_ system, consisting of the cerebro-spinal axis, and
the cerebral and spinal nerves; and the _sympathetic_ system, consisting
of the chain of sympathetic ganglia, the nerves which they give off, and
the nervous trunks which connect them with one another and with the
cerebro-spinal nerves.


THE CEREBRO-SPINAL SYSTEM.


THE CEREBRO-SPINAL AXIS consists of the brain and spinal cord. It lies
in the cavities of the cranium and the spinal column. These cavities are
lined with a very tough fibrous membrane, termed the _dura mater_, which
serves as the periosteum of the bones which enter into the formation of
these parts. The surface of the brain and spinal cord is closely
invested with an extremely vascular, areolar tissue, called the _pia
mater_. The numerous blood-vessels which supply these organs traverse
the pia mater for some distance, and, where they pass into the substance
of the brain or spinal cord, the fibrous tissue of this membrane
accompanies them to a greater or less depth. The inner surface of the
dura mater and the outer surface of the pia mater are covered with an
extremely thin, serous membrane, which is termed the _arachnoid_
membrane. Thus, one layer of the arachnoid envelopes the brain and
spinal cord, and the other lines the dura mater. As the layers become
continuous with each other at different points, the arachnoid, like the
pericardium, forms a shut sac, and, like other serous membranes, it
secretes a fluid, known as the _arachnoid fluid_. The space between the
internal and the external layers of the arachnoid membrane of the brain
is much smaller than that enclosed by the corresponding layers of the
arachnoid membrane of the spinal column.

[Illustration: Fig. 56.
Cross-section of spinal cord.]

THE SPINAL CORD is a column of soft, grayish-white substance, extending
from the top of the spinal canal, where it is continuous with the brain,
to about an inch below the small of the back, where it tapers off into a
filament. From this nerve are distributed fibers and filaments to the
muscles and integument of at least nine-tenths of the body.

The spinal cord is divided in front through the middle nearly as far as
its center, by a deep fissure, called the _anterior fissure_, and
behind, in a similar manner, by the posterior _fissure_. Each of these
fissures is lined with the pia mater, which also supports the
blood-vessels which supply the spinal cord with blood. Consequently, the
substance of the two halves of the cord is only connected by a narrow
isthmus, or bridge, perforated by a minute tube, which is termed the
_central canal_ of the spinal cord.

Each half of the spinal cord is divided lengthwise into three nearly
equal parts, which are termed the anterior, lateral, and posterior
columns, by the lines which join together two parallel series of bundles
of nervous filaments, which compose the roots of the spinal nerves. The
roots of those nerves, which are found along that line nearest the
posterior surface of the cord, are termed the posterior roots; those
which spring from the other line are known as the anterior roots.

Several of these anterior and posterior roots, situated at about the
same height on opposite sides of the spinal cord, converge and combine
into what are called the _anterior_ and _posterior bundles_; then two
bundles, anterior and posterior, unite and form the trunk of a spinal
nerve.

The nerve trunks make their way out of the spinal canal through
apertures between the vertebra, called the _inter-vertebral foramina_
and then divide into numerous branches, their ramifications extending
principally to the muscles and the skin. There are thirty-one pairs of
spinal nerves, eight of which are termed cervical, twelve dorsal, five
lumbar, and six sacral, with reference to that part of the cord from
which they originate.

When the cord is divided into transverse sections, it is found that each
half is composed of two kinds of matter, a white substance on the
outside, and a grayish substance in the interior. The _gray matter_, as
it is termed, lies in the form of an irregular crescent, with one end
considerably larger than the other, and having the concave side turned
outwards. The ends of the crescent are termed the _horns_, or _cornua_,
the one pointing forward being called the _anterior cornu_, the other
one the _posterior cornu_. The convex sides of these cornua approach
each other and are united by the bridge, which contains the central
canal.

There is a marked difference in the structure of the gray and the white
matter. The white matter is composed entirely of nerve fibers, held
together by a framework of connective tissue. The gray matter contains a
great number of ganglionic corpuscles, or nerve-cells, in addition to
the nerve-fibers.

When the nerve-trunks are irritated in any manner, whether by pinching,
burning, or the application of electricity, all the muscles which are
supplied with branches from this nerve-trunk immediately contract, and
pain is experienced, the severity of which depends upon the degree of
the irritation; and the pain is attributed to that portion of the body
to which the filaments of the nerve-trunk are distributed. Thus, persons
who have lost limbs often complain in cold weather of an uneasiness or
pain, which they locate in the fingers or toes of the limb which has
been amputated, and which is caused by the cold producing an irritation
of the nerve-trunk, the filaments, or fibers of which, supplied the
fingers or toes of the lost member.

On the other hand, if the anterior bundle of nerve-fibers given off from
the spinal cord is irritated in precisely the same way, only half of
these effects is produced. All the muscles which are supplied with
fibers from that trunk contract, but no pain is experienced. Conversely,
if the posterior bundle of nerve-fibers is irritated, none of the
muscles to which the filaments of the nerve are distributed contract,
but pain is felt throughout the entire region to which these filaments
are extended. It is evident, from these facts, that the fibers composing
the posterior bundles of nerve-roots only transmit sensory impulses, and
the filaments composing the anterior nerve-roots only transmit motor
impulses; accordingly, they are termed respectively the _sensory_ and
the _motor_ nerve-roots. This is illustrated by the fact that when the
posterior root of a spinal nerve is divided, all sensation in the parts
to which the filaments of that nerve are distributed is lost, but the
power of voluntary movement of the muscles remains. On the other hand,
if the anterior roots are severed, the power of voluntary motion of the
muscles is lost, but sensation remains.

