CORNELL
UNIVERSITY
LIBRARY
FROM
The Estate of
P.R.flacmillanA TEXT-BOOK
GENERAL PHYSIOLOGY
AND
ANATOMY
BY
WALTER HOLLIS EDDY
Chairman of the Department of Biology in the
High School of Commerce, New York
		
NEW YORK CINCINNATI CHICAGO
AMERICAN BOOK COMPANY4
PREFACE
laboratory course in zoology these chapters should be
omitted, except for general reading.
This text is not intended to be used as a substitute
for laboratory study. In my Experimental Physiology
and Anatomy are outlined laboratory exercises which
should be first performed by the pupil himself, if possible,
before he is assigned a lesson in this text. While written
as a supplement to the laboratory guide mentioned, the
text is suited for use with any other laboratory manual
and, where lack of laboratory facilities renders such pre-
liminary work impossible, its description of the laboratory
exercises is sufficiently full to permit its use where a text-
book alone must serve.
The text meets both the optional and required work of
the New York State Syllabus and the requirements of the
Harvard entrance examinations.
Its topical arrangement and the printing of less impor-
tant details in finer type make it adaptable to longer or
shorter courses of study; and in these structural details
the author has endeavored to make the book meet the
needs of all classes of schools.
I wish to acknowledge the helpful suggestions given
me by my colleagues of the High School of Commerce,
and by Professor Geis of Columbia University and Dr.
Darling of Harvard University. I am especially indebted
to my colleague Mr. C. W. Hahn for his very careful read-
ing of the proofs and his many helpful suggestions, and
to my wife for assistance in the revision of grammatical
arrangement and in the mechanical details of the work.
W. H. EDDY.
The High School nv Commerce,
New York.Page
7
12
28
39
51
73
90
107
133
146
159
180
191
212
221
244
252
265
278
294
CON T EN T S
What the Study of Physiology Means. .
Chemical Composition of Living Matter . . . .
The Structural Units of Living Matter . . .
Structural Units of Human Tissues — Histology
Foods and Nutrition .................
Food Accessories; Alcohol and Tobacco
Digestion . .	...................
Digestion (continued)
Absorption . .
Digestion in the Lower Animals .
Blood and Lymph	. . .
Circulation	. .
Circulation (continued)	.	. .
Circulation in the Lower Animals
The Skeleton	. .	...
The Skeleton (continued)	. .
Skeletons of the Lower Animals
Muscles	.......................
Muscles (continued)	.......
Movement in Lower Animals	. .
56
CONTENTS
Page
XXI.	Respiration	  312
XXII.	Respiration in Lower Animals........................340
XXIII.	Excretion .	  352
XXIV.	Skin Structure and Excretion in the Lower Animals 374
XXV. The Nervous System . .	....	.	383
XXVI. The Nervous System (continued)........................411
XXVII. The Nervous System in the Lower Animals .	429
XXVIII. The Special Senses................. ...	436
XXIX.	The Special Senses (continued).....................452
XXX. The Special Senses (concluded)	............ 470
XXXI. Sensation in the Lower Animals ................ 4S0
XXXII.	The Voice.......................................... 486
XXXIII.	Bacteria and Sanitation ........................... 493
Index
505PHYSIOLOGY AND ANATOMY
I. WHAT THE STUDY OE PHYSIOLOGY MEANS.
From our earliest days we are familiar with living
forms. We know that each living form must perform cer-
tain actions every day in order to live; but few of us have
a clear idea of why these actions maintain life, or of the
part each separate action plays in keeping us alive. For
example, if asked why we eat, we answer, “ To keep us
alive,” but that answer does not explain how food gives
life. Again, we see gardeners watering plants, repotting
them in new earth, taking care to place them in sunlight,
and we recognize that all these actions are necessary to
the life of the plants. We know that dogs and cats, horses
and cows, must be fed and cared for to keep them alive
and healthy. In fine, we know that to maintain life all
living bodies, whether plant or animal, must repeat certain
actions continually, or else die ; but we do not know how
these actions produce life. It is this question of “ how ”
and “ why ” that the present study of Physiology is to
answer.
Method of study.-—How shall we begin the study?
An example will indicate the method. If we see a man
moving the handle of a certain machine up and down and
see a stream of water issuing from another part of the
same machine, we know at once that the movement of
the handle in some way causes the flow of water. In
78
WHAT THE STUDY OF PHYSIOLOGY MEANS
order to answer the question “ how ” we should first pull
our pump or a similar machine to pieces and find what
parts it has and how they are placed in relation to one
another. We should call this the study of the structure
of the pump. Next, we should set the machine in motion
and observe the action of the parts and how the move-
ment of one affects the other. We should call this the
study of function, or use of parts. Combining our knowl-
edge of structure and function we might go still further,
and, by experimenting, find that certain actions, such as
lubricating the parts with oil, improve the action of the
machine as a whole. We call this the study of the con-
ditions best suited to the action of the machine.
Applied to the study of living objects such a method of
investigation resolves itself into the following divisions of
research.
First. The study of the structure of the living bodies.
Such a study is called anatomy {ana, up; temnein, to cut)
and is dependent upon dissection.
Second. The study of the use or functions of the various
structures. Such a study is called 'physiology (physiologia,
an investigation into the nature of things) and is dependent
upon experimenting with living structures.
Third. The study of methods that will cause the parts
to act to the best advantage. This study is called hygiene
{hygeia, health) and like physiology is dependent upon
experimentation.
Formulating these three directions we have the follow-
ing definitions:
Anatomy. —The study of the structure of a plant or
animal to determine the parts present and the relative
position of these parts.METHOD OF STUDY
9
Physiology. —The study of the function or use of the
parts of a plant or animal body and their method of action.
Hygiene. —The study of the means best adapted to
maintain the parts and actions of the plant or animal
body in a working or healthful state.
Anatomy.— Like a pump, a living body is made up of
parts arranged in definite positions. It is manifestly
impossible to locate all these parts from the study of the
living body. To get at the internal structure it is neces-
sary to cut or dissect the dead body. In the case of the
human body such dissection is of course impossible for
the high school pupil. It is here that a fact of great im-
portance comes to our rescue, namely, that the arrange-
ment of parts in our bodies is so similar to that in certain
other animal forms that by studying these forms we may
learn of our own structure. In other words, the dissec-
tion of the rat, cat, or rabbit, enables us to know our own
structures.
Even with this difficulty overcome, however, many
parts are so small that the eye cannot make them out.
To remedy this there has been devised an instrument
called the microscope; and so important is this side of
anatomical study that it has received a special name.
Histology (histos, a tissue; logos, speech) is the microscopic
study of plant and animal parts.
Finally, chemists tell us that the parts themselves are
composed of simpler substances called chemical elements.
This branch of our study is called chemical anatomy.
When we have studied a body from these three points
of view, its gross structure, its microscopic structure, and
its chemical composition, we shall have exhausted all
that anatomy has to tell us,10 WHAT THE STUDY OF PHYSIOLOGY MEANS
Physiology. —Anatomy tells us only of the structure
of the lifeless body. It is physiology that tells us of the
action of the parts in the living body. Here the problem
of investigation becomes difficult, but again we call to aid
the fact that vital processes in plants and animals are
similar to those in our own bodies, and the experiments
we employ with the former tell us the workings of our
own body. These experiments are supplemented by the
reports of surgeons of their operations on human bodies,
while another branch of science, called physiological chem-
istry, has taught us how to reproduce artificially many of
the actions which take place normally in our bodies.
Hygiene. —The best engineer is the man who knows
not only how his engine is made and acts, but also how
to keep it in order and to run it to the best advantage.
It is this knowledge — how to run our bodies to the best
advantage, that is, to keep them healthy — that the study
of hygiene gives us. It can be acquired only by taking to
ourselves the experience of those most familiar with the
running of our bodies. There are, however, certain of
these experiences which have been formulated under the
laws of sanitation which will give us a better foundation
for our apprenticeship. Such laws are those that have
been formulated by doctors, physicists, and bacteriologists,
and we shall study them in their proper place.
Relation to other sciences. —The preceding pages indi-
cate in a general way what the scope of our study is to be.
AVhat relation does this study bear to other sciences?
Biology (bios, life; logos, speech) is the name given to
the study of living as distinct from lifeless matter. This
subject naturally groups itself into the study of plants,
botany, and the study of animals, zoology. From what hasBIOLOGY
11
been said already it will be seen that the terms anatomy,
physiology, and hygiene refer to methods of study applied
to either plants or animals, for example, plant physiology,
animal anatomy, etc. —or they may lie restricted to indi-
vidual members of each group, human physiology, horse
anatomy, yeast physiology, etc. In short, physiology,
anatomy, and hygiene are merely one phase of the study
of living matter (biology) and while our present problem
is the human body, we must bear in mind that many of
the laws that hold true of our bodies are merely special
cases of laws which apply to all living organisms.
The great problem which all biologists, zoologists, and
physiologists are seeking to answer is, “ AVhat is life? ”
and in whatever field they may be working, their results
arc valuable and throw light upon this main, great
problem.II. CHEMICAL COMPOSITION OF LIVING
MATTER.
Chemical elements. —The dissection of a plant or
animal shows it to be made up of many different kinds
of materials, some soft, some hard, some elastic. Dis-
section, however, can tell us nothing of the composition
of these separate masses of material, and to find out what
bone, muscle, and fat are made of, we must go to the
chemist. As the result of methods which chemists have
learned to apply to the problem, they tell us that muscle,
fat, bone, wood, and in fact all the other substances of the
universe are built up of simple substances which are called
elements. By examining all the different kinds of matter,
both living and lifeless, to be found in the universe, it has
been found possible to reduce the number of these simple
substances or elements to some seventy-six, and just as
all the words in the language may be built up of the
twenty-six letters of the alphabet, so all matter is built
up by varying combinations of these seventy-six elements.
Some of these elements are very rare, while others are
exceedingly abundant. Some are never found in the
pure state but always combined with other elements,
while others are found both in a free state and in combin-
ation. Of the seventy-six known elements, twelve 1 are
always found in living matter and six of the remaining
sixty-four are sometimes present.
1 The twelve that are always present in living matter are phosphorus,
sulphur, carbon, oxygen, hydrogen, nitrogen, chlorine, potassium,
sodium, calcium, magnesium, and iron.
12PHOSPHORUS
13
Characteristics of chemical elements. —The distinguish-
ing feature of a chemical element is that it cannot be
resolved by any known means into a simpler substance.
Thus while it is possible to resolve a substance like com-
mon salt into two substances known respectively as sodium
and chlorine, no one has ever been able to separate sodium
or chlorine into anything simpler, and these two sub-
stances are, therefore, called elements. Iron, sulphur,
gold, silver, are familiar examples of elements. Since the
chemical elements are the substances out of which all
matter is formed, it will be well for us to examine a few
of them to learn their properties and the manner in which
they combine to form compounds. We shall select for
examination those which are particularly concerned in
the composition of our body materials.
Phosphorus. — (See Ex. III.) This element forms a
large part of our bones; these are, in fact, composed mainly
of a compound of this element with calcium and oxygen.
In its pure state phosphorus has a yellow color and a
waxy consistency. When exposed to the air it gives off
white fumes which have a peculiar odor. These fumes
are a compound of phosphorus with the oxygen of the
air, and to keep phosphorus from combining with oxygen
it is necessary to coArer it with water.
These fumes teach us of a peculiar property of phos-
phorus, namely, its power to combine with another element.
Such power on the part of an element is called its chemi-
cal affinity. All elements possess this affinity for certain
other elements, but the strength of the affinity varies
with the element and with the external conditions.
To this affinity of elements all chemical compounds are
due.14 CHEMICAL COMPOSITION OF LIVING MATTER
Chemical compounds distinguished from physical mix-
tures. — Most of the substances found in nature are
mixtures of several substances. If we examine a piece
of wood we can pick out several different kinds of matter
which are intimately united in the compound wood.
Similarly a piece of flesh shows pieces of fiber, and bits of
fat, blood, and muscle, which are all intimately mixed
together, but each of which possesses properties peculiar
to itself. If we examine a bit of common salt and com-
pare it with the elements sodium and chlorine, we find
that the salt has properties which are distinct from either
the sodium or the chlorine. In short the sodium and
chlorine which have combined to form salt have lost their
identity and individual properties, and the compound
resulting from their union has entirely new properties.
These examples illustrate the distinction between a
mechanical mixture and a chemical compound. The
flesh is a mechanical mixture of fat, blood, etc. The salt
is a chemical compound.
Whenever two or more elements combine, losing their
individual properties and producing a substance with
entirely new properties, the combinaton is called a chemi-
cal compound. The white fumes which result from the
union of phosphorus with the oxygen of the air arc entirely
distinct in properties from the phosphorus and oxygen.
In their formation, therefore, we have an example of the
formation of a chemical compound. If the phosphorus
is exposed to the air in a dark room a dull glow of light
may be seen. This glow is due to the heat generated
in the act of union of the phosphorus and oxygen. A
certain amount of heat is always generated when chemical
elements combine.SULPHUR
15
Sulphur. — (See Ex. IV.) Another important element
in our body compounds is sulphur. Most of us have seen
this element in its pure state and recall its yellow color.
The sulphur used on sulphur matches is the element in
its pure state, and wc may recall the fact that when it
burns it gives off suffocating fumes and a blue flame. In
its free state, this element is found only around volcanic
regions. Combined with other elements (chemical com-
pounds of sulphur) it is extremely abundant. Muscle is
such a compound of sulphur with other elements. Pure
sulphur is odorless and tasteless.
When flesh decays it gives off a disagreeable odor. This
odor is a property of a gas called hydrogen sulphide,
which the sulphur forms as flesh decays, by combining
with the element hydrogen. The ordinary hen’s egg con-
tains sulphur, and in decay this gas is formed abundantly.
The blackening of a silver spoon used in eating an egg is
due to a black compound called sulphide of silver, which
the sulphur of the egg forms with the element silver.
The fumes from binning sulphur (a compound of sul-
phur and oxygen), the bad smelling gas (sulphur and
hydrogen) and the black compound (sulphur and silver)
bring out the fact that a chemical element may have an
affinity for more than one element. The fact that it occurs
free in the air and yet combines with oxygen to form a
compound when heated is an illustration of the way in
which external conditions may affect the affinity of an
element, while the blue flame which results when sulphur
combines with oxygen and which is absent in the forma-
tion of the other two compounds, shows that the amount
of heat generated by the union of two elements may vary
greatly with the elements.16 CHEMICAL COMPOSITION OP LIVING MATTER
Carbon. — (Sec Ex. V.) When we partly burn a piece
of wood, bone, or flesh, we find it transformed into a
black substance which we call charcoal. In nature we find
this same substance in a much harder form and we call
it coal. When we use our lead pencils the mark we make
is due to another form of this substance called graphite.
Finally in the diamond we have a substance very different
in appearance from charcoal, coal, or graphite, but essen-
tially like them in some respects. All of these substances
are forms of a very abundant chemical element called
carbon. So important a, part of all living matter is this
element that the study of its compounds is made a special
branch of chemistry under the name Organic Chem-
istry.
In most of its forms, carbon shows a black color and is
without odor or taste. In its free state it has so little
affinity for other elements that farmers often char the ends
of fence posts before setting them in order to insure against
decay. When heated to a certain temperature, however,
all forms of carbon burn and form a colorless gas, which
may be readily detected by its power to turn clear lime
water milky. This gas is a chemical compound of carbon
with the oxygen of the air and is called carbon dioxide. It
is this gas that is used to charge water in the manufacture
of ordinary soda water. Just as in the union of phos-
phorus and sulphur with oxygen, the formation of this
gas is accompanied by the generation of much heat, and
this fact is made use of in our bodies as well as in our
stoves and furnaces. If we blow our breath through clear
lime water the milky color that results gives unmistakable
evidence that this gas is being constantly formed in our
bodies and given off with the exhaled air.OXYGEN
17
A study of the element carbon demonstrates three
important facts.
First. It is present in large amounts as a part of all
living matter.
Seco7id. Its combination with oxygen to form carbon
dioxide is always accompanied by the production of heat,
and the formation of this gas in our bodies results in main-
taining a constant body temperature.
Third. A chemical element in its free state may occur
in forms which appear very different to the eye, and must
be tested by other means than sight to determine its
identity.
Oxygen. — (See Exs. VII. and VIII.) This important
element occurs free in the air in the form of a colorless,
odorless, and tasteless gas. In its combined form it is
abundant in both living and non-living matter. It is of
great importance to all living processes because it has a
great affinity for other elements, and because, in combin-
ing with other elements, it sets free a great amount of
energy in the form of heat.
While oxygen is present in the air in a free state, it is
so mixed with other gases that special processes must be
resorted to to obtain it pure. The simplest method of
obtaining oxygen is to heat a compound which contains
it. Some of these compounds when so heated break up
and set free the pure gas. Two compounds which are
especially favorable to this method are oxide of mercury
(a compound of mercury and oxygen), and potassium chlo-
rate (potassium, chlorine, and oxygen).
If we heat some oxide of mercury in a tube, as shown
in Figure 1, we note that it gradually disappears. If,
while the heating is going on, we insert the glowing end
EDDY. TUYS. —2.18 CHEMICAL COMPOSITION OF LIVING MATTER
Fig. 1 — Apparatus
for heating mer-
curic oxide.
of a charred match into the mouth of the tube the pres-
ence of the oxygen gas will become evident by the in-
creased brilliancy of the glow, due to
the rapid union of the charcoal with the
liberated oxygen. The presence of the
mercury will be seen in the drops which
condense on the cooled sides of the tube.
Such an experiment not only illustrates
the manner in which a compound may
be separated into its elements, but also
demonstrates an important property of
oxygen, namely, that in its pure state
its affinity for carbon is much greater
than when mixed with other gases in the air.
If we heat a mixture of manganese dioxide1 and potas-
sium chlorate and collect the liberated gas over water as
shown in Figure 2, it is pos-
sible to compare the union
of sulphur and phosphorus
with tlois pure product,
with the result when these
elements combined with
the mixed oxygen of the
air. In every case the
union is much more vio-
lent than when ordinary
air is present, and results in more heat being liberated.
Oxidation and combustion. — Such unions of elements
with oxygen as have been described in the preceding
pages, are spoken of as oxidations and the compounds
1 The manganese dioxide undergoes no change, hut merely facili-
tates the decompositiou of the potassium chlorate.
Fig. 2 — Apparatus arranged for obtain-
ing oxygen.OXIDATION AND COMBUSTION
19
formed are called oxides. For example, a chemist would
describe the formation of carbon dioxide as the oxidation
of carbon to form an oxide. In every case the formation
of an oxide is accompanied by the production of heat, but
sometimes the union of the elements takes place much
more rapidly than at others, and when this occurs the
amount of heat produced is large and often accompanied
by flames or light. Such rapid unions are called rapid
oxidation or combustion, to distinguish them from slower
unions or slow oxidation. All burning then may be ex-
plained as the rapid oxidation of the substance burned,
and it is evident, therefore, that burning cannot take
place in the absence of oxygen. Further, since in burn-
ing the oxygen combines with the substance to form an
oxide, it follows that for a substance to continue burning
implies a continuous supply of oxygen until the substance
has entered completely into combination. It is for these
reasons that we supply our stoves with draughts which
will permit the continuous entrance of oxygen. This
explanation of the true nature of combustion also explains
the methods of extinguishing fires, all such methods aim-
ing to separate, through the aid of some substance which
does not combine with oxygen, the burning substance
from its air or oxygen supply. In short, pouring water on
a fire puts an incombustible substance between the burn-
ing matter and the air, and thus prevents further oxida-
tion. Water is more frequently used than sand or dirt
for this purpose because it is easier to handle, but in
theory one is as effective as the other.
The special interest of oxidation to the physiologist
consists in the fact that it furnishes a means for the pro-
duction of heat, and we shall learn later how the body20 CHEMICAL COMPOSITION OF LIVING MATTER
makes use of oxidation to maintain the body temperature,
while the necessity for a continuous oxidation explains
the presence of carbon in the body and the continuous
entrance of oxygen and exit of carbon dioxide in breathing.
Nitrogen. — Experiments with pure oxygen show that
combustion takes place more rapidly in it than in air.
This implies that air must contain something besides
oxygen, which dilutes it and makes oxidation slower.
What is this substance? A simple experiment answers
this question. (See Ex. IX.). Place a piece of phos-
phorus the size of a pea in an evaporating dish that is
floating on water. Light the phosphorus and thus raise
its temperature to the combining point. Then cover the
dish quickly with a bell jar (Fig. 3). The phosphorus
Further, since the water rises in the jar to take the place
of the consumed oxygen the volume of gas left in the
jar subtracted from its original volume will give us the
proportion of oxygen present in a given volume of air;
roughly one fifth of its volume.
What is this gas that is left? If we examine it by
Fig. 3 — Apparatus arranged for
obtaining nitrogen from air.
will at once begin to combine
with the oxygen in the in-
closed air, and when it stops
burning it will be because there
is no more oxygen to combine
with it to form the white
fumes of phosphorus oxide.
If we let the fumes settle they
will dissolve in the water and
there will be left in the jar
only what was in the air before
the oxygen was taken out.HYDROGEN
21
inserting heated elements as we did oxygen, we find that
while it is colorless, odorless, and tasteless like oxygen, it
has not the power of that element to combine with these
other elements or produce combustion. This residual
gas which is so lacking in chemical affinity is an element
called nitrogen. Like sulphur, however, its lack of affinity
is due rather to lack of the right conditions for combina-
tion than to absence of that power. In nature, nitrogen
occurs very abundantly in combination with other ele-
ments, and some of its compounds form important parts
of our body material, notably muscle or flesh. Owing to
the weakness of its affinity for most elements its compounds
arc apt to break up readily, and on
that account are spoken of as un-
stable compounds. Most of our
explosives, such as gunpowder and
nitroglycerine, owe their explosive
power to the fact that they are
unstable compounds of nitrogen.
Fig. 4—^.apparatus for obtaining hydrogen and oxygen with the aid of an electric
current. B, method of collecting the gases.
Hydrogen. — (See Exs. X. and XI.). If we allow an
electric current to pass through water (Fig. 4, A) which
has had a little acid added to it to make it a conductor22 CHEMICAL COMPOSITION OF LIVING MATTEB
of the current, we find that the current has the power to
separate the water into two gases. One of the gases is
twice as great in volume as the other and shows very
different properties. If we collect these gases (Fig. 4, B)
and examine them we find that oxygen is the smaller vol-
ume. The other gas is colorless, odorless, and tasteless,
like the first, but when we introduce a lighted splinter into
it, it takes fire with an explosion and burns with a very
hot colorless flame. If, instead of using water and the
electric current, we pour sulphuric acid upon zinc (Fig. 5)
the zinc will cause the acid
to split and set free large
quantities of this same
substance which we may
collect and examine more
carefully. Such an exami-
nation will show that not
only does this substance
burn, but that when it
does burn it forms water.
This peculiar gas is a chem-
ical element called hydro-
gen and since burning is
simply the result of the combining of an element with
oxygen, we must conclude that water is not an element
but a compound of hydrogen and oxygen. The explosive
force with which this combination takes place shows that
the affinity of hydrogen for oxygen is very strong. Hydro-
gen has a strong affinity for many other elements beside
oxygen, especially for the element carbon; and its com-
pounds with this element form a very large part of living
matter. Aside from being one of the most active of
Fig. 5 — Apparatus arranged for obtain-
ing hydrogen from zinc and sulphuric
acid.ACIDS, BASES, AND SALTS ; NEUTRALIZATION 23
chemical elements, hydrogen is also noteworthy as being
the lightest chemical element known.
Metallic elements. — All the elements we have con-
sidered so far are called by chemists, nonmetals, to distin-
guish them from another group of elements called metals.
Some of these metallic elements are very familiar to us
under the names of iron, zinc, copper, gold, etc. Beside
these more familiar metals, there are others that form
important parts of our bodies but which very rarely occur
uncombined owing to their greater affinity. _ Such are the
elements calcium, potassium, sodium, and magnesium.
The most important characteristic of metals is that when
they combine, it is with a nonmetallic element, and very
rarely with another metal. Such compounds of metals
and nonmetals fall very readily under two classes, bases,
and salts. All chemical elements are divisible into metals
and nonmetals.
Acids, bases, and salts; neutralization. —When iron is
exposed to the air it takes on a reddish coating that we call
rust. This action is hastened by the presence of moisture.
This rust is merely a compound of the metal iron with
the oxygen of the air or water. Similarly other metals
such as potassium, sodium, and calcium, etc., are able to
unite with the oxygen of air or water and form oxides.
If we dissolve some of these latter oxides in water the
mixtures have peculiar properties. They have a slippery
feel and a soapy taste, and when we put into them a piece
of paper dyed red with the vegetable coloring matter, litmus,
they change the color of the paper from red to blue. Such
mixtures are not solutions but true chemical compounds
of the oxides and the water, and are called bases. If
we put the paper which was colored blue by these bases24 CHEMICAL COMPOSITION OP LIVING MATTER
into vinegar or lemon juice it will turn red again. The
reason for this is that vinegar and lemon juice contain
certain combinations of nonmetallic elements called
acids. The acid present in vinegar is a combination of
hydrogen, oxygen, and carbon, called acetic acid. Some
other powerful acids are sulphuric acid (hydrogen, sul-
phur, and oxygen), nitric acid (hydrogen, oxygen, and
nitrogen), and hydrochloric acid (hydrogen and chlorine).
All acids have a sour taste, and contain no metals in their
composition.
If we add drop by drop some of a base to an acid,
we shall evidently come to a point when the tendency
of the acid to turn litmus red is just counterbalanced by
the tendency of the base to turn it blue. When this point
is reached the base is said to have neutralized the acid,
and the process is called neutralization. Finally, if we
drive off the water of such a neutralized mixture by evap-
oration, we have left a product which turns litmus neither
blue nor red and is called a salt. Analysis of such a prod-
uct shows it to be composed partly of the metal which
was in the base and partly of the nonmetals which were
in the acid. You can make common table salt yourself
by mixing hydrochloric acid and the base caustic soda
(oxide of sodium) in this way. (See Ex. XII.) Salts, or
compounds of metals and nonmetals, form an important
part of the composition of our bodies, bones being a salt
of calcium, phosphorus, and oxygen. Since most of the
minerals in nature, limestone, quartz, etc., are metallic
salts, such salts in the living matter are spoken of as
mineral salts to distinguish them from compounds of
purely nonmetallic elements.MINERALS
25
Chemical Compounds of the Body.
The elements previously enumerated very rarely occur
in an uncombined state in plant or animal matter; the
only important exception to this rule being the element
oxygen, and even that is more abundant in combined
form than in the free state. The various compounds
(acids, oxides, salts, and bases) are so numerous and
varied that it would take too long to enumerate them all.
Fortunately, they admit of classification into a few char-
acteristic groups of life properties. These groups are
called, respectively, minerals, gases, and carbon com-
pounds.
Minerals. —Under this head are grouped water, mineral
salts, and all acids that do not contain carbon. The only
important acid in our bodies under this group is hydro-
chloric, and this is found in small quantities in the gastric
juice of the stomach. Water forms more than fifty per
cent, by weight, of all living matter. In the human body
approximately 59% is water; the bones contain 22%, the
liver 69%, the muscles 75%, and the kidneys 82%, while
the fluids of the body are still richer in water content.
Mineral salts will not burn, and by burning different parts
of the body until nothing but the ash or mineral salts
remain we cannot only demonstrate their presence in
every part of the body, but also determine the form and
proportion present. Such determination shows them to
be of great variety as to kind. The most common are
the chlorides, carbonates, and phosphates of the metals
sodium, calcium, and potassium. These names indicate
the predominant elements in these compounds. Bone ash
also represents nearly 33% by weight of the entire bone,26 CHEMICAL COMPOSITION OF LIVING MATTER
and is composed mainly of calcium carbonate and calcium
phosphate.
Gases. — Oxygen and carbon dioxide occur in all living
matter. They may be free, as in the lungs, or absorbed
in the fluids of the body. These two are the most impor-
tant gases present, but others are present in smaller
quantities.
Carbon compounds. —The compounds of this element
in living matter are so numerous and so important that
they have been given the name of organic compounds,
or life compounds. They may be classified under two
groups, nitrogenous and non-nitrogenous.
Nitrogenous carbon compounds. — As the name indi-
cates, these compounds contain the element nitrogen in
addition to the elements carbon, hydrogen, and oxygen.
They are of many forms. Proteid, which contains sulphur,
in addition to the above mentioned elements, is the name
given to the form which is perhaps most abundant, and
which under the name of albumin and fibrin in the blood,
myosin in the muscles, and casein in milk, is very im-
portant in its relation to vital processes. When proteid
burns it loses part of its carbon and the result is another
nitrogen compound called urea, which is given off as a
waste by the kidneys. Still another form of nitrogen
compound is found in the gelatine of the bones and the
keratin of the hair and nails.
Non-nitrogenous carbon compounds.— In this group
are placed all carbon compounds found in living matter,
which do not contain nitrogen. This group is divisible
into two sub-groups, carbohydrates and fats.
Carbohydrates. — Besides carbon these compounds con-
tain only hydrogen and oxygen, which are always in theNON—NITROGENOUS CARBON COMPOUNDS
27
proportion in which they occur in water, namely, two
parts of hydrogen to one of oxygen. Carbohydrates fall
into two main divisions, the sugars and the starches.
The most important of the sugars is glucose or grape
sugar, found in the blood and in many other parts of the
body. Starch occurs in the body in a form called glyco-
gen, which is stored in the liver. When a carbohydrate
is oxidized it liberates much heat, and the oxidation in
the body of these compounds furnishes us much of our
warmth.
Fat. — This substance is present in the body in varying
amounts. Like the carbohydrates it is a compound of
carbon, hydrogen, and oxygen, and like them it produces
heat when oxidized and thus acts as a body fuel. It
occurs largely in a liquid state in the living body, in the
muscles, the blood, the bones, and connective tissues.
We shall consider the properties of these carbon com-
pounds further, in our study of foods.III. THE STRUCTURAL UNITS OF LIVING
MATTER.
The analyses of the chemist tell merely what elements
or combinations of elements enter into the building of
living matter. The principal interest in such analyses
lies in the proof they give that all living matter, plant or
animal, consists of essentially the same chemical elements.
When we speak of the struc-
tural composition of a given
plant or animal we do not refer
to its chemical make up, but to
the form which these compounds
take. Thus the most evident
structural forms these com-
pounds take in our bodies we
call eyes, ears, bones, flesh, etc.,
while in plants they receive the
name of leaves, roots, stems, etc.
Gross structure of the body.
— Roughly, the body consists
of a head, a trunk, and append-
ages (arms and legs). All these
parts agree in being made up
of soft matter supported by a
framework of hard matter.
Lower animals, like the crayfish
and grasshopper, also consist of hard and soft matter, but
with them the hard matter, when present, is on the
28
Fig. 6— A, diagram of a human
trunk split to show the internal
skeleton. It, diagram of a grass-
hopper split to show the external
skeleton. Hard parts are in
black.GEOSS STKUCTUKE OF THE BODY
29
outside and forms what is called
an external skeleton, while in man
it is internal with the soft parts
packed upon it. If we examine a
diagram (Fig. 7) of the human
body split lengthwise the arrange-
ment of these soft and hard parts
is made clearer. Such a diagram
shows that the head is really a
box of bone resting on top of the
backbone with an external cover-
ing of soft parts, the eyes, ears,
cheeks, and scalp, and containing
within a mass of soft material
called the brain. This latter is
continued down the center of the
backbone in a long cord called the
spinal cord. The trunk is divided
into two cavities, one above the
other. The upper cavity, the
chest, is protected on each side
by arches of bone called the ribs,
on the front by the breastbone,
and is separated from the lower
cavity by a thin membrane called
the diaphragm. The lower cavity
is supported at the base by a
broad basket of bones called the
pelvis and at the back by the back-
bone. The upper cavity is filled
with the heart and lungs, while the lower contains a number
of soft organs (stomach, intestines, kidneys, etc.). When
Fig. 7 — Diagram of a human
trunk split lengthwise; a,
brain and cord cavity; b,
nasal cavity : c, mouth cav-
ity ; d, regions of alimentary
canal; <?, heart in chest cav-
ity or thorax; f, partition
(diaphragm) separating the
chest cavity from the abdom-
inal cavity below.30 THE STRUCTURAL UNITS OF LIVING MATTER
split in the same manner the appendages show a central
cylinder of bone surrounded by solid masses of soft matter
called flesh. In addition to these general regions we find
that the whole body is covered with a soft layer called skin,
with openings at certain points for special structures
such as eyes, ears, and mouth.
Character of the body parts. — When we examine the
parts of the body more in detail, we find that they are
capable of separation into portions that perform some
definite kind of work. For example, in and on the head
are parts called eyes, tongue, brain, etc., each of which
has some special work to do. Again, the body cavities
contain portions called the heart, the lungs, the stomach,
etc., all with special work to perform. These specialized
portions of matter which may be entirely soft or hard, or
contain both soft and hard parts, are called organs. They
are usually of a definite shape or outline and position.
We may define an organ as a portion of the matter of
the body which performs a special kind of work. For
example, the eye is an organ for seeing, the ear for hear-
ing, the heart for pumping, etc.
In some cases several organs combine and work to a
common end, and we call such a collection of organs a
system. For example, the brain, the spinal cord, and
the nerves, are organs combined for the control of our
actions. Likewise the stomach, liver, intestines, etc., all
share in preparing food for the body, and hence constitute
a system. We may define a system as a collection of
organs so arranged as to accomplish some definite end by
their combined actions. Systems are named, usually,
according to the end attained, for example, the digestive
system, the circulatory system, etc.THE CELL
31
Composition of organs. — If we turn now to an exami-
nation of the organs, we find them made up of masses of
material which differ both in appearance and action.
For example, the nose consists of a hard framework of
bone, an outer covering of skin, and underlying portions of
red flesh, blood vessels, and nerves. Other organs, when
similarly examined, are found to contain also these flesh,
blood-vessel, and nerve layers, but in different positions.
These substances of which the organs are built are called
tissues, and the simplest structural units to which we can
reduce the body with the aid of the knife are these tissues.
Tissues then are the layers of matter of similar appear-
ance which combine to form the organs.
If now we call the microscope to our aid, tissue, in turn,
is seen to be made up of tiny bodies of definite structure
and shape called cells.
The cell. —-If we scrape the inside of the cheek with a
Fig. 8 — Plant and animal cells ; A, plant cell (spirogyra); B, animal cells scraped
from the inside of the cheek; C.W., cell wall; C, colored bodies; P, proto-
plasm ; S.B., spiral band ; N, nucleus ; C.S., cell sap.
clean knife and mount the scrapings in a drop of water, for
examination under the microscope (see Ex. XXII.), these32 THE STRUCTURAL UNITS OF LIVING MATTER
scrapings resolve themselves into a collection of irregular
bodies whose most prominent structure is a round particle
imbedded in what seems to be a granular liquid. The
bodies are cells, the particle is a cell nucleus, and the gran-
ular liquid is called protoplasm.
If we examine in the same way a little of the common
green pond scum (Spirogyra) we shall see that it also is
made up of oblong bodies which contain a nucleus and
protoplasm (see Ex. XXIII.). These plant cells differ
from the cheek cells, in having a distinct boundary wall,
and appear to be filled with long spiral bands of a green
color.
Both the bodies mentioned above are typical cells,
Fig. 9 — Diagram of a cell. [After Wilson.] Notice in this diagram of a typical
cell the cell protoplasm, composed of cell plasm and cell lymph, and containing
cell foods, C f. and cell sap C.s. The large spherical nucleus has a true nucle-
olus t.n., a false nucleolus/.n., a nuclear network N.n., and nuclear lymph N.l.
c is the centrosome.
and all plants and animal tissues are built of such struc-
tures. For this reason the cell is called the structural unitLIVING PROTOPLASM; THE AMCEBA
33
of all living matter. Assuming the fact that the above
are typical cell structures we may then define a cell as
follows: A cell is a boxlike body having an external wall
that incloses a liquid called protoplasm, and in which
floats a particle called a nucleus. Cells may vary in size,
shape, and in the thickness of their walls, and may con-
tain other substances beside the liquid protoplasm. Our
examination of the other tissues of the body will determine
whether this definition is complete (see pp. 39-51). Mean-
while let us examine this liquid protoplasm a little more
fully.
Protoplasm. — Some one has said that “ The study of
physiology is the study of protoplasm.” The meaning of
this statement lies in the fact that the action of all living
tissues depends upon the action of this substance. If
that is the case, it is certainly worth while to learn what
we can of its properties. Here, however, a difficulty arises.
It is easy to obtain tissues whose cells are filled with
this substance, but when these tissues are removed from
the plant or animal body in which they belong, the proto-
plasm which they contain is killed and shows only a few
of the properties of the living substance. Fortunately,
however, there exists in nature a tiny animal form, only
about a hundredth of an inch in diameter, which is made
up almost entirely of protoplasm, uninclosed by any wall.
Furthermore, this little animal, called the amceba, can be
picked up from the surface of leaves in the still pools
where it lives and be transferred alive, in a medicine
dropper, to the slide of a microscope, where we can watch
all its actions (see Ex. XXIV.).
Living protoplasm; the amoeba. •—The amoeba is com-
posed of liquid substance (protoplasm) of about the con-
EIJDY. PKYS, -3.34 THE STRUCTURAL UNITS OF LIVING MATTER
sistency of the white of an egg. This liquid itself is
colorless, but while it appears clear at the edges, its inner
part is filled with tiny granules that give it a slightly
grayish tinge. The clear outer part is called ectoplasm
to distinguish it from the granular portion or endoplasm.
These terms also (ecto, meaning outer; endo, meaning
inner) serve to indicate the position of the two forms.
The endoplasm is seen also to contain many round bubble-
like bodies (vacuoles) which are filled, some with water,
some with small particles of food. Besides these we note
a single dark body (nucleus) in which is a smaller dark
spot (nucleolus). This nucleus is actually only another
form of protoplasm, and the ectoplasm and endoplasm
together are spoken of as cytoplasm to distinguish the
nucleus from the liquid parts. In structure this tinyLIVING PROTOPLASM ; THE AMOEBA	35
body is identical with the particle scraped from the cheek,
except that it has no boundary wall. It is, in fact, a single
cell without a cell wall, and is one of the lowest forms
of animal life known.
But we have not yet noted the most important feature
of this tiny fluid body. If we watch it a moment we shall
be able to see that it moves. The cytoplasm will be seen
to flow out first in little projections (pseudopodia) which
then fill as the rest of the mass flows into them, and in
this way the body progresses across the slide. That this
is not mere physical flowing like the running down hill of
water is shown by the fact that it can change its direction.
This power of automatic movement is possessed by no
other fluid known, and in itself is sufficient to make this
fluid remarkable.
If now we press down the cover glass with a needle or-
pin just above the animal, we shall note that the movement
stops and that the animal draws back to form a round
ball. If not disturbed further, it will expand and flow
as before. In this simple action two more properties of
protoplasm are shown; the power of expansion and con-
traction (contractility) and sensitiveness to a stimulus
such as touch (irritability). Now watch the amoeba until
it comes in contact with a particle in the water. It flows
over it and engulfs it, but curiously enough, if the particle
is of vegetable matter it is carried along with the amoeba,
while if sand, it is soon cast out. The reason for this
action is that the amoeba, in selecting what to retain and
what to reject, actually makes a distinction between
foods and substances which are not foods. Further, if
the particle is retained it is soon broken up in one of the
round bubbles (food vacuoles) and this is slowly revolved36 THE STRUCTURAL UNITS OF LIVING MATTER
in the cytoplasm and continually decreases in size until it
finally disappears entirely. What has happened? Simply
this: This curious liquid has selected a food substance,
broken it up, dissolved it, and distributed the dissolved
product all over the body; while the fact that the product
is no longer visible shows that it must have been trans-
formed into protoplasm itself. In other words, proto-
plasm is capable of distinguishing and taking in food, of
dissolving or digesting it, and of making new protoplasm
out of it.
Finally, we may note that one of the vacuoles is larger
than the others and expands and contracts rhythmically.
As it expands this vacuole fills with a liquid that seeps
from the protoplasm, and which the contracting vacuole
forces out into the surrounding water. This liquid is
the waste matter produced by the active protoplasm, and
is formed by the breaking up of the chemical composi-
tion of the old protoplasm. Protoplasm, therefore, not
only can assimilate food, that is, build it up into new
protoplasm, but also has the power to dispose of its waste
products.
Other experiments have taught that the amoeba absorbs
oxygen as well as food, and that in the breaking up of the
protoplasm the carbon which it contains unites with this
oxygen and produces carbon dioxide with a consequent
production of heat and power. The carbon dioxide is
given off as a waste from the surface of the cytoplasm.
We may summarize then, as follows, the properties of
this peculiar fluid, protoplasm, as exhibited in the amoeba:
(a)	Power of automatic movement.
(b)	Sensitiveness to stimuli.
(c)	Power to select food and to take it in.BIOLOGICAL POSITION OF MAN
37
(d)	Power to dissolve and distribute food.
(e)	Power to transform absorbed food into new pro-
toplasm.
(/) Power to remove waste matter.
(g) Power to take in air and give off carbon dioxide.
A review of these properties shows that protoplasm
performs in a simple way practically all the actions that
we perform as human beings. If we recall that all cells
contain protoplasm, and that tissues are made of cells,
organs of tissues, and systems of organs, it is evident that
the activity of this protoplasm must be very closely re-
lated to the actions of the organs and systems themselves,
and hence to the activities of the organism that contains
these systems.
Biological position of man. — Botanists and zoologists
(who study plants and animals) tell us that 'there are
thousands of different kinds of plants and animals, but
that when we examine the structure of these many forms
of living matter, all are found to be capable of reduction
to a common structural unit (the cell) and a common
structural substance (the protoplasm). Furthermore,
they tell us that the only essential difference in structure
between high and low plants and animals is to be found
in the greater number, kind, and complexity of arrange-
ment of the cell units in the higher forms, as compared
with the simpler arrangement, kinds, and numbers in the
lower forms. Man, therefore, is to be considered as an
animal composed of many cells of different kinds and
action. He is placed highest in the living kingdom be-
cause in him we have a cellular organism in its highest
development.38 THE STRUCTURAL UNITS OF LIVING MATTER
Evolution. — The theory has been set forth in what is
known as the doctrine of evolution that all plants and
animals have arisen from a common origin, namely, the
substance protoplasm, and that each form of plant or
animal has developed from a less complex form. According
to this theory living matter once consisted of simple
one-celled forms, and all the complicated forms we now
know have been developed from these, much as the many
kinds of houses in a city have been made by different
arrangements of the bricks.
Classification. — In the animal world it is customary to
group the various forms in an ascending series with the
most complex form (man) at the head of the series. When
this is done it is found that the series is roughly divisible
into two groups, vertebrates, or animals with a back-
bone; and invertebrates, or those without a backbone.
In the former class are included all the fishes, frogs,
reptiles, birds, and mammals. In the latter are the one-
celled protozoans, the worms, mollusks, insects, etc.
Man, in common with horses, cows, rabbits, dogs, cats,
etc., possesses hair, and glands for suckling its young
(mammary glands), and hence belongs to the group of
mammals. It is for this reason, namely, that all mam-
mals are very much alike in structure, that it is possible
for us to use the dissections of other mammals, such as
the cat, rabbit, and the rat, to get a clear idea of our
own structure. Many times in the pages to come we
shall make use of this similarity of structure for illustra-
tions of structural arrangement.IV. STRUCTURAL UNITS OF HUMAN TISSUES —
HISTOLOGY.
The study of the structural units (the cells) that make
up the human tissues, is called microscopic anatomy or
histology. The methods of microscopy make it possible
for us to cut very thin sections of tissue, to stain them
with certain dyes so as to bring out the different parts of
the cells, and finally, to mount them on glass slides in
such a way that they may be examined with the com-
pound microscope. This instrument, by magnifying many
diameters, makes it possible for us to study details of
structure that were otherwise invisible. The various tissues
that are found in the body are found to be capable of clas-
sification into four general classes of tissues: epithelial,
nervous, muscular, and connective. This classification
is based upon the characters of the cells of which the
tissues are formed.
Epithelial tissue. — All surfaces of the body, external
and internal, are protected by layers of membranes. The
external layer we call the skin; the layer which lines the
mouth, digestive organs, and lungs we call the mucous
membrane. When we examine these membranes under
the microscope they are found to be composed entirely of
cells united in layers by a thin cementlike substance,
much as bricks are held together by mortar. Owing to
the fact that the membranes which these cells form are
covering or protective layers, the name of epithelial tissue
has been given to them, and the cells of which they are
3940
STRUCTURAL UNITS OF HUMAN TISSUES
composed are called epithelial cells. As we have already
noted, one of the functions of epithelial tissue is protec-
tion. A second function, exhibited only by certain layers
of this tissue, is secretion. Thus the skin that covers the
flesh and the mucous membrane that lines the mouth,
both prevent injury to the underlying parts, but in addi-
tion to this protective action the mucous membrane
secretes a liquid (the mucus) which keeps that mem-
brane moist and flexible. All epithelial cells consist of a
large oval nucleus floating in protoplasm of a granular or
netted appearance, while the cell walls are extremely thin.
Epithelial tissues are classified according to the shape and
arrangement of the cells which compose them. Thus we
speak of pavement, cubical, columnar, spheroidal epi-
thelium, according as the cells which compose them are
Fig. 11 — Forms of epithelial tissue ; o, 1, cylindrical cells ; a, 2, squamous cells
showing cement connection; 6, ciliated epithelial cells; c, statitied epithelium,
showing, 1, a pavement cell, 3, 4, spheroidal cells ; d, columnar epithelium, com-
posed of ciliated cylindrical and spheroidal cells.
flat, cubical, cylindrical, or spherical in shape (Fig. 11).
When the layers are more than one cell layer deep we call
it stratified epithelium. The cells scraped from the inside
of the cheek (see Ex. XXII.) illustrate the flattened orCONNECTIVE TISSUE
41
squamous form, while a section of the intestine lining (see
Ex. XXV.) illustrates the columnar or cylindrical forms.
Sometimes the cells are modified in various ways. For
example, in the skin, hair, and teeth the external cells
become hard or horny, while the cells that line the breath-
ing passages may develop hairlike projections of proto-
plasm called cilia. These cilia wave back and forth and
thus free the surface of tiny particles of dust, etc. In
secretory epithelium the secreting cells are often goblet
shaped.
Connective tissue. — If we attempt to pull apart a piece
of beef we find that it resists
this action by means of an in-
tricate interlacing of white fibers
that hold the separate parts
together. These fibers make
up what is called intermuscular
tissue, which, in turn, is a form
of connective tissue. These
fibers are not cells, but are
formed in the matter thrown
off by certain cells. They are
of two kinds, white inelastic, and
yellow elastic, fibers, and their
function is to surround and
connect other layers. Hence
the name, connective. It is evi-
dent then that the connecting
is done by the substance given off by cells and not by cells
themselves. This characteristic holds true of all forms of
connective tissue. The cells are small and of importance
only as makers of the connective part which is called inter-
Fig. 12 — Intermuscular connect-
ive tissue. Corpuscles are the
cells which form the fibers.42
STRUCTURAL UNITS OF HUMAN TISSUES
cellular substance since it occurs between cells. The other
important forms of connective tissue are cartilage and
bone.
Cartilage. — An examination under the microscope (see
Ex. XXVI. and Fig. 13) shows that cartilage, or gristle,
consists of a solid mass surrounding pits very much after
the manner of Swiss cheese. The solid mass is the elastic
intercellular part of the connective tissue, and corresponds
to the fibers of the intermus-
cular form. The pits or la-
cunce are inclosed in capsules,
and within these capsules lie
the cells which produce the
solid matter. These cells are
the true structural units of
this form of tissue, and the
solid part is made by them.
As this is thrown out by the
protoplasm of the cells, it col-
lects around them, and thus
forms an important structural
element of our bodies. For
example, the external ear and the nose owe their shape
to underlying layers of this substance, while the ends of
the bones are capped by layers of the same material, and
thus protected from friction and jar.
Bone. — If, in addition to the gristly matter given off
by the cartilage cells, there be added salts of lime, the
intercellular mass becomes hardened, and wc have a struc-
ture called bone. An examination under the microscope
(see Ex. XXVI. and Fig. 14) shows that bone differs from
cartilage in that the intercellular mass is arranged in rings
Fig. 13 — Hyaline cartilage; cap,
capsule in a lacuna; m, matrix
formed by cells ; c, cartilage cell;
n, nucleus.CONNECTIVE TISSUE
43
or lamella?, about central canals (the Haversian canals).
Further, the pits or lacunce which contain the bone-form-
ing cells arc connected by fine tubes (canaliculi). The
explanation of this difference of arrangement is found in
the fact that the cells can give off only substances which
they manufacture from the food brought to them. The
Haversian canals contain blood vessels which bring to the
A	B
Fig. 14 — Bone tissue ; A, piece of bone under the low power of the microscope,
showing rings or lamellae about central cavities (Haversian canals); By piece of
bone highly magnified, showing the lacunae with the fine canaliculi radiating
from them.
bone cells their food and lime. Food and lime ooze out
of the blood vessels and finally reach the cells by way
of the canaliculi. The protoplasm of the cells, then, manu-
factures from the food the cartilage material which it
throws out with the lime, so that it accumulates in layers
about the cells. The result of the similar action of many
cells means a great accumulation of this material, and
hence the production of supports for the soft parts of our
body.44
STRUCTURAL UNITS OP HUMAN TISSUES
Connective tissue, then, consists of cells that manufac-
ture and give off a substance which, in. turn, binds to-
gether and supports other parts of the body. Besides the
kinds mentioned, we have still other forms, named accord-
ing to the character of the intercellular material. Thus,
adipose tissue is a form of connective tissue in which the
thrown off matter consists of drops
of oil or fat, held together by fibers
(Fig. 15).
Muscular tissue. —This tissue, to
which all our motions are due, con-
sists of cells whose protoplasm has
the power of contraction and expan-
sion especially well developed. It is
of two kinds and these kinds differ
both in action and structure.
Striated, or voluntary muscular
tissue, is the name given to the kind
whose expansion and contraction is
under our control. It is made up of
cells that are very long in comparison
Fig. 15 — Adipose tissue;	.
a, connective fibers; 6, with their width and are marked with
ol1 drops‘	transverse bands or striations. Such
tissue forms the muscles of the legs, arms, back, etc.
Nonstriated or involuntary muscle tissue is the name
given to the form whose expansions and contraction are
not under our direct control. It is made up of cells
which are spindle shaped and without transverse stria-
tions. Such is the tissue which forms a part of the walls
of our digestive organs.
Striated muscular tissue. An examination under the
microscope (see Ex. XXVII. and Fig. 16) shows suchMUSCULAR TISSUE
45
muscular tissue to consist of long fibers covered with a
transparent covering or sarcolemma, within which may be
made out several nuclei and two kinds
of bands or strice. These strife appear
as alternate broad, dim stripes and
narrow, light stripes. Both these bands
are made of protoplasm called sarco-
plasm or muscle protoplasm. The
numerous nuclei indicate that a fiber
is a collection of several cells fused
together with a common wall or sarco-
lemma.
Nonstriated muscular tissue. The
muscle layers of the
intestine (see Ex.
XXVII. and Fig. 17)
show us that this tissue consists of a mass
of spindle-shaped cells each containing a
single large nucleus surrounded by a mass
of protoplasm which is striated longi-
tudinally but not transversely. Around
each cell is a thin wall or membrane
similar to the sarcolemma.
Muscle cells then may be described as
cells containing protoplasm, which is so
arranged that when it expands and con-
tracts the whole cell is shortened or ex-
tended. By massing cells of this nature
together this shortening and extension
is made to cause movement of the parts
attached. Evidently the extent of mo-
tion is also increased by this massing, the total short-
Fig. 17 — Muscle
layer of tlie in-
testine (unstria-
ted muscle cells);
n, nucleus of a
single cell.
Fig. 16 — Striated
muscle fiber ; a, ter-
mination of muscle
substance; nt nuclei.46
STRUCTURA-L UNITS OF HUMAN TISSUES
ening being the sum of the shortenings of the respective
cells.
Nerve tissue.—In every human being there are two
organs called the brain and the spinal cord. From these
there extends all over the body a lacework of fibers which
are called nerves. Brain, cord, and nerves.are built of the
substance called nerve tissue,
whose principal purpose
seems to be to originate and
conduct stimulations to vari-
ous parts of the body. These
stimuli act upon the other
tissues of the body and cause
them to either increase, or
cease their special kind of
activity. In this way is con-
trolled the action of the dif-
ferent organs that are made
up of tissues. Nervous tissue
is made up of cells just as are
all other tissues. These cells
are, however, peculiarly ar-
ranged and extended so as to
be continually in contact. For
example, while the muscles of
the foot are entirely distinct
and separated from those of the hand, the nerve cells of
the hand are, on the contrary, comiected with those of the
foot and other parts of the body, in the same way that tele-
graph stations are connected. In other words, if we could
take away all the other tissues of the body we should have
left the nervous tissue in the form of an intricate network,
Fig. 18 — Nerve cell, showing nucleus
in the center of the cell, and having
one axis cylinder or nerve process
and several dendrites.NERVE CELLS
47
all parts of which were connected. It is this connection
that allows us to become conscious of an itching sensation
in the foot, and permits us to direct the muscles of the hand
to rub the affected part. The manner of the connection of
these cells is peculiar to this tissue, and its method we shall
investigate further in a later chapter.
Nerve cells.—As shown under the
microscope, a nerve cell consists (see
Ex. XXVIII. and Fig. 18) of a mass of
protoplasm with a nucleus. The proto-
plasm is drawn out into two forms of
projections, nerve processes and den-
drites. The rodlike nerve processes
(axis cylinders) form the center of a
fiber called a nerve and this latter may
be of great length. These nerves may
branch extensively, and through its
nerve process permit a nerve cell located
in the brain or cord to be in contact
with widely distant points, say in the
finger or foot. By means of the den-
drites, one cell and its nerve may be
brought in contact with another cell and
nerve, and thus the entire set of cells
brought into communication. A nerve
cell with its processes and dendrites is FlG' ‘®—np’
called a neuron. (See Fig. 19.) Just
as the protoplasm of a muscle cell has
the power of expansion and contraction
developed to a great degree, so the pro-
toplasm of a nerve cell has developed the power of con-
ducting impulses of various kinds.
nerve cell; t, ter-
minal brusli; c, d,
nerve fiber with
sheaths surrounding
the axis cylinder.48
STRUCTURAL UNITS OP HUMAN TISSUES
In our study of the nervous system we shall investigate
further this conductive power and the action of nerve
tissue.'
General Properties of Tissues.
There are two general facts to be learned from the study
of the structure of tissue. First, that all parts of our
bodies are made up of tiny structural units or cells.
Second, that the functions of a given form of tissue are
due to the predominance of at least one of the properties
of the protoplasm of its cells over the other properties.
Thus, the protoplasm of the bone cell, like that of the
muscle cell, has the power of absorbing food, building up
new protoplasm, giving off wastes, and forming new cells.
In these respects bone cells are like all other cells. They
differ in that the protoplasm of the bone cell has developed
its power of giving off waste matter over all the other prop-
erties, while the muscle cell has made a specialty of the
power of contraction and expansion. The result is that
a combination of bone cells becomes important in the
body for the character of material it throws off, namely,
bone, while the combination of muscle cells is equally
important from the power of motion which it makes
possible. A comparison may make this point clearer.
Take two men, one a musician, one a carpenter. Each
eats, moves, thinks, and performs all the ordinary actions
of the other. Both have all the properties of a man.
One, however, has trained his hands to one kind of action,
the other, to a different kind, and to this specialization
are due their respective callings. So tissues are but col-
lections of cells whose essential substance is protoplasm.GENERAL PROPERTIES OF TISSUES
49
All these cells possess the common attributes of proto-
plasm. The specialization of one of these properties:
contraction, excretion, or conduction, makes them different,
and gives its special tissue name to the collection of which
these cells are a part.
What happens when we combine bodies of men of dif-
ferent specialties? Take a factory, for example. If it is
a cotton mill, some are carders, some spinners, some
weavers, and the like. Each body performs its own
special work, and all together produce the common result,
the production of cotton cloth. So in our bodies. The
various organs when dissected are found to be composed
of different tissues; that is, collections of specialized cells,
which work together for some common result, such as
seeing, digestion, smelling, and feeling. We may carry
the comparison further. Just as different factories supply
the needs of a community, so in the body we find organs
combined into systems, each organ contributing its action
to a common purpose. Thus the stomach, intestines, liver,
etc., are organs which by their combined action prepare
the food for the body, and hence we group them under
the name of the digestive system. Finally, all these sys-
tems are placed imder the control of a governing board
in the form of the nervous system, and so their work is
coordinated, and all work for the maintenance of the
individual which they make.
The individual man consists of a collection of sys-
tems, organs, tissues, cells, and the total actions of the
individual are but the sum of the actions of his indi-
vidual cells. This division of the work of the body among
different specialized cells is called 'physiological division
of labor. The significance of this term lies in the fact that50
STRUCTURAL UNITS OP HUMAN TISSUES
each set of cells performs special work just as in a cotton
mill, spinners, weavers, and others have their special tasks.
Reproduction of cells. — Before we leave the tissues, one
feature of cell action which they all share must be con-
sidered more fully. Like the parts of a machine, the parts
of our body are continually being broken down by use.
To remedy this there must be continual repair or building
up. Otherwise the body, like the machine, soon becomes
Fig. 20 — Reproduction of a cell; A, diagram of a cell, showing centrosome (a),
and nucleus (n) consisting of nuclear membrane inclosing granular substances
in which are a spherical nucleolus and irregular masses of chromatin ; B to F,
changes in the course of reproduction.
worn out and useless. How is this brought about? The
answer to this question is found in the discovery of a very
peculiar property of protoplasm called reproduction. This
means that an old cell has the power to produce new cells
out of its protoplasm. The manner in which it does this
is illustrated in Figure 20 ; thus a single cell or egg is able
by repeated divisions to produce all the tissues of a body,
and a tissue is able to renew its structure to maintain its
efficiency. When the protoplasm of an individual loses this
power the individual soon becomes worn out and dies.
Life depends upon the reproductive power of protoplasm.V. FOODS AND NUTRITION.
One of the earliest sensations that a child experiences
is hunger or the desire for food. A child comes early to
recognize that this hunger is merely a way the body has
of telling us that it must have food at certain intervals.
Further experience teaches that this food must be of a
certain kind to enable the body to grow and maintain
itself in a state of health and activity. An examination
of other animals and of plants teaches that they too re-
quire food and that without food they die.
The foods of the different plants and animals differ
greatly, however, and what supports life in one does not
necessarily answer the same purpose in another. For
example, the hay on which a horse feeds would be useless
to us, and the fertilizer and water on which a plant flour-
ishes would prove valueless to both the horse and our-
selves. How then shall we define food as a general term
which shall include all classes? Broadly, we may define
it as follows: Anything which an animal or plant takes
into its body as nourishment is food. Applying this defi-
nition to ourselves, food is anything which we eat or
drink that nourishes our body. The number of such
substances is very great. Chemists have analyzed all the
various forms of food, however, and have found that they
are composed of one or more of some six classes of com-
pounds. These compounds then are the structural units
of which all the varied forms of foods are built up. These
5152
FOODS AND NUTRITION
fundamental food units are spoken of as food stuffs, ali-
mentary principles, or nutrients. We shall adopt the last
term in speaking of them.
Nutrients. — Nutrients are. usually classified under the
following names: water, inorganic salts, proteids, carbo-
hydrates, fats, and albuminoids. Since the importance
of a food and its value as nourishment depends upon the
amounts and kinds of these nutrients present, it will be
well to consider some of their properties and the means
by which their presence may bo detected.
Proteids or nitrogenous compounds. — Different vari-
eties and forms of proteid are found in nearly all kinds of
animal and vegetable foods. The exact composition of
proteid is unknown, but the elements of which it is built
are nitrogen, carbon, sulphur, hydrogen, and oxygen. It
occurs in a great variety of forms which receive different
names; for example, myosin and syntonin in meats; egg
albumin; casein of milk and cheese; gluten of grain.
White of egg or egg albumin illustrates well the charac-
teristic properties of proteids.
When egg albumin is heated or treated with acids (see
Ex. XIII.), it is changed from a soluble, sticky liquid to
a white solid, insoluble in water. This change to a solid
form, due to the heat or acids, is called coagulation, and
is a characteristic of many proteids. The solidifying of a
piece of meat under heat is due to the coagulation of the
proteid in it by the heat, while the curdling of milk when
sour is another case of coagulating proteid due to the
presence of lactic acid in the milk.
There are a number of ways in which the presence of
proteid in a food may be detected. The principal chem-
ical tests are as follows (see Ex. XIII, a-d.):CARBOHYDRATES
58
(a)	Xanthoproteic test. Protcid boiled in nitric acid is
dissolved and the solution colored yellow by the action of
the acid. If to this mixture is added enough ammonia to
neutralize the acid the color turns a deep orange.
(b)	Millon’s test. Proteid, brought slowly to boiling
with a solution of mercury in nitric acid, becomes rose
red. The nitric acid solution of the mercury is called
Millon’s reagent.
(c)	Biuret test. If a cold solution of proteid is first
made alkaline, by the addition of caustic soda, and then a
few drops of a very dilute solution of copper sulphate are
added, a violet color appears which deepens in color with
boiling.
Any one of these tests applied to a food which contains
proteid will reveal its presence. Of the three, the xantho-
proteic is best for common use.
Albuminoids. — These substances, an example of which
is the gelatine found in bones, are closely allied to proteids
in composition. This resemblance to proteid is confined
to their composition and the presence of nitrogen as they
vary from proteid, not only in failing to respond to proteid
tests, but also in their action. In this group is included
keratin which forms horn, hair, feathers, nails, and hoofs;
elastin, which forms the elastic fibers of connective tissue;
and certain bodies called enzymes which we shall inves-
tigate later. The different forms vary so much in charac-
teristics that it is impossible to name any one feature
that is distinctive. Of the common foods, soups and
soup stock made from bone marrow are richest in this
nutrient.
Carbohydrates. — Under this name are included the
starches and sugars. These are compounds of carbon,54
FOODS AND NUTRITION
hydrogen, and oxygen, so combined that the hydrogen
and oxygen are always in the same proportion as they
occur in water.
Starch is found in all green plants in the form of gran-
ules and is frequently stored in the cells of grains, seeds,
and stems, as a reserve food supply for the plant. To two
of these plant storehouses, the potato and the corn, com-
merce is indebted for its market supply of the substance.
The principal characteristics of starch are its insolubility
in water, and its transformation to a sticky swollen mass
or paste by the addition of boiling water. It gives a blue
color when treated with a solution of iodine, and this
latter means is usually employed as a test to detect its
presence in foods. (See Ex. XIV.)
There are several different kinds of sugars; cane sugar
or saccharose is the sugar of the market, and as its name
indicates is obtained from the juice of the sugar cane. It
is also obtained from the juice of the sugar beet. Grape
sugar or dextrose (also called glucose) is found in almost
all plant and animal tissues, and it derives its name of
grape sugar from its abundance in grapes. Still another
form is milk sugar or lactose, found in milk. Both sac-
charose and lactose can be changed to dextrose by boiling
with certain acids, notably, hydrochloric. This process is
called inversion.
Grape sugar is characterised by two important prop-
erties, the power to ferment and the power to take up
oxygen. As a result of the first, certain substances, such
as yeast, are capable of breaking it up into alcohol and
carbonic acid gas; while to the second is due the means of
detecting its presence in foods. The most common test
used is a mixture called Fehling’s solution. (See Ex. XV.)MINERAL SALTS
55
When a solution containing grape sugar is treated with a
few drops of this solution, a substance is precipitated,
which, upon heating, gives up part of its oxygen to the
sugar, and turns a reddish brown. If no sugar is present
this loss of oxygen does not occur and there is no con-
sequent colored precipitate. This reaction furnishes a
simple and delicate means of detecting the presence of
grape sugar in a compound.
Fats. —Under this name are included not only what is
ordinarily understood by the term fat, but also animal and
vegetable oils of all kinds. Like the carbohydrates, fats
are composed of carbon, hydrogen, and oxygen, but in differ-
ent proportions. They are all lighter than water and in-
soluble in it. All are readily made liquid by a moderate
amount of heat, while some are liquid at ordinary tempera-
tures. Thus body fat melts at a temperature of 98°
Fahrenheit and is liquid in the living body. (See Ex. XVI.
a.) Fats are soluble in ethers and benzene and when
heated with alkalies form soap. Their presence may be
detected in one of two ways. (See Ex. XVI., b, c.)
(a)	Rub on paper the substance to be tested or place
it on the paper and heat gently. If fat is present a grease
spot will result, which gives the paper a translucent appear-
ance and is readily detected.
(b)	Soak the substance in ether (benzene will do as
well). The ether will dissolve the fat if it is present. Then
filter off the solution and allow the ether to evaporate.
The fat does not evaporate and will remain. This second
test enables us to extract and measure the exact quantity
of fat present as well as to determine its presence.
Mineral salts. —Many forms of salts occur dissolved in
water and are present in all living matter, while other insol-56
FOODS AND NUTRITION
uble forms enter into the structure of important parts of
the body. For example, in the fluids of the body we find
common salt (chloride of sodium) as well as chlorides of
potassium, ammonium, etc., while important parts of our
bones are the insoluble carbonates and phosphates of cal-
cium. Mineral salts all agree in being unburnable, and it
is by burning foods and examining the character and
amount of the unburned residue that we are enabled to
determine the kinds and amounts of salts present in a
given food. (See Ex. XVII.)
Water. —This liquid forms a large proportion of all
fresh vegetable and plant foods. It constitutes nearly 59%
of the weight of our bodies. This latter fact indicates the
great importance of water as a nutrient. The fact that
water becomes a vapor and passes off
in that form into the air when heated,
enables us by drying a substance in a
warm place to determine its water con-
tent. (See Ex. XVIII.) The loss of
weight represents the water evaporated.
Composition of Foods.
The extent to which these nutrients
occur in various forms of foods (see also
Ex. XX.) is indicated in Chart I., on
page 57.
Sources of nutrients. — An examina-
tion of our ordinary food supply reveals
the fact that for most of our nutrients
we are dependent ultimately upon the vegetable world.
Our food, in general, is animal, vegetable, or mineral, in
Fig. 21 — Apparatus ar-
ranged for nutrient
experiment with a
seedling.SOURCES OF NUTRIENTS
57
Chart I.
Food	Comp 2 "S o Sh		OSIT A c* ■*s cn	Other o Carbo- ^ hydrate ^		OF N cS		UTRIE F-4		2. Mineral g		Energy in Calories per Pound	Average Cost per Pound
Bread (White) . . .	8		47.	3		1		37		2		1280.	$.04
Flour . ...	11		66.	4	2	2		15		1	7	1645.	.025
Oatmeal. . ...	12	6	58.	5	4	5	6	15		3		1850.	.05
Rice . .	6		79.	0	4	0	7	13		0	5	1630.	.07
Beans . .	23	1	55.	2		2		12	6	3	1	1615.	.05
Potatoes . ...	2		18.	3		0	2	76		0	7	375.	.0125
Milk . .	4			5	0	4	0	86		0	8	325.	.035
Cheese . .	28	3		1	8	35	.5	30	2	4	2	2070.	.16
Beef (Round)	20	5				10	1	68	2	1	2	805.	.14
Beef (Corned Flank)	14	2				33		49	8	3		1655.	.10
Mutton (Leg)		18	3				19		61	8	0	9	1140.	.18
Veal (Shoulder) . . .	20	2				9.8		68	8	1	2	790.	.20
Pork (Shoulder — fresh)	16					32	8	50	3	0	9	1680.	.16
Pork (Ham) . .	16	7				39	1	41	5	2	7	1960.	.16
Pork (Salt Fat) ....	0	9				82	8	12	1	4	2	3510.	.12
Chicken ... ...	24	4				2	0	72	2	1	4	540.	.20
Eggs ...	14	9				10	5	73	8	0	8	721.	.18
Butter . .	1			0	5	85		10	5	0	3	3615.	.30
Codfish . . . .	15	8				0	4	82	6	1	2	310.	.08
Mackerel . . .	18	2				7	1	73	4	1	3	640.	.12
Oysters . ...	6			3	7	1	2	87	1	2		230.	.25
origin, but if we consider for a moment the kinds of ani-
mals upon which we depend for food it is noteworthy that
these animals themselves feed exclusively upon vegetable
and mineral matter. We may say, then, that with the
exception of some mineral salts and water, the animal
world in general ultimately derives all its supply of nutri-
ents from the vegetable world. When we eat flesh we are
simply using the proteid, carbohydrate, and fat that these
animals have themselves obtained from vegetable matter.
It is in the plant, then, that the chemical compounds58
FOODS AND NUTRITION
which we call nutrients, are produced from the elements or
their inorganic compounds. A few simple experiments
with a seedling (see Ex. XIX.) demonstrate that a plant
grows well when its diet is restricted to water, certain min-
eral salts, and carbon dioxide. The roots take up the
salts and water as solutions, while the leaves absorb the
carbon dioxide from the air. When analyzed, the plant
thus restricted in diet is found to have produced proteid,
starch, sugars, and oils. Plant physiologists tell us that
the simple raw materials are broken up and recombined
chemically into what we call the nutrients, and that a plant
Fig. 22 — J, diagram illustrating the manufacture of nutrients by the plant; 7?,
cycle illustrating the relation of living forms and the mineral world in regard to
nutrient supply.
place. Some of this manufactured nutrient the plant
uses for its own food and some it stores. The stored nu-
trients are in turn taken in by the animals, and by them
A
nBUILDING MATERIAL
59
converted into animal proteid, and carbohydrate. If
now we note what happens when animal bodies and plant
bodies die, we find that in the process called decay these
nutrients are again broken down into mineral salts and
gases and thus complete a cycle. The plant world there-
fore stands between the mineral and the animal world as a
manufacturer of nutrients.
Uses of nutrients. —Very careful experiments have
been performed in the past and are being performed at
the present time to determine the exact use of each kind of
nutrient to the body. What is already known in this
direction may be summarized as follows:
Nutrients serve two purposes in the body. First, they
furnish the material out of which our tissues are built and
repaired. Second, they may be oxidized in the body, and
when this action occurs the result is the production of
either action (movement) or heat; both action and heat
being forms of energy. These two uses are usually ex-
pressed by saying that nutrients furnish the body with
either building material or fuel.
Building material. —All the organs and tissues of the
body are continually undergoing two processes, a building
up and a tearing down. These two processes are spoken
of collectively as metabolism. The building up is called
anabolism, and the tearing down katabolism. Anabolism
and katabolism vary in amount at different periods of our
life. In childhood the anabolism is greater than kata-
bolism and the body increases in size and weight, that is,
it grows. In middle age the two processes are nearly
equal in amount so that the new material merely serves
to repair or replace the worn out parts. Finally, in old
age, katabolism becomes the greater, the new material60
FOODS AND NUTRITION
fails to replace completely the worn out parts, the body
loses in power, and finally its action ceases altogether, and
what we call death follows. To furnish new material for
anabolism is what is meant by the use of nutrients as
building materials.
If we recall that tissues are composed of cells, and cells
of protoplasm, we shall see that this function of nutrients
really resolves itself into the building up and repair of
broken down protoplasm. The particular nutrients that
are best suited to use as building material are proteids
or the nitrogen compounds. The fact that proteid and
protoplasm are nearly identical in composition has a
new significance in the light of the above facts. How
far the other forms of nutrients share in this use as build-
ing material, is at present uncertain. In fact, experiment
has shown that food free from inorganic salts will not
support life, thus demonstrating that these substances
are an essential part of the tissue structure.
Fuel. — Whether carbohydrates and fats are ever used
as building material or not, this is not their principal
function in the body. Their main purpose is to serve as
fuel, and thereby to produce what we call energy, or the
power to do work.
Meaning of energy. — Whenever our muscles move
parts of the body, as in running, leaping, walking, and eat-
ing, we have examples of this power to do work, or energy.
Whence comes this power? An example may help us to
understand better the nature and production of energy.
An energy producing machine with which we are all
familiar is the steam engine. Such an engine derives its
power to do things from the coal which we put in the fire-
box. Here this fuel burns (that is, oxidizes, see p. 18),BUILDING MATERIAL
61
and the result is the production of a form of energy which
we call heat. This heat in turn acts upon water inclosed
in a boiler, and changes it to steam. Steam has the
power of expansion; that is to say, the heat energy has
been transformed into expansive; power, another form
of energy. If the steam is admitted to the cylinder of the
engine and allowed to expand against the piston, its form
of energy (expansion) is transformed into still another
form, thrusting movement. Further, if we attach the
piston to a wheel this form of energy is changed once
more into rotary movement, and that in turn may be
applied to a saw, and thus the engine may be made to
saw wood. In all these steps the process has simply con-
sisted in transforming the energy derived from the burning
fuel into first one form and then another form of energy.
In other words, the steam engine is simply an apparatus
for changing the form of energy derived from combustion.
We may summarize what this example tells us as follows:
First. In the coal we see that energy may exist with-
out manifesting itself. Coal, in other words, has the
power to burn and produce heat even when it is not burn-
ing. This power becomes manifest as soon as the coal
actually begins to burn or oxidize. We express this fact
by saying that energy is of two forms, potential energy
and kinetic energy. Energy is potential when, owing to
its position or composition, a body possesses the power to
do work without manifesting that power. When the
energy manifests itself in the actual doing of work we have
an example of kinetic energy.
Second. Potential energy may be transformed into
kinetic energy, and kinetic energy into potential energy.
Thus the potential energy of the coal to produce heat is62
FOODS AND NUTRITION
transformed into kinetic energy or actual heating when the
coal is burned. Further, heat is transformed into poten-
tial energy or the power to expand, that steam has when
it is not actually expanding.
Third. Kinetic energy can be transformed from one
kind of kinetic energy into another. Thus heat is trans-
formed into expansion, expansion into thrusting, thrust-
ing into rotation, rotation into sawing, etc. Careful
experiment has demonstrated that when energy is trans-
formed from one form to another no energy is lost or
created. The law which expresses this fact is called the
law of the conservation of energy, and applies to all cases
where the production of energy is concerned. It is usually
stated as follows: Energy can be transformed from one
kind to another, but can never be increased, diminished,
destroyed, or created from nothing.
Fourth. The source of energy in a steam engine is the
oxidation of fuel. That is to say, the energy of an engine
is derived fundamentally from the oxidation of certain
chemical compounds. Such oxidation is simply a chem-
ical change, and chemical changes are the sources of most
forms of energy.
In the light of this example let us examine again our
body as a machine for the transformation of energy. Just
as in the steam engine, we find that its source of energy is
to be found in the oxidation of fuel. Such fuel is found
in the carbon compounds known as carbohydrates and
fats which form part of our food. The energy liberated
by the oxidation of these compounds in the body takes
many forms, such as heat, muscular activity, secretion,
etc., and one form may be transformed into another,
since the law of conservation holds here just as every-BUILDING MATERIAL
63
where else. Thus the energy of a sleeping child is poten-
tial and becomes changed to various forms of kinetic
energy when he wakes. Finally, all energy liberated in
the body ultimately manifests itself either in action or
heat, and hence we may say that our actions and our
body temperature are merely manifestations of energy
whose source is the oxidation of the nutrients, carbo-
hydrate and fat. Since some part of our body is always
in action, and all parts must be kept at a constant temper-
ature, it follows that there must always be on hand a
sufficient quantity of fuel to supply this energy. It is
this fact that makes it so important that our daily diet
contain a certain proportion of carbohydrate and fat,
and explains why people w7hose work is largely muscular
require more of these nutrients than those whose labors
are less active.
Other substances than fats and carbohydrates may be
used as fuel, notably both proteids and albuminoids, as
both contain oxidizable carbon. The carbohydrates and
fats, however, are the cheapest and easiest sources of this
energy. It is interesting to note that the carbohydrates
are capable of transformation into fat, and that it is in the
latter state that the reserve energy of the body is stored.
In starvation it is this stored fat that maintains life.
Another striking feature that can be deduced from the
above statements is that proteid is the only universal food,
since it is the only nutrient that combines in itself both
tissue building and fuel qualities.
Value of common foods. —A study of Chart II., shows us
that an important factor in determining the value of a
food is its composition. A food like meat, for example,
owing to its high percentage of proteid, is at once recog-64
FOODS AND NUTRITION
nized to be a valuable tissue builder. Potatoes with their
abundant starch are in the same way put down as fuel or
energy producers. If we consider our own experience,
however, we realize that composition tells us only a little
of the true value of a food. A food, for example, may
contain all the nutrients necessary to the body, and yet
be absolutely valueless to us. Hay, for instance, contains
all the nutrients necessary to support life, and yet man
cannot use it as food. Again, certain foods may be so
rare that their cost prohibits their use. In general, then,
we may say that the value of a food is determined only
when we have considered its composition, its fitness for
our use, and its cost.
The first point that may be noted from Chart II. is
that most food as bought in the market is made up of
nutrient only in part. The remainder may consist of
bone, connective tissue, skin, bran, etc., all of which is
included under the heading of refuse in the chart.
The amount of refuse present is evidently an important
factor as regards the value of a food. It is because of
the fact that eggs, milk, and bread contain almost no
refuse that they are such valuable foods.
A second point to be noted is the water content of
foods. In this respect, animal foods, in general, contain
more water than the vegetable foods. Exceptions to- this
rule are found in potatoes, onions, lettuce, and some other
vegetables, in which the proportion of water is very high.
In meat foods, too, the fatty forms contain less water
than the lean meats. In general, other things being
equal, the higher the proportion of water the less the food
value. Fish on this account are less valuable as food than
ordinary meats.Chart II. — (From Farmers' Bulletin Number 23).
A calorie equals the amount of heat necessary to raise the temperature of h
kilogram of water (2.2 lbs.) from 0 to 1° Centigrade, i.e. lg° Fahrenheit. It
is possible to determine the amount of heat energy that a given amount of any
nutrient can produce in terms of these calories, and hence they furnish con-
venient units to express the fuel value or energy producing power of foods.
EDDY. PHYS. —5	6566
FOODS AND NUTRITION
Again, the fuel value of a food is largely a matter of its
fat and carbohydrate content, though the fact that pro-
teid has a fuel value makes it a factor. Foods rich in these
substances take high rank as energy producers. It is
interesting to note that fats have about two and a quarter
times the energy producing power of either carbohydrate
or proteid. Foods, therefore, which are rich in fats and
poor in water have the greatest fuel value. Meats differ
greatly in this respect, and while the different cuts and
kinds necessarily vary, veal is distinctly the poorest and
pork the richest in fat, while mutton and lamb rank with
the fatter cuts of beef. Chickens and turkeys rank low
in fuel value but high in proteid in spite of their large
proportion of refuse. Vegetable foods, though rich in
carbohydrates, have less fuel value than fat meats.
A comparison of vegetable and animal foods from the
chart is interesting, and the importance of dry seeds and
grains with their small proportion of water and high pro-
portion of nutrient is worthy of note. In most of such
vegetable foods the proportion of nutrient is not far from
87%. Beans and peas are richest in tissue building
material, cereals next richest, and fresh vegetables least
rich. Vegetable foods, therefore, are good tissue builders,
but lack the fuel value of meats owing to the small pro-
portion of fat. They are in general less rich in proteid
than meats. In this comparison the position of bread as
a nearly ideal food is interesting. The flour or vegetable
part with its large proportion of proteid and little water
has its lack of fat and mineral matter remedied by the
addition of lard or butter, milk, and salt, and thus
combines the advantages of the cereal with that of the
fat meat.FITNESS FOB USE
67
Fitness for use. — So many factors enter into this deter-
mination that it is difficult to generalize them. The pres-
ence of the necessary organs for digestion, sex, age, size,
occupation, likes and dislikes, amounts necessary for
nourishment, and conditions of the nutrients all bear on
this question. If we leave out of account all personal
peculiarities the most important of these factors may be
comprised under the head of digestibility. We shall dis-
cuss digestion more in detail in a later chapter. For the
present it is sufficient to state that by digestibility we
mean the relative amount of a food that in a given time
our body is able to put into tissue building or energy pro-
ducing form. Without attempting to detail the methods
by means of which digestibility is determined, we may
state the general conclusions of such experiments as follows:
First. Vegetable proteid is much less readily digested
than animal proteid. That is to say, animal foods will
furnish us with more energy and tissue in a given length of
time than will vegetable foods.
