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[Transcriber's Note: Figures 162-167 have been renumbered. In the
original, Figure 162 was labeled as 161; 163 as 162; etc.]

A Practical Physiology

A Text-Book for Higher Schools

By

Albert F. Blaisdell, M.D.

Author of "Child's Book of Health," "How to Keep Well,"
"Our Bodies and How We Live," Etc., Etc.

Preface.

The author has aimed to prepare a text-book on human physiology for use in
higher schools. The design of the book is to furnish a practical manual of
the more important facts and principles of physiology and hygiene, which
will be adapted to the needs of students in high schools, normal schools,
and academies.

Teachers know, and students soon learn to recognize the fact, that it is
impossible to obtain a clear understanding of the functions of the various
parts of the body without first mastering a few elementary facts about
their structure. The course adopted, therefore, in this book, is to devote
a certain amount of space to the anatomy of the several organs before
describing their functions.

A mere knowledge of the facts which can be gained in secondary schools,
concerning the anatomy and physiology of the human body, is of little real
value or interest in itself. Such facts are important and of practical
worth to young students only so far as to enable them to understand the
relation of these facts to the great laws of health and to apply them to
daily living. Hence, it has been the earnest effort of the author in this
book, as in his other physiologies for schools, to lay special emphasis
upon such points as bear upon personal health.

Physiology cannot be learned as it should be by mere book study. The
result will be meagre in comparison with the capabilities of the subject.
The study of the text should always be supplemented by a series of
practical experiments. Actual observations and actual experiments are as
necessary to illuminate the text and to illustrate important principles in
physiology as they are in botany, chemistry, or physics. Hence, as
supplementary to the text proper, and throughout the several chapters, a
series of carefully arranged and practical experiments has been added. For
the most part, they are simple and can be performed with inexpensive and
easily obtained apparatus. They are so arranged that some may be omitted
and others added as circumstances may allow.

If it becomes necessary to shorten the course in physiology, the various
sections printed in smaller type may be omitted or used for home study.

The laws of most of the states now require in our public schools the study
of the effects of alcoholic drinks, tobacco, and other narcotics upon the
bodily life. This book will be found to comply fully with all such laws.

The author has aimed to embody in simple and concise language the latest
and most trustworthy information which can be obtained from the standard
authorities on modern physiology, in regard to the several topics.

In the preparation of this text-book the author has had the editorial help
of his esteemed friend, Dr. J. E. Sanborn, of Melrose, Mass., and is also
indebted to the courtesy of Thomas E. Major, of Boston, for assistance in
revising the proofs.

Albert F. Blaisdell.

Boston, August, 1897.

Contents.

Chapter I. Introduction
Chapter II. The Bones
Chapter III. The Muscles
Chapter IV. Physical Exercise
Chapter V. Food and Drink
Chapter VI. Digestion
Chapter VII. The Blood and Its Circulation
Chapter VIII. Respiration
Chapter IX. The Skin and the Kidneys
Chapter X. The Nervous System
Chapter XI. The Special Sense
Chapter XII. The Throat and the Voice
Chapter XIII. Accidents and Emergencies
Chapter XIV. In Sickness and in Health
Care of the Sick-Room; Poisons and their Antidotes; Bacteria;
Disinfectants; Management of Contagious Diseases.
Chapter XV. Experimental Work in Physiology
Practical Experiments; Use of the Microscope; Additional Experiments;
Surface Anatomy and Landmarks.

Glossary

Index

Chapter I.

Introduction.

1. The Study of Physiology. We are now to take up a new study, and in
a field quite different from any we have thus far entered. Of all our
other studies,--mathematics, physics, history, language,--not one comes
home to us with such peculiar interest as does physiology, because
this is the study of ourselves.

Every thoughtful young person must have asked himself a hundred questions
about the problems of human life: how it can be that the few articles of
our daily food--milk, bread, meats, and similar things--build up our
complex bodies, and by what strange magic they are transformed into hair,
skin, teeth, bones, muscles, and blood.

How is it that we can lift these curtains of our eyes and behold all the
wonders of the world around us, then drop the lids, and though at noonday,
are instantly in total darkness? How does the minute structure of the ear
report to us with equal accuracy the thunder of the tempest, and the hum
of the passing bee? Why is breathing so essential to our life, and why
cannot we stop breathing when we try? Where within us, and how, burns the
mysterious fire whose subtle heat warms us from the first breath of
infancy till the last hour of life?

These and scores of similar questions it is the province of this deeply
interesting study of physiology to answer.

2. What Physiology should Teach us. The study of physiology is not
only interesting, but it is also extremely useful. Every reasonable person
should not only wish to acquire the knowledge how best to protect and
preserve his body, but should feel a certain profound respect for an
organism so wonderful and so perfect as his physical frame. For our bodies
are indeed not ourselves, but the frames that contain us,--the ships in
which we, the real selves, are borne over the sea of life. He must be
indeed a poor navigator who is not zealous to adorn and strengthen his
ship, that it may escape the rocks of disease and premature decay, and
that the voyage of his life may be long, pleasant, and successful.

But above these thoughts there rises another,--that in studying physiology
we are tracing the myriad lines of marvelous ingenuity and forethought, as
they appear at every glimpse of the work of the Divine Builder. However
closely we study our bodily structure, we are, at our best, but imperfect
observers of the handiwork of Him who made us as we are.

3. Distinctive Characters of Living Bodies. Even a very meagre
knowledge of the structure and action of our bodies is enough to reveal
the following distinctive characters: our bodies are continually
breathing, that is, they take in oxygen from the surrounding air; they
take in certain substances known as food, similar to those composing the
body, which are capable through a process called oxidation, or through
other chemical changes, of setting free a certain amount of energy.

Again, our bodies are continually making heat and giving it out to
surrounding objects, the production and the loss of heat being so adjusted
that the whole body is warm, that is, of a temperature higher than that of
surrounding objects. Our bodies, also, move themselves, either one part
on another, or the whole body from place to place. The motive power is not
from the outside world, but the energy of their movements exists in the
bodies themselves, influenced by changes in their surroundings. Finally,
our bodies are continually getting rid of so-called waste matters, which
may be considered products of the oxidation of the material used as food,
or of the substances which make up the organism.

4. The Main Problems of Physiology briefly Stated. We shall learn in
a subsequent chapter that the living body is continually losing energy,
but by means of food is continually restoring its substance and
replenishing its stock of energy. A great deal of energy thus stored up is
utilized as mechanical work, the result of physical movements. We shall
learn later on that much of the energy which at last leaves the body as
heat, exists for a time within the organism in other forms than heat,
though eventually transformed into heat. Even a slight change in the
surroundings of the living body may rapidly, profoundly, and in special
ways affect not only the amount, but the kind of energy set free. Thus the
mere touch of a hair may lead to such a discharge of energy, that a body
previously at rest may be suddenly thrown into violent convulsions. This
is especially true in the case of tetanus, or lockjaw.

The main problem we have to solve in the succeeding pages is to ascertain
how it is that our bodies can renew their substance and replenish the
energy which they are continually losing, and can, according to the nature
of their surroundings, vary not only the amount, but the kind of energy
which they set free.

5. Technical Terms Defined. All living organisms are studied usually
from two points of view: first, as to their form and structure; second, as
to the processes which go on within them. The science which treats of all
living organisms is called biology. It has naturally two
divisions,--morphology, which treats of the form and structure of
living beings, and physiology, which investigates their functions, or
the special work done in their vital processes.

The word anatomy, however, is usually employed instead of morphology.
It is derived from two Greek words, and means the science of dissection.
Human anatomy then deals with the form and structure of the human
body, and describes how the different parts and organs are arranged, as
revealed by observation, by dissection, and by the microscope.

Histology is that part of anatomy which treats of the minute
structure of any part of the body, as shown by the microscope.

Human physiology describes the various processes that go on in the
human body in health. It treats of the work done by the various parts of
the body, and of the results of the harmonious action of the several
organs. Broadly speaking, physiology is the science which treats of
functions. By the word function is meant the special work which an
organ has to do. An organ is a part of the body which does a special
work. Thus the eye is the organ of sight, the stomach of digestion, and
the lungs of breathing.

It is plain that we cannot understand the physiology of our bodies without
a knowledge of their anatomy. An engineer could not understand the working
of his engine unless well acquainted with all its parts, and the manner in
which they were fitted together. So, if we are to understand the
principles of elementary physiology, we must master the main anatomical
facts concerning the organs of the body before considering their special
functions.

As a branch of study in our schools, physiology aims to make clear certain
laws which are necessary to health, so that by a proper knowledge of them,
and their practical application, we may hope to spend happier and more
useful, because healthier, lives. In brief, the study of hygiene, or
the science of health, in the school curriculum, is usually associated
with that of physiology.[1]

6. Chemical Elements in the Body. All of the various complex
substances found in nature can be reduced by chemical analysis to about 70
elements, which cannot be further divided. By various combinations of
these 70 elements all the substances known to exist in the world of nature
are built up. When the inanimate body, like any other substance, is
submitted to chemical analysis, it is found that the bone, muscle, teeth,
blood, etc., may be reduced to a few chemical elements.

In fact, the human body is built up with 13 of the 70 elements, namely:
oxygen, hydrogen, nitrogen, chlorine, fluorine, carbon, phosphorus,
sulphur, calcium, potassium, sodium, magnesium, and iron. Besides
these, a few of the other elements, as silicon, have been found; but they
exist in extremely minute quantities.

The following table gives the proportion in which these various elements
are present:

Oxygen 62.430 per cent
Carbon 21.150 " "
Hydrogen 9.865 " "
Nitrogen 3.100 " "
Calcium 1.900 " "
Phosphorus 0.946 " "
Potassium 0.230 " "
Sulphur 0.162 " "
Chlorine 0.081 " "
Sodium 0.081 " "
Magnesium 0.027 " "
Iron 0.014 " "
Fluorine 0.014 " "
-----
100.000

As will be seen from this table, oxygen, hydrogen, and nitrogen, which are
gases in their uncombined form, make up ¾ of the weight of the whole
human body. Carbon, which exists in an impure state in charcoal, forms
more than ⅕ of the weight of the body. Thus carbon and the three gases
named, make up about 96 per cent of the total weight of the body.

