NUTRITIONAL
PHYSIOLOGY

BY

PERCY GOLDTHWAIT STILES

ASSISTANT PROFESSOR OF PHYSIOLOGY IN SIMMONS
COLLEGE ; INSTRUCTOR IN PHYSIOLOGY AND PERSONAL
HYGIENE IN THE MASSACHUSETTS INSTITUTE OF TECH-
NOLOGY, BOSTON

ILLUSTRATED

PHILADELPHIA AND LONDON

W. B. SAUNDERS COMPANY

OTIf

Copyright, 1912, by W. B. Saunders Company

Reprinted September, 1914

PRINTED IN AMERICA

PRESS OF

W. B. SAUNDERS COMPANY
PHILADELPHIA

TO

(Srabam Xusfc

WHOSE DELICATE KINDNESS NO LESS THAN HIS

SCHOLARLY POWER MADE MEMORABLE THE

YEAR OF OUR ASSOCIATION

299033

PREFACE

THE making of this book has been a study in elimination.
It is, therefore, necessary at the outset to indicate the
limits of its scope. It is intended that it shall be used
with other books. Supplementary reading upon general
biology, human anatomy, food chemistry, and dietetics
is greatly to be desired. In the field of physiology itself
many fascinating topics are entirely ignored and others
treated in bare outline, with the purpose of subordinating
all else to the subject of nutrition. Chemical formulae
have been excluded from the text and used but sparingly
in the notes.

A certain preliminary knowledge of elementary science
is assumed. The key-word of the following discussion is
" energy." The success of the reader in gaining clear con-
ceptions of what is presented will depend upon his famil-
iarity with the meaning of that term. It is essential that
he shall understand that energy is latent or potential in
those chemical compounds which are susceptible of oxida-
tion. He must have learned to recognize the possibility
of its unending transformation. The more readily he
thinks in terms of molecules, the more profitably he can
read these chapters.

Miss Ruth Bryant, Instructor in Biology in Simmons
College, has borne a part in the work, which is to be de-
scribed as collaboration rather than assistance.

P. G. S.

BOSTON, MASS.,
August, 1912.

5

TABLE OF CONTENTS

CHAPTER I PAGE

INTRODUCTION 11

CHAPTER II
THE ENERGY RELATIONS OF PLANTS AND ANIMALS 20

CHAPTER III

THE NATURE AND THE MEANS OF DIGESTION 28

CHAPTER IV
THE WORK OF MUSCLES AND GLANDS 36

CHAPTER V
REFLEX ACTION 46

CHAPTER VI
THE ALIMENTARY CANAL 54

CHAPTER VII

THE MOUTH — SWALLOWING, SALIVARY DIGESTION 63

CHAPTER VIII
THE MOVEMENTS OF THE STOMACH 70

CHAPTER IX
GASTRIC SECRETION AND DIGESTION '. . . 78

CHAPTER X

THE SMALL INTESTINE: ITS MOVEMENTS, SECRETIONS, AND
DIGESTIVE PROCESSES. .

8 TABLE OF CONTENTS

CHAPTER XI

F-AGE

THE LARGE INTESTINE -98

CHAPTER XII
THE BLOOD 105

CHAPTER XIII
THE CIRCULATION 116

CHAPTER XIV
THE ABSORPTION OF FOOD-STUFFS 128

CHAPTER XV
THE METABOLISM OF FATS AND CARBOHYDRATES 135

CHAPTER XVI

NITROGENOUS METABOLISM 147

CHAPTER XVII
THE REMOVAL OF THE END-PRODUCTS OF METABOLISM 160

CHAPTER XVIII
THE ESTIMATION OF METABOLISM 169

CHAPTER XIX
THE ENERGY OF THE METABOLISM 176

CHAPTER XX

THE FACTORS WHICH MODIFY METABOLISM 187

CHAPTER XXI
THE MAINTENANCE OF THE BODY TEMPERATURE 196

CHAPTER XXII
THE HYGIENE OF NUTRITION . . 204

TABLE OF CONTENTS 9

CHAPTER xxm

PAGE

THE HYGIENE OF NUTRITION (Continued) 217

Water; Meat; Sugar.

CHAPTER XXIV
ALCOHOL 227

CHAPTER XXV

INTERNAL SECRETION 238

CHAPTER XXVI
THE NERVOUS SYSTEM 245

CHAPTER XXVII

THE NERVOUS SYSTEM — ITS HIGHER WORK. . . 255

INDEX.. . 265

NUTRITIONAL PHYSIOLOGY

CHAPTER I
INTRODUCTION

The Modern Emphasis in Biology. — Living things are
transformers of matter and energy. When we say trans-
formers rather than generators we indicate the modern
as contrasted with the old-time view. When we say that
physiology is the physics and chemistry of living matter
we suggest the same significant tendency to bring living
and lifeless matter into direct comparison and to recog-
nize the same laws as operating in both. The teaching
that the same laws do hold sway in the living and the non-
living is covered by the term " mechanism "; the earlier
view that living things are not fairly to be compared with
lifeless, and are to some extent exempt from physical
principles and limitations, is expressed by the word
" vitalism." We have every reason to believe that the
principle of the conservation of energy holds as rigidly
for the plant or the animal as for the clock or the loco-
motive. This is perhaps the most important generaliza-
tion of nineteenth century physiology.