It appears from these experiments, that, when a nerve is irritated, a
change in the arrangement of its molecules takes place, which is
transmitted along the nerve-fibers. But, if the nerve-trunks are
divided, or compressed tightly at any point between the portion
irritated, and the muscle or nerve-centre, the effect ceases
immediately, in a manner similar to that in which a message is stopped
by the cutting of a telegraph wire. When the nerves distributed to a
limb are subjected to a pressure sufficient to destroy the molecular
continuity of their filaments, it "goes to sleep," as we term it. The
power of transmitting sensory and motor impulses is lost, and only
returns gradually, as the molecular continuity is restored.

From what has been said, it is plain that a sensory nerve is one which
conveys a sensory impulse from the peripheral or outer part of a nerve
to the spinal cord or brain, and which is, therefore, termed _afferent_;
and that a motor nerve is one which transmits an impulse from the nerve
centre, or is _efferent_. So difference in structure, or in chemical or
physical composition, can be discerned between the afferent and the
_efferent_ nerves. A certain period of time is required for the
transmission of all impulses. The speed with which an impulse travels
has been found to be comparatively slow, being even less than that of
sound, which is 1,120 feet per second.

The experiments heretofore related have been confined solely to the
nerves. We may now proceed to the consideration of what takes place when
the spinal cord is operated upon in a similar way. If the cord be
divided with a knife or other instrument, all parts of the body supplied
with nerves given off below the division will become paralyzed and
insensible, while all parts of the body supplied with nerves from the
spinal cord _above_ the division will retain their sensibility and power
of motion. If, however, only the posterior half of the spinal cord is
divided, or destroyed, there is loss of sensation alone; and, if the
anterior portion is cut in two, and the continuity of the posterior part
is left undisturbed, there is loss of voluntary motion of the lower
limbs, but sensation remains.

REFLEX ACTION OF THE SPINAL CORD. In relation to the brain, the spinal
cord is a great mixed motor and sensory nerve, but, in addition to this,
it is also a distinct nervous centre, in which originate and terminate
all those involuntary impulses which exert so potent an influence in the
preservation and economy of the body. That peculiar power of the cord by
which it is enabled to convert sensory into motor impulse is that which
distinguishes it, as a central organ, from a nerve, and is called
_reflex action_.

The gray matter, and not the white, is the part of the cord which
possesses this power. This reflex action is a special function of the
spinal cord, and serves as a monitor to, and regulator of the organs of
nutrition and circulation, by placing them, ordinarily, beyond the
control of conscious volition.

[Illustration: Fig. 57.]

If the foot of a decapitated frog is irritated, there is an instant
contraction of the corresponding limb; if the irritation is intense the
other limb also contracts. These motions indicate the existence, in some
part of the spinal cord, of a distinct nerve-centre, capable of
converting and reflecting impulses. It has been found by experiment,
that the same movements will take place if the irritation be applied to
any portion of the body to which the spinal nerves are distributed, thus
giving undoubted evidence that the spinal cord in its entirety is
capable of causing these reflections. Fig. 57 represents the course of
the nervous impulses. The sensory impulse passes upward along the
posterior root, _a_, until it reaches the imbedded gray matter, _b_, of
the cord, by which it is reflected, as a motor impulse, downward along
the anterior root, _c_, to the muscles whence the sensation was
received. This is the reflex action of the spinal cord. There is no
consciousness or sensation connected with this action, and the removal
of the brain and the sympathetic system does not diminish its activity.
Even after death it continues for some time, longer in cold-blooded than
in warm-blooded animals, on account of the difference in temperature,
thus showing this property of the spinal cord. By disease, or the use of
certain poisons, this activity may be greatly augmented, as is
frequently observed in the human subject. A sudden contact with a
different atmosphere may induce these movements. The contraction of the
muscles, or cramp, often experienced by all persons, in stepping into a
cold bath, or emerging from the cozy sitting-room into a chilly December
temperature, are familiar illustrations of reflex movements. It has been
demonstrated that the irritability of the nerves may be impaired or
destroyed, while that of the muscles to which they are distributed
remains unchanged; and that the motor and sensory classes of filaments
may be paralyzed independently of each other.

The reflex actions of the spinal cord have been admirably summed up by
Dr. Dalton, as exerting a general, protective influence over the body,
presiding over the involuntary action of the limbs and trunk, regulating
the action of the sphincters, rectum, and bladder, and, at the same
time, exercising an indirect influence upon the nutritive changes in all
parts of the body to which the spinal filaments are distributed.

THE BRAIN. The brain is a complex organ, which is divided into the
_medulla oblongata_, the _cerebellum_, and the _cerebrum_.

The _medulla oblongata_ is situated just above the spinal cord, and is
continuous with it below, and the brain above. It has distinct functions
which are employed in the preservation and continuance of life. It has
been termed the "vital knot," owing to the fact that the brain may be
removed and the cord injured and still the heart and lungs will continue
to perform their functions, until the medulla oblongata is destroyed.