Second. The amount of fat present in a given animal
food does not indicate exactly its energy producing power,
since, as ordinarily eaten, nearly five per cent of such fat
escapes digestion, fat requiring a long time for its digestion.
Third. Carbohydrates are ordinarily very completely
digestible and hence foods rich in these nutrients become
important energy producers even though their caloric
value is less than that of fat.
Fourth. The amount of food digested in a given time
(its digestibility) is not affected to the extent commonly
supposed, by the addition of flavors or condiments, nor
does the time differ much with people of different habits.
The statements made above are, of course, general, and68
FOODS AND NUTRITION
are all subject to modification when we consider them in
relation to individual tastes and needs. Thus certain
forms of food may be distasteful to certain persons. In
that case a less nutritious food may be more valuable
because of its readier digestibility.
Cost of foods. — The money value of foods is a question
that should have much more attention than is commonly
given to it, and much time has been given to this side of
food values by the Department of Agriculture at Wash-
ington. The table on the following page gives some of
these results in a form which explains itself.
The principles which should guide in the selection of
cheap foods may be stated as follows: “The cheapest
food is that which supplies the most nutriment for the
least money. The most economical food is that which
is cheapest and at the same time best adapted to the
wants of the eater.” 1
These two statements indicate very clearly two of the
guiding principles for the economical purchase of food,
namely, nutritive value and palatableness. It is evident
that only by the study of such tabulated statements as are
given in Charts I. to III. can we determine the relative
nutritive values of foods, and it is in supplying of such
tables that the government is giving valuable aid to the
purchaser.
Adulteration. —Another principle which the purchaser
must consider is the quality or relative purity of foods.
Especial care is necessary in the selection of brands of
canned foods since many of the market brands contain
adulterants or preservatives which are positively danger-
ous to the health of the user. Recent legislative measures
1 Farmers’ Bulletin, 23, p. 20.6U
Chart III. — Pecuniary Economy of Food.
(From Farmer's Bulletin Number 23.)70
FOODS AND NUTRITION
in the passing of Pure Food Laws are steps in the direc-
tion of safe-guarding the public in this respect. The
refusal of the public to purchase any forms of foods
except those which inspection proves to be of good qual-
ity will go far toward remedying the present tendency to
adulterate.
Daily diet. —Observation experiments conducted in
Europe and in the United States have given us figures
which serve as standards in determining the amount of
foods which must be consumed daily by people of almost
every class in order to preserve good health. In Europe
the figures given by Professor Voit of Munich, Germany,
are considered most reliable. Those of Professor Atwater
are considered better suited to conditions in our own coun-
try. The following table indicates the amounts deter-
mined by Professor Atwater:
Conditions	Proteid, Lbs.	Carbo- hydrate, Lbs.	Fat, Lbs.	Calories
Man with light muscular exercise	.22	.88	.22	2989.
Man with moderate work. . .	.28	.99	.28	3570.
Man with active muscular work	.33	1.10	.33	4060.
By comparison with this table and the one on page 65,
daily dietaries may be made up to fit all classes and at the
same time secure variety of diet. (See Ex. XXI.)
Cooking. —Experience teaches us all the importance
of cooking. In general, its scientific value lies in the
fact that it either renders food more palatable or more
digestible.
Methods of cooking meats. Meats are usually roasted,COOKING
71
boiled, broiled, or fried. Of these methods frying is the
poorest as it adds too much fat and tends to make the
meat less digestible. The advantage of roasting or broil-
ing, over boiling, is that both of these former methods tend
to retain the juices and thus prevent loss of nutrient and
flavoring substances. If meat must be boiled, it should
be plunged into boiling water at first, so as to coagulate
the surface proteid, and thus keep in the juices. This, too,
is the reason for a hot fire in broiling and roasting.
On the contrary soups obtain their value from the juices
extracted from the meat and bones and hence the reverse
process is necessary here. In this case the meat should
be put in cold water and the temperature raised gradually.
Very little nutrient is found in soups, and hence stews,
which make use of the meat as well as the broth, are much
better foods.
The value of meat cooking may be summarized as fol-
lows: (a) Heat kills bacteria and parasites, such as tape
worms, which may be present, and thus makes the food-
safer for use. (b) It renders the meat more attractive
and improves its flavor, (c) It renders the fibers softer
and makes the digestion simpler for the organs, (d) It
makes the time required for digestion shorter and thus
enables the organs to digest more nutrient in a given
time.
Vegetable cooking. The principal reason for cooking
vegetables is found in the fact that it renders them more
readily digestible and improves the flavor. Starch as
found in raw vegetables is inclosed in grains which have
a hard indigestible outer shell. Heat causes the starch
to swell and burst these walls and thus enables the diges-
tive fluids to get at it.72
FOODS AND NUTRITION
In boiling vegetables, care must be taken to prevent
the soaking up of water, as this increases the proportion
of water. For this reason the cooking should be as rapid
and with as little water as possible. On this account
steaming is preferable to immersion in water.VI. FOOD ACCESSORIES; ALCOHOL AND
TOBACCO.
In ordinary diet we eat many things which merely add
to the enjoyment of eating. These substances are to be
regarded not as foods but as food accessories. They in-
clude the condiments (pepper, mustard, etc.), the flavors,
the stimulants (tea, coffee, etc.), and the narcotics (tobacco
and opium). Some of these substances are harmless, some
are beneficial, and others positively injurious. Some are
harmless in small quantities, but become distinctly inju-
rious in larger amounts. None of these food accessories
is absolutely necessary to the body, though few people are
willing to eat food entirely without flavor.
Condiments and flavors. — Most of these substances
have a directly beneficial effect either in making the food
more palatable or in stimulating the flow of the fluids
which digest food. Some hasten the absorption of the
food into the blood. They are harmless if not used to
excess and not used to disguise foods whose natural flavors
would indicate their unfitness for use. A few, such as the
salts and the sugars, have a food value also. Used in
excess, they may result in overstimulation of the digestive
fluids or the cultivation of habits of use which destroy the
normal appetite. The continual use of highly seasoned
food destroys the appetite for natural flavors.
The members of this group may be classified as follows:
aromatic condiments (vanilla, cinnamon, nutmeg, etc.),
peppers, alliaceous condiments (garlic, onion juice, mus-
7374
POOD ACCESSORIES
tard, etc.), acid condiments (vinegar, lemon juice, pickles,
etc.), the salts, and sugars.
Stimulants. — This is the name applied to substances
which excite cells to action without furnishing them
material with which to develop heat and energy. They
produce their effect through action upon the nervous
system. Of all the substances included in this group it
may be said that, while in small amounts they are harm-
less and may even be beneficial, in excess they are dan-
gerous to the best health of the body. The main reason
for their injurious action lies in the fact that while a little
stimulation is sometimes necessary, an excess soon causes
a weakening of the powers of the cells affected, and leaves
the body in a worse state than before the use of the stimu-
lants. In this respect, their action is like the application
of the whip to a horse. A little use may produce good
results, but excess soon exhausts the powers of the animal.
It is generally agreed by all health authorities that the
use of stimulants should be avoided by the growing
child.
Tea, coffee, and cocoa or chocolate owe their stimu-
lating effect to certain principles which they contain
(caffein in tea and coffee, theobromin in cocoa and choc-
olate). These principles cause an increase in the blood
flow, the tired cells are supplied with more food from the
blood, and the sense of fatigue disappears for the time.
While such stimulation may be necessary at times for the
adult, it is always better to secure the stimulation through
natural means such as exercise or by allowing the cells
to rest. Beef extracts were formerly supposed to contain
much nourishment. It is now known that their sole
claim to value lies in their action as a stimulant. In casesPOSITION OP ALCOHOL
75
of sickness they furnish the best amount of stimulation
with the least amount of harm, but must never be con-
sidered as a substitute for nutriment.
Alcohol.
Relation of the study of alcohol to physiology. — Owing
to the fact that the use of alcohol is so widespread among
men, and that this use has been so productive of evil, many
societies have been formed throughout the country to
fight this tendency. One of the objective points of the
work of these societies has been to inform the public of
the effects of alcohol upon the human system. To this
end, through legislation, they have made the study of the
effects of alcohol a required part of the course in physi-
ology in the public schools of the country.
Since the science of physiology has as its sole object
the explanation of life processes, so far as the most careful
experiments of competent investigators have made such
explanation possible, we shall consider the effects of alcohol
upon the human system from the following view points
only:
(a)	Its proper position among the substances which
affect the action of the body organs.
(b)	The statement of the effects, direct or indirect, which
its use has upon the action or structure of the organs of the
body.
In the present chapter wTe shall consider alcohol mainly
from the first view point. The statements concerning
these effects represent the views of highest authorities.
Position of alcohol. — Much time has been spent in try-
ing to answer the question as to whether alcohol is a food76
FOOD ACCESSORIES
or a poison. The final answer to this question must rest
upon experiment, and experiments have been conducted
in the past and are still being conducted to establish the
true answer to this question. The results of the past
experiments and the questions involved are outlined in
the following paragraphs:
A poison is defined by the “ International Dictionary ”
as “ any agent which, when introduced into the animal
organism, is capable of producing a morbid, noxious
or deadly effect upon it.”
A food we have defined (see p. 51) as “ anything which
we eat or drink which nourishes the body.” That is to
say, anything from which tissue may be built, or energy
produced without harm to the tissues of a person in a healthy
condition.
The experiments of reliable investigators have estab-
lished the following facts concerning this question:
First. Small quantities of grain alcohol (the amount
varies with the individual) may be completely oxidized
in the body, and by such oxidation energy is produced
exactly similar in kind and effect to that produced by the
oxidation of fats and carbohydrates.
Second. Small amounts of alcohol may be substituted
for equivalent amounts of fats and carbohydrates in the
diet without direct harm to the tissues, and when so substi-
tuted they protect the other nutrients of the body in practi-
cally the same way as do the nutrients for which they are
substituted.
Third. Small amounts of alcohol are absorbed directly
into the blood, that is, require no preparation for absorption.
Fourth. In large amounts alcohol acts as a drug, is
always a poison, and causes direct harm to the tissues.POSITION OF ALCOHOL
77
Fifth. As ordinarily taken, alcohol is not intended as
a food, but for its exhilarating effect upon the nervous
system. Such use is always attended with the danger of
forming habits of excess.
Sixth. Alcohol is not necessary to people in health, and
leads readily to habits of excess. In excess its effects are
far worse than excess of ordinary food.
Seventh. Compared with ordinary food it is very expen-
sive. The amount of alcohol necessary to produce a given
amount of energy costs several times as much as the equiv-
alent amounts of fat and sugar in ordinary foods.
Eighth. In small amounts, it increases the flow of
digestive fluids and may be beneficial in sickness or
disease.
These results teach us that we may consider small
amounts of alcohol as true fuel food, even though it never
acts as a tissue builder. Further, its use in small amounts
does not necessarily mean immediate damage to our sys-
tems, that is, does not injure the tissues directly. Under
these conditions then it is not a poison, but a true fuel food.
The results, however, point still more strongly to the fact,
first, that its use is unnecessary, since other foods furnish
the same amount of energy at a much less cost and at the
same time are attended with none of the dangers of acquir-
ing habits of excess; second, that this habit which alcohol
encourages is a very real and great danger, and that on that
account its general use is to be discouraged. When we
indulge in too much candy we are made sick, and the sick-
ness tends to destroy our craving for such excess. When
we indulge in too much alcohol the effect is not only to
poison the system but to increase the craving. It is the
fearful effect of this habit and the excesses it encourages78
POOH ACCESSORIES
that should make us avoid this unnecessary and costly
food, and on this point all investigators agree.
In stating the position of alcohol, then, we must bear in
mind that while it is definitely established that alcohol in
small quantities must be considered as a fuel food, it is
equally well established that in large amounts it is a poi-
son; that while it induces a flow of digestive fluids, and
thus hastens digestion, its use for such purposes in a healthy
person is unnecessary; that large amounts stop the action
of the juices and thus really delay digestion; and finally
that its tendency to encourage habits of excess makes it an
extremely dangerous substance to take into our systems
except under competent medical advice. If both sides of
the above statement are kept in mind it will be seen that
in determining the true position of alcohol the investiga-
tors have taught us a very forcible argument for total
abstinence.
In fact, most of the great evils resulting from the use of
alcohol have their beginning in the fact that many people
who begin by its moderate use feel no evil effects, what-
ever, as would be the case if alcohol were a true poison in
small amounts. The use, however, soon becomes a habit,
the habit demands ever increasing amounts or the stimu-
lation is no longer felt, the increased amounts soon pass
the danger point, and all the effects of alcohol poisoning,
with its attendant moral evils, follow. We might also sup-
pose that if the amount was always limited no evil results
would follow. The fact that people in general use alcohol
for its stimulating effects and not for food, interferes with
the safety of this view. In other words, while it is probably
true that a small amount taken daily would be oxidized
and produce energy without harm, it is a peculiarity ofCOMPARISON OF ALCOHOL WITH ORDINARY FOODS 79
nerve affecting substances that unless the amount be reg-
ularly increased the nerves fail to respond after a time,
and hence more must be taken, or the object sought fails of
accomplishment, the taker ■ must increase his amount or
lose the pleasing effect on himself. In this way the moder-
ate drinker is often unable to remain moderate. Further,
the safe amount is not the same for all individuals. It
varies with the individual, the time of taking, the physical
health, the presence of other foods, and all these influences
make it almost impossible to prescribe a safe amount for
even single individuals except for limited times, as in
sickness.
In fine, all that we learn of the value of alcohol as a food
and its action on the nerves unites to make more clear the
uselessness and danger of this substance to the ordinary
healthy individual.
Comparison of alcohol with ordinary foods. — Like fats
and carbohydrates, alcohol contains no nitrogen. In com-
parison with other foods, therefore, it is associated with
the above named nutrients as a fuel food as distinct from
proteid or tissue building food. A comparison with fats
and carbohydrates shows many similarities and differ-
ences. Some of the most important follow.
. Before ordinary food can be used by the tissues it must
be changed by digestion. Alcohol is a fermented product,
and the process by which it is made fits it to be directly
absorbed by the blood and used by the tissues without
change. A food of this form is said to be predigested.
The speed with which this absorption takes place and the
alcohol reaches the tissues is one of its most valuable prop-
erties as a medicine.
Fats and carbohydrates can be transformed into body80
POOD ACCESSORIES
fat and stored in the body for future use. Alcohol is never
stored for any length of time in the body, but is oxidized
at once. On this account fats and carbohydrates can
serve as sources of reserve energy, alcohol, never.
Compared with the other fuel foods, the amount of
energy that a given amount of alcohol can produce when
completely oxidized is shown in the following table:
	Alcohol	Proteid	Fat	Carbo- hydrate
Fuel value per pound				
in calories	3130	1S20	4040	1820
In other words, the complete oxidation of a given amount
of alcohol produces 1.73 times as much energy as the same
amount of proteid or carbohydrate and 0.73 as much as
the same amount of fat.
In ordinary digestion not all of the fat, carbohydrate,
and other nutrients which are taken in as food are actually
used by the tissue. The proportion of any substance
which actually reaches the tissues and is used by them is
called the availability of that nutrient. The availability
of the above four nutrients as actual energy producers for
the tissues is as follows: alcohol, 98%; proteid, 70%; fat,
95%; carbohydrate, 92%. In other words, 98% of all
the alcohol taken, up to the safe limit, is at once trans-
formed into energy as against 70%, 92%, and 95% of a
similar amount of proteid, carbohydrate, or fat.
One of the uses of ordinary nutrients is to preserve
other nutrients from oxidation. For example, the carbo-
hydrate which we eat keeps our body from using up the
proteid and fat of the body for its energy supply, or theCOMPARISON OF ALCOHOL WITH ORDINARY FOODS 81
fat eaten keeps the body from using up carbohydrate and
proteid, etc. This protective action is important, as
otherwise, in times of need, the tissues of the body might
be consumed in the demand for energy unless so protected.
It has been conclusively demonstrated1 that alcohol shares
this protecting power with fats and carbohydrates. It
was not until this important fact was demonstrated that
alcohol could be considered as a food, since, if this were
not so, its use might permit the destruction of tissues,
and then the result would be harm to the tissues in every
case. Compared with the other fuel foods, alcohol protects
fat better than carbohydrates, but carbohydrates and fats
both protect proteid better than alcohol. The fact that
carbohydrates and fats can both be stored and held in
reserve for this very purpose gives them an advantage
over alcohol in this respect.
All energy liberated by fuel foods is ultimately trans-
formed in the body into either heat or muscular energy.
The availability of alcohol as a heat producer is greater
than either fats or carbohydrates, but its value as a pro-
ducer of muscular energy is not yet fully determined.
All experiments so far conducted indicate that its action
as a drug tends to interfere with its power in this direction
and make it far less valuable in this respect than either
fat or carbohydrate. To put a concrete case, a laborer
does not get from a given amount of alcohol anywhere
near the power of continued muscular effort that he can
from other fuel, owing to the fact that the drugging effects
which first stimulate the cells, and so weaken their later
1 See Sub-Report of Committee of Fifty, Vol. II., and Howell, Text-
Book of Physiology, 1906, p. 808.
eddy, piiys.—682
FOOD ACCESSORIES
efforts, make the total muscular energy produced less than
it would sometimes appear.
Alcohol as a drug. — The real value of ordinary foods
depends not only upon their tissue building or energy
producing power but also upon their effect upon the
organs of the body. All nutrients affect the action of the
organs more or less directly, and their effects may be
beneficial or harmful according to the conditions or amounts
taken. In small quantities, the effects of alcohol are not
harmful but in larger quantities it acts as a drug. A
drug is a substance which acts upon a special set of cells
either stimulating or retarding their natural action. As
a drug the effect of alcohol is ultimately to impede diges-
tion, dilate the blood vessels, cause loss of heat, and
destroy sensibility. In this respect it is like a powerful
narcotic, and to this narcotic action is due the bad effects
of large amounts. (See Narcotics, p. 86).
Alcoholic beverages. — Alcohol is produced by a process
called fermentation (see p. 102) in fluids which contain
starches and sugars. In this process these two carbo-
hydrates (starch and sugar) are ultimately changed into
carbon dioxide and alcohol. The form of sugar which is
most readily changed into alcohol is what we have called
glucose (see p. 54). This glucose occurs abundantly in
the juices of many fruits, and especially in that of grapes.
Such juices naturally furnish one of the chief sources of
alcoholic drinks or beverages. Drinks made by the fer-
mentation of such fruit juices are called wines, and while
the term is frequently restricted to fermented grape juice,
it actually includes other fermented juices, such as cider,
blackberry wine, etc.
Grains, potatoes, etc., are rich in starch. If a little of aALCOHOLIC BEVERAGES
83
grain, such as barley, is moistened and kept in a warm place
it develops a substance called diastase, which changes the
grain starch into glucose. Such moistened grain may be
heated after the change in the starch has gone on to a
certain point and the action stopped. When this is done,
the dried product with its diastase is called malt. If a
little of this malt is added to ground grain and the whole
mixture moistened, the action begins again and the starch
of the ground grain is in turn transformed into sugar. If
yeast is added, its action causes the mixture to ferment,
and the sugar becomes partly changed to alcohol. In
this manner grains, potatoes, and other starch-contain-
ing substances may be used to make alcoholic beverages,
and the liquids thus produced are called malt liquors.
Beers, ales, and porter belong in this class of alcoholic
beverages.
Finally, if we add yeast to the malted mixture of starch-
containing cereals, potatoes, etc., until all the starch and
sugar are turned to alcohol, and then distil this liquid to
remove some of the water, we get liquors of much higher
proportion of alcohol. Grain mixtures treated in this
way and distilled until they contain nearly 40% of alcohol
produce whiskey. Distillation of wine produces brandy.
Gin is produced by flavoring the distilled liquid of malted
gfains with juniper berries, while rum is produced by
distilling fermented molasses. All these liquors are rich
in alcohol, and are grouped together as distilled liquors.
The kind of grain or juice used and the flavors developed
determine the brands.
We may distil these liquors still further, and remove all
or nearly all the water and flavoring materials, and thus
obtain the alcohol in the free state or with only a small84
FOOD ACCESSORIES
proportion of water. Commercial alcohol contains 95% of
alcohol and 5% of water. Absolute alcohol is about
99.8% pure alcohol.
In speaking of alcohol in the preceding pages we have
reference to what is known as grain alcohol, cologne spirits,
or ethyl alcohol. There are many other forms of alcohol,
notably among them, methyl alcohol or wood alcohol.
This latter, however, has only a few of the properties of
ethyl alcohol and is a poison even in small quantities.
In summary, then, we may state that alcoholic bever-
ages may be classified as wines, malted liquors, or distilled
liquors. The range of alcohol in these forms varies. Wines
contain from 5 to 25%, malted liquors from 2 to 5%, dis-
tilled liquors from 40 to 50%. All contain various other
ingredients which are produced from the fruits and grains
in manufacture and are called on this account extractives.
In addition to these recognized alcoholic beverages there
is a fourth class of compounds which are sold under the
name of patent medicines. While it is true that many
drugs are insoluble unless alcohol is used, these patent
medicines often contain amounts of alcohol which are
out of all proportion to their use as solvents. In fact, the
exhilarating effects of many of these so-called cures are
due solely to the large proportion of free alcohol present,
and the taker is fooled into thinking that he is being cured,
when his “ cure ” is actually alcoholic exhilaration. He
would scorn to take a similar amount of alcohol in a wine.
Investigation exposed so many of these so-called medicines
that the government came to recognize their danger to
the public health, and in the Pure Food Law is a provision
requiring all medicines to bear a placard, on which must
be stated the exact amount of alcohol contained. PeopleSUMMARY OR CONCLUSIONS CONCERNING ALCOHOL 85
should make sure that their medicine list includes none of
these liquors in disguise, since their use may fasten upon
one a habit difficult to overcome and dangerous in the
extreme.
Nutritive value of alcoholic beverages. —The action of
alcoholic beverages is influenced by two factors, first, the
alcohol contained; second, the presence of extractives. It
is undoubtedly true that in certain cases the character of
the extractives may add to the harmful effects of the
beverage, but careful experiment has demonstrated that
in ordinary liquors this action of the extractives may be
neglected, and the harmful effect of the beverage may be
stated to be directly proportional to the amount of alcohol
which it contains.
Summary of conclusions concerning alcohol.— All inves-
tigators agree that while alcohol may be used in small
amounts by certain individuals, without necessarily caus-
ing harm, the amount varies so much with the individual
that it is impossible to fix a universally safe limit.
Because of its tendency to form habits of use, drinks
containing alcohol are much more dangerous for young
people to use than for adults.
Statistics from business corporations, life insurance com-
panies, and insane asylums all show that the use of alcohol,
except as prescribed in cases of sickness, is dangerous. In
other words, while inefficiency, insanity, pauperism, and
ill health may result from other causes, so marked is the
tendency of the use of alcohol to result in one or more of
these evils that its use is recognized in the business world
as a risk. For example, all corporations employing labor-
ers guard against it by stringent rules, and life insurance
companies increase the premiums on life insurance policies86
FOOD ACCESSORIES
for moderate drinkers and absolutely refuse to insure
habitual topers.
It is a very expensive form of fuel food, and all the energy
which its use supplies can be secured with less expense and
no danger from recognized foods.
Finally, its transition from a food in small amounts
to a poison in larger amounts is due to its action as a
narcotic drug.
Narcotics.
In the paragraphs on alcohol we have mentioned the fact
that alcohol in large amounts is a narcotic. Whether it is
a stimulant or a narcotic in small amounts is at present a
disputed point (see p.426). Narcotics is the class name
applied to substances which in small doses relieve pain
and produce a state of rest or sleep, but in larger amounts
may produce stupor, convulsions, and even death. The
sense of fatigue and pain is the method employed by the
cells of our bodies to inform us of their need for rest, or
relief from disturbing influences. Sometimes, as in cases
of surgical operations or painful diseases, it becomes neces-
sary to use narcotics to lessen this sensation of pain. In
such cases, however, the use is purely temporary and to
avoid greater evils. In cases of fatigue the best remedy
and the only one which will completely restore the tired
cells is natural sleep, or such change of occupation as will
give the overworked cells a chance to recover. While
the ordinary person never thinks of using chloroform or
chloral for overcoming the sense of fatigue, there are two
narcotics whose use is frequently resorted to; these, like
alcohol, soon result in the formation of habits which, ifTOBACCO
87
continued, may become dangerous to the health. One of
these narcotics is tobacco, the other opium.
Tobacco.— This very commonly used substance owes
its narcotic effect to the presence of a substance called
nicotine. In a pure state nicotine is a colorless liquid.
When heated in burning tobacco, the nicotine turns to
vapor and is given off in the smoke. Nicotine is a pow-
erful narcotic poison, two or three drops being sufficient
to kill a man. In ordinary usage very tiny amounts actu-
ally enter the system. Its action when taken into the
body for the first time is to produce sickness of the stomach
(nausea) and general discomfort. In this way (by the
nausea) the body tries to tell us of the presence of a
poison. If, however, the use is persisted in, the body
gradually becomes accustomed to its presence, the nerves
which tell of fatigue are gradually dulled, and a grateful
sense of restfulness is produced, which continues until the
effect of the narcotic wears off. It is this pleasing sensa-
tion that constitutes its danger, for when the effect wears
off the sense of weakness returns, and with it a desire for
more, which if yielded to often results in the introduction
into the body of amounts of nicotine great enough to
injure the tissues permanently. The fact that the body
is able to overcome the results of moderate use without
permanent injury, and that pleasant sensations accompany
such use, is responsible for the wide-spread use of tobacco.
Of the various methods of use those will evidently be
least dangerous which introduce the least amount of nico-
tine. In this respect, smoking, when the smoke is not
inhaled, is the least dangerous, and chewing, where more
or less of the juice is swallowed, most dangerous. Fortu-
nately, the frequent spitting which the chewing habit88
FOOD ACCESSORIES
induces renders such a method of use disgusting, and tends
to discourage the method. Cigarettes are made of shreds
of tobacco, and on account of their size may be quickly
smoked and with less immediate effect than a cigar or pipe.
The number required to produce nausea and their cheap-
ness make their use particularly attractive to the young.
This fact and the danger of habit forming make their use
by young people one to be guarded against by every means
possible. The governments of most states recognize this
fact by forbidding their sale to minors. In weakening the
cells the use of tobacco often prevents their full develop-
ment. In this way the building of tissues may be hin-
dered and the body fail to reach its full size. We call this
the stunting effect on growth. It is this stunting that
makes the use of tobacco very dangerous to young people
who are growing, and forms a sufficient reason for prohibit-
ing the use of tobacco to minors, were no other evils result-
ant from its use. In adults this effect is not so noticeable,
but in times of sickness, when the full strength of the cells
is required to resist the ravages of the disease, their weak-
ening by tobacco often lowers their power to such an extent
as to seriously hinder recovery, and a tobacco user may on
this account succumb to a disease which a non-user would
have no difficulty in conquering.
The cure of the tobacco habit is simply a matter of will
power. It naturally becomes more difficult the more
extensive the use. It is undoubtedly better never to have
formed the habit, and every experiment to determine the
effect of tobacco tends to show the needlessness of such use
and the advantage of abstinence.
Opium.— This is a much more powerful narcotic than
tobacco, and the knowledge of its evil effects and habitOPIUM
89
forming tendencies is sufficient to keep most people from
its use. The main danger in this respect comes from the
same practice as the forming of the alcohol habit through
the use of patent medicines. People suffering from head-
aches, or other painful maladies, sometimes seek relief in
medicines whose composition they do not know. These
may contain opium or other dangerous narcotics, and thus
the use becomes a habit without their realizing it. The
quickness with which these powerful drugs act and the
rapid degeneration which results, make such unknown
medicines a positive danger to the community. The best
way to avoid such evils is to never take any medicine
unless it is prescribed by a physician.VII. DIGESTION.
The purpose of food as shown in the preceding chapter,
is to furnish the cells of the body with material out of which
they may either make new cells or develop energy. It is
evident that a piece of beef, while it may contain all the
nutrients necessary to these two processes, is not in the
condition to be taken up by the tiny cells of the body and
used by them for these purposes. The beef also contains
matter which is of value neither as tissue building material
nor as fuel.
In general, then, all foods require special preparation in
order to fit them for use by the cells of the body, and the
steps included in such preparation are called digestive
processes or collectively, digestion. The processes of diges-
tion are of three kinds: First, the separation of a food
stuff into its nutrient and non-nutrient parts. Second,
the transformation of the nutrients into a liquid form
which the cell can absorb. Third, the elimination of the
non-nutrient matter. These three processes are prelimi-
nary to the use of food by any living cell, and while the
means employed must vary with the food, digestion is com-
mon to all living forms as the first step in the manufacture
of food into living matter and activity.
In man the processes of digestion take place in a part of
the body known as the alimentary canal.
The alimentary canal. — This canal is a continuous tube
extending through the body and having two openings, the
mouth at one end and the anus at the other. In some of
90THE ALIMENTARY CANAL
91
Plate I92
DIGESTION
the lower animals, such as the common earthworm, this
tube is almost straight and of the same diameter throughout
the body. In man it is very much twisted and doubled
upon itself, so that its entire length in our body is over
thirty feet. Its internal diameter in man varies at different
points, forming at such points
large cavities (mouth, stomach,
etc.), which are connected by
narrower portions (gullet and
intestines). At certain points
it gives out growths of various
sizes called glands (liver pan-
creas, salivary glands). The
main bulk of this tube and its
outgrowths is packed and coiled
in the abdominal cavity (see
plate I) of the body.
The tube is made up of vari-
ous kinds of tissue, muscular,
connective, epithelial, blood, and
nerve, arranged in such a way as
to form layers of varying thick-
ness. It is lined throughout its
entire length with a layer of epi-
thelial tissue called the mucous
membrane. The various parts of
the tract have been given spe-
cial names, and each part performs a distinct action in
the preparation of food for the body.
While there are many of these parts each having a special
kind of activity, their actions admit of classification under
general headings. For example, the parts known as glands
mi
Fig. 23 — The alimentary canal.THE BUCCAL CAVITY
93
all agree in secreting fluids called digestive fluids. These
fluids, owing to the presence of certain chemical com-
pounds called ferments, dissolve the nutrients with which
they come in contact. Other parts, by rolling about and
crushing the foods, cause them to become mixed with
these fluids, and thus be more readily dissolved. In gen-
eral, then, the functions of
the parts or organs of the
alimentary tract are to se-
crete digestive fluids, or
to dissolve nutrients, or to
crush and mix the nutri-
ents with the fluids. We
shall consider each of these
actions as they occur in
the various parts of the
tract.
Regions of the Alimen-
tary Canal.
The buccal cavity.—(See
Fig. 24.) This is the name
given to that part of the
tract usually called the
mouth. It is bounded in
front and at the sides by
the lips, teeth, and cheeks,
below by the tongue, and
above by the palate. The
bony part of the palate
Fig. 24 — Relation of the buccal and
nasal cavities to the esophagus; n,
brain cavity ; l, hard palate ; f, uvula ;
</, opening of eustacliian tube to ear ; e,
epiglottis ; 7c, tongue ; c, windpipe ; b,
esophagus.
which forms the roof of the
mouth and is called the hard palate, is extended back-
ward'into a soft part (the soft palate) with a pendant94
DIGESTION
called the uvula. The hard palate separates the buccal
cavity from the nasal cavity. .The uvula, which .can be
seen easily by opening the mouth and examining it with a
mirror, forms a curtain which guards the opening from the
mouth cavity into the pharynx or throat cavity. This
opening at the back of the mouth cavity is flanked on either
side by two swollen masses of flesh called the tonsils. All
these cavities are
lined with a thin
membrane which is
continuous with the
outside skin and of
the same kind of
tissue (epithelial).
This membrane con-
sists of a network of
cells carrying blood
vessels and nerves,
and secretes a liquid
called the mucus
which moistens and
lubricates the surface
of the cavities. It is
called the mucous
membrane.
Fig. 25— Teeth in place, showing place of nerve	niOllth. are
supply.
two structures, the
teeth and the tongue, which have a purely mechanical
action in digestion; while connected with the mouth
cavity are several outgrowths (the salivary glands) which
secrete and pour into it a digestive fluid called saliva.
The teeth. — (See Fig. 25 and Ex. XXXIV.) TheKINDS OP TEETH
95
teeth are hard bodies set at intervals in two semicircular
rows of sockets. These sockets are formed for the purpose
in the bones called the jaws. The teeth are surrounded by
a layer of flesh which we call the gums. Though hard like
bone they are not of the same structure but are actually
hardened masses of epithelial tissue. In the life of a man
or woman two sets of teeth are developed. The milk teeth
are the first to appear, and are shed early in life. The
second teeth, or permanent set, begin to appear before the
milk teeth are all gone, and gradually replace them. These
two sets agree in their origin and general structure but
differ in number. There are twenty milk teeth and thirty-
two permanent teeth.
Kinds of teeth. — (See Fig. 26.) Teeth of the same set
Fig. 26 — Kinds of teeth ; a, incisors ; 6, canines; c, bicuspids or premolars;
d, molars.
differ in shape. For example, in the permanent set there
are found in front, eight teeth, with broad chisel-shaped96
DIGESTION
edges. These teeth, four on each jaw, are called incisors,
and are shaped to cut up the food taken into the mouth.
On each side of the incisors is found a tooth with a sharp
point. These teeth, four in number, are called canines
from their resemblance to the teeth of a dog. The shape
of the canines fits them for tearing apart the food which
cannot be easily cut. Next behind the canines, two on
each side of the jaw, are teeth with two pointed surfaces.
These are called the bicuspids or premolars. They serve
the same purpose as the remaining six teeth on each jaw
which are called the molars. The twelve molars have
rough broad surfaces for grinding the food into small pieces.
The third molar on each side of a jaw sometimes does not
appear until after the twentieth year, and these teeth are
often spoken of as the wisdom teeth.
The milk teeth set differs from the permanent set in
having no bicuspids and only eight molars. The numerical
relation of the teeth is often represented by a fraction
whose numerator gives the number of each kind on the
upper jaw, and the denominator expresses the same rela-
tion for the lower jaw. Such a fraction is called a dental
formula. The permanent set formula for man is:
Dental formula =
2 + 2,1 + 1,2 + 2,3 + 3
2 + 2,1 + 1,2 +2, 3+3
= 32 teeth; or, for
half a jaw, dental formula =
2 + 1 + 2 + 3
2 + 1 + 2 + 3
= 16 teeth.
Milk teeth formula =
2	+	2, 1	+	1, 0 + 0, 2	+ 2
2	+	2, 1	+	1, 0 + 0, 2	+ 2
No animal knowm has more kinds of teeth than man.
The number on each jaw differs greatly among differentSTRUCTURE OF THE TEETH
97
animals. For example, in the rabbit the dental formula
is as follows:
j + i f i 2 + 2, 0 + 0, 3+3. 3+3 OG
Rabbit dental formula = ! + 1>0 + o; 2 + 2>3 + 3 = 28.
Structure of the teeth.—(See Fig. 27.) The part of a tooth
above the gum is called its crown. When a tooth is pulled
we find that the crowm is only a
small part of the entire tooth.
At the point where it enters the
gum it narrow's into a part called
the neck and extends below in one
or more projections called the fangs
or roots ivhich anchor it firmly in
its bony socket. The incisors and
canines have only one fang, the
bicuspids may have two, while
the molars may have as many as
five and as few as two.
The internal structure of all
human teeth is practically the
same. If we split a tooth in two, lengthwise, examina-
tion shows that the crown is covered with an external layer
of hard matter called enamel. This thins down at the
neck and is succeeded by a fang covering called cement.
The main bulk of the tooth is formed of a bonelike sub-
stance called dentine which surrounds a small central cavity
called the pulp cavity. During life this cavity is filled
with nerve fiber and blood vessels interlaced in a network
of connective tissue, called the pulp. It is lined with a
layer of epithelial cells which secrete the mineral matter of
which the outer layers are formed. This cavity has a small
EDDY. PKYS.- 7
Fig. 27 — Section of a tooth;
k, crown; e, enamel; n,
neck ; d, dentine ; /, fangs;
c, cement; p, puip cavity
inclosed by dentine.98
DIGESTION
opening at the base of each fang through which the nerves
and blood vessels communicate with the rest of the body.
The blood supplies the epithelial cells with the material
out of which they build the tooth, and the nerves probably
direct this process. In some animals the teeth grow
throughout life, but in man their growth ceases upon the
attainment of a certain size.
Digestive action of the teeth. —The teeth grind and
Pig. 28 — The tongue.
break the food into small particles, and thus prepare it for
the solvent action of the digestive juices. If the food is
not properly chewed its solution is necessarily delayed justGLANDS AND SECRETIONS
99
as a lump of sugar dissolves less quickly than the same
amount of pulverized sugar. The grinding action of the
teeth is called mastication.
The tongue. —(See Fig. 28.) The tongue is a soft, fleshy
mass of matter mainly composed of muscle with a cover-
ing of mucous membrane. It is fastened at its base to a
bone called the hyoid. Its tip projects forward, and is free
and very mobile. It has two functions, one mechanical,
the other sensitive in nature. As a sense organ it is the
seat of taste. As a mechanical organ it mixes the food
with saliva and pushes it into the proper position for chew-
ing and swallowing. It also serves as an indicator of the
health of the digestive system by becoming coated with a
“ fur ” or coat of yellowish mucus whenever the digestion
is impaired. Its healthy color is red.
Chemistry of Digestion.
Both tongue and teeth are merely mechanical agents
which prepare the food for the solvent action of a liquid
called saliva. This liquid is a typical digestive fluid in
that it is produced by a structure called a gland, and
exerts a solvent action through chemical change induced
by means of a contained ferment or enzyme. We shall
understand its action better if first we make clear what a
gland and a ferment are.
Glands and secretions. — (See Ex. XXXII.) Every cell
has the power of taking in food and giving off waste matter.
Evidently, then, the character of the food which a cell
takes from the blood determines, in a way, the kind of
waste which it shall give off. Sometimes the matter so
given off is a solid, and the result is the formation of hard100
DIGESTION
structures like the bones and teeth. When it is liquid or
semiliquid the matter is called a secretion. There are
many kinds of secretion given out by the various cells of
the body, and some of the secretions have an important
part to play in the action of the body. So important are
some of these secretions, in fact, that certain cells are set
aside whose sole work is their preparation. These special-
ized cells often become grouped together in certain regions,
and are then spoken of collectively as glands. A gland,
then, is merely a collection of cells whose special duty is
the giving off of a certain kind of secretion. To this group
of secretions belong all the digestive fluids, and the cells
or group of cells which secrete them are called the diges-
tive glands. In this group are included the salivary
glands, the pancreas, the liver, and the mucous membrane.
The form of tissue which furnishes most of these speci-
alized cells is the epithelial, and the simplest form of a
gland would be a layer of these cells so placed as to receive
food from the blood and give off their secretion. Such a
glandular structure is called a secreting membrane, and an
example is the mucous membrane which lines all parts of
the alimentary tract. (See Fig. 29.)
In so complicated a structure as the body, however, it
is necessary to economize space, and it is easy to see that
one of the simplest methods of doing this is to arrange a
layer in a simple fold or sac. In this form (see Fig. 29)
the secretion is poured out into the central cavity or
lumen, which, when filled, overflows through the mouth
or opening of the gland. Of such a character are the
gastric glands of the stomach, and the simple glands
{crypts) of the intestine. Evidently, the next step in
saving space would be to collect a series of these simpleGLANDS AND SECRETIONS
101
sacs about a common duct, and such arrangements are
called tubular, racemose or tubular-racemose according as
the separate elements are tubelike or branched or a com-
bination of the two. Such glands are called compound
2 0
Fig. 29 — Forms of glands; A and B, simple layers of mucous membrane cells;
1, a simple tubular gland ; 2, a forked tubular gland ; 3, a simple saccular gland
with duct, d ; 5, a saccular gland still more divided ; 6, a, compound racemose
gland with common duct, d.102
DIGESTION
glands, and in this class are the salivary glands, liver, and
pancreas.
Enzymes. — (See Ex. XXXI.) The secretions of the
digestive glands owe their power of dissolving nutrients
to the presence of certain complicated chemical compounds
called enzymes. The action of these enzymes is little
understood at the present time, but their effect is well
known. In general, they all agree in causing a chemical
change in a nutrient without being themselves consumed
in the process.
For example, if we add a little malt (see p. 83), which
contains the enzyme diastase, to some starch paste, the
starch is gradually changed to sugar, but the amount of
the diastase which causes the change is neither increased
nor diminished by the process.
Similarly, yeast added to a sugar solution causes the
sugar to break up into two simpler compounds, alcohol
and carbon dioxide, while the number of yeast plants is
not decreased by the process.
The name ferment was long used to describe these com-
pounds, and hence the action of yeast or bacteria was
described as fermentation. We now know that yeast and
bacteria produce their effect through definite chemical
compounds, and to all such compounds we give the name
enzymes.
The salivary glands. —(See Fig. 30.) There are three
pairs of these glands in man. They arc called the parotid,
the submaxillary, and the sublingual. All these glands
are of the compound, racemose type, and resemble bunches
of grapes in which the grapes are the sacs of secreting
cells, and the connecting stems are the hollow ducts. The
parotid glands lie in front of and under the ear on eachACTION OF THE SALIVAEY GLANDS
103
side of the head, and their ducts enter the mouth on the
inner side of the cheeks, just about opposite the second
upper molar teeth. It is the inflammation and enlarge-
ment of these glands that causes the disease known as
the mumps.
The submaxillary glands lie below the two halves of
the lower jaw, and their ducts open into the mouth at points
near the middle line of the under side of the tongue.
The sublingual glands lie under the mucous membrane
below the tongue and floor of the mouth. They open into
the floor of the mouth by many small ducts.
Action of the salivary glands. —Each cell of a gland
obtains its food sup-
ply from the blood
vessels, and out of
this food it manu-
factures the secretion
known as saliva.
Since the amount of
saliva needed to di-
gest the food we
eat varies with the
amount of food pres-
ent in the mouth, it
is evident that the
secretion of the cells
must be under con-
trol. In this respect
the glands are like a
factory whose output is regulated to meet the conditions of
supply and demand. In the case of the cells this control
is exercised by nerves whose general arrangement is indi-
Fig. 30 — Relation of salivary glands to the
mouth cavity.104
DIGESTION
cated in the diagram. (See Fig. 31.) The results of this
arrangement are as follows: The nerves may be stimu-
lated in several ways: by the contact of the food in the
mouth with the nerve ends, by the odor of the food which
stimulates the nerve ends in the nose, or by the stimula-
points. One impulse goes to the blood vessels and causes
them to dilate, and thus supply more blood and food.
The other impluse is given to the cells, and stimulates them
to greater secretive activity. The result of these two
directing impulses is an increased flow of saliva.
In a similar way, this threefold nerve connection also
accounts for the dryness of the mouth when certain con-
ditions are present. For example, fright, a bad smell, or
unpleasant taste, may start impulses along the nerve
which, instead of increasing the flow of blood and secre-
tion, may constrict the blood vessel and diminish the
secretive activity of the cells.
The saliva. —The secretion produced by the salivary
glands is called the saliva. The liquid found in the mouth
and constituting the spit is not pure saliva, but a mix-
ture of this liquid with a secretion of the mouth lining
called the mucus, together with a number of other more ox-
less solid elements. This mixture is a colorless, cloudy,
Impulse Controlling Size Of Blood
Tube And Amount Of Blood Supply
Fig. 31 — Diagram to illustrate mouth control.
Impulse Controlling Secretion common
Secreting
Cells
Gland Duct
tions of the nerves
at the brain end, as
by thought of food.
When the nerves
are stimulated in
any of these three
ways the impulse
they carry is com-
municated to twoACTION OF SALIVA
105
slightly alkaline liquid, with a ropy consistency due to the
mucin in it, and is often frothy from the presence of air
bubbles. Pure saliva is a clear, viscid, colorless liquid
consisting mainly of water and a certain enzyme called
ptyalin. It is this enzyme which is responsible for the
chemical action of saliva in digestion.
Action of saliva. —Of all the nutrients taken into the
mouth starch is the only one acted upon by the saliva.
If clear saliva is allowed to act upon starch it changes it
to sugar in exactly the same way that the diastase of
malt changes grain starch to sugar. This sugar is, of
course, readily soluble in the watery part of the saliva,
and thus the nutrient starch is forced in solution. That
this transformation is actually produced may be readily
demonstrated by chewing a little dry biscuit for a few
moments. The dry, tasteless mass soon becomes sweet
to the taste as its starch is changed to sugar by the saliva.
The chemical changes involved in this transformation
are rather complicated, but for our purpose it is sufficient
to state that they have their origin in the action of the
enzyme ptyalin, and we can, therefore, say that it is the
ptyalin which produces the change.
Experiments with starch paste and clear saliva (see
Exs. XXXV. a and XXXVI.) show that this transfor-
mation takes place best at the normal mouth temperature
(98° F.), while high temperatures (above 145° F.), or
very low temperatures (below 32° F.), or the presence
of strong acids or alkalies are sufficient to stop completely
the ptyalin action. Further, cooked starch is much more
easily changed to sugar than uncooked, and requires less
time for the action of the enzyme.
Saliva also aids digestion in a mechanical way by fit-106
DIGESTION
ting the food for swallowing. It is impossible to swallow
dry food, and the moistening with the saliva makes this ac-
tion possible, and at the same time acts as a lubricant and
allows it to slip down more easily. In fact, so short is
the time that food spends in the mouth in ordinary eating
that probably very little starch is changed into sugar
there, and the mechanical action, therefore, is the most
important. Saliva, as we shall see later, also aids in
producing the sensation of taste.
Summary op Digestion in the Mouth.
1.	The teeth grind the food into small particles and fit it for
rapid and thorough mixing with the saliva. (Mastication.)
2.	The tongue rolls the food about into position for mastication
and mixes it with the saliva.
3.	The saliva through the action of the ptyalin enzyme con-
verts part of the starch into soluble sugar.
4.	The saliva moistens and lubricates the food mass and thus
fits it for swallowing.
5.	The food leaves the mouth in proper shape for the action of
other juices and with part of its starch converted into sugar.VIII. DIGESTION (continued).
After the action of the mouth the food is ready to be
passed on to the stomach, where the next steps in diges-
tion take place. The first step in this process is the act
known as swallowing or deglutition. In this action the
Fig. 32— Swallowing or deglutition ; a, tongue ; b, pharynx; ct food bolus ; d, soft
palate; e, uvula ; fy epiglottis.
A shows food being pushed back by the tongue. £ shows position of uvula and
epiglottis as food falls into the pharynx.
uvula at the back of the mouth is raised automatically to
close the nasal passage, and the tongue simply pushes the
food backward until it falls or slips into the pharynx
cavity.
Structure of the pharynx cavity. — (See Fig. 24.) This
cavity is a conical bag just back of the mouth cavity and
about five inches in depth. It has seven apertures ; two
into the nasal cavity at the top, two into the ear (the
Eustachian tube openings), one into the mouth, one into
the windpipe (glottis), and one into the esophagus or
107108
DIGESTION
gullet. The glottis opening is protected by a sort of
trapdoor called the epiglottis, which is closed during
swallowing but open at other times to permit the entrance
and exit of air in breathing. The whole cavity is
lined with mucous membrane, whose secretion of mucus
moistens and makes flexible its walls. (Excess of this
mucus produces catarrh.) When the food is swallowed
the epiglottis is closed and the food falls upon the top
of the gullet. Sometimes the epiglottis does not close
quickly enough, and some food enters the air passage.
This produces choking,
that is to say, the for-
cible expiration of air
in the attempt to force
out the misplaced
food. If properly swal-
lowed, the food is
forced along the gullet
to the stomach.
The esophagus. —
The esophagus, or gul-
let, is the name given
to that part of the alimentary tract which begins at
the lower end of the pharynx cavity, and passing through
chest and diaphragm, enters the stomach. This portion
is in the form of a tube of about nine inches in length
and is formed by three layers of tissue, (^ee Fig. 33
and Ex. XLI.) The inside layer is a continuation of
the mucous membrane, and the secretion of mucus here
together with the saliva acts as a lubricator, making the
food slip down easily. The outer layer is formed of
muscles, while between this and the inside layer is a layer
Fig. 33 — Diagrammatic section of the wall
' of the esophagus ; -S'. Mserous membrane ;
L. M.y longitudinal muscles ; Cl Mcircu-
lar muscles ; M. AT., mucous membrane. Be-
tween the circular muscles and the mucous
membrane is a thin layer of connective
tissue.THE ABDOMINAL CAVITY AND CONTENTS 109
of connective tissue. The muscular layer is actually a
double layer, the inner fibers arranged in a circular man-
ner, while the outer ones are extended lengthwise of the
tube. By alternate contractions and expansions of these
muscles the food is forced along the tube to the stomach.
In this action the effect produced is similar to what hap-
pens when a ring slightly smaller than the tube is forced
along a rubber tube; it forces any bodies inclosed in the
tube in the same direction that the ring is moving. The
contractions of the circular and longitudinal bands of
muscles necessary to this action in the gullet have
received the name peristalsis. Through peristalsis of the
esophagus walls the food is always forced toward the
stomach, regardless of the position of the body. The
food undergoes no chemical change in either the pharynx
cavity or esophagus.
The Abdominal Cavity and Contents.
This large body cavity (seep. 29, and plate I), incloses
the greater part of the alimentary canal. (See Ex.
XXXIII.) It is separated from the chest at the top by a
muscular partition called the diaphragm, while its base is
formed of the basketlike bones of the pelvis. At the back
are the spinal column and the lower ribs, while the sides
and front are formed of thick sheets of muscle without
bone support. This cavity is lined throughout with a
very smooth membrane called the peritoneum. This
peritoneum is so arranged that it not only lines the cavity
but also, by extensions, forms folds which support the
various parts of the alimentary canal. These folds are so
arranged that the parts of the canal are hung in it like a110
DIGESTION
stone in a sling. (See Fig. 34.) The entire mem-
brane is lubricated by a watery liquid or serum, and
Fig. 34 — Intestine supported by a fold of the peritoneum (mesentery), showing
method of entrance of blood vessels.
for this reason it is called a serous membrane. It is
also richly supplied with blood vessels. The organs sup-
Fig. 35 — External view of the stomach, showing covering of peritoneum and
muscles ; 1, esophagus ; 2, cardiac end ; 3, pyloric end ; 4, small intestine ; 5, a
portion of peritoneum turned back; 6, longitudinal muscle libers ; 7, circular
muscle fibers; 8, oblique muscle fibers.THE STOMACH
111
ported by this membrane are the stomach and the many
folded intestines.
The stomach.— (See Ex. XXXIII.) This organ is a bag-
like swelling (see Fig. 35) of the alimentary canal lying
just under the diaphragm and across the upper part of the
cavity. The larger end is at the left
side of the body, and into its upper
surface the esophagus opens by an ori-
fice called the cardiac orifice. The right
end of the stomach tapers into a narrow
neck which is comiected with the be-
ginning of the small intestine by an
opening called the pylorus or pyloric
orifice. The whole bag is supported in
a fold of the peritoneum, whose two
parts come together at the lower edge
of the stomach and hang down like
an apron over the front of the ab-
dominal cavity. This apron is known
as the greater omentum, and is fre-
quently the seat of a great accumula-
tion of fat.
The walls (see Ex. XLI.) of the
stomach are composed of four coats or
layers. (See Fig. 36.) The outermost is
a serous membrane (the peritoneum
just mentioned). Next inside this is
a muscular coat composed of three
layers of muscle fibers. Then comes a layer of connective
tissue (the submucous coat), and lining the entire stom-
ach is the mucous membrane. The blood vessels of the
stomach enter between the folds of the peritoneum, and
Fig. 36 — Section of
stomach w a 11; n,
serous membrane
(peritoneum) ; l. m.y
longitudinal mus-
cles ; c. m., circular
muscles ; s. m., sub-
mucous coat; m.m.j
mucous membrane;
d, ducts in mucous
membrane.112
DIGESTION
branch into capillaries in the submucous layer. The mus-
cular coat is made up of unstriped muscle fibers, and varies
greatly in thickness at different parts. It is thickest at
the pyloric end. The outermost layer of fibers is a con-
tinuation of the longitudinal fibers of the esophagus and is
Fig. 37—Longitudinal section of the stomach; a, esophagus ; b, greater curvature;
c, lesser curvature; d, sphincter pylorus; h-i, small intestine. The ridges in
the stomach are folds of mucous membrane.
thickest over the curvatures of the stomach. The circular
muscle fibers of the esophagus are likewise continued, and
form the inner layer of muscle over the stomach. This
circular layer becomes thickest at the pyloric end. Between
these two layers is a very incomplete oblique layer whose
fibers radiate from the cardiac orifice. The thick ring of
circular fibers at the pyloric end is called the sphincter <
pylorus, and controls the entrance of food into the small
intestine.
6GASTRIC GLANDS
113
The mucous coat is a pink membrane which is not elastic,
and, though smooth when the stomach is fully distended,
is thrown into folds when the organ is empty. This mem-
brane derives its pink appearance from the blood vessels
underlying it and becomes much redder
during digestion, as these submucous
capillaries become distended with blood.
When examined with a lens the mucous
membrane is seen to be covered with
shallow pits. These pits are the mouths
of many simple tubular glands (the gas-
tric glands) which secrete the digestive
fluid of the stomach. This fluid is
called the gastric juice. Between these
special glands are the ordinary mucous
glands which secrete the lubricating
mucus.
Gastric glands. —These glands (see
Fig. 38) are simple tubes lined with
epithelial cells, formed by the folding of
the mucous membrane. Between suc-
cessive tubes project layers of the sub-
mucous capillaries. These capillaries
furnish the food out of which the cells
manufacture the fluids and ferments
used in stomach digestion. The cells
of these glands are of two kinds. Those which secrete
the enzymes, pepsin and rennin, are called the chief cells
to distinguish them from another group called the border
ceZlswhich are supposed to secrete hydrochloric acid. The
exact function of these border cells is still unknown. (For
arrangement of these cells see Fig. 38.)
Fig. 38 — A peptic
gland, from cardiac
end of stomach.
Very much magni-
fied. Ay central or
chief cells, which
make pepsin; B,
border or parietal
cells, which make
acid. [From Miller’s
Histology.]
EDDY. PHYS. — 8114
DIGESTION
Digestion in the Stomach.
Gastric juice.—The digestive action of the stomach is
due mainly to the action of the digestive fluid produced by
the gastric glands. This gastric juice is a thin, nearly
colorless fluid, consisting mainly of water and holding in
solution a small proportion of hydrochloric acid and two
enzymes, pepsin and rennin.
As a result of the acid contained it gives a red color to
blue litmus, and, since ptyalin can act only in an alkaline
medium, the saliva swallowed with the food can exert no
digestive action upon starch after the food is acidified.
The flow of the gastric juice begins with the entrance of
food into the stomach, and the flow increases with the
amount of food taken in. This flow may also be stimu-
lated by certain food accessories, by the alkaline saliva,
and by the flavor of the food. Normally, about three
quarts of gastric juice is produced by the glands in twenty-
four hours.
Digestive action of the enzymes.—Pepsin. This enzyme
has the power to transform proteids to a soluble form
called peptone. It acts only in an acid medium, and what
is known as peptic digestion is therefore the result of the
combined action of pepsin and hydrochloric acid. Peptic
digestion varies with temperature. Low temperatures
retard, while very high temperatures may stop completely
the action of the pepsin. Pepsin can be obtained in a
fairly pure state (see Ex. XXXV., b) and with the product
so obtained it is possible to prepare artificial gastric
juice whose action upon various foods can be observed
with great accuracy. (See Ex. XXXVII.) In such experi-
ments the artificial juice is allowed to act upon food prep-DIGESTIVE ACTION OF THE ENZYMES
115
arations contained in glass vessels. The results of such
experiments may be summarized as follows:
The proteid thus treated first swells and softens and
then is slowly transformed into a soluble form of proteid
known as peptone.
This peptone is the result of the combined action of the
pepsin and hydrochloric acid, the food proteids being
broken gradually into simpler and simpler forms, until
finally the soluble peptone results.
Peptic digestion is found to take place best at a tem-
perature of from 98° to 102° Fahrenheit.
The action of peptic digestion is confined wholly to the
proteid parts of the food, and does not directly affect
any other nutrient. In fat meats the albuminous pockets
are dissolved away in this action, and the result is the
liberation of the fat globules inclosed by the pockets,
but the pepsin does not change the fat in any way. In
similar manner starch grains held in albuminous pockets
may be liberated, but the starch itself is not affected.
All these actions of the pepsin and acid which are ob-
served in experiments with glass vessels take place nor-
mally under similar conditions in the stomach.
Rennin. (See Ex. XXXVII., b.) Gastric juice con-
tains a second enzyme called rennin. This enzyme plays
an important part in the digestive action of the gastric
juice upon the particular form of proteid found in milk.
When milk proteid (caseinogen) comes in contact with
gastric juice, the rennin enzyme causes it to coagulate
and separate from the rest of the milk as curd or casein.
This action is exactly similar to the curdling of milk by
acid when it sours. This separation of milk proteid by
the rennin prepares it for the action of the pepsin and116
DIGESTION
acid, and once separated, peptic digestion takes place as
with ordinary proteid. In children whose diet is almost
entirely of milk this rennin action becomes very important.
Hydrochloric acid. (See Ex. XXXTX.) Aside from
the aid which this acid gives in peptic digestion its pres-
ence is important in that it destroys the germs of fermen-
tation and disease, and probably dissolves some mineral
salts. Its action in destroying germs permits the food
to be stored in the stomach for some time without under-
going decay. In this way the stomach may act as a
storehouse of food. While proteids arc practically the
only nutrients digested in the stomach, it is also true that
some of the preparatory actions upon other nutrients take
place here. For example, the heat of the stomach is
sufficient to liquefy the fats of the food, and the digestion
of the proteid covers of starch and fat particles liberates
these nutrients and prepares them for future digestive
changes. Sugar and certain mineral salts may also be
dissolved here by the water and acid of the gastric juice.
The mixture of all these substances together with the
digested and undigested particles of food and the gastric
juice results in the formation of a milky fluid called chyme.
This chyme is from time to time forced out of the stomach
into the small intestine.
Mechanical action of the stomach. — Once in the
stomach the food is entirely cut off from the rest of the
alimentary canal by the contraction of the cardiac and
pyloric sphincter muscles. It was long thought that dur-
ing the period of stomach digestion the food was forced
in continuous circulation round and round the stomach
until sufficiently ground, and mixed with gastric juice to
permit of its discharge into the intestine. Later investi-SUMMARY OF DIGESTION
117
gators showed that while this mixing does take place the
method is somewhat different from what was formerly
supposed. According to these investigators, the stomach
cavity may be divided into two parts, in only one of which
does the actual mixing take place. The cardiac end of
the stomach or fundus shows almost no movement and
acts simply as a storehouse. The contractions start in the
middle of the stomach and run toward the pylorus end,
and only the portion in this end is actually in process of
mixing. These contraction waves are repeated at inter-
vals of two minutes or less, and finally reduce the food
in this end to a thin liquid. At intervals depending upon
the character of the food, the sphincter pylorus relaxes and
portions of the liquid mass are spirted into the intestine
while new material flows forward from the storage end to
take its place. When the food is liquid these relaxations
of the sphincter are very frequent, while for solid food
they occur at longer intervals. The time required for the
stomach to empty itself in this way is from two to five
hours after a meal. The exact time, of course, depends
upon the amount of food taken into the stomach, and ease
with which it may be broken up. After the stomach has
been empty for a certain length of time we feel a sense
of hunger, and this is the signal for a renewal of the food
supply. Meanwhile, the chyme which has entered the
intestine is undergoing further digestive changes.
Summary of Digestion in the Stomach.
1.	The food enters the stomach by the cardiac orifice, and by
the peristaltic action of the muscular walls is mixed with the gastric
juice.
2.	The hydrochloric acid stops the action of the ptyalin and the
digestion of starch, and makes possible the action of the pepsin.118
digestion
It also tends to kill bacteria and prevent fermentation and aids in
the solution of certain mineral salts.
3.	The pepsin converts proteid into soluble peptone, part of
which is absorbed directly by the blood vessels of the stomach.
4.	The rennin coagulates the proteid in milk and prepares it for
the action of the pepsin.
5.	The partly digested proteids, and starch together with the
undigested matter leaves the stomach as an acid mixture called
chyme.
Fig. 39 — The stomach and intes-
tines ; 1, stomach ; 2, duodenum ;
3, small intestine; 4, end of
ileum ; 5, caecum; 6, vermiform
appendix ; 7, ascending colon ;
8, transverse colon ; 9, descend-
ing colon ; 10, sigmoid flexure ;
11, rectum ; 12, anus.
part of the canal consists
Digestion in the Intestine.
While some starch is changed
to sugar in the mouth and some
proteid to peptone in the stom-
ach, the real seat of digestion
in the body is the small intes-
tine. Here all kinds of nutri-
ents are digested, and the final
preparation of the food for use
in the body takes place. Here,
too, the digestible - matter is
separated from the indiges-
tible. In short, even if there
were no preliminary mouth or
stomach action, the small in-
testine would be capable of
performing all the digestive
actions necessary to prepare
the food for the body.
The intestine. — All of the
alimentary canal between the
stomach and the anus is
known as the intestine. This
of a tube of varying diameter,THE SMALL INTESTINE
119
and to its different parts have been given appropriate
regional names. The part next to the stomach, and con-
tinuing for some twenty feet from the pyloric orifice, is
all included under the name small intestine. It varies in
diameter from two inches at the stomach end to less than
an inch where it enters the large intestine. By large
intestine we include the five odd feet of tube extending
from the point where the small intestine is considered to
end to the anus. This portion of the tract has a diam-
eter of from two and a half to one and a half inches. The
relative position, size, and general arrangement of the two
intestines arc shown in Fig. 39.
The small intestine. — (See Ex. XXXII.) The entire
part of the alimentary canal included under the name of
small intestine is held in place by a fold of the peritoneum
called the mesentery. This mesentery is about four inches
in length at its back edge where it is fastened to the spinal
column, and widens to twenty feet at its outer edge. The
intestine is hung in this fold like an arm i$ a sling, and
the outer edge of the mesentery and the intestine are very
much coiled and folded to confine this long tube to the
space adapted for it. The different regions of the small
intestine are indicated by three names. The twelve inches
nearest the stomach receive the name of duodenum, the
succeeding two fifths is called the jejunum, and the re-
mainder is called the ileum. These names are mainly
useful in permitting us to designate special parts of the
tube.
Like the stomach the small intestine has four coats (see
Ex. XLI.) or layers, an external serous layer (peritoneum),
next inside a muscular layer, inside of this a submucous
coat, and a lining of mucous membrane. (See Fig. 40.)120
DIGESTION
The serous coat is formed of the fold of the mesentery.
The muscular coat is in two layers, an outer one of longi-
tudinal fibers and an
inner of circular fibers.
The submucous coat of
connective tissue binds
together the muscle
and mucous membrane
layers just as in the
stomach, and receives
the blood vessels which
enter in a fold of the
mesentery. The mu-
cous coat which lines
the whole intestine is
pink, soft,	and filled
with minute vessels of
the blood	and lym-
phatic system. In the
upper part	of the in-
testine it	is perma-
nently folded into
ridges known as the
FIG. 40-Wall of small intestine; a, b, mucous valvuloe COnniveilteS,
membrane with projecting villi; c, d, connec- .
tive tissue layers (submucosa); e, circular WlllCll increase tile
muscle fibers; e', longitudinal muscle fibers; digestive Surface and
/, serous membrane ; <7, spaces between villi;	0
h,	opening of crypts or simple intestinal glands ; delay the	passage of
i.	blood vessels in villi.	., r ,	.. ,	T
the iood particles. In
the lower part these ridges disappear, and to the eye it
appears smooth. Examined carefully with a lens the
entire inner surface of the intestine is seen to be raised
in minute projections called villi, like the pile of velvet.THE LIVER
121
(See Fig. 41.) We shall consider the structure and func-
tion of these villi more in detail in our study of the absorp-
tion of food. (See p.
134.) Between the bases
of these villi are the
openings of tiny glands
called the crypts of
Lieberkiihn.
Digestive glands of
the small intestine. —
The glands which se-
crete the digestive fluids of
three in kind. Two of these
Fig. 41—Yilli on surface of intestine (much
magnified).
the small intestine are
are large and lie outside
of the intestine, being connected with it by means of a
duct. The other kinds of glands (the crypts) are very
numerous and embedded in the mucous membrane itself.
The two large glands are known as the liver and the pan-
creas, and the ducts from these two glands unite outside
the intestine and pour their combined secretions into
the intestine through a common duct at a point on the
duodenum a few inches from its connection with the
stomach. The secretions of these two large glands are
known as the bile and the pancreatic juice. The relation
of these two glands and the ducts is shown in the diagram
(see Fig. 42). The secretion of the crypts is called the
intestinal juice.
The liver. — (See Fig. 46 and Ex. XXXII.) This is the
largest digestive gland in the body. It is a dark red mass
located in the upper part of the abdominal cavity just
under the diaphragm, and partly overlapping the stomach.
The entire gland is covered with peritoneum, and is divided
by a fissure into two unequal lobes. Of these two lobes,122
DIGESTION
the right is much the larger. These two lobes are con-
nected on the under side by ducts which emerge from
each lobe and fuse into a common duct called the hepatic
Fig. 42—Relation of bile duct and pancreatic duct to stomach and intestine.
duct. On the under side is also located a small sac (the
gall bladder), in which the bile secreted by the liver is
stored until needed by the intestine. From this sac
another duct (the cystic), leads, and fuses with the
hepatic duct to form the common bile duct which opens
into the duodenum. Two large blood vessels (the portalTHE LIVER
123
vein and the hepatic artery) carry blood to the liver,
entering it on the lower side and breaking up inside the
Fig. 43 — The liver.
lobes into capillaries. These capillaries unite again, and
the blood passes out of the liver by the hepatic veins.
Fig. 44 —Two liver globules ; A, section across central hepatic vein ; B, section
splitting the central hepatic vein ; p, capillaries of the portal vein which break
up among the liver cells and finally unite in the central hepatic vein h.124
DIGESTION
The internal structure shows it to be composed of a mass
of many sided bodies called lobules. Each lobule (see
Fig. 44) consists of a mass of cells (hepatic cells) sur-
rounded by a network of capillaries. Branches of the bile
ducts also enter these lobules and receive the bile which
the hepatic cells secrete from the blood. Each hepatic
cell, therefore, is a trqe gland cell, and the combined secre-
tion of all these cells is collected in the branched ducts
and flows downward until it enters the common hepatic
duct. From this point it may take one of two courses.
Fig. 45 — The pancreas ; 1, central duct; 2, pancreatic duct; 3, stomach.
It either enters the common bile duct and flows directly
into the intestine or it backs up and accumulates in the
gall bladder. In this way bile is stored in this bladder
so that when necessary it may be called on to supply the
intestine with increased amounts of bile. In such case,
bile flows from the lobes of the liver and from the gall
bladder directly into the intestine. The bile is a thick
golden-brown liquid of bitter taste. About a quart is
produced daily, and performs very important functions in
digestion.
The pancreas.— (See Fig. 45 and Ex. XXXII.) ThisACTION OF THE PANCREATIC JUICE
125
organ, often called the sweetbread, is an elongated race-
mose gland of a pinkish-yellow color lying along the
greater curvature of the stomach. The cells of this
gland secrete a digestive fluid called the pancreatic juice,
which they pour into a central duct. This duct joins the
bile duct close to its intestinal opening, and bile and pan-
creatic juice mingle and enter the intestine by the com-
mon opening. (See Fig. 42.)
Pancreatic juice.— This is a watery secretion which
contains three very important enzymes. One (trypsin)
changes proteid to peptone, one (amylopsin) changes starch
to sugar, and the third (steapsin) acts upon fats.
Digestion in the Small Intestine.
The partly digested food or chyme which is forced from
time to time through the pyloric orifice of the stomach is
now ready for the action of the various digestive fluids of
the small intestine. (See Ex. XXXVIII.) The digestive
action which takes place in the intestine is, therefore, due
to the combined action of three digestive fluids, pan-
creatic juice, bile, ami intestinal juice. Rhythmic peris-
taltic movements of the muscle walls of the intestine
keep the contents of the intestine in constant motion,
and thus mix them with these combined juices.
Action of the pancreatic juice.— This juice owes its
digestive action to its three enzymes, trypsin, amylopsin,
and steapsin.
Trypsin. (See Ex. XXXVIII., a.) Trypsin is a more
powerful enzyme than pepsin and, unlike that, acts best
in an alkaline medium. Like pepsin, its action is affected126
DIGESTION
by temperature; it is more active at ordinary body tem-
perature (98-102° F.), while it is completely destroyed at
a temperature of 172° to 176° F. Its action upon proteid
is very similar to that of pepsin in that it converts pro-
teid into soluble peptone. It differs from pepsin in that
there is no swelling of the proteid under its action. The
process is rather an eating away of the proteid part, and
in this action the indigestible matter is left in its original
shape to be broken into fragments later by the churning
action of the intestine. It also reduces some albuminoids
to peptone. Most of the proteid nutrient of our food
receives its final preparation for absorption in the small
intestine under the action of this enzyme.
Amylopsin. (See Ex. XXXVIII., b.) Practically all
the starch of foods is digested by the action of this enzyme.
It is possible that some of the ptyalin of the saliva whose
action was stopped by the acidity of the stomach shares
in this digestive action upon returning to the alkaline
medium of the intestine. It would be impossible to
determine this definitely, since amylopsin and ptyalin are
apparently identical in structure and action, and in Ger-
man texts on physiology the name ptyalin is used for
both enzymes. The use of the two names is, however,
convenient in indicating their origin. Both convert starch
to sugar under similar conditions and thus transform it
into a soluble substance.
Steapsin. (See Ex. XXXVIII., c.) This enzyme (also
called lipase), which is extremely sensitive to temperature,
performs two distinct operations upon fats, saponification
and emulsification.
In saponification the fat is first split into glycerine and
fatty acids. These latter unite with the alkalies andACTION OF INTESTINAL JUICE
m
alkaline salt present, and form soaps which are, of course,
soluble.
In emulsification the fat is separated into tiny droplets
and surrounded by the fatty acid or alkaline material in
such a way as to prevent these drops from uniting again.
In this form they may be absorbed directly without further
action. Whether emulsification precedes saponification or
vice versa is still a disputed point.
In general, then, the pancreatic juice first transforms the
acid chyme of the stomach into an alkaline fluid through
the neutralizing action of the alkaline salts in the juice,
and then by the action of the three enzymes it converts
proteids, albuminoids, starches, and fats into substances fit
for absorption. Chyme is neutralized mainly by bile salts.
Action of the bile. — (See Ex. XXXVIII., d.) It has
been found by experiment that when pancreatic juice
is mixed with bile the emulsifying power of the steapsin
is greatly increased thereby. The alkaline f ile salts are
of aid in the splitting of fats and consequently in the
saponification. Its presence is also supposed to prevent,
to a certain degree, the putrefaction of foods, though this
antiseptic power is disputed by some authorities. Finally,
it takes a direct part in destroying the peptic digestive
action of the acid chyme. It not only neutralizes the acid
but also causes a precipitation or separation of the proteids
in such a form that they may be acted upon directly by
the trypsin.
Action of intestinal juice (succus entericus). — This juice
is produced by the simple glands known as the crypts of
Lieberkiihn. It is a yellowish liquid with a strong alka-
line reaction. This alkaline character it derives from
the presence of sodium carbonate, and the presence of this128
DIGESTION
alkali is important in aiding the emulsification of fats.
It varies in quantity in different regions of the intestine,
being scarce in the upper part and very plentiful in the
lower or ileac region. Its specific action is due mainly to
the presence of two kinds of enzymes. One, similar to
amylopsin, aids in converting the starches to sugar. The
other (invertase) converts cane sugar and other forms of
sugar to grape sugar so that all sugars and starches are
finally absorbed as grape sugar.
Summary.
1.	The food enters the intestine as an acid mixture called
chyme.
2.	It is mixed in the intestine with three juices, bile, pan-
creatic juice, and intestinal juice.
3.	The three juices transform the acid chyme into an alkaline
mixture called chyle.
4.	In this alkaline medium the various enzymes of the three
juices transform proteids and albuminoids to peptone; fats to
soap and emulsions; starches and sugars, to grape sugar.
5.	The action of the enzymes is aided by the mechanical action
of the muscle walls.
6.	The resulting mixture is then ready for absorption.
Digestion of the Large Intestine.
That part of the alimentary tract included under the
name of the large intestine is subdivided, like the small
intestine, into regions. These regions are termed the
caecum, the colon, the sigmoid flexure, and the rectum.
The caecum is the name applied to the saclike pouch
into which the small intestine opens at the top by an
orifice, guarded by a valve called the ileocolic valve. AtACTION OF THE LARGE INTESTINE
129
the bottom of this etecum is a wormlike extension a few
inches in length called the vermiform appendix. Inflamma-
tion of this appendix produces the disease known as
appendicitis. The appendix has no known function, and
is supposed to represent a remnant left in man’s develop-
ment. From the point where the small intestine enters
the caecum the intestine extends upward, along the right
side of the abdomen, across the cavity just below the
stomach and down the other side. These portions of the
intestine have the greatest diameter (2J inches) of any
part of the intestine and are called the ascending, trans-
verse, and descending colons according to position. At
the base of the descending colon is an S-shaped bend
called the sigmoid flexure, and from this a straight
portion (the rectum) proceeds to the external opening or
anus.
The large intestine shows the same number of coats as
the small intestine. The muscular layer, however, is not
uniformly developed but arranged in bands separated by
spaces where it is wanting. This arrangement gives the
tube a puckered appearance. The mucous coat has no
valvulae conniventes but is filled with closely set simple
glands similar to the crypts of Lieberkiihn.
Action of the large intestine.— By contractions of the
small intestine its contents are forced through the ileo-
colic valve into the large intestine. Before these con-
tents reach the large intestine, however, nearly all the
nutrient part has been digested and absorbed, and what
passes through the ileocolic valve consists mainly of indi-
gestible material called feces. These faeces consist mainly
of water, cellulose, elastic tissue, and mucin. With these
substances may be present some starch and fat. If this
EDDY. PHYS. —9130
DIGESTION
is the case the digestion and absorption is continued in the
large intestine until finally the residue is collected in the
rectum and then passed out of the body. There are no
distinctive enzymes secreted by the glands of the large
intestine, and what little digestion goes on there is due either
to secretions which pass through from the small intestine
or to bacteria. These bacteria induce fermentation, and
cause the putrefaction of certain parts of the faeces. The
principal function of the large intestine, therefore, is the
removal of indigestible material from the body. In case
of over-feeding the faeces will contain more or less nutrient
matter which the small intestine was unable to digest and
absorb.
Alcohol and Digestion.
Alcohol, in small amounts, stimulates the flow of gastric
juice and saliva, and this increased flow is accompanied
by an increase in the enzyme and hydrochloric acid con-
tent of these juices. Furthermore, whep taken in small
quantities, it is itself very quickly absorbed by the blood
vessels. Hence, the amount which remains in the stomach
at any given time is entirely insufficient to interfere with
the action of the digestive fluid upon the nutrients. From
these statements, then, it would appear that digestion
is aided by the use of small quantities of alcohol.
Wherein lies the danger? Aside from the danger of habit
forming, which is great and in itself sufficient reason for
avoidance of the use of alcohol, the fact that this flow of
juices is due to unnatural stimulation is the dangerous
feature of its use.
Natural foods stimulate the flow only to the extentALCOHOL AND DIGESTION
131
of a normal production of fluid. By normal production
we mean a flow sufficient to use up the materials out of
which the juices are made at a rate not faster than the
body can naturally supply them for the purpose. Since
the supply of this material remains fairly constant in
amount it follows that any unnatural stimulation will
cause this material to be used up faster than it can be sup-
plied, and periods of increased flow must, therefore, be
followed by periods of decreased flow. While tire occa-
sional use of alcohol, therefore, may, as in cases of sickness,
be of advantage, its continued use results ultimately in
decrease in supply of fluids.
Furthermore, the action of a stimulant is such that the
secretive cells soon fail to respond to a slight stimula-
tion, and hence the amount taken must be continually
increased or fail of effect, thus leading to such increase as
seriously to affect other parts of the body, such as the
nerve centers. Finally, in large amounts, the effect of
alcohol is to retard seriously the action of the fluids upon
the foods, and to injure the digestive organs. While, at
first, small amounts of alcohol may aid digestion, they soon
fail of effect, and tend to encourage habits of excess which
retard digestive processes and injure the body.
Alcoholic beverages also often contain other substances
which counteract the advantage of increased flow. Wines,
for example, contain acids, and while the alcohol may
stimulate the flow of saliva, this increased flow is of no
value since ptyalin cannot act in an acid medium, and
simply drains the body of valuable fluid. Since the alcohol
is mainly absorbed directly from the stomach its effect
upon the intestinal secretives is practically nil, and may
be neglected as a factor in digestive processes.TABULAR SUMMARY OF DIGESTION. (See Ex. XL.)
to
IS' utrient	Region of Alimentary- Tract, Where Digested	Digestive Fluid	Enzyme	Digested Product
Starch	Mouth (very little) Sm. intestine Sm. intestine	Saliva Pancreatic juice Succus entericus	Ptyalin Amylopsin Amyloptic enzyme	Grape sugar Grape sugar Grape sugar
Proteid	Stomach Sm. intestine	Gastric juice Pancreatic juice	Pepsin and HCl 1 Trypsin	Peptone Peptone
Albuminoids	Sm. intestine	Pancreatic juice	Trypsin	Peptone
Sugars other than Grape	Sm. intestine	Succus entericus	Invertase	Grape sugar
Pats	Sm. intestine	Pancreatic juice	Steapsin	Soap or emulsion
Soluble salts	Mouth, stomach, and intestine	Water of digestive juices		Salts in solution
Insoluble salts	Stomach	HCl of gastric juice		Soluble salt
Water	Requires no digestive action			
1 HC1 — hydrochloric acid.
DIGESTIONIX. ABSORPTION.
In the processes of digestion, food is broken up and
transformed in various ways into soluble substances,
such as peptone, sugar, etc. This food, however, is still
in the alimentary canal, and far removed from the cells
of the body which it is to repair and build up. In this
respect, then, the alimentary canal is like a big factory
whose whole action is centered in the preparation of food
products. A delivery system is necessary for factory
products to be made available for use, and, likewise, in
the body, a delivery system is necessary to distribute to
all parts of the body the food prepared by the alimentary
canal. In the body such a system is found in the blood
system, and it is evident that the first step necessary to
distribution of food is the transfer of the digested food
matter to the blood. In the business world such a trans-
fer of goods to a delivery system is called shipping. In
the body the processes which are concerned in transfer
of digested food to the blood are all included under the
name of absorption. The object of shipping and absorp-
tion is identical, namely, to get prepared products into
circulation.
Relation of the blood vessels to the alimentary tract. —
The transfer of food to the blood may take place in one
of two ways. It may pass directly into the blood vessels
which line the walls of the intestine and stomach, or it
may enter another set of tubes called the lymphatics.
These lymphatics are closed tubes like the blood vessels,
133134
ABSORPTION
and the food which they collect from the intestine they
convey, by a common duct, to a large vein at the base
of the neck, and place in the blood circulation at that
point. In either case the ultimate disposal of the food
is the same, namely, the putting it into the blood. The
knowledge of the relation of these collecting blood vessels
and lymphatics to the alimentary canal is important to
the understanding of the method of transfer.
In the study of the structure of the stomach and intes-
tine it was noted that beneath the lining of the mucous
membrane and separating it from the muscle layers was
a coat of tissue called the submucous layer.
Further, it was noted that blood vessels entered this
layer through a fold of the peritoneum and within the
layer spread out over the mucous lining in a many branched
network of fine tubes or capillaries. These tiny, thin
walled capillaries, which are thus in contact with the
mucous membrane in all parts of the intestine and stomach,
are the direct receivers of the digested food of these tracts.
These capillaries are all connected and finally fuse into a
single large vein called the portal vein.