7. Chemical Compounds in the Body. We must keep in mind that, with
slight exceptions, none of these 13 elements exist in their elementary
form in the animal economy. They are combined in various proportions, the
results differing widely from the elements of which they consist. Oxygen
and hydrogen unite to form water, and water forms more than ⅔ of the
weight of the whole body. In all the fluids of the body, water acts as a
solvent, and by this means alone the circulation of nutrient material is
possible. All the various processes of secretion and nutrition depend on
the presence of water for their activities.

8. Inorganic Salts. A large number of the elements of the body unite
one with another by chemical affinity and form inorganic salts. Thus
sodium and chlorine unite and form chloride of sodium, or common salt.
This is found in all the tissues and fluids, and is one of the most
important inorganic salts the body contains. It is absolutely necessary
for continued existence. By a combination of phosphorus with sodium,
potassium, calcium, and magnesium, the various phosphates are formed.

The phosphates of lime and soda are the most abundant of the salts of the
body. They form more than half the material of the bones, are found in the
teeth and in other solids and in the fluids of the body. The special place
of iron is in the coloring matter of the blood. Its various salts are
traced in the ash of bones, in muscles, and in many other tissues and
fluids. These compounds, forming salts or mineral matters that exist in
the body, are estimated to amount to about 6 per cent of the entire
weight.

9. Organic Compounds. Besides the inorganic materials, there exists
in the human body a series of compound substances formed of the union of
the elements just described, but which require the agency of living
structures. They are built up from the elements by plants, and are called
organic. Human beings and the lower animals take the organized
materials they require, and build them up in their own bodies into still
more highly organized forms.

The organic compounds found in the body are usually divided into three
great classes:

1. Proteids, or albuminous substances.
2. Carbohydrates (starches, sugars, and gums).
3. Fats.

The extent to which these three great classes of organic materials of the
body exist in the animal and vegetable kingdoms, and are utilized for the
food of man, will be discussed in the chapter on food (Chapter V.). The
Proteids, because they contain the element nitrogen and the others do
not, are frequently called nitrogenous, and the other two are known
as non-nitrogenous substances. The proteids, the type of which is egg
albumen, or the white of egg, are found in muscle and nerve, in glands, in
blood, and in nearly all the fluids of the body. A human body is estimated
to yield on an average about 18 per cent of albuminous substances. In the
succeeding chapters we shall have occasion to refer to various and allied
forms of proteids as they exist in muscle (myosin), coagulated blood
(fibrin), and bones (gelatin).

The Carbohydrates are formed of carbon, hydrogen, and oxygen, the
last two in the proportion to form water. Thus we have animal starch, or
glycogen, stored up in the liver. Sugar, as grape sugar, is also found in
the liver. The body of an average man contains about 10 per cent of
Fats. These are formed of carbon, hydrogen, and oxygen, in which the
latter two are not in the proportion to form water. The fat of the body
consists of a mixture which is liquid at the ordinary temperature.

Now it must not for one moment be supposed that the various chemical
elements, as the proteids, the salts, the fats, etc., exist in the body in
a condition to be easily separated one from another. Thus a piece of
muscle contains all the various organic compounds just mentioned, but they
are combined, and in different cases the amount will vary. Again, fat may
exist in the muscles even though it is not visible to the naked eye, and a
microscope is required to show the minute fat cells.

10. Protoplasm. The ultimate elements of which the body is composed
consist of "masses of living matter," microscopic in size, of a material
commonly called protoplasm.[2] In its simplest form protoplasm
appears to be a homogeneous, structureless material, somewhat resembling
the raw white of an egg. It is a mixture of several chemical substances
and differs in appearance and composition in different parts of the body.

Protoplasm has the power of appropriating nutrient material, of dividing
and subdividing, so as to form new masses like itself. When not built into
a tissue, it has the power of changing its shape and of moving from place
to place, by means of the delicate processes which it puts forth. Now,
while there are found in the lowest realm of animal life, organisms like
the amœba of stagnant pools, consisting of nothing more than minute
masses of protoplasm, there are others like them which possess a small
central body called a nucleus. This is known as nucleated protoplasm.

[Illustration: Fig. 1.--Diagram of a Cell.

A, nucleus;
B, nucleolus;
C, protoplasm. (Highly magnified)
]

11. Cells. When we carry back the analysis of an organized body as
far as we can, we find every part of it made up of masses of nucleated
protoplasm of various sizes and shapes. In all essential features these
masses conform to the type of protoplasmic matter just described. Such
bodies are called cells. In many cells the nucleus is finely granular or
reticulated in appearance, and on the threads of the meshwork may be one
or more enlargements, called nucleoli. In some cases the protoplasm at the
circumference is so modified as to give the appearance of a limiting
membrane called the cell wall. In brief, then, a cell is a mass of
nucleated protoplasm; the nucleus may have a nucleolus, and the cell
may be limited by a cell wall. Every tissue of the human body is formed
through the agency of protoplasmic cells, although in most cases the
changes they undergo are so great that little evidence remains of their
existence.

There are some organisms lower down in the scale, whose whole activity is
confined within the narrow limits of a single cell. Thus, the amœba
begins its life as a cell split off from its parent. This divides in its
turn, and each half is a complete amœba. When we come a little higher
than the amœba, we find organisms which consist of several cells, and a
specialization of function begins to appear. As we ascend in the animal
scale, specialization of structure and of function is found continually
advancing, and the various kinds of cells are grouped together into
colonies or organs.

12. Cells and the Human Organism. If the body be studied in its
development, it is found to originate from a single mass of nucleated
protoplasm, a single cell with a nucleus and nucleolus. From this
original cell, by growth and development, the body, with all its various
tissues, is built up. Many fully formed organs, like the liver, consist
chiefly of cells. Again, the cells are modified to form fibers, such as
tendon, muscle, and nerve. Later on, we shall see the white blood
corpuscles exhibit all the characters of the amœba (Fig. 2). Even such
dense structures as bone, cartilage, and the teeth are formed from cells.

[Illustration: Fig. 2.--Amœboid Movement of a Human White Blood
Corpuscle. (Showing various phases of movement.)]

In short, cells may be regarded as the histological units of animal
structures; by the combination, association, and modification of these
the body is built up. Of the real nature of the changes going on within
the living protoplasm, the process of building up lifeless material into
living structures, and the process of breaking down by which waste is
produced, we know absolutely nothing. Could we learn that, perhaps we
should know the secret of life.

13. Kinds of Cells. Cells vary greatly in size, some of the smallest
being only 1/3500 an inch or less in diameter. They also vary greatly in
form, as may be seen in Figs. 3 and 5. The typical cell is usually
_globular_ in form, other shapes being the result of pressure or of
similar modifying influences. The globular, as well as the large, flat
cells, are well shown in a drop of saliva. Then there are the _columnar_
cells, found in various parts of the intestines, in which they are closely
arranged side by side. These cells sometimes have on the free surface
delicate prolongations called cilia. Under the microscope they resemble a
wave, as when the wind blows over a field of grain (Fig. 5). There are
besides cells known as _spindle, stellate, squamous_ or pavement, and
various other names suggested by their shapes. Cells are also described as
to their contents. Thus _fat_ and _pigment_ cells are alluded to in
succeeding sections. Again, they may be described as to their functions or
location or the tissue in which they are found, as _epithelial_ cells,
_blood_ cells (corpuscles, Figs. 2 and 66), _nerve_ cells (Fig. 4), and
_connective-tissue_ cells.

14. Vital Properties of Cells. Each cell has a life of its own. It
manifests its vital properties in that it is born, grows, multiplies,
decays, and at last dies.[3] During its life it assimilates food, works,
rests, and is capable of spontaneous motion and frequently of locomotion.
The cell can secrete and excrete substance, and, in brief, presents nearly
all the phenomena of a human being.

Cells are produced only from cells by a process of self-division,
consisting of a cleavage of the whole cell into parts, each of which
becomes a separate and independent organism. Cells rapidly increase in
size up to a certain definite point which they maintain during adult life.
A most interesting quality of cell life is motion, a beautiful form of
which is found in ciliated epithelium. Cells may move actively and
passively. In the blood the cells are swept along by the current, but the
white corpuscles, seem able to make their way actively through the
tissues, as if guided by some sort of instinct.

[Illustration: Fig. 3.--Various Forms of Cells.

A, columnar cells found lining various parts of the intestines (called
_columnar epithelium_);
B, cells of a fusiform or spindle shape found in the loose tissue under
the skin and in other parts (called _connective-tissue cells_);
C, cell having many processes or projections--such are found in
connective tissue, D, primitive cells composed of protoplasm with
nucleus, and having no cell wall. All are represented about 400 times
their real size.
]

Some cells live a brief life of 12 to 24 hours, as is probably the case
with many of the cells lining the alimentary canal; others may live for
years, as do the cells of cartilage and bone. In fact each cell goes
through the same cycle of changes as the whole organism, though doubtless
in a much shorter time. The work of cells is of the most varied kind, and
embraces the formation of every tissue and product,--solid, liquid, or
gaseous. Thus we shall learn that the cells of the liver form bile, those
of the salivary glands and of the glands of the stomach and pancreas form
juices which aid in the digestion of food.

15. The Process of Life. All living structures are subject to
constant decay. Life is a condition of incessant changes, dependent upon
two opposite processes, repair and decay. Thus our bodies are not
composed of exactly the same particles from day to day, or even from one
moment to another, although to all appearance we remain the same
individuals. The change is so gradual, and the renewal of that which is
lost may be so exact, that no difference can be noticed except at long
intervals of time.[4] (See under "Bacteria," Chapter XIV.)

The entire series of chemical changes that take place in the living body,
beginning with assimilation and ending with excretion, is included in one
word, metabolism. The process of building up living material, or the
change by which complex substances (including the living matter itself)
are built up from simpler materials, is called anabolism. The
breaking down of material into simple products, or the changes in which
complex materials (including the living substance) are broken down into
comparatively simple products, is known as katabolism. This reduction
of complex substances to simple, results in the production of animal force
and energy. Thus a complex substance, like a piece of beef-steak, is built
up of a large number of molecules which required the expenditure of force
or energy to store up. Now when this material is reduced by the process of
digestion to simpler bodies with fewer molecules, such as carbon dioxid,
urea, and water, the force stored up in the meat as potential energy
becomes manifest and is used as active life-force known as _kinetic
energy_.