The arrangement of the white and gray matter of the medulla oblongata is
similar to that of the spinal cord; that is to say, the white matter is
external and the gray internal; whereas in the cerebellum and cerebrum
this order is reversed. The fibres of the spinal cord, before entering
this portion of the brain, decussate, those from the right side crossing
to the left, and those from the left crossing to the right side. By some
authors this crossing of the sensory and motor filaments has been
supposed to take place near the medulla oblongata. Dr. Brown-Sequard
shows, however, that it takes place at every part of the spinal cord.
The medulla oblongata is traversed by a longitudinal fissure, continuous
with that of the spinal cord. Each of the lateral columns thus formed
are subdivided into sections, termed respectively the _Corpora
Pyramidalia_, the _Corpora Olivaria_, the _Corpora Restiformia_ and the
_Posterior Pyramids_.

The _Corpora Pyramidalia_ (see 1, 1, Fig. 58) are two small medullary
eminences or cords, situated at the posterior surface of the medulla
oblongata; approaching the Pons Varolii these become larger and rounded.

The _Corpora Olivaria_ (3, 3, Fig. 58) are two elliptical prominences,
placed exterior to the corpora pyramidalia. By some physiologists these
bodies are considered as the nuclei, or vital points, of the medulla
oblongata. Being closely connected with the nerves of special sensation,
Dr. Solly supposed that they presided over the movements of the larynx.

[Illustration: Fig. 58.]

[Illustration: Fig. 59.]

The _Corpora Restiformia_ (5, 5, Fig. 59) are lateral and posterior
rounded projections of whitish medulla, which pass upward to the
cerebellum and form the _crura cerebelli_, so called because they
resemble a leg. The filaments of the pneumogastric nerve originate in
the ganglia of these parts.

The _Posterior Pyramids_ are much smaller than the other columns of the
medulla oblongata. They are situated (4, 4, Fig. 59) upon the margin of
the posterior fissures in contact with each other.

The functions of the medulla oblongata, which begin with the earliest
manifestations of life, are of an instinctive character. If the
cerebellum and cerebrum of a dove be removed, the bird will make no
effort to procure food, but if a crumb of bread be placed in its bill,
it is swallowed naturally and without any special effort. So also in
respiration the lungs continue to act after the intercostal muscles are
paralyzed; if the diaphragm loses its power, suffocation is the result,
but there is still a convulsive movement of the lungs for sometime,
indicating the continued action of the medulla oblongata.

The _Cerebellum_, or little brain, is situated in the posterior chamber
of the skull, beneath the _tentorium_, a tent-like process of the dura
mater which separates it from the cerebrum. It is convex, with a
transverse diameter of between three and one-half and four inches, and
is little more than two inches in thickness. It is divided on its upper
and lower surfaces into two lateral hemispheres, by the superior and
inferior vermiform processes, and behind by deep notches. The cerebellum
is composed of gray and white matter, the former being darker than that
of the cerebrum. From the beautiful arrangement of tissue, this organ
has been termed the _arbor vitae_.

The _peduncles of the cerebellum_, the means by which it communicates
with the other portions of the brain, are divided into three pairs,
designated as the _superior_, _middle_ and _inferior_. The first pass
upward and forward until they are blended with the tubercles of the
_corpora quadrigemina_. The second are the _crura cerebelli_, which
unite in two large _fasciculi_, or pyramids, and are finally lost in the
_pons varolii_. The inferior peduncles are the corpora restiformia,
previously described, and consist of both sensory and motor filaments.
Some physiologists suppose that the cerebellum is the source of that
harmony or associative power which co-ordinates all voluntary movements,
and effects that delicate adjustment of cause to effect, displayed in
muscular action. This fact may be proved by removing the cerebellum of a
bird and observing the results, which are an uncertainty in all its
movements, and difficulty in standing, walking, or flying, the bird
being unable to direct its course. In the animal kingdom we find an
apparent correspondence between the size of the cerebellum and the
variety and extent of the movements of the animal. Instances are cited,
however, in which no such proportion exists, and so the matter is open
to controversy. The general function of the cerebellum, therefore,
cannot be explained, but the latest experiments in physiological and
anatomical science seem to favor the theory that it is in some way
connected with the harmony of the movements. This co-ordination, by
which the adjustment of voluntary motion is supposed to be effected, is
not in reality a _faculty_ having its seat in the brain substance, but
is the harmonious action of many forces through the cerebellum.