Again, we may recall that the lining of the small intes-
tine was not smooth, except in appearance, and that the
surface of the mucous lining was raised in tiny projections
called villi. (See p. 120.) A lengthwise section of one of
these villi (see Fig. 46) shows the following structure:
Each villus is seen to be a tiny conical projection of the
mucous membrane (one fiftieth to one eighth of an inch in
length). The outside, or surface exposed to the digestive
fluids, is composed of a single layer of mucous membrane
cells, while the core of the villus is a loose network of
blood capillaries and connective tissue cells, together withABSORBENT VESSELS
135
one or more larger tubes or lymph capillaries. Like the
blood vessels, these latter are branches of larger vessels
in the submucous layer; and fluid which once enters the
lymph capillaries of the villus flows first into these larger
lymph vessels, while
these in turn are col-
lected and fuse into a
single tube about the
diameter of a goose
quill, called the thoracic
duct. This duct passes
up in front of the spinal
column and finally opens
into a large vein in the
neck under the left collar
bone. Owing to the
milky color of the fluid
which is in these vessels
and the absence of blood
they have received the
name of the ladeals,
and this name also
serves to distinguish
them from other parts
of the lymphatic sys-
tem. The blood capilla-
ries as already mentioned are collected together and finally
pour their contents into the portal vein. From this descrip-
tion it must be clear that the digested food of the stomach
and intestine in order to enter the blood and lymph must
pass through two very thin layers, namely, the single layer
of mucous cells, wrhich form the lining of the tracts, and
Fig. 46 — Section of a villus; a, layer of
mucous membrane cells which absorb food
and carry it to the tubes within ; &, an
artery ; c, blood capillaries ; d, a lacteal.136
ABSORPTION
the walls of the blood and lymph vessels themselves. The
manner in which this is accomplished involves a discus-
sion of certain peculiar physical processes known as osmosis
and dialysis.
Osmosis and dialysis.— Tie a thin membrane, such as
parchment or the skin which a butcher uses to hold sau-
sage meat, tightly about the base of a student lamp chim-
ney. Then, into the top of the chimney fit a cork with an
eighth of an inch glass tube passing through a hole in its
center. Before fitting in the cork and tube, fill the base of
the chimney with water in which has been dissolved some
colored salt, such as potassium bichromate. Then insert
the cork and tube. If the parch-
ment is tightly tied there will be
no leakage. Finally, suspend
this whole apparatus in a jar of
clear water so that the liquid in
the jar and the chimney is at
the same level. If these direc-
tions are carefully followed,
the result will be an apparatus
such as is pictured in Fig. 47,
and the liquid in the chimney
will have a reddish-yellow color.
(See Ex. XXIX.) If we let this
apparatus stand for a few hours,
at the end of that time we shall
Fig. 47—Dialyzer.
be able to observe two curious
results. First, the level of the liquid in the chimney will
be higher than that in the jar; second, the clear water in
the jar will have become colored like that in the chimney.
Now it is evident that what has happened is not due toOSMOSIS AND DIALYSIS
137
ordinary leakage from the chimney, for while such explana-
tion might account for the similarity in color of the two
liquids, it would not account for the rise in liquid in the
chimney. What has happened is as follows:
In some way, the soluble coloring matter in the chimney
has passed through the membrane into the jar of clear
water, while some of the water in the jar has passed through
the membrane and into the chimney as evidenced by the
higher level in the chimney. This tendency of two liquids
to mix by passage through a separating membrane is called
osmosis and the experiment just described is an example
of osmosis.
If we continue the experiment and substitute for the
potassium bichromate solution other solutions, such as
grape sugar, white of egg, starch paste, peptone, etc., we
shall obtain varying results. When we test the water in
the jar we find that while grape sugar, peptone, and salt
solutions pass readily through the membrane into the jar,
white of egg and starch paste do not pass through it at all.
In other words, peptone, grape sugar, and salt solutions
are substances which mix with water by osmosis, while
starch paste and albumin do not. Soluble substances
which mix by osmosis are called crystalloids; those which
do not are called colloids. Grape sugar, salt solution,
potassium bichromate solution, and peptone are therefore
crystalloids; starch paste and albumin are colloids. Fur-
thermore, observations of the height to which the water
rises in the chimney in each of the above cases shows that
the rate of exchange is unequal, the water in each case pass-
ing through more rapidly than the crystalloid solution,
and experiment has shown that this rate of passage is deter-
mined by the character of the crystalloid employed. This138
ABSORPTION
relation of the crystalloid solution to the rate of exchange
is spoken of as the osmotic pressure of the crystalloid. To
say that potassium bichromate has a greater osmotic pres-
sure than peptone means simply that the rate of flow of
watqr through a membrane toward the potassium bichro-
mate solution is greater than it is toward peptone solution.
This process of osmosis enables us to separate a crystalloid
from a colloid by putting the mixture in a membrane bag
and suspending the bag in water. Under those conditions
the crystalloid solution will pass through the membrane
while the colloid will not. Separation of a crystalloid
from a colloid by osmosis is called dialysis.
Relation of osmosis to absorption in the body. — If, now,
we refer to the conditions in the intestine we shall note that
the relation of the digested food to the fluid in the lymph
capillaries and blood capillaries fulfils exactly the condi-
tions for osmosis and dialysis. In other words, the mucous
membrane and capillary walls furnish the thin membrane
separating two fluids, and by dialysis and osmosis the solu-
ble crystalloid substances in the intestine are separated
from the colloids and pass into the blood.
In this explanation of the relation of digestion to dial-
ysis is seen a new reason for the action of the various
enzymes upon foods. In other words, they not only
render them soluble but at the same time transform them
into crystalloids in order to make possible this action of
dialysis.
While the ordinary process of osmosis accounts for
the passing of crystalloid material from the alimentary
tract to the lymph and blood, it must be admitted that
it does not fully explain the disposal of proteid matter.
For example, if peptone were a simple crystalloid sub-WHERE NUTRIENTS ARE ABSORBED
139
stance like grape sugar we should expect to find it in
blood. The fact remains, however, that while we know
that peptone leaves the alimentary tract as peptone it
is not only never found in the blood as 'peptone, but when
injected into the blood it acts as a poison. Now, since
the only thing that intervenes between the alimentary
tract and the blood and lymph are the cells which com-
pose the membranes (mucous membrane and capillary
wall cells), it must follow that in passing through these
cells the protoplasm in them acts upon the peptone and
converts it into another and harmless form of proteid
and delivers it in this form to the blood. What the exact
nature of this protoplasmic action is, is still unknown. We
may, however, sum up the facts known as to transfer of
foods as follows:
Summary. Digestion by transforming foods to crystal-
loids makes possible their transfer into the blood and
lymph. This passage may or may not be accompanied
by changes in the character of the substances in solution,
according as the protoplasm does or does not act upon
them in transit. Before leaving this subject, however, it
must be stated that since the membranes concerned are
made up of separate cells, there may be some seeping
of liquid through the spaces between the cells. Such pas-
sage of liquids is called filtration, and is not to be con-
fused with osmosis. Since these spaces are exceedingly
minute, it is safe to say that the osmotic exchange
accounts for the transfer of the major part of the food.
Where nutrients are absorbed.— Proteid. While some
of the proteid is changed to peptone in the stomach,
the amount of peptone absorbed by the blood vessels of
the stomach is relatively small, as most of the peptone is140
ABSORPTION
carried into the small intestine with the chyme. In the
small intestine, therefore, is found the main seat of proteid
absorption. A little may pass into the large intestine and
be absorbed there, but this represents a very small amount
of the whole, less in fact than in the stomach under ordi-
nary conditions. Wherever it is absorbed it passes into
the blood and never into the lymphatics or lacteal system.
The reason for this selection is another of the facts not
yet explained. In passing through the mucous mem-
brane cells, as already mentioned, the protoplasm of those
cells breaks down the peptone into a simpler form of
proteid and delivers it in that form to the blood. While
this action of the protoplasm is not yet fully explained,
indications are that the change takes place in the mucous
membrane cells, and is due to an enzyme present in the
protoplasm. At present we know nothing of what this
enzyme is or its composition. In view of the poisonous
action of peptone upon the blood and the system, its
importance is evident.
Starch and sugar. By the time digestion is completed
in the small intestine, starches and sugars have all been
changed to grape sugar, and this substance passes into
the blood without change. While some may be absorbed
in the stomach and large intestine, the main seat of
absorption is the small intestine, as in the case of proteid.
Like proteid, grape sugar enters the blood, and not the
lacteals.
Fats. Fats are absorbed either as emulsions or soaps,
and instead of entering the blood they are the only form
of nutrient that passes into the lacteals. In passing
through the mucous cells of the villi the soaps are changed
back into emulsions so that the fat which enters the lac-WHERE NUTRIENTS ARE ABSORBED
141
teals is all in the form of an emulsion. It is the milky
character of this emulsion which accounts for the appear-
ance and name of this branch of the lymphatics. The
seat of absorption of fats is the villi of the small intestine,
and the lacteals, which receive it finally, put it into the
blood circulation at the
opening of the thoracic
duct into the left jugular
vein. (See Fig. 4S.)
Water and salts. These
nutrients are absorbed
directly into the blood
mainly in the small and
large intestine. The
Fig. 48 — Diagram of absorption, showing
Splits and tllG water pass how food reaches the blood circulation.
through by osmosis owing
to the osmotic pressure of salts contained in the blood.
In the case of water, the long time required by the food
to pass through the large intestine permits the great
bulk of water to become absorbed in this tract. This
is further evidenced by the fact that while the chyle
which enters the large intestine is very thin and watery,
the matter that is passed out of the rectum is mainly
solid. Very little water or salt is absorbed in the
stomach;
Alcohol. Alcohol is very quickly and completely absorbed
by the blood vessels of the stomach, very little ever pass-
ing into the intestine. This rapid absorption in the
stomach accounts for the quick effect of this substance
upon the body. It also has its beneficial side in that,
as a solvent of certain drugs, it enables them to be put
into circulation quickly.142
ABSORPTION
Summary of nutrient absorption. — In general, the seat of
greatest digestion is the seat of greatest absorption, namely, the
small intestine; the large intestine blood vessels taking up only
water and such food as may have escaped absorption in the small
intestine. Water, salts, grape sugar, and proteid enter the blood
capillaries directly and, except in the case of the proteid, without
change. All these nutrients are thus conveyed to a large vein
called the portal vein which carries them to the liver.
Fats enter the jugular vein by way of the lymph capillaries and
thoracic duct (the lacteals) in the form of an emulsion. They
do not, therefore, enter the liver like the other nutrients. The
nutrients which the portal vein carries to the liver are in the form
of water, dissolved salts, blood proteid, and grape sugar.
Action of the liver as an absorbent organ.— The nutri-
ents brought to the liver by the portal vein undergo still
further changes before entering that part of the blood
circulation which supplies the tissues with food. The
most important of these changes is the transformation of
grape sugar to a starch or glycogen (sometimes called liver
starch). Another change which takes place in the liver
is the elimination of poisonous compounds which may
be absorbed with the foods, and which, if permitted to
circulate to the tissues, would result in disease.
Glycogen.— The cells of the liver take up the grape
sugar brought to it by the portal vein, which itself divides
in the liver into many tiny capillaries. This grape sugar
the liver changes into a carbohydrate of the same compo-
sition as vegetable starch, hence the name liver starch.
This substance, like vegetable starch, may be changed
into grape sugar again by the action of ptyalin and other
diastasic enzymes. Acids also transform it directly
into grape sugar. It may be distinguished from starch
by its action on iodine solutions to which it gives a wine-OTHER SOURCES OE GLYCOGEN
143
red color instead of the blue color of ordinary starch.
The manner in which this glycogen is formed from the
grape sugar of the portal vein is explained as follows:
The protoplasm of the liver cells causes the sugar to lose
water, and the result is a chemical change resulting in
glycogen.
It has been demonstrated that blood which contains
over a certain proportion of grape sugar throws off this
excess through the kidneys, and it is thus lost to the
body as a nutrient. To avoid this loss the liver cells take
up this sugar and convert it into glycogen, which they
store in the body of the cell and feed out to the tissue-
supply system as it is needed. Before they feed it out
they reconvert it into grape sugar, and thus the amount
of sugar contained in the blood which supplies the tissues
never exceeds a certain proportion (0.1% to 0.2%). The
liver, therefore, acts as a storehouse of the carbohydrate
or energy supplying nutrient of the body. How it changes
the glycogen back to sugar is not fully known, but it is
supposed that the liver cells contain an enzyme similar
to diastase, which performs this action, as needed. When
we consider that the carbohydrates furnish the main
part of the energy of the body, this regulative action of
the liver becomes very important to the activity of the
body.
Other Sources of Glycogen.
Experiments have demonstrated that, to some extent, glycogen
may be formed also from proteids. For example, when a person
suffering from diabetes is fed upon purely proteid foods, all carbo-
hydrates being excluded from his diet, sugar is still formed.
This can be explained only on the assumption that it is manu-
factured from proteid. This manufacture is believed to take144
ABSORPTION
place when the peptone passes through the mucous cells; that
there a certain per cent of the proteid is split off as a sugar. If
that is the case, it is evident that this sugar may be transformed
in the liver to glycogen, and hence proteid is to a certain extent a
glycogen former. Some evidence seems to indicate that fat may
also be changed to glycogen, but this has' not been satisfactorily
proved at the present time.
Glycogen is formed in the muscles and in other parts of the
body than the liver. In all these cases, it is made from the grape
sugar in the blood and acts as a local supply of reserve energy.
The great storehouse, however, is the liver.
Poisons.— In the intestines are found many forms of bacteria.
These in turn may bring about fermentation of the food, and in
this way are produced certain injurious substances or poisons
which may be absorbed with the digested nutrients. In that
case they are brought in the blood of the portal vein to the liver,
and one of the important functions of this organ is to destroy
these poisons or return them to the intestine with the bile. If,
however, any interruption of the bile making of the liver occurs,
it loses its power to destroy and give off these poisons, and the
result is that they pass into the tissue-supply blood, and poison
the body. The results of the entry of these poisons into the
system usually manifest themselves in a coated tongue, loss of
appetite, headache, and dullness, and such a condition is called
biliousness. Certain drugs have the power to stimulate the liver
cells and cause a great outpouring of bile which carries with it
the poisons, and thus restores the liver to its normal action and
sometimes removes the source of trouble.
Defaecation.— The removal of the feces from the large
intestine results not only in the removal from the body of
indigestible material, but also has a vital connection with
the poison removing power of the liver. When the bile
formed in the liver is poured into the intestine by the gall
duct it carries with it the poisons which the liver cells have
separated from the blood of the portal vein. If these poi-
sons are allowed to remain in the small intestine they willHVGIENE OF DIGESTION AND ABSORPTION 145
again be absorbed, taken to the liver and so on indefinitely,
or until the power of removal of the liver is exhausted.
The only way in which this can be avoided is for the bile
which contains them to be carried into the large intestine
and out with the feces. It is this fact that makes regular
operations of the bowels necessary and makes constipation
dangerous. In severe cases of constipation stimulants
which increase peristalsis, and thus induce defecation, are
often given, but it must be borne in mind that all such rem-
edies are purely stimulants and should be resorted to only
under the advice of a physician. Regularity and care in
diet will make their use unnecessary in most cases.
Hygiene of Digestion and Absorption.
The body requires the digestion and absorption of a cer-
tain amount of each kind of nutrient daily. (See p. 70.)
If the amount of food taken by a person in a day is only a
little in excess of what is required, this excess is readily
taken care of by the action of the bowels. If, however,
too much is taken, the intestines become clogged and over-
worked, the food is not properly digested, poisons are de-
veloped and not removed, and all the evils of biliousness
and indigestion follow. For this reason a balanced,
moderate diet, taken at regular intervals, becomes abso-
lutely necessary to a satisfactory operation of the digestive
and absorbent system. Americans are especially prone to
over-eat, and many of the ills of our nation have their
source in this intemperance in eating.
EDDY. PHYS. — 10.X. DIGESTION IN THE LOWER ANIMALS.
The process of digestion (as a preliminary step in the
changing of food into living matter or protoplasm) is to
be met with wherever we find living cells. In man the
system has reached its most complex development, but if
we examine the various groups of animals, we find that in
every group the process of changing food into soluble sub-
stances by the aid of enzymes is quite as important and
necessary a step in metabolism as it is in man, though the
organs which perform this action may be much simpler of
structure.
It is interesting, then, to study the methods of digestion
in the lower forms, in a comparative way, from the light
they throw upon the development of the complex human
system. In the pages that follow a few types have been
selected to bring out the manner in which the human sys-
tem has been developed from lower types.
Digestion in one-celled animals.— In animals like the
amoeba and paramcecium (one-celled animals) the food is
taken into the cell in the form of solid particles. In
this respect the protozoans differ from all higher animals.
Digestion proper, therefore, in these animals occurs in
the cytoplasm of the cells themselves, and to distinguish
it from that of other forms is called intracellular digestion.
1 Chapters X, XIV, XVII, XX, XXII, XXIV, XXVII, and XXXI,
are intended for study in connection with a course in zoology. In con-
nection with laboratory work in zoology they furnish necessary data
for comparison. When not preceded by such laboratory work they
should be omitted.
146DIGESTION OF THE AMCEBA
147
While the methods of food taking of one-celled animals
differs greatly, some engulfing the food like the amoeba,
others having a distinct opening or mouth for the purpose,
and rows of cilia to direct the water currents toward the
mouth, the digestion proper is practically similar in method
in all. The amoeba will serve for a type.
Digestion of the amoeba. —With the ingested food is
always taken in some water which collects about the food
and forms a food bubble (food vacuole) in the cytoplasm.
Soon the cytoplasm around the food vacuole begins to
pour into it a secretion which contains hydrochloric acid
and probably an enzyme. This secretion is therefore a
sort of gastric juice. At the same time the vacuole is
kept in motion by the streaming of the cytoplasm. Under
the combined actions of this secretion and movement, the
solid particle is broken up and dissolved and is then ready
to be assimilated directly by the cytoplasm. In this way
new protoplasm is built up, and the animal increases in
size and strength.
In certain one-celled formsthe food vacuoles move in a per-
fectly definite path (see
Fig. 49) through the cy-
toplasm, and this move-
ment probably serves
to distribute the digested
food uniformly, as well as
to break it up. In such
cases the resemblance to
a primitive circulatory
system is evident.
Another way in which these one-celled forms differ is
in the different nutrients which they are able to dissolve.
F.V.
F.v.
Fig. 49 —Path of food particles in a para-
mcecium ; JST, nucleus ; M, mouth ; F. V.,
food vacuoles. Note the decrease in size
of the food vacuoles as the food is digested.148
DIGESTION IN THE LOWER ANIMALS
The amoeba, for example, cannot digest cither starch or
fats, while other forms have no difficulty in doing so.
This is accounted for on the supposition that the amoeba
can produce no starch or fat dissolving enzymes, while
the other forms do produce them.
Digestion in multicellular animals. — As an animal in-
creases in number of cells it becomes manifestly more
difficult for the food to reach the cells. In such animals
we should expect to find various adaptations to remedy
this difficulty.
Hydra and sponges. In the little animal known as the
hydra, and in sponges, the cells which form the animal are
arranged in layers about a central cavity. (See Fig. 50 )
Fig. 50— Longitudinal section of a Hydra; 6, bud which will form a younghydra;
ba, base by which it is attached when not creeping ; ia, mouth ; ov, ovary with
an egg > sp, sperm ary.
Idie cells lining this cavity are provided with hairlike
projections, or cilia, which are in constant motion, and
by their motion create a current which draws the waterDIGESTION IN MULTICELLULAR ANIMALS
149
into the cavity and causes it to circle about, thus bring-
ing the food which it may contain in contact with all the
cells which line the cavity. Many of these cells give off
digestive secretions which dissolve the food matter con-
tained in the cavity, and thus prepare it for absorption.
Such cells represent simple glands, and since the solution
takes place outside of the cells it is called extra-cellular
digestion, and the cavity becomes a sort of digestive
cavity or tract. All animals except the protozoans have
extra-cellular digestion. In the forms mentioned (hydra
and sponge) the waste is thrown out of the lining cells
into the same cavity from which they absorb the food
and is removed by the outgoing currents of water.
Worms. In these forms we have for the first time a
definite alimentary tract or canal. In some, this canal
Fig. 51 — Diagram of a longitudinal section of an earthworm, showing the
central digestive tube.
is open only at one end, while in others it forms a con-
tinuous tube running the length of the body with a true
mouth and anus. Such a continuous tube is found in
the earthworm. In this animal the slitlike mouth at
the front of the body opens into an expanded, muscular
walled cavity just back of it called the pharynx. By
the action of these muscular walls the food is sucked
into the cavity through the mouth, after it has been150
DIGESTION IN THE LOWER ANIMALS
broken up by the lip. From this pharynx, the food is
forced backward by true peristaltic movements of layers
of muscles in the walls of the alimentary canal until it
reaches the anal opening. In traversing this canal it
passes through several modified regions where it is acted
Fig, 52 — Cross section of earthworm, showing central intestine (1) and the dorsal
fold or typhlosole (T); D, dorsal bloodvessel; K, ventral bloodvessel; B. C.t
body cavity.
upon by special apparatus or secretions, and in this way
the digestible material dissolved out and absorbed through
the walls of the canal into the body cavity of the animal,
where a fluid similar to the blood in our bodies receives
it and distributes it to all parts of the body. In the
earthworm the regional division of this tract is of par-DIGESTION IN MULTICELLULAR ANIMALS 151
ticular interest in showing how the needs of the animal
have brought about adaptations of a straight tube to
digestive purposes. For example, the widening at the
pharynx is a modification to suit the earth burrowing
habits of the animal and permit the taking in of food of
a solid character. The saclike enlargement back of the
pharynx, called the crop, permits the food to be stored
for a time in the tract, and enables the animal to eat
more at one time than he can digest, and thus give the
parts a chance for rest. The thick walled portion back
of the crop, called the gizzard, is merely a portion whose
muscle layers are thickened to form a special grinding
organ for macerating the food, since the animal possesses
no teeth. Furthermore, all these modifications are made
in a tube of essentially the same general structure as our
own tracts, since it has an inner layer of secreting cells,
a middle muscle layer, and just outside it is a blood fluid
to receive the absorbed food, as in our own body. In
fact we might almost say that the earthworm’s digestive
tract is a living model of what a simple human tract
might have been before it was modified into stomach
glands, etc.
In the earthworm, therefore, we have a distinct advance
over the hydra and sponge in that certain cells have been
set aside to make a tube in which to digest the food for
the whole body. Further, the manner in which this tube
is constructed and modified indicates the way in which
the complex system of glands, cavities, and organs of the
higher animals could have been developed from such a
simple foundation to fit the needs of the animal. As we
pass from the earthworm to the higher animals the in-
creased amounts of food necessary to furnish a much152
DIGESTION IN THE LOWER ANIMALS
larger number of body cells, the character of the food
required by these cells, the opportunities for getting food,
and many other factors combine to produce these greater
modifications of structure. The nature of these adapta-
Fig. 53 — Section of a fish ; st, stomach ; i, intestine ; <7, gills. Davison, Zoology.
tions to increased needs, however, is not different in
kind from that indicated in the earthworm. A few
types will illustrate the direction which these modifica-
tions have taken in different animals.
Modifications in the digestive tube of the fish.— In this
animal, the skin that lines the mouth has produced out-
growths of a horny character called teeth, which prevent
the food from slipping backward when seized. These
are not like our teeth in that they are not set in sockets,
and are found all over the surface of the mouth. In the
mouth also, is found a fleshy motionless projection called
the tongue, which has in it nerves of taste. It has no
movement and does not aid in swallowing like ours. Back
of the mouth is a widened space or pharynx, in the sides
of which four slits are developed for letting in water to
the breathing organs or gills. From the pharynx theMODIFICATIONS IN THE FROG
158
food passes backward into a U-shaped portion or stomach.
From this stomach the remainder of the tube or intestine
passes at first forward and then turns and extends straight
backward to the anus. Opening into this forward tube
are a number of blind tubes called pyloric creca. There
is also a liver with a large gall bladder connected to this
part of the intestine by a duct. The cieca and liver secrete
digestive fluids which they pour into the alimentary tube.
The principal advance of this form of tract over that of
the worm is seen in the two devices employed to increase
the digestive surface, namely, by the folding and by the
extension of the walls at points to form external glands.
Other adaptations to the new needs are seen in the devel-
opment of a tongue and teeth in the mouth, and in the
increase in size of the stomach portion.
Modifications in the frog.— The most striking adapta-
tions in the digestive tract of the frog are the mouth
structures, the increased coiling of the intestine, and
the division of the tube into the stomach, small and large
intestine. In addition to the liver the frog also possesses
a pancreas.
The mouth of the frog contains no teeth with sockets, but
there is a single series of projections around the upper jaw
which serve, as those of the fish do, to prevent the escape
of the food when swallowing. In the frog, the tongue,
unlike that of the fish, is movable, and plays a distinct
part in swallowing. Unlike ours it is fastened at the
front with the tip pointing down the throat. The end is
sticky and can be extended to catch insects, which it snaps
back into the throat.
The large intestine ends in an enlarged space called a
cloaca into which open also the egg or sperm ducts and the154
DIGESTION IN THE LOWER ANIMALS
urinary tubes. The anus by which this cloaca opens in this
animal, therefore, is the place of discharge of other matter
besides indigestible food. The advance of this animal
over the fish is seen in the increase of digestive surface
Fig. 54 — Section of frog; t, tongue ; l. mouth; st, stomach ; p, pancreas : d. duo-
denum ; si, small intestine ; r, rectum, and b, bladder, opening into a common
cavity, the cloaca; ur, ureter, and ovd, oviduct, also opening into the cloaca;
k, kidney ; or, ovary ; lie, liver. Davison, Zoology.
by coiling, the placing of the teeth in relation to the jaws,
and the greater tendency to regional arrangement.
Adaptations in warm-blooded animals.—- In these animals
(with the exception of birds) are found for the first
time, true teeth in sockets. All such teeth may be
divided into four classes (incisors, canines, premolars,
and molars) as in man, and are placed uniformly upon
the jaws. The kind which predominates in any animal
is determined by the kind of food which it eats. Thus,
squirrels and rats have the incisors greatly developed for
gnawing, while the flesh caters are marked by the predomi-
nance of canines, and the grass eaters by the molars.ADAPTATIONS IN WAEM-BLOODED ANIMALS 155
In birds the teeth are replaced by horny sheaths, the
mastication of the food being provided for by a thick
walled portion of the digestive tract called the gizzard.
Fig. 55 — Skulls, showing teeth ; A, monkey ; B, horse, a grass eater (note number
of molars); <7, dog, a flesh eater (note the prominent canines) ; Z), rodent, «*
gnawer (note the prominent incisors).
While the other parts of the digestive tract are, in
general, similar to that in man, the stomachs of the various
forms show interesting modifications. The stomach of the
rabbit and other rodents, and of the carnivora, is almost
identical with that of man. The birds show a curious
adaptation to compensate for the lack of teeth and the
uncertainty of food supply. In these animals the stom-
ach is in three parts. First, at the base of the gullet
is a sac called the crop in which the food may be stored156
DIGESTION IN THE LOWEK ANIMALS
until needed by the body. By means of this organ the
bird is enabled to eat enough at one meal to last for some
time. Succeeding
this is a compart-
ment called the giz-
zard. When food is
needed it is passed
from the crop into
this gizzard which has
an outer wall of thick
muscles and an inner
horny lining. Inside
it, too, are stones and
other hard objects
which the bird swal-
lows for the purpose
of grinding the food
thoroughly to pre-
pare it for the ac-
tion of the digestive
fluids. When thor-
oughly ground up,
the food passes into
the third compart-
ment, or true diges-
tive stomach, where
the gastric juice is
secreted.
The ruminants or
cud-chewing animals
show still another curious modification. In these animals,
such as the cow, the stomach is divided into four compart-
Fig. 56 — Digestive tract of a bird; a, esophagus ;
b, crop ; c, proventriculus ; cl, gizzard ; e, liver;
/, gall bladder ; y, pancreas ; h, duodenum ;
i, small intestine; k, caeca; m, ureter; w,
oviduct ; o, cloaca.ADAPTATIONS IN WARM-BLOODED ANIMALS 157
ments. The first and largest is called the rumen, and into
this the grass mixed with saliva passes without mastication
as it is eaten. From this rumen the food is passed into a
second compartment
with a honeycombed
wall and called the
reticulum. After the
grazing is over the
food is returned to
the mouth from this
reticulum in the form
of boluses or. round
balls called the cud.
This cud is then thor-
oughly masticated by
the molars and finally
swallowed again. This
time it passes directly
into a third compartment called the psalterium, which is
ridged, and strains the food into the fourth compartment,
the abomasum or rennet stomach. In this abomasum the
actual digestion takes place, and here is secreted the gas-
tric juice. Such an arrangement as this is very evidently
an adaptation to compensate for the greater difficulty of
digestion of such food as grass and hay; but it is to be
noted that in none of these cases is the purpose and
general method of digestion essentially different.
In all, the various structures are working to one common
end, namely, the transforming of food into soluble matter.
And the ultimate method employed is the action of fer-
ments or enzjunes secreted by cells located in the walls or
connected with the digestive tract.
Fig. 57 — Stomach of a sheep (ruminant); e,
esophagus ; Ru, rumen ; R, reticulum ; Ps,
psalterium ; A, abomasum ; P, pylorus; P,
duodenum.158
DIGESTION IN THE LOWEK ANIMALS
In all the warm-blooded animals the intestines are
much longer and more coiled than in the cold-blooded
forms, and the time required for the digestion of vegetable
matter is responsible for the greater length of the intes-
tines in the herbivorous animals as compared with the
carnivora.
In conclusion, we may note that all modifications of
digestive methods are merely adaptations to meet special
conditions. These adaptations may take the direction of
specialized structures for producing digestive fluids or
preparation of food for the action of fluids, or they may
be directed toward increase in surface and the insurance
of more complete digestion. In every case, however, the
transformation of food into soluble matter as a prelimi-
nary to absorption is the end sought, and the ultimate
chemical means is the action of special compounds called
enzymes secreted by specialized cells.XI. BLOOD ANJ) LYMPH.
After digestion and absorption the food is ready for the
use of the cells. Absorption has placed this food in the
blood for delivery to the cells, and it is in its character as
a carrier of digested nutrients and other substances to
the cells that we are to investigate the functions and
structure of the blood.
Blood vessels. —The blood in our bodies is contained
in a set of tubes called blood vessels, and these vessels are
so constructed and connected as to form a completely
closed system. In other words, while the blood absorbs
matter into itself and gives out matter from itself to the
cells of the body, this exchange takes place through the
walls of the blood vessels, and never, except in case of
accident (bleeding), does the blood (as blood) escape from
the system of closed tubes. In this respect it is like a
train on a circular track. We may load goods into this
train and .take goods from it, but the train itself keeps
on continually and, except in case of accident, never
leaves the track. In our body the blood is kept in con-
stant movement within the tubes by the beating of the
heart and for this reason the blood vessels and their con-
tents are spoken of collectively as the circulatory system.
If it were not for this constant motion it would be mani-
festly impossible for food taken in from the alimentary
tract to reach the other parts of the body. Before con-
sidering the way in which this liquid is kept in circulation
we shall first consider its composition.
1591G0
BLOOD AND LYMPH
Blood structure. — We may readily obtain human blood
for study at any time by pricking the end of the finger
with a clean needle. (See Ex.'XLII.) If we examine a
little of the blood so collected on a flat surface we note
that the drop is not of the same color throughout. At the
edge of the drop it shows a yellowish color while the center
is dark red. With the compound microscope we find that
this difference in color is due to the fact that blood con-
sists of two distinct parts, a yellowish liquid called the
plasma, and a vast number of tiny round disks called
corpuscles. The red color of the blood is due to the pres-
ence of these corpuscles (red corpuscles). If we use the
higher powers of the microscope we find that these red
corpuscles are actually biconcave disks (see Fig. 58)
Fig. 58 — Blood corpuscles ; A, red corpuscles in rouleaux ; a, a, colorless corpus-
cles (X400); B, red corpuscles, highly magnified; C, view of edge; Z), three
quarters view; E, red corpuscle swollen with water ; Ft O, Ht distorted red cor-
puscles.
massed together in bands or rouleaux. At the same time,
we may be able to make out one or more colorless bodies
of irregular shape floating among these bands of disks.
These latter are called white corpuscles to distinguish
them from the other more numerous form. They differ
from the other corpuscles in having a nucleus. Both
kinds of corpuscles are cells, but the red corpuscles haveSTUDY OF BLOOD
161
lost their nucleus. The presence of these cells allows us
to consider the blood as a true tissue.
In general, then, blood is a tissue consisting of a yellowish
liquid (plasma) in which float a large number of non-
nucleated cells (red corpuscles) and a few nucleated cells
(white corpuscles).
Properties of blood.—If we mount a drop of blood on
a glass slide and allow it to stand for a few moments, at
the end of that time we note two distinct changes in its
appearance. First, it is darker in color, and second, its
surface is covered with a film which, when pricked with a
needle, shows an elastic consistency. This formation is
called a clot. If we examine the needle puncture on the
finger we shall find that it has ceased to bleed, and is
covered with a similar clot. If we wash this clot off the
finger, bleeding begins anew, and this gives us a hint of
an important use for this clot formation. An examination
with the microscope of the clotted blood on the slide
shows that it is actually a complicated network of slender
threads in which are entangled the corpuscles. Evidently
these threads must have their origin in the plasma, and to
understand their nature it is necessary to make a careful
examination of the plasma. For this purpose a larger
quantity of blood is necessary.
Study of blood. — Fortunately for our purpose, the blood
of all warm-blooded animals is practically identical in
structure with human blood. This fact permits us to use
fresh-drawn blood of a pig or cow. (See Ex. XLIII.)
First, place some blood in a bottle and let it stand for a
time exposed to the air. Put the rest in a basin and whip
it vigorously with twigs. Pour the whipped blood into
another bottle. While these two bottles are standing we
EDDY. PHY3. —11162
BLOOD AND LYMPH
may examine the twigs with which we did the beating.
These will be covered with a stringy elastic substance
which, when washed, is seen to consist of glistening white
strands or fibers. These fibers are composed of a sub-
stance called fibrin, and are identical in composition with
those which were noted in the clot of human blood.
Furthermore, if we test this fibrin with our nutrient tests,
we shall find that it responds strongly to one test only,
namely, that for proteid. In other words, fibrin is a coag-
ulated proteid which was present in the plasma of fresh-
drawn blood in a liquid state. If we examine the bottled
blood after it has stood for awhile we shall find the con-
tents of the bottle filled with fresh-drawn blood in a differ-
ent condition from the whipped blood. The former will
be found to have solidified into a clot of the same shape
as the bottle, and to be floating in a small quantity of
a straw-colored liquid (serum). The whipped blood will
show no clot, and is in the same condition as when we put
it in the bottle.
If we pour off the serum from the first bottle, and then
break the bottle so as to remove the clot entire, we may
examine this clot and study its structure. Such examina-
tion shows it to be made up of a solid mass of fibrin
fibers which inclose masses of corpuscles. The corpuscles
give it its color. The whipped blood fails to clot because
we have removed its fibrin by whipping. From these
observations we must conclude that the serum is plasma
whose fibrin has been removed, and that the presence of
this fibrin is necessary to the formation of a clot.
Our experiments thus far have succeeded in separating
only one of the constituents of blood plasma, namely,
fibrin.STUDY OF BLOOD
163
If our nutrient tests are applied to the serum which
was poured from the bottle containing the clot they fur-
nish the following interesting results:
First. They show it to be rich in proteid.
Second. They indicate small amounts of fats and grape
sugar.
Third. They show the presence of salts.
Fourth. They show that starch is absent.
Fifth. The serum gives a weak alkaline reaction.
These results justify the following conclusions in regard
to the composition of blood plasma:
First. The proteid tests show that the plasma contains
other proteid matter beside the fibrin, since that was
removed by the clotting, and that this proteid matter
does not coagulate like fibrin when the blood is removed
from the living body.
Second. The small amount of grape sugar in circulation
at any time is what we should expect from our study of
the formation of glycogen in the liver (see p. 142), while
the small amount of fat present shows that most of the
fat poured into the blood by the lacteals does not enter
into the general circulation but is used up by the living
tissues.
Third. The absence of starch is accounted for by the
fact that all the starch was changed by digestion into
grape sugar and enters the blood in that form.
Fourth. The presence of salts explains the osmotic pres-
sure of the blood, accounts for its alkaline reaction, and
serves to keep certain proteids in solution.
Finally, the presence of these particular nutrients shows
in an indisputable way that it is the plasma of the blood
which is the carrier of digested nutrients.164
BLOOD AND LYMPH
While our experiments show in a general way the com-
position of the blood, we must go to the chemist for a care-
ful analysis of its structure. Such chemical analyses show
the plasma to be composed of the following substances:
COMPOSITION OF PLASMA.
Plasma
' Water . fFibrinogen
Proteids . \ Paraglobulin (serum globulin)
C Serum albumin
Extractives
-
(Fats
! Sugars
1 Urea
[ Acids
Salts
f Chlorides
I Carbonates ! f
I Sulphates
I Phosphates j
Sodium
Potassium
Calcium
| Magnesium
L Iron
(_ Enzymes and unknown substances
Blood proteids. — Fibrinogen is fibrin in the liquid state. It is,
of course, absent in the serum. It varies in amount from .22% to
.40% in human blood, and to it is due the clotting power of the
blood. (The causes of clotting will be discussed under a separate
heading, see p. 169.)
Serum albumin is the form of proteid which is supposed to have
been formed from the digested proteids of the food. It forms 4.5%
of the blood of man, and is supposed to be the form of proteid used
in supplying the tissue cells.
Paraglobulin forms 3.1% of the blood of man. Its origin and
function are at present unknown.
The two proteids, then, whose functions are definitely known
are serum, albumin, and fibrin.
Blood extractives.—The fats and sugars are evidently absorbed
from the digestive fluids, and as nutrients are carried to the tissues
for use as energy producers.
Urea and acids are merely forms of waste matter given off byTHE SOLID MATTERS IN THE BLOOD
165
the cells to the blood, and their presence indicates another function
of blood, viz., the removal of wastes. (See p. 352, Excretion.)
Blood Salts. The mineral salts are probably absorbed in the
same way as the other nutrients, and their presence accounts for
the osmotic pressure of the blood as well as for the alkaline reaction.
This last is mainly due to the presence of the salt, sodium carbon-
ate. Certain proteids (notably the globulins) are kept in solution
by the presence of salts.
Water of blood. — This liquid is taken in by osmosis from the
digestive tract and other parts of the body, and the liquid character
of blood is mainly due to it. It forms 78% of the entire blood.
The solid matters in the blood- —Having now a general
idea as to the nature of the liquid plasma, let us turn to
the solid parts. We have already noted that blood con-
tains two kinds of corpuscles, the red and the white. In
addition to these solids, blood contains a third set of bodies
called blood plates. These three solids are only slightly
heavier than the plasma, and the movement of the blood
keeps them pretty thoroughly mixed with the plasma. A
knowledge of their origin and function is necessary to a
clear understanding of the blood functions.
Red corpuscles. These bodies to which the red color
of the blood is due, are shown by the microscope to be
biconcave, circular disks which are usually banded to-
gether in rolls like a roll of coins. (See Fig. 58.) They are
very small, having an average diameter of about .0077
millimeter, and while their number varies greatly with the
condition of health the average is five million to a cubic
millimeter of blood in males, and four million and a half
per cubic millimeters in females. When viewed singly
they are seen to be of a vellowish-red color, but in masses
they appear deep scarlet. This red color is due to a
pigment which they contain called haemoglobin. Haemo-166
BLOOD AND LYMPH
globin gives them an important function as carriers of
oxygen from the lungs to the tissues, and their physiology
is largely concerned with the action of this pigment.
In structure they are found to consist of a colorless
substance called the stroma, which* gives shape to the cor-
puscles, and in this stroma is deposited the hemoglobin
together with some water and salts. They contain no
nucleus, and are to be considered as cells which have been
modified to meet the needs of the blood.
Haemoglobin. This pigment consists of two chemical
compounds, haematin and a proteid body. The haematin
has the power when in contact with oxygen to combine
with it and to form an unstable compound which breaks
down when it reaches the tissues, and gives up its oxygen
to the cells. This power of haematin to unite with oxygen
is due to the iron which forms a large part of its chemical
composition. An interesting demonstration of the power
of the red corpuscles to take up oxygen is furnished when-
ever we mix blood with air. For example, shake violently
a tube half filled with blood. The increased redness of the
shaken mixture is due to the taking up of oxygen. In
the body the red corpuscles are brought in contact with
the air in the lungs, and the oxygen which they pick up is
carried by them to the tissues and there given up to the
cells as needed. It is evident, then, that one function of
the red corpuscles is to carry oxygen to the tissues. This,
however, is only one of their functions.
Haemoglobin forms compounds not only with oxygen
but also with carbon dioxide. This compound is called car-
bohemoglobin to distinguish it from oxyhemoglobin. This
compound is formed when the carbon dioxide is absorbed
from the tissue cells. In this way the red corpuscles areORIGIN OF RED CORPUSCLES
167
not only enabled to carry oxygen to the tissues but also to
remove part of the waste carbon dioxide from them.
In summary, then, we may say that the two uses to the
body of the red corpuscles are the carriage of oxygen to
the tissues and the removal of carbon dioxide from them.
The red corpuscles of man and of all mammals, except
the camel family, are biconcave, circular disks without
nuclei. In the camel family, they are elliptical in shape.
They vary, however, in size, and this variation often enables
experts to determine the character of blood stains in murder
trials. The red corpuscles of fishes, amphibia, reptiles,
and birds are oval in shape and usually nucleated.
FIG. 59—Various types of red corpuscles.
Origin of red corpuscles. — The fact that the human corpuscles
are without nuclei shows that they are cells which are in a degen-
erating state and hence cannot last long in circulation. Since168
BLOOD AND LYMPH
they are so necessary to the life of man it is evident that there
must be a constant renewal of the supply from some source. This
source is found in the red marrow of bones. Here are produced
nucleated, colorless cells, called erythroblasts, which multiply
and form daughter cells. These daughter cells finally produce
hsemoglobin in their cytoplasm, and thus form nucleated red
corpuscles. These lose their nucleus and finally enter the blood
stream in the form we have recognized as red corpuscles. The
old corpuscles are continually breaking down, and after undergoing
dissolution in the blood stream, the liberated haemoglobin is
discharged by the liver as bile pigment. It was once thought
that their destruction took place in the spleen, but there is no
evidence to prove this and it is now believed that this destruction
may take place anywhere in the blood stream. In a normal
human being the supply and loss is so balanced as to 'maintain a
fairly constant number of corpuscles in the blood.
The number of red corpuscles in the blood varies not only with
the sexes, as already noted (see p. 165), but also with other con-
ditions. They increase greatly with the altitude, supposedly to
compensate for the reduction in air supply. They often decrease
in number in disease causing a condition known as anemia. At
present the remedies for lack of blood corpuscles usually take
the form of tonics, that is, the giving of medicines, usually rich
in iron, which stimulate the formation of both erythroblasts and
haemoglobin. Exercise in the open air is a far better remedy than
any drug for this condition.
White Corpuscles.—These bodies are not nearly so num-
erous as the red corpuscles being only five thousand to
seven thousand per cubic millimeter. They are colorless
amceboid cells having a nucleus and no definite cell out-
line, moving about in the plasma in much the same man-
ner as an amoeba, which they strongly resemble. They
are of two kinds: those with granular cytoplasm are more
numerous and are called leucocytes ; those without granules
in the cytoplasm are called lymphocytes. The leucocytesTHEORIES OE CLOTTING
169
develop from bone marrow; the origin of lymphocytes is
uncertain. The leucocytes have various functions. Some,
called phagocytes, absorb disease-causing bacteria as then-
food, and thus protect the body
from disease. Others aid in the
absorption of fats and proteids
from the digestive tract. They
also aid in coagulation or clot
forming and finally help to
maintain the normal supply of
proteid in the blood plasma.
Blood plates.— These are cir-
cular or elliptical bodies smaller
than red corpuscles. They are
more numerous than the white
corpuscles, and have been shown
to be capable of amoeboid move-
ments. They disintegrate rapidly
when removed from circulation,
and hence little is known of
their structure. Their principal
function appears to be their
action in clotting, which we will discuss under that head.
Theories of clotting.— In the beginning of the chapter we
noted that blood removed from circulation tended to solidify
into a clot. We noted also that the foundation of this clot was
the coagulated fibrinogen or fibrin, of the blood plasma, and that
the failure of whipped blood to clot was due to the removal of
this fibrinogen by whipping. These experiments, however, merely
give us an explanation of what the clot is made of, namely, of
fibrinogen. The fibrinogen is made to coagulate and forms fibers
of solid proteid matter called fibrin, between whose strands are
caught the corpuscles, and to whose shrinking is due the squeezing
Fig. GO—A phagocyte (white cor-
puscle), A, engulfing a bacte-
rium, B; 1, 2, 3, steps in the
process.170
BLOOD AND LYMPH
out of the serum or defibrinated plasma. What the cause is that
suddenly produces this coagulation of fibrinogen, and the conse-
quent formation of fibrin fibers, the experiment failed to explain.
While this question has not yet been fully answered the theory
which is generally accepted at the present time may be stated as
follows:
The experimental facts upon which the theory is based are
three. First, the coagulation of fibrin is not due to exposure to
air, as it takes place with equal readiness when air is not present.
Second, it occurs only when there are present calcium salts, fibrin-
ogen, and an enzyme called thrombin. Third, the enzyme throm-
bin is not a part of the normal composition of plasma, but is
furnished under special conditions by the combined action of
the blood plates, the leucocytes, and the calcium salts.
The formation of the clot involves the following actions: First,
the breaking down of the blood plates and leucocytes in the
presence of calcium salts results in the formation of a chemical
compound or enzyme called thrombin. The enzyme reaction of
this thrombin causes the coagulation of fibrinogen into fibrin.
The reason that this action occurs only when the blood is removed
from the blood vessels is probably due to the fact that the blood
plates and leucocytes do not disintegrate in sufficient numbers
while the blood is in the body to produce much thrombin. It is
also possible that, in blood which is inclosed in blood vessels there
may be compounds among the unknown constituents of plasma
which neutralize the thrombin as fast as it is formed.
Conditions affecting the blood. — Since the blood fur-
nishes the oxygen and food supply of the tissues, it is
evident that it must be maintained in such a condition
as to perform these functions. Anything, therefore, which
interferes with the number of corpuscles, the presence
of haemoglobin, the oxygen content of the corpuscles, or
the food content of the plasma, is bound to interfere
with its normal functions. The conditions which have
been found to be essential to a healthy condition of bloodBLOOD TEMPERATURE
171
are an abundance of fresh air to secure sufficient oxygen
supply, plenty of sleep to prevent too great a drain by
the tissues upon the food and oxygen it carries, exercise
to allow the proper distribution of food and oxygen to all
parts of the body and the removal of waste, and nutri-
tious food for diet to maintain the proper food content
of the plasma. This latter condition also affects the red
corpuscles as well as the plasma, as without iron it is
impossible to form hremoglobin.
Amount of blood in the body.—- We might expect that the
constant giving up of food and oxygen by the blood would cause
its quantity to vary. So far as known, however, the loss of these
substances and the acquiring of wastes is so balanced that the
quantity is fairly constant, and in a healthy person amounts to
about one thirteenth of the total weight of the body. The amounts
in different parts of the body, however, vary. Roughly, it is
distributed in the following proportions:1
Heart and Lungs....................................25%
Liver................ ...	.........25%
The Skeletal Muscles	 25%
The Other Organs...................................25%
Blood Temperature.— Heat is constantly being pro-
duced in all of the animal tissues of the body, and at the
same time heat is being given off to the outside air from the
surface of the body. The amounts produced and lost
vary in different parts of the body. Now, the blood is
passing constantly from one part of the body to the other,
and hence necessarily carries warmth from the parts where
greater amounts of heat are produced to places where
there is less. In this way it tends to equalize the tempera-
ture in all parts of the body, and the temperature of the
1 Foster, Physiology.172
BLOOD AND LYMPH
blood is therefore the average temperature of the whole
body. In man the production of heat and the loss to the
outside of the body is so nicely balanced that the average
or blood temperature remains almost constant. This
being the case, any rise in this temperature indicates a
more or less serious disturbance in the body. In some
animals, such as the fishes, frogs, and reptiles, the loss of
heat is so great compared with the production that the
temperature of the blood is only slightly above that of the
surrounding air or water. Such animals feel cold to the
touch, and are spoken of as cold-blooded animals. They
are also called variable temperature animals, since their
average temperature varies with that of their surround-
ings. In birds and mammals, including man, the amount
of heat produced is greater and the loss so nicely balanced
that their average temperature is not only kept constant,
but at the same time is higher than that of their surround-
ings and does not vary with the outside temperature.
Such animals feel warm to the touch and are therefore
called warm-blooded.
In man the temperature of a healthy body remains at
about 9S° Fahrenheit, and though it may increase to 103°
or 105° in violent exercise, this is only temporary and it
sinks in a few hours to normal. A continued temperature
of over 100° is usually an indication of fever. This fact
enables the physician to discover at once the presence of
fever by taking a person’s temperature. In birds the
temperature is much higher than in man, these animals
having an average temperature of 105° Fahrenheit.
Summary of blood action.— The blood is usually spoken of as
the nutritive fluid of the body, since its main function is to supply
food or nourishment to the tissues. It has, however, other func-SUMMARY
173
tions as we have seen, and we may summarize its functions as
follows:
(а)	It carries to the tissues the food which has been prepared
by digestion, and introduced into the plasma by absorption.
(б)	It carries oxygen, which it takes from the air in the lungs,
to the tissues and there exchanges it for carbon dioxide which it
carries away. The red corpuscles are the carriers in this case.
(c)	It carries from the tissues to the lungs, kidneys, and skin,
the wastes of the body. Such wastes include carbon dioxide,
urea, acids, etc., and are removed from the body by the organs
mentioned.
(d)	It is the medium for the transportation of the internal
secretions of certain glands.
(e)	It distributes the heat produced in various parts of the
body and thus tends to equalize the body temperature.
Blood in other animals.— Since the function of the blood is the
distribution of food and oxygen, and the collection of wastes, it is
evident that animals composed of a single or few7 cells wall have
no need of such a fluid. In animals like the amoeba, and even
in animals of more complicated cellular structure, such as the
hydra, sponges, and jellyfishes, the digested food and the air are
brought in direct contact with the cells, and hence in these animals
there is no blood and no need of such fluid. As animals increase
in complexity, some of their cells are necessarily so placed that the
food is no longer brought in direct contact with them. In such
animals some system of distribution becomes necessary, and hence
most animals higher than those mentioned have some kind of fluid
that answers the purpose of the blood in our bodies. In many
this blood fluid is not inclosed in tubes and is without corpuscles.
It is to be compared rather with the plasma than with the red
blood of man. Crayfishes, crabs, and insects have such a fluid,
and it is colorless. In the earthworm the blood fluid is red, but
has no red corpuscles, the ha3moglobin being dissolved in the fluid
itself. It is only when we reach the vertebrate or backboned
animals that we find blood containing corpuscles specialized for the
carrying of oxygen and carbon dioxide. In the lower forms of
vertebrates these corpuscles are nucleated. Only in birds and
mammals do we find the non-nucleated corpuscle.174
BLOOD AND LYMPH
The Lymph.
In the pages preceding we have attempted to make it
clear that the materials required by the tissues are brought
to them by the blood, and that the wastes (carbon dioxide,
urea, etc.) given off by the tissues are finally conveyed by
the blood to organs which remove them from the body.
In the lower animals where the blood fluid is not inclosed
in tubes, and bathes all the tissues, this exchange between
tissue cells and blood fluid is a comparatively simple
matter. In man and the higher animals the fact that the
blood is contained always in closed tubes makes it less easy
to see how this exchange takes place. The explanation of
this exchange involves a study of another important liquid
of the body known as the lymph. All the tissue elements
of the body are bathed in this liquid. Furthermore, the
lymph spaces are connected by a set of tubes known as
the lymphatics, which permit the flow of the lymph from
one part of the body to another.
Origin and structure of the lymph. —Examination of
the composition of the lymph contained in the lymph tubes
and spaces shows that it is practically identical in struc-
ture with the plasma of the blood, and in fact it is sup-
posed to be plasma which has in some way passed out
of the blood vessels and become a tissue bathing fluid.
It was once thought that the lymph in the tubes and
the lymph in the spaces were in direct communication.
Recent investigation has shown that the lymph tubes are
closed as completely as the blood vessels, but the lymph
contained in them is identical in structure and origin with
that in the spaces. How has the plasma that is contained
in the blood vessels been able to escape through the wallsORIGIK AND STRUCTURE OF THE LYMPH 175
into the lymph spaces, and how does the lymph of these
spaces pass into the lymph tubes? The answer to this
question involves the whole mechanism by means of which
the tissue elements
receive their food
and get rid of their
wastes. In Figure
61 the relation of a
lymph tube, blood
tube, lymph space,
and tissue element
(cell) to one an-
other is indicated
in a diagrammatic
way. Let us ex-
amine this diagram for a moment, to see what explana-
tion it suggests for this problem.
The first point to be noted is that between the lymph
in the space and the plasma in the blood tube there is only
the thin membrane of the tube wall. Secondly, between
the lymph in the space and the liquid contents of the cell
is a second membrane, the cell wall. Finally, between the
lymph in the space and that in the lymph tube is a third
membrane, the lymph tube wall. If we recall what we
learned in regard to dialysis and osmosis (see p. 136) it
is evident that the above relation satisfies the conditions
necessary to those processes. By osmosis the dissolved
gases and some of the nutrients of the plasma together
with the water, pass through the walls of the blood vessel
into the lymph in the space, while in similar manner
the wastes in this fluid pass into both the blood vessel and
lymph tube. Furthermore, through the cell wall, the176
BLOOD AND LYMPH
nutrients of the lymph in the space are exchanged for the
wastes of the cell. In this manner then the plasma of
the blood tube becomes
lymph and is in turn passed
on into the tube, which
drains it away to other
parts of the body.
Another feature of this
exchange is seen in the
action of the corpuscles.
It will be noted that the
red corpuscles do not leave
the blood tube, but the
white corpuscles or leu-
cocytes are able to pass
through tiny spaces in the
walls. The reason for this
is that the leucocytes are
able to change their shape
and to push themselves
through holes too small
to let the rigid red corpus-
cles pass. Certain proteids
which are not dialyzable
also probably pass through
similar spaces in the blood
tube walls by filtration.
Another feature to be
considered in this diagram
is the disposal of the wastes.
If the liquids were station-
ary the exchange would go on until all reached the same
Fig. 62 — The lymphatic system; a, union
of left jugular and subclavian veins, and
point of union with the thoracic duct,
b; c, right jugular and subclavian veins,
and point of union of right lymphatic
system ; d, receptacle for food absorbed
from the intestine; the oval bodies are
glands.LYMPH FLOW
177
composition. The fact that the lymph tubes are continu-
ally draining off the liquid they receive, and that the
plasma is being constantly renewed, together with the vary-
ing activity of the cell, makes the composition of the three
liquids subject to constant change. It is this change that
gives a definite direction to the flow. While it is true that
some substances pass from the lymph tubes into the space,
and from the space into the blood tube, the prevailing
direction of flow is from blood tube to space to lymph tube,
and in this way most of the cell wastes are carried into the
lymph tubes. What then becomes of these wastes which
enter the lymph tubes?
The lymphatics. —The lymph tubes just referred to are
only the ultimate tiny endings of a branch-
ing network of tubes called the lymphatics.
From all parts of the body these tiny tubes
(lymph capillaries) are collected together, and
finally empty their contents into two large
trunks on each side of the body. These
trunks are of different size, the one on the
left side with its branches being much the
larger. It is this left branch that includes
the system of branches called the lacteals
(see p. 135). Both the right and left branches
open finally into two veins on the right and left
sides of the neck. In short, then, the waste
laden lymph is finally drained from all parts of
the body and poured into the blood again at
such points that the wastes must pass through
the lungs before entering the general circula-
tion and are finally eliminated by the kidneys.
Lymph flow. — Unlike the blood system, there is no
EDDY. PHYS. —12
Fig. 63 —Lymph
tubes, showing
valves, d.178
BLOOD AND LYMPH
apparatus for pumping lymph. The lymph vessels are,
however, provided with, valves which allow it to flow only
toward the veins. While the flow is irregular and slow the
pressure in the capillaries is sufficient to keep it in motion,
and the movements of muscles that press upon the tubes
also aid in driving it onward to the veins.
Since the lymph is laden with waste, it will be recognized
that anything which interferes with this flow is bound to
disturb the health of the body
by the over-accumulation of
waste in the spaces. Muscular
exercise is necessary, since it
maintains the lymph flow.
At irregular intervals the
tubes open into spongy bodies,
known as lymph glands or
nodes. These nodes are spongy
masses of fibers filled with cor-
puscles similar to the white cor-
puscles of the blood. They are
supposed to have two functions,
first, the removal of certain
wastes from the lymph, such
as bacteria, which the corpuscles
devour, and second, the produc-
tion of the corpuscles them-
selves. In other words, these
glands filter the lymph as it
passes through them, and tend to remove harmful bac-
teria in this way. Occasionally these nodes become over-
charged with poisonous material, and break down, forming
abscesses or boils.
Fig. 64 — A lymph node.SUMMARY OP THE LYMPH RELATIONS
179
Summary of the lymph relations. — From the preceding it is
evident that the most important action of lymph is to act as a
middleman for the exchange of nutrients and wastes between the
blood on the one hand and the cells on the other. Furthermore,
the waste matter is in turn taken up by the lymphatics, which
drain it off into the veins. In cases of dropsy the failure of the
drainage system to remove these wastes causes an accumulation
of lymph in the spaces, and often causes the body to swell to an
enormous extent. This relation of the lymph spaces and lym-
phatics also explains why medicines injected under the skin are so
quickly distributed to all parts of the body, the lymphatics taking
it up and distributing it to the tissues much more quickly than
if it had entered the system through the digestive tract.
Certain parts of the body pour into the lymph more of their
wastes and secretions than others, and this accounts for the varia-
tion in composition of lymph in different parts of the body, but
the foundation of the fluid is the blood plasma.XII. CIRCULATION.
Anatomy of the Heart and Blood Vessels.
Enough has been said in our study of the blood to make
clear that its function is the distribution of food and
oxygen to the tissue cells, and the removal of waste from
these cells. In other words, its purpose in the body is
distribution. Furthermore, it is clear that in order that
distribution be made to all parts of the body, the blood
must penetrate to all these parts, and its flow to and
from these points must be continuous. Such conditions
are satisfied by inclosing the blood in tubes of varying
size and structure, so arranged as to enter all parts of the
body, and connected in such a way that a pump placed
in one part of the system will force the blood in a constant
stream from the pump to these places and back to the
pump again. In our body this closed system of blood
vessels is spoken of collectively as the circulatory system,
while the pump which keeps the blood in motion is called
the heart. The various kinds of tubes are called veins,
arteries, and capillaries, according to the direction of blood
flow. Let us first examine the structure and workings of
the heart, or pump.
Anatomy of the heart.—(See Ex. XLVII.) This organ
is a hollow, muscular sac about the size of a large pear,
lying in the chest cavity just above the diaphragm, and
immediately behind the lower two thirds of the breast-
bone (see Fig. 65). It is shaped somewhat like a cone
180ANATOMY OF THE HEART
181
with the smaller end below and slightly to the left of the
breastbone. It is this position of the point that makes the
Fig. 65 —External view of the heart; L, L, lungs; RA, right auricle; LA, left
auricle; RV, right ventricle; LV, left ventricle; IVC, inferior vena cava;
SVC, superior vena cava; Ao, aorta; PA, pulmonary artery ; Tr, trachea.
beating more distinctly felt on the left side of the body,
but the bulk of the organ is in the middle of the cavity.
The heart is surrounded by a sac called the pericardium.182
CIRCULATION
This is conical like the heart, but the broad end is down-
ward and attached to the diaphragm. Both the inside of
the pericardium and the outside of the heart are covered
with smooth slippery membranes, while between these two
layers is a fluid which moistens the surfaces and lessens
the friction between them as the
heart beats. The heart is also lined
internally with fibrous membrane
while the walls between these outer
and inner membranes are composed
of a peculiar kind of striped muscle
tissue whose cells differ markedly
from the striped cells of ordinary
voluntary muscle. These muscular
walls contain nerves, blood vessels,
and connective tissue. Their thick-
ness varies markedly in different
parts of the organ.
Internal structure of the heart. —
(See Fig. 66.) On opening the
heart, it is found to be completely
separated into two distinct halves
by a longitudinal partition or
septum. Each of these halves,
which are right and left in posi-
tion, is divided by transverse partitions into an upper
and lower cavity. The cavities which are at the broad
end of the heart are thin walled and flaplike in appear-
ance. From a fancied resemblance to the ears of a dog
they are called auricles. The other two cavities and
the thick walls of muscle which surround them, make up
the bulk of the heart. These latter cavities are called
Fig. 66 — Relation of cavities
in one side of the heart.EXTERNAL. ANATOMY OF THE HEART
183
ventricles. In the partition between the right auricle and
right ventricle is an opening guarded by a three-cornered
valve known as the tricuspid, valve. The left auricle and
left ventricle communicate by a similar opening, and the
valve which guards this opening is called the mitral valve
from its resemb-
lance to a miter.
The auricles re-
ceive the blood
from veins, and
this blood passes
from the auricles
into the ventri-
cles through the,
valves. The ven-
tricles in turn, by
contraction of
their muscular
walls, force this
blood out of the
heart into arter-
ies and thus over
the body. It is
the pumping ae- Fig. 67 — External view of the heart; 1, right ventricle;
2, left ventricle ; 3, riglit auricle ; 4, left auricle ; 5,
aorta; 6, pulmonary artery ; 7, innominate artery;
8 carotid arteries; 9, subclavian artery; 10, supe-
rior vena cava 11, pulmonary veins.
stitutes what is
known as the heart beat. The two sides of the heart
beat in unison.
External anatomy of the heart. — (See Fig. 67.) When
the heart is examined from the outside, the position of
these four internal cavities is seen to be outlined by grooves
tion of these ven-
tricles which con-184
CIRCULATION
on the surface. The two flaplike auricles at the top of
the heart are separated from the ventricles by a deep
groove, while a shallow furrow marks the division of the
right and left ventricles, and the position of the septum.
On the outside may also be seen a number of large
blood vessels which extend from the top of the heart. In
addition to these there are smaller blood vessels (the coro-
nary system) which branch
over the surface of the
heart and supply it with
blood. The outside is also
more or less coated with fat.
Relation of blood vessels
and heart cavities. — If
we push bristle seekers
through the external blood
vessels, and then carefully
dissect away the two sides
of the heart we can make
out the course of the blood
through these vessels and
the heart. Let us first
examine the right side.
(See Fig. 68.)
Following the course
of the seekers we find
that two large veins,
called the superior vena
cava and inferior vena
cava, open into the right auricle, while at the back of the
auricle is the opening of a small vein (the coronary vein).
Blood which enters the heart from these three veins passes
Fig. 68—Fight 6ide of the heart (dis-
sected) ; 1, superior vena cava ; 2, infe-
rior vena cava; 3, right auricle; 4,
tricuspid valves ; 5, papillary muscle
fastening the threadlike chorda? ten-
din®, which are, in turn, fastened to
the edges of the valve; 6, pulmonary
artery; 8, aorta; 9, branches of the
aorta; 10, left auricle; 11, ventricle;
12, semilunar valves.BLOOD VESSELS AND HEART CAVITIES
185
through the opening guarded by the tricuspid valve into
the right ventricle. This tricuspid valve is so constructed
as to open to admit the blood from the auricle when the
ventricle walls expand, and to prevent its flow backward
into the auricle when the ventricle contracts. It is in the
form of a curtainlike flap
which is fastened by chords
{chordae tendince) to raised
prominences on the ven-
tricle (;■papillary muscles).
If we follow the seekers
from the right ventricle
they extend outward
through a single artery
known as the pulmonary
artery, since it conveys
blood from the ventricle
to the lungs. This artery
has at the base series
of valves or pockets of
a half-moon shape {semi-
lunar valves). These valves
are so arranged as to per-
mit the flow of blood
from the ventricle into
the artery when the ven-
tricle contracts, and pre-
vent its backward flow when the ventricle relaxes.
The left side of the heart shows the following connec-
tions (see Fig. 69): Into the left auricle open four veins
(the right and left pulmonary veins) so-called because they
bring blood to the auricle from the right and left lungs.
Fig. 69 — Left side of the heart (dis-
sected) ; 1, pulmonary veins ; 2, papil-
lary muscles ; 3, cut surface of the left
ventricle walls ; 4, semilunar valves at
the entrance to the aorta ; 5, pulmo-
nary artery ; 6, mitral valve ; 7, aorta ;
8, vena cava; 9, branches of the aorta.186
CIRCULATION
From this auricle the blood passes into the left ventricle
through an aperture similar to that between the right
auricle and ventricle. This opening is guarded by the
mitral valve, which is fastened and operates in a manner
similar to the tricuspid valve. From the left ventricle a
single large blood vessel opens, called the aorta or great
artery. This, like the pulmonary artery, is guarded from
back flow at its base, by semilunar valves. Near its base,
also, separate two smaller arteries (the coronary arteries)
which convey blood to the substance of the heart itself.
Arteries, veins, and capillaries. — (See fix. XLIV.) In
the preceding paragraphs we have used the terms artery
and veins to indicate blood vessels, but without distin-
guishing clearly between them. What is the meaning of
the terms artery, vein, and capillary as applied to blood
vessels?
An examination of the back of the hand will show us a
number of dark, bluish colored lines, which give no sensa-
tion of beating when we place the finger upon them.
These lines are actually blood tubes lying underneath
the skin, and their color and lack of beat or pulse indicates
that they are veins. If now, we place our finger on the
inside of the wrist we shall find that there is a certain spot
where a regular beating or pulse ic felt. That spot marks
the presence of an artery near the surface, and if we look
closely we can make out a thin line less blue than the
surrounding veins.
While almost all parts of the body surface show blue
veins the pulse spots are very few, the most prominent
being located on the inside of the wrists, the temples, and
the under side of the knee. This indicates that the arter-
ies are less near the surface than the veins and are thusarteries, veins, and capillaries
187
better protected. The necessity for this is found in the
fact that the bleeding of a cut artery with its pulsing blood
is much more difficult to stop than a bleeding vein.
If we prick any spot on the skin with a needle we get
bleeding, even though no arteries or veins are visible.
This is due to the fact that a network of tiny tubes, too
small to see with the eye, and called hair tubes or capilla-
Fig. 70 — Capillary circulation in the web of a frog’s foot (X 100); a, Z>, small
veins, d, capillaries in which the corpuscles are seen to follow one another in
single series ; c, pigment cells in the skin.
ries, because of their tiny dimensions, connect veins and
arteries. The relation of arteries, veins, and capillaries
may be observed to good advantage by stretching the
living web of a frog’s foot or tadpole’s tail under the
objective of a compound microscope. (See Ex. XLY. and
Fig. 70.) The blood vessels, which are transparent, show188
CIRCULATION
clearly the streams of blood plasma and corpuscles in
motion. With closer attention we can make out which
tubes are arteries, veins, and capillaries. The blood in
the arteries flows in spurts or pulses. The capillaries
show a steady streaming motion and are so small that the
corpuscles move in almost single file through them. The
veins are as large as the arteries but the blood in them
flows in a steady stream. If, now, we observe the direc-
tion of flow in the several vessels, it will be seen that the
blood in the arteries divides and passes into the small
capillaries, while from them in turn it passes on into the
veins and through these toward the bod}' again. In
other words, the capillaries are the connecting links be-
tween the arteries and veins, and in passing through them
the blood loses its spurting method of flow and is converted
into a steady continuous flow.
The difference in color between the arteries and veins
is explained by the color of the blood in the two tubes.
The blood in the arteries is bright red, being full of oxygen,
while that in the veins has a darker color owing to the
larger proportion of carbon dioxide present. This latter
blood is not blue, and that color is due to certain light
effects.
Summary. 1. Arteries convey blood from the heart, and veins
return the blood to it
2.	Arteries contain much oxygen and little carbon dioxide,
veins contain much carbon dioxide and little oxygen.
3.	Blood in arteries moves in spurts (that is, it pulses), while
that in capillaries and veins moves in a steady stream.
4.	Arteries are better protected than veins by being in general
placed farther below the surface. (This does not apply to the
main vein trunks.)STRUCTURE OF BLOOD VESSELS
189
Structure of blood vessels.—(See Ex. XLYI.) Arteries.
A cross section of an artery (see Fig. 71) shows it to be
made up of three layers of
tissue. The outer coat is
composed of fibrous con-
nective tissue, which is
tough, and a strong pro-
tective layer. This coat
contains also some elastic
fibers that enable it to
expand under the pulsing
of the blood. The middle
layer consists of a mixture
of unstriped muscle cells
and elastic fibers, and is
thicker than the other two
layers. The inner layer is made of flattened, epithelial
cells, and between it and
the middle layer are a
varying number of bands
of membranous and elastic
tissue.
Veins. The structure of
the veins is practically
identical with that of the
arteries (see Fig. 72) ex-
cept that the outer coat is
thickest and the two inner
layers are much less de-
veloped. Owing to this
difference they are much
less elastic, and readily collapse when empty. The
Fig. 72 —Cross section of a vein ; c, outer
fibrous sheath ; m, middle muscle and
elastic tissue layer; e, inner epithelial
cell layer.
Fig. 71 — Cross section of an artery; c,
fibrous connective tissue, outer sheath;
m, muscle cells and elastic fibers; e,
epithelial cells, with bands of tissue
separating them from the middle layer.190
CIRCULATION
toughness and thickness of the outer coat gives them
great strength.
Many veins also possess semilunar valves formed by
pockets located at intervals on the inner surface of the
tubes. These are so arranged (see Fig. 73)
that the edge toward the heart is free and
prevents a backward flow in the tube by pres-
sing out and bridging the tube under backward
pressure. These valves are found chiefly in the
superficial and muscle supply veins. The ar-
teries have no such valves except at the
entrance into the heart. The location of these
valves may be recognized by the knotted ap-
pearance of the veins at these points.
Capillaries. As the arteries decrease in size
the outer and middle coats gradually disappear
until, in the capillaries, only the inner layer
semi] unar is left. This is composed of flattened cells
valves.	cemented together at the edges. Such a
structure is evidently well adapted to the process of
osmosis, dialysis, and filtration.XIII. CIRCULATION (continued).
The Flow of Blood, and Blood Systems.
In the preceding chapter we have seen the way in which
the various parts of the circulatory system are put together.
In the present chapter we shall investigate the cause of
the blood flow and its course in the blood vessels.
Heart action. — (See Fig. 66.) Enough of the structure
of the heart has been outlined to show that the actual
driving of the blood into the arteries is due to the con-
tractions of the ventricles. Before these ventricles contract
it is necessary that they be filled with blood, and this
requires a preliminary contraction on the part of the auri-
cles. The order of these contractions is as follows: The
action begins at the mouth of the large veins (the venae
cavae). The blood is flowing steadily from these veins into
the auricles and dilating them. Next the tricuspid and
the mitral valves are forced open by the blood pressure
in the auricles and the ventricles fill. Now the auricles
contract and send a wave into the ventricles dilating them
still further and checking the flow from the veins by back
pressure, while the mitral and tricuspid flaps float upward.
As the ventricle becomes fully filled, it, in turn, begins to
contract, and these flaps are suddenly pressed upward
until they cover the opening tightly and prevent further
flow from the auricle into the ventricle and also back
flow from the ventricle into the auricle. The chordae
tendinae prevent the flaps from being forced back through
the opening. The blood in the ventricles is now under
191192
CIRCULATION
pressure and forcing down the semilunar valves at the
mouths of the arteries; the blood from the ventricles enters
these arteries with a spurt. While this ventricle contrac-
tion is going on, the auricles begin to expand and the blood
flows from the veins into them. Before they finish ex-
panding the ventricles also begin to expand, while the semi-
lunar valves of the arteries close to prevent back flow from
them. During this period of expansion the mitral and
tricuspid valves begin to open, and both auricles and ven-
tricles to fill with blood. This goes on until the auricles
begin to contract again, and this cycle is repeated indefi-
nitely. The position of the valves may be summarized as
follows:
(a)	As the auricles contract, the mitral and tricuspid
valves are open; the semilunar valves are closed.
(b)	As the ventricles contract, the mitral and tricuspid
valves are shut tightly. The semilunar valves are open and
blood is entering the arteries.
(c)	During the pause before the next contraction the
mitral and tricuspid valves are open to permit the filling of
ventricles, while the semilimars are closed to prevent
back flow from the arteries.
The name of diastole has been given to the period when
the auricles and ventricles are expanding, while the period
of contraction of these organs is called the systole. Between
these two periods there comes a time when neither is con-
tracting or expanding. Using these terms the various
stages of a “ cardiac cycle ” or heart beat may be ex-
pressed as follows:
First. An auricular systole.
Second. A ventricular systole.
Third. Heart pause (both organs in diastole).PULSE
193
Sounds of the heart.— If one listens carefully to the heart beat,
by placing the ear against the chest, two distinct sounds may be
distinguished. The first is lower in pitch than the second and
of longer duration. It is supposed to be produced by two factors,
the vibration of the tricuspid and mitral valves in closing, and the
vibration of the contracting muscle mass. The second sound is
sharper and is due to the closing of the semilunar valves. The
first sound is essentially a systolic sound, while the second is dia-
stolic. These two sounds are of great aid to the physician in deter-
mining cases of heart trouble. For detecting variations in this
sound an instrument called the stethoscope is used, and this method
of examination is called auscultation.
Pulse.— When the ventricles contract the blood is
forced into the arteries with a spurt. If the arteries,
capillaries, and veins were rigid tubes this sudden pressure
would be transmitted instantaneously throughout the
entire system, and a quantity of blood would enter the
auricles exactly equal to that which left the ventricles.
Owing to the elasticity of the walls of the blood vessels
this is not the case. Part of the pressure is used up in
expanding the walls. Since the arteries are subject to
the most direct effect of this systole the reason for their
thicker walls is apparent. This elastic character of the
arteries also results in the pressure being transmitted in
a wave instead of directly. The results of this may be
pictured as follows: As the ventricle contracts the ex-
pressed blood stretches that portion of the artery nearest
the heart. As the ventricle expands and this pressure is
removed this enlarged part of the artery begins to contract
slowly, and the pressure is transferred to another point
further along in the artery. As the next systole occurs
this process is repeated, and thus the effect of the pressure
or wave is gradually transmitted throughout the whole
EDDY. PHYS. — 13.194
CIRCULATION
arterial length. In this tvay each portion of the artery
wall expands to a maximum and then contracts, at inter-
vals corresponding to the systolic and diastolic periods
of the heart. The disappearance of this wave in the
capillaries and veins is explained when we consider how
the arteries branch into smaller and smaller tubes, which
are farther and farther removed from the seat of pressure
until in the capillaries the wave has become so split up as
to be barely perceptible, and in the veins is converted into
a steady flow. In this respect it is like a water wave,
which becomes less apparent the farther we go from the
center of the disturbance. It is this wavelike character
of the flow of the blood in the arteries that enables us to
feel a pulse at points where the arteries are near the sur-
face. What we actually count in taking the pulse is the
time between successive periods of pressure and relaxa-
tion, and since these correspond to expansion and
contraction of the heart they enable us to detect the
rate of beat of the heart. ' The physician uses the
pulse point in the wrists and temples to furnish him
Avith valuable information about the heart and circula-
tion.
Pulse rate.— The rate of the pulse or heart beat is subject to
great variation under different conditions of health and disease.
It is this fact that makes it a valuable indicator of the body con-
dition to the physician.
Age, sex, size, atmospheric pressure, all have a distinct effect
upon the pulse rate. The average rate for the adult man is
seventy beats per minute, and for the adult woman seventy-eight
to eighty per minute, showing that sex affects the rate. Tall
individuals have a slower rate than short persons, rvhile large
bodies have a slower rate than small bodies. The human rate is
most rapid in infancy, sinks rapidly at first, and then more slowlyPULMONARY CIRCULATION
195
to a minimum in adult life, rising slowly as we enter extreme old
age. The following table gives the average rates:
At birth........................140	beats per minute.
Infancy.........................120
Childhood.......................100
Youth ...........................90
Adults....................... 75-70	“
Old age..........................70	“
Extreme age...................75-SO	“
Systems op Circulation.
From what has been said it follows that the blood which
passes through any given chamber of the heart must ulti-
mately return to that same chamber. Owing, however,
to the division of the heart into two parts, which beat in
unison, and the branching of the blood vessels, the actual
course of the blood requires some further explanation.
For convenience, the blood vessels that are concerned in
the transfer of the blood from the right to the left side of
the heart, and that have their branches in the lungs, are
spoken of collectively as the pulmonary system. Those
concerned with the transfer of blood from the left side of
the heart to the right side, and whose branches take it
all over the body, are spoken of as the systemic system.
Pulmonary circulation.— When the blood enters the
right auricle it is partly exhausted of its oxygen and is
dark and purplish in color, owing to the abundance of car-
bon dioxide which it contains. In this condition it enters
the right ventricle, and when that organ contracts it is
forced into the 'pulmonary artery which in turn breaks up
into two branches leading to the right and left lung, and
these into smaller branches within the lungs. These pul-196
CIRCULATION
monary arteries convey the blood from the right ventricle
directly to the lungs. In the many capillaries, into which
the branches of the pulmonary artery divide in the lungs,
the blood takes on a new supply of oxygen and loses its
carbon dioxide. These capillaries are collected into largerTHE SYSTEMIC CIRCULATION
197
and larger veins and finally bring back the oxygen laden
blood to the left auricle by four veins (two from each lung).
These veins are called the right and left pulmonary veins
respectively. From the left auricle the blood passes into
the left ventricle, and in this manner the transfer of blood
from the right to the left side of the heart is brought about.
In our study of the lungs we shall discuss more in detail the
manner in which it loses its carbon dioxide and takes on
oxygen.
In this circulation it will be noticed that it is the pul-
monary artery which contains the carbon dioxide laden
blood, while the pulmonary veins contain the bright red,
oxygen laden blood. Owing to this fact the only universal
definition that will apply to all arteries and veins is one
which is based on the direction of blood flow. Arteries
convey blood away from the heart, veins toward the heart.
The systemic circulation. — From the left ventricle this
oxygen laden blood is pumped into a large artery called
the aorta. The aorta subdivides into many branches
which carry the blood to all parts of the body. In the
tissues these arteries subdivide into capillaries, and these,
in turn, collect into veins through which the blood finally
enters two large veins (the superior and inferior venae
cavce) and is returned to the right auricle by them. In
this system the blood is transferred over the body and
from the left to the right side of the heart.
Since it is this system that carries food and oxygen to
the tissues and transfers their wastes and carbon dioxide
to the excretory organs, the relations of the branches is
important to a clear understanding of the blood flow of
this system. As it leaves the heart, the great aorta bends
first to the left, then downward through the rear part of198
CIRCULATION
Kxq. 75 —The arterial systemTHE SYSTEMIC CIRCULATION
199
the chest cavity, finally piercing the diaphragm and en-
tering the abdominal cavity. The change in direction as
it leaves the heart results in an arch called the arch of the
aorta. From the top of this arch on the right side is given
off a large branch which, in turn, soon divides into two
parts. One of these two parts (the right carotid artery)
supplies blood to the right side of the head. The other
part passes under the collar bone, down the arm, and at
the elbow divides into the radial and ulnar arteries thus
supplying the right arm with blood. It is the radial artery
whose pulsings are felt in the wrist.
From the top of the arch on the left side are given off
two branches, one of which (the left carotid) supplies blood
to the left side of the head, while the other branches as on
the right side and supplies blood to the left arm.
From the aorta, near the heart, arise also the coronary
arteries which, branching over the surface of the heart,
supply the tissues of that organ with blood. As the aorta
curves downward through the chest cavity it gives off
branches which supply the lung tissues, the esophagus, and
the muscles of the ribs and spinal column.
From the aorta in the abdomen are given off five main
branches. The first (cceliac axis) divides into three parts
which supply the stomach, spleen, and liver respectively.
Two other branches (the mesenteries) carry blood to the
walls of the intestines. The remaining two branches carry
blood to the kidneys. The main trunk of the aorta con-
tinues to the rear of the abdominal cavity and finally
divides into two branches which each enter a leg and
supply these extremities with blood.
All these branches of the aorta subdivide first into smal-
ler arteries and then into capillaries, which are in turn col-200
CIRCULATION
lected into veins by which the blood is returned to the right
auricle of the heart.