16. Epithelium. Cells are associated and combined in many ways to
form a simple tissue. Such a simple tissue is called an epithelium or
surface-limiting tissue, and the cells are known as epithelial
cells. These are united by a very small amount of a cement substance which
belongs to the proteid class of material. The epithelial cells, from their
shape, are known as squamous, columnar, glandular, or ciliated. Again, the
cells may be arranged in only a single layer, or they may be several
layers deep. In the former case the epithelium is said to be simple; in
the latter, stratified. No blood-vessels pass into these tissues; the
cells derive their nourishment by the imbibition of the plasma of the
blood exuded into the subjacent tissue.

[Illustration: Fig. 4.--Nerve Cells from the Gray Matter of the
Cerebellum. (Magnified 260 diameters.)]

17. Varieties of Epithelium. The squamous or pavement epithelium
consists of very thin, flattened scales, usually with a small nucleus in
the center. When the nucleus has disappeared, they become mere horny
plates, easily detached. Such cells will be described as forming the outer
layer of the skin, the lining of the mouth and the lower part of the
nostrils.

The columnar epithelium consists of pear-shaped or elongated cells,
frequently as a single layer of cells on the surface of a mucous membrane,
as on the lining of the stomach and intestines, and the free surface of
the windpipe and large air-tubes.

The glandular or spheroidal epithelium is composed of round cells or
such as become angular by mutual pressure. This kind forms the lining of
glands such as the liver, pancreas, and the glands of the skin.

The ciliated epithelium is marked by the presence of very fine
hair-like processes called cilia, which develop from the free end of the
cell and exhibit a rapid whip-like movement as long as the cell is alive.
This motion is always in the same direction, and serves to carry away
mucus and even foreign particles in contact with the membrane on which
the cells are placed. This epithelium is especially common in the air
passages, where it serves to keep a free passage for the entrance and exit
of air. In other canals a similar office is filled by this kind of
epithelium.

18. Functions of Epithelial Tissues. The epithelial structures may be
divided, as to their functions, into two main divisions. One is chiefly
protective in character. Thus the layers of epithelium which form the
superficial layer of the skin have little beyond such an office to
discharge. The same is to a certain extent true of the epithelial cells
covering the mucous membrane of the mouth, and those lining the air
passages and air cells of the lungs.

[Illustration: Fig. 5.--Various Kinds of Epithelial Cells

A, columnar cells of intestine;
B, polyhedral cells of the conjunctiva;
C, ciliated conical cells of the trachea;
D, ciliated cell of frog's mouth;
E, inverted conical cell of trachea;
F, squamous cell of the cavity of mouth, seen from its broad surface;
G, squamous cell, seen edgeways.
]

The second great division of the epithelial tissues consists of those
whose cells are formed of highly active protoplasm, and are busily engaged
in some sort of secretion. Such are the cells of glands,--the cells of the
salivary glands, which secrete the saliva, of the gastric glands, which
secrete the gastric juice, of the intestinal glands, and the cells of the
liver and sweat glands.

19. Connective Tissue. This is the material, made up of fibers and
cells, which serves to unite and bind together the different organs and
tissues. It forms a sort of flexible framework of the body, and so
pervades every portion that if all the other tissues were removed, we
should still have a complete representation of the bodily shape in every
part. In general, the connective tissues proper act as packing,
binding, and supporting structures. This name includes certain tissues
which to all outward appearance vary greatly, but which are properly
grouped together for the following reasons: first, they all act as
supporting structures; second, under certain conditions one may be
substituted for another; third, in some places they merge into each other.

All these tissues consist of a ground-substance, or matrix, cells, and
fibers. The ground-substance is in small amount in connective tissues
proper, and is obscured by a mass of fibers. It is best seen in hyaline
cartilage, where it has a glossy appearance. In bone it is infiltrated
with salts which give bone its hardness, and make it seem so unlike other
tissues. The cells are called connective-tissue corpuscles, cartilage
cells, and bone corpuscles, according to the tissues in which they occur.
The fibers are the white fibrous and the yellow elastic tissues.

The following varieties are usually described:

I. Connective Tissues Proper:

1. White Fibrous Tissue.
2. Yellow Elastic Tissue.
3. Areolar or Cellular Tissue.
4. Adipose or Fatty Tissue.
5. Adenoid or Retiform Tissue.

II. Cartilage (Gristle):

1. Hyaline.
2. White Fibro-cartilage.
3. Yellow Fibro-cartilage.

III. Bone and Dentine of Teeth.

20. White Fibrous Tissue. This tissue consists of bundles of very
delicate fibrils bound together by a small amount of cement substance.
Between the fibrils protoplasmic masses (connective-tissue corpuscles)
are found. These fibers may be found so interwoven as to form a sheet, as
in the periosteum of the bone, the fasciæ around muscles, and the capsules
of organs; or they may be aggregated into bundles and form rope-like
bands, as in the ligaments of joints and the tendons of muscles. On
boiling, this tissue yields gelatine. In general, where white fibrous
tissue abounds, structures are held together, and there is flexibility,
but little or no distensibility.

[Illustration: Fig. 6.--White Fibrous Tissue. (Highly magnified.)]

21. Yellow Elastic Tissue. The fibers of yellow elastic tissue
are much stronger and coarser than those of the white. They are yellowish,
tend to curl up at the ends, and are highly elastic. It is these fibers
which give elasticity to the skin and to the coats of the arteries. The
typical form of this tissue occurs in the ligaments which bind the
vertebræ together (Fig. 26), in the true vocal cords, and in certain
ligaments of the larynx. In the skin and fasciæ, the yellow elastic is
found mixed with white fibrous and areolar tissues. It does not yield
gelatine on boiling, and the cells are, if any, few.

[Illustration: Fig. 7.--Yellow Elastic Tissue. (Highly magnified.)]

22. Areolar or Cellular Tissue. This consists of bundles of delicate
fibers interlacing and crossing one another, forming irregular spaces or
meshes. These little spaces, in health, are filled with fluid that has
oozed out of the blood-vessels. The areolar tissue forms a protective
covering for the tissues of delicate and important organs.

23. Adipose or Fatty Tissue. In almost every part of the body the
ordinary areolar tissue contains a variable quantity of adipose or
fatty tissue. Examined by the microscope, the fat cells consist of a
number of minute sacs of exceedingly delicate, structureless membrane
filled with oil. This is liquid in life, but becomes solidified after
death. This tissue is plentiful beneath the skin, in the abdominal cavity,
on the surface of the heart, around the kidneys, in the marrow of bones,
and elsewhere. Fat serves as a soft packing material. Being a poor
conductor, it retains the heat, and furnishes a store rich in carbon and
hydrogen for use in the body.

24. Adenoid or Retiform Tissue. This is a variety of connective
tissue found in the tonsils, spleen, lymphatic glands, and allied
structures. It consists of a very fine network of cells of various sizes.
The tissue combining them is known as adenoid or gland-like tissue.

[Illustration: Fig. 8.--Fibro-Cartilage Fibers. (Showing network
surrounded cartilage cells.)]

25. Cartilage. Cartilage, or gristle, is a tough but highly elastic
substance. Under the microscope cartilage is seen to consist of a
matrix, or base, in which nucleated cells abound, either singly or in
groups. It has sometimes a fine ground-glass appearance, when the
cartilage is spoken of as hyaline. In other cases the matrix is
almost replaced by white fibrous tissue. This is called white
fibro-cartilage, and is found where great strength and a certain
amount of rigidity are required.

Again, there is between the cells a meshwork of yellow elastic fibers, and
this is called yellow fibro-cartilage (Fig. 8). The hyaline cartilage
forms the early state of most of the bones, and is also a permanent
coating for the articular ends of long bones. The white fibro-cartilage is
found in the disks between the bodies of the vertebræ, in the interior of
the knee joint, in the wrist and other joints, filling the cavities of the
bones, in socket joints, and in the grooves for tendons. The yellow
fibro-cartilage forms the expanded part of the ear, the epiglottis, and
other parts of the larynx.

26. General Plan of the Body. To get a clearer idea of the general
plan on which the body is constructed, let us imagine its division into
perfectly equal parts, one the right and the other the left, by a great
knife severing it through the median, or middle line in front, backward
through the spinal column, as a butcher divides an ox or a sheep into
halves for the market. In a section of the body thus planned the skull and
the spine together are shown to have formed a tube, containing the brain
and spinal cord. The other parts of the body form a second tube (ventral)
in front of the spinal or dorsal tube. The upper part of the second tube
begins with the mouth and is formed by the ribs and breastbone. Below the
chest in the abdomen, the walls of this tube would be made up of the soft
parts.

[Illustration: Fig. 9.--Diagrammatic Longitudinal Section of the Trunk and
Head. (Showing the dorsal and the ventral tubes.)

A, the cranial cavity;
B, the cavity of the nose;
C, the mouth;
D, the alimentary canal represented as a simple straight tube;
E, the sympathetic nervous system;
F, heart;
G, diaphragm;
H, stomach;
K, end of spinal portion of cerebro-spinal nervous system.
]

We may say, then, that the body consists of two tubes or cavities,
separated by a bony wall, the dorsal or nervous tube, so called
because it contains the central parts of the nervous system; and the
visceral or ventral tube, as it contains the viscera, or general
organs of the body, as the alimentary canal, the heart, the lungs, the
sympathetic nervous system, and other organs.

The more detailed study of the body may now be begun by a description of
the skeleton or framework which supports the soft parts.

Experiments.

For general directions and explanations and also detailed suggestions for
performing experiments, see Chapter XV.

Experiment 1. _To examine squamous epithelium._ With an ivory
paper-knife scrape the back of the tongue or the inside of the lips or
cheek; place the substance thus obtained upon a glass slide; cover it
with a thin cover-glass, and if necessary add a drop of water. Examine
with the microscope, and the irregularly formed epithelial cells will be
seen.

Experiment 2. _To examine ciliated epithelium._ Open a frog's
mouth, and with the back of a knife blade gently scrape a little of the
membrane from the roof of the mouth. Transfer to a glass slide, add a
drop of salt solution, and place over it a cover-glass with a hair
underneath to prevent pressure upon the cells. Examine with a microscope
under a high power. The cilia move very rapidly when quite fresh, and
are therefore not easily seen.

For additional experiments which pertain to the microscopic examination of
the elementary tissues and to other points in practical histology, see
Chapter XV.

[NOTE. Inasmuch as most of the experimental work of this chapter
depends upon the use of the microscope and also necessarily assumes a
knowledge of facts which are discussed later, it would be well to
postpone experiments in histology until they can be more
satisfactorily handled in connection with kindred topics as they are
met with in the succeeding chapters.]