The _Cerebrum_ occupies five times the space of all the other portions
of the brain together. It is of an ovoid form, and becomes larger as it
approaches the posterior region of the skull. A longitudinal fissure
covered by the dura mater separates the cerebrum into two hemispheres,
which are connected at the base of the fissure, by a broad medullary
band, termed the _corpus callosum_. Each hemisphere is subdivided into
three lobes. The anterior gives form to the forehead, the middle rests
in the cavity at the base of the skull, and the posterior lobe is
supported by the tentorium, by which it is separated from the cerebellum
beneath. One of the most prominent characteristics of the cerebrum is
its many and varied _convolutions_ These do not correspond in all
brains, nor even on the opposite sides of the same brain, yet there are
certain features of similarity in all; accordingly, anatomists enumerate
four _orders of convolutions_. The first order begins at the _substantia
perforata_ and passes upward and around the corpus callosum toward the
posterior margin of that body, thence descends to the base of the brain,
and terminates near its origin. The second order originates from the
first, and subdivides into two convolutions, one of which composes the
exterior margin and superior part of the corresponding hemisphere, while
the other forms the circumference of the _fissure of Sylvius_. The third
order, from six to eight in number, is found in the interior portion of
the brain, and inosculates between the first and second orders. The
fourth is found on the outer surface of the hemisphere, in the space
between the sub-orders of the second clasp. A peculiar fact relating to
these convolutions is observed by all anatomists: mental development is
always accompanied by an increasing dissimilarity between their
proportional size.

The cerebral hemispheres may be injured or lacerated without any pain to
the patient. The effect seems to be one of stupefaction without
sensation or volition. A well-developed brain is a very good indication
of intelligence and mental activity. That the cerebrum is the seat of
the reasoning powers, and all the higher intellectual functions, is
proved by three facts. (1.) If this portion of the brain is removed, it
is followed by the loss of intelligence. (2.) If the human cerebrum is
injured, there is an impairment of the intellectual powers. (3.) In the
animal kingdom, as a rule, intelligence corresponds to the size of the
cerebrum. This general law of development is modified by differences in
the cerebral texture. Men possessing comparatively small brains may have
a vast range of thought and acute reasoning powers. Anatomists have
found these peculiarities to depend upon the quantity of gray matter
which enters into the composition of the brain.

In the cerebro-spinal system there are three different kinds of reflex
actions. (1.) Those of the spinal cord and medulla oblongata are
performed without any consciousness or sensation on the part of the
subject. (2.) The second class embraces those of the tuber annulare,
where the perception gives rise to motion without the interference of
the intellectual faculties. These are denominated purely _instinctive_
reflex actions, and include all those operations of animals which seem
to display intelligent forethought; thus, the beaver builds his
habitation over the water, but not a single apartment is different from
the beaver homestead of a thousand years ago; there is no improvement,
no retrogression. Trains of thought have been termed a third class of
reflex actions. It is evident that the power of reasoning is, in a
degree, possessed by some of the lower-animals: for instance, a tribe of
monkeys on a foraging expedition will station guards at different parts
of the field, to warn the plunderers of the approach of danger. A cry
from the sentinel, and general confusion is followed by retreat. Reason
only attains its highest development in man, in whom it passes the
bounds of ordinary existence, and, with the magic wand of love, reaches
outward into the vast unknown, lifting him above corporeal being, into
an atmosphere of spiritual and divine Truth.

[Illustration: Fig. 60.
Section of the brain and an ideal
view of the pneumogastric nerve
on one side, with its branches, _a_.
Vertical section of the cerebrum.
_b_. Section of the cerebellum, _c_.
Corpus callosum. _d_. Lower section
of medulla oblongata. Above
_d_, origin of the pneumogastric
nerve. 1. Pharyngeal branch. 2.
Superior laryngeal. 5. Branches
to the lungs. 4. Branches to the
liver. 6. Branches to the stomach.]

THE CRANIAL NERVES. From the brain, nerves are given off in pairs, which
succeed one another from in front backwards to the number of twelve. The
_first_ pair, the _olfactory_ nerves, are the nerves of the sense of
smell. The _second_ pair are the _optic_, or the nerves of the sense of
sight. The _third_ pair are called the _motores oculi_, the movers of
the eye, from the fact that they are distributed to all the muscles of
the eye with the exception of two. The _fourth_ pair and the _sixth_
pair each supply one of the muscles of the eye, on each side, the fourth
extending to the superior oblique muscle, and the sixth to the external
rectus muscle. The nerves of the _fifth_ pair are very large; they are
each composed of two bundles of filaments, one motor and the other
sensory, and have, besides, an additional resemblance to a spinal nerve
by having a ganglion on each of their sensory roots, and, from the fact
that they have three chief divisions, are often called the _trigeminal_,
or _trifacial_, nerves. They are nerves of special sense, of sensation,
and of motion. They are the sensitive nerves which supply the cranium
and face, the motor nerves of the muscles of mastication, the
_buccinator_ and the _masseter_, and their third branches, often called
the _gustatory_, are distributed to the front portion of the tongue, and
are two of the nerves of the special sense of taste. The _seventh_ pair,
called also the _facial_ nerves, are the motor nerves of the muscles of
the face, and are also distributed to a few other muscles; the _eighth_
pair, termed the auditory nerves, are the nerves of the special sense of
hearing. As the _seventh_ and _eighth_ pairs of nerves emerge from the
cavity of the skull together, they are frequently classed by anatomists
as one, divided into the _facial_, or _portio dura_, as it is sometimes
called, and the _auditory_, or _portio mollis_. The _ninth_ pair, called
the _glosso-pharyngeal,_ are mixed nerves, supplying motor filaments to
the _pharyngeal muscles_ and filaments of the special sense of taste to
the back portion of the tongue. The _tenth_ pair, called the
_pneumogastric_, or _par vagum_, are very important nerves, and are
distributed to the larynx, the lungs, the heart, the stomach, and the
liver, as shown in Fig. 60. This pair and the next are the only cerebral
nerves which are distributed to parts of the body distant from the head.
The _eleventh_ pair, also called _spinal accessory_, arise from the
sides of the spinal marrow, between the anterior and posterior roots of
the dorsal nerves, and run up to the medulla oblongata, and leave the
cranium by the same aperture as the pneumogastric and glosso-pharyngeal
nerves. They supply certain muscles of the neck, and are purely motor.
As the glosso-pharyngeal, pneumogastric, and spinal accessory nerves
leave the cranium together, they are by some anatomists counted as the
_eighth_ pair. The _twelfth_ pair, known as the _hypoglossal,_ are
distributed to the tongue, and are the motor nerves of that organ.