The blood supplied to the head by the carotids is re-
turned to the heart by two large veins (the jugulars) pass-
ing down each side of the neck; while from the arms come
two other veins. These four veins finally unite to form
the superior vena cava which opens into the right auricle
of the heart. In a similar manner various veins return the
blood from the lower part of the body and unite in one
common vein (the inferior vena cava) which also opens into
the right auricle of the heart It is this lower part of the
venous system which collects the food from the digestive
system.
The portal circulation. — The veins which are found in
the walls of the stomach and intestine do not convey the
blood directly to the in-
ferior vena cava. After
collecting the food from
these organs these veins
are collected into a single
large vein (the portal
vein) which passes di-
rectly to the liver. Here
this vein splits up into
capillaries again, which
in turn collect again into a second vein (the hepatic
vein). This latter pours its blood into the inferior- vena
cava. It is to be noted in this case that the capil-
laries of the liver arise from and unite again into veins,
and that the food is thus forced to pass through two sepa-
rate sets of capillaries before entering the heart. This
branching of the portal vein into capillaries is the only
Fig. 76 — Diagram of the portal circulation.THE LOADING OF THE BLOOD WITH NOURISHMENT 201
example in the body of a vein’s connection with another
vein through capillaries. In all other cases the capillaries
connect veins and arteries.
It is this arrangement of portal-vein capillaries which
permits the liver to act as a storehouse of carbohydrate
material, and to regulate the amount of that material
which shall enter the general circulation at any given time.
The renal circulation. — The blood which is returned
to the left side of the heart by the pulmonary veins has
been freed of its carbon dioxide only. Hence the blood
which passes into the aorta is still rich in urea and other
body wastes brought to the heart by the venae cavae. Much
of this waste is diverted into the artery branches which
carry blood to the kidneys (the renal arteries). In these
organs, by means which we shall discuss more fully under
the excretory system, this waste is removed, and the
renal veins, therefore, return to the inferior vena cava
blood which is nearly free of body wastes.
Changes in blood composition. — In passing through the
various systems of circulation, portal, renal, systemic,
and pulmonary, the blood undergoes distinct changes in
composition. Having in mind the parts of the body which
these systems supply, we may now summarize these
changes as follows:
The loading of the blood with nourishment. — 1. Food
enters the blood at two points in its circulation.
(a) All kinds of food, except the fats, enter the capil-
laries of the stomach and intestines and are collected
and carried to the liver by the portal vein. In the
liver, the capillaries into which this vein divides
permit the liver cells to separate from the rest of the202
CIRCULATION
blood the carbohydrate, together with certain poisons
(see p. 142). The carbohydrate thus separated is
either stored in the liver as glycogen, or given out
as needed in the form of grape sugar to the hepatic
vein. This hepatic vein passes this grape sugar and
the other unchanged nutrients into the general cir-
culation by opening into the inferior vena cava.
The poisons are passed into the liver secretion or
bile and returned to the intestine with that fluid for
removal with the feces.
(b) Fats are collected by the lacteals, and enter the
veins by way of the thoracic duct at a point just under
the left collar bone where the left jugular and left
arm vein join. Thence they pass into the heart by
the superior vena cava.
2. Oxygen enters the blood in the lungs, and is con-
veyed to the heart by means of the pulmonary veins. We
shall consider this phase of the circulation more in detail
in our study of respiration.
The loading of blood with waste. — All veins carry
wastes (COs, urea, etc.), which they collect from the tis-
sues. This waste either enters the blood directly by way
of the capillaries, or else enters the lymphatics and thus
the veins by the thoracic duct or right lymphatic duct.
The right lymphatic duct enters at the joining of the
right jugular vein and the right arm vein.
Unloading of nourishment by the blood.— All arteries1
carry food and oxygen to the tissues which they supply,
and deliver these substances to the cells of the tissues
1 The pulmonary veins and arteries are exceptional in that the pul-
monary veins carry oxygen laden blood, and the pulmonary arteries,
carbon dioxide laden blood.ARTERIAL AND VENOUS BLOOD
203
through the thin walls of the capillaries in the manner
already described (see p. 175). The plasma composition
indicates that it is the part of the blood which carries the
food to the tissue cells.
Unloading of wastes by the blood.— The carbon dioxide
given off by the tissues is removed from the blood almost
entirely in the lungs. It is noteworthy that the lungs do
not free the blood of wastes in general, and on that account
should not be said to 'purify the blood. In other words,
their great function is to exchange oxygen for carbon
dioxide or aerate the blood.
Most of the urea and true wastes of the body are
removed by the kidneys and the skin, this waste being
collected from all parts of the body by the veins and
conveyed to these organs by special arteries, such as the
renal artery to the kidneys.
Arterial and venous blood.— From the description given
above it is readily seen that the blood in the arteries and
veins varies in composition in different parts of the body.
In general, however, we may say that the veins contain
less oxygen and more carbon dioxide than the arteries.
The redder color, due to the larger proportion of oxygen
present, has led to the custom of representing arteries
by red, and veins by blue lines in a diagram of circulatory
systems. The notable exceptions to this rule are the
pulmonary veins and arteries. The pulmonary vein con-
tains very little carbon dioxide, while the pulmonary
artery contains very little oxygen.
In general, then, the terms arterial and venous are used
to indicate relative amounts of oxygen and carbon dioxide
in the blood. In that sense the pulmonary vein contains
arterial blood, and the pulmonary artery, venous blood.204
circulation
Regulation of Blood Flow.
Heart control. — The amount of blood which passes
through a given part of the circulatory system at any one
time is, of course, regulated primarily by the heart beat.
This beat, which is produced by rhythmic contractions of
the heart muscles, is ordinarily very regular. Its rate,
however, is subject to certain external influences. For
example, a decrease in blood pressure in the arteries is
associated with an increase in heart rate and vice versa.
Muscular exercise also increases the heart rate. Even so
little an effort as a change in position is sufficient to change
the rate, that of a standing position being higher than
that of a sitting or reclining one.
The purpose of these changes in rate is readily under-
stood when we consider that tissues which are active are
using up more food and creating more waste than still
tissues, and consequently require a greater flow to supply
this demand.
Mental states and emotions, blood content, and tem-
perature, all affect the rate of heart beat. Some lower
this rate, others increase it. Some, like temperature, cause
an increase up to a certain limit, and then above that
limit produce a decrease. Physicians take advantage of
these factors and introduce into the blood certain drugs
which may accelerate or lower the rate of beat, and thus
can control it to a certain extent in cases of disease.
The manner in which all these special conditions are
able to affect the heart is through what we call nerve
action. The muscle walls of the heart are connected by
nerves to other parts of the body, and when one set of
nerves is stimulated by any of the above means, the nerve-ARTERY CONTROL
205
ends In the heart muscle receive this impulse and increase
the rate of contraction and expansion of the cells. When
another set of nerves is stimulated, they, in turn, cause
the contraction to be reduced in rate. These are like
telegraph wires connecting stations. One station is the
heart where messages are received. The others are found
in the brain and in the body tissues, and the messages
sent from these latter points control the action of the
heart. Of the two kinds of nerves those that decrease
the rate are called heart inhibitors; and those that in-
crease the rate, heart accelerators. By their aid the rate
of flow is adjusted to the needs and conditions of the
tissue cells.
Artery control.— The walls of the arteries, as well as
of the heart, are supplied with the ends of what are called
the vaso-rnotor nerves. Under the normal action of these
nerves the walls of the arteries have a certain elasticity
which responds to the blood pressure; stretching when
the pressure is increased and contracting as the pressure
is removed. When these vaso-motor nerves are stimu-
lated further the effect produced may be a still greater
contraction of the arter}r walls and a consequent decrease
in the blood content of the contracted artery, or a paralysis
of the walls which causes them to lose their elastic tension
and stretch. In this way it is possible for conditions,
external or internal, to affect these nerves and produce
stimuli, which, in turn, control the blood supply of any
given part of the body. The nerves which, when stimu-
lated, cause the contraction of the w’alls are called the
vaso-constrictors. Those which, when stimulated, pro-
duce paralysis of the walls and thus allow them to stretch
are called the vaso-dilators. This stretching and contract-206
CIRCULATION
ing of the artery walls with the consequent increase and
decrease in blood must necessarily cause the capillaries,
whose walls are also elastic, to become stretched. Hence
variation in the tension of the artery walls is always
accompanied by similar variations in the blood supply
to those capillaries which the arteries feed.
The conditions which may thus affect these vaso-motor
nerves are many. Heat to a certain limit, exercise, the
presence of food in the digestive tract, etc., all affect the
vaso-dilator nerves and cause the artery walls to relax.
Cold, on the contrary, affects the vaso-constrictors and
causes the contraction of the walls. Mental emotions in
some cases cause relaxation of the walls, and in other
cases, contraction. Thus, blushing is due to overcharg-
ing of the facial capillaries from relaxation of those artery
walls which supply these capillaries; while paleness is due
in the same manner to contraction of the supplying artery
walls. In both cases the mental emotions are responsible
for the nerve stimulus.
Injury to tissue cells invariably results in a relaxation of
the walls of the arteries supplying the affected part, and
thus permits an increase in the blood supply to the injured
area. Such an increase is spoken of as congestion.
Congestion and inflammation. — A simple case of con-
gestion is seen when we scratch the skin with a pin, the
scratched surface becoming reddened from the action of the
injured cells upon the artery supply. If the injury is
severe and the cells are killed, swelling often follows the
redness, and painful sensations may result.
Such congestion, with its attendant swelling, is merely
an indication of the first steps in the repair of the injury.
The flow of blood to the injured part is soon followed, inCONGESTION AND IMELAMMATION
207
this case, by the outpouring of plasma and white blood
corpuscles (leucocytes) through the capillary walls into the
lymph spaces. This extraordinary flow is due to changes
in the cement which holds together the capillary cells com-
posing the walls, and may in some instances be accompanied
by the outpouring of red corpuscles.
In simple injuries the result of this outpouring of plasma
is to stick together the old cells and cause them to grow and
unite, while the leucocyt.es and injured cells are absorbed
anti the injury thus repaired.
If, however, the injury is extensive or bacteria are in-
troduced into the injured area, a more extensive action is
involved. The growth of the bacteria or the nature of the
injury may cause the death of other cells in the neighbour-
hood, and the formation of a mass of dead tissue results.
In this event the white corpuscles proceed to kill or absorb
the bacteria and dead tissue cells with the formation of a
mass of matter usually designated as pus. At the same
time new cells are formed and a new tissue grows in place
of the old one. This pus may be removed by the lympha-
tics or absorption into the blood vessels, or it may soften
the tissue about it and discharge, as in an abscess. In
the latter event, the white corpuscles either devour the
bacteria and cells, or give off ferments which dissolve them.
Meanwhile the living cells about the injured part arc ex-
cited to growth and reproduction, and thus new cells pro-
duced which unite and replace the old tissues.
The processes here are evidently two, the dissolving
and removal of old matter through the agency of the white
corpuscles, and the formation of new tissue cells to replace
the injured ones. The manner in which the old matter is
removed varies with the location of the injury. These208
CIRCULATION
two processes of repair are called inflammatory processes,
and inflammation is usually defined as “ the local attempt
at repair of an injury,”
Examples of simple inflammatory processes.—When the mucous
membrane is injured by exposure to cold, there results a conges-
tion in the membrane. This congestion may be complicated by
the presence of bacteria. In such congestion the formation of
pus is accompanied by an increased activity and secretion of the
mucous cells. The combined secretion and pus collects in the
passages inclosed by these membranes and forms the discharge
common to colds. This congestion of the mucous membrane
may be brought about by the action of the cold upon the vaso-
motor nerves which control the artery supply to the skin. In this
event their action is to cut off the blood supply to the skin, and
the excess of blood thus cut off is forced to accumulate in the
mucous membranes and produce congestion. If this congestion
and inflammatory process is confined to the nasal passages the
result is called a cold in the head, or catarrh. If in the throat,
it may result in swelled tonsils and a sore throat. If in the bron-
chia passages, it is called bronchitis, while in the lungs it may
produce pneumonia. When certain bacteria attack the throat
membranes the result may be severe. Such an instance is seen in
diphtheria.
Another example of inflammatory processes is found in the
pimples and boils. In these cases, bacteria are introduced in some
way into the underlying layers of the skin where they cause the
death of the surrounding cells. Congestion results, and the leuco-
cytes dissolve the dead cells and devour the bacteria forming a
greater or less quantity of pus. The pus thus formed softens the
surface layers of skin and forms a head or weak spot on the
surface through which the pus may ultimately be discharged.
“Taking cold in a boil” is actually a case of accidental intro-
duction of bacteria into a partly discharged boil, and the resultant
formation of new pus, and has no connection whatever with
cold.
The introduction, by the blood, of bacteria into the internalTREATMENT OP CUTS AND BRUISES	209
tissues, internal disturbances of function, and many other causes,
may bring about so extensive a congestion and accumulation of
poisonous matter that the body is unable to remove it. In that
event the presence of this matter in the body may produce various
forms of disease, such as fevers, growths, etc.
Treatment of cuts and bruises. — When the capillaries
are cut the bleeding is very slow and is usually easily
checked by the natural clotting of the expressed blood.
The cutting of an artery or vein, however, is a much more
serious matter, and owing to the spurting of the blood the
cut artery is more difficult to treat than the vein. If
the artery or vein is small the mere pressing together of the
walls with a bandage may check the flow sufficiently, to
permit the formation of a clot. When the artery or vein
is large the blood pressure is too great to allow the clot to
form and special measures are necessary. The object of
all such measures is to secure sufficient pressure upon the
blood vessel to force the cut edges together and diminish
the blood flow sufficiently to permit the formation of a
clot. Owing to the direction of flow in an artery, this
pressure must be applied on the side of the cut nearer the
heart. In case of a vein the pressure should be applied
on the side farther from the heart. In some cases simple
bandaging fails to stop the flow and the clot is washed
away as fast as it forms. In such cases an application
called a tourniquet is resorted to. This consists of a band-
age applied on the proper side of the cut and twisted tight
with a stick so that the walls of the blood vessels are pressed
together at a point above the cut. The blood being shut
off from the cut ends, a dangerous loss of blood is thus
prevented. In emergencies persons may thus be saved
from bleeding to death since all the tools required are a
EDDY. PHYS. —14.210
CIRCULATION
strip of cloth and a stick. The tourniquet is of course
only a temporal'}’ measure and is to be employed only as
an emergency remedy until the surgeon can treat the
wound. Surgeons usually close large wounds by tying the
ends of the cut blood vessels with clean catgut. Such
bindings are called ligatures.
When the ends of a vein are cut they may grow together
again and thus circulation be restored through the original
channel. Artery ends, on the contrary, do not grow to-
gether but the ends close and the branches which are in
communication above and below the cut enlarge and replace
the original channel.
All cuts should be kept clean and free from bacteria.
Weak solutions of certain germicides can be obtained from
druggists and used to wash and protect the cut from these
dangerous visitors. Blood poisoning is one of the serious
results of neglect of these precautions.
The healing of a cut or bruise will take place without
pain or inflammation if kept clean.
Effect of alcohol upon the circulation. — The most
marked effect of small amounts of alcohol is the dilation of
the surface capillaries and their filling with blood. The
result is a sensation of warmth or glow due to the stimu-
lation of the surface heat nerves. Alcohol produces this
effect by its action on the nerve centers, which, in turn,
stimulate the vaso-dilator nerves of the arteries that supply
these capillaries, and not by any direct action upon the
blood vessels.
In this case the sensation of warmth is deceitful, since
the bringing of the blood from deeper parts of the body to
the surface permits the escape of heat and a slight lower-
ing of the body temperature.EFFECT OF ALCOHOL UPON THE CIRCULATION 211
In larger doses alcohol produces effects in one of two
ways:
(a)	It may cause the deatli or degeneration of the cells
with which it comes in contact. For example, it may
cause the cells of the heart to become charged with fat and
thus weakened, or it may make the cells of the artery walls
brittle and unable to withstand pressure, or finally, it may
actually poison and destroy the cells of certain tissues.
(b)	When brought in contact with the cells of the
tissues it may so injure them that congestion and inflam-
mation results. This inflammation may cause a disturb-
ance of the natural action of the congested part due to
the swelled condition, or it may stimulate unnatural growth
on the part of the surrounding tissue cells. Another bad
result of this congestion may be the stretching of the arte-
ries to such an extent that they are unable to recover their
natural size, and thus- produce continued congestion in the
part affected.
The extent of the injury which alcohol may produce in
the body by either one of these means, that is, directly
or indirectly, is a subject for a medical text, and we need
only outline here the method of the action. In brief, then,
it is evident that alcohol in either large or small doses acts
upon the nervous centers and through them upon the
vaso-dilator nerves, producing abnormal distribution of
the blood, and that in large doses it is actually poisonous
to the cells with which it comes in contact. While the
extent of the injury produced by small doses may be only
temporary, continued use or excess causes such changes
in the distribution of blood as to be extremely dangerous
to the system.XIV. CIRCULATION IN THE LOWER ANIMALS.1
In the one-celled animals, such as the amoeba, the food
is digested in the cell protoplasm itself, and oxygen is
absorbed directly from the surrounding air or water. In
like manner the wastes and carbon dioxide are removed
from the cell and cast into the surrounding medium. In
such animals a circulatory system is evidently unneces-
sary. In fact, the only suggestion of it is found in the
movement of the protoplasm in the cell by means of 'which
the absorbed food is distributed throughout the cell. This,
however, is entirely an internal cell circulation, and not a
system for bringing food to the cells as in man.
In the sponges and coelenterates each cell is able to
obtain its air supply and food supply directly, and no
special circulatory system is necessary. When we come
to animals possessing many layers of cells it is evident
that some cells will be so placed as to be unable to secure
food and air directly or to rid themselves of waste. Some
special apparatus, therefore, becomes necessary to supply
these cells with food and to remove their wastes. In
this way there arises the circulatory system as we have
considered it in our definition. Some of the systems in
the lower animals are interesting from the light they throw
upon the development of our own complex network of
tubes.
Insect circulation. — In the grasshopper, we see one of
the simplest forms of a circulatory system. In this animal
1 See Footnote on p. 146, Chapter X,
212INSECT CIRCULATION
213
the food passes from the digestive tract directly through
the walls of the stomach and intestine into the body
cavity. Here it would remain were it not for a tube with
several swellings along its surface extended along the
dorsal surface of the body cavity. This
tube, with its swellings, is a sort of
muscular pump or heart, closed at the
rear end and open in front. The swel-
lings expand and contract rhythmi-
cally. As they expand the liquid of
the body cavity is sucked in through
openings along the sides. These open-
ings are protected by valves which open
inward. As the swellings contract these
valves close, and the inclosed fluid is
thus forced forward and out the front
end of the tube toward the head. In
this manner the body fluid is kept in
motion, and by means of the cavities
between the tissues it flows around
and bathes all the tissues, and finally
returns to its starting point, thus sup-
plying the body cells with food and
receiving their wastes. This waste is
absorbed by kidney tubes which extend
into the body cavity from the intes-
tine, and the absorbed fluid is excreted
through the intestine with the fa?ces.
The liquid circulated contains no red corpuscles, as the
air supply is furnished by an entirely independent set
of tubes. There are no arteries or veins, the blood flow-
ing through the spaces of the body from the mouth of
Fig. 77 —Portion ot
an insect’s blood
tube (dissected); a,
b,	muscular walls ;
d, valves between
the compartments;
c,	valve covering the
general opening into
the body cavity;
through this valve
the liquid enters.214
CIRCULATION IN THE LOWER ANIMALS
the tube to its starting point. The heart is simply a
contractile structure whose office is to keep the liquid in
motion.
The earthworm. — In this animal we have a slight
advance over the insect. It possesses a true closed sys-
tem of tubes conveying a red blood. It consists of two
tubes running lengthwise of the body, one dorsal and
one ventral. The dorsal blood vessel can be easily made
out through the transparent skin of the living earthworm,
and observation shows that the red liquid which it con-
tains moves in pulses toward the head. At the anterior
Fig. 78 — Diagram of a longitudinal section of an earthworm ; h, one of the fine
pulsating arches which pump blood from the dorsal blood vessel into tlid ventral
blood vessel.
end of the animal are five pairs of arches or blood vessels,
which branch from each side of the dorsal blood vessel
and, encircling the esophagus, open below it into the ven-
tral blood vessel. These pairs are arranged, one behind
the other, like rings from the sixth to the eleventh segment.
These five pairs of tubes are called the aortic arches,
and they pulsate in unison, forcing the blood to flow
from the dorsal blood vessel into the ventral. The course
of the blood, then, is from the rear of the body along the
dorsal blood vessel to the head; downward, through the
arches to the ventral vessel, and through* this toward the
rear of the body again. Both dorsal and ventral bloodTHE EISH
215
vessels give off branches, and while the exact course of
the blood flow from the ventral vessel is at present un-
known, it is supposed that in some way it is ultimately
brought back into the dorsal vessel and thus completes
its circuit. The branches of the two main vessels sub-
divide into capillaries, and by means of these the blood
is brought into contact with the various tissues of the
body, and thus food is distributed and wastes collected.
The red color of the blood is due to the hremoglobin
dissolved in the fluid, the only corpuscles
present being a few white ones.
Besides this inclosed fluid, we find the
body cavity filled with a similar liquid to
that found in the grasshopper, and it is
known that part of the absorbed food
enters the inclosed system and part is
distributed by the cavity fluid, so that we
actually have two methods of circulation in
this one animal.
The fish. — It is only in the vertebrate
animals that we find a true blood with red
corpuscles. The simplest vertebrate system
is found in the fish. In this animal the
heart is a simple structure consisting of a
single ventricle and auricle, and is located
in a pericardial cavity separated from the
rest of the body organs by a thin mem-
brane. The blood is driven from the
ventricle through an artery (the aorta)
to the gills. Here the artery branches into capillaries
and through the thin walls of these is effected the inter-
change of oxygen and carbon dioxide between the water
d
Fig. 79—Plan of
circulation in a
fish ; a, auricle ;
b, ventricle; c,
branchial arte-
ry; e, branchial
veins ; (I, gills ;
/, aorta; g, vena
cava.216
CIRCULATION IN THE LOWER ANIMALS
and the blood. These capillaries ultimately return the
blood to a dorsal artery which branches and carries the
blood to all parts of the body, where the division of the
branches into capillaries permits the giving up of food and
the removal of the waste of the tissues. These capillaries
are now collected into veins, and the blood is returned by
these tubes to the auricle of the heart to repeat this cycle
indefinitely. As in man the food of the digestive tract is
absorbed by capillaries which unite to form the portal vein,
and this absorbed food is carried to the liver by this vein.
Here the portal vein produces capillaries, and these, after
branching throughout the liver, unite again into the hepatic
vein and are carried directly to the auricle by this tube.
In this circulation the single auricle and ventricle cor-
respond to the right side of the human heart, and the heart
receives and expels only the venous blood. The friction
in the gill capillaries retards the flow so greatly that the
circulation in the arteries of the fish is very sluggish.
The red corpuscles of the fish are nucleated, and it also
possesses a lymphatic system.
The frog.— In the frog the blood is aerated in the lungs,
and the heart lias two auricles and one ventricle. The
right auricle, as in the fish, receives blood from the veins
which bring it from all parts of the body to this division of
the heart. The left auricle receives aerated blood from
the lungs. It is to be noted that the frog has advanced
over the fish in having the aerated blood returned to the
heart before pumping it over the body, and as we would
expect, this arrangement allows a little faster flow through
the arteries. Since, however, there is only one ventricle,
the two kinds of blood found in the auricles must become
mixed in this chamber, and the arteries in consequence areTHE BIRDS
217
filled with mixed blood. By an ingenious arrangement of
arteries and valves the blood is not evenly mixed, but the
part which is pumped to the head is richest in oxygen, that
to the body organs next richest, and that to the lungs and
skin is poorest in oxygen and richest in carbon dioxide.
As in the fish, the red corpuscles are nucleated. In its
early life the frog breathes by gills.
The reptiles.— These animals have, in
general, the same sort of circulatory
system as the amphibians. There is,
however, one modification in the struc-
ture of the heart that shows an ad-
vance. In most reptiles the ventricle
is partly divided by a partition, and
this partition tends to keep separate
the aerated and non-acrated blood so
that the aorta which arises from the
left side of the ventricle receives blood
freer of carbon dioxide than the pulmo-
nary arteries which arise from the right
side. This partition is interesting as a
step toward complete separation of the
two kinds of blood. The red corpus-
cles are nucleated.
The birds.— In these vertebrates the
ventricle is completely divided by the
partition, and the bird heart, like that
of man, is four chambered. This' fact
alone would enable us to classify birds
as higher forms than reptiles. The
rate of flow in birds is much more rapid than in man, and
the temperature of the blood is much higher. This is due
Fig. 80.— Plan of circu-
lation in amphibians
and reptiles ; a, right
auricle receiving ve-
nous blood from the
system ; 6, left auri-
cle receiving arte-
rial blood from the
lungs; c, common
ventricle; d,f, c, sys-
temic arteries, capil-
laries, and veins ; <j,
pulmonary artery;
k, h, pulmonary cap-
illaries and vein.218
CIRCULATION IN THE LOWER ANIMALS
to the tremendous muscular exertions that the bird is com-
pelled to make in the process of flying. Another change
is the turning of the aorta to the right instead of to
the left. The red corpuscles of this animal are not
nucleated, and in this respect the bird is nearer man
in the scale of animal life.
The mammals.—In all the mammals
the circulatory system is practically
identical with that of man, and the red
corpuscles are without nuclei, but differ
in shape.
Significance of these comparisons.—
The lessons that these comparisons
teach are two. First, they show us
that while the apparatus may be differ-
ent in construction in the different
animals, the purpose of the structures
found are identical in all animals,
namely, to distribute food and oxygen
and remove wastes. Second, the fact
that they differ only in minor details
and in adaptation to special conditions
of life show us that man bears a very
close relation to other animals and dif-
fers from them only in the greater per-
fection of his organs. It is by compari-
sons such as these that men have been
enabled to trace the history of the
development of living forms on the
earth, just as by studying all the different musical instru-
ments that have ever been produced it is possible to learn
the history of instrument making in the arts. In short,
Fig. 81—Plan of cir-
culation in birds and
mammals ; right
auricle receiving
blood from the sys-
tem ; b, left auricle
receiving arterial
blood from the
lungs; c, right ven-
tricle ; c' left ventri-
cle ; t7, e, /, systemic
arteries, 'veins, and
capillaries; g, pul-
monary artery ; k, h,
pulmonary capilla-
ries and vein.SIGNIFICANCE OF THESE COMPARISONS
219
each form of animal that shows an advance over another in
the structure of its organs aids in locating the time of its
development and its relation to other forms. The group-
ing of animals according to their relative development of
structure is the basis of the classification of animals in
the zoology of to-day, and the view that each form of
animal has developed from a lower form is called the
Doctrine of Evolution.Plate 11 — Hand. From an X-ray photograph. Overton, Applied PhysiologyXV. THE SKELETON.
In the preceding pages we have considered at some
length the means by which nutriment and oxygen are
distributed to the various tissues of the body. It will
be well for us to consider now two of these most important
tissues, the bones and the muscles, since to their action
we owe all our voluntary and involuntary movements.
The body framework.— If we examine certain of the
lower animals, such as the jellyfishes and worms, we find
that, while they have a body form, that form is subject
to distinct changes when subjected to pressure. In other
words, they are compressible to a great degree. This is
owing to the absence of any hard matter in their bodies,
that is, they have no skeleton or framework. Other ani-
mals, such as the insects, lobsters, and starfishes have a
framework, but it is on the outside of the body. While
such a skeleton is preferable to none at all, it is an incon-
venient arrangement, since in most of these forms it does
not permit any growth on the part of the animal, without
splitting of the hard layers, and thus growth in these
animals is attended with periods when the soft parts of
the body are exposed. Skeletons such as these are called
exoskeletons. In the higher animals the problem of secur-
ing rigidity and protection of the soft parts, and at the
same time allowing growth, has been solved by the devel-
opment of an inner framework or endoskeleton of bone.
By the aid of an apparatus known as the X-ray machine
(see Plate II) it is possible to take a picture of parts of
221222
THE SKELETON
the living body which shows only faint outlines of the
soft parts and brings out clearly the bony framework of
these parts. With the aid of this apparatus, or by the
Fig. 82 — Sea urchin shell, an exoskeleton.
removal of the soft parts, a definite framework of bone,
cartilage, and gristle is revealed, to which the shape of the
human body is due, and which we call the human skeleton.
This skeleton is composed of over two hundred separate
bones, so joined as to fulfil the following distinct purposes:
(a) They give shape and rigidity to the body. (b) They
protect the delicate organs and soft parts of the body,
(c) They provide .levers on which the attached muscles
may act, and thus produce motion. The skeleton of man
and the higher animals is peculiar in the development of
a special chain of bones called the vertebra:, which are
so interlocked as to form a long flexible column, the spinal
column, and this column is the central portion of the
framework. All animals that possess such a structure are
called vertebrates. Man is the highest type of a vertebrate
animal.BONE REGIONS
223
Bone regions. — On the top of
this long, curved spinal column
is mounted a box-shaped collec-
tion of bony plates which is
called the skull. This box in-
closes the delicate brain and
forms the framework of the face.
The chest, or thorax, is formed
by a series of long, curved, flat
bones called the ribs, which curve
from the vertebral at the back
around to the front of the body.
Here, they are united to a flat
structure, half bone, half gristle,
called the breastbone. This ar-
rangement secures the protec-
tion of the heart and lungs.
The abdominal cavity of the
body has no side walls of bone,
but at the bottom the soft parts
are supported by a basket-shaped
collection of bones connected to
the lower end of the spinal
column and known as the pelvis.
Finally, the arms and legs contain
a central axis of bones, which, in
the case of the arms, are con-
nected to the spinal column by
a girdle known as the shoulder
or pectoral girdle, and, in the
case of the legs, by the pelvic
bones or pelvic girdle. The
SPINAL
COLUMN224
THE SKELETON
Fig. 84 — Spinal column; A, side view; 7?, rear
view ; C\ 1-7, cervical vertebra*; D, 1-12, dor-
sal vertebras; L, 1-5, lumbar vertebrae ; S, sac-
rum ; Co, coccyx.
bones that compose
the column and those
that frame the thor-
acic and abdominal
cavities are spoken
of collectively as the
axial skeleton. The
bones which form the
appendages (arms and
legs) arc called the ap-
pendicular skeleton.
The Axial Skele-
ton.
The spinal column.
— The bones that
make up this long,
curved column are
called the vertebrae.
These vertebrae are
twenty-four in num-
ber and arc divided
into three regions.
The seven nearest
the skull are called
rcrrical vertebrae and
constitute the cervi-
cal or neck region.
The next twelve are
called flic dorsal ver-
tebra' and carry theA DORSAL VERTEBRA
225
twelve pairs of ribs. This is called the dorsal region.
The remaining five are very large and are called the lum-
bar vertebrae. Together they make up the lumbar or
loin region. Below these lumbar vertebrae is a single large
bone called the sacrum. This is actually composed of
five vertebrae, separate in the young child but fused in the
adult. On the end of this sacrum is a curved tip of bone
(the coccyx) formed of four separate bones fused together.
(For regions, see Fig. 84.)
Structure of the vertebrae. — (See Ex. XLIX., c.) The
vertebrae are all irregular bones with a central portion
called the body, and many projections or processes. The
first two cervical vertebrae are formed for a speciid purpose
and are called the atlas and axis. The remaining verte-
brae have the same general form, differing only in size.
We shall first consider a typical vertebra and then note
the variations in the other forms.
A dorsal vertebra. — Viewed from above (see Fig. 85),
i
A	B
Fig. 85 — Dorsal vertebra; A, top view; B, side view; 1, centrum; 2, neural
cavity; 3, spinous process ; 4, transverse processes.
it shows the following regions: First, a central mass of
bone called the body, or centrum, which, in the human
body, extends toward the ventral side. From this on each
side an arch of bone curves backward inclosing a cen-
EDDY. PHYS. —15.226
THE SKELETON
tral or neural cavity, through which the spinal cord passes.
This arch is called the neural arch, and from it are given
off seven processes or projections of bone.
One extends from the very center of the arch dorsally,
and is called the spinous process. It is the end of this pro-
cess that one can feel as he passes his finger down the
middle of the back. Two lateral processes extend from
each side of the neural arch, and to these the ribs are
attached. They are shorter than the spinous process.
The remaining four processes are simply raised projec-
tions lined with cartilage, and arc called articular pro-
cesses. Two face dorsally, two ventrally, and they connect
Fig. 86 —I, cervical vertebra; II, lumbar vertebra ; 1, centrum; 2, neural cavity;
3, spinous process ; 4, transverse processes ; A, top view ; B, side view.
with similar processes on other vertebra;, thus permit-
ting the bones to articulate and move upon one another.
The centrum is smooth on each side, and between succes-ATLAS AND AXIS
227
sive centra are inserted pads of cartilage. The centra
and pads bear the weight of the body. On the centra
are also two cartilage-lined sockets which support a part
of the rib.
The cervical vertebrae. — These are very similar to the
dorsal vertebrae except for the absence of the rib articu-
lations. The transverse processes are poorly developed
and are perforated. The centrum is small, and the spinal
process short and often forked. They are specially devel-
oped to carry the head and give freedom of action.
The lumbar vertebrae. — These are the largest of all
the vertebrae and have no rib attachments. Their spinous
O
Fig. 87—A, atlas; B, atlas and axis; a, 6, articulations with occipital bone;
c, transverse ligament; o, odontoid process.
and transverse processes are short and stout, and well
adapted for the attachment of strong muscles. The heavy
centra allow them to support weight.
Atlas and axis. •—- The two top cervical vertebrae have a
special function to perform, namely, to support the skull
and permit its free movement from right to left and for-
ward and back. To satisfy this requirement they show a
special modification. The top one, known as the atlas,
is fastened to the skull. It has a small centrum and a
large neural arch. This arch, or ring, is divided by a228
THE SKELETON
transverse ligament into a dorsal and ventral portion.
The spinal cord lies in the dorsal portion, while an upright
process (the odontoid) of the second vertebra or axis
project^ through the lower larger portion. On the top of
the atlas are two broad facets lined with cartilage on
which the skull rests. By this arrangement, as the skull
is turned to right or left by the muscles the atlas rotates
about the peglike odontoid process of the axis, which
thus forms a sort of pivot. In nodding, the surfaces a
and b (see Fig. 87) of the atlas, permit the skull to rock
back and forth on them. This odontoid process is a devel-
opment of the centrum of the second vertebra.
The spinal column as a whole. — The total length of the
column is about twenty-eight inches in the average man.
Its curvature may be readily observed from a glance at
the figure (see Fig. 84). This curvature and the cartilage
pads between the centra give springiness to the column,
while the articulations permit a certain amount of play to
the bone. The curvature also prevents a sudden jar to
the base of the brain. The greatest amount of movement
is between the cervical vertebra?, but the possibility of
bending the whole back indicates the flexibility of the whole
column.
The placing of the heavier vertebra? at the bottom of
the column is an adaptation to compensate for the greater
weight upon that part. The neural arch protects the
spinal cord, and the apertures between the arches and
openings in the vertebrae themselves protect the nerve
roots which branch from the cord.
The ribs and sternum. — The ribs are flat, double-curved
bones, twenty-four in number, and arranged in pairs on
each side of the twelve dorsal vertebrae. They are jointedTHE BIBS AND STEENUM
229
to the vertebrae by two processes, one of which rests
against the transverse process and the other against the
centrum. At the front of the body the curved ends of
the first seven pairs end in cartilages which unite them
to the flat breastbone, or sternum. The eighth, ninth,
and tenth pairs end in similar cartilages, but these are
fused together into one common cartilage which is at-
tached to the lower
part of the breast-
bone. The eleventh
and twelfth pairs are
not attached, and are
called floating ribs.
The sternum is a
flat, irregular bone
formed of three parts
in the adult. The
upper piece is broad,
and the collar bones
fit into notches in the
top. On the sides are
two other notches to
which the first pair
of ribs is attached.
The middle piece is
narrower, and to it
are attached the fourth, fifth, and sixth pairs of ribs. The
last piece has no rib attachments. The ribs are so
arranged as to permit the expansion and contraction of
the organs of the chest cavity and at the same time to
protect the inclosed organs. We shall consider their
action further in our study of respiration.
Fig. 88 — Ribs and sternum.230
THE SKELETON
The Appendicular Skeleton.
The pectoral girdle.— Attention has been called to the
fact that the ends of two bones called the collar bones
(iclavicles) fit into the notches at the top of the sternum.
These bones may be readily located by the fingers in front
and on each side of the neck. These bones extend from
the sternum to the shoulders, where their ends form part of
the socket for the upper arm bone. The other parts of
the socket are formed by two large, triangular bones which
a
Fig. 89 — The pectoral girdle; a, backbone ; b, scapula; c, shoulder joint; d, arm
bone; e, clavicle ; /, rib ; g, breastbone (sternum).
fit against the back and are held in place by muscles and
ligaments. These triangular bones are called the shoulder
blades, or scapulas (see Fig. 89). The scapulas, clavicles,
and the upper arm bones on each side of the body meet at
a common point (the shoulder joint), and the only attach-
ment of these three bones to the axial skeleton is the place
where the clavicles are jointed to the sternum. Such an
arrangement permits of great play to the arms and shoul-
ders. Since the arrangement of clavicles and scapulars is
such as to encircle the upper part of the thoracic cavity,
these four bones are called the pectoral girdle.
The pelvic girdle.— On the sacrum arc two large facets,
or surfaces, to which are attached, one on each side, twoTHE PELVIC GIRDLE
231
large, irregular bones called the iliac or pelvic bones. These
bones curve forward to form a basketlike support for the
soft parts of the abdominal cavity. Two other bones are
a
Fig. 90—The pelvic girdle; A, diagram of bone relations; B, the bones of the
girdle ; a, sacrum ; b, hip joint; c, leg bones ; d, pubis ; e, ischium ; /, ilium.
attached to the front of each ilium. One of these, curving
downward, forms a loop and is called the ischium. The
other fuses with the ilium and is called the pubis. The
meeting of the edges of the pubes completes the arch, or202
THE SKELETON
girdle. In each ilium is a deep socket into which fits the
round head of the upper leg bone.
Like the arms, the legs are attached to the axial skel-
eton by a girdle of bones, but, unlike the pectoral girdle,
this pelvic girdle is very rigid, and the movements of the
legs are confined to the bones of these members.
The arms and legs.— The bones of the arms and legs
correspond very closely in number and arrangement. The
upper part of the arm
contains a single long
bone (the humerus), the
upper end of which is
jointed at the shoulder
to the surface of the
clavicle and scapula.
To the lower end of the
humerus are jointed two
bones (radius and ulna)
which form the forearm,
and this joint is called
the elbow.
In the leg the upper
bone, or femur, is jointed
to the pelvic bones; it
humerus of the upper
arm, but the socket where it joins the ilium (the hip
joint) is much deeper than the shoulder joint. The join-
ing of the two bones of the lower leg to the lower end
of the femur forms the knee. These bones arc called the
tibia and fibula. While the radius and ulna arc of nearly
equal size, the tibia, on the contrary, is much larger than
the fibula, and forms the main support of the lower leg.
corresponds in position to theBONES OF THE HEAD
233
The lower ends of the radius and ulna are jointed to eight
irregular bones called the carpals, and these eight carpals
are so joined as to permit of one sliding a little over the
surface of the other. This sliding gives some flexibility
to the wrist, of which these bones form the support.
In the ankle are seven irregular bones which correspond
to the carpals and are called the tarsals. One of these
tarsals is much larger than all the others, and forms the
heel. The tarsals are
less movable than the
carpals.
Extending from the
wrist are five rela-
tively long bones
• called the metacar-
pals which form the
hand. In the foot the
instep is formed by
five similar bones
called the metatar-
sals.
The fingers and
thumb are formed of
fourteen bones called
the phalanges. The thumb has only two phalanges, the
fingers three. In the foot, the toe bones are also called
the phalanges; the great toe showing two, the others three.
Bones of the head.— The bones which form the head are
divided into two groups, those which form the brain box
being called the cranial bones, while those that make up
the face and jaws are called facial bones. The two together
constitute the skull■
Fig. 92 — The skull; 1, frontal bone; 2, parietal
bone ; 3, occipital bone; 4, temporal bone; 5,
nasal bone; 6, malar bone; 7, superior maxil-
lary bone; 8, lachrymal bone; 9, inferior maxil-
lary bone.234
THE SKELETON
Cranium.—This is a box almost spherical, formed of eight
bones. The bone that forms the forehead is called the
frontal bone. Back of this, forming the top and part of
the sides of the box, are two large parietal bones; while
the back of the box is a single large bone, the occipital.
The occipital has a large opening (the foramen magnum)
through which the
brain communi-
cates with the
spinal cord. Two
smaller bones, the
temporals, and part
of the wedge-
shaped sphenoid,
complete the sides
of the box. In the
temporals are cavi-
ties which contain
the delicate parts
of the ears. The
lower part of the
sphenoid bone, to-
Fig. 93 —The skull; 1, frontal hone; 2, parietal	,i
hone; 3, temporal hone ; 4, sphenoid hone ; 5, eth- £GtllGl vi It 11 ali-
moid bone ; 6, occipital hone ; 7, superior maxillary otllGT boilG C&IIgcI
(upper jaw) hone; 8, malar hone; 9, lachrymal
hone; 10, nasal hone; 11, inferior maxillary (lower t-llG cthlYlOlCl, form
jaw)bone-	the base of the
cranium and separate it from the face portion. On each
side of the foramen magnum are large facets which articu-
late with those of the atlas.
The face bones.— The face is composed of fourteen
bones, of which twelve are paired. One pair, the molars,
or cheek bones, form the base of the eye socket and theUSES OF THE SKULL
235
upper part of the cheek. Another pair, the nasals, form
the front and sides of the nose, while between these is a
partition formed by a single bone, the vomer. The other
paired bones of the face are: (a) The maxilla or upper jaw
bones which carry the upper teeth and form most of the
hard palate. This palate separates the nasal cavity from
the mouth cavity. (5) Back of these maxillae are a pair
of bones which complete the hard palate and are called the
palate bones, (c) Two small, thin bones, which form the
part of the eye socket nearest the nose, are called the lach-
rymal or tear bones. (d) The inferior turbinate bones, or
the spongy bones which lie on the inside of the nose,
form the outer wall of each nasal chamber.
A single large bone, the mandible, forms the lower jaw,
and completes the list of face bones.
Besides these bones of the face and cranium, there are
in the ear of the human being, three small, separate bones
known from their shape as the hammer, the anvil, and the
stirrup bone. Another separate bone, the hyoid, forms
the support for the base of the tongue. The mandible is
the only movable bone of the skull, all the others being
dovetailed together by teethlike projections (sutures) into
one rigid structure.
Uses of the skull.— This rigid box with its rounded
surface forms an ideal protection for the delicate brain.
The major part of the nose and ears is made of flexible
cartilage, thus protecting these projecting parts from
injury b}r blows. The sockets that hold the eyes are of
just the right depth to catch a blow upon the frontal and
malar bone without allowing it to strike the eye, while
the delicate parts of the inner ear are protected by being
placed in cavities in the temporal bone. The opening at236
THE SKELETON
the base of the skull which communicates with the neural
cavity of the spinal column permits the connection of
the brain and spinal cord without exposing these delicate
structures to blows or pressure; while the form of the
joint permits free play of the head in all directions.
Bone Structure.
Shapes of bones.— These two hundred odd bones of the
skeleton may be classified into three groups according to
Fig. 94 —Kinds of bones ; A, long bone (femur) ; B, flat bone (scapula) ; (7, irreg
ular bones (tarsals).
shape. These are the long bones, the flat bones, and the
irregular bones. To the first group belong the humerus,
femur, radius, tibia, ulna, fibula, metacarpals, etc. To
the second group belong the ribs, sternum, patellas, scap-
ula3, ilia, and the skull plates. In the third group are
included the carpals, tarsals, facial bones, vertebra, sacrum,
and coccyx.STRUCTURE OF A LONG BONE
237
Structure of a long bone.— (See Ex. XLIX., b.) A
leg of lamb or a soup bone furnishes a good example for
the study of such a bone. Examination
shows the following external features;
At each end is an enlargement called
the head, connected by a relatively nar-
row, cylindrical shaft. The enlarged
heads are usually more or less roughened
except at the very ends to allow the
attachment of muscles, while the ends
are coated with layers of cartilage to
form a smooth articulating surface where
the bone is joined to another bone.
The central shaft is smooth and smaller
in diameter than the ends, and usually
functions mainly for support. Very few
muscles are attached to this part. All
parts, except those covered with carti-
lage, are encased in a thin membrane
(the periosteum). This periosteum is a
layer of thin, tough, connective tissue
which distributes blood and through
whose agency most of the new bone is
formed.
If we cut this bone lengthwise, we
find the following internal structure:
The two heads are filled with a spongy
mass of bone tissue. Over the whole
outside is a thin layer of very hard,
compact bone, thinnest on the heads,
and thickening to form the main part
of the shaft walls. In the center of the shaft is a cavity
Fig. 95 — A long bone
split lengthwise
head; b, shaft; c,
sponge bone and
marrow.238
THE SKELETON
lined with a thin layer of spongy tissue and filled with a
soft, fatty, reddish yellow marrow. Tins marrow also fills
the spaces in the spongy tissue of the heads. It is this mar-
row that forms the red corpuscles of the blood. It is rich
in fat and gelatin (an albuminoid), and it is this last sub-
stance that forms the thickening matter of soup stock. The
structure of the long bone is excellently adapted to combin-
ing lightness with strength. It is a law of mechanics that
a given amount of material in the form of a hollow cylinder
produces a stronger structure than the same amount of
material molded into a solid rod. The bone, therefore,
by its hollow structure, secures the maximum strength
with the minimum of material. While it is true that they
would be stronger if they were kept of the same size but
made solid throughout, it would require great effort to
move such heavy structures. The structure of the heads,
also, gives strength and lightness at the same time, while
their enlarged surfaces and roughened projections fit
them for the attachment of the various muscles and
tendons. These adaptations fit the long bones to form
the framework of movable parts, such as the legs and
arms, that must be strong and, at the same time, light
enough to admit of rapid movement.
Structure of a flat bone.— (See Ex. XLIX., a.) A
sheep’s rib serves well as a type of this form of bone.
Such a bone may show slight enlargements or heads at
one or both ends, but these heads are much smaller in
proportion to the rest of the bone than in the case of the
long bone, and are entirely lacking in case of the skull
plates. In the rib, there is only one such head, namely,
the one attaching the bone to the vertebra of the spinal
column. The other end terminates in a mass of cartilage.STRUCTURE OF A FLAT BONE
239
The shaft, or body, of these bones is not cylindrical, but
flattened like a blade. It is this character of the shaft
that gives these bones their name. The vertebral end
I'M. 96 —liibs.
of a rib shows two cartilage-covered surfaces or articula-
tions that mark the points where the rib is jointed to the
dorsal vertebra. The rest of the bone is covered with
periosteum like the long bone.
The internal structure is best examined by sawing the
bone crosswise at different points. The structure appears
to be uniform throughout. The entire interior is filled
with the same spongy bone tissue that was present in the
heads of the long bone, while the outer walls are composed of
layers of hard bone. All flat bones show these layers
of periosteum, hard bone, and spongy bone. In the case of
the ribs, the spongy layer is usually filled with marrow,
but the quantity is necessarily small as compared with
that found in the long bones.
The flat bones of the skull and pelvis form excellent
supports and protection to the inclosed parts; while the
flatness and thinness of the ribs, and their flexibility,
permit the covering and movement of the organs of the
thorax without making the chest heavy and bulky.240
THE SKELETON
Irregular bones.— The general features of the irregu-
lar bones, such as the vertebrae (see Ex. XLIX., c, and
p. 225), have already been discussed. All irregular bones
show adaptations for the attachment of muscles or articu-
lations. They have an outer covering of either cartilage
or periosteum, with inner coats of hard and spongy bone.
They are without cavities except for the spaces between
the spongy tissue.
Chemical composition of bone.—If we place a clean rib in
a bottle with a 20% solution of hydrochloric acid and let it
stand for a few days, we shall find, upon removing it from
the acid, that its character has changed. (See Ex. L.) It
has the same shape as before, but is so soft as to be easily
cut with a knife, and so flexible as to permit of its being
tied in a knot. The substance of which it is now composed,
is cartilage or gristle, and this substance is identical in
structure with the cartilage that formed the pads for the
articulations. Evidently, the acid has taken away the
stiffening material.
If we burn a rib in a fire we note another change. The
bone is again without change in form, but is very white
and brittle. If we put this brittle structure into hydro-
chloric acid it entirely dissolves.
These experiments demonstrate that bones are made
of two substances: one, cartilage which will burn but is
insoluble in acid; the other, a mineral salt, of lime forma-
tion, which will not burn but dissolves readily in acid.
Another feature that the experiments teach us is that these
two substances are so mixed in the living bone that the
removal of one or the other does not alter the shape of the
bone. To the cartilage component of bone is due evidently
its flexibility, while the lime salt is responsible for its stiff-NUTRITION OR BONES
241
ness. All bones have this compound structure, those
in which the lime is most abundant being stiffer than those
in which the cartilage predominates. Bones of old per-
sons contain less cartilage and more lime than those of
young people, and hence break more easily.
If we saw a soup bone lengthwise and boil it in water
there will appear on the surface a rich collection of oil
drops. These come from the marrow of the bone, which is
very rich in fats. If, now, the water in which the bone
was boiled is allowed to cool, a solid, jellylike mass forms
whose jellylike character is due to the presence of albuminoid
material. This albuminoid material, usually in the form of
gelatin, forms the nutrient portion of soups. Food tests
also show that bones contain a small proportion of other
nutrients and some flavoring material, and when such a
mixture is combined with vegetables the combination makes
a very nutritious food. Bones alone, however, are not to
be considered as very rich in nutrient material.
Nutrition of bones. — The bones of a child are much
richer in cartilage than those of an adult. As the child
grows to an adult the bones increase in size and in mineral
matter. The manner in which this growth takes place
becomes clear only when we understand the relation of
the bone structure to the circulatory system. A glance
at the microscopic structure of bone shows it to be pene-
trated by a series of canals (the Haversian canals) by means
of which the blood vessels of the periosteum are able to
penetrate to all parts of the bone. From these canals the
blood is enabled to radiate by a series of cross channels,
or canaliculi. These canaliculi are at right angles to the
canals, and connect them with a series of cavities called
the lacunre which are arranged in concentric circles about
EDDY. PHYS. — 16242
THE SKELETON
the canals as centers. (For location of these parts, see
Fig. 97.) In these lacunae are the living bone cells
which absorb the food and transform it into bone mate-
rial. The lime part of this secretion forms in concentric
rings about these lacunae and the layers of this material
A
Fig. 97 — A, transverse section of bone (x 150) a, a, Haversian canals, b, b, lacunar :
B, longitudinal section of bone (X 100) ; a, a, Haversian canals ; 6, 6, lacunar : C,
bone lacunar (x COO); c, lacunar; cl, canaliculi.
are called lamellae. When the bones are young, the secreted
matter from the cells is largely cartilage; later, the mineral
part is given off more freely and the structure becomes
more rigid. In the marrow, as already mentioned, are
formed the red corpuscles which are added to the circu-NUTRITION OF BONES
243
lating blood in the bones. In general, bone is formed
by the bone cells located in the lacunae, and the cartilage
and lime out of which its main structure is formed are
to be understood as secretions produced by these cells
out of the food supplied to the cells by the blood. This
blood is brought into the bones through the periosteum
and the Haversian canals, and the canaliculi are simply
channels for permitting the distribution of the capillaries
and their contents to the bone cells.
In the child also, many bones are separate, which in the
adult fuse together. In the skull, for example, the parie-
tals and frontals do not meet at a point in the top of the
skull for some time. This spot is called the fontanelle
and often is not completely closed until the child has
attained an age of two years. Again, the bones of the
sacrum are five in number and separate in the child;
likewise the four bones of the coccyx, and the breast bones.
All of these bones fuse by growth, and such fusion is called
ankylosis. It is this tendency of bones to fuse so readily
in youth that makes a broken bone knit so much more
quickly in a child than in an adult; while in old age the
fusing power is almost entirely lost.XVI. THE SKELETON (continued).
Joints.
In the living body the various bones are closely united
to one another by various kinds of articulations or joints.
Some of these joints permit movement of the combining
bones, while others are rigid and form a fixed or immov-
Fig. 98 — Forms of joints ; A, hinge joint (elbow); B, ball and socket joint (hip);
C, immovable joint (skull sutures).
able joint. For convenience it is customary to classify all
joints as movable or immovable. To the first class belong
such unions as the shoulder and hip joints, the knee,
elbow, wrist and ankle joints, the finger, toe, and jaw
articulations, etc. Examples of the other class are found
in the sutures of the skull plates and the symphyses of
the pelvic bones with the sacrum. The word sutures
is restricted to dovetailed joints, and symphyses to those
in which the united bones have their edges simply pressed
together.
2Ustructure oe a joint	245
Movable joints. — There are four classes of movable
joints. To these, according to the method of joining, are
given the names, ball and socket, hinge, gliding and pivot
joints. The best examples of the first type are the joints
of the arms and legs with the shoulder girdle and
pelvic girdle respectively. The knee and elbow illustrate
the second type. The third type is found in the joinings
of the bones which make up the wrists and ankles (carpals
and tarsals). These bones slide over one another and
thus give flexibility to these
parts. A fourth type is illus-
trated by the union of the axis
and atlas and the ulna and
radius at the elbow.
Structure of a joint. — The
joint in a leg of lamb fur-
nishes a very good example
of the structure of a joint.
After all the flesh is removed
the two bones will be seen
to be held together by tough
bands of connective tissue
called ligaments. These bands
are so arranged as to permit
of motion in certain direc-
tions, and at the same time
prevent too great a separation of the bone ends. Cut-
ting away these bands, the smooth ends of the bones
with their caps of cartilage are seen. These caps prevent
friction where the two surfaces rub together. As the
ligaments are cut there escapes a thin, slimy liquid of the
consistency of a white of egg. This liquid is called the246
THE SKELETON
synovial fluid. It is secreted by a thin membrane cover-
ing the inside of the ligaments (the synovial membrane),
and acts as lubricating fluid to the heads of the bones of
the joint. The movement of the bones is produced by
muscles which are attached to them by inelastic cords
called tendons. We shall discuss these attachments fur-
ther in our study of the muscles. The roughened and
enlarged heads of the joining bones arc special adapta-
tions to the need of surface for the attachment of the
ligaments and muscles.
All movable joints show the above structure, namely,
smooth cartilage pads where the bones are in contact, syno-
vial fluid as a lubricant, ligaments to hold the bones in
place, and a synovial membrane to line these ligaments and
Fig. 100— Section of ball and socket joint.
produce the synovial fluid. The direction of movement of
a joint is determined by the form of union and the methodIMMOVABLE JOINTS
247
of attachment of the ligaments. In a ball and socket joint
the rounded head of one bone fits into a more or less con-
cave cavity in the other. Such a joint allows free movement
about this socket in every direction. The hip and shoulder
joints illustrate this type. In the hinge joint the bones
are so connected as to permit of motion in only two direc-
tions, just as a door swings upon a hinge. The elbow and
knee joints illustrate this type. In gliding joints the bones
slide over the surfaces of one another. The union of the
carpals in the wrist and of the tarsals in the ankle are ex-
amples in point. The lower jaw shows a combination of a
hinge and a gliding joint. The up and down movements
being a true hinge action while the power to protrude the
lower jaw beyond the upper is due to its gliding action.
In the pivot joint one bone remains stationary and the
other rotates upon it. When we turn the wrist from right
to left this motion is made possible by such a pivotal joint
at the wrist at the point where the ulna and radius are in
contact. In this case, the radius rolls over the end of the
ulna carrying the hand with it while the ulna remains sta-
tionary. The movement of the atlas over the surface of the
fixed axis is another illustration of this type. (See p. 227)
Immovable joints.— These are of two kinds, sutures
and symphyses. The interlocking of the skull plates illus-
trates the former t3rpe. Here the plates are held in close
union by interlocking projections much as a board is dove-
tailed to fit another board. The projections of one bone
fitting into the spaces between projections of the other.
The symphysis type is illustrated in the union of the
ilia to the sacrum. Here the two smooth surfaces of the
bones are pressed tightly together and held in place by
muscles and ligament.248
THE SKELETON
Still another form of union is seen in the joining of the
ribs to the sternum. Here a band of cartilage connects
the two bones, and the flexibility of the cartilage permits
of some motion.
CLASSIFICATION OF JOINTS.
Kind of.Joint	Names of Uniting Bones	Region of Joint
Hinge joints .	Humerus with ulna and radius	Elbow
	Carpals with metacarpals	Wrist
	Phalanges		Fingers and Toes
	Femur with tibia		Knee
	Tibia and fibula with tarsals	Ankle
	Mandible with temporal	Jaw
Hall and Socket	Humerus with clavicle and	
joints.	scapula		Shoulder
	Femur with ilia	Hip
Gliding joints .	Carpals with carpals	Wrist
	Knee cap and femur . .	Knee
	Tarsals with tarsals ....	Ankle
	Vertebrae with vertebrae	Spinal column
	Mandible with temporal . .	Jaw
Pivot joints . .	Radius with ulna ....	Wrist
	Axis with atlas		Neck
Sutures . . .	Skull plates		Skull
Symphyses	Ilia with sacrum		Back
Other joints	Ribs and Sternum ....	
Hygiene of the skeleton.— The formation of bone is
directly dependent upon the amount of lime salts in
the blood supply. On this account it is essential that
children, whose percentage of bone stiffening matter is
naturally small, should have a diet rich in these mineral
salts. Milk is especially rich in such material, and on this
account, as well as for its generally nutritious composition,
furnishes an ideal food. The disease known as rickets is
a result of such lack of bone-making material in the blood.
Bones do not increase in length and size uniformly, theHYGIENE OE THE SKELETON
249
heads and shafts being separated by a cartilagenous area
which does not solidify until late in life. On this account
bones of the young are especially subject to distortion
from pressure which, if continued, may result in permanent
change of shape or deformity. Another factor which makes
young bones sensitive to pressure is the small proportion
of lime in them. Bow legs, for example, are a result of
allowing children to walk too soon, and thus subjecting the
bones to the weight of the body before they are sufficiently
stiffened to resist this pressure. Sitting for long periods
at a bench in schoolroom when the bench is too high to
allow the feet to touch the floor may permanently bend
the thigh bones. A still more common deformity is what
is known as round shoulders. The bending of the body
over books at school, or crouching attitudes in writing,
helps to increase the natural curvature of the spine and
produce this unsightly deformity.
The ease with which bones may be molded is illustrated
in the practice of dentists in applying pressure to the jaw-
bones to straighten teeth. Braces, to remedy round shoul-
ders, are examples of this same plasticity of bone to external
pressure.
One of the worst cases of deformity from pressure is
found in the tight lacing of corsets as practiced by many
women. As a result, the ribs are bent and the decrease
in the size of the waist is accompanied by crowding of the
chest and abdominal regions to such an extent as to hinder
the action of the inclosed organs.
Still another effect of pressure applied to joints is injury
to the tissues and ligaments which result in the formation
of painful swellings, called bunions. Tight shoes are often
responsible for these painful deformities.250
THE SKELETON
Fractures.— When too great pressure is applied to a bone it
can bend no farther, and the result is a break. Such a break
tears the periosteum and injures the surrounding tissue cells,
nerves, and blood vessels. The result is a painful and serious
injury. Such a break is called a fracture. The surgeon remedies
such fractures by placing the two ends of the bones together and
binding them firmly in place with splints or a plaster cast. When
kept in this position for a sufficient length of time, the bone cells
produce new bone material and the two ends are soon as firmly
united as before the break. In young people this union takes
place very quickly, as their bone cells are in a naturally active
condition. In older persons this power of growth has become
weakened, and a long time is required for the knitting together
of the ends. Owing to the large proportion of lime in the bones
of old people, their bones are much more brittle than those of
children and much more easily fractured.
If a bone is broken in two without splintering, it is called a
simple fracture. When the break is not a clean break but splin-
ters are formed, it is called a splintered fracture. If the broken
end of the bone or bones work their way out through the flesh
and are exposed, the fracture is called a compound fracture.
Sprains.— When joints are bent more than is natural the
ligaments and surrounding muscles are stretched and torn. The
result is the production of inflammation with swelling and pain.
Such an injury is called a sprain. The remedy requires the reduc-
tion of the swelling and inflammation, and hot or cold applica-
tions will usually produce the desired result.
Dislocations.— When the bending of a joint is so great as to
force the ends of the bones apart, the result is called a dislocation.
Accompanying such dislocations is the tearing and stretching of
ligaments and muscles and all the results of a sprain. The first
step in the repair of such an injury is to replace the dislocated
bones in their original position. This may often be done by
simply pulling on the bone until it settles back into its place.
This result is usually indicated by a snap as the ends come to-
gether. After the dislocation is repaired the swelling and inflam-
mation may be reduced by methods similar to those employed in
treatment of sprains.JOINT DISEASES
251
Joint diseases.— Inflammation of the synovial membrane often
produces a great flow of liquid and a painful distention of the
joint. This often occurs in rheumatism. Again, waste matter
often accumulates in the synovial membranes and cartilage, and
results in another painful disease known as gout.
Sometimes bacteria enter the joint and produce a discharge
of yellow matter with painful swelling. Such a case is that known
as tuberculosis of the joints.
If the deposit of lime matter is too great it often accumulates
in the joint and fuses the ends of the bones together, making
them rigid and the joint immovable. Such a disease is called
ankylosis of the joints.XVII. SKELETONS OF THE LOWER
ANIMALS.1
The most striking point of difference between the higher
and the lower animals is the difference in their skeleton or
supporting framework. In fact, it is customary to sub-
divide all animals into two groups (vertebrates and inver-
tebrates) on the basis of the presence or absence of a
backbone.
Invertebrate animals are characterized either by the
entire absence of skeletal structures, as in the slugs, worms,
jellyfishes, etc., or by the presence of an external protec-
tive layer called an external skeleton. A few, such as
Fig. 101 —Protozoans (foraminifera) with external skeleton.
the cuttle fishes, have an internal framework, but it is
not in any way similar to the internal skeleton of the
vertebrate animals. Vertebrates all possess an internal
1 See Footnote, p. 146, Chapter X.
252SKELETONS OP INVERTEBRATES
253
jointed skeleton whose central structure is the flexible
column of bones called vertebrae.
Skeletons of Invertebrates.— In certain of the protozoans
the cytoplasm secretes a layer of substance which, collect-
ing over the surface of the animal, forms a protective
outer membrane or wall. In certain cases this membrane
becomes the seat of deposit for lime or siliceous material,
and this deposit gives it a firm character which increases
its power of support and protection. (See Fig. 101.)
In the sponges certain cells in the body produce spicules
of lime, or silica, or spongin, which are embedded in the
walls of the animal and form an interlacing network. The
ordinary sponge of commerce consists of the spongin
skeleton of such animals with the animal parts removed.254
SKELETONS OF THE LOWER ANIMALS
In certain of the coelenterates, such as the coral polyps,
the walls secrete a limestone matter, which increases to
such amounts that the animals which produce it finally
lie in crevices of this hardened secretion, which thus forms
masses often larger than the animals themselves.
These secreted skeletons are all more or less rigid, and
while they function as supports, or protective material, they
Fig. 103 —A coral colony; living polyps in limestone which they have secreted.
are entirely without joints. Such a skeleton is evidently
entirely unadapted for animals that are to move about in
search of food, and whose parts
must be capable of movement.
Such animals require a modifica-
tion of this rigid, one-piece skele-
ton, and in the starfish we find
a simple change in this direction.
These animals are covered with
an external limestone secretion which is in the form of
plates, but these plates are small and are united by con-
Fig. 104 — Arrangement of
plates in a starfish ; M, soft
membrane ; P, hard plates.SKELETONS OF INVEUTEBRATES
255
nective tissue of a flexible character, so that, like a chain-
mail shirt, the parts of the body may bend and still be
protected and supported.
The mollusks (oysters, clams, snails, etc.) have a cer-
tain portion of the body, called the mantle, whose function
it is to secrete hard material, and this material collects to
form a structure called a shell. These shells are external
and completely in-
close the soft body.
Those mollusks
that require an
opening for the en-
trance of food and
air, and that, un-
der ordinary condi-
tions, are station-
ary, secure this re-
sult by having the
shell hinged and
provided with mus-
cles which can open
and close it when
necessary. Others, such as the snails, have the shell of one’
piece, but so mounted that the animal can carry it on his
back when moving about, and when disturbed can with-
draw the body into it until the disturbing factor has
disappeared. Still other forms of mollusks have the shell
inside the body, where it serves to give the body a cer-
tain stiffness, as in the cuttlefish and squid. These
internal shells are of no use in protecting the soft parts
of the body.
In the arthropods (insects and lobsters, etc.) we have
h
Fig. 105 — Left shell of clam; h, liinge ligament; t, c,
t', hinge teeth ; a, apoints of attachment of the
muscles, by means of which the clam closes its shell.256
SKELETONS OE THE LOWER ANIMALS
the most perfect type of external skeleton found. In
these animals the skeleton is formed of a horny material
called chitin, which completely incloses all parts of the
body, except the apertures for the mouth and other open-
Fig, 106 — External jointed skeleton (lobster).
ings. By making this secretion less stiff at points, very
perfect joints are formed which allow a great freedom of
movement to the legs and body. The arthropods are,
in fact, animals encased in a suit of excellently jointed
armor. This form of external skeleton, while excellent as
a protection, and well adapted to movement, necessarily
restricts the growth of the animal. In other words, when
the crustacean increases in size he is in the same situation
as the small boy in a tight suit of clothes, and the result is
the same, he bursts the clothes and must have a new and
larger suit supplied. The manner in which this is done
is by withdrawing the body from the old, broken covering,
and after expansion, allowing the new one to form fromTHE FISH SKELETON
257
secretions of the soft skin. Meanwhile the animal is sub-
ject to all sorts of dangers from its enemies, since between
the time of shedding of the old skin and the forming of
the new it is wholly unprotected. This is one reason why
it so difficult to raise enough lobsters to supply the
demands of the people, although the number of eggs
hatched each year is enormous. The young are eaten
between molts by all sorts of enemies.
Vertebrate skeletons. — Fishes, frogs, reptiles, birds, and
mammals all agree in the possession of a jointed, internal
framework, and differ only in the number of the bones, the
composition of the bones, and the arrangement with
regard to one another. All possess a central column or
backbone made up of separate pieces called vertebrae, and
on this column are hung the other bones of the body.
The fish skeleton.— The skeleton of the fish differs from
that of the other Vertebrates in containing a higher pro-
Fig. 107— Skeleton of a fish ; note the great number of bones.
portion of cartilage. A few forms show remnants of an
external skeleton in the form of the scales which in the
garpike are so hard as to offer some protection to the
underlying parts.
EDDY. PHYS. —17258
SKELETONS OF THE LOWER ANIMALS
The absence of lime in the bones of the fish makes them
extremely flexible, and fits the body for the bending nec-
essary to swimming motion. Furthermore, the support
which the water furnishes to the body makes such hard-
ness unnecessary as a supporting structure. Another fea-
ture of the fish skeleton is the great number and small size
of the bones. (See Fig. 107.)
The frog skeleton. — In the amphibians, of which the
frog is a type, we find a marked difference in the composi-
tion and number of
the bones, while the
arrangement of the
separate bones much
more nearly follows
the order in our own
bodies. The explana-
tion of tills change is
found in the adapta-
tion necessary to fit
the animal for a land
existence. As soon as
Fig. 108- Skeleton of a frog-	the animaJ ig removed
from the support of
water, the flexible, cartilage bones are unable to support
the body, and we should expect, therefore, to find the bones
of the frog more rigid than those of the fish. The new
method of motion requires the modification of fins into
legs and arms, and the front legs of the frog are supposed
to have developed as modifications of what we call the
pectoral fins of the fish. In the same way, the hind legs
of the frog are modifications of the pelvic fins of the fish.
Another reason for the rigid character of the frog skeletonREPTILE SKELETONS
259
is the necessity for a stiff framework against which the legs
may push in propelling the body. If the backbone were
too flexible it would simply bend when the legs pushed
against it, and the animal would not move any more than
does an object when pushed with a feather.
The frog skeleton will be seen to differ from that of the
higher forms of animals, though not so much as from that
of the fish. For example, the frog has a much shorter back-
bone than is found in higher forms, the ribs arc wanting,
and the skull is made up largely of face bones with a very
tiny cranium or brain box. As a machine for support and
motion, it is very primitive compared with our skeleton.
The legs are so attached that the thigh bones point out-
wrard from the body instead of being under the part to be
supported. The shoulder girdle is rigid and the elbows
point outward, which makes it awkward to use the front
legs as pushing agents. In fact, in the frog these front
legs are of small use in locomotion on land; the greatly
elongated hind legs being so arranged as to make leaping
the natural mode of progress.
Reptile skeletons.— The reptiles differ from the amphibi-
ans in possessing ribs (see Fig. 109), which have been de-
Fig. 109— Skeleton of a crocodile.
veloped to protect the sides of the body and to aid in the
operation of the lungs as breathing organs. In the snakes
the legs are lacking and there is no breastbone. In all the260
SKELETONS OF THE LOWED, ANIMALS
other forms, there are a breastbone and well developed legs.
These legs, however, are poor organs of locomotion and
support, for, like the frog’s, the thighs point outward, and
instead of raising the body from the ground the legs are so
short that the belly rests on the ground and must be dragged
rigid, to form a firm support for the wing joints, while the
greatly elongated breastbone, with its deep keel, is an excel-
lent adaptation for the attachment of the powerful flying
muscles. Both the birds and reptiles have a larger cranium
than the frogs or fishes. Another feature is the arrange-
ment of the legs. In the bird the toes are the only part of
the foot that is in contact with the ground, for the heel is
over the surface. The
elbows point backward
more than in the frog,
and the arm, or foreleg,
is, in this respect, an
improvement over the
similar structure in the
frog.
Fig. 110 —A bird skeleton (vulture).
Bird skeleton. — The
bird skeletons are in-
teresting as studies in
adaptation. In these
animals everything has
been sacrified to light-
ness and a flying habit.
The bones are hollow
and contain no marrow.
The arms are developed
as wings, the shoulder
girdle is sturdy andMAMMAL SKELETONS
261
raised and the metacarpals are limited to one bone. The
S-shaped position of the thigh bone, tibia and metacarpal
forms a splendid arrangement for preventing a jar when
the bird alights.
Mammal skeletons. —The most striking differences
between the mammal structures and the human skeleton
have to do with the adaptation of the body to an erect
position, and the consequent freeing of the arms from the
necessity of support. The development of the hand as a
grasping organ, and the relations of the bones in the different
forms of the arm and leg structure are interesting also as
showing adaptations to different modes of use. The fol-
lowing figures illustrate the relations of the bones of the
Fig. Ill — Typical fore limbs ; A, of man ; B, of cow ; C, of cat; D, of horse ; E, of
whale ; F, of bat; G, of mole.
arms and legs in animals of different habits; the important
point to be observed is the manner in which the same262
SKELETONS OF THE LOWER ANIMALS
bones are modified to serve different uses. Nature’s
economy in this respect is remarkable. For example, in
Figure 111 note the arrangement of the hands in the mole
and their adaptations for digging. Note, also, in the cat
the development of the finger and toe nails into claws for
tearing. The flipper of the whale shows a structure very
similar to that of the fins of the fish; in the horse and
the cow, the development of the nail into hoofs and the
reduction of the metacarpals into a single bone support,
Fig. 112—Typical skulls ; A, of man ; B, of cow ; C, of bat; D, of mole ; E, of
cat; F, of horse : G, of whale.
is interesting. In the bat, the enormously elongated
phalanges serve as a framework for the membranous wings.
Note, also, the differences in the legs of these animals
from that of man. In man, the whole foot rests on the
ground; while in the cat, the horse, and the cow the toes
alone support the body, thus permitting the development of
powerful thigh and calf muscles at points where they willGENERAL CONCLUSIONS
263
not interfere with the movement by their bulk and yet
give tremendous power to the pushing action of the legs.
Finally, a comparison of the skulls (see Fig. 112) is
interesting as an illustration of the progression in size of
the brain box, and the increase in intelligence as we go
higher in the scale; while the jaws with their teeth modi-
fications show every variety of adaptation to different
foods and modes of eating.
General conclusions. —We may note, in conclusion,
that the bones of all vertebrate animals are remark-
ably similar in arrangement and position, but that every
animal shows modifications in structure and position,
that fit his particular framework to his needs.264 4
MUSCLES
Plate III — Muscles.XVIII. MUSCLES.
Kinds, Structure, and Composition.
In the skeleton we have a well-jointed framework that
of itself has no power of movement. In order to move
the different bones and produce the different contractions
and expansions of the organs of the body, a special tissue,
called the muscles, is devised. Muscles, then, are respon-
sible for all movement in our bodies.
Kinds of muscle tissue.— Every boy knows that when
he bends his forearm at the elbow a swelling appears on
the upper side of the upper arm. This swelling, which the
boy speaks of as his “ muscle,” is, in fact, a separate mass
of muscle tissue and known technically as the biceps
muscle. Its action illustrates two characteristics of
muscle tissue. First, the production of motion by con-
traction. Second, that contractions are under control,
for in this case the boy determines when they shall contract.
The muscles which form the walls of the intestines and
produce the grinding movements that we call peristalsis,
do so by contraction also, but their action differs from the
action of the biceps and triceps in that it is not under the
control of the will. We cannot stop the action as we wish.
It was formerly customary to make this matter of con-
trol the basis of classification of muscles. Muscles whose
contractions required no act of will on our part were called
involuntary muscles. Those which arc subject to our
direct control, like the muscles of the fingers, arms, and
legs, were called voluntary muscles. The difficulty of this
265266
MUSCLES
classification arises from the fact that some muscles are
both voluntary and involuntary, for example, the muscles
that close the eyelids.
Furthermore dissection has shown that all muscle
tissue in one body may be put into one of three classes,
each with a distinct structural formation and it is this
structural classification we shall adopt. These three classes
are known as striated, non-striated, and heart muscle.
These three kinds of muscle tissue combined constitute
nearly half of the entire weight of the body, and in their
action is consumed the greater part of the food and oxygen
supply of the body.DISTRIBUTION AND FORM
267
Striated Muscles.
Distribution and form.— (See Ex. LIII.) If we remove
carefully the skin of any vertebrate animal, we find masses
of flesh or meat just below this skin. This flesh is composed
mainly of striated muscle tissue. We may note further,
that this mass of flesh is composed of separate bundles of
varying size and shape, each bundle covered with a glisten-
ing sheath and attached to the bones only at certain points.
Such a bundle constitutes a separate muscle, and there are
five 'hundred of such separate bundles or muscles in our
body. In their sum they determine the form of the animal
body. Although there are so many of these muscles, they
admit of classification under comparatively few heads.
The type forms are as follows:
Bellied muscles. These muscles, of which the biceps
muscle already referred to is a type, receive their name
from their shape. They are spindle-shaped bundles with
a swelling or belly in
the center, and taper
at each end into one
or more white, in-
elastic cords called
tendons (see Fig. 114).
These tendons serve to attach the body of the mus-
cle to the bones which it is to move. As these muscles
contract, the belly swells and the spindle shortens, draw-
ing the two attached ends nearer together. If these ends
are attached to movable bones, these bones must necessa-
rily move as the spindle shortens. In some cases, the bones
at each end move. In others, only one of the bones moves.
The end of the muscle that is attached to the bone268
MUSCLES
that moves most is called the insertion of the muscle.
(See Fig. 115, A.) That attached to the end which moves
Fig. 115—A, bones of the arm, showing origin and insertion of biceps (a) and
triceps (b) muscles; B, » digastric muscle.
least, is called the origin of the muscle. By following
with the fingers the curve of the biceps muscle, the tendon
that attaches it to the radius bone of the forearm and its
place of attachment to this bone can be readily felt. The
other end is covered with other masses of muscle, but dis-
section shows it to end in two tendons which are attached
to the shoulder blade. In bending the forearm, it is
evident that the radius moves most; hence the radius end
of the muscle is its insertion in this case, and the shoulder
blade its origin.
It must be noted, however, that these terms insertion and
origin are merely relative terms. For example, in the oper-
ation of “ chinning ” oneself on a horizontal bar the
radius bone remains fixed and the shoulder blade is the
bone moved. In this case the radius end is spoken of as
the origin of the muscle, and the shoulder-blade end as the
insertion. Most muscles, however, are so arranged that
one end is always the insertion and the other the origin.
Many bellied muscles have a single tendon at each end.GROSS STRUCTURE OF STRIATED MUSCLE 269
Others divide at one end into two tendons, and are called
biceps or two-headed muscles on that account. Still others
divide into three parts and are called triceps muscles. In
another form of muscle there are two bellies attached by
tendons and with tendons at each end. Such a form is
called a digastric muscle (see Fig. 115, B); while others
show as many as four bellied portions, and are called poly-
gastric muscles.
Not all muscles end in tendons. In some cases the
muscle ends are attached directly to the bones. Nearly all
the muscles of the arms, legs, hands, and feet are of the
bellied variety.
Flat muscles. On the trunk are found many muscles
which are flat and attached directly to the bones. Such
muscles are called flat muscles. They are of nearly uni-
form thickness throughout, and are found in places where
the belly would interfere with the action of other organs
or make the part too bulky. The muscles on the chest
are of this variety.
Arrangement of striated muscles. — The bellied muscles
of the body are usually arranged in pairs (see Fig. 115, A).
Those which bend bones about a joint as pivot when they
contract are called flexors. The biceps muscle is a type
of a flexor muscle. Those, on the contrary, which by con-
traction, extend a bone about a joint as pivot, are called
extensors. The triceps muscle, whose insertion is the end
of the ulna and whose origin is the humerus and scapula,
is an example of this type. The flexing and extending of
the forearm is due to the alternate contraction and relaxa-
tions of these two muscles and illustrates the typical paired
arrangement.
Gross structure of striated muscle. — (See Ex. LIV.).270
MUSCLES
A bellied muscle, dissected from the leg of a rat or frog,
illustrates clearly the gross structure of a muscle. Exam-
Fig. 116. — Flat muscles of the back.
ination of such a dissected muscle shows it to be entirely
inclosed in a connective tissue sheath called the 'perimy-
sium. This sheath is thin and transparent, and, at theGROSS STRUCTURE OF STRIATED MUSCLE 271
edges of the muscle, fuses into a glistening, white, inelas-
tic cord called the tendon. These tendons are very use-
ful in the body, because they permit the contracting or
belly part of the muscle to be placed at some distance
from the part to be moved, and the inelastic cords trans-
mit these contractions without loss of power to the parts
to which they are attached, and, at the same time, this
arrangement avoids bulkiness. Thus, the fingers are
flexed and extended by muscles whose bodies are located
in the forearm, and whose contractions are transmitted
to the finger bones by tendons attached to the bones at
one end and to the muscles at the other. The extensor
tendons on the back of the hand can be readily followed
to their origin and insertion. It is evident that such an
arrangement gives much greater delicacy and ease of
manipulation to the hand than would be the case if
each finger were attached directly to a muscle. This
tendon method of attachment is especially noticeable
in the limbs where bulkiness
is most undesirable.
If we boil a piece of the
dissected muscle or a piece
of lean beef (the bellied
muscle of a beef creature),
it readily separates into a
series of smaller prism-
shaped bundles, and these
may be easily picked apart.
Examination of one of these
prism-shaped bundles shows
it to be covered with the same sheathlike perimysium which
in fact is merely a partitionlike extension of the outer peri-
Fig. 117 —Bundles of striated muscles
cut across ; /, /, several bundles bound
together to make up the main muscle.272
MUSCLES
mysium. If we proceed further, and cut across the end
of one of these bundles, or fasciculi, we find it to be divided
into smaller bundles by perimysium partitions, and these,
in turn, into still finer partitions and bundles. These
ultimate bundles may be teased apart into a certain
number of microscopic muscle fibers too small to be
examined with the naked eye. These fibers are the struc-
tural elements of the muscle. It is the combined contrac-
tion and expansion of these fibers that produces the com-
plete muscle action.