Chapter II.

The Bones.

27. The Skeleton. Most animals have some kind of framework to support
and protect the soft and fleshy parts of their bodies. This framework
consists chiefly of a large number of bones, and is called the
skeleton. It is like the keel and ribs of a vessel or the frame of a
house, the foundation upon which the bodies are securely built.

There are in the adult human body 200 distinct bones, of many sizes and
shapes. This number does not, however, include several small bones found
in the tendons of muscles and in the ear. The teeth are not usually
reckoned as separate bones, being a part of the structure of the skin.

The number of distinct bones varies at different periods of life. It is
greater in childhood than in adults, for many bones which are then
separate, to allow growth, afterwards become gradually united. In early
adult life, for instance, the skull contains 22 naturally separate bones,
but in infancy the number is much greater, and in old age far less.

The bones of the body thus arranged give firmness, strength, and
protection to the soft tissues and vital organs, and also form levers for
the muscles to act upon.

28. Chemical Composition of Bone. The bones, thus forming the
framework of the body, are hard, tough, and elastic. They are twice as
strong as oak; one cubic inch of compact bone will support a weight of
5000 pounds. Bone is composed of earthy or mineral matter
(chiefly in the form of lime salts), and of animal matter
(principally gelatine), in the proportion of two-thirds of the former to
one-third of the latter.

[Illustration: Fig. 10.--The Skeleton.]

The proportion of earthy to animal matter varies with age. In infancy the
bones are composed almost wholly of animal matter. Hence, an infant's
bones are rarely broken, but its legs may soon become misshapen if walking
is allowed too early. In childhood, the bones still contain a larger
percentage of animal matter than in more advanced life, and are therefore
more liable to bend than to break; while in old age, they contain a
greater percentage of mineral matter, and are brittle and easily broken.

Experiment 3. _To show the mineral matter in bone_. Weigh a large
soup bone; put it on a hot, clear fire until it is at a red heat. At
first it becomes black from the carbon of its organic matter, but at
last it turns white. Let it cool and weigh again. The animal matter has
been burnt out, leaving the mineral or earthy part, a white, brittle
substance of exactly the same shape, but weighing only about two-thirds
as much as the bone originally weighed.

Experiment 4. _To show the animal matter in bone_. Add a
teaspoonful of muriatic acid to a pint of water, and place the mixture
in a shallow earthen dish. Scrape and clean a chicken's leg bone, part
of a sheep's rib, or any other small, thin bone. Soak the bone in the
acid mixture for a few days. The earthy or mineral matter is slowly
dissolved, and the bone, although retaining its original form, loses its
rigidity, and becomes pliable, and so soft as to be readily cut. If the
experiment be carefully performed, a long, thin bone may even be tied
into a knot.

[Illustration: Fig. 11.--The fibula tied into a knot, after the hard
mineral matter has been dissolved by acid.]

29. Physical Properties of Bone. If we take a leg bone of a sheep, or
a large end of beef shin bone, and saw it lengthwise in halves, we see two
distinct structures. There is a hard and compact tissue, like ivory,
forming the outside shell, and a spongy tissue inside having the
appearance of a beautiful lattice work. Hence this is called cancellous
tissue, and the gradual transition from one to the other is apparent.

It will also be seen that the shaft is a hollow cylinder, formed of
compact tissue, enclosing a cavity called the medullary canal, which is
filled with a pulpy, yellow fat called _marrow_. The marrow is richly
supplied with blood-vessels, which enter the cavity through small openings
in the compact tissue. In fact, all over the surface of bone are minute
canals leading into the substance. One of these, especially constant and
large in many bones, is called the _nutrient foramen_, and transmits an
artery to nourish the bone.

At the ends of a long bone, where it expands, there is no medullary canal,
and the bony tissue is spongy, with only a thin layer of dense bone around
it. In flat bones we find two layers or plates of compact tissue at the
surface, and a spongy tissue between. Short and irregular bones have no
medullary canal, only a thin shell of dense bone filled with cancellous
tissue.

[Illustration: Fig 12.--The Right femur sawed in two, lengthwise. (Showing
arrangement of compact and cancellous tissue.)]

Experiment 5. Obtain a part of a beef shin bone, or a portion of a
sheep's or calf's leg, including if convenient the knee joint. Have the
bone sawed in two, lengthwise, keeping the marrow in place. Boil,
scrape, and carefully clean one half. Note the compact and spongy parts,
shaft, etc.

Experiment 6. Trim off the flesh from the second half. Note the
pinkish white appearance of the bone, the marrow, and the tiny specks of
blood, etc. Knead a small piece of the marrow in the palm; note the oily
appearance. Convert some marrow into a liquid by heating. Contrast this
fresh bone with an old dry one, as found in the fields. Fresh bones
should be kept in a cool place, carefully wrapped in a damp cloth, while
waiting for class use.

A fresh or living bone is covered with a delicate, tough, fibrous
membrane, called the periosteum. It adheres very closely to the bone,
and covers every part except at the joints and where it is protected with
cartilage. The periosteum is richly supplied with blood-vessels, and plays
a chief part in the growth, formation, and repair of bone. If a portion of
the periosteum be detached by injury or disease, there is risk that a
layer of the subjacent bone will lose its vitality and be cast off.[5]

30. Microscopic Structure of Bone. If a very thin slice of bone be
cut from the compact tissue and examined under a microscope, numerous
minute openings are seen. Around these are arranged rings of bone, with
little black bodies in them, from which radiate fine, dark lines. These
openings are sections of canals called _Haversian canals_, after Havers,
an English physician, who first discovered them. The black bodies are
minute cavities called _lacunæ_, while the fine lines are very minute
canals, _canaliculi_, which connect the lacunæ and the Haversian canals.
These Haversian canals are supplied with tiny blood-vessels, while the
lacunæ contain bone cells. Very fine branches from these cells pass into
the canaliculi. The Haversian canals run lengthwise of the bone; hence if
the bone be divided longitudinally these canals will be opened along their
length (Fig. 13).

Thus bones are not dry, lifeless substances, but are the very type of
activity and change. In life they are richly supplied with blood from the
nutrient artery and from the periosteum, by an endless network of
nourishing canals throughout their whole structure. Bone has, therefore,
like all other living structures, a _self-formative_ power, and draws from
the blood the materials for its own nutrition.

[Illustration: Fig. 13.

A, longitudinal section of bone, by which the Haversian canals are seen
branching and communicating with one another;
B, cross section of a very thin slice of bone, magnified about 300
diameters--little openings (Haversian canals) are seen, and around
them are ranged rings of bones with little black bodies (lacunæ), from
which branch out fine dark lines (canaliculi);
C, a bone cell, highly magnified, lying in lacuna.
]

The Bones of the Head.

31. The Head, or Skull. The bones of the skeleton, the bony framework
of our bodies, may be divided into those of the head, the trunk,
and the limbs.

The bones of the head are described in two parts,--those of the
cranium, or brain-case, and those of the face. Taken together,
they form the skull. The head is usually said to contain 22 bones, of
which 8 belong to the cranium and 14 to the face. In early childhood, the
bones of the head are separate to allow the brain to expand; but as we
grow older they gradually unite, the better to protect the delicate brain
tissue.

32. The Cranium. The cranium is a dome-like structure, made up
in the adult of 8 distinct bones firmly locked together. These bones are:

One Frontal,
Two Parietal,
Two Temporal
One Occipital,
One Sphenoid,
One Ethmoid.

The frontal bone forms the forehead and front of the head. It is
united with the two parietal bones behind, and extends over the forehead
to make the roofs of the sockets of the eyes. It is this bone which, in
many races of man, gives a dignity of person and a beauty of form seen in
no other animal.

The parietal bones form the sides and roof of the skull. They are
bounded anteriorly by the frontal bone, posteriorly by the occipital, and
laterally by the temporal and sphenoid bones. The two bones make a
beautiful arch to aid in the protection of the brain.

The temporal bones, forming the temples on either side, are attached
to the sphenoid bone in front, the parietals above, and the occipital
behind. In each temporal bone is the cavity containing the organs of
hearing. These bones are so called because the hair usually first turns
gray over them.

The occipital bone forms the lower part of the base of the skull, as
well as the back of the head. It is a broad, curved bone, and rests on the
topmost vertebra (atlas) of the backbone; its lower part is pierced by a
large oval opening called the _foramen magnum_, through which the spinal
cord passes from the brain (Fig. 15).

The sphenoid bone is in front of the occipital, forming a part of the
base of the skull. It is wedged between the bones of the face and those of
the cranium, and locks together fourteen different bones. It bears a
remarkable resemblance to a bat with extended wings, and forms a series of
girders to the arches of the cranium.

The ethmoid bone is situated between the bones of the cranium and
those of the face, just at the root of the nose. It forms a part of the
floor of the cranium. It is a delicate, spongy bone, and is so called
because it is perforated with numerous holes like a sieve, through which
the nerves of smell pass from the brain to the nose.

[Illustration: Fig. 14.--The Skull]

33. The Face. The bones of the face serve, to a marked extent, in
giving form and expression to the human countenance. Upon these bones
depend, in a measure, the build of the forehead, the shape of the chin,
the size of the eyes, the prominence of the cheeks, the contour of the
nose, and other marks which are reflected in the beauty or ugliness of the
face.

The face is made up of fourteen bones which, with the exception of
the lower jaw, are, like those of the cranium, closely interlocked with
each other. By this union these bones help form a number of cavities which
contain most important and vital organs. The two deep, cup-like sockets,
called the orbits, contain the organs of sight. In the cavities of the
nose is located the sense of smell, while the buccal cavity, or mouth, is
the site of the sense of taste, and plays besides an important part in the
first act of digestion and in the function of speech.

The bones of the face are:

Two Superior Maxillary,
Two Malar,
Two Nasal,
Two Lachrymal,
Two Palate,
Two Turbinated,
One Vomer,
One Lower Maxillary.

34. Bones of the Face. The superior maxillary or upper jawbones
form a part of the roof of the mouth and the entire floor of the orbits.
In them is fixed the upper set of teeth.

The malar or cheek bones are joined to the upper jawbones, and help
form the sockets of the eyes. They send an arch backwards to join the
temporal bones. These bones are remarkably thick and strong, and are
specially adapted to resist the injury to which this part of the face is
exposed.

The nasal or nose bones are two very small bones between the eye
sockets, which form the bridge of the nose. Very near these bones are the
two small lachrymal bones. These are placed in the inner angles of
the orbit, and in them are grooves in which lie the ducts through which
the tears flow from the eyes to the nose.