THE GREAT SYMPATHETIC.


A double chain of nervous ganglia extends from the superior to the
inferior parts of the body, at the sides and in front of the spinal
column, and is termed, collectively, the system of the _great
sympathetic_. These ganglia are intimately connected by nervous
filaments, and communicate with the cerebro-spinal system by means of
the motor and sensory filaments which penetrate the sympathetic. The
nerves of this system are distributed to those organs over which
conscious volition has no direct control.

[Illustration: Fig. 61.
Course and distribution of the great Sympathetic Nerve]

Four of the sympathetic centers, situated in the front and lower
portions of the head, are designated as the _ophthalmic,
spheno-palatine, submaxillary_ and _otic ganglia_. The first of these,
as its name indicates, is distributed to the eye, penetrates the
_sclerotic membrane_ (the white, opaque portion of the eyeball, with its
transparent covering), and influences the contraction and dilation of
the iris. The second division is situated in the angle formed by the
sphenoid and maxillary bone, or just below the ear. It sends motor and
sensory filaments to the palate, and _velum palati_. Its filaments
penetrate the carotid plexus, are joined by others from the motor roots
of the facial nerve and the sensory fibres of the superior maxillary.
The third division is located on the submaxillary gland. Its filaments
are distributed to the sides of the tongue, the sublingual, and
submaxillary glands. The otic ganglion is placed below the base of the
skull, and also connects with the _carotid plexus_. Its filaments of
distribution supply the internal muscles of the _malleus_, the largest
bones of the _tympanum_, the membranous linings of the tympanum and the
_eustachian tube._ Three ganglia, usually designated as the _superior,
middle_, and _inferior_, connect with the cervical and spinal nerves.
Their interlacing filaments are distributed to the muscular walls of the
larynx, pharynx, trachea, and esophagus, and also penetrate the _thyroid
gland_. The use of this gland is not accurately known. It is composed of
a soft, brown tissue, and consists of lobules contained in lobes of
larger size. It forms a spongy covering for the greater portion of the
larynx, and the first section of the trachea. That it is an important
organ, is evident from the fact that it receives four large arteries,
and filaments from two pairs of nerves.

The sympathetic ganglia of the chest correspond in number with the
terminations of the ribs, over which they are situated. Each ganglion
receives two filaments from the intercostal nerve, situated above it,
thus forming a double connection. The thoracic ganglia supply with motor
fibres that portion of the aorta which is above the diaphragm, the
esophagus, and the lungs.

In the abdomen the sympathetic centers are situated upon the _coeliac_
artery, and are termed, collectively, the _semilunar coeliac ganglion_.
Numerous inosculating branches radiate from this center and are called,
from the method of their distribution, the _solar plexus_. From this,
also, originate other plexi which are distributed to the stomach, liver,
kidneys, intestines, spleen, pancreas, supra-renal glands, and to the
organs of generation. Four other pairs of abdominal ganglia connected
with, the lumbar branches are united by filaments to form the semilunar
ganglion.

The sympathetic ganglia of the pelvis consist of five pairs, which are
situated upon the surface of the sacrum. At the extremity of the spinal
column this system terminates in a single knot, designated as the
_ganglion impar_.

Owing to the position of the sympathetic ganglia, deeply imbedded in the
tissues of the chest and abdomen, it is exceedingly difficult to subject
them to any satisfactory experiments. A few isolated facts form the
basis of all our knowledge concerning their functions. They give off
both motor and sensory filaments. The contraction of the _iris_ is one
of the most familiar examples of the action of the sympathetic system.

In the reflex actions of the nerves of special sense, the sensation is
transmitted through the cerebro-spinal system, and the motor impulse is
sent to the deep-seated muscles by the sympathetic system. Physiologists
enumerate three kinds of reflex actions, which are either purely
sympathetic, or partially influenced by the cerebro-spinal system. Dr.
Dalton describes them as follows:

_First_.--"Reflex actions taking place from the internal organs, through
the sympathetic and cerebro-spinal systems, to the voluntary muscles and
sensitive surfaces.--The convulsions of young children are often owing
to the irritation of undigested food in the intestinal canal. Attacks of
indigestion are also known to produce temporary amaurosis [blindness],
double vision, strabismus, and even hemiplegia. Nausea, and a diminished
or capricious appetite, are often prominent symptoms of early pregnancy,
induced by the peculiar condition of the uterine mucous membrane."