Microscopic structure of striated muscle.— Each muscle
fiber, when examined under the microscope, is found to
be from J inch to 1J inches in length, but only yfo inch
to jj-q inch in diameter. They are spindle-shaped and
taper to a point at each end. They evidently do not
extend the entire length of the bundle, which may be a
foot or more in length, but are packed together, side by
side, the pointed ends of one fiber fitting in between the
ends of others, so that each fiber extends lengthwise of
the bundle.
The fiber of a striated muscle may always be recog-
nized by its striped or striated appearance. These stripes
extend transversely across the fiber and pass entirely
through it. They are of two kinds, a broad, bright band,
and a narrower, darker band. These dark and bright
bands alternate. With the high powers of the micro-
scope can also be made out dim longitudinal striations.
If we press upon one of these fibers with a needle point
it is seen to be covered with a transparent membrane or
sarcolemma which surrounds the striped, contractile,
inner substance. This outer coat or sarcolemma is a
tough transparent membrane. Visible through the sar-M1CROSCORIO STRUCTURE OF STRIATED MUSCLE 273
colemma and scattered over the body of the fiber are sev-
eral oval bodies which indicate the true nature of the
fiber. These oval bodies are nuclei and the fiber is actu-
D
Fig. 118 —A, muscle fiber with sarcolemma removed and broken at one end into
itsfibrillse ;B, separate fibrillse ; C, muscle fiber breaking into disks ; D, muscular
fiber, the contractile substance of which (a) is torn, while the sarcolemma (b) has
not given way (x 350).
ally a composite muscle cell. Such a cell is called a multi-
nuclear cell and the sarcolemma is the common cell wall.
Viewed as a multinuclear cell the above structures bear
the following relations to the cell as we know it. The
sarcolemma is the common cell wall. The contractile
substance is protoplasm in which the power of contraction
and expansion has become greatly developed. Such
specialized protoplasm is called sarcoplasm, or muscle-
EDDY. PHYS. — 18274
MUSCLES
protoplasm. The oval bodies are nuclei. The faint,
lengthwise striations indicate a peculiar, structural divi-
sion of the sarcoplasm. Examination shows that it is
divided into long hardened portions or fibrils, which
extend lengthwise of the cell, and the spaces between these
fibrils are filled with a liquid portion more like ordinary
protoplasm. It is the faint outlines of these fibrils that
give the lengthwise striations. These columns are also
called sarcostyles to distinguish them from the liquid
sarcoplasm. Each sarcostyle is divided transversely into
disks of alternate dark and bright appearance, and these
combined disks are what give the cross striated appear-
ance. Both the sarcostyles and the sarcoplasm are merely
modified forms of protoplasm. The part played in muscle
action by these various elements of the fiber cell will be
discussed later under muscle action. (See p. 278.) Af
present, all we need note is that the substance of a
muscle cell has a highly specialized power of contraction,
and that the combined contractions of many cells united
into bundles and these into other bundles, is responsible
for the ultimate action of the muscle, just as the combined
pull of many men in a tug of war results in one motion of
the rope. Most striated muscles are under the control of
the will and were formerly included under the heading
of voluntary muscle.
Unstbiated Muscles.
Muscles of this class are found in the walls of the diges-
tive tract and organs, the blood vessels, the genital and
urinary organs, etc. Such muscles show no tendons, but
form expanded membranes that surround cavities. There-
fore they have no definite origin or insertion. The factSTRUCTURE OF UN STRIATED MUSCLE
275
that their contractions and relaxations are controlled by
nerve centers which are not under direct will controlled to
their being called involuntary muscles.
Structure.— The membranes which unstriated mus-
cles form are not in bundles like the striated muscles,
but are composed of masses of single, spindle-shaped cells
stuck together by a cementlike substance and held in
place by an interlacing net-
work of connective tissue.
These cells are only about
eh inch m length, and	n, nucleus,
while they show a longi-
tudinal striation there are no cross striations. The cells
are covered with a cell wall of transparent mem-
brane and inclose a single nucleus. They are,
therefore, to be considered as single and not com-
posite cells. The protoplasm which they contain
has the power of contraction in the direction of
their long axis, and since they are usually arranged
about a cavity their contraction and relaxation re-
sults in the reduction or enlargement of the cavity.
Heart Muscle.
The muscular walls of the heart are composed
of a tissue which is intermediate in structure
between that of the striated and unstriated
types. It consists of single, nucleated, branched
cells united to make a meshwork. These cells
show cross striations, but these striations are less
distinct than those of the skeletal muscles. The
rate of contraction of the heart-muscle cells is less than
Fig.120
— Mus-
cle cells
of the
heart.276
MUSCLES
that of the ordinary7 striated tissue and greater than that
of the unstriated. It is also remarkable for the rhythmic
character of its contractions.
Composition of Muscle.
Applying the common food tests to muscles we find
them to be composed mainly of proteid and water with a
small quantity of the salts, carbohydrates, fats, nitro-
genous wastes, and enzymes.
Muscle proteids.— There are two forms of muscle proteid: one
known as myosin, the other as myogen. The myogen is most
abundant in living muscle, for it is present to three or four times
the bulk of the other. Both are coagulated by heat, but the
myogen requires a higher temperature (131°-149° F.) than the
other (113°-122° F.). Both are similar to the fibrinogen of the
blood, for when they are allowed to stand at ordinary tempera-
tures they are capable of forming a clot, and of assuming an in-
soluble form called myogen-fibrin or myosin-fibrin, just as fibrin-
ogen is transformed into fibrin by the clotting of blood. Dead
muscle then consists mainly of clotted myosin and myogen, or
myosin-fibrin and myogen-fibrin. This clotting is supposed to
explain what is known as “ rigor mortis ” or the stiffening of the
muscles after death.
Muscle carbohydrates.— The carbohydrates present in muscle
are sugar and glycogen. It is supposed that muscle has the power
to convert sugar into glycogen and glycogen into sugar as needed.
In action the glycogen is first transformed into sugar and then
consumed in the production of the energy7. We can assume then
that the glycogen in the muscle represents sugar brought to it
by the blood, converted by it and stored in the form of glycogen
until the muscle needs it. It is a reserve supply of fuel in the
muscle.
Muscle Enzymes.— Muscles contain a number of enzymes.
The following have been identified: A proteolytic enzyme capableNUTRITION OF MUSCLES
277
of digesting proteid, and an amylotic enzyme which changes glycogen
to sugar, a glycolytic enzyme which destroys sugars, a fat enzyme
or lipase which breaks up fats, and finally an enzyme which causes
the clotting of myogen and myosin after death.
Inorganic muscle constituents. — Muscle contains many salts
which aid in securing an interchange of nutrition by osmosis and
dialysis with the lymph and blood through the sarcolemma, and
they also give to the muscle its irritability and sensitiveness to
nerve stimuli.
Water forms 75% of the living muscle.
Pigments and other components. — The red color of muscle is
due partly to the blood it contains, and partly to a pigment, sim-
ilar to haemoglobin, called myohocmatin, which probably aids the
muscle cell to absorb oxygen.
Muscle also contains several organic acids and nitrogenous
wastes.
Nutrition of Muscles. — Muscles are penetrated every-
where by blood vessels. From the larger blood vessels
smaller branches are given off, and these end at the
capillaries which surround the finest bundles of the muscle
mass. From the lymph which comes from these blood
vessels, the muscle fibers select their food and give off
their wastes to it. (See p. 175.) From the blood they
also receive oxygen and give up the carbon dioxide to it.XIX. MUSCLES (continued).
Their Action.
Our study of the structure of the muscles has taught us
that movement is produced by the contraction of the
muscles which are connected to
movable bones or so arranged as
to inclose cavities. Further, we
have learned that this power of
contraction is located in tiny,
microscopic fibers or cells, the
sum of whose combined contrac-
tions is the movement of the en-
tire bundle. Finally, we have seen
that all parts of these tiny cells
are not equally endowed with this
power, but that it is limited
to certain contractile substances
found in the cell fibers.
Study of a frog’s muscle. — (See
Ex. IV.) If we dissect away the bel-
lied muscle known as the gastroc-
nemius or calf muscle of a frog’s
Fig. 121 —Frog’s ieg, dissected	]	shall ftnc[ a white
to snow the muscle and nerve	D
connections; </, gastrocnemius threadlike fiber lying close to its
pe, etc., other muscles. surface. This fiber branches into
many separate threads, and these,
in turn, enter the body of the muscle. By means of certain
stains it is possible to follow these branches and find that
278STUDY OF A frog’s MUSCLE
279
Nerve Fiber
Fig. 122 — Nerve endings in striated
muscle fibers.
ultimately they end in the walls of the muscle fibers them-
selves. The large thread, with its branches, is a nerve, and
in the living frog the large end is continued backward to
the spinal cord through which
it connects with the brain. If
we remove all the muscles from
the leg of a freshly killed frog,
except the gastrocnemius, then
arrange such an apparatus as
shown in Fig. 123, and bring the
two ends of an electric circuit
in contact with the nerve, the
muscle will suddenly shorten and
then relax again as though the frog were alive. If now
we remove the ends of the wires from the nerve a second
contraction and relaxa-
tion takes place. Evi-
dently the shock of
making and breaking
the electric current is
transmitted by the nerve
to the muscle fiber and
acts as a stimulus to
the contractile sub-
stance which it con-
tains. Furthermore, the
experiment shows us
that a stimulus, such
as was given by the
electric current that we
sent over the nerves to the fiber, is sufficient to set in con-
traction at one time all the muscle fibers of such a mass.
Fig. 123 — Nerve muscle preparation; s, set
screw ; c, clamp , /, femur ; to, gastrocne-
mius muscle ; n, n, sciatic nerve ; h, hook ;
l, lever; t, electrodes ; b, battery.280
MUSCLES
Such a simple experiment also enables us to understand
how a desire formulated in the brain may be transmitted
as a nerve impulse to the muscle cells and thus produce
movement on the part of the muscle which receives such
impulse. Every muscle in our body is supplied with a
nerve connection, the ultimate ends of the nerves being
embedded in the muscle fibers while the main trunks are
collected into the great trunk called the spinal cord, and
through that are in connection with the brain. The brain
or cord, therefore, is the starting point of the impulse that
causes the contractions of the muscle fibers, and this
arrangement puts all the complicated movements of the
body under their control.
The power of cells to respond to stimulus is called irri-
tability. By irritabilty in general we mean the power of
a cell when stimulated to exhibit that particular form of
activity for which it has been specialized. Muscle cells,
for example, have lost many of the characteristics of a free
moving amoeba but have developed the power of con-
traction to a high degree. A secreting cell, on the other
hand, has little power of contraction but a highly developed
power of secretion. Hence, the irritability of the muscle
cell shows itself in contraction when stimulated; the
gland cell, on the other hand, manifests increased secretory
power when stimulated. In other words, each possesses
irritability, but the expression of that irritability depend^
upon the manner in which the cell is specialized. Any
impulse which can produce an expression of the cell’s
specialized power is said to excite its irritability.
If we plunge the ends of the electric wires directly into
the muscle substance the mass will contract and relax as
before, but not to so great an extent. It is evident fromCOMPARISON OP MUSCLE AND STEAM ENGINE 281
this result that the nerves are especially well adapted for
the carriage of impulses to the fibers, though they are
not the sole means of stimulating the fibers. The sudden
contraction of a muscle when struck is thus explained as
merely a response of the cells to another form of stimu-
lus. The nerves, however, constitute the normal means
of transmitting an impulse to the muscle from out-
side.
Method of muscle contraction under stimulus. — If, in
place of a simple apparatus, such as that described in the
preceding paragraph, we substitute an instrument which
can apply a rapid succession of shocks to the nerve end,
it is possible to make the stimuli so rapid that the second
contraction will begin before the first relaxation has ended.
Under these conditions the muscle rapidly shortens imtil
it finally reaches a point where it can contract no more.
Successive stimuli will, then, merely serve to maintain
it in a contracted state until finally the contractile power
becomes exhausted, and extension occurs in spite of stim-
ulation. Such a progressive series of contractions are
spoken of collectively as a tetanic contraction. It is
supposed that in the living body all nerve impulses are
of this tetanic character, as the time of a simple contrac-
tion is only about a tenth of a second, and very few move-
ments are of. so short duration.
Comparison of Muscle and Steam Engine as
Energy Producers.
The substance in the muscle cells has the power of irri-
tability, and the nerves are the natural conductors of the
stimuli which excite this irritability. What happens in
a muscle fiber or cell when this stimulus is received? We282
MUSCLES
can best understand this by comparing the results with
what takes place in an ordinary steam engine;. The com-
parison of the methods of energy production in the two
cases are shown in the following table.
The first step in the production of energy is combustion.
TABLE OF COMPARISON OF STEAM ENGINE AND
MUSCLE CELL.
Conditions Necessary to Combustion in —
The Steam Engine
1.	The presence of a combus-
tible chemical compound,
i.e., one which can combine
chemically with oxygen
(Fuel). Coal, wood, or
other carbon containing
matter.
The Muscle Cell
1. The presence of a combus-
tible chemical compound,
i.e., one which can combine
chemically with oxygen
(Fuel). The carbon com-
pounds, glycogen, sugar,
proteid.
2.	Some initial impulse (kind-
ling) necessary to start
rapid oxidation or com-
bustion. Application of
match flame or other kind-
ling material.
3.	There must be present a
sufficient supply of oxygen
to permit of rapid union
with the fuel. Supplied by
draughts of various kinds.
2.	Some initial impulse neces-
sary to start oxidation
(kindling). The nervous
impulse received by the cell
substance.
3.	There must be present a
sufficient amount of oxygen
to permit of rapid union
with the fuel. Supply se-
cured from the red cor-
puscles of the blood and
from reserve supply stored
in cells.COMPARISON OF STEAM ENGINE AND MUSCLE CELL 283
Results of Combustion in —
The Steam Engine	The Muscle Cell
1. Energy in form of heat.	1. Energy in form of heat.
2. Carbon dioxide as result of the union of the carbon in the fuel with the oxygen.	2. Carbon dioxide as the result of the union of the carbon in the fuel with the oxygen.
3. Wastes in the form of ashes or other unoxidizable ma- terial.	3. Wastes in form of lactic acid, urea, etc.
Disposal of the Products of Combustion in —	
The Steam Engine	The Muscle Cell
1.	Part of the heat energy
transforms the water in a
boiler to steam and shows
itself in the power of expan-
sion that this steam has. If
this expansion is allowed to
act upon a piston rod it is
transformed into motion.
2.	Part of the heat energy is
lost as heat, and merely
warms the surrounding air
and parts of the engine.
3.	The carbon dioxide passes
off with the smoke through
the chimney or smoke stack.
4.	The wastes pass out partly
as smoke through the chim-
ney, and partly as ashes
into the fire box.
1.	Part of the heat energy acts
upon the fibrils (sarco-
styles) or other contractile
matter in the cell and
causes it to shorten. The
result is a shortening of the
whole cell, i.e., contraction
or motion. When the heat
supply stops, this contrac-
tile matter relaxes.
2.	Part of the heat energy
escapes as heat, and thus
raises the temperature of
the body.
3.	The carbon dioxide passes off
in the blood and is removed
from the body by the lungs
and skin.
4 The wastes are carried off by
the blood and lymph and
removed from the body by
the excretory organs, skin
and kidneys.284
MUSCLES
Let us examine this analogy more closely to see
how this explanation of the production of contraction
and expansion in a muscle cell fits in with known condi-
tions for muscular activity.
In the engine, if the supply of fuel becomes lessened,
the amount oxidized is necessarily less and the power
to do work becomes diminished. Likewise, in the muscle,
any interference with the food supply must decrease the
amount of fuel and lessen the working power of the mus-
cles. It is essential, therefore, that the muscles be pro-
vided with a good supply of blood, and that this blood
contain the proper kind of food for fuel. It is for this
reason that athletes and others who use their muscles
extensively are limited to a diet rich in the foods required
for muscle building and combustion. Meats, coarse breads,
eggs, vegetables, fruits, supply all the important carbo-
hydrates and proteids. Fats and pastries are usually
excluded, not because they do not contain energy pro-
ducing material but because they are difficult to digest.
Tobacco and alcohol also tend to diminish the irrita-
bility of the cells, and should be excluded from the diet.
What is good for the athlete is evidently well suited for
the growing boy and girl, and, in general, the food of all
people should contain the materials suited to the upbuild-
ing of muscle cells and, at the same time, fitted to provide
the fuel necessary for their action.
Again, if the fire of the engine is not supplied with a
good draught, that is, a constant supply of oxygen, the
production of heat must necessarily stop. So, in the
body, the muscles must be supplied constantly, not only
with food-laden blood, but also with oxygen-laden blood.
This means that the lungs, from which this supply isEXERCISE
285
obtained, must be in good condition, and that the air which
we take into them must be rich in oxygen. Bad ventilation
diminishes the oxygen content of the blood, and diminishes
the working powder of the muscles by just so much.
Every one knows that it is necessary at times to let
the engine fires go out in order that the accumulated
wastes may be removed, and in order that the heated
parts may cool. So, with the muscles, the constant use
of the cells soon causes an accumulation of waste too
great for the circulation to remove, the body becomes
overheated, and soon the muscle loses its power to respond
to a stimulus. Such a condition is called muscle fatigue.
We feel tired, and this tired feeling is an indication that it
is time to rest and allow the cells to recover from their
activity. If we heed this call and permit the used part
to rest for a while, the blood is able to remove the accumu-
lated wastes during this period, and stock the cells writh
new fuel, and, in general, restore the muscle cells to their
working condition. On this account, children and manual
laborers who use their muscles much more constantly
than brain workers, require much longer rest periods.
Often a change of work will accomplish a restoration of
the muscles, provided the new work calls into play a
different set of muscles and allows the others to rest. A
certain amount of sleep, however, is necessary for every
healthy man, woman, or child.
Exercise.— Muscles possess another property besides
contractility, namely, elasticity. For example, if w'e sus-
pend weights at one end of a freshly dissected frog’s
muscle, the muscle readily stretches to accommodate the
increase in weight, and as readily returns to its original
length when the weight is removed, provided we do not286
MUSCLES
allow the weights to exceed a certain limit. In our bodies
a healthy muscle is always in this state of elastic tension,
as is shown by its shortening when the tendon which
attaches it is cut. This tense condition makes the muscle
respond much more quickly to stimuli, and is of great
value in mechanical movements of the body. Muscles
which are not used tend to lose this elastic tension and to
become soft and flabby. The only method of remedy is
to give them plenty of use or exercise. Exercise, there-
fore, is of value in keeping our muscles in a proper state
of tension, and, at the same time, aids in the proper cir-
culation of food and oxygen material to the used parts.
Care should be taken in the selection of the exercise that
it shall affect all the muscles of the body and not center
upon any one set of muscles. Rowing and running are
particularly valuable as forms of exercise which call into
play many muscles; and walking is one of the best forms,
since it exercises the body and leg muscles and the lung
and heart muscles, and is not violent enough to over-
fatigue any given set. What are called “ setting up ”
exercises in the army and in our schools are particularly
good, since they have been devised to fit every part of the
body and to give a general toning up of the system rather
than to develop any one set of muscles.
Another feature of exercise that should be borne in
mind is that it should be regular. A little regular exer-
cise each day, calling into play as many muscles as pos-
sible, is far more valuable than prolonged exercise one day,
followed by a long period of no exercise. Further, since
active muscle makes great demands upon the blood supply,
we should not exorcise immediately after eating when the
blood is needed elsewhere.THE MECHANICS OF MUSCLE ACTION
287
The Mechanics of Muscle Action.
In a steam engine the energy liberated by combustion
is in the form of heat. Only pait of this energy can be
converted into motion and into the lifting of weights or
overcoming of resistance of various kinds. The rest of
the energy is lost as heat. In the best steam engines the
amount of heat energy that can be converted into work
represents only ten to fifteen per cent of the entire energy
liberated by the combustion. We express this relation
of the useful energy to the total energy liberated in any
given machine, as the efficiency of the machine. A steam
engine, then, has an efficiency of only 10% to 15%. Ex-
periments with the muscles have demonstrated that the
ordinary muscle has an efficiency of something like 33%.
In other words, nearly a third of the energy liberated in
the combustion of fuel in the muscle cell can be converted
into useful work, while the rest is lost in heat. While,
perhaps, this amount seems small compared with the
heat loss, if we compare the muscle with the best steam
engines it will be seen that it is a remarkably efficient
machine. This efficiency represents an average, and dif-
fers in individuals.
The simplest method of expressing this useful energy in
any machine is in terms of the weights or resistance which
is overcome and the distance through which the action
takes place. Thus, the energy necessary to raise a pound
one foot is expressed as one foot pound of energy. In other
words, work is equal to the force exerted, times the distance
through which it acts. In formula:	Work = Force
X Distance. It is evident from this formula that a given
amount of energy may move a large body a short distance288
MUSCLES
and a small body a long distance without varying the
total value of the work done. Thus with ten foot pounds
of energy we may raise a ten-pound weight one foot or a
one-pound weight ten feet.
Men have contrived various pieces of apparatus called
machines by means of which weights, and the distances
they are moved, may be varied without changing the total
amount of energy required. One of the simplest of these
machines is the lever.
Laws of the lever. — A lever is a rigid bar turning about
a fixed point or pivot. There are three classes of levers
known respectively as first, second, and third class levers.
Fig. 124 — Levers ; I, lever of the first class ; II, lever of the second class ; III,
lever of the third class ; W, weight ; F, fulcrum, or pivot; P, power.
In a first-class lever the pivot is between the weight to be
raised and the force applied to raise it. (See Fig. 127.) In
the second-class lever the weight is between the pivot and
the power, while in the third class the power is between the
pivot and the weight. Tn each of these classes the points
nearest the pivot evidently move a less distance than those
farther away, and by arranging at different distances from
the pivot the power and weight applied, it is possible to
make a small power balance a large weight and vice versa.LEVERS IN THE BODY
289
Expressed in terms of distance from the pivot this relation
is known as the law of levers.
Weight times the perpendicular distance of its point of
application from the pivot equals the power required to
balance it times the perpendicular distance of its point of
application from the pivot.
Thus, in a lever of the first class a pull of one pound ap-
plied two feet from the pivot will exactly balance a weight
of two pounds applied one foot from the pivot. Further,
in moving the two-pound weight through a circular arc one
foot in length, the one-pound power moves through a two-
foot arc, and the actual amount of energy exerted by the
power and weight is the same. (See Ex. LVI.)
On this account levers are very useful in mechanical
operations where they enable a person to move a large
weight a small distance by applying a small power through
a longer distance. The actual amount of energy exerted
at each end of the lever remains the same.
Levers in the body. — When we come to examine the
method of application of the muscles to the jointed bones
of our body we find that nature has made use of this law
of levers in a very remarkable way. For example, in the
forearm we have a lever of the third class. (See Fig. 115, A.)
Here the bones form the rigid bar, the elbow is the pivot,
and the power is applied by the biceps muscle attached
between the hand and elbow. With this arrangement the
muscle is able by a short contraction to raise a weight in
the hand no little distance. In this case the amount of
force exerted by the muscle to raise a given weight can
easily be calculated by applying the law of levers.
Furthermore, if we put in the hand a weight which
the muscle is just able to raise, the calculation enables
EDDY. PHYS. — 19290
MUSCLES
us to get the maximum strength of the muscle. (See
Ex. LVI.)
All classes of levers are illustrated in the muscle attach-
ments to the bones in our bodies. (See Fig. 121.) Levers
of the first class are least numerous. The action when the
toes are pressed downward, and the head lifted are ex-
amples of this type. Those of the second class are com-
paratively few also. The action of the foot when the body
is raised on the toes is a case in point. Here the Achilles’
tendon, which connects the calf muscle with the heel, exerts
the pull; the ankle supports the weight of the body, and
the toes are the pivot. Levers of the third class are most
abundant. The lifting of the forearm, as already described,
-the movement of the jaws in eating, the bending of the
toes and fingers, and the bending of the knee, are all ex-
amples of this class. It is to be noted that while this form
of lever requires relatively greater muscle pull to the
amount of weight raised, it is of advantage in the body
not only in the range of motion it permits, but also because
it allows bulky muscle bellies to be placed where they will
least interfere with freedom of action.
Standing, walking, and running.— It is a law of physics
that a rigid body will remain in a given position of rest as
long as a line drawn from its center of gravity toward the
center of the earth falls inside of the supporting surface.
If such a body is tipped it will return to this position of
rest, provided it is not tipped so far that the line falls out-
side of this basal support. It is this second fact which
makes it possible to tip a broad-based body farther than a
narrow-based one without danger of upsetting it. Now,
in the body, the center of gravity lies in the pelvic region
near where the sacrum connects with the last lumbar ver-STANDING, WALKING, AND RUNNING	291
tebra, and if the body were rigid it would remain erect
as long as the line from the center of gravity fell within
the area of the feet. As the feet were spread apart and
the base support correspondingly increased,
the more difficult it would be to upset the
body. If, then, we had a rigid structure the
only precaution necessary to secure erect
position would be to fulfil this law of
physics. But in our body there are many
joints, and the tendency of the whole system
is to bend and pitch the body forward. To
remedy this it is necessary for the muscles
that control the movements of the bones to
be so fixed as to keep these joints rigid.
This is accomplished by opposing the various
flexor and extensor muscles so as to hold the
bones of the legs and body in an upright
position and prevent movement about the
joints. To do this requires complete nerve
control, and this control is only acquired
by much practice as is evidenced by the
difficulty which children experience in their
first attempts to stand upright. The reel-
ing of the drunken man is another evidence
that the erect position is the result of fig. ms-Arrange-
nervous control since, in such case, the	of the muscles
alcohol has disturbed the nerve centers which keeP tlie
body erect.
only.
Some one has described walking as the act of falling
down and recovering oneself repeated at regular intervals.
This is an apt description, for as the flexors bend the leg
muscle, the body falls forward upon the other foot as a292
MUSCLES
pivot, and this forward fall is stopped only by swinging the
flexed leg forward and allowing the foot to strike the
ground some distance ahead. Then, the body weight is
shifted to this foot and the operation is repeated with the
other leg. In ordinary walking not only the leg flexors are
used but also those of the toes and foot, and the body is
usually more or less bent at the waist while the arms swing
freely about the shoulder sockets. The feature of this
combination of movements which makes it difficult to the
child is that these various movements must be so correlated
that they take place in regular sequence, and the power to
do this comes only with complete nerve control.
Running differs from walking in that the advancing foot
alights on the toes alone, and there are instants when
neither foot is in contact with the ground. The body is
bent more, the pressure of the toes thrusts the body forward
with more force and the rate of action of all the muscles is
more rapid. Accompanying this greater rapidity of action
is a greater demand for oxygen on the part of the muscles,
and a consequent increase in the rate of breathing. For
this reason running forms a valuable type of exercise for
the development of the lung power and as a stimulant to
circulation.
Effect of alcohol upon muscular activity.— The exact
effect of alcohol upon muscle action is not yet fully deter-
mined. Sufficient evidence has been collected, however,
to show that the combined action of alcohol upon the mus-
cle cells and upon the nerve centers which control these
cells is such as to make it less valuable than other fuel
foods for the production of energy and activity. And it
has been shown further that while the immediate effect of
small doses may result in temporary stimulation, this periodeffect of alcohol upon muscular activity 293
of increased activity is soon followed by a period of exhaus-
tion.
In general, then, the total effect of alcohol upon muscle
activity is such as to make the total amount of work per-
formed less with its use than without it, while through its
effect upon the nerve centers which control the muscles
its continued use tends to destroy the power of precise
movement and to lower the user’s endurance. In other
words, it is a very poor source of energy as compared with
other fuel foods.XX. MOVEMENT IN LOWER ANIMALS.1
An examination of a muscle cell demonstrates that the
power of contraction and expansion which it possesses is
actually a property of the protoplasm of which
the cell is composed. We should expect, then,
to find in all living matter some power of
■movement, since all are built up of cells, that
is, protoplasm. In other words, motion is an
essential characteristic of all living protoplasm.
It does not follow, however, that this mo-
tion of protoplasm must necessarily result in
the movement of the animal whose cells con-
tain protoplasm, any more than the move-
ment of water in a bowl necessarily results
in the motion of the bowl. In fact, in many
of the cells of our bodies and in those of
plant and animal bodies in general, the pro-
toplasm of the individual cells shows move-
ment, while the cells in which it is inclosed
remain stationary. It is necessary, therefore,
in speaking of movement to distinguish be-
motion of protoplasm by itself and
protoplasm
within the locomotion, or the movement of the body or
cell from one position to another. The motion
of protoplasm within a stationary cell is very important
as a means of distributing material within the cell, but
Fig. 126—A
plant cell >
the arrows
indicate the
streaming of tween
1 See Footnote, p. 146, Chapter X.
294CAUSES OF LOCOMOTIOH
295
in our present discussion we are more directly concerned
with locomotion.
Causes of locomotion in animals.— The locomotion of
all animal forms can usually be explained as due to one
of four causes:
(a)	A change of position due to the action of some force
outside themselves, such as currents of water or air. This
sort of movement is, of course, passive movement as far
as the animals are themselves concerned, since they take
no part in its production.
(b)	A change of position due to change in the cell com-
position, as a result of which there is a gain or loss in
specific gravity. Such change allows the buoyant force
of the water to move the body up or down. This is not
a passive movement, since the changes in composition
determine the action of the buoyant force.
(c)	A change in position due to active secretion on the
part of the cells composing part or all of the animal, and
an accumulation of this secretion in such amounts and
in such a manner as to force the body forward.
(d)	A change in position due to expansions and con-
tractions of part or all of the cell bodies composing the
animal, these movements being so arranged as to produce
a movement of the whole animal.
The first cause needs little discussion, as it is due to no
physiological action <3f the animals affected. The other
three need some explanation to make their action clear.
Locomotion due to changes in specific gravity. — Changes
of position of this character are usually confined to water-
dwelling forms. Protoplasm is slightly heavier than
water, and an animal, therefore, which rests upon the
bottom and has no apparatus for raising itself must re-296
MOVEMENT IN LOWER ANIMALS
main there unless it can produce such changes in its com-
position as to make it lighter than the water it displaces.
Certain one-celled animals have the power to accumulate
carbon dioxide within the cell bodies to such an extent as
to decrease their specific gravity. Thus, the body be-
comes lighter than the liquid it displaces, and the buoyant
force of the water floats it upward. Further, they can
discharge this gas, increase their specific gravity, and sink,
as they have become heavier than the liquid displaced.
In this way many one-celled, water-dwelling animals are
able to change their position without the aid or use of
special organs of locomotion. In the higher animals,
Fig. 127 — Swim bladder of the fish.
the swim bladder of the fish and the connection of the
lungs of the bird with cavities of the body, by means of
which air can be introduced into the body, are apparently
parallel cases, but it must be noted that in these cases
the decrease in body specific gravity is due to the action
of special muscles and not to changes in the composition
of the protoplasm of the cells. The result, however, is
the same.
Locomotion due to secretion. —There are certain forms
among the one-celled animals and plants that are able to
creep over surfaces without contractile movements, or any
apparent organs of locomotion. Examination shows thatLOCOMOTION DUE TO CONTRACTILITY
297
in certain eases this creeping movement is due to active
secretion. In one case which has been especially well
studied (the one-celled gregarines), it has been shown
that, out of the food supplied to it, the protoplasm of the
cell secretes a slimy substance, just as the cells of the
salivary glands produce saliva. This slimy substance is
forced out of the cell body and accumulates at the rear
of the animal. Here it sticks to the surface and hardens,
and thus by its accumulation and resistance forces the
body along slowly before it.
In the higher forms of animals we find no cases of motion
due entirely to secretion. In the snails, however, the
body secretes a slimy mucus which forms a surface for
the action of the muscular foot and which we see as a shiny
trail when it hardens. The common house fly also secretes
a sticky fluid on the bases of the foot, by means of which
it is enabled to walk over smooth surfaces. The web of
the spider over which it travels, and by means of which it
raises and lowers itself, is a secretion of certain glands of
the body. These cases are evidently not cases of motion
due to secretion alone, but simply instances of where
secretion aids other methods. Motion due to secretion
alone is confined to a few one-celled forms.
Locomotion due to contractility.— Practically all of
the free-moving animal forms owe their ability to change
position to the power of contractility exercised by the
protoplasm of part or all of the cells of the body. We
can distinguish three kinds of locomotion due to the
contractility of protoplasm:
(a)	Amoeboid movements.
(b)	Ciliary movements.
(c)	Muscular movements.298
MOVEMENT IN LOWER ANIMALS
Amoeboid movements. —In the study of the amoeba it
was noted that this animal moves by means of pseudo-
podia. These pseudopodia are merely extensions of the
body protoplasm into which the rest of the protoplasm
flowed. In other words, the amoeba extends its proto-
Fig. 128 — Various shapes of an amceba in motion.
plasm at a definite point on the surface, and then, by con-
traction, forces the rest of the protoplasm to flow in that
direction. The point where the pseudopods are pro-
duced varies with conditions and, since the animal has
no cell wall, theoretically these pseudopodia may be pro-
duced anywhere. In general, however, they are so formed
as to carry the animal toward food or away from irrita-
ting causes, and this fact shows that the expansions and
contractions are in some way a response to a stimulus.
The result of this peculiar kind of contractility of proto-
plasm is to produce a slow creeping motion, and all animals
which move in this slow creeping fashion with the aid of
pseudopodia are said to show amoeboid movement. In
some of the forms of protozoans which have walls about
the body, amceboid movement is still possible by extending
the pseudopodia through openings in the walls. The
little shelled form known as arcella and common in all
fresh water is an example of such a type. (See Fig 130.)
In the higher forms we find no cases of amceboid move-
ment on the part of the animals as a whole; but often the
cells of a part of the body may exhibit this method ofCILIARY MOVEMENTS
299
motion. In our own body, for example, the movement
of the white corpuscles is a case in point. To this amce-
Fig. 129 — White corpuscles escaping from blood tube by amoeboid motions.
boid movement these corpuscles owe their power to pass
through spaces in the walls of the blood vessels through
which the smaller but rigid, red corpuscles cannot pass.
Ciliary movements.— Amoeboid movement is at best a
very slow method of changing position, and is interesting
mainly in showing us that free protoplasm has the power
of contractility. We should expect, then, to find in the
higher protozoans a more rapid
method of progression. Such
a modification in motile organs
we do find in many of the pro-
tozoans, and these organs are
called cilia.
In the arcella, movement
is produced by pseudopodia
which are projected through an opening in the cell wall, and
these can be extended and withdrawn as necessity requires.
-Shell
’seudopod
Fig. 130—An arcella creeping by
pseudopod.300
MOVEMENT IN LOWER ANIMALS
In the slipper animalcule or paramoecium (see Fig. 131),
which is a higher type of protozoans than
the arcella, the body wall is covered with
permanent projections of protoplasm
called cilia from their resemblance to
hairs. These hairlike structures are
composed of protoplasm like the pseud-
opodia, but, unlike them, they cannot
be withdrawn, and in contracting and
expanding they move at a different
rate and in only two directions. As
it contracts the cilium is bent quickly
and sharply backward; relaxing, it re-
turns much more slowly to an upright
position. Many cilia, acting in concert
all over the body, push the body
through the water just as oars do a
boat. A peculiarity of ciliary action,
however, is that they do not all
contract at once, but the movement
of one follows that of the one preced-
ing in definite order, so that in action
they resemble the waving of a field of grain in a wind,
each cilium corresponding to a single stalk of the grain.
Very rapid movement is
made possible by these
Fig. 131 — Paramce-
cium ; c} cilia; cvr
contractile vac-
uole ; n, micro-nu-
cleus ; m, macro-
nucleus; g, gullet;
/, food vacuole; z,
modified cilia (tri-
chocysts).


Fig. 132 — Ciliary movement.
structures, and their
permanent form permits the body to be completely
covered with a wall, and at the same time this rigid
wall furnishes a resistant structure which the cilia can
push through the water much more readily than would
be possible if the body were plastic.CILIAKY MOVEMENTS
301
Sometimes the cilia are replaced by a single long pro-
jection which performs movements of a whip-lash char-
acter. Such modifications are called flagella, and are found
Fig. 134—Colony of flag-
ellated protozoans with
Fig. 133 — A flagel-	two flagella,
lated protozoan ; F,
flagellum;	2V, nu-
cleus ; By body of
cell.
in many one-celled plant and animal forms. (See Figs.
133 and 134.)
In many of the higher animals, including man, are
found cells provided with similar, ciliary projections, but
here the action of the cilia is not to produce movement
of the cells themselves. On the contrary, their function
here is to produce currents in the liquid that bathes them
or to remove obstacles from the surface of the cells. Their
action may be compared to the motion of the oars when302
MOVEMENT IN LOWER ANIMALS
the boat is tied. In such case, the result is that the
water, and not the boat, is moved. In many of the lower
animals the digestive cavities are lined with such ciliated
cells, and so the currents of water which they create
cause the food, floating in the water, to be brought within
reach by the currents set up. In our bodies, the walls
that line the bronchial tubes are
lined with such ciliated cells, and
here they serve to free the surface
of the membrane from particles of
dust which might produce harm if
they were allowed to remain. In
fact, it is only in the case of the
protozoans and a few of the young
stages of the lower animals that cilia
are used to move the body of the
animal itself. In all other forms
their function is the setting up of
currents or the removal of particles.
Cilia and flagella are to be con-
sidered, then, as merely a modified portion of protoplasm
in which the power of rhythmic contraction and expan-
sion has been specially developed. They are chiefly
interesting, therefore, as indicating the manner in which
protoplasm can become developed into a motile organ.
Muscular movement.— Muscle cells differ from ordinary
cells in that they contain certain modifications of pro-
toplasm called fibrillse. These fibrilla: are thin threadlike
fibers embedded in the fluid protoplasm of the cell, and may
be variously arranged as to direction, but are always par-
allel to one another. In certain instances these fibrillse
are divided into segments by cross lines or striae, and these
Fig. 135 — Ciliated epi-
thelial cells lining the
bronchial tubes; a, basal
membrane ; b, spherical
epithelial cells; c, cili-
ated cells ; </, cilia.MUSCULAR MOVEMENT
303
give the name of striated muscle fiber to muscle masses
made up of such cells. Again, the fibrillse may be entirely
free from such cross lines, and the muscle masses composed
of such cells are called non-striated muscle fiber. We have
already noted in man that the muscles are distinguished
by these differences.
The simplest form of a muscle fibrilla is the unstriated
form found in the cell bodies of certain of the protozoans.
Fig. 136 — Stentor, a, one-celled protozoan in three stages of contraction ; the
black lines are muscle fibers.
In the stentor (see Fig. 136) these fibrillse are arranged
parallel to one another so that by contraction they shorten
the whole cell. In other one-celled forms, such as the
vorticella, the fibrillse are united into a bundle which
extends the length of the stalk. The contraction of this304
MOVEMENT IN LOWER ANIMALS
bundle is sufficient to draw back the whole body of the
animal.
These simple forms of fibrillae give us the key to their
character. They are, in fact, merely specially developed
portions of protoplasm in which the power of rapid and
powerful contraction, has been developed to a high rate,
leaving the rest of the protoplasm of the cell to perform the
ordinary functions of the cell. In the higher forms of
animals the cells which contain these fibrillae are almost
completely filled by them, and such cells are set aside in
the body to perform the action of contraction and expan-
sion and produce movement, while the work of digestion,
etc., is given to other cells. In
general, then, muscle cells are
simply cells which have devel-
oped to a remarkable extent
the modified forms of protoplasm
called fibrillae. It must be evi-
dent, then, that whether we have
amoeboid, ciliary, or muscle move-
ment, the source of the power is
the same, namely the contractile
power of protoplasm. In this
connection it may be noted that
the striated fibrillae have the power
of rapid contraction much more
highly developed than it is in the
non-striated. The enormous ex-
tent to which this rapidity of
contraction may be developed is seen in the action of
the striated wing muscles of the insects (three hundred
to four hundred contractions per second in the fly).
Fig. 137 — Vorlicella. This
protozoan has a single fiber
in the stalk, which by con-
tracting draws the animal
down as in B.ORGANS OF LOCOMOTION FOR WATER LIFE 305
Arrangement of muscle fibers.— The muscles of all ani-
mals lower than the vertebrates are parallel fibers, and are
never united into bundles like those of man. in no case
are they found bound together by a connective tissue, or
united to hard parts by tendons.
All the muscles of the starfish, worm, and oyster
family are of the unstriated variety. All those of the
lobster, spider, and insect tribes (Arthropoda) are stri-
ated.
Organs of locomotion.— In many of the worms and
lower forms, and even in some of the higher forms, such as
the snake, motion is secured by undulations of the body
as a whole. Most animals, however, have special organs of
locomotion. The number of these organs varies, but, in
general, the higher we go in the animal scale the greater the
perfection and the fewer the number. Thus, while a star-
fish has hundreds of tube feet, and a centiped many legs,
the two arms and legs of man are much more effective
as means of progress. While it is impossible to consider
in detail all the organs of locomotion in the animal world,
we may consider for a moment the characteristics common
to those adapted for special surroundings. All the organs
of locomotion about to be described depend upon the action
of variously arranged muscle layers, that is their move-
ments are muscular movements.
Organs of locomotion for water life.— Since the water
supports nearly all the weight of the body, we should
expect to find the organs of locomotion of aquatic animals
ill adapted for supporting the body. For example, the
cilia of the protozoan, or the fins of the fish, are not in-
tended to support the body, but to push it through the
water; while even the webbed feet of the duck are placed
EDDY. PHYS. —20306
MOVEMENT IN LOWER ANIMALS
so far back on the body as to make walking awkward,
though admirably placed for swimming.
All organs made for movement in the water are wTell
Fig. 138 — The fins of a fish.
adapted to pushing the body through the water. The
broad tail of the lobster and the flexible abdomen enable
it to shoot the body backward by bend-
ing this portion sharply forward, and
then slowly straightening it. The fins
of the fish and the flukes of the whale
show the same paddlelike character, while
the webbed feet of the frogs and aquatic
birds are adaptations for pushing against
water. In all these paddlelike organs,
too, the broad surface is so constructed
as to be opposed to the water in push-
ing, while by bending, it is enabled to
return to position without offering much
resistance to the water.
An interesting modification of the water
motion is seen in the squid where the
broad tail serves simply to steer with, while the motion
is produced by ejecting spurts of water by contraction
Fig . 139— Movement
of the tail tin of a
fish in swimming.OEGANS OF LOCOMOTION FOE A FLYING LIFE 307
of the body walls, and forcing this ejected water through
a funnel at the front of the body.
Organs of locomotion for a flying life.— All animals
which fly require special adaptations of structure to
lighten the body, and also very powerful muscles to keep
the body in motion, while the form of paddle or wing used
to push the body through the air must be much broader
in proportion to the size than were those used in pushing
against the water.
The bodies of flying insects are lightened by air sacs
and cavities distributed through the body, while in birds
similar air sacs and hollow light bones serve them for the
same purpose. Feathers in the bird are adaptations for
a body covering which secure lightness and protection at
the same time.
The wings of insects are expansions of the skin or body
h
Pig, 140—Cross section of an insect; k, heart; a, alimentary canal; n, nerve
ganglia; t, air tubes (tracheae) which carry air to all parts of the body and form
the veins of the wings.308
MOVEMENT 'IN LOWER ANIMALS
covering, stretched upon a frame of hollow tubes of the
same material. They are moved by muscles lying in the
thorax, and these muscles are remarkable for their rapid
rate of contraction. The number of wings in the insects
varies from two to four, and they also vary in size from
the tiny wings of the gnats to the relatively enormous
surfaces of the butterfly wings. In some forms, such as
the beetles, the front pair is adapted solely for a protec-
tive covering, while the flying is done with the second
pair. The dragon fly is the most perfect flying machine
known, being able to fly forward, backward, or remain
poised in the air at will, while its movements are marked
with great speed and accuracy.
In birds the arms are modified into wings by a special
covering of feathers. These feathers are so arranged as
to allow the air to pass through them as the wing is raised
and to prevent such passage on the down stroke. This
Fig. 141 — Wings of the bat, formed by expansions of the skin over the length-
ened finger bones.
arrangement enables them to strike the air very strongly
at each down stroke, and thus to force the body upward
and forward. The muscles which operate these wings areLAND AND WATER BOTTOM LIFE
809
powerful, and the breastbone is keeled to permit their
attachment to it. The speed of some birds in flying is
extremely great, a hawk being able to fly at the rate of
one hundred and fifty miles an hour.
The only mammal that flies is the bat, and here the
wings are formed by expansions of skin stretched over
the arms and legs as frames, while the fingers are enor-
mously developed for this framework. (See Fig. 141.)
Organs of locomotion for land and water-bottom life.—
We find many curious adaptations in the animals which
move on land or over the bottom of the sea or ponds.
The earthworm progresses by first contracting the cir-
cular muscles and thus extending the body. Then, as
the longitudinal muscles are contracted, the bristles on
the under side of the body are inserted into the surface
over which it is creeping; as a result, the shortening of
the body produced by the muscle contraction causes the
body to be dragged forward.
The leeches use the same muscles, but fasten the ends
of the body by means of suckers, so that their motion is a
progression of loops.
Starfishes use their tube feet in a similar manner, first
by lengthening the foot and attaching it by the sucking
disk at the end, and then dragging the whole body for-
ward by contraction.
In the mollusks, motion is produced by a creeping disk
or foot which in the clam ploughs through the sand and
forces the sharp-edged shell forward, while in the snail the
muscles contract and expand in such a way as to wrinkle
the bottom of the foot and thus push the body forward.
Many land animals and sea-bottom animals have jointed
legs of some kind, operated by powerful muscles, usually310	MOVEMENT IN LOWER ANIMALS
arranged in pairs, as in our own bodies. In the crus-
taceans these legs may be as many as twenty, some being
moved backward as the others
move forward. The motion of the
six legs of the insect is illustrated
in Figure 142.
Most land vertebrates, except
the snakes, have four legs, the two
front ones corresponding to our
arms, and the two hind ones to
our legs. In some forms the legs
are so placed that the animal
stands upon the tips of the toes
as in the horse, cat, dog, etc. In others, as in man and
the bears, the whole foot is on the ground. Nearly all
Fig. 142 — Motion of tlie legs
of a fly in crawling.
Fig. 143—Feet of mammals adapted to different methods of locomotion ; A, foot
of bear ; B, foot of seal; C, foot of lion.
vertebrates have more powerful leg muscles than man,
and can move much more rapidly. The legless snakesLAND AND WATER-BOTTOM LIFE
311
move by writhing and by pushing the edges of the ventral
scales against rough surfaces.
In all running vertebrates the hind legs are much more
muscular than the fore legs or arms, and are the principal
organs of locomotion; the front legs being mainly for the
support of the front part of the body.XXI. RESPIRATION.
The activity of a muscle cell was seen to be dependent
upon a constant supply of food and oxygen. What is
true of a muscle cell is true of every cell in the body. All
require food and oxygen, and all give off carbon dioxide
and wastes. In our study of the digestive and circulatory
Fig. 144—Position of the lungs in the thorax; A, lung; B, heart; D, pulmonary
artery ; E, trachea.
systems we saw the manner in which food and oxygen
reached the cells of the body and how the wastes were un-
312RESPIRATORY ORGANS
313
loaded from them. Finally, we traced the source of the
oxygen supply to the lungs of the body, and the wastes to
the kidneys. The organs which are concerned in the sup-
ply of oxygen to the cells and in the removal of the carbon
dioxide from the body are spoken of collectively as the
respiratory or breathing system.
Respiratory organs.—The term respiration (re, again,
and spiro, I breathe) in-
volves two distinct actions.
The taking of air into the
body is called inspiration,
or inhaling. The throwing
out of the body of carbon
dioxide and the air which
has been freed of oxygen,
is called expiration, or ex-
haling. These two pro-
cesses of respiration are
performed by various kinds
of organs in the lower ani-
mals, but in man and in all
the other vertebrates, ex-
cept the fishes, the principal
organs of respiration are the
lungs. Connecting with the
lungs, which are located in
the thoracic cavity, are
several passages and tubes
which bear a part in the
process and are to be in-
cluded in our term respiratory organs. The relation
of these parts to the lungs can be best understood
Fig. 1.45 — Relation of the buccal and
nasal cavities to the esophagus; n,
brain cavity; Z,hard palate uvula;
g, opening of Eustachian tube to ear;
e, epiglottis ; fc, tongue; c, windpipe;
b, esophagus.314
RESPIRATION
by following the course of the air as it passes through
them.
Air enters the body by way of the nostrils or mouth and
passes backward into the pharynx or throat cavity. Here,
two passages are open to it: one, the gullet, leads to the
stomach; the other, the trachea, or windpipe, leads to the
lungs. As we inhale, the walls of the esophagus contract,
and the air is sucked down the windpipe and enters the
lungs. Just how this suction is produced we shall investi-
gate later. For the present let us examine these passages
a little more in detail. The figure (see Fig. 145) shows the
arrangement of these parts with relation to one another.
If we examine it carefully we note that the two external
nostrils are merely openings into a large cavity (the nasal
cavity) located just above the roof of the mouth and con-
necting at the back with the pharynx. The nostril en-
trance to this cavity is guarded by many hairs which serve
to catch and remove dust particles from the air. The
cavity itself is lined with a layer of mucous membrane, the
secretion from which aids in catching particles of dust and
germs that have escaped the_ hairs. This membrane is
also lined with many blood vessels, and the heat which
escapes from them serves to warm the air before it comes
in contact with the delicate membranes of the lungs.
These facts show clearly that the nose is adapted for the
entrance of air to the body, and that while mouth breath-
ing permits the air to reach the lungs, it has none of the
safeguards against impurities and cold that are found in
the nasal passage. Mouth breathing is a habit to be
avoided.
The location of the top of the windpipe may be easily
found by applying the fingers to the outside of the throatRESPIRATORY ORGANS
315
where a hard swelling may be felt, called the Adam’s
apple. This is the larynx, or voice box, and is on the top
of the windpipe. The trachea itself may be felt below this
projection and recognised by the rings of cartilage which
encircle it and prevent its collapse as the air is sucked in.
The entrance to the larynx and trachea is guarded by a
lid called the epiglottis which is always open to admit
Bronchial
tubes.
Larynx.
Fig. 146 — Diagram of trachea and connections.
air, except in the act of swallowing, when it closes to pro-
tect the tube from entrance of food.
At its lower end the trachea splits into two branch
tubes, or bronchi, which, in turn, split into smaller and316
RESPIRATION
smaller branches called bronchial tubes, and finally end
in thin-walled sacs called 'pulmonary sacs. The bronchial
tubes and the sacs are all inclosed in two baglike bodies
covered with a membrane, and located in the chest cavity
one on each side of the heart. These two bodies, the
framework of which consists of the branches of the trachea,
are what we designate by the name of lungs, and the pul-
monary sacs of these lungs are the ultimate destination
of the inhaled air. The structure of the windpipe and
lungs of man are practically identical with those of all
other mammals, and the dissection of the chest cavity of
such a mammal (rat, cat, or rabbit) gives one a very clear
idea as to the structure of our own organs, and their posi-
tion with relation to the other organs of the body.
Position and structure of the lungs.— (See Ex. LVIII.)
If we remove the skin from the chest and throat of a mam-
mal the windpipe and the ribs will be laid bare. Between
the ribs and connecting them are layers of muscle (the
intercostals), and on moving the point of the breastbone
up and down the layers will be seen to give a peculiar
movement to the ribs. We can produce this same move-
ment by inserting a glass tube in the entrance of the
trachea and inflating the lungs with the breath. As we
do this we shall also note a downward movement of a
membrane which separates the chest cavity from the
abdominal cavity. This membrane is the diaphragm, and,
together with the ribs, it performs an important action
in breathing movements. The relation of these boundary
walls of the chest cavity are shown in Figure 144.
Cutting the ribs on one side close to the breastbone,
and pressing them back, the lungs and heart which fill the
cavity are seen. The lungs appear as two pinkish bags,STRUCTURE OF TRACHEA
317
one on each side of the heart, and are held together at the
top by the two bronchi or main branches of the trachea.
If we inflate these two bags with the aid of the glass
tube and then tie the trachea tightly, the lungs may be
separated from the cavity and floated upon water. This
Fig. 147—Ribs and muscles ; a, ribs ; 6, cartilages ; c, junction of bone and carti-
lage; d, bony portions, and e, cartilaginous portions of breastbone ; A, external
intercostal muscle; B, internal intercostal muscle. In the middle the external
muscle has been removed.
power to float has earned them the name of “ lights ”
from the butcher. The dissection of these separated lungs
shows the following structural features.
Structure of the trachea.— The trachea is a stiff, cylin-
drical tube covered on the outside with a layer of fibrous,
connective tissue in which are embedded the stiffening
rings of cartilage. The interior of the tube is lined with
a single layer of mucous membrane, the innermost cells of318
RESPIRATION
which are provided with cilia. (See Fig. 135.) These cilia,
together with the mucus secreted by the cells, serve to catch
any particles of dust that may have passed the nasal cavity.
The cilia are in constant motion, bending quickly upward
and then slowly returning to place, and in this manner
forcing the collected particles steadity upward toward the
pharynx.
The cartilage rings do not completely inclose the tube,
but are shaped like the letter C, the open part being at
the back and leaving a portion of the tube soft and com-
pressible. The gullet lies against this soft groove, and
thus room for expansion is given as the food passes down
the gullet.
Structure of the lungs.— These are merely membranous
sacs with an internal framework composed of the branch-
ing bronchial tubes and air sacs. These treelike tubes
are bound together with a soft, pink connective tissue,
rich in elastic fibers, and over the
whole is stretched an elastic mem-
brane known as the pleura. The
bronchi and bronchial tubes are
continuations of the trachea, and
the larger ones resemble it in struc-
ture with the exception that the
cartilage rings are not so regularly
arranged. As the tubes become
smaller their walls become thinner,
and finally the cartilage rings dis-
appear entirely in the smaller
tubes, while the mucous membrane
is reduced to a single layer of ciliated cells. The air
sacs, in which the smallest tubes end, are composed of
Fig. 148—Two air sacs (b)
into which a bronchial
tube (a) opens.BKEATHING
819
a thin, elastic outer coat of connective tissue and a lining
of flat, mucous-membrane cells without cilia. Between
these two layers is a network of tiny, thin-walled capil-
laries whose blood is thus separated from the air in the
sac by only the thin capillary wall and the layer of flat
cells.
The whole structure is very elastic, and readily expands
and contracts within the limits of the chest cavity as air
enters and leaves it.
The pleura.— The pleura covers both the right and
left lung. This pleura is a serous membrane, and is a
complete cover for the lungs except at the point where
the bronchi enter. Here, instead of fusing with the walls
of the bronchi, it is turned back and lines the interior
of the chest cavity. Between the pleural cover of the
lung and the pleural lining of the chest cavity is thus
formed a cavity which is so small as to require onlj'’ a few
drops of liquid to fill it. This cavity is filled during life
with a liquid of the nature of lymph, secreted by the
pleura, and as the lungs are inflated in breathing, this
liquid serves to lubricate the pleural surfaces where they
rub against one another and to prevent friction.
Breathing.— Two processes are involved in breathing,
inhaling and exhaling. Inhaling consists of drawing the
air from the outside through the nasal passages, trachea,
bronchi, and finally distributing it through the bronchial
tubes until it reaches every part of the lungs and fills the
air sacs. Exhaling reverses this process and expels the
air in the air sacs through the above passages in the re-
verse order to the outside. As the walls of the chest cav-
ity enlarge in inhaling, the lungs fill with air; while in
exhaling, the walls contract, the lungs are compressed,320
RESPIRATION
and the air is forced out of them by the passages above
noted. W hat causes the lungs to expand and fill with
air, and, since the lungs themselves contain no muscles,
what parts of the body are concerned in this process?
The answer to this question is to be found in the structure
and movements of the walls of the chest cavity.
Structure of the chest cavity.— The chest cavity is
conical in shape, with the point of the cone at the top,
and is completely inclosed on all sides. The upper part
and sides of the cone are formed of the ribs, intercostal
muscles, sternum, and backbone, all covered over with
muscles and skin, while the floor of the cavity is com-
posed of the membrane called the diaphragm. These
walls are not rigid, but permit of movement in two defi-
nite directions. As we inhale, the sides of the cavity can
be felt to move outward, while the diaphragm is pulled
downward, forcing the contents of the abdominal cavity in
the same direction, and causing the belly to swell under
the pressure. As we exhale, the ribs move inward and the
diaphragm moves upward, causing the belly to be drawn
in. The causes of these movements arp to be found by
studying the shape of the ribs and the muscular connec-
tions.
In the dissection of the chest cavity, the muscles between
in
Fig. 149 — Diagram of rib movement in breathing ; AB, vertebral column ; CD and
EF, ribs attached to movable breastbone, DF; a, b, external and internal in-
tercostal muscles ; c, neck muscles.STRUCTURE OF THE CHEST CAVITY
321
the ribs were noted under the name of intercostals. From
the study of the skeleton it was noted that the ribs were
jointed to the backbone at one end and to the breastbone,
by cartilages, at the other; while the position of the ribs
was such that the sternal ends were lower than the dorsal
ends. Finally, muscles attach the upper ribs to the
neck vertebrae If, now, these latter muscles and the inter-
costals contract simultaneously, the effect is to lift the
breastbone and the sternal ends of the ribs upward. To
do this is to widen the chest cavity from front to back
and from side to side. The action of these muscles is
rhythmic and automatic. They account for the move-
ment of the ribs, but not of the diaphragm.
The diaphragm is a membrane which forms a domelike
roof for the abdominal cavity, with the top of the dome
extending normally into the chest cavity. It is composed
of thin, muscular tissue, and a tendon is attached at
the apex of the dome. From this tendon, muscle fibers
radiate downward and outward, and are fastened to the
lower ribs, to the breastbone, and to the backbone at
their outer ends. When
these fibers contract the
result is that the dome
of the membrane is pulled
downward out of the chest
cavity and compresses the
contents of the abdominal
cavity, thus increasing the
depth of the chest cavity
from top to bottom. This
contraction of the dia-
phragm muscles and those which pull the ribs upward
EDDY. PHYS.— 21.
7 A
Fig. 150 — Position of tlie diaphram (A)
in breathing.322
KESPIRATIOH
takes place simultaneously, as does the expansion. The
combination of these two movements is responsible for
the filling and emptying of the lungs with air. Now, it is
easy to see how the return to position of the ribs and
diaphragm might squeeze the lungs, and cause them to
expel the air; but, as the walls of the chest cavity are not
fastened to the walls of the lungs directly, it requires a
more detailed study to understand how this outward
movement causes the lungs to expand and become filled
with air. How does the en-
largement of the chest cavity
cause the lungs to fill with
air?
Causes of inhaling and ex-
haling.—A simple mechanical
device will make clear the
relation of the chest enlarge-
ment to the inflation of the
lungs. (See Ex. LIX.) An
ordinary bell jar, open at the
top and bottom, will repre-
sent the chest cavity. A
piece of sheet rubber stretched
across the bottom will repre-
sent the diaphragm. Finally,
a toy rubber balloon tied to a
glass tube will be the lung,,
and the tube the trachea.
To make the comparison com-
plete we shall put the balloon into the bell jar with the
tube fitted into the stopper. Our apparatus will now
look like Figure 151. If we have made our connections
Fig. 151 — Apparatus to illustrate
breathing movements ami their
effect upon the lungs. The rubber
diaphragm corresponds to the dia-
phragm in the body ; the handle to
the tendon; the balloon to the
lungs ; the tube to the trachea; the
bell jar to the walls of the thorax.
As the handle is lowered the air
flows down the tube and inflates
the balloon.CAUSES OF INHALING AND EXHALING
323
well, the jar is now air-tight, and the only connection
of the interior of the balloon with the outer air is through
the glass tube. This apparatus represents the relation
of the lungs to the chest cavity, since the sides of the
balloon are not in any way connected with the rubber
sheet (diaphragm) or with the sides of the bell jar (the
rib walls). Xow pull the rubber sheet downward. The
balloon becomes inflated as air enters through the tube
(trachea). Let the sheet return to place, and the balloon
collapses, expelling the air through the glass tube. What
is the explanation? The answer is found in what the
physicists call the laws of air pressure. They tell us that
every square inch of surface exposed to the air is under a
pressure of fifteen pounds. At the start of our experiment,
therefore, both the inside and outside of the balloon are
under equal pressure. Since no air can enter the bell jar,
when we lower-the diaphragm the air inside it must occupy
a greater space, and its pressure on each square inch of the
balloon surface becomes less. The only way in which
the pressure can be equalized is for air to rush in from
the outside and distend the balloon until the volume of
air in the jar is the same as before. Then the inflow
stops, since both surfaces of the balloon are again at the
same pressure. When the diaphragm returns to position
the air in the jar is compressed, and the surface of the
balloon exposed to this compressed air is now under a
greater pressure than the outside air can exert upon the
inside through the glass tube, and the walls are gradually
forced together with the outpouring of the contents of
the balloon, until again the pressure on the outside of the
balloon is the same as on the inside. Meanwhile, the
balloon has performed all the movements of the lungs in824
RESPIRATION
inhaling and exhaling, and this process may be repeated
as often as we lower and raise the rubber diaphragm.
In the chest cavity the conditions are practically iden-
tical with that of cur apparatus, but the lungs are much
more sensitive to changes in pressure than are the balloon
walls. One reason for this is that the chest cavity volume
is increased not only by the downward movement of the
diaphragm but also by the upward movements of the ribs.
Furthermore, the lungs fit closely against the walls of the
cavity, and thus a slight increase in volume is immediately
felt by all parts of the lungs, with a correspondingly greater
expansion.
The movements of contraction and expansion, on the
part of the chest walls, take place rhythmically about fifteen
to eighteen times per minute, which is the normal rate
of breathing. In violent exercise this rate may increase
to sixty or seventy times per minute. The rate also varies
with the age, that for a baby being about forty, wdiile this
rate gradually decreases, until, at eighteen years of age, it
is practically the same as for an adult. Much more mus-
cular effort is exerted in inhaling than in exhaling, there
being practically none in normal exhaling.
Physiological Effects of Breathing.
Composition of inspired and expired air.— The study of
ordinary air (see p. 20) shows it to be composed mainly
of two gases, oxygen and nitrogen. Careful chemical
analysis shows the presence of small quantities of other
chemical elements and compounds, such as argon, krypton,
carbon dioxide, and water. Of these, only the water and
carbon dioxide have any physiological significance. Ex-COMPOSITION OF INSPIRED AND EXPIRED AIR 325
pressed in volume per cent the following table represents
the composition of ordinary air:
Inspired Air.1
Oxygen..................................... 20.96%
Nitrogen ‘................................. 79.00%
Carbon dioxide........................ 00.04%
If we examine the air which is expelled from the lungs
we find certain remarkable changes in it. If we breathe
upon a thermometer bulb or a mirror surface, two proper-
ties peculiar to expired air are apparent. The first is the
increase in water content as shown by the film of vapor
that collects upon the bulb. The second is the rise of tem-
perature as indicated by the rise of the mercury column of
the thermometer. A comparison with the body tempera-
ture shows that the expired air has been raised to this body
temperature.
Evidently then, breathing involves a loss of water and
heat to the body, and thus plays an important part in the
.regulation of the temperature and water content of the
body.
If we blow expired air through a tube into a tumbler of
limewater, the latter soon becomes milky, indicating that
a large proportion of carbon dioxide is present. (See p. 16.)
If we collect some of the expired air over water and intro-
duce a lighted match into it, the flame soon goes out, indi-
cating the loss of a large amount of its oxygen, and hence
its power to support combustion. Expressed chemically,
1 The very small per cent of argon and krypton is included in the
nitrogen estimate. The percentage of water is so small as to be neglected
in the above table.326
RESPIRATION
the composition by volume of expired air is shown in the
following table:
Expired, Air.
Oxygen.......................................16.02%
Nitrogen...................................  79.00%
Carbon dioxide............................... 4.38%
Water........................................ 0.60%
A comparison of this table with that for inspired air shows
that in the lungs the air loses some 4.94% of oxygen, and
takes on some 4.34% of carbon dioxide and 0.6% of water.
The proportion of nitrogen remains unchanged. Expressed
in terms of a day’s breathing, the body obtains for the tis-
sues, in a day, about two pounds of oxygen, and loses in
that time some two pounds of carbon dioxide and a half a
pound of water.
Expired air also contains traces of ill-smelling, organic
compounds. These are what give the bad smell to a poorly
ventilated room that has been occupied for some time by
people. They render the air dangerous and poisonous to
breathe. A bad digestion also adds to the ill-smelling
compounds which, emerging from the gullet, mix with the
expired air and make a fetid breath, while decaying teeth
add still other odoriferous compounds.
Blood changes in the lungs.— The tables above indi-
cate only the changes which take place in the inspired
and expired air. They do not explain their purpose.
To do this we must recall the structure of the lungs.
It will be remembered that the inspired air ultimately
reaches the air sacs and that these sacs are lined with
thin walled capillaries. The blood, which has been cir-
culating through the body tissues, comes to these capilla-BLOOD CHANGES IN THE LUNGS
327
ries from the heart (pulmonary arteries). The corpuscles
of this blood are laden with the carbon dioxide which they
have absorbed from the tissues. When these corpuscles
reach the lung capillaries their carbon dioxide passes, by
diffusion, through the
walls of capillary and
air sac into the cavi-
ties of the sacs. At the
same time, the haemo-
globin in the corpus-
cles absorbs the oxygen
of the inspired air in
the sacs. Similarly, by
dialysis, the organic
compounds and water
are given off to the
sacs from the blood
plasma. In this way
the blood in the capil-
laries is enabled to
renew its supply of
oxygen, rid itself of its burden of carbon dioxide
and excess of water, and finally to return to the heart
(pulmonary veins). From there it is sent out to the
tissues again. This interchange, therefore, accounts for
the manner in which the oxygen gets into blood, and carbon
dioxide out of it. It is to be noted in this action that,
except for the tiny amounts of organic compounds given
off, no true wastes pass from the blood to the air in the air
sacs. On this account, breathing should not be spoken
of as the purifying of the blood. AVhat actually happens
is the aerating of the blood, that is, an exchange of carbon
the capillary network which covers them,
and at a the structures which intervene be-
tween the air and the blood are indicated ; 1,
mucous membrane of the air cell ; 2, submu-
cous meshwork; 3, wall of capillary; 4,
plasma in capillary ; 5, red blood corpuscle.328
RESPIRATION
dioxide for oxygen. The purifying or removal of wastes
from the blood is performed by an entirely different set
of organs, which we shall consider more in detail in a
following chapter.
The whole respiratory system, then, is simply a device for
getting the blood into thin walled tubes and these tubes
in contact with air, in order that the process of osmosis
may bring about the exchange of the two gases, oxygen
and carbon dioxide.
Breathing Habits.
Costal and abdominal breathing.— In man, in quiet,
normal breathing, the operation is mainly confined to dia-
phragm movement, the ribs expanding very little. In
woman, on the contrary, quiet breathing is carried on
mainly by the rib movements. These two kinds of breath-
ing are spoken of usually as abdominal (diaphragm) and
costal (rib) breathing. It is not believed that the two
kinds are to be associated primarily with the sex, but, on
the contrary, it is believed that the costal method of the
woman has resulted from the tightness of her clothing as
compared with that of a man. The use of corsets and
tight fitting dress waists tends to restrict the expansion of
the abdomen, and forces the use of the upper ribs for the
purpose. If such tight dressing is canned to excess, as in
tight lacing, not only is abdominal breathing prevented,
but even rib action is interfered with, and thus the entire
breathing action or chest cavity enlargement may be inter-
fered with. Such interference results in insufficient air
supply to the lungs and body tissues, and the distorting of
the skeleton, and subjects the abdominal organs to suchLUNG CAPACITY
329
great pressure that disease of these parts may result. Such
tight clothing is evidently a menace to the health of the
body and is to be deplored.
It is especially important that the growing child should
have free play of the chest cavity, and in exercising the
increased demands of the muscles for oxygen can be met
only by allowing free play to the lungs and walls of the
chest cavity.
Lung capacity.— The amount of air respired in a breath-
ing depends, of course, upon the
extent of chest enlargement, and
the size of the individual. In
ordinary breathing, the amount
of air respired by all individuals
represents only a small propor-
tion of the total capacity of the
lungs. Even in deep expira-
tions, the lungs are never en-
tirely emptied of air. The figures
for an average adult are as fol-
lows:
After the fullest possible in-
spiration the lungs contain about
three hundred and thirty cubic
inches of air. After the fullest
possible expiration following such
an inspiration they still contain
1	J	#	Fig. 153 — Lung capacity.
one hundred cubic inches of air.
The difference, two hundred and thirty cubic inches of air,
represents the greatest possible amount of air that can
enter and leave the lungs in a single respiration. This is
called the vital capacity of the lungs. In quiet breathing
? A ,	y /, '/ / .
"GomplemYn tdla
T/V J?"
Air
/.- 100 cu. in.
y	/
Tidal Air
30 cu. in.
Reserve
Air
100 cu. in.
Rrsidind -1
100-icir.yiinM330
RESPIRATION
only about thirty cubic inches is inhaled and exhaled in a
single respiration. This is called tidal air. The one hundred
cubic inches left in the lungs after the fullest possible
expiration is called residual air.
The significance of the small amount of tidal air and
the presence of a certain constant amount of residual air
is found in the fact that such a condition allows the blood
exchange to take place continually instead of at intervals
as would be necessary if the lungs were entirety emptied
at each expiration. Further, the possibility of varying
the amount of air content permits the lungs to accommo-
date the air supply to the demand of the tissues, that is,
when exercising the increased amount of oxygen needed
is met by deeper inspiration and expiration.
Hygiene of Breathing.
We have already noted that in order that the air be
properly warmed and cleared of dust before entering the
lungs it should pass through the nasal cavities rather
than through the mouth. We have also seen that the
action of the lungs is dependent upon the enlargement of
the chest cavity, and hence tight clothing or other inter-
ference with such enlargement is necessarily injurious.
Like muscles, too, the capacity of the lungs and their
size are increased by use. Exercise increases both the
blood rate and the breathing rate. Now, in quick, short
breathing, only the air in the upper part of the lungs is
changed, while the increased rate of blood flow shoots the
corpuscles through the capillaries so rapidly that they do
not have time to effect a satisfactory exchange of oxygen
and carbon dioxide unless there is a large and constantMODIFICATIONS OF BREATHING
331
supply of fresh air to draw upon. Deep breathing exer-
cises soon accustom the lungs to fuller expansions and
contractions under these conditions, and thus bring the
air to greater areas of capillaries. Runners, whose lungs
have been so trained, can run much longer distances and use
their muscles to greater advantage than those who have
neglected such training. “ Second wind” is simply the
automatic response of the lungs by increase of rhythmical
expansions and contractions. Again, since the breathing
depends upon the action of the chest muscles and those of
the diaphragm, exercise tends to develop these muscles
and thus directly develop lung capacity. People who
have well developed lungs and muscles are said to be long-
winded, and are usually more healthy than short-winded
ones.
Modifications of breathing.— When any obstacle such as dust
or food enters the trachea, the abdominal muscles contract forcibly
and compress the lungs. The glottis opens suddenly and the air
forcing out the matter gives an effect called coughing. Some-
times inflammation of the lining of the bronchial tubes may cause
this action in bronchitis. In like manner irritation of the nasal
membranes causes the air to be forcibly expelled through the
nose. Such an operation is called sneezing. Blowing is a long,
forcible expiration through a small opening in the lips or through
the nose. If the object which causes coughing is of sufficient
size to stop the trachea, as when food slips by the epiglottis, the
expirations increase in frequency and force, and we call the result
choking. In this last, the supply of air to the lungs may be com-
pletely cut off if the obstacle is not soon removed, and strangula-
tion, or death from lack of oxygen, result. When the air supply
is cut off by inclosure in a room without ventilation, or by immer-
sion in a gas or water, the result is spoken of as suffocation. Both
strangulation and suffocation are the results of oxygen starvation.
Laughing, crying, and sobbing are merely rapid successions of382
RESPIRATION
short expirations followed by a long inspiration. Hiccoughs are
caused by a sudden contraction of the diaphragm. Yawning is a
long inspiration and expiration through the open mouth while the
throat muscles are held rigid. Sighing is similar, except that the
inspiration is through the nose, and the muscles are suddenly
relaxed after the inspiration, thus making a sound. Snoring is
produced by the inspired air passing over the soft palate and
setting up a vibration. Spitting is a use of expired air to force
matter from the mouth. It may be done by the action of the
mouth muscles alone.
In general, then, all these modifications of respiratory move-
ments are essentially reflex, that is, they take place without any
act or will on our part, and often in spite of our desire to the con-
trary. Spitting is an exception to this rule, though that may
become a habit. Most of them, with the exception of hiccoughs,
are under our control to a certain extent.
Asphyxiation and artificial respiration. — Asphyxiation is death
from suffocation or the lack of oxygen supply. It may be brought
about in various ways. Blocking the tracheal entrance by pres-
sure applied to the outside of the throat, as in strangulation, is
one method. Exposure in a medium containing no oxygen is
another. Exposure to gases which have a greater affinity for the
hemoglobin of the blood than oxygen, and hence turn it out of
the corpuscles, produces the same result. Carbon monoxide or
the gas produced in burning charcoal is an example of such a
cause. In general, any cause that deprives the lungs of their
normal supply of oxygen will eventually produce asphyxiation.
When the tissues find their oxygen supply reduced in any of
the above ways, the first visible symptom of this lack is seen in
the deeper and more rapid breathing. Then the breathing loses
its regularity, and soon convulsions follow. Gradually these con-
vulsions spread until other muscles beside those concerned in
breathing are affected, and the whole body is thrown into violent
spasms. Finally the muscles become exhausted, the movements
decrease and finally cease altogether, consciousness goes and the
animal is soon still in death unless remedies of a powerful char-
acter are soon applied. What these remedies shall be is largely
determined by the cause of the asphyxiation, and resource shouldVENTILATION
333
be had at once to a physician. In one method of asphyxiation,
however, namely drowning, it often becomes necessary to apply
remedies before the attendance’ of the physician can be secured.
Owing to this fact, and because the methods illustrate the typical
process, the next paragraph is devoted to consideration of such
remedies.
Artificial respiration.— When a person has become unconscious
from immersion in water, or is in a partly drowned state, it is often
possible to imitate respiratory movements, and thus restore normal
breathing and life. All such methods depend upon what is called
artificial respiration. One of the simplest methods of the arti-
ficial breathing remedies is as follows: Remove all the clothing
from the upper part of the body and turn the patient upon his
face, place a roll of cloth under the pit of the stomach and then
press down hard upon the back in order to force the contents of
the stomach out of the mouth. Now turn the body on its back
and set up artificial breathing movements by first grasping the
arms above the elbows and raising them above the head. Next
lower them gently to the sides, at the same time pressing the chest
to expel the air and water in the lungs. Repeat these movements
regularly about fifteen times to the minute. See that the tongue
is well drawn out while this is going on. The first symptoms of
life will show in a faint, pink color under the finger nails and in
the lips. Continue these artificial breathing movements until the
body takes them up automatically and regularly.
The action involved in these operations depends upon the fact
that in raising and lowering the arms the chest walls are made to
move in the same manner as in normal breathing, and gradually
the muscles which control these movements take up their functions
again and the patient is restored. The pulling forward of the
tongue opens the larynx and stimulates the nerves which control
the respiratory movements.
The above method is only one of many for the restoration of
partially asphyxiated victims. All methods depend upon the
forcing of air in and out of the lungs until the exhausted muscles
are restored to their normal activity.
Ventilation.— Since the lungs are constantly exchanging large
quantities of carbon dioxide laden air for oxygen, it is evidently334
RESPIRATION
necessary that the air in the rooms in which we live be renewed
frequently. If this air is not constantly renewed the lungs are
forced to inhale air which has lost part of its oxygen, and the pre-
liminary stages of asphyxiation are soon reached. A feeling of
dullness, labored breathing, and headache are the warnings which
tell us of the need of change of air in a close room, and these warn-
ings should be heeded. All schoolrooms and public halls are
now provided with systems of ventilation, that is, apparatus
forcing in new air and removing the old. Houses not provided
with such systems should have the windows opened frequently,
and the air should be allowed to circulate freely until the renewal
is secured. During sleep the demand for oxygen is even greater
than in the waking hours, since that is the time for the restoration
of exhausted tissue. No one should sleep in a room whose win-
dows are tightly closed and the means of ventilation thus cut off.
The basis of all ventilation methods is found in the fact that
cool air is heavier than warm air. For example, the air in a room
is usually much cooler at the floor than near the ceiling. If, now
we provide an exit for the warmer air at the top of the room,
and an entrance for the outside air, the outside air will rush in and
the warm air will be forced out. Thus the supply for the room will
be renew'ed. Precaution against draughts is important, since the
sudden chilling of any part of the body may produce colds. The
greater the difference in temperature between the outside and
inside air the more rapid the flow, hence in winter much smaller
openings are necessary for ventilation than in summer.
All furnace-heated houses should be provided with cold air
boxes, so that the air in the pipes may receive constant renewal
of oxygen as it circulates.
Air in a room cannot be changed oftener than three times in
an hour without producing dangerous draughts. On that account,
schoolrooms where fans are used to force air into the room should
have this rate carefully regulated, and also should not be so
crowded as to make a more frequent change necessary.
Dusting.— We have seen that the approaches to the lungs are
well provided with apparatus for removing dust from the inhaled
air. Such dust irritates the delicate membranes of the lungs if
allowed to come in contact with them and may also contain diseaseADENOIDS
335
germs. On this account sweeping and dusting should be so per-
formed as to remove dust from the room and not to fill the air
with it. Sweeping raises into the air the dust from the carpets
and furniture. Before the room is used again plenty of time
should be allowed for this dust to settle and the air to clear. Then
the dust should be removed from the furniture on which it has
collected, with a damp cloth, and not with a brush or feather
duster, as these latter do not remove it but simply throw it into
the air again. Opening windows while sweeping permits currents
of air to remove most of the dust particles, and at the same time
renews the oxygen.
• Stone cutters and others who work in dust-laden rooms often
suffer from serious diseases as the result of inhaling these particles.
Disease of the Respiratory Organs.
Adenoids.— This is the name given to certain grapelike
swellings or growths which develop in the upper part of
the pharynx just behind the nasal opening. They often
form in children, and if allowed to grow soon close the
opening to the nose, and thus make mouth breathing
necessary. They may even develop to such an extent as
to close the Eustachian tube leading from the pharynx
to the ear, and thus produce deafness. While they are not
in themselves dangerous, and often shrink and disappear
as the child grows older, they may indirectly cause much
harm as well as annoyance to the growing child. In
children, the bones are in process of formation, and mouth-
breathing often produces an unsightly deformation of
the jaws. Adenoids also render the child liable to colds,
since air which enters by the mouth is not properly warmed,
and thus may chill the delicate membranes, and so pro-
duce sore throats, tonsilitis, etc. They are growths which
may be easily removed, and children suffering from them
should receive proper treatment for their removal at once.336
RESPIRATION
Inflammatory diseases. — Sudden changes of temperature
affect the vaso-motor nerves, and thus the blood supply to
certain parts of the body. When the skin is suddenly
chilled in any way the blood vessels of the mucous mem-
brane, which lines the air passages, become charged with
blood. If under such conditions these membranes become
infected with bacteria, inflammation results. When this
inflammation is limited to the nasal cavity we call the result
a cold in the head. When the pharynx membrane is
inflamed a sore throat is the result. A chest cold is due to
inflammation of the windpipe and bronchial linings. If
this inflammation spreads to the linings of the bronchial
tubes, we call it bronchitis, while infection of the air sacs
themselves produces pneumonia. The reactions involved in
these various types of inflammatory diseases of the mucous
membrane have already been discussed in the paragraphs
on inflammation. (See p. 206). All forms are accom-
panied by great secretion of mucus, which tends to clog
the air passages and render breathing difficult.
In certain cases the pleura which lines the thoracic cavity
becomes inflamed, the two surfaces rub together, produc-
ing severe pains, while the secretion of pleural liquid
increases to an abnormal amount and produces pressure
upon the lungs. Such a disease is called pleurisy, and can
often be relieved only by tapping the pleural cavity and
extracting the excess of fluid collected there.
The treatment for inflammatory diseases depends upon
the reducing of inflammation and the restoration of the
blood supply and secretion to their natural amount. In
severe cases a physician should always be consulted.