The palate bones are behind those of the upper jaw and with them form
the bony part of the roof of the mouth. The inferior turbinated are
spongy, scroll-like bones, which curve about within the nasal cavities so
as to increase the surface of the air passages of the nose.

The vomer serves as a thin and delicate partition between the two cavities
of the nose. It is so named from its resemblance to a ploughshare.

[Illustration: Fig. 15.--The Base of the Skull.

A, palate process of upper jawbone;
B, zygoma, forming zygomatic arch;
C, condyle for forming articulation with atlas;
D, foramen magnum;
E, occipital bone.
]

The longest bone in the face is the inferior maxillary, or lower jaw.
It has a horseshoe shape, and supports the lower set of teeth. It is the
only movable bone of the head, having a vertical and lateral motion by
means of a hinge joint with a part of the temporal bone.

35. Sutures of the Skull. Before leaving the head we must notice the
peculiar and admirable manner in which the edges of the bones of the outer
shell of the skull are joined together. These edges of the bones resemble
the teeth of a saw. In adult life these tooth-like edges fit into each
other and grow together, suggesting the dovetailed joints used by the
cabinet-maker. When united these serrated edges look almost as if sewed
together; hence their name, sutures. This manner of union gives unity
and strength to the skull.

In infants, the corners of the parietal bones do not yet meet, and the
throbbing of the brain may be seen and felt under these "soft spots," or
_fontanelles_, as they are called. Hence a slight blow to a babe's head
may cause serious injury to the brain (Fig. 14).

The Bones of the Trunk.

36. The Trunk. The trunk is that central part of the body which
supports the head and the upper pair of limbs. It divides itself into an
upper cavity, the thorax, or chest; and a lower cavity, the
abdomen. These two cavities are separated by a movable, muscular
partition called the diaphragm, or midriff (Figs. 9 and 49).

The bones of the trunk are variously related to each other, and some of
them become united during adult life into bony masses which at earlier
periods are quite distinct. For example, the sacrum is in early life made
up of five distinct bones which later unite into one.

The upper cavity, or chest, is a bony enclosure formed by the
breastbone, the ribs, and the spine. It contains the heart and the lungs
(Fig. 86).

The lower cavity, or abdomen, holds the stomach, liver, intestines,
spleen, kidneys, and some other organs (Fig. 59).

The bones of the trunk may be subdivided into those of the spine, the
ribs, and the hips.

The trunk includes 54 bones usually thus arranged:

I. Spinal Column, 26 bones:
7 Cervical Vertebræ.
12 Dorsal Vertebræ.
5 Lumbar Vertebræ.
1 Sacrum.
1 Coccyx.

II. Ribs, 24 bones:
14 True Ribs.
6 False Ribs.
4 Floating Ribs.

III. Sternum.

IV. Two Hip Bones.

V. Hyoid Bone.

37. The Spinal Column. The spinal column, or backbone, is a
marvelous piece of mechanism, combining offices which nothing short of
perfection in adaptation and arrangement could enable it to perform. It is
the central structure to which all the other parts of the skeleton are
adapted. It consists of numerous separate bones, called vertebræ. The
seven upper ones belong to the neck, and are called cervical
vertebræ. The next twelve are the dorsal vertebræ; these belong to
the back and support the ribs. The remaining five belong to the loins, and
are called lumbar vertebræ. On looking at the diagram of the backbone
(Fig. 9) it will be seen that the vertebræ increase in size and strength
downward, because of the greater burden they have to bear, thus clearly
indicating that an erect position is the one natural to man.

[Illustration: Fig. 16.--The Spinal Column.]

This column supports the head, encloses and protects the spinal cord, and
forms the basis for the attachment of many muscles, especially those which
maintain the body in an erect position. Each vertebra has an opening
through its center, and the separate bones so rest, one upon another, that
these openings form a continuous canal from the head to the lower part of
the spine. The great nerve, known as the spinal cord, extends from
the cranium through the entire length of this canal. All along the spinal
column, and between each two adjoining bones, are openings on each side,
through which nerves pass out to be distributed to various parts of the
body.

Between the vertebræ are pads or cushions of cartilage. These act as
"buffers," and serve to give the spine strength and elasticity and to
prevent friction of one bone on another. Each vertebra consists of a body,
the solid central portion, and a number of projections called processes.
Those which spring from the posterior of each arch are the spinous
processes. In the dorsal region they are plainly seen and felt in thin
persons.

The bones of the spinal column are arranged in three slight and graceful
curves. These curves not only give beauty and strength to the bony
framework of the body, but also assist in the formation of cavities for
important internal organs. This arrangement of elastic pads between the
vertebræ supplies the spine with so many elastic springs, which serve to
break the effect of shock to the brain and the spinal cord from any sudden
jar or injury.

The spinal column rests on a strong three-sided bone called the
sacrum, or sacred-bone, which is wedged in between the hip bones and
forms the keystone of the pelvis. Joined to the lower end of the sacrum is
the coccyx, or cuckoo-bone, a tapering series of little bones.

Experiment 7. Run the tips of the fingers briskly down the
backbone, and the spines of the vertebræ will be tipped with red so that
they can be readily counted. Have the model lean forward with the arms
folded across the chest; this will make the spines of the vertebræ more
prominent.

Experiment 8. _To illustrate the movement of torsion in the spine,
or its rotation round its own axis_. Sit upright, with the back and
shoulders well applied against the back of a chair. Note that the head
and neck can be turned as far as 60° or 70°. Now bend forwards, so as to
let the dorsal and lumbar vertebræ come into play, and the head can be
turned 30° more.

Experiment 9. _To show how the spinal vertebræ make a firm but
flexible column._ Take 24 hard rubber overcoat buttons, or the same
number of two-cent pieces, and pile them on top of each other. A thin
layer of soft putty may be put between the coins to represent the pads
of cartilage between the vertebræ. The most striking features of the
spinal column may be illustrated by this simple apparatus.

38. How the Head and Spine are Joined together. The head rests upon
the spinal column in a manner worthy of special notice. This consists in
the peculiar structure of the first two cervical vertebræ, known as the
axis and atlas. The atlas is named after the fabled giant who
supported the earth on his shoulders. This vertebra consists of a ring of
bone, having two cup-like sockets into which fit two bony projections
arising on either side of the great opening (_foramen magnum_) in the
occipital bone. The hinge joint thus formed allows the head to nod
forward, while ligaments prevent it from moving too far.

On the upper surface of the axis, the second vertebra, is a peg or
process, called the _odontoid process_ from its resemblance to a tooth.
This peg forms a pivot upon which the head with the atlas turns. It is
held in its place against the front inner surface of the atlas by a band
of strong ligaments, which also prevents it from pressing on the delicate
spinal cord. Thus, when we turn the head to the right or left, the skull
and the atlas move together, both rotating on the odontoid process of the
axis.

39. The Ribs and Sternum. The barrel-shaped framework of the chest is
in part composed of long, slender, curved bones called ribs. There
are twelve ribs on each side, which enclose and strengthen the chest; they
somewhat resemble the hoops of a barrel. They are connected in pairs with
the dorsal vertebræ behind.

The first seven pairs, counting from the neck, are called the _true_ ribs,
and are joined by their own special cartilages directly to the breastbone.
The five lower pairs, called the _false_ ribs, are not directly joined to
the breastbone, but are connected, with the exception of the last two,
with each other and with the last true ribs by cartilages. These elastic
cartilages enable the chest to bear great blows with impunity. A blow on
the sternum is distributed over fourteen elastic arches. The lowest two
pairs of false ribs, are not joined even by cartilages, but are quite free
in front, and for this reason are called _floating_ ribs.

The ribs are not horizontal, but slope downwards from the backbone, so
that when raised or depressed by the strong intercostal muscles, the size
of the chest is alternately increased or diminished. This movement of the
ribs is of the utmost importance in breathing (Fig. 91).

The sternum, or breastbone, is a long, flat, narrow bone forming the
middle front wall of the chest. It is connected with the ribs and with the
collar bones. In shape it somewhat resembles an ancient dagger.

40. The Hip Bones. Four immovable bones are joined together so as to
form at the lower extremity of the trunk a basin-like cavity called the
pelvis. These four bones are the sacrum and the coccyx,
which have been described, and the two hip bones.

[Illustration: Fig. 17.--Thorax. (Anterior view.)]

The hip bones are large, irregularly shaped bones, very firm and
strong, and are sometimes called the haunch bones or _ossa innominata_
(nameless bones). They are united to the sacrum behind and joined to each
other in front. On the outer side of each hip bone is a deep cup, or
socket, called the _acetabulum_, resembling an ancient vinegar cup, into
which fits the rounded head of the thigh bone. The bones of the pelvis are
supported like a bridge on the legs as pillars, and they in turn contain
the internal organs in the lower part of the trunk.

41. The Hyoid Bone. Under the lower jaw is a little horseshoe shaped
bone called the hyoid bone, because it is shaped like the Greek
letter upsilon (Υ). The root of the tongue is fastened to its bend,
and the larynx is hung from it as from a hook. When the neck is in its
natural position this bone can be plainly felt on a level with the lower
jaw and about one inch and a half behind it. It serves to keep open the
top of the larynx and for the attachment of the muscles, which move the
tongue. (See Fig. 46.) The hyoid bone, like the knee-pan, is not connected
with any other bone.

The Bones of the Upper Limbs.

42. The Upper Limbs. Each of the upper limbs consist of the upper
arm, the forearm, and the hand. These bones are classified
as follows:

Upper Arm:
Scapula, or shoulder-blade,
Clavicle, or collar bone,
Humerus, or arm bone,

Forearm:
Ulna,
Radius,

Hand:
8 Carpal or wrist bones,
5 Metacarpal bones,
14 Phalanges, or finger bones,

making 32 bones in all.

43. The Upper Arm. The two bones of the shoulder, the scapula
and the clavicle, serve in man to attach the arm to the trunk. The
scapula, or shoulder-blade, is a flat, triangular bone, placed point
downwards, and lying on the upper and back part of the chest, over the
ribs. It consists of a broad, flat portion and a prominent ridge or
_spine_. At its outer angle it has a shallow cup known as the _glenoid
cavity_. Into this socket fits the rounded head of the humerus. The
shoulder-blade is attached to the trunk chiefly by muscles, and is capable
of extensive motion.