_Second_.--"Reflex actions taking place from the sensitive surfaces,
through the cerebro-spinal and sympathetic systems to the involuntary
muscles and secreting organs.--Imprudent exposure of the integument to
cold and wet, will often bring on a diarrhea. Mental and moral
impressions, conveyed through the special senses, will affect the
motions of the heart, and disturb the processes of digestion and
secretion. Terror, or an absorbing interest of any kind, will produce a
dilatation of the pupil, and communicate in this way a peculiarly wild
and unusual expression to the eye. Disagreeable sights or odors, or even
unpleasant occurrences, are capable of hastening or arresting the
menstrual discharge, or of inducing premature delivery."

_Third_.--"Reflex actions taking place through the sympathetic system
from one part of the body to another.--The contact of food with the
mucous membrane of the small intestine excites a peristaltic movement in
the muscular coat. The mutual action of the digestive, urinary, and
internal generative organs upon each other takes place entirely through
the medium of the sympathetic ganglia and their nerves. The variation of
the capillary circulation in different abdominal viscera, corresponding
with the state of activity or repose of their associated organs, are to
be referred to a similar nervous influence. These phenomena are not
accompanied by any consciousness on the part of the individual, nor by
any apparent intervention of the cerebro-spinal system."

* * * * *




CHAPTER XIII.

THE SPECIAL SENSES.

SIGHT.


The eye is the organ through which we perceive, by the agency of light,
all the varied dimensions relations, positions, and visible qualities of
external objects.

The number, position, and perfection of the eyes, vary remarkably in
different orders, in many instances corresponding to the mode of life,
habitation, and food of the animal. A skillful anatomist may ascertain
by the peculiar formation of the eye, without reference to the general
physical structure, in what element the animal lives. Sight is one of
the most perfect of the senses, and reveals to man the beauties of
creation. The aesthetic sentiment is acknowledged to be the most
refining element of civilized life. Painting, sculpture, architecture,
and all the scenes of nature, from a tiny way-side flower to a Niagara,
are subjects in which the poet's eye sees rare beauties to mirror forth
in the rhythm of immortal verse.

In the vertebrates, the organs of vision are supplied with filaments
from the second pair of cranial nerves. In mammalia, the eyes are
limited to two in number, which in man are placed in circular cavities
of the skull, beneath the anterior lobes of the cerebrum. Three
membranes form the lining of this inner sphere of the eye, called
respectively the Sclerotic, Choroid, and Retina.

The _Sclerotic_, or outer covering, is the white, firm membrane, which
forms the larger visible portion of the eyeball. It is covered in front
by a colorless, transparent segment, termed the _cornea_, which gives
the eye its lustrous appearance. Within the sclerotic, and lining it
throughout, is a thin, dark membrane termed the _Choroid_. Behind the
cornea it forms a curtain, called the _iris_, which gives to the eye its
color. The muscles of the iris contract or relax according to the amount
of light received, thus enlarging or diminishing the size of the
circular opening called the _pupil_. The _Retina_ is formed by the optic
nerve, which penetrates the sclerotic and choroid and spreads out into a
delicate, grayish, semi-transparent membrane. The retina is one of the
most _essential_ organs of vision, and consists of two layers. A
spheroidal, transparent body, termed the _crystalline lens_, is situated
directly behind the pupil. It varies in density, increasing from without
inward, and forms a perfect refractor of the light received. The space
in front of the crystalline lens is separated by the iris into two
compartments called respectively the _anterior_ and _posterior
chambers_. The fluid contained within them, termed the _aqueous humor_,
is secreted by the cornea, iris, and ciliary processes. The space behind
the crystalline lens is occupied by a fluid, called the _vitreous
humor_. This humor is denser than the other fluids and has the
consistency of jelly, being perfectly transparent. "The function of the
crystalline lens is to produce distinct perception of form and
outline."[3] The transparent humors of the eye also contribute to the
same effect, but only act as auxiliaries to the lens.

[Illustration: Fig. 62.]

The figure on the next page represents the course of the rays of light
proceeding from an object _a b_, refracted by the lens, and forming the
inverted image _x y_ on the screen. All rays of light proceeding from
_b_ are concentrated at _y_, and those proceeding from _a_ converge at
_x_. Rays of light emanating from the center of the object _a b_ pursue
a parallel course, and form the center of the image. Rays of light
passing through a double convex lens converge at a point called the
_focus_. In the organ of vision, if perfect, the focus is on the retina,
which serves as a screen to receive the image or impression. We have a
distinct perception of the outline of a distant hill, and also of a book
lying before us. The rays of light we receive from these objects cannot
have the same focus. How, then, can we account for the evident
accommodation of the eye to the varying distances? Various theories have
been advanced to explain this adjustment; such as changes in the
curvature of the cornea and lens; a movement of the lens, or a general
change in the form of the eyeball, by which the axis may be lengthened
or shortened.

[Illustration: Fig. 63.]