But while scientific workers are now seeking to analyze
the reactions of organisms in accordance with the data
furnished by chemists and physicists whose work has
been with materials not living, it is probable that the diffi-
culties of their problems are better appreciated than was
the case a few years ago. Living matter is found to be
more complex in structure and more varied in response

11

12

: j» $T R I T I Ols A L "Pi YSIQLOG Y

than had been supposed. Physiologists are bound to be
modest in their claims for progress. They are ignorant
of many factors at work in even the simplest forms of
plant and animal life. And the mystery of consciousness
with its relation to nervous systems seems ever to defy
approach.

Free-living Cells. — About seventy years ago, at a time
when investigators were profiting by important im-
provements in microscopes, it was found that the larger

Fig. 1. — Four types of free-living animal cells: A is the ameba,
distinguished for its changeable form; B, the euglena, shows the
peculiar feature known as a flagellum, a writhing filament, which
is its means of locomotion; C is the paramcecium, or " slipper ani-
malcule," which has a ciliated surface; D is the interesting form
known as the stentor.

plants and animals are made up of structural units as-
sembled in vast numbers. These units are generally
much too small to be seen without magnification. They
are called cells, a term which is not especially appropriate,
but not likely to be abandoned. Many microscopic
forms, such as the swarming Infusoria of pools and ditches,
are cells leading an independent existence. It will be
helpful to consider what are the characteristic activities
of such cells. They are for the most part equally charac-
teristic of the higher forms.

INTRODUCTION 13

The free-living animal cell takes something from its en-
vironment and returns something to it. It takes into it-
self a variety of organic substances together with small
quantities of mineral salts. These constitute its food.
It receives also a supply of oxygen. This is not ordinarily
reckoned as a food and for a good reason. The term food
is best restricted to material which can serve constructive
purposes or at least be stored in the cell. The function
of oxygen is not to promote constructive processes, but to
release energy, a process of decomposition in which the
stores of the cell are sacrificed. The process in which
oxygen reacts with substances within the cell, giving rise
to simple oxidized products in place of complex material
rich in potential energy, is called respiration. (The word
is, indeed, frequently used as a synonym for breathing,
but we shall use it in its chemical sense.) Respiration
is often compared with combustion, and while the two
are not identical in all their stages, the fact remains that
the initial and the final conditions are essentially the same
for both. The release of energy is generally just as great
in the physiologic change as in the actual burning of like
quantities of the cell constituents.

The free-living animal cell is thus an accumulator of
fuel and a furnace in which it is burnt. But this is a very
imperfect comparison, for it has in addition the property
of self-repair, and under favorable circumstances capacity
for growth and reproduction. Cells multiply by cleaving
into two similar parts, and the tendency to do this after
a certain increase in size usually limits very definitely the
dimensions to which a single cell may attain. When
growth is taking place it is evident that not all the food
is serving as fuel; a certain portion is becoming incor-
porated with the more permanent substance of the cell
and is so changed as to become entirely typical protoplasm.
The process through which food becomes an integral part
of the cell is called assimilation. The word emphasizes
through its root-meaning the attainment of likeness to
the material of the cell and indirectly implies that the food

14 NUTRITIONAL PHYSIOLOGY

was originally foreign in its nature. We use the expression
in much the same sense when we speak of the assimilation
of immigrant peoples. The word nutrition is used in
about the same way as assimilation, the only distinction
being that we speak of the nutrition of cells (or cell-
aggregates), while we speak of the assimilation of food,
the former term referring to the structure nourished and
the latter to the supplies worked over for the purpose.
The word digestion is best restricted to the preliminary
stages of the assimilation process; its application will be
defined later.

Respiration has been said to be a process in course of
which compounds are decomposed that their potential
energy may be made available. The greater part of the
released energy appears as heat. A smaller part mani-
fests itself in movements through which resistances are
overcome. The facts in regard to the production of
energy are naturally better known for the larger animals
than for the free-living cells, but " the whole is equal to
the sum of its parts." The energy from respiration may
in exceptional cases become kinetic, in the form of light
or electric discharge (firefly, electric eel). The energy
which shows itself in movement is of particular interest
to us. Motion, when exhibited by animal cells, is almost
always the expression of contraction, the word being used
in a physiologic sense. So used, contraction does not mean
diminution of volume, but does mean diminution of sur-
face and active shortening in one or more dimensions.
Although an increase in other dimensions attends such
changes of form, we do not talk of the " expansion " of
cells. It is the contraction which is the positive and
forcible element in the movement. When this is said we
intentionally leave out of account some types of movement
occurring commonly in the plant world, in course of which
the cells actually change their volume through gain or loss
of water. Among free-living cells the type of move-
ment may be " ameboid," that is, a flowing of the cell
contents to conform to an ever-changing outline. Con-