Other germ diseases.— Inhaled air often contains disease
germs of various kinds. If these germs lodge in the mem-OTHER GERM DISEASES
337
branes of the air passages, they may grow and produce
colonies of bacteria. These colonies in their growth may
break up the surrounding tissues or may produce poisons
or toxins which are absorbed by the blood and carried to
parts where they produce serious harm and may even pro-
duce death.
Tuberculosis or consumption is one of these bacterial
diseases. The bacteria which cause it are rod-shaped
bodies, known as the bacillus tuberculosis. These bacteria
infest the air passages and sacs and gradually destroy the
surrounding tissues. Unless removed, they eventually
produce death. The sputum which a consumptive coughs
up consists of masses of these bacteria and broken down
tissue, and soon sets free the bacilli if allowed to dry on the
floor or street; these may then be blown about by the
air and thus be inhaled. It is on this account that Boards
of Health make strenuous rules against spitting in public
places, and are also coming to insist more and more upon
the isolation of consumptives in special sanatoriums where
they will not be able to spread this dangerous disease.
Diphtheria, membranous croup, and la grippe are all
germ diseases produced by special kinds of bacteria which
settle in the throat and grow there. These forms produce
their effect by poisons called toxins, which they give off, and
which enter the blood, sometimes with fatal results. An
effective cure for diphtheria has been found in a liquid called
diphtheria anti-toxin; when this is introduced into the cir-
culation it destroys the toxin produced by the bacteria.
All diseases that are produced by bacteria are called
communicable diseases, since they may be transmitted
from one person to another by the air which carries the
dread germs. Some of these bacterial diseases are produced
EDDY. PHYS. — 22.338
BESP1KATION
by bacteria which enter the body through the blood or
food; but it is evident that the air furnishes a natural
highway for them, and tliis is another reason why we
should make sure that our air supply is as pure as possible,
and that we keep our respiratory organs in a healthy state,
in order that the germs which enter may be removed before
they have time to form colonies.
Alcohol and respiration.— It is often claimed that alco-
hol increases the rate of respiration, and is thus a valuable
element in cases where great demands for oxygen are made
by the tissues, as in heavy muscular exertions. Careful
experiment has shown that alcohol does increase the rate
of respiration and the amount of oxygen taken into the
body for a period of an hour or more after its use; while
highly flavored wines and brandies have an even greater
effect in this direction than pure alcohol. When, however,
the manner in which alcohol causes this increase in oxygen
supply is investigated, it is seen to be a harmful rather than
a helpful agent. The reason for this may be stated as
follows:
Alcohol causes a congestion of the surface blood vessels
and a consequent loss of heat to the body. It also causes
temporary increase in the activity of the digestive tissues
and the muscles. This loss of heat and action of the tissues
creates a demand for more oxygen on the part of the body,
and this demand acts upon the respiratory center and
makes the rate of breathing faster, in order to furnish the
increase in oxygen. In other words, if it were not for the
action of the alcohol on the tissues there would be no in-
crease in the breathing rate, and the increase in oxygen
thus secured is more than used up in offsetting the loss of
heat and in supplying the needs of these unnaturally activeALCOHOL AND RESPIRATION
339
tissues. Hence the tissues which are being used in exer-
cise are worse off than before the alcohol was taken.
In short, the taking of alcohol as a means for increasing
oxygen supply is like giving a man an increase of salary
and at the same time increasing his expenses by a
larger amount. In the end he is worse off than in the
beginning.
It is the above result that has made athletes and those
who have severe muscular efforts to perform abandon alco-
hol as a means of aid to their action.XXII. RESPIRATION IN LOWER
ANIMALS.1
The activity of tissue cells demands that they receive a
constant supply of oxygen; while the result of oxidation
in them is the production of quantities of carbon dioxide
which must be as constantly removed to permit of their con-
tinued activity. In other words, respiration is a property
of protoplasm, and wherever there is active protoplasm
there must be respiration. Since plants and animals are
all built up of cells containing active protoplasm, respira-
tion becomes a property of all living matter. The rate
of action of the cells depends upon the completeness and
speed with which this exchange of oxygen and carbon
dioxide takes place.
In all the lower organisms the mere contact of the cells
with a medium which contains oxygen — such as air or
water — is sufficient for the exchange to take place.
When, however, the organisms increase in size and the
number of cells, many cells are necessarily forced into
places where such contact is impossible. To these cells
the oxygen must be brought in some wav, and so in most
plants and animals there are found special structures
whose purpose is the distribution of oxygen to the tissue
cells and the collection of carbon dioxide from them.
Such apparatus wherever found, whether in plant or
animal, is called a respiratory system. We shall consider
1 See Footnote, p. 146, Chapter X.
340PROTOZOANS, SPONGES, AND CCELENTEEATES <541
a few of these systems in the animals lower than man and
see in what they resemble his or differ from his.
Sources of oxygen.— All animal forms 1 obtain their
oxygen from one of two sources, — from air or from water.
Water has the power to dissolve certain gases with which
it is in contact, and thus water which is exposed to air
for any length of time is bound to contain more or less
oxygen in solution. Animals which have the power to
extract this dissolved oxygen from water for use by their
cells, are often called water-breathing, to distinguish them
from those forms which take their oxygen from the air
or air-breathing forms. In both air and water breathers
the process is the same, namely, the extraction of the
oxygen for the use of the tissue cells.
Tvpes of Respiration in Invertebrates.
Protozoans, sponges, and coelenterates.— All protozoans
(one-celled animals), in their active state, live in water, or
fluids containing water, and extract their oxygen from
this water. With them, there is no need for blood or other
respiratory apparatus, since each cell is in contact with
the water, and can take in oxygen and give off carbon
dioxide as needed.
The sponges and ccelenterates, such as the jellyfishes,
anemones, etc., are built up of many cells, but through
the use of cilia or other means the currents of water which
bring food to the internal cells are so directed as to bathe
1 Certain plant forms are known, called the Anterobia and limited
mainly to certain groups of bacteria, that are able to live in media that
contains no oxygen. This does not mean that these organisms are able
to live without oxygen, but it is supposed that they have the power to
extract oxygen from the food which they receive.342
RESPIRATION IN LOWER ANIMALS
every cell, and thus each cell can effect its exchange of
gases directly, in the same manner as the single cell of the
protozoan.
The starfish and its allies.— In the starfish we have the
simplest kind of a respiratory system. In these animals
a tube is found which conveys water to various parts of
Fig. 154. — Vertical section of one arm of a starfish ; b, ampulla; d, stone canal;
i, radial ambulacral vessel; ft, feet. — Davison Zoology.
the body. The same tube is used in distributing water
to the tube feet (see p. 305), and this tube serves not only
to make locomotion possible, but also, by the water which
it carries, to distribute oxygen to various parts of the star-
fish body and to receive carbon dioxide from them. These
starfishes also have soft membranelike extensions of the
skin that line the body cavity and project as pouches, or
pockets, between the plates on the upper surface of the
body. Oxygen is absorbed from the sea water into the
body cavity through these thin, membranous pockets,
and carbon dioxide passes out through them, also. The
starfish, therefore, has two entirely distinct methods of
respiration. It is to be noted that while this animal also
possesses a partly developed circulatory system the oxy-
gen distribution is not dependent upon it. This water-
tube system of distribution is also entirely distinct from theCLAMS AND OTHER MOLLUSKS
343
food-distributing system. Such an apparatus is a true
adaptation for respiratory purposes.
Worms.— The worms obtain their oxygen supply by
absorbing it from the air or water through the skin, which
is thin and moist, and in direct contact with
the air or water in which they live. In some
cases this skin is produced into pouches or tufts
in order to increase the respiratory surface, and
such tufts are called gills. After the oxygen
has passed into the body cavity through the
gills it must be distributed to the tissue cells.
In some worms this distributing is performed
by the digestive fluids. In others, such as the
earthworm, this oxygen passes through the thin
walls of a blood tube where it unites with
haemoglobin dissolved in the blood fluid, and,
in this case, the blood acts as a true distribu-
tor of the gas to the cells. This blood also
collects the carbon dioxide, which is passed
out through the skin in the same manner that
the oxygen enters it.
In the earthworms we find the first ex-
ample of the blood acting in connection with
the skin as a medium for the distribution of	Theiob-
oxygen and carbon dioxide. It is to be noted	gVo™’-
that in this case the skin is the respiratory organ ingtuft-
and the blood plasma the distributor, since the
blood contains no corpuscles. In order to absorb oxygen
the skin must be very thin and moist.
Clams and other mollusks.— In the clam and the oyster
there is a heart and a definite system of branching blood
vessels. Some of the smaller branches pass into four flaps344
RESPIRATION IN LOWER ANIMALS
of soft skin (in the clam two on each side of the body)
called gills. These gills are thin-walled membranes or
extensions of the skin which
cover the body of the clam,
and thus the water in which
they lie and which passes over
them is separated from the
blood by only this thin skin
and the thin walls of the ves-
sels. Oxygen is able to pass
easily through these mem-
branes and enter the blood
just as it does through the air-
sac linings and capillary walls
of our lungs. The blood of the
clam contains no corpuscles
and no haemoglobin and is
colorless. It does, however,
contain a substance similar
to haemoglobin called haemo-
cyanin, which takes up the oxygen and distributes it. At
the same time the blood passes its carbon dioxide out to
the water and is thus aerated. This aerated blood passes
from the gills back to the heart, and is then pumped over
the body to the tissues, finally returning to the gills, and
so on indefinitely.
Here	in	the	clam,	then, we have a definite structure
developed	solely	for	respiratory purposes, and showing
practically the same relation between blood and the oxygen-
supplying medium as that seen in our air sacs. The clam,
also, has a pair of tubes which cause a constant stream of
water to pass into and out of the shell and over the gills,
Fig. 156 — Diagrammatic section of
a clam (Anodonta); a, lobes
of mantle; b, gills, showing
transverse partitions ; c, ventricle
of heart; d, auricles ; e, pericar-
dium ; f9 g, kidneys ; h, venous
sinus ; k, foot; A, branchial, or
pallial, chamber ; B, epibranchial
chamber.INSECTS
845
so that the supply of oxygen and the removal of carbon
dioxide is constant. In the oyster the system is similar,
except that there are no tubes, as the movement of the
sea water is sufficient for this renewal and removal.
In the air breathing mollusks, of which the snails are a
type, the blood vessels line a cavity in the body, and this
cavity is connected with the outside by an opening. This
cavity is an infold of the skin instead of an outfold, but its
action is identical with that of the gill. Into this aperture
the snail draws a bubble of air, and even the water snails
creep to the surface for this purpose, and sink to the bot-
tom again after filling the cavity. The blood vessels
which line this cavity effect an interchange of gases with
this air just as the gills do with the water, and after all
the oxygen is removed the walls of the cavity contract
and force out the bubble, now laden with carbon dioxide,
and take in a new supply of air. This is the simplest form
of a lung known.
Insects.— In the insects we find air breathers having a
respiratory system unlike that of any other animal. These
air breathers have openings called spiracles along the sides
of the abdomen. By movements of the body these spir-
acles can be made to open and close rhythmically. They
are actually the mouths of a system of branching tubes or
tracheae which penetrate to all parts of the body, and thus
carry air directly to the various body tissues. These tubes
are strengthened internally by a spiral thread which
prevents their collapse as the air rushes in and out. The
insects are the only animals that carry air directly to all
parts of the body. The movements of the abdomen
maintain a constant circulation of air through these tubes,
in and out of the spiracles. This system is entirely inde-346
RESPIRATION IN LOWER ANIMALS
pendent of the blood system, and in these animals the blood
has practically no part in the exchange of gases.
Gills.— All the higher animals, including fishes, frogs,
reptiles, and mammals, breathe by the aid of gills or lungs
according as they are water breathers or air breathers.
A gill in its simplest
form is a fold of skin,
usually lined with capil-
laries and so placed that
the oxygen - supplying
water shall pass over it,
while the thinness of the
skin and capillary walls
permits the exchange of
oxygen and carbon di-
oxide between the blood
and the water. The
pouchlike extensions of
the starfish membrane
differs from this defini-
tion only in the lack of
blood capillaries. In
this case the exchange
of gases is between the
body cavity and the
water. The blood tubes
appear first in the earth-
worm where the whole
skin acts as a gill.
In the clam we find the simple fold relation, exactly as
in our definition, and the clam gill may, therefore, be
taken as typical.
Fig. 157 —The respiration system of an insect;
a, an air sac ; ft, a tracheal branch ; c, a spira-
cle or breathing pore.GILLS
347
The fringed gill appears first in the animal scale, in
the crayfishes, lobsters, and crabs. Here, folds of skin
Fig. 158. — The lobster, showing fringed gills (h).
lined with blood tubes are attached to the bases of the
legs and inclosed in a cavity formed by the shell. Instead
of a flat fold, however, the skin is broken up into tiny,
cylinderlike projections or fringes, thus increasing enor-
mously the absorbing surface without increasing the
space occupied by the gills. This fringing, then, is an
adaptation for increase of absorbent surface without in-
crease in space occupation, and is made necessary by the
increased demand for oxygen on the part of the animal.
In these forms, the cavity is constantly supplied with
water, and the movement of the legs causes the gills
to wave about and thus bring all surfaces of the fringes in
contact with it. Though this gill chamber, and the appa-
ratus for circulating water through it, may be called ac-
cessory respiratory apparatus, it bears the same relation to
breathing as do the rib walls and diaphragm in our body.
The fringed gill reaches its highest development in the
fishes. Here, the gills are placed in the sides of the head,
supported on arches of bone and in such a position that the
fish can maintain a steady stream of water over these gill848
RESPIRATION IN LOWER ANIMALS
fringes by constantly taking in and forcing out water
through slits in the side of the head. The rich supply of
blood capillaries in
these fish gills is in-
dicated by their red
color. The blood
of the fish passes
from the ventricle of
the heart into these
capillaries; there ex-
changes its carbon
dioxide for oxygen
and then passes di-
rectly over the body
without first return-
ing to the heart, as
in our bodies.
While the gill is well adapted to gas exchange with water
the small proportion of oxygen found in water makes that
element a poor source of oxygen, and all animals higher
than the fishes obtain their supply of this gas from a me-
dium much richer in it, namely, air. Such animals extract
their oxygen supply from air by structures called lungs.
Lungs.—- Just as we defined a gill as an external fold of
the skin, so we may define the lung as an internal fold
of the skin lined with capillaries and adapted for the
exchange of oxygen and carbon dioxide between the blood
and the ah’.
The lung of the snail is the simplest type. Here, the
single, internal pocket, lined with blood vessels, may be
conceived of as having been developed by a single infold
of the external skin. So slow is the exchange of gases in
Fig. 159—Gills and heart of the perch, exposed
by removal of gill cover on left side; a, first of
the fonr bony arches which carry the gills; b,
gills ; blower edges of gills on the right side)
h, heart.LUNGS
349
this animal, and so small the demand for oxygen by the
tissues, that a single bubble of air furnishes oxygen enough
for a considerable period of time.
In the frog, which in its earlier stages breathes by gills,
we find a much more complex lung than this. Here, in-
closed in the thoracic part of the body cavity, are two sacs
connected at the throat by a common tube and lined with
blood vessels. These sacs have their inner surface raised
in many ridges that increase their internal surface without
increasing the size. As we go higher in the animal scale,
these ridges increase in depth, until finally, in mammals,
the interior of the lung sac is subdivided into thousands
of tiny air sacs connected by tubes to the common trachea.
The frog has no diaphragm, and in order to force air into
the lungs he first fills the mouth cavity through the nasal
passages, then, closing these passages and elevating the
throat, forces this mouthful of air down the tube into the
lungs. The air is expelled from these sacs by simply con-
tracting the body walls. In the frogs, too, we find the first
development of the trachea as a voice-producing organ.
In the reptiles, instead of a ridged sac with a central
cavity, the ridges are developed so as to divide the sac into
pockets, and these pockets connect with the trachea by
tubes. In this case, the two branches of the trachea (the
bronchi) extend through the lung sac, and the branches
or bronchial tubes are given off to the pockets from the
bronchus as lateral branches.
In birds we find the first development of true air sacs,
* and a branching system of bronchial tubes similar to ours.
The bird differs from man in having the lungs connected,
by extensions, with air cavities in various parts of the body,
and through these it is able to give the body a buoyancy350
RESPIRATION IN LOWER ANIMALS
which helps it in flight. Its method of breathing differs
from ours, there being no diaphragm, and the expansions
and contractions are
produced by the
thoracic muscles. The
voice box of these
birds is at the bottom
of the trachea instead
of at the top, and is
called a syrinx in-
stead of a larynx.
The mammals have
lungs practically iden-
tical with ours. As
we ascend the animal
scale, then, we see a
constant increase in
the absorbent surface
of the respiratory
organs, and this in-
crease keeps pace
with the increased de-
mand for oxygen. All lungs arc alike in having the blood ca-
pillaries separated from the air by a thin membrane through
which the exchange of gases may take place, and in having
some method of introducing new air into these sacs and
removing it at regular intervals, in order to maintain a
constant supply of oxygen to the animal. In all verte-
brates the oxygen extracted from the air is carried to the
tissues by the blood corpuscles which contain haemoglobin
for this purpose. The increase in the number of mem-
branes in the lungs is simply an adaptation for protec-
Fig. 160— Lungs of a bird; a, trachea; b, bronchi;
c*, lungs; d, apertures communicating with air
cells.COLD-BLOODED AND WARM-BLOODED ANIMALS 351
tion, or is made necessary by the increase in surface by
foldings.
Cold-blooded and warm-blooded animals. —By cold-
blooded animals we mean those in which the temperature
of the blood is only slightly higher than that of the sur-
rounding environment. These animals feel cold to the
touch, as they are of lower temperature than our bodies.
This cold-blooded condition is dependent upon the amount
of oxidation which takes place in the body, and fishes,
frogs, and reptiles have a slower rate than that of birds
and man. The higher temperature of birds and mammals
is evidently due to the fact that the greater perfection of
the respiratory apparatus permits a more rapid exchange
of oxygen and carbon dioxide in these animals than in the
cold-blooded forms. As a rule, cold-blooded animals are
more sluggish in their movements than warm-blooded ones.XXIII. EXCRETION.
In the preceding chapters we have traced in detail the
processes by means of which food and oxygen are obtained
and distributed to the cells of our tissues. We have also
seen that the function of some of this food is to make
new protoplasm, while other food substances free energy
by uniting chemically with oxygen in the process called
oxidation, and thus give to the protoplasm its varied
power of action, secretion, contraction, or the like. The
making of food into new protoplasm we call assimilation,
while the production of energy we call oxidation. We
give the name of constructive metabolism or anabolism to
all the processes which lead up to the production of activity
on the part of the cells. When oxidation takes place two
results follow, namely, the activity of the cell through the
energy produced, and the production of wastes such as
carbon dioxide and urea. This breaking up of the cell
through oxidation is called destructive metabolism or
katabolism. Anabolism and katabolism, which together
make up metabolism, are going on continually as long as
the cells remain active. We have seen in our study of
respiration how the cells receive their oxygen. We have
also noted that the blood carries the carbon dioxide back
to the lungs, and that these organs remove it from the
body. How are the other wastes removed from the cells?
To answer this question we must study another set of
organs whose main function is the removal from the body
of waste products of the cell. All organs concerned in
352THE SKIN
353
this removal of body wastes are grouped together under
the name of the excretory system. The lungs, by remov-
ing the carbon dioxide, may be included in this system,
but the removal of the principal body wastes is the work
of the kidneys and the skin.
The skin—If we examine the surface of the body care-
fully we find it everywhere covered with a layer of flexible,
elastic tissue which we call the skin. The hair and nails
are modifications of this tissue and are included in the
study of skin structure.
Examination of a section of the skin taken from any
surface of the body shows it to consist of two distinct
layers. The upper layer (epidermis, or cuticle) is thinner
than the lower layer (dermis, or corium), and contains no
blood vessels. Hence, it is possible to prick and even tear
off portions of the epidermis without causing bleeding.
This epidermal layer varies greatly in thickness in different
EDDY. PHYS. — 23354
EXCRETION
Fig. 162 — Portion of the epi-
dermis, showing the transi-
tion from the lower living
cells to the upper horny
scales.
parts of the body, being thickest at points where there is
greatest friction, as on the sole of the foot and the palm
of the hand.
The inner layer, or dermis, contains the blood vessels,
nerves, and various kinds of glands, and is easily recognised
by its pink color.
Epidermis.— Examination with a microscope shows this
layer to be composed of many cells held together by a
sort of cement. The lowest layer is
made up of a single row of active,
columnar, epithelial cells. These
cells absorb nourishment from the
blood and lymph of the dermis,
and, by growth, produce other cells
which are gradually pushed up-
ward. In this upward movement
they gradually lose their columnar shape, and become
flattened, so that the upper layers of the epidermis consist
mainly of flattened cells. Gradually, too, the protoplasm
of these cells dries up and they are transformed into flat,
horny scales which make up the uppermost layer of the
epidermis. These outer horny scales are being constantly
worn away and renewed from below, and the epidermis,
as a whole, shows transition stages of cells from the active
columnar layers near the dermis, to the dead horny outer
layer above. Scattered among these cells may be found
occasional cells which contain pigment or coloring matter.
These are called pigment cells, and in the negro race they
are very abundant and give the black color to the skin.
The distinction between a blonde and a brunette is merely
a difference in the number of these pigment spots or cells.
Freckles are due to the collecting of these cells in spots.THE DERMIS OR CORIUM
355
Examination of the epidermis on the hand shows it to
be raised in ridges parallel to one another, and so arranged
as to form various patterns. Since these patterns do not
change throughout life they serve to identify criminals.
A hand lens also shows that the epidermis is perforated
at frequent intervals. These perforations, or pores, are the
openings of tiny ducts which extend downward to the
sweat glands of the dermis and discharge the perspiration.
In other places the epidermis is pierced by hairs which
also have their source in the dermis.
As has been already noted, no blood vessels enter the
epidermis; the active cells receiving their food from the
lymph of the dermis. Nerve fibers are wanting in the upper
layers of the epidermis, but fibers do extend into the lower
cells, so that when this layer is separated from the dermis,
as in a blister, the prick of the needle can still be felt by the
epidermal covering of the blister though it produces no
sensation of pain.
The dermis or corium.— The dermis or true skin is com-
posed mainly of a network of connective tissue rich in
elastic and white fibers. In tanning operations it is this
la}'-er that is transformed into leather. The dermis is
loosely connected to underlying parts by a layer of fibers
called the subcutaneous layer, and this loose connection
permits the skin to move freely over the underlying flesh.
Sometimes this subcutaneous layer is the seat of fat
deposits, especially in the layers that cover the abdomen.
These fat layers tend to protect the body from cold. The
surface of the dermis that is in contact with the epidermis
is raised in a great many tiny projections called papilla).
These papillae are arranged in rows and are the cause of
the ridged appearance of the epidermis. Some contain a356
EXCRETION
knot of capillaries which are supplied with blood from
a small artery and empty into small veins. They enable
Fig. 163 — Dermal papilla*; the black lines represent capillaries, and the white
lines, nerves. The nerves end in bundles called corpuscles.
the blood to be brought near the active, epidermal cells.
The other papilla' contain delicate sense organs connected
by nerves with the central nervous system. The func-
tion of this latter kind will be discussed more fully in our
study of the nervous system. Some are sensitive to touch,
and some to cold, and to them is due the powrer of skin to
respond to external sensory stimuli. They are especially
abundant at the ends of the fingers, thereby giving to these
members their delicate perception of touch. We may dis-
tinguish these two kinds of papillae by calling one kind
blood papilke and the other nerve papillae. Evidently the
dermis is well provided with blood vessels and nerves.
Except for these papillae, the dermis is usually smooth, but
in old age the absorption of fat from the subcutaneous
layer permits the skin to shrink and form wrinkles.
Besides these papillae the dermis show's other structures,
sweat and sebaceous glands, while hairs and nails are siin-SWEAT GLANDS
857

ply modifications of normal skin
layers, and have their origin in the
dermis.
Sweat glands. — If we trace a
spiral duct which leads downward
from a pore in the epidermis, we
shall find that it passes through the
dermis also and ends finally in the
subcutaneous layer as a coil. This
knotted coil is surrounded by a
network of capillaries, and the com-
bination is called a sweat gland.
These coils are lined with a secreting
membrane of epithelial cells, and the
power of the skin to excrete wastes
in the form of sweat or perspiration
is due to their activity. Their ac-
tion is as follows: From the blood
brought to them by the capillaries
the cells select the salts and nitrog-
enous wastes (urea). These wastes,
together with water and some salts,
are passed on by the cells as a secre-
tion to the inside of the tubes where
they collect, and finally leave the
bodjr through the epidermal ends
of the ducts or pores. This action
may take place so slowly as to be
invisible to the eye (insensible per-
spiration), or in summer, and in violent exercise, drops
may collect on the surface of the body (sensible perspira-
tion). By placing the skin against a cool surface, such as
Fig. 164 — Sweat gland; a, epi-
dermis; b, dermis; c, sub-
cutaneous layer; d, sweat
gland; e, mouth of sweat
duct (spiral duct).358
EXCRETION
a mirror, we can condense the perspiration, and thus
demonstrate that even when invisible to the eye the pro-
cess is always going on.
Sebaceous glands.— These are small, saclike ducts em-
bedded in the dermis, and opening usually by a small duct
near the mouth of the hair follicles. They are lined with
epithelial cells whose substance becomes gradually charged
with oil drops. Finally the protoplasm disappears, the
cells break down and their oil is discharged through the
duct, new cells taking the place of the old ones and con-
tinuing the process indefinitely. While these glands are
always found associated with hair follicles, they are also to
be found in parts of the skin free of hair. In these cases
the oil discharged spreads over the skin and moistens and
softens it. When such ducts become stopped by dirt the
contents become hardened and the ducts become distended
and form black spots on the skin.
Hairs.—A vertical section of a hair shows the following
structure: The external hair is a long, hollow shaft made
of an extension of the epidermis and embedded at its base
in a pocket of the dermis called the hair follicle. This
pocket or follicle extends downward into the subcutaneous
tissue, and at its base is a special papilla which forms the
hau and which is abundantly supplied with nerves and
blood vessels. The follicle is lined with a layer of epider-
mal cells, and the upper part of the hair shaft is composed
almost entirely of the horny layers of the epidermis, and
hence this part is not sensitive to cutting. The nerves at
its base in the papilla render the hair sensitive to touch, and
give the sharp sensation of pain when the hair is pulled.
The relation of the various parts of the hair to the normal
skin layers is shown in Figure 165. The color of the hairNAILS
359
is due to pigment cells lying in the
hair shaft.
Attached to the sides of the
hair follicle are oblique strips of
unstriped muscle by means of
which the hair may be moved.
When these muscles are con-
tracted by cold, the skin becomes
roughened, and this condition is
due to the erection of the hairs.
Opening into the side of each hair
follicle is the duct of a sebaceous
gland. The glossy appearance of
the hair is due to the oil poured
out by this gland.
Hairs may be foimd on practi-
cally all parts of the skin except
the sole of the foot and the palm
of the hand. Their development
varies greatly with the individual.
On the head they form a good
protection from blows.
Nails. — The ends of the fingers
and toes are protected by thicken-
ings of the horny layer of the
epidermis called nails. These nails fig. i®. —section of a hair; a,
hair shaft showing central
are very similar to hairs in their medulla or core; sebaceous
formation. Their relation to the ££
epidermal and dermal layers is the hair is seen tlie p»pilla
. which forms it.
shown in Figure 166. The base is
embedded in a fold of the skin, and at its root is a portion
of the dermis called the matrix which, by pushing out360
EXCRETION
new cells, constantly increases the length of the nail. At the
same time the dermis just beneath the bed of the nail
forms new cells which add to its thickness. The matrix
Fig. 166—Sections of nails. A, median longitudinal section (X 4); a, the fold at
the base of the nail; b, the nail; c, the bed of the nail. B, transverse section
( X 4); a, lateral folds on each side of the nail; 6, the nail; c, bed of nail showing
papillae. C, portion of bed of nail (X 200); e, nail, formed of horny dermal scales;
d, deep layers of epidermis; o, papillae.
may be compared to the hair papilla, and injury to hair or
nail will be repaired so long as the hair papilla or the nail
matrix is uninjured. The nail shows the following areas:
The part embedded in the dermal pocket is called theACTION OF THE SKIN
361
root. The visible part consists of a body or bed of the
nail which is attached to the dermis, and a free edge.
Near the root is a half moon, or limula, which is whiter
than the rest. This difference in color is due not to the
nail itself which is transparent, but to the fact that the
blood papilhe are less numerous under this part of the
nail.
General physiology of the skin.— The principal uses of
the skin may be classified under the following heads:
(a)	It affords protection to the underlying muscles and
organs, and at the same time, by its loose connection, per-
mits free movement of these underlying parts.
(b)	It permits the evaporation of water from the sur-
face of the body, and thus regulates the temperature of
the body. In hot weather, or when one is exercising, the
excess of internal or external heat is attended with profuse
perspiration, and this heat is used to evaporate the water
of the perspiration, and hence does not raise the body
temperature. By this means the body temperature is
maintained at an even rate of about 98° Fahrenheit.
(c)	Its underlying laj^ers of fat also prevent too great
a loss of heat from the underlying parts.
(id) It contains sensory papilla; of various kinds which
are connected with the central nervous system (spinal
cord and brain), and these papilla; permit the response of
various parts of the surface to external stimuli.
(e) It is an excretory organ, and by means of the sweat
and sebaceous glands it is able to extract wastes from the
blood and remove them to the outside of the body.
Action of the skin as an excretory organ.— The precise
composition of sweat is difficult to determine, as it is
usually mixed with the secretion of the sebaceous glands362
EXCRETION
It is a very thin liquid, slightly heavier than water (Sp.
Gr. 1.004), and usually shows an alkaline reaction from
the dissolved salts present. These salts consist mainly
of alkaline phosphates and sulphates and sodium chloride.
The organic wastes (urea, uric acid, etc.), which are pro-
duced by the breaking down of the proteid parts of the
tissue cells, are present only as mere traces, though as the
result of muscular work the amount of urea thus thrown
off may become considerable. The principal waste given off
by the sweat glands, however, is water, and it is as a
remover of water from the body that the skin is espe-
cially important. The amount of water thus removed
varies greatly with conditions of temperature, and with
the mental and physical states. It has been calculated
that the average amount of sweat given off in twenty-
four hours by a normal person is between two and three
quarts.
The sebaceous glands contribute an oily liquid that
sets in a cheesy mass upon exposure to the air. It con-
tains fats, soaps, and some proteid materials. It varies
in quality in different parts of the body, that of the ear
constituting the ear wax. Its principal function is to
soften the skin and hairs, and protect the skin from exter-
nal moisture, while it also tends to prevent loss of heat
by evaporation.
In some of the lower animals the skin also excretes car-
bon dioxide and takes in oxygen, thus acting as a respira-
tory organ. In man the amount thus given off by the
skin is small under ordinary conditions. In profuse per-
spiration, however, the amount of carbon dioxide removed in
this way becomes considerable. Under ordinary conditions
the removal of carbon dioxide takes place in the lungs.BATHS
363
Hygiene oe the Skin.
The various excretions of the skin tend constantly to
deposit increasing layers of waste on it's surface. The
accumulation of this waste clogs the pores and prevents
the escape of further wastes. Since these wastes must
leave the body somewhere, the result is the overworking
of the kidneys and lungs, while the lack of water evapora-
tion permits an excessive increase of body temperature.
If, however, the pores are clogged with material which
conducts heat there results a lowering of body temperature
that produces death when any considerable portion of the
skin has been painted or varnished.
To avoid any of these dangers, it is evidently imperative
that the skin be bathed frequently, and thus the excess
of waste removed and the pores kept open. Cleanliness
of person, therefore, becomes one of the first laws of health.
Baths. — The oily secretions and the grimed-in dirt require
more than the mere application of cold water for their removal.
Soap contains an alkali which dissolves these oils and loosens the
dirt. On this account exposed parts of the body should be
washed with soap and warm water at least three times a day,
and the whole body at least once a week. Care, however, must
be taken in selection of the soap used, as many contain an excess
of alkali which tends to injure the skin. Baths not only serve to
cleanse the skin, but, if properly taken, tend to strengthen and
invigorate the whole system. For this purpose a cold bath is
better than a warm one, since warm baths tend to open the
pores more widely, to dilate the skin arteries, and thus increase the
perspiration and render the body liable to colds. On the other
hand, cold baths tend to contract the arteries at first, leaving the
skin pallid. This effect is soon followed by a reaction, the arteries
fill with blood, and the skin begins to glow. This glow may be
increased by a thorough rubbing with a rough towel which at364
EXCRETION
the same time removes the scales of dead epidermis. If the bath
is continued too long a second reaction takes place, the skin
becomes pale and chilled, and the .person becomes depressed as
much as from excessive warm bathing. A warm bath followed
by a cold shower is often beneficial in producing a healthy glow.
When we say cold baths we do not mean that the temperature
must be excessively low. Some persons cannot stand the shock
of too cold water, and for them water which just feels cold to the
skin may produce better results than colder water. The salts
in sea water tend to stimulate the skin nerves, and thus aid in
producing the glow. Salt baths may often be taken with good
effect when cold fresh water depresses. Cold baths should not be
taken when the body is depressed, as they cause too much loss of
heat. They should never be taken immediately after a meal,
as they draw to the surface the blood required for digestion, and
thus may produce serious indigestion. Shower baths take less
heat from the body than immersions, and also give a greater
stimulus to the skin nerves. Rightly indulged in, the cold bath
tends to keep the skin clean and to invigorate the system, at the
same time rendering the person less liable to colds. A judicious
mixture of cold and warm baths is best for maintaining the
body in the best possible state.
Care of the complexion. — The use of paints, powders, and
external applications for the beautifying of the complexion is
injurious, for the reason that, like dirt, they clog the pores of
the skin. Many of them, too, contain poisonous substances
which may be absorbed through the pores and produce disease.
Good exercise and frequent baths will keep the skin glowing in
health, and render it far more beautiful than any artificial appli-
cations.
Care of the hair. — The hair is naturally kept soft and glossy
by the secretion of the oil glands. If it is not washed frequently,
and the scalp rubbed thoroughly dry after the washing, this oil
will collect, mix with the perspiration and dead-skin scales, and
clog the pores of the scalp. Washing, therefore, is important in
removing this accumulation, while the rubbing stimulates the
blood supply and thus furnishes food to the hair papillae. Leav-
ing the hairs wet at the roots tends to rot them.BURNS AND BLISTERS
365
Care of the nails. — Children are very apt to bite the nails.
This not only gives the hands a ragged appearance, but also is
apt to tear the nails loose from the bed and thus permit germs,
which collect in the dirt under the nails, to get into the blood
and produce sores. For this reason also it is better to remove this
dirt with a dull instrument, as a sharp blade is apt to cut the
skin. The epidermis around the base of the nail also tends to
grow to the nail, and as the nail lengthens the skin is torn and
hangnails are produced. When this is the case, they should be
cut back close to the skin. They may be prevented by loosen-
ing the fold of skin about the base of the nail with a dull instru-
ment.
Burns and blisters. — Blisters are caused by friction or heat
which kills the lower layers of the epidermis and permits the
lymph and water to collect between the dermis and the epidermis.
If this distended epidermis is pricked with a clean needle and
the water allowed to escape, the two surfaces will eventually knit
together again. Care must be taken not to tear the upper layer,
as this permits the dermis to become uncovered, and often pain-
ful sores result. Ordinarily the epidermis will not absorb poi-
sonous germs. When this layer is removed or the dermis is cut
the blood vessels are exposed, and in this case they should be
covered with some germicide material to prevent the absorption
of germs into the blood. The germs which cause lockjaw and
blood poisoning are very frequently absorbed from lack of pre-
cautions in this direction, and may even produce fatal results.
One of the simplest germicides for this purpose is a 1% to 2%
solution of carbolic acid. A cut should be cleansed first with
water and if dirty, with carbolic acid solution. Then bandage it
with cloths which are strictly clean. (Small cuts should receive as
much care as large ones in this respect.)
Burns tend to produce blisters and kill the skin cells. They also
bring about inflammation. All burned places should be kept from
contact with the air, because this treatment lessens the pain and
reduces the inflammation. For this purpose baking soda or vase-
line is a useful application. Acid burns should be treated with
weak alkalies which neutralise the acids, while burns caused by
alkalies should receive neutralisation with weak acids.366
EXCRETION
Corns. — When the skin on the feet or the hands is rubbed,
the parts protect themselves by increased development of the
epidermis and thus form horny spots or callouses. Sometimes,
Fig. 167__Section of a corn. Note the thickened epidermis over one papilla, and
the manner in which it presses into the sensitive dermis below. Overton,
Applied Physiology.
on the feet, these spots grow inward, and a point of hardened cells
forms which presses inward into the sensitive dermis. The re-
sult is a painful formation called a corn.
Other Excretory Organs.
The urinary organs. — On each side of the backbone, in
the part of the abdominal cavity a little below the lower
ribs, lie two bean-shaped bodies about four inches long, two
inches wide, and one inch thick. These two organs
are the kidneys. They are embedded in a thick layer of
fat, and from the curved-in portion of each emerges a tube
called the ureter. These ureters are continued downward
and enter a baglike body covered externally with muscle
and lined with mucous membrane. This body is the blad-UENEKAL FUNCTIONS OF THE KIDNEY
St57
der. From the bladder a common duct called the urethra
extends downward to the external urinary opening.
Each kidney receives
from the abdominal
aorta an artery called
the renal artery. This k
artery breaks up in the
kidney into fine capil-
laries which are eventu-
ally collected into a vein
called the renal vein.
The two renal veins,
one from each kidney,
ultimately return the
blood to the inferior
vena cava. The renal
arteries and veins enter
and emerge from the
kidneys at the same
point as the ureters.
General functions Of Fig. 168 — The kidneys and connections* k,k,
kidneys; ry, renal vein; ra, renal artery;
the kidney.---±116 kid- ufu, ureters; 6, bladder.
neys receive the waste-
laden blood of the body through the renal arteries.
Out of this blood they take the Avaste material (mainly
urea) and form of it a secretion called urine (urea dis-
solved in water). The waste-freed blood then returns to
circulation by w'ay of the renal veins, Avhile the urine is
poured from the kidneys into the bladder as fast as it is
formed. When a sufficient quantity has collected in this
bladder, the muscular walls contract and the liquid is ex-
pelled from the body by way of the urethra and urinary368
EXCRETION
opening. Unlike the skin, the action of these bodies is
purely excretory, and to them is due the purification of
the blood.
Structure of the kidney.— The ordinary lamb’s kidney
which may be obtained from the butcher serves well to
demonstrate the structure of this important organ.
Removing the fat from the outside we find the reddish-
brown	kidney	within,	still	surrounded	with	a trans-
parent membrane	or	capsule.	This membranous	capsule is
called the renal capsule.
Remove	this	and we
have the kidney.
Viewed from the out-
side it shows a bean-
shaped outline, being
convex on one side and
concave on the other.
In the depression on the
concave	side	may be
made out the connect-
ing renal vein and
artery and the ureter
at the point where they
enter the body of the
kidney. If we cut the
kidney lengthwise the
cut surface shows the
following areas (see
Fig. 169). As it enters the kidney the narrow ureter
is seen to widen into a cavity called the pelvis. From this
pelvis extend branches forming what are called calices.
The solid portions show two layers. The outer is called
RA
Fig. 169 — Median longitudinal section of a
kidney; Ct, cortex; M% medulla; P, pelvis;
U, ureter; RA, renal artery; Pyt pyramids
of Malpighi.STRUCTURE OF THE KIDNEY
869
the cortex, and is the darker and more compact of the two.
The inner layer is called the medulla. This medullary
substance is less red than the cortex, and is divided into
pyramids whose small ends are toward the pelvis and
whose bases are toward the cortex. These pyramids are
called the pyramids of Malpighi.
The renal artery divides in the hilus (the name given to
the depression), and its branches extend between the pyra-
mids to the cortex. While they
give off a few branches to the
pyramids they end in a much
richer network of capillaries in
the cortex, and to this fact is due
the deeper color of the cortex.
These cortex capillaries are in
turn collected into veins which
ultimately empty into the common
renal vein, following a path similar
to that of the arteries through the
kidneys.
The pyramids of the medullary
layer are composed of fine tubular
glands. At the points of the
pyramids are found opening into
the calioes the mouths (the papillae)
of many tiny tubes (the urini-
ferous tubules). These tubules
extend backward into the med-
ullary layer, branching frequently,
and thus forming the pyramid.
Ultimately these branches end in the cortex, where they
swell out and form a spherical capsule (Malpighian capsule.)
EDDY. PHYS.-24
Fig. 170 — A uriniferous tu-
bule; r, cortical portion of
kidney; g, medullary por-
tion; p, a pyramid; /, a Mal-
pighian capsule; II and V,
twisted portions of tubule;
III, IV, VI- VIII, straight
portions of tubule; IX, open-
ing of tubule into pelvis at
top of pyramid.370
EXCRETION
The entire length of the tube, its branches and capsules,
is lined with epithelial gland cells. The relation of these
parts is shown in Figure L72. Into each capsule enters a
tiny branch of the renal artery and there breaks up into
a twisted knot of capillaries called a glomerulus which
Fig. 171 — A Malpighian capsule;
v. a, small branch of renal
artery breaking up in the glom-
erulus (y.l) into capillaries,
which in turn unite into the
renal vein (r.e); e, tubule; a,
epithelium over the glomeru-
lus ; 6, epithelium lining the
capsule.
Fig. 172—Relation of parts in kid-
ney; a, artery; vein; c, capil-
laries about the tubule; </, glom-
erulus; l, tubule.
nearly fills the capsule. These capillaries pour their con-
tents into a tiny vein, which, after leaving the capsule,
breaks up again in a network about the walls of the urinif-
erous tubules. This network fuses again into a single tube
which connects with the renal vein.
Excretory action of the kidney.— The cells which line
the capsules and upper parts of the tubules are true gland
or secreting cells. Those in the lower part of the tubule
are purely for protection. The cells of the capsule secrete
only water and inorganic salts which they receive from theTHE URINE
371
blood of the glomeruli. It was once supposed that the
glomerulus was simply a filter, and that the waste and salts
passed through by filtration and osmosis. Present the-
ories tend to prove that the cells of the capsule actually
secrete this water and salt material first received from the
blood. The cells of the upper parts of the tubules secrete
the organic wastes (urea, etc.) winch they obtain from the
capillaries surrounding these parts and which is dissolved
in the water to make the urine. The water, salts, and
organic wastes accumulate slowly, and drop by drop they
pass through the length of the tubes into the calices and
pelvis. Here they enter the ureter, and the combined
liquid is stored in the bladder pending removal. The blad-
der is covered with a layer of muscle and peritoneum, and
lined with mucous membrane, and the mouth of the urethra
is closed by a sphincter muscle which opens from time to
time to permit the escape of the urine.
The urine.— Analysis of the urine shows it to consist
of a yellowish liquid slightly heavier than water and con-
sisting of about 96% ivater and 4% solids. The principal
solid constituent is a nitrogen waste called urea, and the
most important excretory action of the kidneys is their
power to remove the nitrogen wastes of the body.
In the metabolism of the tissue cells the nutrients are
broken up into water, salts, carbon dioxide, and nitrogen
wastes. We have already seen that the water as sweat,
is largely excreted by the skin, and the carbon dioxide by
the lungs. The kidneys, therefore, are the great nitro-
genous waste disposers. The total amount of water, salts,
and nitrogenous wastes given off by the kidneys in twenty-
four hours varies from forty to sixty fluid ounces or one
thousand five hundred grams. Of this, water forms 96%,372
EXCRETION
urea 2.2%, other nitrogenous wastes 0.17%, mineral salts
about 1.6%.
The liver as an excretory organ. — We learned (see p.
144) that through the bile the liver has the power to
remove various poisons and other wastes. In this respect
it is also an excretory organ. It is now believed that the
urea or nitrogenous ivaste is not formed in the kidneys,
but is present in the blood which flows to the kidneys,
these simply secreting it from that blood. It has been
further proved that urea is formed, in pail at least, in the
liver and then sent in the blood to the kidneys for removal.
Just how the wastes of the cells are made into urea, and
where it takes place, is still uncertain, but it is already
demonstrated that the liver performs an important part
in this operation.
Hygiene of the kidneys.— The skin excretes very little
urea and much water. The kidne3rs, on the other hand,
excrete much urea. "When much water is given off by the
skin, as in summer, less is given off by the kidneys; while
in winter, when perspiration is less, the amount of water
disposed of by the kidneys is correspondingly increased.
The fact that more blood passes to the skin in summer
also has an effect in this direction. Further, the perspira-
tion and blood supply to the skin is governed mainly by
the external temperature, while the amount of water
excreted by the kidneys is governed partly by the amount
of water swallowed. Now a decrease below a certain
limit in the amount of water excreted by the kidneys
tends to prevent the proper washing away of the urea,
while, if not washed away, the accumulation of this urea
colors the urine and tends to clog the action of the
kidneys.ALCOHOL AND ITS EFFECT
373
For the above reasons, persons suffering from kidney
troubles may often remove this trouble by increasing the
amount of water drunk. Urea itself is not poisonous, but
when the nitrogen of the tissues is not properly oxidized
other substances are produced which pass in the blood to
the kidneys. The kidneys try to excrete these products,
but often become overworked in the attempt, and these
poisonous substances collect and produce the disease
known as Bright’s disease. An excess of sugar in the diet
often brings about an imperfect oxidation, and patients
with this disease often have their sugar supply cut down
for that reason. Exercise, which maintains a healthy
oxidation of the tissues, indirectly helps to maintain the
health of the kidneys.
Alcohol and its effect upon the skin and kidneys.— One
of the first effects of alcohol drinking is to cause congestion
of the surface blood vessels. When the use is occasional,
this congestion may pass off and the skin return to its
normal condition. If continued, however, the constant
use of alcohol causes the blood vessels of the skin to be-
come permanently dilated, the skin becomes red and
often blotched with pimples, due to poor nutrition of the
skin cells and the over-accumulation of wastes.
Alcohol also causes more or less congestion in the blood
vessels of the kidneys, and the result may be that the
kidneys are overworked, with results seriously menacing
the health of the individual. All these effects, of course,
vary in proportion to the amount of alcohol consumed.XXIV. SKIN STRUCTURE AND EXCRETION
IN THE LOWER ANIMALS.1
In comparing the skin of man with that of the lower
animals we shall have to modify our conception of the
word skin. In man we used the word to describe a two-
layered system of tissues whose function was fourfold: pro-
tection of underlying parts, excretion of wastes, the end
organ of various sense nerves, and the regulator of the
body temperature. When we speak of the skin of the
lower forms, we have in mind only one characteristic of
the human skin, namely, a protective layer of some kind.
In the consideration of the different groups of animals it
will be of interest to note the manner in which the body
coverings of the various forms approximate to the complex
structure of man.
Protozoans.— Since the protozoan consists of only one
cell, it can evidently have no skin in the sense of a tissue
structure. Many protozoans, however, do develop a wall
or outer covering for their protoplasm which in some re-
spects fulfils the functions of the skin of higher forms. For
example, in the thin membrane or cuticle, which is secreted
by the ectoplasm of the paramoecium, we have a layer
which protects the contents of the cell, and at the same
time permits the exchange of oxygen and carbon dioxide
to take place through it. In permitting the escape of
carbon dioxide it is as truly a means of excretion as the
thicker membranes of other forms.
‘See Footnote, p. 146, Chapter X.
374"WORMS AND ARTHROPODS
375
Sponges and ccelenterates.— In these animals the body
is composed of only two layers of cells, and for the most
part the action of these cells is on the same plane as that
of the protozoan. In the sponge, the variation between
the inner and outer cells results in division of labor, and
the outer cells are more protective and less digestive in
their action.
In some of the coelenterates, however, we find a simple
form of covering which approaches closely in structure
the skin of man. In certain of the polyps, for example,
we find the outer ectoderm layer of cells more or less com-
pletely divided into two layers. Where this occurs the
outer layer is composed of cells very similar to those of
the epidermis, while the inner layer may contain muscle
fibers, nerves, touch organs, and certain simple glands.
Such a covering is evidently much more than a protective
form, and in fact performs several functions identical in
nature with those of the human skin.
Mollusks.— In the oyster, clam, snail, and other members
of this group we find the covering of the body composed
of a true two-tissued layer. The outer layer is com-
posed of epithelial cells, while the underlying part is made
up of connective tissue in which are found muscle fibers,
blood vessels, and nerve ends. This skin also acts very
effectively in the discharge of carbon dioxide from the
body. It differs from other forms of skin by the presence
of special shell-secreting glands and thus is the source of
the hard exoskeleton which we call the shell.
Worms and arthropods.— In the members of these two
great classes we find also a true skin. The outer body
layer, or hypodermis, as it is called in these groups, has
the power of secreting a thin layer of material, which may376
SKIN STRUCTURE AND EXCRETION
be so thin, in the earthworm, as to permit of the action of
respiration, while, in the insects or crustaceans, it may be so
thick, or become so loaded with salts of lime, as to form
a hard external skeleton for the animals. When we speak
of an insect’s shedding its skin, we mean that it sheds this
outer secretion or exoskeleton. The underlying layer or
hypodermis remains and serves to secrete the new layer.
This hypodermis is also found to contain glands of various
kinds, and may even show layers of nerve and muscle
fibers. The hard layer of the insect and lobster is evi-
dently not a satisfactory substance for permitting the
excretion of wastes, and very little excretion or respiration
takes place through the skin of such forms.
Body coverings of vertebrates.— All vertebrates possess
a dermis and epidermis similar in structure and function
to that of man. The epidermis always consists of epi-
thelial cells only, while the dermis is made up mainly of
connective tissue cells and fibers, and may also inclose
muscle fibers. While the various glands of the skin are
composed of epithelial cells they are found most often
embedded in or under the dermis, and have their opening
by ducts on the surface of the epidermis. The epidermis
is in two layers, as in man; the outer being composed of
Fig. 173—Skin of fish (diagram); a, dermis with vertical fibers, &; c, gristly layer;
rf, surface scales; e, layer containing limestone; /, scale pit.
dead cells or cuticle, with an underlying layer of living
cells. In some vertebrates tills outer layer, or cuticle,BODY COVERINGS OP VERTEBRATES
377
becomes so thick and horny as to form a true exoskeleton,
namely, the scales of the crocodile and the horny layers of
the turtle. Again, in the amphibians it may be so thin that
respiration takes place through it with readiness.
The skins of vertebrates differ mainly in the structures
produced by modifications of the
epidermis and the forms of glands.
For example, the scales of the fishes
and the bony horn-covered plates
of the turtle are of like origin.
Both consist of a bony plate de-
veloped by the dermis, and covered
with a horny portion of the outer
layer of epidermis. The feathers of
the bird and the hairs of man are
likewise identical in origin, being
merely modifications of the epider-
mal layers.
In like manner, by infolding of
the epithelial cells of the lower epi-
dermis, glands of various forms
are built; such are the sebaceous
and sweat glands of man, the mam-
mary glands or milk glands of all
mammals, the various scent glands
of different forms, and the slime
glands of the amphibians and some
of the fishes. Some of these glands
are purely excretory in fimetion,
while others are protective (scent
glands) or produce secretions of value in animal opera-
tions; for example, the slime glands of the amphibians
Fig. 174—A feather; ay quill;
b, shaft; e, vane; d, down;
e,/, upper and lower orifice.378
SETS' STRUCTURE AND EXCRETION
make the body difficult to grasp, and at the same time
moisten the outer layer so as to permit exchange of carbon
dioxide and oxygen. Claws, spines, fur, hoofs, horns, etc.,
are all modifications of the horny outer part of the epidermis,
and each arises from a dermal papilla, or papillae, just as
do the hair and nails of man. The dermal layer of animals
is often much thicker than that of man, and when tanned
it forms what we call leather.
Excretory- Organs of the Lower Animals.
Aside from the use of the skin as an excretory organ,
there are no structures comparable to kidneys in any of the
lower animals until we reach the worms and mollusks.
The protozoans carry on the removal of wastes by simply
throwing out the wastes of metabolism into the surround-
ing water or liquid in which they five. As the animals
become larger, the cells multiply in numbers, and it becomes
more difficult for each cell to get rid of its wastes. If this
waste thrown out by the cells remains in the body it may
produce serious harm to the tissues. The solution of this
difficulty is met in the development of glands whose func-
tion is to take the collected wastes of the cells from the
circulatory system, to transform these wastes into a secre-
tion of uniform character, and then remove this secretion
to the outside of the body.
.All such glands, of whatever character or structure, are
identical in plan with the air sac of the lung, the sweat
gland of the skin, and the kidney of man. In all, the cells
which do the secreting are placed in such relation to the
blood system that the body wastes (carbon dioxide, water,
or urea) may be readily absorbed, and when transformedNEPHKIDIA
379
into a secretion shall pass into some kind of a duct through
which it escapes to the outside. The function of the kid-
ney of man is the removal of nitrogenous wastes, that is,
the removal of the wastes resulting from metabolism of
proteid in the protoplasm. We have already discussed
the adaptations of the lower animals for the removal of
carbon dioxide under the head of the respiration. It re-
mains to examine the nitrogen waste removers of these
lower forms, and in the following paragraph we shall con-
sider only the organs which perform the same action as
the human kidneys.
Nephridia.— One of the simplest forms of kidney is found
in the group of mollusks and earthworms. It is called a
nephridium, and as the earth-
worm is perhaps the most fa-
miliar type we shall describe the
organs as seen in this animal.
If we turn the earthworm over
and examine its ventral surface
we can make out a double row
of pores, tW'O to a segment, ex-
tending from the head end to
the tail end. If we dissect the
worm and trace the internal
connections of these pores we
find them to be the mouths of fig. 175—a nephridium; s,sep-
long coiled ducts which after ‘U'"; F, funnel; O, pore or ex-
twisting about in the body
cavity, end in funnel-shaped, ciliated mouths in the seg-
ment just in front of the segment in which the pore is
located. (See Fig. 176). Further examination shows this
duct to be formed in part by secreting cells and surroundedS80
SKIN STRUCTURE AND EXCRETION
by branching blood vessels. Its action is as follows. The
waste from the body cells of the segment collect in the
Fig. 176—Cross section of earth worm showing nephridia in position; B. Cbody
cavity; c, cutis; e, epidermis; c. m. & l.m. muscle layers; -.V, nephridia; ex.p.
excretory pores; S, setae; Z>, dorsal blood vessel; /, intestine; V, ventral blood
vessel; N. V., ventral nerve cord.
cavity or are absorbed into the blood circulation. Those
wastes of a nitrogenous character which remain in the cav-
ity of the segment pass directly into the ciliated end of one
or the other of the nephridia, and, after action by the cells,
pass out of the pore as a secretion. Similarly, the wastes
which entered the blood are eventually brought to the
branches which surround this tube, and, being absorbed
from the blood by the secreting cells, are made into a secre-
tion, and in turn pass down the duct and out the pore. InVERTEBRATE KIDNEYS
881
this way each of these coiled tubes absorbs wastes either
from the body cavity or blood, transforms it into a secre-
tion, and pours it into a duct and out the pore. The action
is the same in every way as that of a single tubule and cap-
sule of the kidney. (See Fig. 171.) The kidney of the clam
is a coiled tube almost identical in its structural features
with this nephridium of the earthworm, while the green
glands of the crustacean are supposed to act in a similar
manner.
Kidneys of insects.— These animals show a modification
of the nephridium plan in that the tubules are arranged in
a ring about the intestine.
The ends of the tubes float
in the body cavity of this
animal, and absorb the
waste from that, while their
mouths open, not to the
outside, but into the in-
testine, so that the nitrogen
wastes pass out with the
faeces through the anus.
Vertebrate Kidneys.— In
all the vertebrates we find
some form of kidney. These
kidneys diff er from a nephrid-
ium mainly in that they
are a combination of many
secreting tubules under one covering, and they differ from
that of man only in the manner in which the ducts termi-
nate. For example, the ducts of the bird and reptile kidney
open into an enlargement of the rectum (lower end of in-
testine) called the cloaca, and in these forms the anus is
Fig. 177 — Kidney tubes of an insect; k,
kidney tubes; a, esophagus; b, crop; c, d,
stomach; e, intestine; /, large intestine;
g, anus.382
SKIN STRUCTURE AND EXCRETION
the common external opening for the kidney and the intes-
tine. In mammals the openings are for the most part dis-
tinct, the kidney opening being called the urinary opening.
The development of the bladder is simply a device for pre-
venting continuous secretion and permitting the animal
to control this operation of removal from the body.
In general, then, whether we are dealing with the
nephridium of the worm or the sweat gland of man, the
means of disposal of waste are practically identical in action,
and the kidneys are to be considered as simply a collection
of separate excretory glands enormously increased in number
to provide for the increase in waste, and having a common
duct for removal.XXV. THE NERVOUS SYSTEM.
Structure.
The study of the preceding pages has taught us the part
played by the various organs and structures in our body
in the operation of tearing down and building up the
human machine, and the manner in which these operations
keep it in running shape. We have also seen that all
these systems are interrelated, the work of one being
directly dependent upon the activity of another, while in
our life we are conscious of the fact that these many parts
are all acting harmoniously and with comparatively little
attention on our part. For example, in so simple an
operation as walking, every system in our body is con-
cerned. The muscles contract and expand, and in so doing
produce motion, consume oxygen and nutriment, and give
off wastes and carbon dioxide. The necessity for food
must be met by a quickened flow of blood, while the in-
crease in appetite warns us to supply the digestive system
with more nourishment. The lungs mus.t expand and con-
tract to furnish the blood its oxygen, and the excretory
system must increase its activity a little to dispose of the
wastes. Even in our sleep these systems are all more or
less active, and the needs of one part of the bodjr can be
met only by special effort on the part of all the others.
Manifestly such a harmonious action would be utterly
impossible unless the organs had some means of com-
munication by means of which the needs of one might be
383384
THE NERVOUS SYSTEM
Plate IY.— Nervous system.GENEKAIj COMPOSITION OP THE NEKVOUS SYSTEM 385
communicated to the others. Some central system of con-
trol must also exist, in order that the various activities of
the body may be directed by us. The system of organs
in our body by means of which this direction and control
are exercised is spoken of collectively as the nervous system.
The relation of this system to voluntary and involuntary
actions and the structure of the parts will be considered
in the following paragraphs.
General composition of the nervous system.— The ner-
vous system may be said to consist of three parts: nerve
centers, nerves, and nerve-end organs. The nerve centers
in our bodies are the brain, the spinal cord, and little
patches of nervous matter scattered in different parts of
the body, called ganglia. The nerves arise from these
centers, and with their branches penetrate all organs and
tissues of the body. Finally, each nerve ends in some
form of organ called an end organ. Some of these organs
are very simple in structure; others, such as those at
the ends of the sight and hearing nerves, are exceedingly
complex.
For convenience, the cord and brain, together with the
nerves which arise in them, are spoken of as the central
nervous system, while the sets of ganglia on each side of
the spinal column, with their nerve connections, are called
the sympathetic system. All nervous centers are, how-
ever, in communication, and these two systems are sepa-
rated in name for convenience of study, and not because
of any true separation in the systems.
In general, then, the brain and cord may be likened to
the “ central ” of a telephone system, the nerves to the
wires, and the end organs to the individual telephones.
Such an arrangement makes it necessary that all communi-
EDDY. PHYS.—25386
THE NERVOUS SYSTEM
cations from one organ to another, or from one part of an
organ to another, be made through the brain and cord.
If we enlarge our idea of this
arrangement and conceive of
the brain and cord as not only
switching messages from one
part of the body to another,
but also of being able to modify
the character of these messages,
we shall have a true picture of
the manner in which the brain
and cord control and harmonize
the actions of all parts of our
bodies.
The nervous system of the
frog. — A dissection of the ner-
vous system of the frog is easily
made, and since it is essentially
similar in kind to ours, a study
of this system forms a good in-
troduction to the study of the
human system.
As in our body, the brain is
inclosed in the skull, and is con-
tinued downward as the spinal
cord through the spinal cavities
of the vertebra. In this posi-
tion the system is well pro-
tected from external pressure.
If this bony covering be carefully removed so as not to
injure the contents or its connections, a structure, such as
shown in Figure 178 is seen.
Fig. 178 — Nervous system of frog;
a, cerebral hemispheres (fore-
brain) b, olfactory lobes; c, eyes;
d, midbrain; e, optic lobes; /,
hindbrain (cerebellum); <7,
medulla; h, i, spinal cord; I to
X, first to tenth pair of cranial
nerves; 1 to 10, spinal nerves.THE FROG’S BRAIN
387
The frog’s brain.— The brain may be divided into three
parts, called respectively, the forebrain, midbrain, and hind-
brain. From these parts of the brain are given off ten
pairs of nerves called cranial nerves. These nerves are
made up of fibers called nerve fibers. Those cranial nerves
whose fibers bring impulses or messages to the brain are
called afferent or sensory nerves. Those whose fibers con-
Fig. 179 — Frog brain; /, top view; A, olfactory nerves; B, olfactory lobes; C, D.
forebrain; E, midbrain; F, optic lobes; G, hindbrain; H, medulla; /, cord; J,
K, L, M, N, nerves; II, ventral view; III, lateral view.
vey impulses from the brain are called efferent or motor
nerves. Several of these nerves contain both efferent and
afferent fibers, and such are called mixed nerves.388
THE NEKVOUS SYSTEM
The forebrain.— This consists of two elongated bodies
lying side by side and called the cerebral hemispheres.
Together, they constitute the cerebrum. At their ex-
treme front are two swellings, from which a pair of cranial
nerves extend to the nose. The fibers of these nerves are
afferent, and carry sensoiy impulses from the nose to these
lobes of the brain. They give to the animal the sense of
smell. On this account the lobes of the brain from which
these nerves arise are called the olfactory lobes, and the
nerves are called the olfactory nerves. The olfactory are
the only nerves connected directly with the cerebral
hemispheres.
The midbrain.— Just back of the hemispheres is the
midbrain. This is covered on the top and sides by two
swellings called the optic lobes, so named because the
sensory nerves from the eye enter them. They are the
receiving points of sensations of light. The optic, or sight
nerves (the second pair of cranial nerves) enter these
lobes on the under side, and are crossed so that the nerve
from the right eye enters the left lobe, and vice versa.
These optic lobes also communicate with the forebrain
by means of which we will speak later, but the optic nerves
have no direct connection with any part of the brain except
these lobes.
The third pair of cranial nerves also arises from the
midbrain, and, unlike the first two pairs, it carries impulses
from the brain instead of to it (that is, it is a motor or
efferent nerve). The nerve fibers of this third pair control
four of the six muscles which move the eyeballs and also
some of the internal eye muscles. They are called the
oculo-motor nerves.
The hindbrain.—The rest of the brain, which extendsTHE SPINAL CORD OF THE FROG
389
backward to the beginning of the spinal cord, is called the
hindbrain. It shows two regions. The transverse ridge
just back of the optic lobes is called the cerebellum, while
the wedge-shaped portion back of this, from which the
spinal cord arises, is called the medulla oblongata. Seven
pairs of cranial nerves arise from this medulla. The fourth
pair, known as the trochlears, are very small, and carry mo-
tor impulses to one of the muscles which move the eyeballs.
The fifth pair, or trigeminals, are very large. They belong
to the class of mixed nerves. Some of their fibers bring
sensory impulses from the teeth and face to the brain.
Other fibers carry motor impulses to the muscles of the
jaws and eyelids. The sixth pair, or abducents, carry mo-
tor impulses to a muscle of the eyeball. The seventh pair,
like the fifth, are facial nerves of a mixed character, and
carry impulses to and from certain parts of the face. In
the higher animals and in man this nerve is purely a motor
nerve, and controls the muscles which give expression to
the face. The eighth pair, or auditory nerves, are sensory,
and carry impulses of hearing from the ears. The ninth
pair, or glossopharyngeal, supply the tongue and pharynx,
and are mixed nerves. The tenth pair, or the vagus nerves,
have the widest distribution of any. They are also mixed
nerves, and their fibers supply the lungs and air passages,
the heart and blood vessels, and the organs of digestion.
Finally, the centers of all these cranial nerves are in
communication with parts of the forebrain, and the con-
sciousness of sensations has its seat in that organ, though
the nerves are not directly attached to it.
The spinal cord of the frog.— The cord of the frog, which
is merely an extension of the medulla, is much shorter
than that of man. From it are given off some ten pairs390
THE NERVOUS SYSTEM
of spinal nerves whose fibers are distributed to various
parts of the body. . Figure 178 illustrates the method of
branching of these nerves.
Comparison of the frog’s spinal cord and brain with that
of higher forms.— The spinal cord and brain are to be con-
sidered as one structure, the brain lobes and swellings being
simply modifications of the more uniform spinal cord.
These modifications become much more marked in the
higher forms, and instead of lying directly behind one an-
other the increase in size and the lack of space compels these
brain lobes to become more or less twisted and doubled upon
one another. If we keep the parts of the frog’s brain in mind,
however, we shall have no difficulty in recognizing the
homologous parts of the human central system, as, aside
from the folding and variation in size, they are almost iden-
tical, part for part.
Let us now examine the human brain with this com-
parison in mind, and see if we can identify the structures
found there.1
The human brain.— The brain of man is inclosed in that
part of the skull called the cranium, and it fills this part
completely. At the base of this cranium is an opening
called the foramen magnum through which it is continued
downward as the spinal cord. Various apertures in this
bony box permit the entrance and escape of the cranial
nerves.
Brain membranes.— When the bony plates of the cra-
nium are removed, the brain is found to be covered with
three membranes. The outermost (the dura mater) has
1 The brain of the sheep is easily obtainable, and is so nearly identical
with that of man in structure that it serves well for laboratory work.
(See Ex. LXIII.)BRAIN MEMBRANES
391
a roughened outer surface which fits closely to the inner
surface of the bones, and forms the inner periosteum of
these bones. The brain side of this membrane is smooth,
Fig. 180 — Human brain in place; C, C, wrinkled surface of the cerebrum; Cb,
cerebellum; M.Ob, medulla; B, bodies of vertebrae, and 6', their spinous processes
inclosing the spinal cord.
and the whole membrane is thick and fibrous. It lines the
whole cranial cavity, and ingrowths from this membrane
support and invest all parts of the brain. At the foramen
magnum it is continued as the outer coat of the cord. The
blood vessels which supply this membrane are, for the most392
THE NERVOUS SYSTEM
part, distributed on the side next to the bones. In fact,
their principal function Is to supply nourishment to the
bones, not to the brain.
A second membrane, called the arachnoid, fits against the
smooth inner surface of the dura mater. This is a delicate,
transparent, serous membrane. Beneath it, enveloping
the brain itself, is a third, thin membrane, called the pia
mater. This pia mater is composed of a fine network of
small blood vessels held together by tissue, and these
blood vessels supply the nourishment and remove the
wastes of the brain itself. It is practically impossible to
remove this membrane, as it covers all the folds of the
brain.
These three membranes and the cranial bones furnish a
very efficient protection for the delicate matter of the brain
proper.
The brain proper.— Like the frog brain, it consists of
three portions, forebrain, midbrain, and hindbrain, but
these parts are so bent and doubled upon one another that
they form a figure similar to an interrogation point (?).
The two hemispheres of the forebrain occupy the bulk of
the cranial cavity and correspond in position to the top,
front, and part of the back of the curved portion of the
interrogation point. The olfactory lobes are tucked away
at the very front of the curve. In man, the forebrain con-
stitutes nine tenths of the bulk of the entire brain.
The midbrain, with its optic lobes, is projected forward
in a position corresponding to the curve at the top of the
stem of the interrogation point. When viewed from the
under side, this projection brings the optic lobes just back
of the olfactory lobes.
The hindbrain has a much more developed cerebellumEXTERNAL FEATURES OF THE HUMAN BRAIN 393
than the frog’s has, and this organ is here divided into two
lobes by a longitudinal partition. Underneath the cere-
bellum, and extending downward, is the wedge-shaped
medulla. In comparison with our figure the hindbrain
occupies the position of the stem of the interrogation point,
and if this were continued downward the extension would
represent the spinal cord.
External features of the human brain.— Viewed from
the top, the two hemispheres of the forebrain (the cere-
brum) are seen to be much convoluted and folded, thus
increasing then surface without increasing the space occu-
pied. The two hemispheres are pressed closely together,
the line of separation
being marked by a fissure
(the longitudinal fissure).
Viewed from the bottom
we can make out the con-
tinuation of this fissure
back to the midbrain, and
on each side of it appear
the olfactory lobes. (See
Fig. 182.) Just back of
these lobes are the crossed
optic nerves (the optic
chiasma) which enter the
optic lobes. Back of this
chiasma is a band of white
matter (the pons varolii)
which connects the two lobes of the cerebellum and
covers the top of the medulla. Underneath this pons,
the medulla divides into two stalks (the crura cerebri)
which spread out to form the cerebral hemispheres,394
THE NERVOUS SYSTEM
The cerebellum shows a wrinkled surface different in
appearance from the folded cerebrum. Between its lobes
Olfactory Bulb----
{to which is attached
the Olfactory Nerve)
Pituitary Body____
Optic Nerve----
Optic Chiasma-----
Oculomotor Nerve-—^
Trochlear Nerve---
Trigeminal Nerve---
Pons Varolii---
Abducens Nerve- -
Facial Nerve—
Auditory Nerve- __
Glossopharyngeal Nerve__
Vagus Nerve—
Spinal accessory Nerve—
Hypoglossal Nerve-
Medulla Oblongata---
First Spinal Nerve—
Cerebellum—
Spinal Cord—
Second Spinal Nerve—
Fig. 182 — Human brain viewed from below.
is seen the lower part of the medulla whose lower end nar-
rows and is continued to form the spinal cord.
Along the under side can also be made out twelve pairs
of cranial nerves, two more than in the frog’s, brain. The
first ten pairs are connected to the same regions as those
of the frog and have the same functions. The eleventh
and twelfth arise from the medulla and are known respec-
tively as the spinal accessory and hypoglossal. The spinal
accessory is a motor nerve and supplies the muscles of the
shoulders. The twelfth, or hypoglossal, is also motor andINTERNAL FEATURES OF THE HUMAN BRAIN 395
supplies the muscles of the tongue. The first spinal nerve
of the frog corresponds to this nerve in action.
Internal features of the human brain.— If we split the
brain in two by a vertical
cut, parallel to and a little
to the left of the longi-
tudinal fissure, we can
make out the following
parts on examining the
right side. (See Fig. 184.)
The fissure is seen to con-
tinue downward to a white
fibrous body which binds
the two hemispheres to-
gether. This is called the
corpus callosum. This band
curves in such a way as
to inclose a space actu-
ally outside the brain and
often called the " fifth ”
ventricle. This fifth ven-
tricle is further inclosed by two thin membranes (the
septa lacida). The forward curve of this connecting body
below the fifth ventricle forms the roof of the midbrain,
and this part is called the fornix. Under the fornix is
the cavity of the midbrain or the third ventricle. This
third ventricle communicates with cavities in the right and
left hemispheres (the second or lateral ventricles) by two
openings in the fore part (the foramen Monroe). In the
figure, the opening into the right ventricle only appears.
The midbrain cavity is also extended forward in a funnel-
shaped tube called the infundibulum.
Fig. 183 — Origin of cranial nerves; H,
hemispheres; C. S. corpora striatum; P,
pineal body; Pt, pituitary body; C. Q. cor-
pora quadrigemina; Cb, cerebellum; Thy
optic thalamus; M, medulla; / to XII, the
pairs of cranial nerves; Sp, 1, 2, spinal
nerves.396
THE NERVOUS SYSTEM
The lateral ventricles in man are frequently named the
first, and second ventricles.
Extending from one side to the other of the third ven-
tricle is a solid cord called the median commissure. The
sides of the cavity are formed by the optic lobes, or thalami,
Foramen of Monro
„	„	\ Third Ventricle
Corpus Callosum	\	,	Pineal Bod,J
\ Fornix \	\	\	Cerebrum
-Corpora
Quadrigemina
Cerebellum
Optic Chiasma
Pituitary Body /
Oculomotor Serve /
Tons Varolii
Aqueduct /	/
Medulla Oblongata /
Fourth Ventricle
f-----Spinal Cord
Fig. 184 — Vertical section of brain.
and its floor is formed by the crura cerebri, or stalks, of the
cerebral hemispheres. We have already noted that these
stalks are the forward extensions of the medulla. At the
rear of the third ventricle is a passage called the iter, or
aqueduct, leading to the fourth ventricle, a small cavity in
the hindbrain. The top and sides of this iter are formedINTERNAL EEATDRES OE THE HUMAN BRAIN 397
by four little bodies called the corpora quadrigemina. Its
floor is the medulla and the pons Varolii.
The fourth ventricle projects upward toward the
cerebellum, and the internal structure of this organ shows
a branching, treelike core of white matter (the arbor
vitce) surrounded on the outside by gray matter. The
lobes of the cerebellum are only partly separated, the
fissure dividing them being very shallow. The floor and
sides of the fourth ventricle are formed by the medulla,
and this cavity is continued backward as the neural
cavity of the spinal cord.
From the above description it will be seen that the
brain is actually a hollow organ with a continuous
cavity surrounded by various outgrowths or walls and398
THE NERVOUS SYSTEM
connecting all regions of the brain with the neural
cavity of the spinal cord. In life this cavity (the ven-
tricles) is filled with a liquid known as the cerebrospinal
fluid and the entire cavity is lined with
a layer of ciliated epithelium. The
third, fourth, and lateral ventricles are
also supplied with blood from a net-
work of blood vessels called the choroid
'plexus. Rupture and hemorrhage of the
blood vessels of the lateral ventricles pro-
duces pressure upon the inside of the
hemispheres and is one cause of apoplexy.
If we cut slices at right angles to the
longitudinal fissure in either hemisphere,
we find that, like the cerebellum, the
solid part of the cerebral hemispheres
is composed of two kinds of matter,
white and gray, surrounding a central
cavity (the lateral ventricle). The out-
side of the hemispheres and folds is
called the cortex and is composed of
gray matter; the white matter forming
the core of this part of the brain.
The human spinal cord. — This cord,
which is an extension of the medulla,
extends from the foramen magnum
through the spinal cavities of the ver-
tebrae to the articulation of the first
and second lumbar vertebrae. From
here it narrows off into a slender filament which runs
back to the end of the neural canal behind the sacrum.
It is nearly cylindrical in form, being slightly flattened
Fig. 186 — The human
spinal cord and its
branches.THE HUMAN SPINAL COED
399
dorso-ventrally, and has an average diameter of three
quarters of an inch, with a length of about seventeen
inches. It is largest in the neck region from the third to
the first dorsal vertebrae, and there is a second expansion
Fig. 187—Cross section of spinal cord; 1, anterior fissure; 2, posterior fissure, 3,
central canal; 4, 5, commissures; 9, posterior root; 10, anterior root; a, e, gray
matter. The outer layer is the pia mater, covering the white matter and send-
ing into it blood vessels.
in the last dorsal vertebra. Externally, it is covered
with three membranes like the brain: the dura mater,
arachnoid, and pia mater.
The dura mater is not in close contact with the bones,400
THE NERVOUS SYSTEM
as in the brain, but is separated from their periosteum
by a layer of fat and a network of blood vessels. It does
not send partitions into the cord as it does into the brain.
The arachnoid is also not in close contact with the pia
mater, but forms a loose bag around it. The pia mater
closely invests the surface of the cord and, as in the brain,
supplies it with blood.
The cord itself shows two fissures, a wide shallow anterior
one and a narrow deep posterior one, thus dividing it into
two right and left halves. The posterior fissure is not
a true groove but rather a partition of connective tissue.
Each half of the cord gives off at intervals, from the top
and bottom, nerve roots which unite at a short distance
from the cord into common nerve trunks. (See Fig. 188.)
These trunks or spinal nerves are arranged in pairs in the
cervical and dorsal regions and emerge through openings
between the vertebrae. At the lower end of the cord they
are crowded together in parallel bundles. There are, in
all, thirty one pairs of these nerves. Each spinal nerve
trunk splits up into man)^ smaller nerves and these into
still smaller nerves which are distributed to various partsINTERNAL STRUCTURE OF THE CORD
401
of the limbs and trunk, thus bringing all parts of the body
in communication with the cord. They are also connected
on each side of the cord with the system of nerve centers
called the sympathetic
ganglia. (See Fig. 189.)
Internal structure of
the cord. — If we sec-
tion a portion of the
cord in such a manner
as to split a pair of
spinal nerves (see Fig.
188), we can make
out the following de-
tails: Like the brain,
the cord is seen to
consist of white and
gray matter with this
difference, that, in the
cord, the white matter
is on the outside. The
inner gray matter is in
the form of a letter H.
Two horns of this II
project forward (the
anterior cornua) and
two backward (the pos-
terior cornua). The cen-
tral part, correspond-
ing to the cross bar of the H, shows a small cavity in its
center which is the continuation of the ventricle cavities
of the brain. From the posterior and anterior cornua
emerge nerve fibers which unite to form the roots of the
Fig. 189 — Roots of a dorsal spinal nerve and its
union with the sympathetic system; c, c, anterior
fissure of cord; «, anterior root; /», posterior root
with spinal ganglion; s, sympathetic, and e, its
double connections with spinal nerve a' by a
white and a gray filament.
EDDY. PHYS. — 26402
THE NEEVOTJS SYSTEM
spinal nerves. The places where these roots emerge
appear as four grooves on the surface of the cord. The
posterior roots show a swelling just back of the point where
they unite with the anterior fibers. This swelling is
called the spinal ganglion. Experiment has shown that
the anterior roots are composed mainly of efferent or
motor fibers and convey motor impulses to the muscles,
while the fibers of the posterior root are afferent or sensory
and bring sensory impulses to the cord. The spinal
nerves which contain both kinds of fibers are, therefore,
mixed nerves.
Structure of a spinal nerve trunk and fiber. —If wre cut
a cross section of a spinal nerve we find it made up of
many bundles of nerve fibers. The entire trunk is sur-
rounded with a sheath of connective tissue. Inside this
sheath, each bundle of fibers is wrapped in a sheath of con-
nective tissue called the 'perineurium. We may remove
this sheath and separate the bundles into the nerve fibers
themselves.
In these ultimate nerve fibers, thus separated, we have
the actual conductors of impulses, and our dissection shows
that the spinal nerve may aptly be compared to the cables
of a telephone station where each cable consists of bundles
of separate wires, both bundles and wires being care-
fully insulated from one another.
Examination of a single nerve fiber shows it also to be
composed of layers. In the middle is a core, or axis, com-
posed of modified protoplasm. This is called the axis cy-
linder, and it is this cylinder which transmits the impulse.
Surrounding this is a sheath called the medullary or myelin
sheath. This sheath varies greatly in thickness in different
fibers and is often segmented, forming the so-called “ nodesSPINAL NERVE TRUNK AND FIBER
403
of Ranvier.” Outside this myelin sheath is a thin mem-
brane called the neurilemma, and comparable to the sar-
colemma of the muscle fiber. Lying under
the neurilemma are found nuclei, one for
each segment of the medullary layer. These
two outer layers of the fiber do not conduct
impulses, and while their function has not
been absolutely determined, they may be
compared to the insulation which surrounds
an electric wire. The axis, evidently, is
the wire. In some nerve fibers the medul-
lary layer is lacking, the axis being sur-
rounded by the neurilemma only.
All nerve fibers originate either in the
nervous system or in the sympathetic
ganglia. Those from the former are all
medullatcd; some of those of the latter are
non-inedullated. All fibers admit also of
classification as afferent or efferent, since,
in a given fiber, the direction in which the
impulse travels is always the same. Mixed
nerves, therefore, are composite bundles of afferent and
efferent fibers.
The words afferent and efferent simply indicate the di-
rection in which the impulse travels. Owing to the fact
that efferent fibers all carry impulses which produce motion,
the words efferent and motor are equivalent in describing
a given fiber, while for the same reason the words afferent
and sensory may be interchangeably applied to fibers
which convey sensory impulses.
Finally, the two classes of fibers may be subdivided
again on the basis of the effect of their impulses. For
_ Node of
Ranvier
\\-Neurilemma
Neuraxon
~~or Axis
Cylinder
Medullary
~~Sheath
Fig. 190—Section
of a nerve.404
THE NERVOUS SYSTEM
■Solar Plexics
example, a motor fiber transmits an impulse which may
retard or increase a motion, wliile a sensory fiber transmits
impulses which may increase or decrease sensation. Those
fibers, either motor or sensor}', which excite to greater
motion or sensation, are called excitatory. Those which
decrease motion or sensation
are called inhibitory. This
inhibition or excitation is
supposed to be determined,
not by the nature of the im-
pulse, but by the tissue in
which the fiber ends.