Two facts comprise all the positive knowledge which we possess on this
subject. Every person is conscious of a muscular effort in directing the
eye to a near object" as a book, and of fatigue, if the attention is
prolonged. If, now, the eyes be directed to a distant object, there will
result a sense of rest, or passiveness. By various experiments it has
been proved that the accommodation or adjustment of the eye for near
objects requires a muscular effort, but for distant objects the muscles
are in an essentially passive condition. An increase in the convexity of
the crystalline lens is now admitted to be necessary for a distinct
perception of near objects. We may give two simple illustrations, cited
by Dr. Dalton in his recent edition of Human Physiology. If a candle be
held near the front of an eye which is directed to a distant object,
three reflected images of the flame will be seen in the eye, one on each
of the anterior surfaces of the cornea and lens, and a third on the
posterior surface of the latter. If the eye is directed to a near
object, the reflection on the cornea remains unchanged, while that on
the anterior surface of the lens gradually diminishes and approximates
in size the reflection on the cornea, thus giving conclusive evidence
that, in viewing a near object, the anterior surface of the crystalline
lens become _more convex_, and at the same time approaches the cornea.
Five or six inches is the minimum limit of the muscular adjustment of
the eye. From that point to all the boundless regions of space, to every
star and nebulae which send their rays to our planet, human vision can
reach. It is the sense by which we receive knowledge of the myriads of
worlds and suns which circle with unfailing precision through infinite
space.


HEARING.


[Illustration: Fig. 64.
Internal and external ear. 1. External ear. 2. Internal
auditory meatus. 3. Tympanum. 4. Labyrinth.
5. Eustachian tube.]

Hearing depends upon the sonorous vibrations of the atmosphere. The
waves of sound strike the sensitive portions of the ear, and their
impressions upon the auditory nerves are termed the sensations of
hearing. The ear is divided into three parts, called respectively the
External, Middle, and Internal ear.

The external organs of hearing are two in number, and placed on opposite
sides of the head. In most of the higher order of vertebrates, they are
so situated as to give expression and proportion to the facial organs,
and, at the same time, to suit the requirements of actual life.

The _External ear_ is connected with the interior part by a prolongation
of its orifice, termed the _external auditory meatus_. In man, this
gristly portion of the auditory apparatus is about one inch in length,
lined by a continuation of the integument of the ear, and has numerous
hairs on its surface, to prevent the intrusion of foreign substances.
Between the external MEATUS and the cavity of the middle ear is the
_membrana tympani_, which is stretched across the opening like the head
of a drum. The _tympanum_, or ear-drum, communicates with the pharynx by
the _eustachian tube_, which is a narrow passage lined with delicate,
ciliated epithelium. On the posterior portion it is connected with the
_mastoid cells_. Three small bones are stretched across the cavity of
the tympanum, and called, from their form, the _malleus, incus_ and
_stapes_, or the hammer, anvil, and stirrup. Agassiz mentions a fourth,
which he terms the _os orbiculare_. Each wave of sound falling upon the
membrana tympani, throws its molecules into vibrations which are
communicated to the chain of bones, which, in turn, transmits them to
the membrane of the _foramen ovale_. The three muscles which regulate
the tension of these membranes are termed the _tensor tympani, laxator
tympani_, and _stapedium tympani._

The _Labyrinth_, or _Internal_ ear, is a complicated cavity, consisting
of three portions termed the _vestibule, cochlea_, and _semi-circular
canals_. The vestibule is the central portion and communicates with the
other divisions. The labyrinth is filled with a transparent fluid,
termed _perilymph_, in which are suspended, in the vestibules and
canals, small membranous sacs, containing a fluid substance, termed
_endolymph_ (sometimes called _vitrine auditive_ from its resemblance to
the vitreous humor of the eye). The filaments of the auditory nerve
penetrate the membranous tissues of these sacs, and also of those
suspended at the commencement of the semi-circular canals. These little
sacs are supposed to be the seat of hearing, and to determine, in some
mysterious way, the quality, intensity and pitch of sounds.

The determination of the _direction_ of sound is a problem of acoustics.
Some have contended that the arrangement of the semi-circular canals is
in some way connected with this sensation. But this supposition,
together with the theory of the transmission of sound through the
various portions of the cranial bones, has been exploded.

From the foregoing description, it will be seen that the labyrinth and
tympanum are the most essential parts of the organs of hearing. In
delicacy and refinement this sense ranks next to sight. The emotions of
beauty and sublimity, excited by the warbling of birds and the roll of
thunder, are scarcely distinguishable from the intense emotions arising
from sight. It is a remarkable fact, that the refinement or cultivation
of these senses is always found associated. Those nations which furnish
the best artists, or have the highest appreciation of painting and
sculpture, produce the most skillful musicians, those who reduce music
to a science.


SMELL.


[Illustration: Fig. 65.
1. Frontal sinus. 2. Nasal bone. 3. Olfactory
ganglion and nerves. 4. Nasal branch
of the fifth pair. 5. Spheno-palatine ganglion.
6. Soft palate. 7. Hard palate, _a_.
Cerebrum, _b_. Anterior lobes, _c_. Corpus
callosum. _d_. Septum lucidum. _f_. Fornix.
_g_. Thalami optici. _h_. Corpora striata.]