INTRODUCTION 15

tractile power may be limited in other cases to parts of
cells, as in those forms which swim by the lashing of
slender projections known as flagella, or by the waving
of an animated nap or pile upon their surfaces, the cilia,
of which more will be said. In all cases of energetic
movement we feel justified in assuming that the source
of the power, is in destructive chemical reactions, and
that a draft is being made upon the fuel stores of the
cells.

The Association of Cells. — When many cells are massed,
as in the body of a worm, the situation of the single unit
differs significantly from that of the cell leading an inde-
pendent existence. First of all, its environment is made
for it to a great extent by other cells. A very small
minority are in direct contact with the outside world;
the great majority are submerged among their fellows.
The typical cell is, therefore, shut in from food supplies
of the casual sort on which the free-living cell depends.
It is remote from the oxygen of the surrounding air or
water. A cell so situated would perish were it not for one
of the most striking features of the larger organisms, a
moving liquid medium, which bathes the cells and acts
as a common carrier. This fluid supplies food and oxygen
and removes wastes.

The cells composing the body of any animal are of a
common descent, but they have taken on widely different
characters and have become adapted to particular func-
tions. The cell which is in itself a complete living thing
must perform all the essential activities for itself — the
preparation of crude food, locomotion, etc. In the
multicellular animal the individual cells have come to be
far more restricted in their powers. Many have become
passive structures serving only for mechanical support or
surface protection. Such cells may or may not be living.
Others, while clearly alive, have ceased to perform cer-
tain functions. With limited exceptions movement is
exhibited only by those systematically arranged cells
which form the contractile tissues. Almost all the cells

16

NUTRITIONAL PHYSIOLOGY

require their food to be in solution and of a few standard
forms. In other words, the primitive capacity to digest
and assimilate every kind of nutriment has been lost, but
by a wonderful co-operative activity the internal medium
has been made a depot of those particular foods which can
still be utilized.

Fig. 2. — Drawings like the above are almost always made from
tissues which have been prepared and colored by special means to
make clear, minute features: a Represents an ovum or egg-cell, the
typical cell may be assumed to tend toward this spheric form; b is a
cell from a compact tissue, to show how mutual pressure produces
a faceted or polyhedral form; c is a contractile element such as
occurs in the walls of the alimentary canal, it illustrates an elon-
gated cell; d is an epithelial or lining cell of the order found on the
inner surface of blood-vessels; this is an example of extreme flatten-
ing; e, from the nervous system, exhibits the possibility of a branch-
ing development.

As an animal grows larger its directly exposed surface
becomes smaller in proportion to its weight. The trans-
fers which must take place between the organism and the
external world require ample surfaces, and they are secured
by infoldings of the body wall at different places. The
lining of the alimentary tract is an example of such an
infolding and provides a large area for absorption. Among
the higher forms the lining of the lungs constitutes a vastly

INTRODUCTION 17

extended surface for gaseous exchange. The glands are
organs in which are concealed great surfaces through
which products of cell activity find an outlet.

The specialization which groups the cells of the animal
body in a number of classes each with its definite work to
do also entails the dependence of each class upon the
others. If we compare the life of a savage with that of a
civilized man we shall find an analogy not too far fetched
to be helpful. We have seen that the free-living cell is
self-sufficient, and, indeed, its chances of survival are bet-
ter when there are but few of its own kind in the neighbor-
hood. Such organisms are competitive rather than co-
operative. Somewhat in the same way the solitary savage
may be capable of self-maintenance, having the skill to
find and prepare his food and after a fashion to shelter
and clothe himself. The civilized man is accustomed to
look to many other men — and women — to supply his needs.
Yet if the man, like the cell, loses something of ruggedness
and resourcefulness through becoming a member of a com-
plex society, he evidently gains time and opportunity to
concentrate his efforts upon a special pursuit. It is very
much the same with the cell. Our analogy fails, as such
devices are prone to do, when we consider how the civil-
ized man may become a hermit or a Robinson Crusoe,
whereas no single cell detached from one of the higher
forms can exist by itself for any length of time.