The clavicle, or collar bone, is a slender bone with a double curve
like an italic _f_, and extends from the outer angle of the shoulder-blade
to the top of the breastbone. It thus serves like the keystone of an arch
to hold the shoulder-blade firmly in its place, but its chief use is to
keep the shoulders wide apart, that the arm may enjoy a freer range of
motion. This bone is often broken by falls upon the shoulder or arm.

The humerus is the strongest bone of the upper extremity. As already
mentioned, its rounded head fits into the socket of the shoulder-blade,
forming a ball-and-socket joint, which permits great freedom of motion.
The shoulder joint resembles what mechanics call a universal joint, for
there is no part of the body which cannot be touched by the hand.

[Illustration: Fig. 18.--Left Scapula, or Shoulder-Blade.]

When the shoulder is dislocated the head of the humerus has been forced
out of its socket. The lower end of the bone is grooved to help form a
hinge joint at the elbow with the bones of the forearm (Fig. 27).

44. The Forearm. The forearm contains two long bones, the
ulna and the radius. The ulna, so called because it forms
the elbow, is the longer and larger bone of the forearm, and is on the
same side as the little finger. It is connected with the humerus by a
hinge joint at the elbow. It is prevented from moving too far back by a
hook-like projection called the _olecranon process_, which makes the sharp
point of the elbow.

The radius is the shorter of the two bones of the forearm, and is on
the same side as the thumb. Its slender, upper end articulates with the
ulna and humerus; its lower end is enlarged and gives attachment in part
to the bones of the wrist. This bone radiates or turns on the ulna,
carrying the hand with it.

Experiment 10. Rest the forearm on a table, with the palm up (an
attitude called supination). The radius is on the outer side and
parallel with the ulna If now, without moving the elbow, we turn the
hand (pronation), as if to pick up something from the table, the radius
may be seen and felt crossing over the ulna, while the latter has not
moved.

[Illustration: Fig. 19.--Left Clavicle, or Collar Bone. (Anterior
surface.)]

45. The Hand. The hand is the executive or essential part of the
upper limb. Without it the arm would be almost useless. It consists of 27
separate bones, and is divided into three parts, the wrist, the
palm, and the fingers.

[Illustration: Fig. 20.--Left Humerus.]

[Illustration: Fig. 21.--Left Radius and Ulna.]

The carpus, or wrist, includes 8 short bones, arranged in two rows of
four each, so as to form a broad support for the hand. These bones are
closely packed, and tightly bound with ligaments which admit of ample
flexibility. Thus the wrist is much less liable to be broken than if it
were to consist of a single bone, while the elasticity from having the
eight bones movable on each other, neutralizes, to a great extent, a
shock caused by falling on the hands. Although each of the wrist bones has
a very limited mobility in relation to its neighbors, their combination
gives the hand that freedom of action upon the wrist, which is manifest in
countless examples of the most accurate and delicate manipulation.

The metacarpal bones are the five long bones of the back of the hand.
They are attached to the wrist and to the finger bones, and may be easily
felt by pressing the fingers of one hand over the back of the other. The
metacarpal bones of the fingers have little freedom of movement, while the
thumb, unlike the others, is freely movable. We are thus enabled to bring
the thumb in opposition to each of the fingers, a matter of the highest
importance in manipulation. For this reason the loss of the thumb disables
the hand far more than the loss of either of the fingers. This very
significant opposition of the thumb to the fingers, furnishing the
complete grasp by the hand, is characteristic of the human race, and is
wanting in the hand of the ape, chimpanzee, and ourang-outang.

The phalanges, or finger bones, are the fourteen small bones arranged
in three rows to form the fingers. Each finger has three bones; each
thumb, two.

The large number of bones in the hand not only affords every variety of
movement, but offers great resistance to blows or shocks. These bones are
united by strong but flexible ligaments. The hand is thus given strength
and flexibility, and enabled to accomplish the countless movements so
necessary to our well-being.

In brief, the hand is a marvel of precise and adapted mechanism, capable
not only of performing every variety of work and of expressing many
emotions of the mind, but of executing its orders with inconceivable
rapidity.

The Bones of the Lower Limbs.

46. The Lower Limbs. The general structure and number of the bones of
the lower limbs bear a striking similarity to those of the upper limbs.
Thus the leg, like the arm, is arranged in three parts, the thigh,
the lower leg, and the foot. The thigh bone corresponds to the
humerus; the tibia and fibula to the ulna and radius; the ankle to the
wrist; and the metatarsus and the phalanges of the foot, to the metacarpus
and the phalanges of the hand.

The bones of the lower limbs may be thus arranged:

Thigh: Femur, or thigh bone,

Lower Leg:
Patella, or knee cap,
Tibia, or shin bone,
Fibula, or splint bone,

Foot:
7 Tarsal or ankle bones,
5 Metatarsal or instep bones,
14 Phalanges, or toes bones,

making 30 bones in all.

[Illustration: Fig. 22.--Right Femur, or Thigh Bone.]

47. The Thigh. The longest and strongest of all the bones is the
femur, or thigh bone. Its upper end has a rounded head which fits into the
_acetabulum_, or the deep cup-like cavity of the hip bone, forming a
perfect ball-and-socket joint. When covered with cartilage, the ball fits
so accurately into its socket that it may be retained by atmospheric
pressure alone (sec. 50).

The shaft of the femur is strong, and is ridged and roughened in places
for the attachment of the muscles. Its lower end is broad and irregularly
shaped, having two prominences called _condyles_, separated by a groove,
the whole fitted for forming a hinge joint with the bones of the lower leg
and the knee-cap.

48. The Lower Leg. The lower leg, like the forearm, consists of
two bones. The tibia, or shin bone, is the long three-sided bone
forming the front of the leg. The sharp edge of the bone is easily felt
just under the skin. It articulates with the lower end of the thigh bone,
forming with it a hinge joint.

The fibula, the companion bone of the tibia, is the long, slender
bone on the outer side of the leg. It is firmly fixed to the tibia at each
end, and is commonly spoken of as the small bone of the leg. Its lower end
forms the outer projection of the ankle. In front of the knee joint,
embedded in a thick, strong tendon, is an irregularly disk-shaped bone,
the patella, or knee-cap. It increases the leverage of important
muscles, and protects the front of the knee joint, which is, from its
position, much exposed to injury.

[Illustration: Fig. 23.--Patella, or Knee-Cap.]

49. The Foot. The bones of the foot, 26 in number, consist of
the tarsal bones, the metatarsal, and the phalanges. The
tarsal bones are the seven small, irregular bones which make up the
ankle. These bones, like those of the wrist, are compactly arranged, and
are held firmly in place by ligaments which allow a considerable amount of
motion.

One of the ankle bones, the _os calcis_, projects prominently backwards,
forming the heel. An extensive surface is thus afforded for the attachment
of the strong tendon of the calf of the leg, called the tendon of
Achilles. The large bone above the heel bone, the _astragalus_,
articulates with the tibia, forming a hinge joint, and receives the weight
of the body.

The metatarsal bones, corresponding to the metacarpals of the hand,
are five in number, and form the lower instep.

The phalanges are the fourteen bones of the toes,--three in each
except the great toe, which, like the thumb, has two. They resemble in
number and plan the corresponding bones in the hand. The bones of the foot
form a double arch,--an arch from before backwards, and an arch from side
to side. The former is supported behind by the os calcis, and in front by
the ends of the metatarsal bones. The weight of the body falls
perpendicularly on the astragalus, which is the key-bone or crown of the
arch. The bones of the foot are kept in place by powerful ligaments,
combining great strength with elasticity.

[Illustration: Fig. 24.--Right Tibia and Fibula (Anterior surface.)]

[Illustration: Fig. 25.--Bones of Right Foot. (Dorsal surface.)]

The Joints.

50. Formation of Joints. The various bones of the skeleton are
connected together at different parts of their surfaces by joints, or
articulations. Many different kinds of joints have been described, but the
same general plan obtains for nearly all. They vary according to the kind
and the amount of motion.

The principal structures which unite in the formation of a joint are:
bone, cartilage, synovial membrane, and ligaments. Bones make
the chief element of all the joints, and their adjoining surfaces are
shaped to meet the special demands of each joint (Fig. 27). The joint-end
of bones is coated with a thin layer of tough, elastic cartilage. This is
also used at the edge of joint-cavities, forming a ring to deepen them.
The rounded heads of bones which move in them are thus more securely held
in their sockets.

Besides these structures, the muscles also help to maintain the
joint-surfaces in proper relation. Another essential to the action of the
joints is the pressure of the outside air. This may be sufficient to keep
the articular surfaces in contact even after all the muscles are removed.
Thus the hip joint is so completely surrounded by ligaments as to be
air-tight; and the union is very strong. But if the ligaments be pierced
and air allowed to enter the joint, the union at once becomes much less
close, and the head of the thigh bone falls away as far as the ligaments
will allow it.

51. Synovial Membrane. A very delicate connective tissue, called the
synovial membrane, lines the capsules of the joints, and covers the
ligaments connected with them. It secretes the _synovia_, or joint oil, a
thick and glairy fluid, like the white of a raw egg, which thoroughly
lubricates the inner surfaces of the joints. Thus the friction and heat
developed by movement are reduced, and every part of a joint is enabled to
act smoothly.

52. Ligaments. The bones are fastened together, held in place, and
their movements controlled, to a certain extent, by bands of various
forms, called ligaments. These are composed mainly of bundles of
white fibrous tissue placed parallel to, or closely interlaced with, one
another, and present a shining, silvery aspect. They extend from one of
the articulating bones to another, strongly supporting the joint, which
they sometimes completely envelope with a kind of cap (Fig. 28). This
prevents the bones from being easily dislocated. It is difficult, for
instance, to separate the two bones in a shoulder or leg of mutton, they
are so firmly held together by tough ligaments.

While ligaments are pliable and flexible, permitting free movement, they
are also wonderfully strong and inextensible. A bone may be broken, or its
end torn off, before its ligaments can be ruptured. The wrist end of the
radius, for instance, is often torn off by force exerted on its ligaments
without their rupture.

The ligaments are so numerous and various and are in some parts so
interwoven with each other, that space does not allow even mention of
those that are important. At the knee joint, for instance, there are no
less than fifteen distinct ligaments.