The plexus.— The fact just
noted, that the spinal nerves
are composed of bundles of
fibers, explains how it is pos-
sible for them to split up into
smaller nerves and ultimately
into separate fibers, just as we
may unravel the strands of a
rope. In many regions of the
body some of these unravelled
nerve fibei’S from several
spinal or ganglionic nerves
interlace to form a meshwork, and such networks are called
plexuses. In this fusion, new combinations of fibers are
often made, and nerves extending from a plexus may con-
tain fibers from several different spinal or ganglionic
nerves. Evidently the stimulation of such a nerve will
result in messages to several different parts of the central
system, and vice versa. For example, the nerves supply-
ing the legs arise from large plexuses, and are thus supplied
—'Abdominal
Aorta
—L. Sympathetic
Chain
1—It. Sympathetic
Chain
Hypogastric
Plexus
r^JJiao A.
Fig. 191 — Tlie solar and hypogastric
sympathetic plexuses.CELL THEORY OF NERVE STRUCTURE; NEURA 405
with fibers from many different parts of the central sys-
tem. The advantage of such a combination is seen in the
manner in which a single nerve is able to coordinate very
complex movements, the messages from the different cen-
ters of control being concentrated, as it were, by this
arrangement.
One of the largest plexuses is located just above the pit
of the stomach, and is called the solar plexus because smaller
plexuses radiate from it like the rays of the sun. A blow
on this area paralyses many regions, and
may even produce death.
Cell theory of nerve structure; neura. —
The presence of nuclei under the neuri-
lemma at once suggests the idea that
nerve fibers bear some relation to cells.
Examination of the gray matter of the
cord or brain in which these fibers orig-
inate gives us further information on
this point, and to the relations thus
established is due the present theory of
nerve structure and action.
Preparations of gray matter show it
to be rich in nucleated cells of irregular
outline. (See Fig. 192.) From the angles
of this outline project fine branches of
protoplasm, which may subdivide again
and again to form treelike growths, or
which may continue some distance with-
out branching, in the form of a cylinder.
The branching outgrowths are called
dendrites, while the cylinders are called axis cylinders, or
Fig. 192 — Nerve cells
from the cortex of the
cerebral hemispheres;
Ax axis cylinder.
axones.406
THE NERVOUS SYSTEM
Nerve cells, wherever found, agree in this general struc-
ture. In some the production of dendrites is much less
than in others. Some also have one axon only, while
others may have from two to several. Those cells with one
axon arc called unipolar, with
two, bipolar, and with more
than two, multipolar. If we
trace the course of one of
these axones we find that it
may extend a long distance
from the parent cell with or
without branching, and, what
is most important, such ax-
ones are the axis cylinders
of the nerve fibers.
We see then that a nerve
fiber is to be considered as an
elongated, axis cylinder, or
axon, which has become cov-
ered with one or more pro-
tective sheaths. When the
axis cylinder branches, these
branches are called collaterals,
while the terminal branches
in which they all end are
called the terminal brushes.
A nerve cell with its dendrites,
axones, sheaths, collaterals,
and terminal brushes, is
spoken of collectively as a
neuron. Such a unipolar neuron is shown in Figure. 193.
Theory of neuron action. — The action of an amoeba
free axis cylinder; b, axis cylinder
surrounded by neurilemma alone;
c, axis cylinder surrounded by
medullary sheath alone; d, axis
cylinder surrounded by the sheath
and neurilemma and divided into
segments (by constrictions called
the nodes of Ranvier).THEORY OF NEURON ACTION
407
when touched with a needle point demonstrates that
protoplasm is capable of responding to stimulation. Since
the result is the same no matter where the protoplasm is
touched, it follows that it is also capable of conducting
impulses thus set up. In other words, responses to certain
forms of stimulation and conduction of impulses is a proD-
erty of protoplasm. When we examine the cytoplasm of
a nerve cell we find it to be made up of two kinds of proto-
plasm. One kind resembles closely that of the amoeba.
The other kind seems to be in the form of threadlike fibrils.
The axon protoplasm is almost purely of the fibrillar kind.
This fibrillar protoplasm is found to be able to transmit
impulses much more rapidly than ordinary protoplasm.
In other words, it is protoplasm with a highly developed
power of conduction. The discovery of this structural
peculiarity of nerve cells enables us to think of them as
cells whose protoplasm has been specially developed for
conducting impulses.
Keeping in mind this fact, and also the structure rela-
tions in a neuron, the formulation of the theory of neuron
action may be expressed in the following laws:
First law. An impulse may be started wherever there
is protoplasm, but externally stimulated impulses have
their origin usually in the terminal brushes of the axones
or in the dendrites, while the internally stimulated im-
pulses arise in the body of the nerve cells. In general,
then, the nerve fiber or axon is not a source of impulses,
though in cases of inflammation of the sheaths it may’
become so.
Second law. An impulse started in any part of the
protoplasm of a neuron is conducted by the fibrillar pro-
toplasm to every other part of the neuron protoplasm.408
THE NERVOUS SYSTEM
Thus, an impulse started at the end of an axon is trans-
mitted directly to the protoplasm of the cell body and
through that to the extreme ends of the dendrites and
any other axones that may arise from that cell body.
Similarly, an impulse arising in the dendrites or cell body
is likewise transmitted over the axones to their termina-
tions. Bearing in mind that impulses usually originate in
one of three places, the cell body, the dendrites, or the ter-
minations of an axon, it will follow that the principal func-
tion of the nerve fiber is conduction. This characteristic
is still further evidenced by the fibrillar character of the
axon protoplasm.
When two points are connected by two neura the arrange-
ment is as follows: The dendrites of one neuron are so
placed as to be brought in contact with the terminations
of the axon of the second. In this relation the course of
the impulse takes one of two directions, and this direction
may be formulated in the third law.
Third law. An impulse which originates in the termi-
nations of the axon of one neuron, and has been conducted
to the dendrites of the nerve cell of that neuron, may pass
from those dendrites into the terminals of the axon of
the second neuron, and thus on to the dendrites in the
final cell of the series. Similarly, an impulse originating
in the cell body or dendrites of one neuron may pass, by
way of the axon terminations, into the dendrites of the
connecting neuron, and thus on to the final axon termi-
nations of the series.
These three laws enable us to explain the method of
impulse transmission in all forms of nerve action; while
neura may differ in the number of axones, in the length of
the axones, and in location, the laws of action given aboveDISTRIBUTION OE NEURA
409
hold true for all classes. In connection with the third
law of action it may be noted that whenever there is a
passage of impulse from one neuron to another there is at
this point a delay in the transmission, and sometimes
several impulses must reach a cell before it can overcome
this resistance between dendrites and axon contacts and
discharge into the next cell. We may compare this delay
to the effect of placing a small barrier in the path of a
stream. The effect is not to stop the stream, but to check
it until the volume of water is great enough to push aside
the barrier. The normal rate of nerve discharge is about
ten impulses per second.
It will be noted finally that the above laws give no
explanation of the manner in which nerve impulses origi-
nate. The manner in which externally stimulated im-
pulses originate will be considered in our study of the
sensory end organs. The manner in which impulses
originate in the cell body is still unknown. It is believed,
however, that just as oxidation of fuel and metabolism
account for the contraction of muscle protoplasm, so these
nerve impulses set up in the nerve cells result from certain
chemical changes in the composition of the cell proto-
plasm. The fact that nerve cells require food and become
fatigued by overwork justifies this belief, but the exact
nature of the metabolic changes involved is unknown.
Distribution of neura.— In the central nervous system,
the cell bodies of the neura which originate there are
found mainly in the gray matter of the cord and in the
cortex of the brain. The axones of these cells may
extend far beyond the limits of this system, or again they
may be confined within the central system. The spinal
and cranial nerves represent bundles of axones which410
THE NERVOUS SYSTEM
originate from these centra] system cells and connect them
with more or less distant parts of the body. Many of
the axones found in the white matter of the cord and
brain, however, never leave the central system, and the
function of these is to connect the brain cells with the
cells of the cord or with those of other parts of the brain.
In general, then, the gray matter of the central system
consists mainly of nerve cells, while the white matter is
largely composed of nerve fibers. Each cell gives out from
one to several axones, and the entire nervous system may
be thought of as very complicated network of inter-
connected neura.XXVI. THE NERVOUS SYSTEM (continued).
Function.
From the nervous system we gain control of the
organs of the body, and to its action we owe our
power to adjust ourselves to our surroundings in general
and to maintain relations with other human beings and
objects. In securing to us this adjustment to our envi-
ronment, the nervous system manifests many forms of
activity. For example, the interconnection of the fibers
permits the transmission of messages of control and super-
vision. The end organs of various forms give rise to
special kinds of impulses, and when these are transmitted
to the centers, the structures found there are able to trans-
late them into sensations and thus furnish information
in regard to our surroundings which is of vital importance
in determining the character of the control impulses.
Finally, the nerve centers are the origin of those mental
operations which we call will, emotion, thought, etc.
In short, the verjr individuality of a man is a function of
his nervous system.
The explanation of the many actions which we group
under the common name of mind activities is properly
the province of a science called psychology. In the study
of the physiology of nerve action we are concerned only
with the changes which take place in the nervous ele-
ments of structure in giving rise to these operations and
the manner in which these changes occur. Owing to the
411412
THE NERVOUS SYSTEM
delicacy of the nervous structure and the difficulties in
the way Of experiment our physiological knowledge of
the nervous system is extremely limited, and, in fact,
much has still to be learned of the physiology of nerves
and nerve cells. In the following pages we shall simply
try to indicate a few of the facts which have been deter-
mined up to the present time, the manner in which such
knowledge has been acquired, and its bearing upon our
common activities.
Action, voluntary and reflex.— The action of every
organ of our body is dependent upon nerve control. The
simplest division that can bo made of the actions of
the body separates them into tvro classes. Those which
require the exercise of the will and are associated with
consciousness are called voluntary actions. Those which
take place without any effort of will and may or may not
be accompanied by consciousness we call reflex. All such
actions as the picking up of objects with the fingers and
the avoiding of obstacles in walking, in short, all actions
which involve a conscious choice on our part belong to
the voluntary type. All such actions as the regulation
of the beating of the heart, the withdrawal of the hand
from a hot object, etc., in which the action takes place
without any consciousness of choice on our part, belong
to the class of reflexes.
The relation which consciousness may bear to such
actions is brought out in the two examples of reflex
action cited. In the beating of the heart consciousness
is entirely lacking; while in the withdrawal of the burned
hand we may be conscious of the pain and the move-
ment, but ordinarily the movement has occurred before
the pain is felt, and hence, in this case, while conscious-THE KEFLEX AKC
413
ness may accompany the action, it does not determine the
action. Many actions which are at first voluntary may
become reflex by repetition; and, in fact, one of the funda-
mental features of education is concerned in making reflex
or habitual, actions which in the child are voluntary and
require effort of will on his part.
The reflex arc.— The spinal cord is the seat of most of
the reflex actions of the organs of the trunk and limbs.
The relation of the nerve elements in simple reflexes which
have their origin in the cord, is shown in Figure 194, and
the relation there shown is typical of all reflex actions. In
this figure it will be seen that the posterior root of the spinal
nerve has on its surface a swelling which was called the
spinal ganglion. This ganglion contains many bipolar
nerve cells whose axones extend in different directions.
One axon passes out and forms one of the sensory fibers of
the spinal nerve. It terminates in some form of an end
organ. The other terminates in a brush in the gray
matter of the cord. Likewise, in the anterior cornua of the
cord is found a unipolar nerve cell whose dendrites are in414
THE NERVOUS SYSTEM
contact with the terminal brush of the sensory fiber. The
axon of this nerve cell passes out through the anterior root
and becomes one of the motor fibers of the spinal nerve.
It terminates in a muscle.
If, now, the sensory end organ of such a system is stimu-
lated, as when the finger is brought in contact with a hot
stove, the impulse passes up over the axon or fiber to the
ganglion cell and through that is transmitted to the ter-
minal brush of the other axon in the cord. From this
brush it passes into the dendrites of the motor cell, and
this cell in consequence discharges an impulse over its axon
to the muscle and causes that to contract.
In this simplest form of reflex action only two neura are
involved; one neuron conveying the sensory impulse and
the other the motor impulse. Most of our reflexes are
more complex than
this, but their com-
plexity consists in the
fact that they involve
more than two neura
and not in any essen-
tial difference in ac-
tion. The manner in
which a single sensory
impulse may stimulate
several motor fibers
is seen in Figure 195.
Here the sensory axon
splits up in the cord into collaterals, and each collateral dis-
charges through a different motor cell. Such an arrange-
ment explains how several muscles can be refiexly thrown
into action by a single sensory impulse. It must be noted,
Fig. 195 — Diagram showing how one sensory
nerve may stimulate several motor nerves in
a simple reflex.THE VOLUNTARY ARC
415
however, that the production of the impulses is essentially
the same as in the simpler case.
The reflex arcs of the cranial nerves are similar in action
to those of the cord, and involve sensory and motor
neura. Inasmuch as in this action the impulse travels in
a circuit, the name of reflex arc has been given to such
arrangements as above.
The voluntary arc.— When we come to examine what
takes place in a voluntary action we find that the process
is not very different from that of the reflex action, except
in the time required for transmission. Figure 196 shows a
typical voluntary arc. In such a circuit the path of the416
THE NERVOUS SYSTEM
impulse is as follows: End organ to cord, cord to brain
centers by connecting neura, brain centers to other brain
centers by other connecting neura, and finally back from
the last brain center to the motor cell in the cord and over
its axon as a motor impulse to the muscles. In such a cir-
cuit it often happens that the cord motor cell is so near the
sensory fiber brush that a division of the discharge is made,
part of the impulse passing over the long circuit noted above,
while the other part passes directly over to the motor cell
and thus establishes a short-circuited reflex. In traveling
the long circuit to the brain no motor discharge can take
place until the impulse is returned from that region. Such
a circuit is characteristic of all deliberate actions. In the
case of the division of the impulse, while part of the im-
pulse may continue on to the brain and produce conscious-
ness, the short circuited impulse is able to produce the
action perhaps even before consciousness is aroused. In
the case of the burned finger the importance of this latter
arrangement is evident in regard to speed of action. This
short circuiting also illustrates how a voluntary action may
become reflex, for if gradually the bulk of the discharge
follows the shorter circuit the less necessary becomes the
long circuits, and it may ultimately be abandoned entirely.
In general, then, a voluntary circuit differs from a reflex
circuit only in the length and in the number of neura
involved. The tracing of all these connecting neura and
their distribution in the central nervous system forms a
part of the study of the nervous system in which much
work still remains to be done. Almost nothing is known
as yet about the character of the changes which take place
in the brain cells and the manner in which past experiences
can be stored and modify the nervous discharges. BySPINAL COED
417
experimenting with lower animals, and by the study of
human beings whose nerve tracts have become diseased in
part, it has been found possible to locate the tracts where
sensory impulses are translated into consciousness, and
also the points of origin of control impulses for different
organs.
Localization of functions in the central nervous system.—
Such experimental studies as noted above have demon-
strated that a sensory impulse produces no conscious
sensation unless it reaches a certain area of the brain, and
if this area is destroyed and the transmitting fiber with
its end organ preserved intact, no sensation results. Sim-
ilarly, it has been found that certain organs and parts of
the body are under the control of definite areas of the
brain and cord, and these areas must be in a healthy con-
dition or the action of these organs is seriously interfered
with. The determination of these areas of sensation and
control is called the localization of functions, and the
method by which results have been obtained is largely
through experiment with lower animals.
Spinal cord.— If we cut the spinal cord of the frog just
back of the medulla, the frog will continue to live and the
heart will continue to beat, but he ceases to breathe or
swallow. The animal lies flat on his belly and loses all
power of voluntary movement or conscious sensation. He
is still able, however, to respond to external stimuli with
definite movements. Thus, if we irritate the skin, the
animal will make definite and skilful movements to re-
move the irritating body; if we pinch the toe, the foot is
withdrawn. From these and similar experiments it has
been conclusively demonstrated that the cord is the seat
of those reflex movements which concern responses to
EDDY. PHYS.—27418
THE NERVOUS SYSTEM
externally applied stimuli, and is also the path of sensory
impulses to the brain. In higher animals similarly treated
this reflex response is less apparent, and indicates that in
such animals the reflex action of the cord is more depen-
dent upon control from higher parts of the system.
Cerebellum.— Experiments upon the cerebellum of
the pigeon develop the following facts concerning this
organ. When all the cerebellum is removed the animal
can still execute voluntary movements, but these move-
ments instead of being definite and coordinated are sprawl-
ing, uncertain, and throw the animal into all sorts of
grotesque positions, while it is totally unable to fly. From
these experiments and others upon higher forms it appears
that while the cerebellum does not originate motor im-
pulses it does have the power to regulate and coordinate
these impulses, and is thus responsible for orderly move-
ment. An animal still retains sensation when the cere-
bellum is sliced away.
Cerebrum.— In the cortex of the cerebral hemispheres
is the seat of all the psychical reactions which we include
under the name of intelligence, memory, will, and the
emotions. It is also the seat of conscious sensation, and in
it originate the impulses which produce voluntary move-
ments. Involuntary movements or reflexes may, how-
ever, take place when these hemispheres are removed. A
pigeon whose hemispheres are removed becomes a stupid,
drowsy creature, which, when thrown into the air, recovers
its equilibrium and flies, and it may even be made to feed
by properly applied external stimuli. If starved, how-
ever, it becomes restless and pecks aimlessly at the ground,
but does not seem to be able to locate or recognize food
for itself. In other words, it has lost all those responsesHISTORY OR CEREBRAL LOCALIZATION
419
which depend upon memory of past experiences, that is,
intelligent responses. It shows no signs of fear or pleasure,
and while it responds to sensory stimuli, these responses
are purely reflex and not accompanied by true conscious-
ness.
History of cerebral localization.— When it was estab-
lished that the cerebral cortex was the seat of psychical
activities, the question at once arose as to whether all
parts of the cortex were equally capable of exercising these
faculties, or whether different parts controlled specific
faculties. The idea of separate areas for separate func-
tions was first presented by Franz Joseph Gall, who divided
the cortex into areas of special activity, and put forth the
theory that the more developed any mental quality was
the larger and more prominent became the cortical area
which produced it. Further, since the cortex fits closely
to the cranium, the relative prominence would be indicated
externally by the shape of the skull. From this position
arose the practice of phrenology or the determination of
mental qualities by examination of cranial prominences.
Opposed to the position of Gall was the view of Flourens,
who held that all parts of the cortex were capable of pro-
ducing all kinds of mental qualities, and that when one
part was removed the remaining parts supplied the quali-
ties originally centering in that part. Without going into
this controversy we may present the modern views on the
subject as follows:
First. The general view that functions are localized
has been definitely established, and it is possible to map
the cortex of the brain and thus indicate areas which,
when stimulated, will produce definite actions.
Second. The idea of Gall that the more marked the420
THE NERVOUS SYSTEM
development of a function the larger and more prominent
the area has been proved false.
Third. The interconnection of areas has been shown
to be so intimate that, although normally specific areas
control specific functions, an injury to one part may affect
all the others.
In other words, the cerebrum is composed of many
organs intimately associated with one another and inter-
dependent. In general, the left hemisphere controls the
Fig. 197 — Diagrams illustrating localization of functions in the cerebrum; /, outer
surface of left hemisphere; II, inner surface of same. Motor areas are shaded;
Cc, corpus callosum; Fr, frontal lobe; Oc, occipital lobe; Pa, parietal lobe; J?o,
fissure of Rolando; Sy, Sylvian fissure; Te, temporal lobe.
right side of the body and vice versa. The chief motor and
sensory areas of the hemispheres are shown in Figure 197.
Functions of the medulla.— This is the center of those
reflexes which control the respiratory and circulatory
organs. When a cut is made anterior to the medulla
the animal continues to breathe and the heart to beat
for some time after the operation. If, however, the cut
is made at the point where the medulla joins the cord or
it is destroyed entirely, heart beat and breathing soon
cease and rapid death follows. Other important reflexes
i
TrFUNCTIONS OF THE SYMPATHETIC SYSTEM 421
are also located in this organ which is the origin of many
of the cranial nerves.
Functions of the sympathetic system.— This name was
originally given to the chain of ganglia extending along
each side of the spinal column under the mistaken idea
that it was a pathway for all the so-called sympathetic
(reflex) actions of distant organs. This system was sup-
posed to arise from the brain by branches connected
with the fifth and sixth cranial nerves. It is now known
that this system consists merely of a collection of ganglia
or nerve cells which are connected with one another and
with the spinal nerves. While it is customary to limit
the term to the ganglia on each side of the spinal column,
it actually includes many other ganglia with or without
names which lie in different regions of the body. Accord-
ing to Professor Langley, the efferent fibers from the
sympathetic and related ganglia supply the unstriped
muscle tissues, the cardiac muscles and the glands, in short,
the muscles of the involuntary organs of the body, such
as the walls of the intestines. He has given the name
of autonomic to this system to indicate that these fibers
are to a certain extent independent of the central nervous
system. All the sympathetic ganglia are connected with
the central nervous system by medullated fibers from
the spinal or cranial nerves, but these fibers do not pass
directly to the unstriped muscle tissue. On the con-
trary, they transfer their impulses to a ganglion cell and
the impulse passes out through a fiber (autonomic fiber)
of this ganglion cell which is usually non-medullated.
The connection of these fibers with the spinal cord is
illustrated in Figure 189. Unlike the fibers which supply
the striated muscles, practically the entire set of autonomic422
THE NERVOUS SYSTEM
fibers is removed from the control of the will. Under
ordinary conditions they are probably excited reflexly,
but the course of the afferent fibers is at present little
understood. Most of these reflexes are also unconscious
actions, and include such movements as the dilation and
contraction of the arteries, the movements of the intestines,
and the secretory activities of the digestive glands. The
entire field of action and method is still very little under-
stood.
Hygiene of the Nervous System.
As has been stated, the manner in which nerve cells
generate impulses or give rise to psychical reactions is
unknown. We do know, however, that generation of
impulses and mental activity is in some way dependent
upon the metabolism of the nerve cells. From this, and
the fact that nerve cells, like all other cells, are made of
protoplasm, it follows that lack of nourishment or of air is
bound to affect their activity, overwork to fatigue them,
and rest to restore them. Comparison of the nerve cells
of a child with those of an adult show that the latter are
much more branched and more intimately connected than
those of the child, which is another indication of the rela-
tion between increased nervous action and metabolism.
Nerve food.— It used to be thought that nerve cells required
special kind of foods for their development, notably, phosphorus;
and fish, which are rich in phosphorus, were supposed to constitute
an ideal brain food. It is now known that their protoplasm is
built up of identically the same sort of materials as are used by other
cells, and hence a food which is nourishing to the laborer is equally
so to the brain worker. The important point to be kept in mind
is that there must be a sufficient quantity of food, and that theBEST
423
organs of digestion and circulation, etc., must be kept in good
order for its preparation and distribution. In short, whatever
tends to produce a healthy body also tends, indirectly, to main-
tain a good condition in the nervous system. Exercise, therefore,
is as valuable to the brain worker as it is to the laborer, and per-
haps even more so, in that the latter is apt to neglect muscular
activity, while the very work of the laborer tends, to keep his
system in order.
Air supply.— The chemical changes which take place in the
nerve cells are essentially similar to those in the muscle cells, in
that the production of energy or action involves the oxidation of
cell substance and food. It follows, therefore that not only must
the nerve cells receive food but also a plentiful supply of oxygen.
The brain responds very quickly to a lack of air by a feeling of
dullness and headache, and it is extremely important that our
school and living rooms be well ventilated. Exercise that tends
to develop the lung power is likewise valuable.
Rest.— Like the muscle cells, again, the nerve cells accumu-
late wastes and consume food by continued activity. It is imper-
ative, therefore, that they be allowed periods of rest when the
supply of food may be renewed and the excess of waste removed.
Sometimes, as with the muscles, a change of work will permit one
set of cells to rest while another is working, and thus a change
of occupation is a rest. But no person can work continuously
and maintain a healthy nervous system, and for that reason sleep
is nature’s greatest remedy for tired nerves. Children whose
nerve cells are growing should have plenty of sleep in order that
their nerve cells may store up sufficient food in these rest periods
to meet the daily needs, and at the same time permit of growth
and branching. On this account adults whose cells have attained
their normal size need less sleep than children.
Nerve cells are like muscle cells in another particular, in that
they degenerate by disuse and lose their irritability. The person
who is too careful of overtasking his nervous system may carry
this care to excess and lose his mental power entirely. It is this
need of constant nerve exercise which gives especial value to a
general education, since the variety of subjects in such a course
insures the normal development of all parts of the system.424
THE NERVOUS SYSTEM
Diseases of the nervous system. — Nerves. The sheaths which
inclose the axis cylinder may become the seat of inflammation.
The result is intense pain and various forms of nervous disease,
according to the extent and nature of the inflammation. Among
diseases of this nature are included all forms of neuritis, tooth-
ache, etc.
Spinal cord. — Injuries to the spinal cord or the introduction
of certain forms of bacteria may result in the destruction of the
whole or part of this organ. The result is paralysis of the
parts supplied by the affected portion. Such diseases are usually
slow in action and often incurable.
Brain. — Anything which affects the nerve cells of the brain,
such as pressure, narcotics, worry, lack of work or sleep, etc., is
bound to result in disturbed mental action. Such influences may
result in sleeplessness, hysteria, lack of self-control, irritability,
and even in total loss of control or insanity. Interference with
the blood supply and pressure may produce fits or apoplexy. In
short, so sensitive is the brain to external influences that its encase-
ment in the bony cranium is none too strong a protection for this
delicate structure and any interference with its structure is attended
with serious results.
Alcohol and the Nervous System.
Stages in intoxication.— It is a fact known to everyone
that alcohol in any quantity affects the nervous system
almost immediately. It is noticeable that moderate quan-
tities have apparently a stimulating effect upon the various
centers of speech, thought, and action. The winebibber
becomes exhilarated, his speech flows more rapidly, his wit
appears keener, and he is conscious of a sense of freedom
from restraint. If the amount of alcohol is increased con-
tinuously, this stage of apparent stimulation is soon followed
by excessive exhilaration. The subject becomes still more
talkative and his freedom from restraint manifests itselfTHE PROBLEM OF THE PHYSIOLOGIST
425
in excesses of various kinds. He laughs immoderately at
remarks only slightly humorous and in many ways shows
that his powers of judgment are becoming dulled. Grad-
ually this dullness spreads. His speech becomes thick and
uncertain and a stage of dullness and drowsiness of all his
mental faculties follows. He may still be able to walk, but
his inclination is to be quiet, and, if permitted to do so, he
passes rapidly into a stupid, drunken slumber. Finally,
the loss of control over his speech spreads to include the
centers of voluntary movement. He loses control of his
legs and is no longer able to walk, staggering blindly when
he attempts to do so. Such a condition represents what
are usually the final stages of acute intoxication or drunk-
enness. If, however, the doses of alcohol are still con-
tinued they reach a point when the involuntary centers
which control the breathing and movements of the heart
become affected, and in this case death of course must re-
sult.
The problem of the physiologist.— The problem of the
physiologist is to account for these well-known effects and
to tell exactly what is the action of the alcohol at each step
of the process. All are agreed that alcohol in large or con-
tinued doses ultimately dulls or paralyses the nervous cen-
ters, and that this narcotic effect is felt first in the higher
centers. After these centers are dulled the paralysis grad-
ually spreads until it includes the motor centers and ulti-
mately the vital processes, such as breathing and heart
beat. Thus, in the description given, it is easy to see that
the centers of judgment are the first to become dulled, and
that the effect of this is to remove the speech and motor
centers from control so that they seem to be stimulated.
That this is not a true stimulation, but the first step in426
THE NEBVOUS SYSTEM
paralysis, is seen in the speed with which these centers
become dulled, while the tendency to attack the higher
centers first is shown by the fact that speech is lost before
control of movements.
In regard to the effect of alcohol in small doses there are
at present two different opinions. One school of physi-
ologists holds that alcohol in small doses is a true brain
stimulant, and that the exhilaration felt as a result of such
doses is accompanied by increased keenness of judgment,
and that, while temporary in its effect, this effect is not due
to loss of control by paralysis of higher centers.
The other school claims that alcohol is never a stimulant,
but always a narcotic, whether in large or small doses.
They explain the evident exhilaration which follows small
doses as due to the same causes as the excessive exhilara-
tion, namely, that it results from a partial paralysis of the
centers of judgment and control. They say that, by re-
moving the inhibiting influences from the speech and other
centers, such paralysis inevitably causes in these centers a
freedom of restraint which manifests itself in apparent
stimulation. In short, this school claims that alcohol from
its entry into the system is never a stimulant, but always a
narcotic in its effect upon the nervous system as a whole.
The determination of the true position in this regard
must come as a result of experiment, and the results of
experiment have so far been extremely conflicting. Not
enough has been done as yet to settle the question for either
side, and all that can be said at present is that by far the
larger number of physiologists take the view that alcohol
is always a narcotic, even in small amounts.
Alcohol and time reactions.— Many experiments have
been made to determine the effect of alcohol on the timeALCOHOL AND DISEASES OE THE NERVOUS SYSTEM 427
required for a nervous impulse to travel over the fibers.
All such experiments require the subject to perform a cer-
tain muscular action upon receiving a certain sensory stim-
ulus, and the measure of the time which elapses between
the receipt of the sensory impulse and the performance
of the action is taken to indicate the rate of transmission of
the impulse —the “reaction time ” as it is called. By com-
paring the reaction time of a person under normal condi-
tions with the time when under the influence of definite
amounts of alcohol the effect of alcohol can be inferred.
The results of such experiments are not absolutely con-
clusive in regard to the effects of small amounts of alcohol.
All, however, tend to show that larger doses of alcohol
lengthen the reaction time for all mental processes. The
importance of this fact has led all corporations employing
men in places where quickness of action, clearness of per-
ception, close attention to duty, and the like are required,
to forbid absolutely the indulgence in alcoholic drinks
while performing such duties. Such actions on the part
of corporations are significant warnings to us of the danger
of this alcohol habit and its tendency to destroy efficiency.
Alcohol and brain workers.— It follows directly from the
preceding that alcohol is not of advantage to the brain
worker, and leads to confusion of judgment. One unfor-
tunate effect of its use is that in dulling certain control
centers it often produces in the indulger the delusion that
he is actually keener in perception, when actually this
apparent keenness may be only an indication of impaired
powers.
Alcohol and diseases of the nervous system.— Continued
use of alcohol lowers the resistance of the body to disease,
and it is a well-known fact that hard drinkers often sue-428
THE NERVOUS SYSTEM
cumb to diseases from which a normal person would easily
recover.
Many cases of insanity are reported as due to alcohol,
but while it is undoubtedly true that the use of alcohol is
in many cases indirectly the cause, the exact relation to
this mental disease is not yet definitely known. The only
form of mental disturbance which is a direct result of
alcoholic indulgence is the violent disturbance known as
delirium tremens.
It is also a well-known fact that nervous weakness can
be transmitted by heredity, and thus children of drinking
parents often develop mental disease which is due to this
inherited weakness.XXVII. THE NERVOUS SYSTEM IN THE
LOWER ANIMALS.1
Nervous tissue, wherever it exists, is in the form of
nerve cells and fibers, as in man. Frequently the nerve
cells are collected in masses which are called ganglia, and
Fig. 198 — A nerve ganglion. The round bodies are nucleated nerve cells; the
fibers are nerves.
in this chapter the use of the word ganglion may be taken
to mean a collection of nerve cells. The fibers are always
developed as processes of the nerve cells.
Protozoans and coelenterates.—The one-celled animals can
evidently have no true nervous system. Their protoplasm,
however, is sensitive to stimuli and conducts impulses.
Such a condition is interesting mainly as showing the rela-
tion of ordinary protoplasm to the specialized protoplasm
of the nerve cell and as indicating the origin of the ner-
vous system. The first appearance of nerve cells and
1 See Footnote, p. 140, Chapter X.
429430 THE NERVOUS SYSTEM IN THE LOWER ANIMALS
fibers is found in the ccelenterates. In the hydra are cer-
tain scattered cells which are partly muscular and partly
nervous in function. In the jellyfishes we find a network
of fibers and cells scattered over the disk. The general
purpose of such systems seems to be to give sensitiveness
to the animals and thus aid them in securing food.
Echinoderms.— In the starfish and its allies we have a
true system. In these animals we find a ring of five gan-
glia surrounding the mouth and connected by a nerve cord.
Each ganglion gives rise to radial nerves which extend
along the arm opposite the
ganglion. The peculiarity of
this system is that it actually
consists of three systems, each
of which has its own combina-
tion of cells and fibers and sup-
plies a special part of the body.
These three systems are abso-
lutely independent of one another
in action. We may compare the
nervous control of the starfish to
a city with three absolutely in-
dependent telephone companies,
each company with its own “central” and wires. Such
an arrangement is absolutely unknown in any other group
of animals.
Mollusks. — The nervous system of such animals as the
clam and its allies consists of several ganglia located in
different parts of the body and interconnected by nerve
trunks. Each ganglion controls special parts of the body,
and this arrangement is spoken of as a scattered system
of control. The arrangement in the clam consists of two
Fig. 199 — Diagram of nervous
system of starfish; r, nervous
ring around mouth; 11, radial
nerves to each arm, ending in
the eye.MOLLUSKS
431
compound ganglia (cerebro-pleural) located on the
right and left sides of the esophagus and connected by
a nerve cord called a commissure. Each
of these ganglia actually consists of two
cell masses. The two cerebral masses give
off nerves which control the head parts
while the pleural masses control the neigh-
boring parts of the body. Each com-
pound ganglion gives off also two nerve
trunks which pass backward along the
same side of the body as the ganglion
from which they originate. The right and
left trunks of one pair unite with a gang-
lion (visceral) near the siphon end of the
animal. The right and left trunks of the
other pair unite with a ganglion in the foot,
called the pedal ganglion. The pedal gang-
lion sends out fibers which direct the
movements of the foot, while the visceral
ganglion, through its fibers, controls the
internal organs and the parts near it. In
such a system it is evident that we have
control stations located near their seat of
control and, unlike those of the starfish, all
are interconnected into one system by the nerve trunks.
We may compare such a system with a city telephone
system in which each district has its own “central” but
all the centrals are connected.
In the snail we find an interesting modification of the
clam system. Here the main ganglia are collected in
a ring about the esophagus, w'hile additional ganglia are
found scattered through the body. In the cuttlefish this
Fig. 200 — Ner-
vous system of
clam; c, cere-
b r o-p 1 e u r a 1
ganglia; c',
cerebral com-
mis8ure; p,
pedal ganglion;
ps, visceral
ganglion; p'
and psf, nerve
trimks.432 THE NEEVOUS SYSTEM IN THE LOWEJR ANIMALS
ring of ganglia is actually inclosed in a cartilage box, thus
giving it a still closer resemblance to a brain.
Worms.— In the worms we find two main ganglia
usually located in the head end and united by a com-
missure. In the simpler worm forms these ganglia are
the only ones in the body, and fibers from them supply all
parts of the body.
In the earthworm these two ganglia are represented
by a single two-lobed mass located just above the esopha-
gus and called on this account the supra-esophageal
Fig. 201 — Nervous system of earthworm; g, ganglion; v.n. ventral nerve cord.
ganglion. From this compound mass or “ brain ” a
ring of cord encircling the esophagus unites it with a
ganglion mass under the esophagus called the sub-esopha-
geal ganglion. Finally, a double cord extends from this
latter ganglion along the ventral side of the body. This
cord shows a series of small ganglia at intervals corres-
ponding to segments. Each of these small ganglia is the
source of motor and sensory fibers which supply that
particular segment, and these segment ganglia are the seat
of the reflex actions. It is not difficult, then, to compare
such a system with the one in man. The supra-esophageal
ganglion may be compared to the brain, the ventral cord
to the spinal cord, and the smaller ganglia to the centers
of reflex actions and sources of spinal nerves in the cord.VEKTEBIIATE SYSTEMS
433
This brain ganglion and cord constitute a central system
in distinction to the fibers which arise from the small
ganglia and which correspond to our peripheral system.
Arthropods.— In the lower arthropods, such as the
lower forms of crustaceans, and the wormlike larva1 of
insects, the nervous system is practically
identical with that of the earthworm. In
the higher artlnopods such as the higher
crustaceans and insects, the plan is the same,
but many of the ganglia are fused together
in the head and thorax making this end of
the system much more brainlike in structure.
This head mass also gives nerves which
supply the eyes and antennae much as our
cranial nerves are given off from the brain.
The ventral cord shows fewer ganglia, and
each ganglion furnishes motor and sensory
fibers to larger areas than in the worms.
In short, while this system is similar in
composition to that of the worms the ten-
dency is to centralize control in the head.
Vertebrate systems. — In the vertebrates
the most remarkable changes are the develop-
ment of the true brain as an organ of mental
operations, and the transfer of the whole
system to the dorsal side of the body where
it is protected in the skeletal structures.
The essential features of the systems found in the
different forms of vertebrates are identical, the variation
being in the direction of complexity. Figures 203 and
204 illustrate the manner in which the complex brain
of man has been developed, and the relative stages in
EDDY. PHYS.—28
l|k
Fig. 202—Ner-
vous system
of a cater-
pillar.434 THE NERVOUS SYSTEM IN THE LOWER ANIMALS
development seen in the vertebrates lower than man.
The feature which strikes the eye as most vital in this
Fig. 203—A comparison of vertebrate brains; O.L. olfactory lobes; C.B., cere-
bral hemispheres; O.L. optic lobes; C.L. cerebellum; M, medulla; S.C. spinal
cord.
comparison is that the changes have arisen mainly in
order to accommodate the increasing brain surface to the
limits of the inclosing cavities. For example, in the
Fia. 204 — Comparative wrinkling of cerebral surface of cat (A), and man (R).VERTEBRATE SYSTEMS
435
lower vertebrates the increase in size has been met by
doubling and folding one part upon the other, while in the
higher forms the wrinkling or furrowing of the cortex
gives increased surface.
Along with these physical changes has come increased
mental power, until, in man, we have an animal preeminent
over all others by virtue of the superior mental powers.XXVIII. THE SPECIAL SENSES.
Touch, Taste, and Smell.
In the preceding chapters we have discussed, mainly,
the mechanism by which sensory and motor impulses are
conveyed and controlled; and the areas of the central
system where these sensory impulses are received and
translated into conscious sensation. The manner in which
impulses originate and their character requires a study of
the arrangements at the surface ends of the afferent fibers.
Such arrangements are grouped together mider the name
of end or sense organs. They are all specialized struc-
tures adapted for response to certain kinds of stimuli only,
and are named according to the effect of the impulses
upon the consciousness. Thus, organs which produce
sensations of sight are called sight organs, those which
produce sensations of touch, tactile organs, etc. It must
be distinctly borne in mind, however, that all of these
organs merely originate impulses, and it is.only when these
impulses have traveled over the afferent fibers and stim-
ulated certain tracts of the brain cortex that we become
conscious of the sensations which they produce. In short,
the sense organ is simply a structure adapted for response
to a special kind of stimulus; and the impulse sent out by
this stimulus, the brain interprets as sight, pain, heat,
etc., according to the quality of the impulses.
Laws of sensory impulses.— There are three laws of
sensory impulses which arc applicable to all kinds of sen-
sory fibers.
436LAWS OF SENSORY IMPULSES
437
First law. A sensory fiber wherever stimulated, car-
ries only one kind of impulse. Thus a touch nerve fiber,
whether stimulated in its end organ or elsewhere always
produces the sensation of touch, never that of sight or
hearing.
Second law. In consciousness an impulse is located as
coming from the natural source of such an impulse. Thus,
in an amputated leg, pressure upon the cut end of the
nerve is felt by the brain as though it came from the
original end organs of that nerve. In this way may be
explained many of the instances where pains are felt in
the toes or fingers even after these parts have been
amputated.
Third law. The increase in any given sensation from a
definite kind of impulse does not take place uniformly
with the increase in stimulation. On the contrary the
amount of stimulation has to increase by a definite quan-
tity before any increase in sensation is felt. A man named
Weber investigated this relation between stimuli and sen-
sation, and the law is often called Weber’s Law on that
account. An illustration will make clear its application.
If we hang a thirty-gram weight to the finger we get a
certain feeling of pressure. If we now add a half-gram
weight no increase in pressure is felt, and it is only when
we add a whole gram that the increase is actually felt.
If, now, we start with sixty grams we have to add two
grams before the increase is felt; that is, one thirtieth of
the original weight. In this case, the stimulus increase
necessary to sensation is one thirtieth of the original weight.
The amount of increase in a stimulus necessary to in-
creased sensation varies greatly with different fibers. It
is possible to reduce by practice the increase in stimulus438
THE SPECIAL SENSES
required by any given fiber. This relation of amount of
stimulus to sensation is one of the factors which determine
the sensitiveness of a sense center. The tips of the fin-
gers, which require much less increase in stimulus to pro-
duce sensation than the skin of the back requires, are said
to be more sensitive than the latter.
Classification of sense organs.— The end of every affer-
ent nerve is a sense organ, but for convenience these end
organs are grouped under various heads according to the
region where they end and the quality of their impulses.
The following scheme presents the commonly accepted
classification:
CLASSES OF SENSE ORGANS.
Name of Group	Location of End Organs		Quality of Impulse
		r	Pressure
Cutaneous .	Skin and mucous membrane		Warmth
	of the mouth			Cold
		L	Pain
Internal . .	Internal membranes and muscles	' .	Pain Muscle sense Hunger and thirst
Gustatory	Tongue 			Taste
Olfactory	Nasal passages 			Smell
Visual . . .	Eves			Sight
Auditory . .	Ears			Hearing
Cutaneous sensations.— It was formerly customary to
group all the sensations which were produced by the
impulses from the sensory nerve endings in the skin as
touch sensations. Modern experiments have shown that
touch sensations show four distinct qualities, namely, pres-
sure, warmth, cold, and pain, and that this difference inDISTRIBUTION OF CUTANEOUS SENSE AREAS 439
quality depends not upon the nature of the stimulus but
upon the particular fiber which is stimulated. In other
words, the fibers and end organs which
transmit impulses of warmth are as distinct
from those which transmit impulses of
pressure as they are from those of sight,
hearing, and S7nell. For this reason the
old term, touch sensation, has been re-
placed by the word cutaneous (skin) sen-
sations, and the variety of end organs in-
cluded under this head are to be considered
as four distinct kinds of sense organs.
Cutaneous end organs. — The sensory fibers which end in
the skin show several varieties of end organs. Some form
networks in the outer dermis where the axis cylinders lose
their sheaths, and from this network extend out into the
epidermis as tiny naked filaments. Others show special-
ized structures which may be classified under three heads,
as tactile cells, tactile bulbs, or tactile corpuscles. At pres-
ent it is impossible to tell by the look of a cutaneous end
organ what kind of impulse it will transmit. In other
words, unlike the fibers which end in the eyes and ears, we
cannot be sure from their structure whether a given form of
end organ produces pressure impulses or heat impulses,
pain or cold.
Distribution of cutaneous sense areas. — The most inter-
esting fact concerning cutaneous senses is that all parts
of the skin are not equally sensitive to all kinds of stimuli.
For example, in a given area of skin there will be spots which
respond only to pressure, and not to cold or heat. Other
spots in the same area will respond to heat and not to pres-
sure, cold, or pain. In short it is possible to map any given
Fig ‘205. — A touch
corpuscle in a
skin papilla; C,
corpuscle; iV,
nerve fibers.440
THE SPECIAL SENSES
portion of the skin and to locate on such a map by different
colored dots the position of the different sources of sen-
sation. The relation of these spots to the fibers may be
summarized as follows: (a) The surface of the skin is di-
vided into spots or areas in the center of which is the end-
ing of a sensory fiber, (b) Each of the four cutaneous
senses has its own areas and fibers, and stimulation of one
of these spots produces only one kind of sensation, heat or
cold, warmth or pressure, (c) The pain spots are most
numerous and the warmth spots least numerous. The
ends of the pain fibers are nearest the surface and those
of the warmth fibers farthest below the surface. (d) Dis-
crimination between two stimuli of the same kind depends
upon the stimulation of two distinct fibers. Thus, we
feel the pressure of two points as two only when each
point stimulates a different pressure fiber from the other,
(e) Fibers of a given kind are much nearer together in certain
parts of the skin than in others, and sensitiveness in any
given direction depends partly upon the nearness together
of the fibers.
Pressure. — The pressure spots are, next to the pain spots,
the most numerous. In those portions of the body which
are covered with hair the pressure fibers terminate in a
ring surrounding the hair follicle. These rings, therefore,
form the end organs. In other parts of the body the tactile
corpuscles seem to function as end organs. These cor-
puscles are particularly abundant on the tips of the fingers
where they underlie the dermal papilla'. It is estimated
that on the entire body exclusive of the head region there
are about five million pressure spots. These spots are
very close together on the tips of the fingers and less near
in other parts of the body.PRESSURE
441
The delicacy of pressure sense can be measured in two
ways, by determining the least amount of pressure necessary
to arouse a sensation, and by locating the least distance
apart at which two points can be felt as two. The method
of determining the first consists in varying the pressure
upon various portions of the body until the least amount
necessary to produce a sensation in any given part is de-
termined. The results of such tests show that the skin of
the face, forehead, and temples is most sensitive of all the
skin areas to the feeling of pressure. The back of the hand
is more sensitive in this respect than the tips of the fingers.
Two milligrams of pressure is necessary to arouse sensation
on the forehead, while from five to fifteen milligrams is
necessary when applied to the finger tips. Such tests
simply determine the relative sensitiveness to stimuli of
the parts tested.
To test the power of discriminating between two pres-
sure stimuli, the method consists in applying to the surface
two points and determining how near together these points
can be placed and still felt as two. The relative sensitive-
ness of different areas of the body to such a test is given in
the following table: 1
Area	Distance of Points Apart
Tip of tongue . 		1.1 millimeters
Tip of finger 		2.3
Middle of palm		8 to 9 millimeters
Forehead		22.6 millimeters
Back of hand		31.0
Forearm		40.6
Chest 		45.
Middle of back		07.7 “
1 N.B. In both methods of test the subject is blindfolded,442
THE SPECIAL SENSES
The above results show that while less stimulus is neces-
sary to arouse a sense of pressure in the face than in the
fingers, the power to discriminate is particularly acute in the
finger tips and tongue. This second method is also of service
in locating the distribution of nerve ends, since to feel two
points as two means that two nerves are being stimulated.
Another interesting feature of this discrimination ense
is that it can be increased with practice, notably in the
case of the blind. This power is probably due to in-
creased branching in the nerve ends thus increasing the
number of spots on a given area.
Temperature sensations.— These are of two kinds, heat
and cold, each with separate spots of stimulation and
separate fibers. Their location may be determined by
passing gently over the skin of the blindfolded subject,
metal points which are colder or
warmer than the skin surface. As
this is done there will be aroused
at definite points distinct sensa-
tions of heat or cold, which are at
the same time entirely distinct from
pressure sensations. If a certain
area be explored in this manner
and the spots located by ink spots,
of different color, say red for cold
spots and black for warm, the
extent of the nerve ends and the
relative number of heat and cold
nerves for a given area can readily
be determined. Figure 206 shows such a map. The results
of such tests applied to all areas of the body may be sum-
marized as follows:
Fig. 206 — A portion of tlie
skin mapped for hot and
cold spots; white spots, cold;
black spots, hot.PAIN SENSATIONS
443
The cold and hot spots are always distinct, some portions
of the skin being sensitive only to cold, some to heat. The
cold spots are much more numerous and more sensitive
to stimuli than the heat spots. The sensitiveness to
stimuli varies for different areas of the body, the order
of sensitiveness to temperature being as follows: tip of
tongue, eyelids, forehead, cheek, lips, limbs, trunk. Tem-
perature sense is always relative to the temperature of
the skin, that is, objects colder than the skin stimulate
cold spots and feel cold, and vice versa. Hot and cold spots
may be stimulated by mechanical and chemical stimuli.
Menthol, for example, gives a cold sensation. They may
also be stimulated from within, the skin feeling hot in
case of fevers being a case in point. No distinctive end
organs for these sensations have as yet been discovered.
Pain sensations.— It used to be thought that pain was
the result simply of excessive stimulation of sensory fibers.
Thus, if a pressure nerve was stimulated by too great a
weight the result was a sensation of pain added to that of
pressure but conveyed by the same impulse. It is now
believed that pain sensations of the skin at least are pro-
duced by special impulses which pass over special fibers
and that these fibers are distributed in spots as are the
other cutaneous nerve ends. The results of careful experi-
mentation in locating and determining the peculiarities of
the skin pain spots may be summarized as follows:
Summary.— (a) Pain spots are distinct from and more numerous
than other cutaneous sense spots. (6) Pain spots of different regions
vary greatly in the amount of stimulus necessary to produce a
sensation. Those of the eye being much more sensitive than
those of the fingers. All, however, require much greater stimulus
than those of temperature or pressure, (c) The nerve fibers444
THE SPECIAL SENSES
which supply the pain spots have no distinctive end organs so far
as known. (d) They may be stimulated by any of the usual
forms of cutaneous stimuli, such as temperature or pressure, pro-
vided these st muli are of sufficient intensity, (e) The location
of a pain stimulus by the brain is aided by the pressure and tem-
perature stimuli, parts which have lost these centers still retain-
ing the sense of pain but the subject has difficulty in locating
the seat of stimulation.
Internal Sensations.
The membranes of the internal organs, such as the
mucous membranes that line the alimentary tract and the
lungs and the membranes that invest the muscles or
cavities of the body, have no pressure, warmth, or cold
fibers. Increase in internal temperature or pressure can
be detected, therefore, only by its effect upon the cutan-
eous fibers. These membranes are, however, supplied with
pain fibers and other fibers whose function it is to supply
the brain with information as to the health and needs of
these parts. Very little is known of these latter fibers or
their end organs, their method of distribution or stimula-
tion. The sensations of hunger, thirst, general health-
fulness or weakness are all produced by impulses passing
over these fibers, and their function as indicators of the
condition of our internal organs is extremely important.
At present, we know only of the presence of these fibers
and admit our ignorance of their end organs or properties.
Gustatory Sensations.
Distribution of fibers and end organs.— The sensation of
taste is produced by impulses sent over special nerve
fibers whose ends are distributed over various parts of
the mouth cavity and particularly in certain regions ofDISTRIBUTION OF FIBERS AND END ORGANS 445
the tongue. If the tongue is protruded and its surface
examined with the aid of a mirror it is possible to dis-
Fig. 207 — The tongue; K, filiform papillse; I, fungiform papilke; Z, circumval-
late papillse.
tinguish many raised points on the surface of its mucous
membrane. These raised points are called papillae, and are
of three forms—the circumvallate, fungiform, and filiform.
Of these, the circumvallate are the largest and least numer-
ous. They appear as circular extensions of the membrane
and are surrounded by a little depression or ditch. They
are only seven to twelve in number, and lie on the upper
surface near the root of the tongue.446
THE SPECIAL SENSES
The fungiform papilke are so called from their resem-
blance to a mushroom, and consist of small, rounded ele-
vations supported on short, slender stalks. They are
found all over the middle and fore part of the tongue on
the upper surface.
The filiform papillm are most numerous and the smallest,
and appear as little pointed projections scattered all over
the upper surface and edges of the tongue.
In the walls of the cir-
cumvallate papillae, in
some of the fungiform
papillae, and in portions
of the pharynx, palate,
and epiglottis, and even
in the vocal cords, are
peculiar structures known
as taste buds. These taste
buds are oval bodies composed of a number of over-
lapping epithelial cells (protective)
surrounding a core of elongated or
taste cells ivhich project through a
pore at the top some stiff hairlike
processes and are connected at the
base with the taste nerve fibers.
The hairlike extensions constitute
the part of these organs which is
directly stimulated by the tasted
substance. The impulse thus origi-
nated is passed through the body
of the taste cells and finally stim-
ulates the terminal branches of the taste nerve fibers
themselves. These taste buds, therefore, may be con-
Taste Cells
Fig. 209—A, isolated taste
bud, from whose upper
free end project the ends
of the taste cells; B, sup-
porting or protecting cell;
C, taste cell.
Fig. 208—Section of circumvallate papilla.KINDS OF TASTE SENSATIONS
447
sidered as the specialized end organs of the sensory taste
nerve fibers.
The perception of taste is produced by the connection
of these fibers and the brain through the fifth and ninth
pairs of cranial nerves.
Stimulation and sensitiveness.— These taste buds can
be stimulated only by substances in solution, and in dis-
solving solids the saliva of the mouth plays an important
part in the production of taste sensations. Dry salt or
sugar placed upon the tongue remains tasteless until
dissolved by the saliva. The sensitiveness of the taste
buds to stimuli is influenced by many external factors as
well as by the concentration and character of the dis-
solved substance. The result of experiments in this
direction may be summarized as follows:
Summary.—(a) The taste buds are most sensitive at a tem-
perature of from 50 to 86° Fahrenheit. Very high or very low
temperatures tend to paralyse the taste buds and thus lessen or
destroy the sensation of taste.
(6) Rubbing the substance to be tasted against the tongue
increases the sensitiveness of the taste buds. This accounts for
our habit of rubbing the tasted food against the walls of the mouth
with the tongue.
(c) The sensitiveness varies with the character of the tasted
substance. The order of greater sensitiveness in this respect
being bitterness, sourness, sweetness, saltness.
Kinds of taste sensations.— While our taste sensations appear
to be very varied in kind, it has been demonstrated that they are
actually only four in number, namely, bitter, acid, sweet, and
salt. The bitter taste is most developed at the back of the tongue,
the sweet taste at the tip of the tongue, the acid or sour taste
at the sides, and the salt taste is nearly equally distributed.
Many of our taste sensations are combinations of these four pri-
mary sensations with odor or sight sensations This fact can be148
THE SPECIAL SENSES
easily demonstrated by closing the nasal cavities and bandaging
the eyes when the sensation of taste produced by certain foods
will often disappear entirely. This fact also accounts for loss
of taste in cases of colds in the head, the nasal passages being
clogged by mucus. Certain substances taste sweet when first
taken into the mouth and bitter when swallowed. This is due
to the fact that these substances are capable of stimulating both
sweet and bitter taste buds and also demonstrates that these
primary sensations are carried by different afferent nerve fibers.
Olfactory Sensations.
Distribution of fibers and end organs.— The nasal cavity
is located just above the hard palate. It communicates
Fig. 210—Section of nose showing outer wall of right nasal cavity; u, b, c, d,
interior of nose; k, olfactory bulb giving off nerves to inside of nose.
with the external air through the two nasal openings and
with the pharynx by a single posterior opening. TheDISTRIBUTION OF FIBERS AND ORGANS
449
walls of this cavity are formed by the inferior maxillary
palate, the nasal and ethmoid bones, and is lined through-
out with mucous membrane. In this membrane are dis-
tributed the fibers of the first pair of cranial nerves (the
olfactories), and through
these fibers impulses of
smell are carried to the
olfactory lobes. These
fibers end in the upper
part of the membrane of
this cavity in special end
organs of smell. The lin-
ing of the nose is, there-
fore, divided into a lower
or respirator}7 membrane
composed mainly of cili-
ated epithelial cells, and an
upper or olfactory mem-
brane, rich in nerve fibers,
and devoid of ciliated cells.
The end organs themselves
consist of the ends of the
fibers developed into long,
olfactory cells, and these
cells are protected on all
sides by surrounding nucleated, epithelial cells. The
olfactory cells show a long, nucleated cell body some-
what like a taste cell, and, like that, ending in tiny hair-
like projections at one end, while the other is continuous
with the nerve fiber. The hairlike processes are first
stimulated, and transmit this stimulus to the fiber which
conveys the impulses to the brain.
EDDY. PHYS. —29
Fig. 211 — Olfactory cells from lining of
nose; O, O, olfactory cells terminating
nerve fibers; Ey E, ordinary epithelial
cells of the mucous membrane; Hy
sensitive cilia affected by odor.450
THE SPECIAL SENSES
Stimulation and sensitiveness.— The explanation of the
stimulation of an olfactory nerve fiber is as follows:
It is supposed that all odoriferous bodies give off par-
ticles in the form of vapor, which latter when dissolved in
the liquids of the mucous membrane stimulate the sensi-
tive hairs of the end organs. The vapor may be given
off by a near or distant body, but the question of whether
it be smelled or not is determined by the currents of air
which carry this vapor. If the air is inhaled it carries
this vapor through the nasal passages, where the dis-
solved particles produce the sensation in the manner
described. Similarly, swallowed bodies may send their
vapors up into the nasal passages through the posterior
openings, and stimulate in the same manner.
The sensitiveness of the olfactory nerves to stimuli
varies greatly in different persons and between persons
and animals. While in man the sensitiveness is not so
great as in dogs and many of the lower vertebrates, it is
still very acute. By taking odoriferous substances and
diluting them to known amounts it has been shown that
the sense is so keen in the average man that one part of
camphor in four hundred thousand parts of air can still be
readily detected, while odors like musk and vanilla can
be diluted to eight and ten million parts of air and still be
detected. These olfactory cells, however, soon become
exhausted by continuous stimulation and cease to react.
Their sensitiveness is also affected by lack of secretion on
the part of the membranes.
Kinds of olfactory sensations.— Smell sensations are usually
classified as agreeable and disagreeable, according as their effect
is pleasant or unpleasant, and these effects undoubtedly serve to
guard the person from evil atmospheres. This is hardly a trueKINDS OF OLFACTORY SENSATIONS
451
classification, however, as many odors are agreeable to some
persons and intensely disagreeable to others. It has not yet
been determined whether there are, as in taste, fundamental or
primary odors out of which others are compounded, but it seems
reasonable to suppose that such is the case, and that special fibers
convey special kinds of odor impulses.XXIX. THE SPECIAL SENSES (continued).
Visual Sensations.
The eyes arc the external organs by means of which
visual impulses are produced. So complex are these organs
that we must first study their anatomy and some of the
laws of light before we can appreciate the manner in
which they produce visual impulses.
Form and protection of the eyeball. — The eyes con-
sist of two nearly
spherical bodies called
the eyeballs, which,
together with certain
protective structures,
completely fill the two
sockets in the facial
bones which we call the
orbits or eye sockets.
Both eyeballs and
their accessory parts
are identical in struc-
ture, hence a description
of one will serve for
both.1 In front the
eyeball is protected by
movable folds of the
skin called the eyelids. These lids are lined with a
1 A sheep’s skull with the bony parts and eyeballs uninjured serves
well for a demonstration of the structure of the eyeballs. (See Ex. LXX.)
452
Fig. 212—Section of eyeball in orbit;
frontal bone; 2} cheek bone; 3, eyebrow; 4,
eyelids; 5, folds of the conjuctiva lining eye-
lids and covering front of eye; 6, 7,8, muscles
moving eyeball; c, cornea; a, aqueous humor;
it iris; l, lens; r, vitreous humor; ck, choroid
layer; r, retina; a, sclerotic layer; v, optic
nerve.FORM AND PKOTECTKH OF THE EYEBALL
453
mucous membrane called the conjunctiva,1 which is also
folded back to cover the front of the eyeball. The
lids are movable, and when lowered form a protective
curtain in front of the eyeball. Their edges arc lined with
sebaceous glands, which, by secreting oil, prevent their
sticking together when closed. A row of hairs also fringes
these lids and serves to keep out dust and furnish a slight
shade. These lids are closed by the contractions of a cir-
cular muscle (the orbicularis) which acts in part reflexly,
as when an object is suddenly brought toward the eye or
head.
As these lids rest against the front of the eyeball, friction
would naturally result between the two surfaces when they
rub together. The mucus secreted by the membrane partly
prevents this by acting as a lubricant. There is in addi-
tion a small gland about the size of an almond (the tear
gland) located on the
outer and upper side of
the orbit, and this gland
secretes a salty liquid
called the tear fluid or
the lachrymal fluid. The
gland is also known as
the lachrymal gland.
This fluid flows over
the front of the eyeball
assisted in -its progress
by the motion of the
lids until it reaches
the inner angle of the lids next to the nose. Here it is
drained off into a small duct (the lachrymal chid) which
1 Inflammation of this lining causes conjunctivitis, or pink eye.
Fig. 213 — View of right eye and tear appa-
ratus; G, tear gland; C, C, upper and lower
tear ducts; B, common nasal tear duct.454
THE SPECIAL SENSES
empties it into the nasal passages. Not only does this
fluid aid in lubricating the surface of the ball and eyelid,
but in passing continuously across the exposed part of the
eyeball it removes from the surface dust or other foreign
particles which may tend to collect there. Sometimes this
fluid is poured forth very copiously, as when the membrane
covering the eyeball is irritated, or the person is under strong
emotion. Under such circumstances the ducts cannot
drain it off fast enough, and it accumulates and rolls over
the edges of the lids down the cheeks. Under ordinary
conditions of flow, the oily secretion at the edge of the lids
is sufficient to prevent its escape in this way.
Back of the lids the eyeball itself rests upon a fatty layer
which lines the orbit. Connected to it is the optic nerve
which enters the eyeball from the back through an opening
in the socket. This nerve carries the visual fibers to their
centers in the brain. The eyeball is also held in place and
moved by six muscles. These muscles are controlled by
motor impulses sent from the brain along the fibers of the
third, fourth, and sixth cranial nerves. The attachment of
sides of the eyeball, and, by contracting and relaxing alter-
nately, turn the eye toward or away from the nose. Two
(the inferior and superior rectus) are attached to the lower
in}. VOlique
Fig. 214 — Muscles of the eye-
ball.
these muscles shows an ingenious
mechanical adaptation to the
structure of the orbit. (See Fig.
214.) Four (rectus muscles) are
straight muscles attached at one
end to the eyeball and at the
other to the bottom of the eye
socket. Two (the internal and
external rectus) are attached to theSTRUCTURE OF THE EYEBALL
455
and upper surface of the eyeball respectively, and by their
action turn the eyeball upward or downward. The other
two muscles (the inferior and superior oblique) serve to
rotate the eyeball outward or inward. The inferior oblique
has its origin near the front of the orbit on the inner side
and passing under the eyeball is inserted on the outer side.
Its contraction rotates the eyeball outward. The superior
oblique has its origin at the back of the orbit like the rectus
muscles, passes forward just above the superior rectus and
its tendinous end, after first passing through a loop or pulley
formed by a fibro-cartilage on the upper edge of the orbit,
turns outward over the top of the eyeball, and is inserted
in the eyeball at a point on the inner side. By this arrange-
ment the contraction of a straight muscle rolls the eye
inward.
The combined action of these six muscles is responsible
for all the movements of the eyeball. The muscles of the
two eyeballs contract and relax in pairs, the internal rec-
tus of one eye contracting at the same time as the external
rectus of the other, and similarly for all the others. In this
■way, both eyes are turned in the same direction at the same
time.
As has been already mentioned, the eyelids are closed by
the orbicularis muscle. The upper lids, however, are pro-
vided with elevator muscles attached to the back of the
orbit, and these muscles pull the upper lids open farther
than would be possible by the extension of the orbicularis
alone.
Structure of the eyeball.— The eyeball itself, when de-
tached from its supports, is a nearly spherical sac of about
an inch in diameter. It is covered, except at the points
where the optic nerve enters and in front, with an opaque456
THE SPECIAL SENSES
white coat known as the white of the eye or sclerotic coat.
In this layer are the blood vessels which supply the coat with
Fig. 215 — Section of the eyeball; Sc, sclerotic coat; Can, conjunctiva; C, cornea;
/, iris; 0, optic nerve; Ch, choroid layer; C.P, ciliary processes; Cm, ciliary
muscles and ligament; R, retina; L, crystalline lens; A, aqueous humor; Vt
vitreous humor.
nourishment. At the front this layer is replaced by the
transparent layer of mucous membrane (the infolded con-
junctiva) and a transparent modification of the white of the
eye called the cornea. Back of this cornea is a colored,
circular, muscular curtain called the iris.
If we split the eyeball vertically so as to split the optic
nerve it shows the following internal structure. The
conjunctiva in front folded back from the eyelids. Over
the whole surface, and covering the outside of the optic!
nerve, is the sclerotic layer just mentioned, with its trans-
parent window or cornea in the front. Inside the scleroticSTRUCTURE OF THE EYEBALL
457
is a second layer, called the choroid, composed of loose con-
nective tissue, and colored black by pigment particles
similar to those in the skin. At the front where the cornea
joins the sclerotic this middle layer is thrown into plaits
(iciliary processes) beyond which it is continued as a
muscular curtain (the iris). This iris is composed of cir-
cular and radial unstriped muscle fibers, is colored by
pigment, and has a circular aperture in the center (the
pupil). At its outer edge it is firmly united with the
sclerotic layer by a ligament called the ciliary ligament,
and to the ciliary process of the choroid. The contraction
of its circular fibers decreases the size of the pupil, while
the contraction of the radial fibers reverses this action.
By means of this variation in the pupil, the amount of
light which enters the eye is kept under control.
The third layer of the eye is an extension of the optic
nerve called the retina. It consists of the endings of the
optic nerve fibers and a supporting layer of connective
tissue. The entire layer is very thin and lines the whole
interior of the eyeball with the exception of the front,
where it stops at the edge of the ciliary process. This
retina is the sensitive layer of the eye and the one in which
the visual impulses originate.
Just behind the iris is a biconvex, transparent body,
called the crystalline lens. The front surface of this lens
is slightly flatter than the back and is in contact with the
inner surface of the iris. Between it and the cornea is a
space which is filled with transparent watery liquid called
the aqueous humor. The entire cavity of the eyeball back
of this lens is filled with a glassy semi-solid mass called the
vitreous humor. The lens itself is surrounded by a thin
membranous capsule, and is kept in place by a circular458
THE SPECIAL SENSES
band (the suspensory ligament) which attaches the edges
of the capsule to the ciliary process of the choroid layer.
Between the ligament and the edge of the choroid layer arc
a series of muscles (the ciliary muscles). When these
muscles contract they draw forward the edge of the choroid
layer, thus causing the suspensory ligament to relax. This
contraction permits the lens to shorten its diameter and
become thicker and more bulging. In this way the shape
of the lens may be changed, and this affects its image-
forming power.
Functions of the various eye structures.—The human
eye is constructed essentially like a camera, and we can
Fig. 216—An eye model.
obtain an idea of the function of its parts by study of the
parts of a camera or eye model such as shown in Figure
216. This shows a box painted black on the inside and
having at the front a small aperture into which may be
fitted perforated cardboard disks of different sized aper-
tures. Inside the box is a biconvex lens of glass so
mounted as to bring its center opposite the center of the
cardboard aperture, and to be capable of movement for-
ward and backward. It also contains a ground glass
screen similarly mounted, and adjustable to forward andFUNCTIONS OF THE VARIOUS EYE STRUCTURES 459
backward movement. If, now, we place this box with the
aperture pointing toward an illuminated object and move
the lens backward or forward, wc find that for a certain
position of the lens there appears on the ground glass
screen an inverted picture of that object. In other words,
this combination of lens, screen, and aperture, with its
light-excluding walls enables us to project inverted images
of external objects upon the screen. If we should replace
the screen in this apparatus by a sensitive photographic
plate we should obtain a permanent impression of the
image or picture. Our box would then fulfil all the func-
tions of a camera.
Now, the human eye is essentially such a sort of image
producer as this box or camera. The only difference is
that, instead of a sensitive photographic plate capable of
one exposure and producing one picture, we have a ner-
vous structure, the retina, which receives these impressions
and transmits them to the brain as nerve impulses. This
retina is always ready to receive impressions; the human
camera is always “ loaded.” But the resemblance to the
camera does not stop with the retina. The pupil of the
eye with its iris serves to regulate the amount of light
which shall enter, just as the cardboard disks or stops of
a camera do. The sclerotic layer corresponds to the
wooden or metal walls of the box, and like those gives shape
and strength to the structure.- The black, inner choroid
coat, by means of its pigment, absorbs light, and prevents
internal reflection, just as does the black paint on the
inside of the camera box. Finally, the cornea and lens cor-
respond to the lens of the camera, and, like that, produce
inverted images upon the sensitive surface or retina.
The points of difference between the eye and a camera460
THE SPECIAL. SENSES
may be noted as follows: First, the retina and lens of the
eye are fixed in position. The external aperture is pro-
tected by the transparent cornea and thus makes the
eye dust-proof. The air is replaced by the transparent
aqueous and vitreous humors, but this does not affect the
comparison, since these humors are transparent. They are
made necessary by the fact that layers of the eye are not
stiff like the wood of the box and would collapse were it
not for the presence of these fluids. Finally, the lens
and retina are fixed in position, and the eye, therefore, must
resort to special means for producing pictures of near and
distant objects and bringing them all to focus upon the
retina. To understand this last process it is necessary that
we first understand how a lens produces images.
rays distributed all over the receiving over the receiving Surface,
sul£ace'	and the total effect is a
jumbling together of all these images so that it is im-
possible to detect anything but a mass of light. (See
Fig. 217 — Scattered images produced by
How pictures are formed
by lenses. — Every point
of a luminous body trans-
mits light in the form of
rays. These rays pass
from the luminous body
in straight lines and in
every direction. Wherever
a ray of light strikes it
produces an image of the
point from which it came.
In the ordinary course of
light rays, therefore,
images are formed allHOW PICTUKES AEE FOKMED BY LENSES -161
Fig. 217.) Now, each ray is supposed to be made up of
vibrations of a substance which fills all space and is called
ether. The rate at which this ether vibrates determines
the color of the light ray. These vibrating lines or rays
when they pass from one transparent substance into
another of different density are bent in definite directions,
the shape and density of the transparent body determin-
ing the direction of bending. Glass structures called
lenses are simply masses of transparent matter so shaped
as to control the direction of the bending in rays which
pass from air into them. By means of them it is possible
Fig. 218—Effect of lenses on rays of light; A, a concave lens spreading the rays;
B, a convex lens bending the rays to a point.
to concentrate the rays which pass through them (con-
vex lenses) or to spread the rays (concave lenses). (See
Fig. 218.)
The point to v'hich rays are converged by a convex lens,
or from which they seem to be spread by a concave lens,
is called the focus of those rays and the action of the
lens is called focussing. By its shape, a convex lens is
able to focus the rays which it receives from a luminous
body, so that the rays, which come from a certain point on
the object shall, after passing through it, all converge at a
certain spot. In this vcay the lens is able so to collect the462
THE SPECIAL SENSES
scattered rays of a luminous body that a sharp but in-
verted image of the object is thrown upon any screen
placed at the point of convergence. (See Fig. 219.) In
Fig. 219—Convex lens of eye forming an image by converging the rays on the
retina.
a camera a glass lens serves this purpose. In the eye, the
cornea and crystalline lens accomplish the same result.
Accommodation.— In a camera with a lens of a definite
shape the images formed by the converging rays are
focussed at different distances back of the lens according
as the objects which produce the rays are near to or far
from the lens. To secure a sharp image for any given
object such instruments are provided with mechanical
adjustments by means of which either the lens or the
screen may be moved until the distance between them is
such that the screen is exactly at the point of convergence
of the rays. Such an adjustment is called accommodation.
This accommodation to distant and near objects may also
be secured by varying the thickness of the lens, and in the
eye it is this latter method that is employed. As has
been seen, the cornea and retina (screen) are fixed. By
means of the ciliary muscles and suspensory ligament,EFFECT OF THE IMAGES UPON THE RETINA 463
however, the shape of the crystalline lens may be so
changed as to secure this accommodation, and the invol-
untary movements of these muscles en-
ables the human eye to accommodate itself
automatical!}'- to near and distant objects.
(See Fig. 220.)
The eye a self-supporting camera. — In
all physical details, then, the eye acts as a
camera, and forms inverted images of objects
upon the surface of the retina, exactly as the
camera does upon the sensitive plate. Being
living tissue, however, it requires nourish-
ment for its cells, and hence the choroid
layer not only serves by its color to pre-
vent internal reflection but through its
nourishes the tissue cells of the inner eye.
Fig. 220 — Dia-
gram to illus-
trate changes
of the lens in
accomodation.
blood vessels
The sclerotic
layer also contains blood vessels which supply it with
food.
Effect of the images upon the retina.— The actual sen-
sation of sight is located not in the eye but in the occipital
lobes of the cerebral hemispheres where the visual impulses
of the optic nerves are ultimately received. The optic
nerve fibers in fact originate in these regions, passing from
here to the anterior corpora quadrigemina (see Fig. 184,
page 396), and are continued from here to a junction called
the optic commissure. At this point many of the fibers
cross, thus connecting the left eye with the right hemisphere,
and vice versa. Some, however, do not cross, so that if the
center of vision in one of the hemispheres is destroyed, there
will still be sight in both eyes. This crossing of the main
fibers is called the optic chiasma. After passing through
it the fibers inclosed in two right and left trunks (the optic464
THE SPECIAL SENSES
nerves) pass to the back of the orbits,
eyeball, and, spreading out their fibers,
where they enter the
have their endings in
the end organs of the
retina. The projec-
tion of an image upon
the retina stimulates
these end organs and
thus arouses the im-
pulses of vision.
How the brain cells
interpret the impul-
ses produced by an
inverted image into
a sensation of sight
is unknown.
Structure of the
retina. —The optic
nerve fibers enter
the retina on its
anterior surface.
Near this surface
the fibers connect
with the axis cylin-
ders of large gang-
Fig. 221 — A, Diagram of structure of the retina lioncells. These Cells
seen under tlie compound microscope; 2?, the .
essential nervous elements; C, the direction of	Oil OtnOF CVlin-
light stimuli; 1, internal membrane; 2, nerve	which. COMiect
fibers; 3, nerve cells; 4, terminal brushes of
neura; 5, inner nerre cell layer; 6, 2d set With. Other Cells (see
terminal brushes; 7, nerve cells; 8, external mem- -r-v <-)<y\ \ rl +1
brane; 9, rods and cones; 10, pigment layer.	^	)j	tliese
connecting cells and
cylinders finally terminate in modified nerve cells at the
very back of the retina next to the choroid layer. This lastPECULIARITIES OP SIGHT SENSATIONS
465
layer of cells has received from their shape the name of the
rods and cones. They are separated from the choroid
layer by a thin layer of pigment. It is these rods and cones
which the light vibrations of the projected image stimu-
late, and after passing through this complicated system of
cells the impulses finally enter the optic nerve fibers and
pass on to the brain. A peculiar feature of this arrange-
ment is seen in the fact that though the rays pass over the
connecting cells to reach the rods and cones they produce
no impulses until they reach this area. The rods and cones
are believed to convey different qualities of visual impulses.
The cones are believed to produce the impulses which give
color sensations, while the rods produce merely degrees of
light and darkness.
Peculiarities of the retina. — Not all parts of the retina are
equally sensitive to light. The region where the optic nerve enters
the eye is a little to one side of the middle line. This spot, of
course, has no rods and cones, and objects whose images fall upon
this spot are invisible. It is called, therefore, the blind spot. The
rest of the retina has a varying sensitiveness. That part of it
directly back of the pupil in the mid line, naturally receives con-
tinuously more rays than the other parts. At this point (see
Fig. 215) is a yellow spot about a quarter of an inch in diameter,
where the retina is thinnest, and the rods and cones most numer-
ous. This is the region of keenest vision.
Peculiarities of sight sensations. — Sight sensations are pro-
duced almost immediately upon turning the eyes toward an object,
but the retina has the power to retain images upon it for a short
interval (about one eighth of a second). Owing to this fact,
impulses which succeed one another rapidly give the effect of
continuous sensation. For example, falling raindrops appear as
lines, flames swung in a circle as continuous rings of light, and
moving pictures give the effect of a continuous movement. Owing
to this same cause, two revolving colors give a mixed sensation,
black and white producing gray, etc.
EDDY. PUYS. — 30466
THE SPECIAL SENSES
The sensitiveness of the retina is readily exhausted. Hence, if
we look at a bright light for a short time that portion of the retina
upon which the image falls soon becomes exhausted and loses its
power to transmit impulses. If, now, we turn the eye from the
light to a white surface, there appears to be a black spot on it, due
to the rays which fall upon the blinded spot, in the retina.
Color sensations are due mainly to a difference in the quality
of the impulses, and these differences are due to different rates
of vibration, each color having its own time of vibration. White
light is a mixture of the impulses of all colors, blackness the
absence of all light impulses There are also many different
shades of color in sensation which are produced by combining
impulses from two primary colors Thus, green is a mixture of
yellow and blue. Lack of ability to distinguish colors is called
color blindness. In persons thus afflicted the light waves of
several different colors may produce the same sensation in the
retina. This affliction is due to a defect in the cones of the
retina.
In binocular vision, images of the object are projected on both
the retinas, but the impulses thus set up and transmitted by the
two sets of nerve fibers give us only a single sensation in the brain.
This shows that the sensation of sight is not a simple action but
the result of association and fusing of impulses in the brain
cells.
The intensity of the illumination increases the intensity of the
stimulus and determines the brightness of the sensation. Very
strong light or very dim light tends to strain the eye and should
be avoided. Individuals differ among themselves and from the
lower animals in the amount of light necessary to arouse sensa-
tion. Cats, for example, are able to distinguish objects with an
illumination far less than that required by human eyes. Certain
animals, such as the owls, whose eyes respond to the weak light of
the night, become almost blind in the bright light of day. The
retinal end organs may be stimulated by other means than by
light waves. Blows on the eyes or head may produce sensations
of light, and we “ see stars,” as the saying goes. The shadows
cast by blood corpuscles of the eye blood vessels often modify the
effects of vision.NEARSIGHTEDNESS AND FARSIGHTEDNESS 467
Hygiene of the Eyes.
The eyes are extremely sensitive organs, and their satis-
factory working depends upon the health of the general sys-
tem. Poor circulation, which results in lack of nourishment,
as in disease, often affects the eyes. Biliousness often ren-
ders the eyeballs yellow and sore. Nervous diseases also
disturb their functions. Filth and dirt or rubbing the eyes
with dirty hands may often cause diseases of the eyeball
and lids. To remove the dirt which accumulates, the eyes
should be bathed as carefully as other parts of the body.
For this purpose warm or tepid water is better than cold.
Reading in a strong light or dim light or overuse tends
to strain the muscles which operate the lenses and eyeballs
and thus weaken them. Flickering light especially re-
quires overexertion on the part of these muscles in accom-
modation and should be avoided. Often the eye is defective
in structure, and unless this defect be corrected may react
upon other parts of the system producing severe headaches
and other nervous troubles.
All eye troubles may be classed as eye defects or eye
diseases. The most common defects of the eye are near-
sightedness, farsightedness, and astigmatism.
Nearsightedness and farsightedness. — In a normal eye images
of both near and distant objects are, through the accommodation
of the lens, projected exactly upon the surface of the retina. When,
however, the eyeball is too short or too long, that is, abnormal in
shape, the image instead of being formed on the retina is pro-
jected 'either in front or back of it, and the result upon the
vision is a blurring often accompanied by headache. When the
image is formed in front of the retina the defect is called near-
sightedness (myopia). The defect may be corrected by the
use of spectacles made with divergent lenses, which spread the468
THE SPECIAL SENSES
rays which enter the eye just enough to overcome this defect.
In farsightedness (hypermUropia) the image is projected back of
the retina, and convergent glasses are necessary to remedy this
difficulty. In both of these cases the amount of convergence or
Fig. 222—Position of retina in near and far sight; B, in natural sight; G, in far
sight; C, in near sight.
divergence exerted by the spectacle lenses should be only such as
to counterbalance the defect of the lens in this particular.
The selection of glasses, therefore, should always be the result of
prescription by an oculist who has determined by careful exam-
ination just how much correction is needed. Many eyes are ruined
by careless selection of glasses.
Astigmatism. — A very common defect in the eyes of children
and adults is an irregular curvature in the surfaces of the cornea
or lens, or in both. The result of this is that rays which pass
through the affected parts are brought to a focus at a different
point from those which pass through the unaffected parts. This
produces a blurred image on the retina, and indistinct vision.
The eye muscles become strained in the constant attempts they
make to change the focus of the lens and secure a clear image, and
this strain often results in chronic headache. This defect receives
the name of astigmatism. It can be corrected only by the use of
glasses whose surfaces are so formed as to correct these irregu-
larities in surface, and the selection of these glasses should be based
upon careful examination by an oculist.