Co-ordination. — We have emphasized above the services
rendered to the organism by its internal medium. The
composition of the circulating fluid is influenced by the
ever-varying activities of all the organs and tissues.
Accordingly, a contribution made to this medium by
any group of cells may conceivably modify the conduct
of any other group. We shall meet with numerous
instances of such influence exerted through the chemical
products of one organ upon another or upon the system as
a whole. When we speak of an animal as an individual
we imply that the parts of the body constantly interact.
It is this interaction which makes it appropriate to regard
2

18 NUTRITIONAL PHYSIOLOGY

the body with all its complexity as still forming a unity
rather than a colony. The term co-ordination is employed
to express this purposeful working together of all parts
for the advantage of the whole. The means of co-ordina-
tion may be chemical as just now suggested, but a more
conspicuous agency in the highly developed types is that
of the nervous system. While we must postpone until
a later time any detailed description, we ought to indicate
at this point the essential contrast between the two modes
of co-ordination. One part of the body may affect an-
other through the actual despatch of material to it.
When the influence is through the nervous system instead
of through the circulation there is no transfer of material.
The nerves were once supposed to convey a fluid of strange
properties, but the fact is established that they transmit a
form of energy and not of matter. (The temptation to
think of the nerves as conductors of electric currents and
to compare the nervous mechanism with a telephone
system is strong. Guardedly used, the comparison is
valuable, but it is a symbolic rather than a literal repre-
sentation of the facts. " Nerve impulses," so called, are
not electric currents in the ordinary sense.)

In a great nation the prosperity of any section must de-
pend to a large degree upon the commercial exchanges
.taking place between that section and others. Its fac-
tories may depend upon the mines of another province
for coal and upon still another for raw material. Much in
the same way a single organ of the human body is de-
pendent upon others. A muscle, for instance, profits by
the prepared foods or fuels forwarded to it from the di-
gestive tract and by the oxygen borne to it from the lungs.
The blood in this case is serving the purpose which is ef-
fected by trains and steamers in the case of the nation.
Nor do we find lacking in our illustration an analogy for
the nervous communication between parts of the living
body. The type of such intercourse is furnished by the
telegraphic messages which pass incessantly from place
to place. News, in itself immaterial, may affect the course

INTRODUCTION 19

of local events just as surely and much more quickly than
can the material exports of another region. Reactions
produced through the nervous system are correspondingly
sharp and prompt in developing.

Blood and Lymph. — In the bodies of the higher animals
the internal medium may be described as existing in two
forms. In direct contact with the majority of the cells
there is a comparatively stagnant fluid, the lymph.
From this they draw their needful supplies of oxygen and

Fig. 3. — B is a blood-vessel of the smallest size — a capillary —
with walls of flattened cells like that in Fig. 2, d. The blood flow-
ing within is removed from direct contact with the cells (C, C), but
dissolved substances may pass from one to the other through the
capillary wall and the medium of the lymph (L).

food; into it they discharge their waste. The limited
resources of the lymph at a given point would be quickly
exhausted were it not that the blood is passing close by in
vessels whose delicate walls permit the passage of material
in both directions. The blood is in rapid movement and
it is constantly renewing the oxygen of the lymph with
fresh portions just brought from the lungs. It is at the
same time receiving from the lymph the accumulated
waste.

CHAPTER II

THE ENERGY RELATIONS OF PLANTS AND
ANIMALS

A CHEMICAL reaction can usually be assigned to one of
two classes. Either it is exothermic, that is, attended by
the evolution of heat, or it is endothermic, in which case
heat must be supplied to cause its occurrence. When
heat is evolved the products of the reaction are generally
simpler and more stable than the original material. The
most important reactions of this class are the oxidations.
Heat, or other forms of energy applied from without, may
effect the synthesis of complex substances rich in fuel
value from initial material comparatively destitute of
potential energy.

Broadly speaking, the constructive chemical processes
in nature are the work of the higher plants. Animals, as
well as those forms of plant life which lack pigment, carry
on for the most part reactions of a destructive character.
This makes evident the dependence of all other forms of
living matter upon the pigmented plants. It is through
the agency of light-waves, a form of kinetic energy, that
the synthetic reactions resulting in the formation of starch
and other energetic compounds are made possible. When
light is excluded from the green plant it has no advantage
over the animal, but pursues a similar course of decomposi-
tion. In fact, there is always going on in the plant,
even when its constructive activity is most marked, an
undercurrent of an opposite trend. Early writers com-
monly exaggerated the supposed contrast between the
chemical conduct of plants and that of animals. They
were disposed to deny that an animal could execute any
20

ENERGY RELATIONS OF PLANTS AND ANIMALS 21

synthesis whatever. It is true that animal cells make no
useful application of the energy showered upon them by
the sun's rays. Nevertheless they do carry on synthetic
reactions, although to a limited extent. Since much energy
is released within such cells by the prevailing oxidative
changes it is not difficult to see that some portion of it may
be applied to promote endothermic reactions. When a
hydraulic ram supplies with water a house considerably
above the level of the stream which operates the device,
we understand that the result is made possible because a
great deal of water falls that a little may rise. The prin-
ciple of the conservation of energy is not violated here,
nor is it when animal cells erect from a portion of their
food molecular structures more complex and energetic
than anything in their current supply. The formation of
fat from sugar is a case in point. Weight for weight, the
fat is more highly endowed with potential energy than is
the sugar, but we must take into account the fact that the
quantity of sugar entering into this common transforma-
tion is much greater than the quantity of fat which can
be produced. The energy in the product is more concen-
trated, but not absolutely larger in amount than it was in
the sugar.