53. Imperfect Joints. It is only perfect joints that are fully
equipped with the structures just mentioned. Some joints lack one or more,
and are therefore called imperfect joints. Such joints allow little or no
motion and have no smooth cartilages at their edges. Thus, the bones of
the skull are dovetailed by joints called sutures, which are immovable.
The union between the vertebræ affords a good example of imperfect joints
which are partially movable.

[Illustration: Fig. 26.--Elastic Tissue from the Ligaments about Joints.
(Highly magnified.)]

54. Perfect Joints. There are various forms of perfect joints,
according to the nature and amount of movement permitted. They an divided
into hinge joints, ball-and-socket joints and pivot joints.

The hinge joints allow forward and backward movements like a hinge.
These joints are the most numerous in the body, as the elbow, the ankle,
and the knee joints.

In the ball-and-socket joints--a beautiful contrivance--the rounded
head of one bone fits into a socket in the other, as the hip joint and
shoulder joint. These joints permit free motion in almost every direction.

In the pivot joint a kind of peg in one bone fits into a notch in
another. The best example of this is the joint between the first and
second vertebræ (see sec. 38). The radius moves around on the ulna by
means of a pivot joint. The radius, as well as the bones of the wrist and
hand, turns around, thus enabling us to turn the palm of the hand upwards
and downwards. In many joints the extent of motion amounts to only a
slight gliding between the ends of the bones.

55. Uses of the Bones. The bones serve many important and useful
purposes. The skeleton, a general framework, affords protection,
support, and leverage to the bodily tissues. Thus, the bones of
the skull and of the chest protect the brain, the lungs, and the heart;
the bones of the legs support the weight of the body; and the long bones
of the limbs are levers to which muscles are attached.

Owing to the various duties they have to perform, the bones are
constructed in many different shapes. Some are broad and flat;
others, long and cylindrical; and a large number very irregular
in form. Each bone is not only different from all the others, but is also
curiously adapted to its particular place and use.

[Illustration: Fig. 27.--Showing how the Ends of the Bones are shaped to
form the Elbow Joint. (The cut ends of a few ligaments are seen.)]

Nothing could be more admirable than the mechanism by which each one of
the bones is enabled to fulfill the manifold purposes for which it was
designed. We have seen how the bones of the cranium are united by sutures
in a manner the better to allow the delicate brain to grow, and to afford
it protection from violence. The arched arrangement of the bones of the
foot has several mechanical advantages, the most important being that it
gives firmness and elasticity to the foot, which thus serves as a support
for the weight of the body, and as the chief instrument of locomotion.

The complicated organ of hearing is protected by a winding series of
minute apartments, in the rock-like portion of the temporal bone. The
socket for the eye has a jutting ridge of bone all around it, to guard the
organ of vision against injury. Grooves and canals, formed in hard bone,
lodge and protect minute nerves and tiny blood-vessels. The surfaces of
bones are often provided with grooves, sharp edges, and rough projections,
for the origin and insertion of muscles.

[Illustration: Fig. 28.--External Ligaments of the Knee.]

56. The Bones in Infancy and Childhood. The bones of the infant,
consisting almost wholly of cartilage, are not stiff and hard as in after
life, but flexible and elastic. As the child grows, the bones become more
solid and firmer from a gradually increased deposit of lime salts. In time
they become capable of supporting the body and sustaining the action of
the muscles. The reason is that well-developed bones would be of no use to
a child that had not muscular strength to support its body. Again, the
numerous falls and tumbles that the child sustains before it is able to
walk, would result in broken bones almost every day of its life. As it is,
young children meet with a great variety of falls without serious injury.

But this condition of things has its dangers. The fact that a child's
bones bend easily, also renders them liable to permanent change of shape.
Thus, children often become bow-legged when allowed to walk too early.
Moderate exercise, however, even in infancy, promotes the health of the
bones as well as of the other tissues. Hence a child may be kept too long
in its cradle, or wheeled about too much in a carriage, when the full use
of its limbs would furnish proper exercise and enable it to walk earlier.

57. Positions at School. Great care must be exercised by teachers
that children do not form the habit of taking injurious positions at
school. The desks should not be too low, causing a forward stoop; or too
high, throwing one shoulder up and giving a twist to the spine. If the
seats are too low there will result an undue strain on the shoulder and
the backbone; if too high, the feet have no proper support, the thighs may
be bent by the weight of the feet and legs, and there is a prolonged
strain on the hips and back. Curvature of the spine and round shoulders
often result from long-continued positions at school in seats and at desks
which are not adapted to the physical build of the occupant.

[Illustration: Fig. 29.--Section of the Knee Joint. (Showing its internal
structure)

A, tendon of the semi-membranosus muscle cut across;
B, F, tendon of same muscle;
C, internal condyle of femur;
D, posterior crucial ligament;
E, internal interarticular fibro cartilage;
G, bursa under knee-cap;
H, ligament of knee-cap;
K, fatty mass under knee-cap;
L, anterior crucial ligament cut across;
P, patella, or knee-cap
]

A few simple rules should guide teachers and school officials in providing
proper furniture for pupils. Seats should be regulated according to the
size and age of the pupils, and frequent changes of seats should be made.
At least three sizes of desks should be used in every schoolroom, and more
in ungraded schools. The feet of each pupil should rest firmly on the
floor, and the edge of the desk should be about one inch higher than the
level of the elbows. A line dropped from the edge of the desk should
strike the front edge of the seat. Sliding down into the seat, bending too
much over the desk while writing and studying, sitting on one foot or
resting on the small of the back, are all ungraceful and unhealthful
positions, and are often taken by pupils old enough to know better. This
topic is well worth the vigilance of every thoughtful teacher, especially
of one in the lower grades.

58. The Bones in After Life. Popular impression attributes a less
share of life, or a lower grade of vitality, to the bones than to any
other part of the body. But really they have their own circulation and
nutrition, and even nervous relations. Thus, bones are the seat of active
vital processes, not only during childhood, but also in adult life,
and in fact throughout life, except perhaps in extreme old age. The final
knitting together of the ends of some of the bones with their shafts does
not occur until somewhat late in life. For example, the upper end of the
tibia and its shaft do not unite until the twenty-first year. The separate
bones of the sacrum do not fully knit into one solid bone until the
twenty-fifth year. Hence, the risk of subjecting the bones of young
persons to undue violence from injudicious physical exercise as in rowing,
baseball, football, and bicycle-riding.

The bones during life are constantly going through the process of
absorption and reconstruction. They are easily modified in their growth.
Thus the continued pressure of some morbid deposit, as a tumor or cancer,
or an enlargement of an artery, may cause the absorption or distortion of
bones as readily as of one of the softer tissues. The distortion resulting
from tight lacing is a familiar illustration of the facility with which
the bones may be modified by prolonged pressure.

Some savage races, not content with the natural shape of the head, take
special methods to mould it by continued artificial pressure, so that it
may conform in its distortion to the fashion of their tribe or race. This
custom is one of the most ancient and widespread with which we are
acquainted. In some cases the skull is flattened, as seen in certain
Indian tribes on our Pacific coast, while with other tribes on the same
coast it is compressed into a sort of conical appearance. In such cases
the brain is compelled, of course, to accommodate itself to the change in
the shape of the head; and this is done, it is said, without any serious
result.

59. Sprains and Dislocations. A twist or strain of the ligaments and
soft parts about a joint is known as a sprain, and may result from a
great variety of accidents. When a person falls, the foot is frequently
caught under him, and the twist comes upon the ligaments and tissues of
the ankle. The ligaments cannot stretch, and so have to endure the wrench
upon the joint. The result is a sprained ankle. Next to the ankle, a
sprain of the wrist is most common. A person tries, by throwing out his
hand, to save himself from a fall, and the weight of the body brings the
strain upon the firmly fixed wrist. As a result of a sprain, the ligaments
may be wrenched or torn, and even a piece of an adjacent bone may be torn
off; the soft parts about the injured joint are bruised, and the
neighboring muscles put to a severe stretch. A sprain may be a slight
affair, needing only a brief rest, or it may be severe and painful enough
to call for the most skillful treatment by a surgeon. Lack of proper care
in severe sprains often results in permanent lameness.

A fall or a blow may bring such a sudden wrench or twist upon the
ligaments as to force a bone out of place. This displacement is known as a
dislocation. A child may trip or fall during play and put his elbow
out of joint. A fall from horseback, a carriage, or a bicycle may result
in a dislocation of the shoulder joint. In playing baseball a swift ball
often knocks a finger out of joint. A dislocation must be reduced at once.
Any delay or carelessness may make a serious and painful affair of it, as
the torn and bruised parts rapidly swell and become extremely sensitive.

60. Broken Bones. The bones, especially those of the upper limbs, are
often fractured or broken. The _simple_ fracture is the most common
form, the bone being broken in a single place with no opening through the
skin. When properly adjusted, the bone heals rapidly. Sometimes bones are
crushed into a number of fragments; this is a _comminuted_ fracture.
When, besides the break, there is an opening through the soft parts and
surface of the body, we have a _compound_ fracture. This is a serious
injury, and calls for the best surgical treatment.

A bone may be bent, or only partly broken, or split. This is called "a
green-stick fracture," from its resemblance to a half-broken green stick.
This fracture is more common in the bones of children.

Fractures may be caused by direct violence, as when a bone is broken at a
certain point by some powerful force, as a blow from a baseball bat or a
fall from a horse. Again, a bone may be broken by indirect violence, as
when a person being about to fall, throws out his hand to save himself.
The force of the fall on the hand often breaks the wrist, by which is
meant the fracture of the lower end of the radius, often known as the
"silver-fork fracture." This accident is common in winter from a fall or
slip on the ice.

Sometimes bones are broken at a distance from the point of injury, as in a
fracture of the ribs by violent compression of the chest; or fracture may
occur from the vibration of a blow, as when a fall or blow upon the top of
the head produces fracture of the bones at the base of the brain.[6]

61. Treatment for Broken Bones. When a bone is broken a surgeon is
needed to set it, that is, to bring the broken parts into their natural
position, and retain them by proper appliances. Nature throws out between
and around the broken ends of bones a supply of repair material known as
plastic lymph, which is changed to fibrous tissue, then to cartilage, and
finally to bone. This material serves as a sort of cement to hold the
fractured parts together. The excess of this at the point of union can be
felt under the skin for some time after the bone is healed.