Cataract. — Degeneration of the lenses at points renders them
opaque. This opacity, whether it occurs in spots or over the
whole surface of the lens or its inclosing capsule, is known as
cataract. The extent determines whether it produce partial orEYE DISEASES
469
total blindness. Removal of the eye lens completely, and the
wearing of special forms of spectacles, permits the spectacle lens
to act as a substitute for this eye lens and form images on the
surface of the retina as before. Since these glass lenses have no
power of accommodation, a different pair must be used for reading
and the examination of near objects than is used for distant
objects. They are of course a makeshift, but infinitely preferable
to blindness.
Presbyopia. — In old age the lenses become stiffened and do not
bulge enough. Hence convergent lenses are necessary, as in far-
sightedness. Such stiffening is called presbyopia.
Eye diseases. — Diseases of the eye include the formation of
ulcers or scars on the cornea, the failure of the muscles of the
iris to expand or contract, inflammation of the conjunctiva (pink
eye and granulated lids), and the wasting of the optic nerve.
Most are curable provided the optic nerve is not permanently
injured.XXX. THE SPECIAL SENSES (concluded).
Auditory Sensations.
As with visual sensations the sensory fibers of the nerves
of hearing have their endings in a pair of external organs
called the ears. By means of these ears the waves of sound
are made to produce impulses of hearing just as the eyes
transform waves of light into visual impulses.
Structure and form of the ears.— The ears are to exter-
nal appearance merely two twisted flaps of skin-covered
cartilage attached one on each side of the head. These
external flaps are, however, only the external parts of the
organ whose sensitive parts are embedded in cavities of
the temporal bones. For convenience in description, we
divide the ear into three parts, which we call the external
ear, the middle ear, and the inner ear. Since both ears are
FlG. 223—Diagram of the ear.
470THE MIDDLE EAR
471
exactly alike, a description of one applies in every particu-
lar to the other.
The external ear.— The oval, folded, and roughly funnel-
shaped external ear consists, as has been mentioned, of a
projecting structure of cartilage covered with a layer of
skin and blood vessels, and bound to the side of the head by
bands of muscle. These muscles in some individuals and
in many of the lower animals are capable of voluntary
movement. The irregular outre surface of this ear con-
verges near the center to form a funnel-shaped canal about
an inch long. This canal is lined with skin whose sebaceous
glands secrete a peculiar form of fluid which on exposure
loses its water and thickens into a bitter yellow wax (ear
■wax). This wax serves to keep moist a membrane stretched
across the bottom of the canal (called the tympanic mem-
brane) . This membrane is covered externally by a thin
layer of skin, and is extremely flexible. The external flap
collects the sound waves, acting in this respect like the horn
of an ear trumpet, and concentrates them upon this ear
drum which is thus set in vibration.
The middle ear.— The tympanic membrane forms the
external covering of a small cavity called the middle car,
or tympanum. This cavity is lined with mucous mem-
brane and filled with air. A narrow tube (Eustachian
tube) connects it with the pharynx. In swimming, water
often passes from the pharynx through this tube into the
middle ear and produces ringing in the cars, until it is
allowed to run out. On the inner surface of the cavity
are two small openings covered with membrane. One is
called, from its shape, the round window, or fenestra rotunda.
The other which is oval in shape is called the fenestra
ovalis. Between the tympanic membrane and the fen-472
THE SPECIAL SENSES
estra ovalis there is strung across the cavity a chain of
three bones called from their shape the hammer {malleus),
the anvil {incus), and the stirrup {stapes) bones. One end
of the malleus rests against the tympanic membrane, while
its other end rests against the incus. The incus, in turn,
articulates, at its farther end with the stapes, whose interior
end rests against the membrane of the fenestra ovalis.
These bones, together with the air, permit vibrations of
the tympanic membrane to be transmitted across the
middle ear to the membrane of the fenestra ovalis. The
Eustachian tubes play a part in this transmission in per-
mitting the escape of air into the throat when the pres-
sure becomes too great, and by admitting it return the
membrane to position. If it were not for this outlet a
sudden increase of pressure on the tympanum might burst
the drum or one of the inner membranes. Its action also
increases the sensitiveness of the ear drum to vibrations,
as is seen in the deafness
that results when this tube
is clogged, as by mucus in
a cold. The middle ear is
called the tympanum.
The inner ear. — The
fenestra ovalis and fenestra
rotunda are the openings
by which the tympanum
communicates with an ir-
regular cavity of the tem-
poral bone called the
osseous, or bony, labyrinth. This labyrinth and its contents
constitute the inner ear.
The entire labyrinth is lined with a membrane which
Fig. 224— Bony labyrinth.THE MEMBRANOUS LABYRINTH
473
secretes a fluid called the perilymph. It contains a closed
membranous sac, or capsule, called the membranous laby-
rinth. This sac fills almost completely the spaces of the
bony labyrinth. The bony labyrinth is usually divided
into three areas. The central part, about an eighth of an
inch in diameter, is called the vestibule. The part back of
this consists of three tubes called the semicircular canals.
The part in front of the vestibule is a single tube
coiled like a snail shell and called the cochlea. Into the
osseous labyrinth enter the branches of the eighth pair of
cranial nerves (auditory).
The bony labyrinth.— (See Fig. 224.) The vestibule lies
just back of the tympanum, and communicates with it by
the membrane-covered fenestra ovalis. The three semi-
circular canals lie toward the back of the head and com-
municate with the vestibule by five openings, two of the
canals uniting before entering the vestibule. The cochlea
lies forward of the vestibule. It is a spiral tube consisting
of two and a half turns about a central pillar. The large
end opens into the vestibule. It communicates with the
tympanum by the membrane-covered fenestra rotunda.
The membranous labyrinth.—This closed membra-
nous sac lies in the bony labyrinth surrounded by
the perilymph of that cavity. It conforms very closely
to it in shape. Internally it secretes a fluid called the
endolymph, which fills it and gives it shape. In
this liquid are tiny particles of lime called otoliths. The
membranous vestibule is divided into twm sacs, the utricu-
lus and the sacculus, connected by a small opening. The
utriculus is the larger and from it arise three membranous
semicircular canals. The sacculus is smaller and commun-
icates with the membranous cochlea. The membranous474
THE SPECIAL SENSES
semicircular canals are about one third the diameter of
the bony canals which they occupy. One of these is hori-
zontal when the body is upright. The others are vertical,
but at right angles to one another. All three have swel-
lings at the front where they join the utriculus, called
ampullae. At the back these swellings are absent, and the
vertical canals fuse before entering the utriculus. The three
ampulla? and the walls of the utriculus and sacculus are
supplied with branches of the auditory nerves which pene-
trate the membrane and attach it at these points to the
bony walls.
Into the bony cochlea projects from the central pillar a
thin shelf of bone dividing it partially into two sections.
Attached to the edge of this shelf is the membranous
cochlea, forming a triangular tube with the base of the
triangle against the opposite wall. It thus divides the
bony cochlea into three compartments. One of these
compartments (the vestibular passage) copimunicates
with the bony vestibule. Another, the tympanic passage,
has its base against the fenestra rotunda. The middle
passage, or membranous cochlea (sometimes called the
cochlear canal), communicates with the sacculus. The
vestibular and tympanic passages are filled with perilymph,
the cochlear canal with endolymph. Along a groove in
the bony shelf passes a branch of the auditory nerves
whose fibers penetrate the membranous cochlea and end
in special end organs known as the fibers of Corti. These
fibers are the seats of origin of sound impulses.
Nature of sound stimuli and path of sound in the ear. —
Just as light stimuli are produced by waves of vibrating
ether, so sounds are produced by waves of vibrating air.
Any vibrating body can produce these waves, and if theyTHE ORGANS OP CORTI AND SOUND IMPULSES 475
reach the ear they will stimulate the auditory nerve and
produce the sensation of sound. When these waves of air
reach the ear they are first collected by the external ear
and concentrated upon the tympanic membrane. This
membrane is thus set in vibration in unison with the vibrat-
ing body, which caused the waves. Its vibrations are in
turn communicated by the bones and air of the middle
ear to the fenestra ovalis of the inner ear. The mem-
brane covering this in turn sets in vibration the peri-
lymph which fills the spaces of the bony labyrinth and
thus distributes these vibrations to all parts of the inner
ear. The rotunda by bulging out when the ovalis moves
in, and vice versa, permits the vibration of the other-
wise incompressible perilymph. Finally, the vibrations
of the perilymph are communicated to the membranous
labyrinth and its endolymph and thus reach the fibers of
Corti where they start the auditory impulses.
The organs of Corti and sound impulses.— While the
auditory nerves send branches to all parts of the mem-
branous labyrinth, the impulses of hearing appear to
originate almost exclusively in the cochlear division of this
nerve. This division gives off many fibers which ter-
minate in the sensory cells of the organ of Corti.
This organ of Corti is located inside the membranous
cochlea. It consists of three parts, the rods, the sensory
cells, and the supporting cells. The arrangement of these
parts is seen in the accompanying figure. (See Fig. 225.)
Notice that the rods are arranged in pairs, three thousand
to five thousand in all, and are so constructed as to inclose
a passage, their bases being expanded while the bodies
lean toward one another. Against these rods on either
side lean the sensory cells with hairlike extensions reach-476
THE SPECIAL SENSES
ing into the endolymph. These latter cells receive the
terminal brushes of the fibers and are supported below
by the layer of cells called the supporting cells.
Fig. 225 — Diagram of cross section of a single coil of the cochlea.
The action of this organ is not fully understood. All
that can be stated definitely of the action and the manner
in which sound impulses are interpreted may be summarized
as follows:
Summary.— (a) The vibration of the perilymph being commu-
nicated to the endolymph through the membrane, sets up in it
similar vibrations. These vibrations, in turn, move the hairs of
the sensory cells and stimulate the fiber brushes, thus starting
impulses.THE MEMBRANOUS VESTIBULE AND THE CANALS 477
(6) Not all the hair or sensory cells are affected by external
vibrations. The region inclosed by the rods, and the membrane
upon which they rest, does not respond as a whole to the vibra-
tions of the perilymph, that is to say, when a particular vibration
is set up in the perilymph only certain regions of the rod in-
closed portion vibrate with it. As a result, only certain ones of
the sensory cells are stimulated, and it is supposed that ability of
the brain to distinguish different sounds depends upon the fibers
which are thus stimulated.
(c)	The character of the vibrations produced by a vibrating
body, and hence set up in the perilymph may vary in three ways,
in pitch or tone, in loudness, and in quality. These variations
are distinguishable by the brain through variations in the impulses
which the sensory cells transmit to it.
(d)	The final interpretation of sound impulses has been located
in certain convolutions of the temporal lobe of the cerebrum,
which is connected by association fibers with the medullary origin
of the auditory nerves. The exact area is much more difficult to
localize than the sight area.
(e)	The two organs of hearing produce in the brain not two
sensations but one. Their position on each side of the head aids,
however, in locating the source of vibrations.
(j) The sensory cells become fatigued by continuous stimula-
tion in much the same manner as do the retinal cells of the eye.
Functions of the membranous vestibule and the canals. —
Divisions of the auditory nerve supply the utriculus,
sacculus, and canals with fibers. The endolymph of these
parts is also continuous with that of the cochlear part, and
they are surrounded by the perilymph of the bony laby-
rinth. Further, the fibers of these divisions of the auditory
nerve end in the ampulla;, utriculus, and sacculus in
sensory hair cells similar to those of the cochlea. In spite,
however, of these similarities of equipment, these hair
cells do not appear to transmit ordinary sound impulses.
The present theory of the action of these parts may be478
THE SPECIAL SENSES
stated as follows: As the body or head changes position,
the endolymph contained in the various canals and the
sacs is subjected to pressure from different points. It
thus tends for a given position to press upon definite
sensory cells of the sacs and ampullae. This pressure is
transmitted to the brain as an impulse by the nerve fibers
attached to these cells, and these impulses enable the brain
to become conscious of the relative position of the body,
and thus to exert the necessary control upon the muscles
which coordinate movements. The sensitiveness of these
sensory cells to such pressure is increased by the presence
in the endolymph of the tiny particles of lime (otoliths)
which act upon the hairs. This sense of position is often
classified as a separate sense, though it is located in the
ear.
Hygiene of the Ear.
The ordinary injuries to the ear consist mainly of
damage to the outer or middle ear. The inner ear is so
well protected that it is almost impossible for external
causes to affect it. Among the injuries to the outer ear
which may be avoided by care is the over-accumulation of
ear wax or dirt on the tympanic membrane. Such accumu-
lation increases the thickness and destroys the sensitive-
ness of the membrane to vibrations. Ear wax allowed to
accumulate in this way often hardens, and thus produces
deafness. Washing the ear frequently with tepid water will
remove the cause and thus prevent this trouble.
Colds or throat troubles may cause stoppage of the
Eustachian tube by mucus or even inflammation of its
lining and that of the middle ear. This results some-
times in the formation of abscesses, which may be simplyHYGIENE OE THE EAE
479
painful, or, by weakening the membranes, may cause them
to break, and thus produce total deafness. Loud noises,
likewise, may cause too great pressure upon the drum,
while adenoids or throat growths often clog the Eustachian
tube and produce deafness.XXXI. SENSATION IN THE LOWER ANIMALS.1
Sensation may be defined as the perception of stimula-
tion. For example, when a hard body presses against our
finger it stimulates a nerve impulse which passes to the
brain and is there interpreted. As a result we perceive the
presence and action of the hard body. In the higher forms
of animals the seat of consciousness is the brain, and the
various organs where the sensory nerve fibers end are con-
nected by these fibers with the brain. It is to be noted
here that sensation does not take place necessarily when a
sensory fiber is stimulated. It is only when those im-
pulses reach the brain and are perceived that we have a
true sensation. In short, the seat of sensatibn is the brain,
and not the sense organ, and we must distinguish sharply
between mere stimulation of sensory impulses and their
perception by the brain as sensations.
Sensation in the protozoans.—It has been known for a long
time that the one-celled animals have in their protoplasm
a very sensitive substance which reacts to various external
stimuli. Whether these reactions indicate true sensation,
that is, whether they are accompanied by any consciousness
on the part of the animals or not, has caused much specu-
lation in the past. In attempting to answer this question
two theories have been advanced. One school holds that
these tiny organisms do actually possess the rudiments of
consciousness, and that their reactions to stimuli indicate
the true conscious choice on their part. The other school
1 See Footnote, p. 140, Chapter X.
480THE SENSE OF TOUCH
481
holds that these reactions are simply the results of physical
or chemical changes produced in the protoplasm by the
stimuli, and that the movement toward food, and away
from particles which are not food, can be fully explained by
the laws of physics or chemistry, and no more indicate con-
sciousness than movements of iron particles to a magnet
prove consciousness in the particles.
The latter school has conducted exhaustive experiments
to demonstrate its point and has proved pretty conclu-
sively that many of these responses to stimuli on the part of
protozoans are the result of the chemical or physical action
of the stimulus upon the protoplasm. Thus the move-
ment of an amceba toward a food particle they explain as
due to chemical attraction between the compounds in the
protoplasm and the food, and not to conscious choice of the
amoeba. Recent investigations (notably those of Professor
Jennings of the University of Pennsylvania) have shown
responses that are difficult to explain by these natural
laws and the question is still unsettled.
At any rate, these reactions to stimuli result in move-
ments beneficial to the organism, and hence, whether they
be indications of consciousness or not, they correspond to
the function of sensation in higher forms.
Development op the Sensory Organs.
The sense of touch.— This sense is found in all ani-
mal forms. In the ccelenterates it is confined to a few
sensory cells which are located in special regions of the
body. In forms higher than the coelenterates it is pretty
generally distributed over the whole body covering, though
some parts are more sensitive than others. In every case
EDDY. PHYS. — 31482
SENSATION IN THE LOWER ANIMALS
stimulus applied upon a given area of covering affects a
sensory cell, or cells which are themselves the termination
of touch nerve fibers and control the area stimulated.
Taste and smell.— The sense organs of taste and smell
are very difficult to distinguish in the lower forms. When
found they consist usually of pits or depressions in the
body covering, lined with sensory cells which are ciliated,
and are the terminations of taste or smell fibers. Often
the location of these pits is the sole indicator as to their
function.
In the higher forms of life these sensory cells are still
seen in the hair cells of the taste buds or olfactory buds.
They are, however, distributed in connection with different
organs with a view to putting them in the path of odors
or tastes. Thus, the ciliated taste buds of our tongue,
while very similar in structure to the olfactory cells, are in
a position to come in contact with food rather than odors,
while the cells of the nose are in the path of inspired air
which brings them odors rather than taste stimuli.
Sensitiveness of these taste or smell cells varies greatly
in different animals. Many animals, for example, can
detect the presence of an enemy or of food by smell alone,
when man would be utterly unaware of its presence.
Hearing.— In the lower forms, the organs of hearing
consist of vesicles or cavities which may be open to the
outside air or water or may be inclosed in the body cover-
ing. These vesicles are lined with sensory ‘ciliated cells,
and filled with a fluid in which float usually one or more
particles of limestone formation called otoliths. Each of
the sensory cells is the termination of an auditory nerve
fiber, and when a sound wave strikes the body it sets in
vibration the fluid and otoliths of these cavities. ThisVISION
483
movement on the part of the fluid and particles stimulates
the sensory cells by rubbing against the hairs, and thus
produces sensation. Such organs are called otocysts, and
arc to be compared with the vestibule and semicircular
canal structures of our ear rather than with the cochlea.
In the vertebrates all, except the lowest fishes, possess
a structure similar to the inner ear of man located usually
in the head. The external ear, however, is a structure
peculiar to mammals. The cochlea and canals of the
human ear are supposed to have developed from the
simple vesicle of the lower forms. It is supposed, for
example, that the vestibule, with its utriculus and sacculus,
corresponds to the original vesicle from which the canals
and cochlea have arisen as outgrowths. The arrange-
ment of the sensory cells, and the presence of otoliths in
these parts, is very suggestive of the otocyst of the lower
form.
Vision.—-The simplest form of a visual organ consists
of a pigment spot overlying a sensory cell which is the
termination of a nerve fiber. The pigment spot absorbs
the light and converts it into a stimulus to the sensory
cell. Such spots are found on the disks of the jelly-
fishes and in the ray tips of a starfish.
The next step is to cover the pigment spot with a trans-
parent thickening or lens. This lens by forming an
image permits the animal to distinguish form instead of
simply shapes of light and darkness as when the pigment
only is present. In the snail we have such simple eyes
or ocelli. Here the lens is in the form of a sphere covered
with a transparent cornea and its image is projected upon
a nervous network which we may consider as a simple
retina. Over this network is the pigment layer. In the484
SENSATION IN THE LOWER ANIMALS
snail these simple eyes are two in number and are located
at the ends of the tentacles. In the scallop they are
numerous and are located on the edges of the mantle.
The next step in advance is found in the insect. Here
thousands of ocelli are massed together side by side to
form what is called a compound eye. (See Fig. 226.)
Fig. 226—Compound eye of insect; A, section; a, optic ganglion; 6, secondary
nerves; c, retina; d, pigment layer; c, optic nerves; B, group of ocelli; J\ bulb of
optic nerve; g> pigment layer; h, vitreous humor; i, cornea.
Each ocellus points at a different angle and hence while
the whole eye is fixed the range of vision is extensive.
In such an eye each ocellus forms its own image independ-
ently of its neighbours. These images are all collected
by the brain into a single perception much as are the
images of our two eyes.
The nearest approach to the structure found in man and
in all other vertebrates among the invertebrate forms is theVISION
485
eye of the squid. Here we find a lens with a humor-filled
cavity, retina, and other layers much as in man. In the
squid eye, however, the rods are on the side of the retina
nearest the lens and they are supplied with optic fibers
from the back of the eye.
The eyes of the vertebrates are essentially alike in
structure. The principal differences are in the means
used to secure accommodation and range of vision.XXXIT. THE VOICE.
In the study of the ear we noted that sounds are pro-
duced by setting in vibration various objects, which
vibrations are propagated as waves of air. When the
air waves reach the ears they produce impulses of sound.
In our study of the respiratory system we noted that in
the human body the expired air was capable of producing
sounds and that the various modifications of these sounds
were called the voice. Combining these two observa-
tions we should expect to find in our bodies some struc-
ture in which vibrations of different pitch, loudness, and
Fig. 227 — The larynx; A, front view; B, side view; C, back view.
quality could be set up by expired air. Such an instru-
ment, in fact, is found in the.larynx or cartilage box at the
486 *STRUCTURE OF THE LARYNX
487
top of the trachea or windpipe. This box is prominent
externally as a projection which we call the Adam’s apple.
Structure of the larynx.— The larynx or voice box is
composed of cartilages, membranes, and muscles. The
front and sides are formed of a single large cartilage (the
thyroid). The curved front of this cartilage has a pro-
jection at the top like the lip of a pitcher (the Adam’s
apple). It curves around on each side in broad, flat
sheets, which end above and below as projecting points.
The two upper points are attached by ligaments to the
hyoid bone and the two lower projections fit into sockets
in a second cartilage called the cricoid.
This cricoid cartilage is a complete ring. The broad
part of this ring forms the back of the larynx, while the
narrow part encircles the front of the box just below the
thyroid. On the top of the broad part of the cricoid are
two triangular cartilages (the arytenoids) whose bases
form true joints with the cricoid and whose apices extend
upward. By means of the joints all these cartilages can
move freely upon one another. Beside these four hyaline
cartilages there are five yellow cartilage structures. One,
the epiglottis, is attached to the top and front of the
thyroid, and projects upward to form a lid for the box.
Two others form small horn-shaped projections on the top
of the arytenoids and the remaining two are inclosed in
folds of the membranous lining of the box.
The box is lined with a smooth layer of mucous mem-
brane. This layer is smooth, except at the point where the
glottis or pharynx opening is located. Here it is thrown
outward in two sets of ridges. The upper ridges are called
the false vocal cords. They have nothing to do with voice
production, but help to close the glottis. The two lower488
THE VOICE
ridges are made up mainly of elastic tissue running in the
membrane from the front angle of the thyroid cartilage
to their attachments at the base of the arytenoid cartilages.
These ridges are the true vocal cords. They have a
glistening white color and sharp smooth edges. In ordi-
Fig. 228—I, The vocal cords in the larynx as seen with larynxoscope; A, when
voice is being produced; B, when no voice is produced; e, epiglottis; co, vocal
cords; cvs, false vocal cords; a, s, w, elevations due to cartilages; /, root of
tongue. II, A, the cords alone when voice is being produced; B, vocal cords
when no voice is produced.
nary breathing they are separated, forming a V-shaped
opening to the glottis.
The arytenoids are united to one another and to the
cricoid cartilages by bands of muscle. By means of these
muscles they may be pulled forward, backward, or to one
side. These muscle movements affect the cords attached
to the arytenoids, pulling them apart, or by their back-
ward movement varying the cord tension.
Vocalizing.— In ordinary breathing the arytenoids areLAWS OF SOUND VIBRATION
489
drawn forward and to one side and the vocal cords are
thus separated. In this position they do not vibrate, and
hence produce no sound. In whispering they are in the
same position and the sounds produced are in this case
due entirely to the position of the lips and tongue.
In talking, shouting, singing, however, these cords are
drawrn inward and backward by the cartilage muscles, and
the expired air is thus forced through the narrower slit
between their edges. This sets the edges in vibration
producing sound. How this sound is varied in pitch,
quality, and loudness requires some knowledge of the laws
of vibrating bodies.
Laws of sound vibration. — The production of sound depends
upon two factors. The first is a vibrating body. The second is a
layer of air between this body and the sense organs which produce
the impulses. Under these conditions the vibrations of the body
are transmitted by the air in the form of waves. If the body
which vibrates does so regularly and continuously, a musical tone
is produced. If irregularly or at irregular intervals, the effect is
called noise. Sound sensations differ in three characteristics,
pitch or tone, loudness, and quality. This variation in sensation
finds its explanation in corresponding variations in the manner
of vibration of the sounding body.
(a)	Loudness. Of two vibrating bodies vibrating at the same
rate, the one whose vibrations are widest are most audible. Thus,
a body struck hard produces wider vibrations and hence a louder
sound than when struck lightly.
(b)	Pitch. The number of vjbrations a body makes in a given
time determines the pitch or tone of the sound. Thus, a tuning
fork whose prongs vibrate two hundred and fifty-six times a second
produces the pitch called middle C of the piano, while the next
note above it in the musical scale requires two hundred and
eighty-eight vibrations per second. Vibrating bodies differ in size
and elasticity, and this difference determines their rate of vibra-
tion. A large-sized string vibrates slower, and thus gives a lower490
THE VOICE
tone than a small one, while a tightly stretched string gives a
higher note than one of the same size but less taut.
(c) Quality. A piano string and a violin string may be of the
same size and tension and be struck with the same force. In
that event they will give the same pitch and have the same loud-
ness. We are still able, however, to detect that one is a violin
string and the other a piano string. This difference between
sound producers of the same pitch and loudness we call quality. It
is due to the fact that few bodies vibrate simply and as a whole.
Most bodies have not only a vibration as a whole which gives what
is called the fundamental note, but separate parts of the bodies
vibrate more or less independently producing distinct waves,
which combine with the fundamental and are called overtones.
No two bodies produce these overtones in the same way, owing to
differences in size, shape, or material, and hence the complex
tone produced is characteristic of each vibrating body and enables
us to distinguish between various sources of sound.
Resonance.— The quality of a sound may be changed and its
loudness increased by reenforcing the original vibration with
others of a similar pitch. Thus, a violin string stretched over a
sounding board sets the air of the sounding board in vibrations
of the same pitch, reenforcing and altering the quality and loud-
ness of the original string vibrations. Such reenforcement is
called resonance and the reenforcing body is called a resonator.
Application of sound laws to vocalizing.—The vocal cords
furnish the vibrating bodies which are struck or set in
vibration by the expired air. The air cavities of the
throat, pharynx, nasal cavity, and mouth are the
resonators.
By movements of the larynx cartilages the tension of
these cords can be changed, and thus the pitch is regulated.
The force with which the air is expired determines the
width of vibration and the loudness, while loudness and
the quality of the tones may be still further modified by
changing the shape of the resonating cavities.HYGIENE OF THE VOCAL ORGANS
491
Articulate speech is the result of voluntary changes in
these resonating cavities, and the organs most concerned in
producing the changes are the lips, tongue, and teeth.
Vowels are open-mouth sounds, with corresponding
changes in the lip and cheek positions. Consonants are
closed-mouth sounds, and tongue, lips, teeth, and cheeks
aid in their production. Harshness, softness, etc., are due
to differences in the quality of the vocal cord vibrations
and the character of the resonating cavities
In a child the resonating cavities and larynx are smaller
than in adults. At about thirteen to fifteen years of age
the larynx widens rapidly and the cords lengthen. The
result is a deepening of tone and a change of voice.
The vibrations of the cords are under voluntary control,
and it is possible by practice to control these vibrations
and the shape of the resonators so as to improve greatly
the clearness and quality of the voice.
Hygiene of the Vocal Organs.
Since the voice is under voluntary control it is important
that children be taught to control it properly. This teach-
ing must be mainly by example, and the best training is
secured by giving the child good examples to follow. Sing-
ing lessons are also valuable in teaching by exercise the
proper control of the vocal muscles, even though the child
never become a singer. Exercise which develops the lung
capacity also increases the volume and resonance of the
voice.
If the lining of the larynx becomes inflamed through
colds or irritation the vocal sounds are at once affected.
Similarly, inflammation of the walls of the resonators or air492
THE VOICE
passages reacts upon the voice. On that account colds of
all sorts manifest their presence almost immediately in the
quality of the vocal sounds. The ordinary precautions
against disease of these passages, therefore, act as pre-
ventives of vocal troubles.XXXIII. BACTERIA AND SANITATION.
Whenever moist foods, such as milk, eggs, meats, bread,
etc., are exposed to warm air for any length of time changes
take place in their composition which render them unfit for
food. These changes we designate as souring, fermentation,
decay, mold formation, etc., according to the character of
the change. Suclr changes are produced not by the action
of the air, but by the presence in the air of certain vegetable
organisms commonly called germs. Analysis of these
germs shows that there arc three forms of plants which may
cause changes in food, and that these three forms belong to
the classes yeasts, molds, or bacteria. While these three
forms of plants are very different in their appearance and
action, they show striking similarity in the manner in which
they bring about changes in foods. Of these three forms,
the bacteria are perhaps the most varied in their effect; and
since they illustrate the action of all, as well as being vitally
connected with the health anti disease of the human ani-
mal, we are justified in devoting part of our study of phy-
siology to the action of these plant forms.
Experimental study of bacteria. — (Sec Fix. LXXII.) If
we prepare three test tubes by placing in one chopped hay
and water, in another some milk, and in a third some wheat
and water, and allow all three to stand uncovered for some
time in a warm room, we may observe the following changes
in the contents of the tubes:
The hay solution becomes turbid, and a scum forms on its
surface, while from the mixture comes a disagreeable odor.
493494
BACTEEIA AND SANITATION
The milk gradually curdles and becomes sour to the
smell and taste.
The meat gives off fetid odors and changes in color
and appearance.
If, now, we mount some of the liquid from any one of
these three tubes on a slide under a compound microscope,
we find it swarming with curious-shaped little bodies myriad
in number and with sharply defined outlines. These bodies
are extremely small beings, only	zo hoo of an inch
in diameter. Examined with the highest powers of the
microscope they are seen to consist of a cell wall sur-
rounding a bit of protoplasm, while scattered through
the protoplasm are tiny particles which color more strongly
than the other parts, when stained, and are supposed to
correspond to the nuclear material of other cells. These
tiny bodies are bacteria.
Definition and forms of bacteria.— Study of preparations
such as the above, and of other sources, shows that there are
many different species of bacteria, all differing in shape and
action, but all showing the characteristic wall of cellulose
and the scattered nuclear material. It is this cellulose wall
that inclines us to place these forms among the plants. We
can, therefore, define bacteria as follows: One-celled vege-
table organisms having a definite unchanging cell wall and
containing nuclear material in the form of scattered frag-
ments which is never combined in the form of a single
nucleus. Certain forms show cilia and arc capable of move-
ment. Others move only as they are carried about by the
air or liquids in which they arc present.
In identifying and naming bacteria, use is made of the
unvarying character of the outline. Experience has also
shown that definite actions are always associated with defi-BACTERIA AS LIVING CELLS
495
nite shaped cells. Thus, bacillus tuberculosis is the name
given to the particular bacterium which causes tubercu-
losis. In this name the bacillus refers
to the shape of the cell (bacillus = rod
shaped), while the second name indi-
cates the specific action of this form.
Similarly, stophylococcus pyogenes
means spherical cells producing pus
formation. Spirillum volutans means
corkscrew-shaped cells which whirl in
their motion through liquids, etc.
(See Fig. 230).
Bacteria as living cells. — Bacteria
contain no chlorophyll (or green col-
oring matter) and, therefore, cannot
make starch. On this account they
must find their food ready made.
Living organisms which obtain their
food at the expense of other organ-
isms are called parasites, and the liv-
ing organisms upon which they live are called their hosts.
If, however, the source of food be no longer living, as was
the case of the chopped hay in the tube, the organism which
feeds upon it is called a saprophyte. Bacteria are all para-
sites or saprophytes. Those which take their food from
living cells, such as the diphtheria and consumption forms,
belong to the parasitic forms, while those which live upon
dead matter, such as dead animal or plant structures,
belong to the saprophytic forms.
Like all living cells, bacteria take in food and oxygen, and
give out wastes. They are rendered inactive by cold and
the presence of certain chemicals, while most of them are
Fig. 229 —Bacteria, highly
magnified; a, the germ
of typhoid fever, stained
to show the cilia; b, a
spiral ciliated form; c, a
rod-shaped form, in
chains; d, a spherical
form.— a, b, from Eng-
LEit and Prantl.496
BACTERIA AND SANITATION
killed by exposure to high temperature, such as the boiling-
point of water. They require water to develop in, and
when there is no water present they become inactive, and
in a dried condition may be easily borne about by the winds.
This drying, however, does not kill them, anti when trans-
planted to a moist place they become active again and
grow.
The most remarkable feature of their action is the ra-
pidity with which they reproduce. Like all single cells,
they reproduce by division. In bacteria, however, this pro-
cess of division is much simpler than in higher forms of
cells and is accompanied by no nuclear changes. When a
bacterium has reached a certain size the walls begin to fold
in at the middle, a partition forms, and the two halves break
apart to form two individuals. Each of these halves ab-
sorbs food until it becomes of normal size and then splits
again. In active forms this whole process may be repeated
as frequently as once in thirty minutes, and it has been
calculated that one bacterium can produce at this rate a
colony of over 200,000,000,000,000 in twenty-four hours.
It is this marvellous rate of development that makes the
introduction of a single disease causing bacterium into our
system so dangerous. A single diphtheria bacillus can
alone produce little damage, but if allowed to develop the
increase may become sufficient to cause serious damage in
a short time.
Occurrence of bacteria and infection.—Bacteria are found
in earth, air, and water, and in fact may be present prac-
tically everywhere on the earth’s surface. Certain condi-
tions are, however, more favorable to their development
than others, and fortunately the malignant forms arise only
under conditions which are subject to control.HOW BACTERIA PRODUCE DISEASES
49T
How bacteria produce diseases and other effects. — In the
act of obtaining their food bacteria may bring about chemi-
cal changes in the material from which they take it, thus
transforming that material into new and simpler com-
pounds.
Again, they may give off as wastes, materials which are
poisonous to the host upon which they live. Such poisons
are called toxins.
Finally, they may by rapid reproduction become so
numerous as to clog effectually the part of the host where
they are present, and thus interfere with the normal
action of that part.
All bacterial effects may be traced to one of these three
causes. For example, the souring of milk is brought about
as follows: The lactic bacteria found in milk break up the
sugar in the milk in the attempt to obtain food and trans-
form it into lactic acid. This acid causes the proteid part of
the milk to coagulate or curdle, and gives the sour taste and
odor. Again, the bacteria which collect in meat break up
the complex proteid matter into simpler compounds and
thus reduce it to simple mineral compounds. Both of
these examples illustrate the first mentioned effect of
bacteria.
The action of the diphtheria bacterium illustrates the
second method. In this case, the bacterium gives off a
toxin, or poison, which enters the blood and disturbs the
action of the organs of the body. In the case of diphtheria,
a substance has been found which if injected into the blood
destroys the effect of the toxin, and is called on that account
an anti-toxin.
The formation of tubercles by the massing of the tuber-
culosis forms in the lungs, thereby breaking down the
EDDY. PHYS. — 32498
BACTERIA AND SANITATION
tissues and clogging the air sacs, is an example of the third
method of action. Frequently a given bacterium combines
these methods of effect. In every case, however, the or-
ganism produces its effect through some result of its meta-
bolism, and these secondary effects, such as poisoning or
clogging, decay, and the like, are the result of the attempt of
one organism to take its food from another.
Beneficial bacterial action.— It must not be thought
that all bacteria are harmful. If it were not for decay
of waste materials, for example, the farmer would be un-
able to raise any crops. Decay in his case means the trans-
formation of dead plant and animal matter into simpler
compounds, such as are acceptable as food to growing
plants, and without bacteria the supply of such food in the
soil would be soon exhausted. Again, in the manufacture
of many dairy products, such as butter and cheese, the bac-
teria perform a very important part in the manufacture.
Certain forms of bacteria have also been found which, if
introduced into the soil with certain plants, can transform
the nitrogen of the air into soluble nitrates which the
plants can use, and such forms are extremely valuable.
Harmful bacterial action.— The forms of bacteria which
are most harmful to man are those which produce disease,
and the following list gives us some idea of the diseases
which are now known to be due to bacterial action:
Bubonic plague
Asiatic cholera
Fowl cholera
Hog cholera
Diphtheria	Lockjaw
Erysipelas	Consumption
Glanders	Typhoid fever
Boils and carbuncles
The forms of bacteria which are beneficial in causing de-
cay of waste material may be harmful to food supplies. TheMEDICAL METHODS
499
methods employed in the preservation of foods are devices
employed to prevent the action of bacteria upon food
stuffs.
Relation of business methods to bacteria.— In the para-
graphs on the action of bacteria as a living cell we noted
certain conditions which were favorable to their develop-
ment, and vice versa. The methods used in the preserva-
tion of food depend upon a proper use of the facts there
noted. For example, in cold storage methods we have the
application of the principle that bacteria are rendered in-
active by cold. Foods, therefore, will not decay so long as
they are kept sufficiently cold to secure this inactivity. We
also noted that bacteria need water to develop in. Dried
meats, therefore, and foods preserved in salt or sugar, either
are free from this necessary water or have the water re-
placed by solutions of salt or sugar, which are not favor-
able to the action of the plants. The packing of foods in
air-tight cans which have been submitted to temperatures
of boiling water meets two conditions unfavorable to bac-
teria. The heating of the food and can kills the germs that
might be present, while the sealing prevents the bacteria
in the ah from entering after the foods are cooled. Certain
chemicals also kill bacteria, and it has been customary in
the past to make use of some of these as preservatives of
food. This latter method should always be carefully in-
spected, since often the chemicals used may be in such
quantity and of such a nature as to be themselves dan-
gerous to health. The pure food law in some of its clauses
covers just this ground.
Medical methods.— The human body is extremely well
protected from disease-causing bacteria, since the only ave-
nue of approach to most of its organs is through the blood;500
BACTERIA AND SANITATION
and to get into the blood is a difficult matter for bacteria,
since the skin is of such structure as to prevent passage of
the germs. In the case of wounds or breaks in the skin,
these serve as excellent passages into the blood for bacteria.
The entrance of bacteria into the body is spoken of as in-
fection, and the part where they enter is said to be infected.
It becomes, therefore, extremely important that a wound
which is healing be kept free from these dangerous germs.
Medical men have discovered certain chemicals which
when applied to wounds do not prevent healing but at the
same time do prevent the development of bacteria. Such
substances are called antiseptics. All wounds should be
first washed as clean as possible with warm water and
then bandaged with one of these antiseptics. In this way
antiseptics such as 0.1 per cent and 0.05 per cent solutions
of carbolic acid and corrosive sublimate, may prevent such
serious complications as tetanus or lockjaw, and blood-
poisoning. It should be borne in mind that small wounds
are quite as dangerous as large ones in this respect and
should be treated as carefully.
In operations requiring the use of instruments everything
used must be freed from bacteria. The process usu-
ally consists in treating the instruments and clothing worn
as well as the hands to the action of various chemicals or
antiseptics. Thus, the instruments are frequently soaked
in solutions of carbolic acid or corrosive sublimate or are
submitted to the action of hot steam. Materials made
bacteria free in this manner arc said to be sterilized or dis-
infected. Both terms indicate the same result, but the
word disinfected is more frequently applied to the result of
chemical treatment, while sterilizing more often refers to
the action of heat. In removing a splinter with a needleSANITATION
501
a simple method of precaution consists in sterilizing the
needle by holding it in a flame for a moment. The ban-
dages used in binding up wounds should all be ‘sterilized in
some manner.
Disinfection.— While this term means to free from bac-
teria, it is frequently used to refer to the methods in which
objects arc freed from disease germs. After a patient has
been sick of diphtheria or other germ disease it becomes
necessary as a matter of safety to the community that all
clothing, furniture, and surroundings of the sick person be
freed of germs, since their presence there might mean their
introduction into some other person. There are various
methods of performing this disinfection, but one of. the
simplest consists in sealing up the room and burning sul-
phur in it. The fumes of burning sulphur are sufficient to
kill all forms of harmful bacteria. There arc other kinds
of disinfectants beside sulphur, and any form which kills
bacteria is entitled to the name. In use it is imperative
that the disinfectant be brought in direct contact with the
germs. The mere presence of a disinfectant in a room is
not a sufficient safeguard, and often disinfectants are sold
which give out an odor but do not necessarily come in con-
tact with the germs for a sufficient length of time to kill
them. It is quite as important to know how to use a dis-
infectant as to know its name. One of the best disinfec-
tants is plenty of bright sunlight.
Sanitation.
It is important that we know how to treat cases of
bacteria-caused disease. It is still better to prevent such
disease altogether. To a certain extent every one is respon-
sible for the proper care of his person and property. In a502
BACTERIA AND SANITATION
community, however, there are always some people who are
careless in this regard, and unless the enforcement of such
care be placed in the hands of specific authorities and
backed by legal penalties for disobedience the action of one
individual may result in great harm to the community.
The department of a government, either city, town, or state,
which has in charge the care of the public health is called
the Sanitary department. In a city the work of sanitation
is usually divided among several departments, for example,
The Board of Health, The Street Cleaning Department, etc.
The extent to which a city board of health looks after the
health of its citizens is seen in the following summary of
the duties of such a board:
Summary of duties of a board of health.— The enforce-
ment of all laws, ordinances, and regulations relating to
the safety of human life and the care and protection of
health.
Exclusive control of the treatment of infectious and
contagious diseases and of the public hospitals used for
such cases.
May cause unsafe or unsanitary buildings to be vacated.
May condemn and destroy buildings that are themselves
unfit for habitation or that make adjacent buildings unfit.
May destroy tainted or dangerous food.
May prohibit the sale of adulterated milk or other drinks
or foods.
May prescribe and enforce rules concerning the sanitation
of tenements, lodging houses, shops, and dwellings.
May be called upon to deal with sources of disease and
infection and to keep the streets and houses free from mat-
ter dangerous to health.
It must insure the pinity of the drinking water.CONCLUSION
503
Iu short, it has full power in any case where the action of
an individual tends to menace the health of the community.
Department of street cleaning.— This department is en-
trusted with the care of the streets and public highways of
the city, and the removal of garbage, rubbish etc., from
such streets.
Conclusion.— To attempt to indicate all the ways in
which a community cares for the health of its individuals
would be impossible in the limits of this book. It should
be borne in mind by every citizen, however, that unless he
himself takes an interest in maintaining his own health
such carelessness may react upon the community, and that
it is the duty of every individual to inform himself of the
laws which govern the healthy action of his own body.INDEX.
A.
Abdominal cavity, 109, 223.
Abducent nerve, 389.
Abomasum, 157.
Abscess, 207.
Absorption, 133.
definition of, 133.
hygiene of, 145.
of nutrients, 142.
summary of, 142.
Accommodation of eye, 402.
Acids, 23, 24.
Adam’s apple.
Adaptation of digestive organs,
154.
of limbs, 261.
of skulls, 262.
Adenoids, 335.
Adipose tissue, 44.
Adulteration, 6S.
Afferent nerves, 403.
Affinity, chemical, 13, 15.
Air, composition of, 20.
expired, 324, 325.
inspired, 324, 325.
Air breathers, 341.
Air sacs, 319.
Albumin, 241.
Albuminoids, 53, 241.
Alcohol:
as a beverage, 82.
as a drug, 82.
as a food, 75, 76, 77, 79, 85,
86.
as a fuel, 78.
as a producer of heat and
energy, 78, 80, 81, 293.
505
Alcohol —
as a solvent, 84, 141.
effect upon brain workers, 427.
effect upon circulation, 210,
211.
effect upon digestion, 130,131.
effect upon gastric juice, 130,
131.
effect upon muscular activity,
284, 292.
effect upon nerve control, 211.
effect upon nervous system,
427.
effect upon respiration, 338.
effect upon saliva, 131.
effect upon the heart, 211.
effect upon the intestines, 131.
effect upon the kidneys, 373.
effect upon time reactions,
426.
effect upon veins and arteries,
211.
narcotic or stimulant, 426.
percentage in beverages, 84.
percentage in patent medi-
cines, S4.
relation of study to physi-
ology, 75.
Alimentary canal, 52, 90.
functions of, 93.
in earthworm, 150.
regions of, 92, 93.
Alkalies in saponification, 126.
Amceba, circulation in, 212.
digestion of, 146, 147.
movement of, 298.
study of, 33.
Amoeboid movement, 297, 298.506
INDEX
Ampulla, 474.
Amylopsin, 125, 126.
Anabolism, 59, 352.
Anaerobia (footnote), 341.
Anatom y, 11.
Anemia, 16S.
Ankle, bones of, 233.
Ankylosis, 243.
of joints, 251.
Antiseptics, 500.
Anti-toxin, 337.
Anus, 129.
Anvil bone, 235.
Aorta, 186, 197, 199.
Aortic arches, 214.
Apoplexy, 39S.
Appendages of the body, 30.
Appendix, vermiform, 129.
Aqueduct or iter, 396.
Aqueous humor, 457.
Arachnoid membrane, 392.
Arbor vita?, 397.
Arms, bones of, 232.
Arterial blood, 187, 188.
Arteries, 186-189.
control, 205.
pulmonary, 185, 195.
pulse in, 186.
renal, 201, 367, 368.
structure, 1S9.
Arthropods:
body covering, 375.
circulation in, 212, 213.
excretion in, 381.
nervous system of, 433.
respiration in, 346.
Articulations, 244.
Arytenoid cartilages, 487.
Asphyxia, 332.
Assimilation, 175.
Astigmatism, 123.
Atlas vertebra, 225, 227.
Auditory sensation, 470-479.
Auricles, 182-184.
action of, 191, 192.
Auscultation, 193.
Autonomic, system, 421.
Axial skeleton, 224.
Axis cylinders, 47, 402.
Axis vertebra, 225-227.
B.
Bacteria, 493-503.
action of, 498.
as living cells, 495.
experimental study of, 493.
forms of, 494.
metabolism of, 496.
occurrence, 496.
relation to blood poisoning,
210.
relation to business methods,
499.
relation to disease, 497.
relation to fermentation in
large intestine, 130.
relation to inflammation, 207,
208.
relation to medical methods,
499.
relation to nervous diseases,
424.
relation to respiratory dis-
eases, 337.
reproduction of, 496.
size of, 494.
structure of, 494.
Ball and socket joints, 230, 231.
Bases, 23, 24, 25.
Baths and bathing, 363.
Beef extract, nutritive value of,
74.
Bellied muscles, 267.
Biceps muscle, 206, 209.
Bicuspid teeth, 95, 96.
Bile, 122, 124, 127.
Bile duct, 124.
Biliousness, 144.
Binocular vision, 466.
Biology, 10.INDEX
507
Birds, circulation in, 217.
digestive system of, 150.
heart of, 217, 218.
respiration in, 349.
skeleton of, 260.
Biuret test for proteid, 53.
Bladder, 367, 371.
Bleeding, 209.
Blind spot in eye, 465.
Blood, 159-179.
absorption into, 134, 135.
aerating of, 203, 327.
arterial, 1S7, 1SS, 203.
changes in lungs, 326.
clotting of, 161.
cold, 351.
composition of, 201, 202.
conditions affecting, 170.
experimental study of, 161.
extractives, 164.
flow, 191, 194-196, 204.
in other animals, 173.
in veins, arteries, and capil-
laries, 187, 188.
loading of, with nutrients,
201, 202.
loading of, with wastes, 202.
plasma, 161.
poisoning, 210, 365.
properties of, 161.
purifying, 203, 327.
quantity and distribution,
171.
relation to circulation, 159.
relation to respiration, 326.
structure, 160.
systems, 191.
temperature, 171.
unloading of nutrients from,
202.
unloading of wastes from, 203.
venous, 203.
warm, 351.
Blood corpuscles,
red, 160, 165, 167, 238.
white, 167.
Blood plates, 165, 169.
Blood proteids, 164.
Blood vessels, 159.
of the heart, 184.
of the skin, 356.
relation to alimentary canal,
130, 134.
See Arteries, Veins, Capillaries.
Blowing, 331.
Boards of Health, duties of, 502.
Body, parts of, 30.
Body cavity in earthworms, 150.
Body covering of lower animals,
374-376.
Body framework, 221.
Boils, 208.
Bones, 221-243.
composition of, 240.
diseases of, 248-249.
effects of age upon, 241.
forms of, 236, 237.
fractures of, 250.
functions of, 222.
regions of, 223.
structure, 42, 43, 237-243.
Bone cells, 243.
Border cells, 113.
Botany, 10.
Bow legs, 249.
Bowel movements, 145.
Brain:
development, 434.
diseases, 424.
external features of, 393.
forebrain, 388.
frog, 388.
hindbrain, 388.
human, 390.
internal features of, 395.
membranes, 390.
midbrain, 388.
structure, 392-397.
Breastbone, 223.
Breathing, 328.
abdominal, 328.
causes of, 322, 323.508
INDEX
Breathing—
costal, 328.
habits, 328.
hygiene of, 331.
modifications of, 341.
mouth, 314.
movements, 320.
rate of, 324.
relation of sex to, 324.
Bright’s disease, 373.
Broken bones, 250.
Bronchi, 315.
Bronchial tubes, 316.
Bronchitis, 208, 336.
Buccal cavit3r, 93.
Bunions, 249.
Burning (combustion), 19.
Burns and blisters, 365.
C.
Caecum, 12S.
Csecapylonic in fish, 153.
Calcium, 13, 26.
Calices of kidney, 368.
Canals, semicircular, 473, 477.
Canalicule, 43, 241.
Canines (teeth), 95, 96.
Capacity of lungs, 329.
vital, 329.
Capillaries, 186, 1S7, 188.
in the lungs, 319.
seat of absorption, 134.
structure of, 189.
Capsules (bony), 42.
Carbohaemoglobin, 166.
Carbohydrates, 26, 53.
as fuel, 60.
digestion of, 132.
Carbohydrates in muscle, 276.
oxidation of, 27.
Carbon, 16, 25, 26.
Carbon dioxide, 16, 26.
excretion bv skin, 362.
See Respiration.
Cardiac cycle, 192.
Cardiac muscles, 266, 275.
Cardiac nerves, 205.
Cardiac orifice (of stomach),
111.
Carnivora, teeth of, 155.
Carotid artery, 199.
Carpals (bones), 232.
Cartilage, a form of connec-
tive tissue, 42.
articulations, 227, 240.
in bones, 222, 240.
Casein and caseinogcn, 26,
115.
Cataract, 46S.
Catarrh, 208.
Cavities, body, 29.
Cells, air, 319.
auditory, 476.
defined, 33.
Cells, forms of, 31.
in tissues, 31.
muscle, 282, 283, 2S4.
nerve, 47, 405, 406.
smell, 449.
structural units, 32.
structure of, 32.
taste, 446.
Central nervous system, 385.
Centrum (of vertebra;), 225,
226.
Cerebellum, 393.
Cerebellum, function of, 418.
Cerebral hemispheres, 392.
distribution of gray matter
in, 398.
Cerebral localization, 419.
Cerebro-spinal fluid, 398.
Cerebrum, 392, 393.
functions of, 418.
Cervical vertebrae, 224, 226,
227.
Charcoal, 16.
Cheek bones, 234.
Chemical compounds defined,
14.INDEX
509
Chemical compounds, dis-
tinguished from mixtures,
14.
Chemical compounds in the
body, 25.
Chemical compounds, relation
to affinity, 13.
Chemical elements, 12,13, 28.
Chest cavity, 320.
Chiasma optic, 393.
Chief cells, 113.
Chocolate, 74.
Choking, 331.
Chordae tendinae, 1S5, 19.
Choroid layer of eyes, 457.
Choroid flexus, 398.
Chyle, 141.
Chyme, 116.
Cigarettes, 88.
Cilia in epithelial tissue, 41.
in hydra, 14S.
use in digestion, 147.
use in lungs, 318.
Ciliary ligament, 457.
Ciliary movement, 297-299.
in protozoans, 300, 301.
in higher forms, 302.
Ciliary muscles, 458.
Ciliary processes, 457.
Circulation of blood, 180, 211.
Circulation effects of alcohol
upon, 210.
in lower animals, 211, 219.
portal, 200.
pulmonary, 195.
renal, 201.
significance of, 218.
systemic, 197.
systems, defined, 180.
Circumvallate papillae, 445.
Clavicles, 229, 230.
Cloaca, 153, 381.
Clotting of blood, 161, 169.
Coal, 16.
Coccyx, 225.
Cochlea, 473.
Cocoa, 74.
Coelenterates, 341, 375, 429.
Coeliac axis, 199.
Coffee, 74.
Cold-blooded animals, 35.
Colds, 20S, 336.
Collar bone, 229, 230.
Colloids, 137.
Colon, 128, 129.
Color production, 466.
blindness, 466.
Combustion, 18,19.
in muscles, 282, 283.
Complexion, care of, 364.
Condiments, 73.
Cones and rods, 465.
Congestion, 206, 207.
Conjunctiva, 453, 456.
Connective tissue, 41, 42, 43.
Consciousness, 418.
Conservation of energy, 62.
Constipation, 145.
Contractility, 35.
as a cause of locomotion, 297.
of muscles, 28.
Cooking, 70.
Coral, skeleton of, 254.
Corium, 355.
Cornea, 456.
Corns, 366.
Coronary arteries, 186.
system, 184,199.
veins, 184.
Corpora quadrigemina, 397.
Corpora striata, 395.
Corpus callosum, 395.
Corpuscles, bone, 42.
red, blood, 160, 165, 167,
238.
white, blood, 167.
Cortex of brain, 405.
Cortex of kidney, 368.
Corti, fibers of, 474.
organs of, 475.
Costal cartilages, 229.
Coughing, 331.510
INDEX
Cranial bones, 233.
Cranial nerves, 388, 389, 394.
Cranium, 234, 390.
Cricoid cartilages, 487.
Crop, 151,156.
Croup, 337.
Crura cerebri, 393.
Crustacea, skeleton of, 256.
Crying, 331.
Crypts, 120.
Crystalloids, 137.
Cud, 156.
Cutaneous sensations, 43S, 439.
Cuticle, 354.
Cuts, 209, 365.
Cystic duct, 122.
Cytoplasm, 34.
D.
Defalcation, 144.
Deglutition, 107.
Delirium tremens, 42S.
Dendrites, 47.
Dental formula;, 96.
Dentine, 97.
Dentition, 96.
Dermis, 354-356.
Dextrose, 54.
Diabetes, 143.
Dialysis, 136, 13S.
Dialyzer, 136.
Diamond, 16.
Diaphragm, 109, 180, 316, 320,
321.'
Diastase, S3.
Diastole, 192.
Diet, daily, 70.
Digestibility of nutrients, 67.
relation to food value, 67.
Digestion, 90-132.
chemistry of, 99.
defined, 90.
extra cellular, 149.
hygiene of, 145.
Digestion —
in lower animals, Chapter X,
146.
of the mouth, 106.
of the large intestine, 128,
129.
of the small intestine, 118,
125-128.
of the stomach, 114.
peptic, 114-116.
summary of, 132.
Diphtheria, 20S, 337.
Disinfection, 500, 501.
Dislocation of joints, 250.
Distillation, S3.
Distilled liquors, 83.
Division of labor, 49.
Dorsal vertebra;, 224-226.
Ducts bile, 124.
cystic, 124.
thoracic, 135, 141.
Duodenum, 119.
Dura mater, 390.
Dust and dusting, 334.
E.
Ears, bones of, 235.
drum, 471.
external, 471.
hygiene of, 478.
inner, 472.
middle, 471.
passages of, 474.
structure and form, 470.
vestibule, 473.
wax, 471.
Earthworm, blood in, 215.
circulation of, 214.
digestion of, 149.
excretion in, 379.
Earthworm respiration in, 343.
Ectoderm, 375.
Ectoplasm, 34.
Elastic fiber, 41,XUDEX
511
Elbow joint, 232.
Elements, chemical, 12, 13,
28.
Emulsion, 127.
Enamel (tooth), 97.
Endoplasm, 34.
Endoskeleton, 221.
Energy, conservation of, 62.
definition of, 60.
kinetic, 62.
muscular, 282, 2S3.
potential, 61.
sources of, 59, 61.
transformation of, 61.
Enzymes, defined, 102.
in muscle, 276.
Epidermis, 353, 354.
absence of blood in, 305.
cells of, 354.
nerves of, 355.
Epiglottis, 108, 315, 487.
Epithelial tissue, 39, 40.
Erythroblasts, 168.
Esophagus, action of, 109.
structure of, 108.
Ethmoid bone, 234.
Eustachian tube, 107, 471, 472.
Evolution, 3S, 21.9.
Excretion, 352-373.
in lower animals, 374-382.
Exercise, forms of, 286.
relation to muscles, 285, 286.
relation to breathing, 329.
Exhaling, 319, 322, 323, 324.
Exoskeleton, 221.
Extractives, alcoholic, S4.
Eye, 452-469.
diseases, 469.
functions of, 458.
hygiene of, 467.
muscles of, 454.
position of, 452.
structure of, 452-455.
Eyeballs, 452-455.
Eyesockets, bones of, 235.
Eyelids, 453.
F.
Face bones, 233-235.
Facial nerve, 389.
Faeces, 129, 144.
Fangs (tooth), 97.
Far sight, 468.
Fatigue muscle, 285.
Fatigue, nerve, 423.
retinal, 466.
smell, 450.
Fats, 26, 27, 60.
absorption of, 140.
digestion of, 126, 127.
emulsification of, 127.
forms of, 55.
in bones, 241.
in muscle, 276.
saponification of, 126.
tests for, 55.
Fatty acids, 126.
Feathers a form of skin, 377.
Fehling’s solution, 54, 55.
Femur, 232.
Fenestra ovalis, 474.
rotunda, 471.
Fermentation, 82, S3, 102.
Fermented drinks, S3.
Ferments, 102.
Fibers, muscle, 302, 303.
nerve, 402, 404.
Fibrus and fibrinogen, 26, 162.
Fibula, 232.
Filiform papillre, 445.
Filtration, 139.
Fingers, bones of, 233.
Fissures of brain, 393.
Fish, bones of, 257.
circulation of, 215.
digestion of, 152.
gills of, 215, 348.
heart of, 215.
respiration of, 348.
Flagella, 301.
Flavors, 73.
Flat bones, 239.512
INDEX
Floating ribs, 229.
Fontanelle, 243.
Foods, 51, 89.
cost of, 68, 69.
composition, 56-59.
determination of fitness, 67.
preservation, 499.
values, 63.
Food accessories, 73.
Food stuffs, 53.
Food vacuoles, 147.
Foot, bones of, 233.
Foramen magnum, 234, 390.
Foramen Monroe, 395.
Forebrain, 388.
Fornix, 395.
Fractures, 250.
Freckles, 354.
Frog, bones of, 257, 258.
brain of, 3S7.
circulation of, 216.
digestion of, 153.
heart of, 216.
nerves of, 3S6, 387.
respiration of, 348.
Frontal bone, 234.
Fuel, 60, 282.
Fundus of stomach, 117.
Function defined, 8.
Fungiform papillae, 445.
G.
Gall bladder, 122.
Gases in the body, 25, 26.
Gastric glands, 113.
Gastric juice, 114.
effect of alcohol upon, 130,
131.
Gelatin, 26, 241.
Germicides, 210.
Germs, 493.
Gills, 343, 144, 346-348.
Gizzard, 151, 155.
Glands, 99-101.
in hydra, 149.
Glands —
lachrymal, 453.
lymphatic, 178.
oil (see Sebaceous).
salivary, 94, 102, 103, 104.
sebaceous, 358.
sweat, 357.
wax, 471.
Glomerulus of kidney, 370.
Glossopharyngeal nerve, 389.
Glottis, 107.
Glucose, 54.
Gluten, 52.
Glycerine, 126.
Glycogen, 27.
in liver, 142,143.'
in muscles, 276.
Grape sugar, 54,143.
Graphite, 16.
Gustatory sensations, 444.
H.
Habit, 413.
Haemocyanin, 344.
Haemoglobin, 165, 166.
Hair, 358.
care of, 364.
follicles, 359.
nerves of, 358.
papillae, 358.
Hammer bone, 225.
Hand, bones of, 233.
Hard palate, 93, 235.
Haversian canals, 43, 241, 242
Head, bones of, 233.
Headache cures, S9.
Health, Board of, 502.
Hearing, 470.
in lower animals, 482.
Heart beat, 183, 191, 204, 205.
muscle, 266, 275.
nerves, 205.
relation to lungs, 312, 316.
sounds, 192, 193.
structure, ISO, 183.INDEX
513
Heat generation, 14,19.
relation to oxidation, 19, 27.
sensation, 442.
Heel bone, 233.
Hepatic artery, 123.
cells, 124.
duct, 122.
vein, 123, 200.
Herbivora, 15S.
Hiccoughs, 332.
Hindbrain, 3S8.
Hinge joint, 245.
Hip bones, 232.
joint, 230, 231.
Histology, 9, 39.
Host, 495.
Humerus bone, 232.
Humor, aqueous, 457.
vitreous, 457.
Hunger, 117.
Hydrochloric acid, 113, 116.
Hydrogen, 15, 21, 22.
Hygiene, 8, 9, 10.
of bones and joints, 248.
of circulation, 206.
of digestion, 145.
of the ears, 47S.
of the eyes, 467.
of muscles, 285, 286.
of nervous system, 422.
of respiration, 330.
public, 501, 502.
Hyoid bone, 375.
Hypodermis, 375.
Hypoglossal nerve, 394.
I.
Ileocolic valve, 128.
Ileum, 119.
Iliac bones, 231.
Images, 460, 463.
Incisors (teeth), 95, 96.
Incus bone, 472.
Indigestion, 145.
Infection, 496.
Inflammation, 206, 208.
Infundibulum, 395.
Inhaling, 319, 322-324.
Inhibitory nerves, 205, 404.
Insects, circulation of, 212, 213.
excretion of, 381.
respiration of, 346.
Inspiration, 319, 322, 324.
Instep, bones of, 233.
Intercellular material, 44.
Intercostal muscles, 317.
Internal sensations, 444.
Intestine, small, 118, 119-121.
large, 128-130.
Intestinal juice, 121, 127, 128.
Intoxication, 424.
Invertose, 128.
Involuntary muscle, 266.
Iris, 456, 457.
Irritability, 35, 280.
Ischium, 231.
Iter, 396.
J.
Jaw bones, 235.
Jejunum, 119.
Joints, 244-251.
diseases of, 249, 250, 251
dislocations of, 250.
forms of, 244-248.
structure of, 245.
Jugular veins, 141, 200.
K.
Katabolism, 59, 352.
Keratin, 26.
Kidneys, 366-372.
of insects, 381.
of vertebrates, 38.
sugar, disposal of, 143
Knee joints, 232.514
INDEX
L.
Labyrinth of ear, 473.
Lachrymal bones, 235.
duct, 453.
glands, 453.
Lacing, 249.
Lacteals, 135, 141.
Lactose, 54.
Lacunas (bone), 42, 43, 241.
La grippe, 337.
Lamellae (bone), 43.
Large intestine, 128, 129.
Larynx, 315, 486, 4.87.
Lateral ventricles, 395.
Laughing, 331.
Leather, 355, 378.
Legs, bones of, 232.
Lens, action of, 460.
crystalline, 457.
Leucocytes, 16S, 207.
Levers, 288, 289.
Ligaments, function of, 245.
transverse, 227.
Light, effect of, 460, 461, 463.
nature of, 461.
Lime, in bones, 240.
Lime water and C02, 16.
Limula, 361.
Lipase, 126.
Litmus, 23.
Liver, as an absorbent organ,
142.
as an excretory organ, 142,
372.
as a storehouse of energy,
143.
structure of, 121-123.
veins of, 201.
Liver cells, 123.
Liver starch, 142.
Lobes, optic, 38S, 392.
Locomotion, causes of, 295.
due to contractility, 297.
due to secretion, 296.
due to specific gravity, 295.
Locomotion, organs of, 305-
309.
Loudness, 489.
Lumbar vertebrae, 225, 226,
227.
Lungs, blood changes in, 326.
capacity of, 329.
capillaries of, 319.
development, 348-350.
excretory action of, 353.
structure of, 312-319.
Lymph, 174-179.
capillaries, 177.
exchange, 175.
flow of, 177, 178.
formation of, 174.
glands, 178.
nodes, 178.
relation to absorption, 135,
175.
relation to assimilation, 175.
relation to osmosis, 175.
relation to plasma, 174.
tubes, 177.
valves, 177.
Lymphatics, in villi, 133, 134.
system of, 176.
Lymphocytes, 168.
M.
Malars (bones), 234.
Malleus (bone), 472.
Malpighi, capsules of, 369.
pyramids of, 369.
tubules of, 369.
Malt, 83.
Manuals, 21S, 261, 350.
Man, biological position of, 37.
Mandibles (bones), 235.
Manganese dioxide, 18.
Marrow (bone), 237.
Maxillae (bones), 235.
Meats, cooking of, 71.
Medulla oblongata, 394, 420.INDEX
515
Medullary layer of kidney, 369.
sheath of nerves, 402.
Membrane, mucous, 94, 314.
Membranes of brain and cord,
392, 400.
of tympanum, 472.
synovial, 246.
Mercuric oxide, 17.
Mesenteric arteries, 199.
Mesentery, 119.
Metabolism, 59, 352.
Metacarpals (bones), 232.
Metatarsals (bones), 232.
Metallic elements, 23.
Microbes (see Bacteria).
Midbrain, 388.
Milk, 57, 64, 248.
Milk glands, 377.
Milk teeth, 96.
Millon’s reagent, 53.
Minerals, 24, 25.
Mineral salts, 55, 56.
Mitral valves, 183, 186, 191.
Mixtures distinguished from
chemical compounds, 14.
Molars (teeth), 95, 96.
Mollusks, 255, 375, 430.
Motor nerves, 403.
Mouth digestion, 106.
Movement, amoeboid, 297, 298.
ciliary, 297, 299.
in lower animals, Chapter XX,
294.
muscular, 297.
Mucous membrane, 94, 314.
Mucus, 94.
Muscles, 265-293.
action of, 27S, 287, 288.
attachment to bones, 240,
268.
bellied, 267.
biceps, 206, 209.
comparison with engine, 281-
283.
composition of, 276, 277.
contraction of, 278, 281.
Muscles —
digastric, 268.
efficiency of, 287.
fatigue of, 285.
forms of, 267-269.
heart, 266.
involuntary, 265, 266.
nerve endings in, 279.
non-striated, 266, 274, 275.
papillary, 185.
rib, 317.
striated, 266, 267, 269.
triceps, 269.
voluntary, 265, 266, 274.
Muscle cells, 282, 283, 284.
Muscle fibers, 272, 273, 305.
fibrils, 274.
Muscular movements, 297-302,
303, 304.
Muscular tissue, 44, 265, 266.
Musical sounds, 489.
Myogen, 277.
Myopia, 467.
Myosin, 26, 276.
N.
Nails, 359, 360, 365.
Narcotics, 86.
Nasal (bones), 235.
Near sight, 467.
Nephridia, 379.
Nerve cells, 47, 405, 406.
Nerve control, 291.
Nerve diseases, 424.
Nerve endings in muscle, 279.
Nerve fibers, 402-404.
excitating, 404.
inhibitory, 404.
medullated, 403.
Nerve muscle preparation, 279.
Nerve processes, 47.
Nerves, afferent, 403.
cranial, 3S9.
dermal, 356.516
INDEX
Nerves —
efferent, 403.
epidermal, 354.
motor, 403.
sensory, 403.
vaso-motor, 205.
Nervous system, 382-428.
central, 385.
composition, 385.
diseases of, 424.
effects of alcohol upon, 424.
functions of, 411.
hygiene of, 422.
in lower animals, Chapter
XXVII, 429.
sympathetic, 385.
Nervous tissue, 46.
Neura, 47, 405, 406.
theory of action, 406-409.
Neural arch, 226.
Neural cavity, 225, 226, 397.
Neuralgia, 424.
Neurilemma, 403.
Neuron (see Neura).
Neutralization, 23, 24.
Nicotine, 87.
Nitrogen, 20.
Nitrogenous compounds, 26, 52.
Nodes of Ranvier, 402.
Non-metallic elements, 23.
Non-nitrogenous compounds,26.
Non-striated muscle, 44,45,266,
274, 275.
Nose, bones, 235.
Nose, sense organs of, 448.
Nucleus, 32, 34.
Nucleolus, 34.
Nutrients, 52.
absorption of, 139.
forms, 52,
in blood, 163.
in bone, 241. j
in foods, 57.
sources of, 56.
uses of, 59.
Nutrition, 51-72.
O.
Oblique muscles, 110.
Occipital bone, 234.
Ocelli, 483, 484.
Oculo-motor nerve, 388.
Odontoid, process, 227.
Odors, 450.
Olfactory lobes, 392, 393.
nerves, 388.
sensations, 448.
Omentum, 111.
Opium, 88, 89.
Optic chiasma, 393.
lobes, 388,392.
nerve, 388.
thalami, 396.
Orbit of eye.
Organic chemistry, 16.
compounds, 26.
Organs, 30, 31.
of Corti, 475.
Osmosis, 136, 138, 163.
Otocysts, 483.
Otoliths, 473.
Oxidation, 18,	19, 282 (see
Respiration).
Oxides, 19, 25.
Oxygen, 13, 17, 18, 19, 26 (see
Respiration).
Oxyhaemoglobin, 166.
P.
Pain, sensations, 443.
Palate, bones, 235.
hard, 93, 235.
soft, 93,
Pancreas, 124, 125.
Pancreatic juice, 125, 127.
Papillae, dermal, 355, 356.
hair, 358.
nail, 360.
taste, 445.
Papillary muscles, 185.INDEX
517
Paramoecium, 146, 147.
Parasite, 495.
Parietal bones, 234.
Parotid glands, 102.
Patent medicines, 64.
Pectoral girdle, 223, 230.
Pelvic girdle, 230, 231.
Pelvis, 223, 232.
of kidney, 368.
Pepsin, 113, 115.
Peptic digestion, 117.
Peptone, 115,137,140.
Pericardium, 181.
Perilymph, 473.
Perimysium, 270, 271.
Perineurium, 402.
Periosteum, 237, 240, 241.
Peristalsis, 109, 125.
Peritoneum, 109.
Perspiration, 357,362,363.
Phagocytes, 169.
Phalanges, 232.
Pharynx, 94, 107.
Phosphorus, 13.
Phrenology, 419.
Physiology, 7, 8, 9, 10.
Physiological chemistry, 10.
Pia mater 392.
Pigment cells, 354.
Pitch, 489.
Pivot joint, 247.
Plant physiology, 11.
as a source of nutrients, 58.
Plasma, 161.
Pleura, 319.
Pleurisy, 336.
Plexus, 404, 405.
Pneumonia, 208, 336.
Poisons, 144.
Pons varolii, 393.
Portal circulation, 200, 201.
Portal vein, 123, 134, 142.
Potassium chlorate, 17.
Premolars (teeth), 95, 96.
Presbyopia, 469.
Pressure sensations, 440, 441.
Proteid, 52.
absorption of, 138, 139.
action of trypsin upon, 126.
action of gastric juice upon,
114-116.
blood, 163, 164.
composition of, 26.
digestion of, 126, 114-116.
forms of, 26, 52.
muscle, 276.
sugar formers, 144.
tests for, 53.
Protoplasm, 32-36.
movement of, 294.
Protozoans, 252, 341, 374, 429,
480.
Psalterium, 157.
Pseudopodia, 35, 298.
Ptyalin, 105.
Pubis (bones), 231.
Pulmonary artery, 185, 195.
capillaries, 196.
sacs, 316-319.
veins, 185, 197.
Pulp cavity (tooth), 97.
Pulse, 193,' 194, 195.
Pupil of eye, 457.
Pure Food Law, 84.
Pus, 207.
Pyloric orifice, 111.
Pylorus, 112.
Pyramids of Malpighi, 369.
Q-
Quality of sound, 490.
R.
Radial artery? 199.
Radius bone, 232.
Rectum, 128.
Reflex action, 412.
arc, 413.518
INDEX
Renal artery, 201, 367, 368.
circulation, 201.
veins, 201, 367, 368.
Rennin, 113.
Reproduction of cells, 50.
Reptiles, 217, 259, 349.
Residual air, 329.
Resonance, 490.
Respiration	(see Breathing),
312-339.
artificial, 332, 333.
effects of alcohol upon, 338.
in lower animals, Chapter
XXII, 340.
organs of, 313, 314, 320, 324.
Respiratory organs, 313, 314.
diseases of, 335.
Rest, 423.
Reticulum, 157.
Retina, 457, 463-465. _
Ribs, action in respiration, 320.
cartilages of, 229.
floating, 229.
muscles of, 317.
processes, 229.
relation to thorax, 223.
types of bone, 238, 239.
Rickets, 248.
Rigor mortis, 276.
Rodents, teeth of, 155.
Rods and cones, 465.
Rods of Corti, 475.
Round shoulders, 249.
Rumen, 157.
Ruminants, 156, 157.
Running, 290, 291.
S.
Saccharose, 54.
Sacculus, 473.
Sacrum, 225, 231.
Saliva, action of, 105.
composition, 94, 104.
effect of alcohol upon, 131.
Salivary glands, 94, 102, 103,
104.
Salts, absorption of, 141.
blood, 163, 164.
characteristics, 23-25.
Sanitation, 10, 493, 501, 502.
Saponification, 126.
Saprophyte, 495.
Sarcolemma, 45, 272, 273, 274,
277.
Sarcoplasm, 45, 273.
Sarcostyles, 274.
Scapula, 230, 232.
Sclerotic layer of eye, 456.
Sebaceous glands, 356, 358,
471.
Secretion, 99.
a property of epithelium, 40.
as a cause of motion, 296.
Semicircular canals, 473, 477.
Semilunar valves, 185,186, 191,
192.
Selection of food, 36.
Sensation, 436-485.
auditory, 470.
classification of, 438.
cutaneous, 438.
gustatory, 444.
in lower animals, 480-485.
internal, 444.
olfactory, 448.
pain, 443.
pressure, 440.
temperature, 442.
visual, 452.
Senses, special {see Sensation).
Sensory organs, development of,
481.
Septa, lucida, 395.
Serous membrane, defined, 110.
Serum, 162, 163.
Sheath, medullary, 402.
Shoulder blades, 230.
Shoulder joints, 232.
Sighing, 332.
Sight sensation, 452, 465.INDEX
519
Sigmoid flexure, 128.
Silver sulphide, 15.
Skeleton, 221-251.
appendicular, 230.
axial, 224.
bones of, 222.
external, 29, 221, 252.
hygiene of, 248.
in lower animals, 252-263.
internal, 29. 221, 252.
Skin, 353-363'.
as an excretory organ, 361.
of lower animals, 374-382.
functions of, 361.
hygiene of, 363.
modifications of, 353-356.
structure of, 353.
Skull, 223, 233.
uses of, 235, 236.
Smell, sensation of, 448-450.
in lower animals, 482.
Snails, 345.
Sneezing, 331.
Soaps, 126, 140.
Sobbing, 331.
Sore throat, 208, 335.
Sound, 474, 489, 490.
Special senses (see Sensation).
Speech, 486.
Sphenoids, 234.
Sphincter pylorus, 112, 117.
Spinal accessory nerve, 394.
Spinal column, 222, 224, 228.
Spinal cord, 398.
center reflex action, 417.
connection with sympathetic
system, 401.
diseases of, 424.
functions of, 417.
membranes of, 399.
of the frog, 389, 390.
structure, 401.
Spinal nerves, 400, 401, 402.
Spinous process, 226.
Spiracles, insect, 346.
Spitting, 332.
Spleen, 168.
Sponges, 148,212,237,253,341,
378.
Sprains, 250.
Spyrogyra, 32.
Squameous epithelium, 40.
Standing, 290, 291.
Stapes (bone), 235, 472.
Starch, absorption, 140.
composition, 27, 53, 54.
digestion of, 105, 126.
in blood, 163.
Starfish, 254, 342.
Steapsin, 125, 126.
Sterilizing, 500.
Sternum, 228, 229.
Stimulants, 74.
Stirrup bones, 235.
Stomach, as a storehouse, 116.
curvatures of, 112.
enzymes of, 113.
glands of, 113.
method of action, 117.
structure, 111.
Strangulation, 331.
Striated muscle, 266, 267, 269,
270, 272-274.
Stroma, 166.
Structure, 8, 28.
Subcutaneous layer, 355.
Sublingual glands, 102.
Submaxillary glands, 102.
Submucous coat, 111.
Succus entericus, 127.
Suffocation, 331.
Sugar, composition, 27.
forms, 53-55.
in blood, 163, 164.
in muscle, 277.
tests for, 54, 55.
Sulphur, 15.
Suspensory ligament, 458.
Sutures, 235, 243, 247.
Swallowing, 107.
Sweat, 356.
Sweat glands, 356, 357.520
INDEX
Sympathetic nervous system,
421.
Symphyses, 243, 247.
Synovial fluid, 245, 246.
membrane, 246.
Systematic circulation, 198-200.
Systems, defined, 30.
Systole, 192.
T.
Tarsals (bones), 232.
Taste sensation, 444-447.
Taste buds, 446.
Taste in lower animals, 482.
Tea, 74.
Tears, 453.
Tear glands, 453.
Teeth, 94-96.
in lower forms, 152-155.
structure, 97.
use in digestion, 98.
Temperature, body, 19, 171,
172, 363.
sensations, 442.
Temporals (bones), 234.
Tendons, 238, 267, 271.
Theobromin, 74.
Thoracic duct, 135, 141.
Thorax, 223.
Thyroid cartilage, 487.
Thumb, bones of, 233.
Tibia (bone), 232.
Tidal air, 330.
Time reactions, 426.
Tissue, 31, 39, 4S.
Tobacco, 87, 88, 284.
Toes, 233.
Tongue, 94, 99, 444, 445.
Tonsils, 94.
Tonsilitis, 335.
Touch, 440, 481.
Tourniquet, 209.
Toxins, 497.
Trachea, 315, 317, 31S.
Trachete, insect, 346.
Triceps muscle, 269.
Tricuspid valves, 183,	185,
191.
Trigeminal nerve, 389.
Trochlear nerves, 389.
Trunk, 29.
Trypsin, 125, 126.
Tuberculosis, pulmonary, 337.
of the joints, 251.
Turbinate bones, 235.
Tympanum membrane, 471.
Tympanum, 471.
Typhlosole, 150.
U.
Ulna, 232.
Ulnar artery, 199.
Urea, 26, 352, 357, 362, 371,
372.
Ureter, 366.
Urethra, 367.
Urine, 371.
Urinary organs, 366.
Utriculus, 473.
Uvula, 94.
V.
Vacuoles, 34.
Vagus nerves, 389.
Valvulae conniventes, 120.
Vaso-motor nerves, 205.
Vegetables, cooking of, 71.
Veins, 186-190.
Vena cava, 184, 191, 197, 200.
Venous blood, 203.
Ventilation, 333.
Ventricles (heart), 183, 148,
191, 192.
Ventricles of the brain, 395,
396, 397.
Vermiform appendix, 129.INDEX
521
Vertebra, 222-240.
cervical, 224, 226, 227.
dorsal, 224, 225, 226.
lumbar, 225, 226, 227.
Vertebrates, 222, 252, 376, 433:
Vestibule of ear, 473.
Vestibular passage, 474, 476.
Villi, 120, 134, 135.
Viscera, 29.
Vision, 452.
in lower animals, 4S3.
Visual sensation, 452.
Vocal cords, 488.
organs, 486.
Vocalizing, 488.
Voice, 486-492.
Voice, hygiene of, 491.
Voluntary action, 412.
arc, 415.
muscle, 266.
Vomer, 235.
W.
Walking, 290, 291.
Warm blood, 351.
Water, absorption of, 141.
as a nutrient, 56.
breathers, 341.
composition, 22, 25.
Wax glands, 471.
Wind pipe, 315.
Wisdom teeth, 96.
Worms, 343, 375, 432.
Wrinkles, 356.
Wrist, 233.
X.
Xanthoproteic test, 53.
X-ray, 220-221.
Y.
Yawning, 332.
Yellow spot of eye, 465.
Z.
Zoology, 10.OUTLINES OF BOTANY
$1.00
By ROBERT GREENLEAF LEAVITT, A.M., of
the Ames Botanical Laboratory. Prepared at the request
of the Botanical Department of Harvard University
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Edition with Gray’s Manual of Botany.................................2.25
THIS book icovers the college entrance requirements in
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«U The work begins with the study of phanerogams, taking
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chapter of laboratory work, followed by a descriptive chapter.
Morphology is treated from the standpoint of physiology and
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AMERICAN BOOK COMPANY
(■74)A NEW ASTRONOMY
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(181)TEXT-BOOKS ON GEOLOGY
By JAMES D. DANA, LL.D., late Professor of Geology
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Geological Story Briefly Told .	............. .	. $1.15
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(161162")A BRIEF COURSE IN
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(3H312)Cornell University Library
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A text-book in general physiology and an
gy and a
3 1924 031 168 705
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