There are many species of green plants which are uni-
cellular, just as there are many single-celled animal forms.
It is suggestive to consider the reciprocal relations of one
such plant and a solitary animal cell living beside it. A
constant and pressing need of the animal is oxygen. Now
oxygen is freely produced by plants when they avail
themselves of the energy of light to carry on constructive
processes. To this extent, then, the proximity of the
plant to the animal is advantageous to the latter. This
ceases to be true when light is succeeded by darkness.
The animal meanwhile is giving off oxidized products, of
which carbon dioxid is the most abundant. This, to-
gether with water, is the very material out of which the
plant can build its stores of starch and sugar. The out-
put of the animal includes also various compounds of

22 NUTRITIONAL PHYSIOLOGY

nitrogen. These, as well as the carbon dioxid, may be of
service to the plant, although to be strictly accurate it must
be added that they require some modification, usually
effected in nature through the action of bacteria. In
view of these exchanges one is tempted to the hasty con-
clusion that a single-celled green plant and a single-
celled animal form a balanced system capable of con-
tinued maintenance — in short, a microcosm. There is,
however, a fatal obstacle to the continuance of the part-
nership— the animal's need of organic food can only be
satisfied by the sacrifice of the plant. To have a truly
balanced system of an enduring character we must assume
a multiplication of cells descended from the original uni-
cellular plant, providing a surplus of vegetable tissue for
the animal's consumption.

The give and take which has been illustrated for single
cells is proceeding on a vast scale everywhere. It is
hard to realize that the great harvests which support
the races of mankind were formed for the most part from a
gas present only very sparingly in the atmosphere, from
water, and from the mineral salts of the soil. While we
rely partly upon animal food (meat, milk, and eggs),
this does not alter the fact of our absolute dependence
upon the green plants, which in turn owe their growth to
the translated energy of the sun. It is amusing to note
the apparent travesty upon our human life which can be
read into the contrasted conduct of green plants and of
animals. The plants are conserving, while the animals
are spendthrift. Plants create and distribute wealth.
They seem like the thrifty and industrious members of
society upon whose charity others less competent depend.
One is reminded irresistibly of the parent at home and
the son at college. If the father does not literally subsist
upon a diet of carbon dioxid and water that the son may
have protein and alcohol, the approach to a parallel is too
close for complacent attention.

The Body and the Diet. — Turning now to the definite
problems of human nutrition, let us consider the respective

ENERGY RELATIONS OF PLANTS AND ANIMALS 23

make-up of the body and the diet. There must evidently
be a degree of similarity between them, inasmuch as the
one has been built from the other. Similar compounds
are met with in both, but, as we shall find, in quite unlike
proportions. The body is mainly water. Water makes
up about two-thirds oLthe_ total, weight and forms even a
larger percentage of the most active tissues. No material
reduction of its quantity can be tolerated. Even after
death from thirst the amount is surprisingly little dimin-
ished. In the diet also water occupies the first place. It
is likely to constitute fully five-sixths of the daily income.
Its most obvious services are in connection with the
absorption of food in solution and the removal of dissolved
wastes. By its evaporation from the skin and the breath-
ing passages it helps to keep the body temperature from
rising above its normal level.

Second to water among the substances which compose
the body we find the group of bewilderingly complex
compounds known as the proteins. A protein always
yields the five elements — carbon, oxygen, nitrogen,
hydrogen, and sulphur1 — when subjected to analysis.
Some members of the group contain phosphorus also.
Merely to mention these constituent elements is to give
no proper conception of the intricate manner in which
they must evidently be combined. To appreciate this
we need to consider the very long list of cleavage products,
in themselves rather complex, which can be obtained by
the decomposition of protein from a single source. The
physiologic chemist is somewhat in the position of a person

1 The elements occur in proteins in about the following percentages:
C 53, O 22, N 16, H 7, S 1 per cent. Phosphorus when present
amounts to 1 per cent, more or less. It is quite impossible to con-
vey an adequate impression of the complex fashion in which the five
or six elements are combined. Many years ago the following formula
was suggested for hemoglobin, the red protein of the blood, which is
exceptional in containing iron:

C758H120;A28N195FeS3.

It is not seriously maintained that these large numbers are precisely
correct, but the order of their magnitude is probably typical.