Next in order of delicacy, and more closely allied with the physical
functions, is the sense of smell. Delicate perfumes, or the fragrance of
a flower, impart an exhilarating sensation of delight, while numerous
odors excite a feeling of disgust. The organ of smell is far less
complicated in its structure than the eye or the ear. It consists of two
cavities having cartilaginous walls, and lined with a thick mucous coat,
termed the _pituitary membrane_, over which are reflected the olfactory
nerves. Particles of matter, too minute to be visible even through the
microscope, are detached from the odorous body and come in contact with
the nerves of smell, which transmit the impressions or impulses thus
received to the brain. Fig. 65 shows the distribution of the olfactory
nerves in the nasal passages. The nose is supplied with two kinds of
filaments which are termed respectively nerves of _special_ and nerves
of _general sensation_. Compared with the lower animals, especially with
those belonging to the carnivorous species, the sense of smell in man is
feeble. The sensation of smell is especially connected with the
pleasures and necessities of animal life.


TASTE.


The sense of taste is directly connected with the preservation and
nutrition of the body. A delicious flavor produces a desire to eat a
savory substance. Some writers on hygiene have given this sense an
instinctive character, by assuming that all articles having an agreeable
taste are suitable for diet. The nerves of taste are distributed over
the surface of the tongue and palate, and their minute extremities
terminate in well developed _papillae_. These _papillae_ are divided
into three classes, termed, from their microscopic appearance,
_filiform_, _fungiform_ and _circumvallate_. The organ of taste is the
mucous membrane which covers the back part of the tongue and the palate.
The papillae of the tongue are large and distinct, and covered with
separate coats of epithelium. The filiform papillae are generally long
and pointed and are found over the entire surface of the tongue. The
fungiform are longer, small at the base and broad at the end. The
circumvallate are shaped like an inverted V and are found only near the
root of the tongue; the largest of this class of papillae have other
very small papillae upon their surfaces. It is now pretty satisfactorily
established that the circumvallate, or fungiform papillae are the only
ones concerned in the special sense of taste.

The conditions necessary to taste are, that the substance be in solution
either by artificial means, or by the action of the saliva; and that it
be brought in contact with the sensitive filaments imbedded in the
mucous membrane. The nerves of taste are both _general_ and _special_ in
their functions. If the general sensibility of the nerves of taste is
unduly excited, the function of sensibility is lost for some time. If a
peppermint lozenge is taken into the mouth, it strongly excites the
general sensibilities of taste, and the power of distinguishing between
special flavors is lost for a few moments. A nauseous drug may then be
swallowed without experiencing any disagreeable taste.

Paralysis of the facial nerve often produces a marked effect in the
sensibility of the tongue. Where this influence lies has not been fully
explained; probably it is indirect, being produced by some alteration in
the vascularity of the parts or a diminution of the salivary secretions.


TOUCH.


By the sense of touch, we mean the _general sensibility of the skin_.
Sensations of heat and cold are familiar illustrations of this faculty.
By the sense of touch, we obtain a knowledge of certain qualities of a
body, such as form consistency, roughness, or smoothness of surface,
etc. The tip of the tongue possesses the most acute sensibility of any
portion of the body, and next in order are the tips of the fingers. The
hands are the principal organs of tactile sensation. The nerves of
general sensibility are distributed to every part of the cutaneous
tissue. The contact of a foreign body with the back, will produce a
similar _tactile_ sensation, as with the tips of the fingers. The
sensation, however, will differ in _degree_ because the back is supplied
with a much smaller number of sensitive filaments; in _quality_ it is
the same.

* * * * *




CHAPTER XIV.

CEREBRAL PHYSIOLOGY.


By means of the nervous system, an intimate relation is maintained
between mind and body, for nervous energy superintends the functions of
both. The fibres of nervous matter are universally present in the
organization, uniting the physical and spiritual elements of man's
being. Even the minutest nerve-rootlets convey impressions to the dome
of thought and influence the intellectual faculties. We recognize
_muscular_ force, the strength of the body, _molecular_ force, molecules
in motion, as heat, light, chemical force, electricity, and _nervous_
force, a certain influence which reacts between the animal functions and
the cerebrum, thus connecting the conditions of the body with those of
the mind. We cannot speak of the effects of mind or body separately, but
we must consider their action and reaction upon each other, for they are
always associated. There are many difficulties in understanding this
relationship, some of which may be obviated by a study of the
development of nervous matter, and its functions in the lower orders of
organization.

Within the plant-cells is found a vital, vegetable substance termed
bioplasm, or protoplasm; which furnishes the same nutritive power as the
tissues of the polyp and jelly fish. Many families of animals have pulpy
bodies, and slight instinctive motion and sensibility, and in proportion
as the nervous system is developed, both of these powers are unfolded.
Plants have a low degree of sensibility, limited motion, respiratory and
circulatory organs. Animals possess quicker perceptions and
sensibilities, the power of voluntary motion, and, likewise a rudimental
nervous system. Some articulates have no bony skeleton, their muscles
being attached to the skin which constitutes a soft contracting
envelope. One of the simplest forms of animal life in which a nervous
system is found, is the five-rayed star-fish. In each ray there are
filaments which connect with similar nerve-filaments from other rays,
and form a circle around the digestive cavity. It probably has no
conscious perception, and its movements do not necessarily indicate
sensation or volition. In some worms a rudimentary nervous system is
sparingly distributed to the cavities of the thorax and abdomen, and, as
in the star-fish, the largest nerve-filament is found around the
esophagus, presiding over nutrition.

[Illustration: Fig. 66.]

A higher grad