With old people a broken bone is often a serious matter, and may cripple
them for life or prove fatal. A trifling fall, for instance, may cause a
broken hip (popularly so called, though really a fracture of the neck of
the femur), from the shock of which, and the subsequent pain and
exhaustion, an aged person may die in a few weeks. In young people,
however, the parts of a broken bone will knit together in three or four
weeks after the fracture is reduced; while in adults, six or even more may
be required for firm union. After a broken bone is strong enough to be
used, it is fragile for some time; and great care must be taken,
especially with children, that the injured parts may not be broken again
before perfect union takes place.[7]

62. The Effect of Alcohol upon the Bones. While the growth of the
bones occurs, of course, mainly during the earlier years of life, yet they
do not attain their full maturity until about the twenty-fifth year; and
it is stated that in persons devoted to intellectual pursuits, the skull
grows even after that age. It is plainly necessary that during this period
of bone growth the nutrition of the body should be of the best, that the
bones may be built up from pure blood, and supplied with all the materials
for a large and durable framework. Else the body will be feeble and
stunted, and so through life fall short of its purpose.

If this bony foundation be then laid wrong, the defect can never be
remedied. This condition is seen in young persons who have been underfed
and overworked. But the use of alcoholic liquors produces a similar
effect, hindering bone cell-growth and preventing full development.[8]
The appetite is diminished, nutrition perverted and impaired, the stature
stunted, and both bodily and mental powers are enfeebled.

63. Effect of Tobacco upon the Bones. Another narcotic, the
destructive influence of which is wide and serious, is tobacco. Its
pernicious influence, like that of alcohol, is peculiarly hurtful to the
young, as the cell development during the years of growth is easily
disturbed by noxious agents. The bone growth is by cells, and a powerful
narcotic like tobacco retards cell-growth, and thus hinders the building
up of the bodily frame. The formation of healthy bone demands good,
nutritious blood, but if instead of this, the material furnished for the
production of blood is poor in quality or loaded with poisonous narcotics,
the body thus defrauded of its proper building material becomes undergrown
and enfeebled.

Two unfavorable facts accompany this serious drawback: one is, that owing
to the insidious nature of the smoky poison[9] (cigarettes are its worst
form) the cause may often be unsuspected, and so go on, unchecked; and the
other, that the progress of growth once interrupted, the gap can never be
fully made up. Nature does her best to repair damages and to restore
defects, but never goes backwards to remedy neglects.

Additional Experiments.

Experiment 11. Take a portion of the decalcified bone obtained from
Experiment 4, and wash it thoroughly in water: in this it is insoluble.
Place it in a solution of carbonate of soda and wash it again. Boil it
in water, and from it gelatine will be obtained.

Experiment 12. Dissolve in hydrochloric acid a small piece of the
powdered bone-ash obtained from Experiment 3. Bubbles of carbon dioxid
are given off, indicating the presence of a carbonate. Dilute the
solution; add an excess of ammonia, and we find a white precipitate of
the phosphate of lime and of magnesia.

Experiment 13. Filter the solution in the preceding experiment, and
to the filtrate add oxalate of ammonia. The result is a white
precipitate of the oxalate of lime, showing there is lime present, but
not as a phosphate.

Experiment 14. To the solution of mineral matters obtained from
Experiment 3, add acetate of soda until free acetic acid is present,
recognized by the smell (like dilute vinegar); then add oxalate of
ammonia. The result will be a copious white precipitate of lime salts.

Experiment 15. _To show how the cancellous structure of bone is
able to support a great deal of weight_. Have the market-man saw out a
cubic inch from the cancellous tissue of a fresh beef bone and place it
on a table with its principal layers upright. Balance a heavy book upon
it, and then gradually place upon it various articles and note how many
pounds it will support before giving way.

Experiment 16. Repeat the last experiment, using a cube of the
decalcified bone obtained from Experiment 4.

[NOTE. As the succeeding chapters are studied, additional experiments
on bones and their relation to other parts of the body, will readily
suggest themselves to the ingenious instructor or the thoughtful
student. Such experiments may be utilized for review or other
exercises.]

Review Analysis: The Skeleton (206 bones).

/ / 1 Frontal,
/ / 2 Parietal,
/ I. Cranium | 2 Temporal,
/ (8 bones) | 1 Occipital,
/ \ 1 Sphenoid,
| \ 1 Ethmoid.
|
| / 2 Superior Maxillary,
The Head | / 2 Malar,
(28 bones). | / 2 Nasal,
| II. Face | 2 Lachrymal Bones,
| (14 bones) | 2 Palate Bones,
| \ 2 Turbinated,
| \ 1 Vomer,
\ \ 1 Lower Maxillary.
\
\ / Hammer,
\ III. The Ear | Anvil,
\ (6 bones) \ Stirrup.

/ / 7 Cervical Vertebræ.
/ / 12 Dorsal Vertebræ,
/ I. Spinal Column | 5 Lumbar Vertebræ,
| (26 bones) \ Sacrum,
| \ Coccyx.
The Trunk |
(54 bones). | / 7 True Ribs,
| II. The Ribs | 3 False Ribs,
| (24 bones) \ 2 Floating Ribs.
|
\ III. Sternum.
\ IV. Two Hip Bones.
\ V. Hyoid Bone.

/ / Scapula,
/ I. Upper Arm | Clavicle,
| \ Humerus.
|
The Upper Limbs | II. Forearm / Ulna,
(64 bones). | \ Radius.
|
| / 8 Carpal Bones,
\ III. Hand | 5 Metacarpal Bones,
\ \ 14 Phalanges.

/ I. Thigh Femur.
/
| / Patella,
The Lower Limbs | II. Lower Leg | Tibia,
(60 bones). | \ Fibula.
|
| / 7 Tarsal Bones,
\ III. Foot | 5 Metatarsal Bones,
\ \ 14 Phalanges.

Chapter III.

The Muscles.

64. Motion in Animals. All motion of our bodies is produced by means
of muscles. Not only the limbs are moved by them, but even the movements
of the stomach and of the heart are controlled by muscles. Every part of
the body which is capable of motion has its own special set of muscles.

Even when the higher animals are at rest it is possible to observe some
kind of motion in them. Trees and stones never move unless acted upon by
external force, while the infant and the tiniest insect can execute a
great variety of movements. Even in the deepest sleep the beating of the
heart and the motion of the chest never cease. In fact, the power to
execute spontaneous movement is the most characteristic property of
living animals.

65. Kinds of Muscles. Most of the bodily movements, such as affect
the limbs and the body as a whole, are performed by muscles under our
control. These muscles make up the red flesh or lean parts, which,
together with the fat, clothe the bony framework, and give to it general
form and proportion. We call these muscular tissues voluntary
muscles, because they usually act under the control of the will.

The internal organs, as those of digestion, secretion, circulation, and
respiration, perform their functions by means of muscular activity of
another kind, that is, by that of muscles not under our control. This work
goes on quite independently of the will, and during sleep. We call the
instruments of this activity involuntary muscles. The voluntary
muscles, from peculiarities revealed by the microscope, are also known as
striped or striated muscles. The involuntary from their smooth, regular
appearance under the microscope are called the unstriped or non-striated
muscles.

The two kinds of muscles, then, are the red, voluntary, striated
muscles, and the smooth, involuntary, non-striated muscles.

66. Structure of Voluntary Muscles. The main substance which clothes
the bony framework of the body, and which forms about two-fifths of its
weight, is the voluntary muscular tissue. These muscles do not cover and
surround the bones in continuous sheets, but consist of separate bundles
of flesh, varying in size and length, many of which are capable of
independent movement.

Each muscle has its own set of blood-vessels, lymphatics, and nerves. It
is the blood that gives the red color to the flesh. Blood-vessels and
nerves on their way to other parts of the body, do not pass through the
muscles, but between them. Each muscle is enveloped in its own sheath of
connective tissue, known as the fascia. Muscles are not usually
connected directly with bones, but by means of white, glistening cords
called tendons.

[Illustration: Fig. 30.--Striated (voluntary) Muscular Fibers.

A, fiber serparating into disks;
B, fibrillæ (highly magnified);
C, cross section of a disk
]

If a small piece of muscle be examined under a microscope it is found to
be made up of bundles of fibers. Each fiber is enclosed within a
delicate, transparent sheath, known as the sarcolemma. If one of
these fibers be further examined under a microscope, it will be seen to
consist of a great number of still more minute fibers called
fibrillæ. These fibers are also seen marked cross-wise with dark
stripes, and can be separated at each stripe into disks. These cross
markings account for the name _striped_ or _striated_ muscle.

The fibrillæ, then, are bound together in a bundle to form a fiber, which
is enveloped in its own sheath, the sarcolemma. These fibers, in turn, are
further bound together to form larger bundles called fasciculi, and
these, too, are enclosed in a sheath of connective tissue. The muscle
itself is made up of a number of these fasciculi bound together by a
denser layer of connective tissue.

Experiment 17. _To show the gross structure of muscle._ Take a
small portion of a large muscle, as a strip of lean corned beef. Have it
boiled until its fibers can be easily separated. Pick the bundles and
fasciculi apart until the fibers are so fine as to be almost invisible
to the naked eye. Continue the experiment with the help of a hand
magnifying glass or a microscope.

67. The Involuntary Muscles. These muscles consist of ribbon-shaped
bands which surround hollow fleshy tubes or cavities. We might compare
them to India rubber rings on rolls of paper. As they are never attached
to bony levers, they have no need of tendons.

[Illustration: Fig. 31.--A, Muscular Fiber, showing Stripes, and Nuclei, b
and c. (Highly magnified.)]

The microscope shows these muscles to consist not of fibers, but of long
spindle-shaped cells, united to form sheets or bands. They have no
sarcolemma, stripes, or cross markings like those of the voluntary
muscles. Hence their name of _non-striated_, or _unstriped_, and _smooth_
muscles.

The involuntary muscles respond to irritation much less rapidly than do
the voluntary. The wave of contraction passes over them more slowly and
more irregularly, one part contracting while another is relaxing. This may
readily be seen in the muscular action of the intestines, called
vermicular motion. It is the irregular and excessive contraction of the
muscular walls of the bowels that produces the cramp-like pains of colic.

The smooth muscles are found in the tissues of the heart, lungs,
blood-vessels, stomach, and intestines. In the stomach their contraction
produces the motion by which the food is churned about; in the arteries
and veins they help supply the force by which the blood is driven along,
and in the intestines that by which the partly digested food is mainly
kept in motion.

Thus all the great vital functions are carried on, regardless of the will
of the individual, or of any outward circumstances. If it required an
effort of the will to control the action of the internal organs we could

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