24 NUTRITIONAL PHYSIOLOGY

who sees the various parts of a watch — the wheels, springs,
jewels, etc. — lying loose upon the table of the watch-
maker. He may gain a fair notion of the intricacy of the
watch, though he may be very far from knowing how the
parts were related in the time-piece. We do well to use
the plural number in speaking of the proteins, for all recent
work tends to emphasize the distinctive molecular pat-
tern which characterizes each form and makes it differ
definitely from every other. " There is one flesh of men
and another of beasts and another of fishes and another of
birds." This is excellent and altogether modern chemical
biology. The presence of the element nitrogen in the
proteins distinguishes them sharply from the other promi-
nent compounds in both the body and its daily income.
Nitrogen makes up about 16 per cent, of protein, and the
value of this figure in calculations will be apparent later.
It has been said that the proteins are second only to water
in their abundance in the body. This is not true of the
diet unless we have to do with a carnivorous animal.
Among the herbivora, and almost always among men, the
second place jnjthe list of supplies is occupied by the
carbohydrates.

Third in quantity among the constituents of the body
we find in most individuals the mineral compounds.
These would not make so large a proportion if it were
not for the_skeleton. _Bone is a tissue in which salts of
lime are abundantly present. But in all the other tissues
and in the fluids too we find a variety of salts, and it is
well established that their presence is not accidental, but
' a matter of moment. Sodium, potassium, calcium, and
magnesium at least, perhaps other bases also, must be
kept in certain balanced relations if the life processes are
to go on. The acids represented are chiefly hydro-
chloric, phosphoric, and carbonic. Sodium chlorid,
the one salt which we take pains to add to our food,
is the one most abundant in blood and lymph. Potas-
sium rather than sodium compounds predominate in the
cells.

ENERGY RELATIONS OF PLANTS AND ANIMALS 25

Next in amount to the salts in the body of average build,
and not uncommonly exceeding them, are the fats.1 The
word " fat " is used sometimes in a chemicaTand some-
times in an anatomic sense. In the first case it denotes a
compound of carbon, hydrogen, and oxygen, having a
formula of a certain type. In the other usage the mean-
ing is " adipose tissue," a form of connective tissue rich in
such compounds. Fats have familiar physical characters.
They are not soluble in water or to any great extent in the
fluids of the body. They pass from solid to liquid form
at moderate temperatures; the fats of the human body
are regarded as in a fluid state when under the influence of
its warmth. No other common physiologic compounds
have so much latent energy awaiting release by oxidation.
Fats are more plentiful in apparently lean individuals than
might be judged. A considerable store of adipose tissue
is to be found in any condition short of imminent starva-
tion.

It has been said above that carbohydrates usually have
the leading place among the. solid matters of the diet.
This is owing to the large proportion of vegetable foods
generally consumed. In the animal body the occurrence
of carbohydrate is rather scant, and it is one of the chief
problems of the physiologist to account for the daily dis-
appearance of a great quantity of these compounds in the
economy of the organism. The reader may already foresee
what we shall later explain in detail, that this disappear-
ance of carbohydrate is due in part to the fact that it is
the fuel most constantly called upon to evolve energy, and
in part to the ease with which the tissues transform to fat
a surplus of these substances. Under the head of carbo-
hydrates we distinguish the starches and the sugars.

1 Fats are compounds which can be resolved into glycerin and
organic acids. Those of chief interest in nutrition are the glycerids
of palmitic, stearic, and oleic acids. The first mentioned has a com-
position indicated by the formula C3H6(OOCH31C15)3. The others
are nearly related. The three common fats differ in their melting-
points and in other respects. They are mingled in definite propor-
tions to form the body fat of each animal species.

26 NUTRITIONAL PHYSIOLOGY

Starches1 are of high molecular complexity, imperfectly
soluble, and tasteless. Sugars are cleavage products of
starches or, if they occur apart from previously existing
starches, closely resemble such cleavage products. They
are of definite and moderate molecular weight, they are
soluble, crystallizable, and sweet. They contain the same
elements which are found in fats — carbon, hydrogen, and
oxygen — but the percentage composition is wholly differ-
ent and the structure of the molecule also. While the
carbohydrates are energetic, their fuel value is less than
half that of fats.

We have now named this list of substances as uniting
to form the body — water, proteins, mineral matter, fats,
and carbohydrates — the order suggesting their relative
abundance. We have said that in the diet water also
takes the first place, but that carbohydrates ordinarily
take the second. Either proteins or fats may stand third
among the constituents of the diet. Often the amounts
of the two are found to be about equal. A possible diet
comprises 100 grams of protein, 100 grams of fat, and 250
grams of carbohydrate, illustrating this equality. The
mineral matter in the ration is not likely to exceed 20
grams a day. Both the body and its income, of course,
contain in small amounts substances which do not fall
into any of the classes named. Such miscellaneous
organic compounds are conveniently grouped as extrac-
tives. Many of them are nitrogenous and represent dis-
integration products of proteins. Reference to their
significance will be made from time to time.

At the very outset the double service of food to the

1 The three elements in starch are "present in proportions repre-
sented by the formula C6H]0O5. But the number of atoms in the
molecule is not correctly indicated by this formula; it requires to
be multiplied throughout by an unknown, but considerable number.
Sugars are of two common classes: the disaccharids, with the
formula CpHnOu, and the monosaccharids or hexoses, with the
formula CeH12O6. Cane-sugar, malt-sugar, and milk-sugar are
disaccharids. The hexoses of direct interest in nutrition are glucose
(also called dextrose), fructose (or levulose), and galactose (a deriva-
tive of milk-sugar).

ENERGY RELATIONS OF PLANTS AND ANIMALS 27

organism was indicated. It represents both building mate-
rial and fuel. If we liken the living body to a power-house,
we see clearly how both kinds of supplies are required.
Coal is the most bulky supply of the power-plant and the
one on which its operation most immediately depends.
But there must also be new parts to replace those which
wear out. The up-keep of the building calls for new
wood-work, for paint, etc. A certain difficulty is encoun-
tered in the attempt to show parallel conditions in the
case of the body because the separation of the two func-
tions is here much less sharp. Protein food, on the whole,
has a peculiar title to be regarded as building material,
but it is also an entirely available fuel. It is as if planks
and beams designed primarily to repair the structure of the
power-house were fed into its furnaces. The suggested
comparison must not be pressed too far, for it conveys
an impression of wanton destructiveness which we cannot
assume to be just in the case of the organism. There are
some minor supplies brought into the power-house which
are not fuels nor precisely materials for repair. The oil
is an example. Some of the extractives of the diet occupy
an analogous position, being neither sources of energy nor
of construction, but nevertheless favorably affecting the
course of events. This comes near to our conception of a
drug in relation to the processes of life.

CHAPTER III
THE NATURE AND THE MEANS OF DIGESTION

IT has been said that one of the results of the specializa-
tion of cells is the loss of the primitive power to receive
and utilize all kinds of food. The blood brings to the tis-
sues of the body food of certain standard forms, and it is
only these which can be used. The attempt to add various
soluble foods to the blood by direct injection into the
circulation has shown that in most cases such foods are
offered in vain to the living cells. Milk introduced in
this way is not a practical means of nutrition. Cane-sugar
added in measured amounts to the blood is excreted
promptly and in almost undiminished quantity by the
kidneys. Thus it becomes clear that foods introduced
directly into the blood are frequently treated like waste
products, while the same foods after transformation in the
alimentary canal are entirely acceptable to the body cells.
The function of the alimentary canal is to work over the
many foreign forms of nutriment into a few forms of the
native type. From day to day the diet may be of quite
variable character, but its variations hardly show them-
selves in the composition of the blood.

The term digestion is usually applied to those changes in
the food-stuffs which precede absorption. To cover sub-
sequent changes we use the word_metabolism.

One of the more evident characteristics of digestion is
that it is a refining process. It effects a separation of the
useful and the useless portions of the ration. This is
a very conspicuous fact with the herbivora, whose food
contains much woody material from which the available
nutriment must be laboriously extracted. With man-
kind, especially under modern conditions, a great part of

28

THE NATURE AND THE MEANS OF DIGESTION 29

this work of separation is accomplished in the preparation
of food, both industrial and domestic. In this way the
task of the digestive organs is lightened, and, as we are
often told, overeating is made easy. Some foods are
quite devoid of residues when successfully digested and
absorbed.

The early writers, having little knowledge of chemistry,
were naturally led to make much of the mechanical re-
duction of food in the alimentary tract. Mastication sub-
divides the food, and it was held that the later opera-
tions, especially those of the stomach, were essentially
further grindings of a similar sort. Such mechanical proc-
esses as do occur continue to be of interest, but they are
now seen to be preliminary to actual digestion. More-
over, we shall see that it is easy to assume that they are
of a more positive nature than is really the case. The
human stomach is not a mill, though the gizzard of a
bird may be fairly described by that word.

In the eighteenth century the emphasis passed from
mechanical factors to the process of dissolving the food.
Solution is plainly one of the features of digestion, but it
is a somewhat superficial one. Of course, it is natural to
believe that solid food must become liquid before it can
penetrate the intestinal wall, but mere solubility, as we
have already seen, is not a guarantee of fitness for the
use of the cells. Freely soluble foods like cane-sugar and
milk-sugar require to undergo digestive changes just as
definite as those carried out in the case of fats or coagulated
proteins. It is often stated that the object of digestion is
to produce diffusible substances. This statement is in-
adequate, for diffusibility like solubility does not in itself
determine the utility of a food. The sugars mentioned
above are sufficiently diffusible, and the changes which
they undergo before absorption serve a more fundamental
purpose than the mere hastening of their passage through
the lining of the intestine.

In the light of modern chemical knowledge we can be
somewhat specific in regard to the molecular aspects of the

30 NUTRITIONAL PHYSIOLOGY

digestive processes. They are probably always cleavages,
large molecules giving rise to smaller. When the original
molecule is of extraordinary size, as with proteins and
starches, these cleavages have a serial character and a
number of intermediate products must accordingly be
formed. That is to say, the earlier products are in turn
subjected to digestion. Such cleavages are generally, if
not always, hydrolytic, that is, water enters into the
jeaction and its elements are found combined in the
products. For the simpler instances of digestion, as in
the case of fats and of the disaccharids,1 we can write
precise chemical equations. We cannot do this with the
same accuracy for the starches, and we are still farther
from being able to express the exact manner in which the
protein molecule undergoes hydrolysis. Yet we have
sufficient evidence that the digestion is generally of a uni-
form type.

Some constituents of the diet need no digestion. This
is the case with the mineral salts, so far as they are ab-
sorbed, with the simple sugars (monosaccharids), and with
alcohol. It is hardly necessary to say that water is also
ready for reception into the body fluids. The numerous
extractives are for the most part absorbed in the form in
which they are eaten. A diet entirely predigested seems
not to be practicable. If one were prepared it would have
to contain advanced decomposition products of the pro-
teins, which are bitter to the taste, and an amount of sugar
which would be cloying and subject to fermentation.

Digestion is anticipated to some extent by changes in
our food which precede its actual arrival in the canal.
The ripening of fruits and vegetables, as well as the corre-
sponding processes in meat, illustrate this point. The
influence of cooking is not so constantly of a sort to
initiate digestion, yet in many instances it is so. For

1 The following equation illustrates the hydrolysis of a disaccharid:

CI2H2Ai + H20 -= 2C6H1206.

This means that one molecule of malt-sugar, reacting with one mole-
cule ol wa,ter, gives rise to two molecules of glucose.

THE NATURE AND THE MEANS OF DIGESTION 31

example, when meat is boiled the common variety of con-
nective tissue in it is converted to gelatin. This change
is a typical hydrolysis, and if it were not executed in ad-
vance it would be an early task of the gastric juice. When
cooking is attended by considerable drying of the food it is
less likely to count definitely toward digestion. Most
proteins are coagulated by heat, and this change to solid
form seems opposed to the general course of events in the
alimentary system. The action of microscopic organisms
upon food substances is in line more or less with normal
digestion; the maturing of cheese is an example. But
bacterial action may depart so far from the normal de-
composition as to generate products of strongly poisonous
properties, the so-called ptomains being among them.

The Means of Digestion. — Hydrolytic cleavages closely
resembling those of animal digestion may be caused to
occur in various ways. Boiling food-stuffs with acids ac-
complishes this. So does treatment with alkalis. Similar
results follow the application of superheated steam. But
the striking fact is that such changes as are brought about
in the laboratory by violent reagents, high temperatures,
or both in conjunction, are caused to take place in the
stomach and the intestine by bland juices acting at the
mild temperature of the body. The changes effected by
these juices are often modified by the simultaneous
activity of bacteria, but the presence of the latter is to be
regarded as accidental and non-essential.

The power to digest foods has been known for a long
time to reside in the secretions which enter the alimentary
tract. It was at first necessarily estimated simply by
observing the progressive solution of solid food. The
intimate nature of the process has become appreciated
more recently. Comparison of the juices from different
sources shows that they are individual and specific to the
extent that each one, as a rule, acts upon certain classes
of food and not on all. There is sufficient evidence for
the belief that when a juice digests two or more classes
of food-stuffs it contains separate and distinct reagents

32 NUTRITIONAL PHYSIOLOGY

for the performance of each line of work. We do not know
the precise chemical nature of the active bodies, " diges-
tive principles " as they were formerly called, but we know
a great deal about the conditions of their working.

When a digestive secretion has a single well-marked
effect upon one sort of food material we say that it contains
an enzyrrie.. We are thus naming a body which we know
chiefly by its power to promote a certain chemical reaction.
It is not many years since an able writer protested against
this confident reference to a substance where it is a property
rather than a compound which we are observing. It will
be admitted that it is doubtful whether anyone has ever
seen that which is an enzyme and nothing else. What
we see and handle are solutions possessing characteristic
powers or dry preparations capable of furnishing such
solutions. But it has seemed altogether reasonable to
connect the property with a substance and we shall con-
tinue to do so. Acting on this basis, we say of a_ juice
which hydrolyzes starch that it contains a diastatic enzyme,
and of one that acts in a parallel fashion on proteins that
it contains a proteolytic enzyme.. When the action is
upon fats we call the enzyme lipolytic. or, using the sub-
stantive, we call it a liyase. It is unfortunate that there
is much confusion at present in the use of such terms;
there are a great many more in use than there need be.
The simplest plan is, perhaps, to fall back upon our Saxon
and speak of enzymes as starch-splitting, protein-splitting,
sugar-splitting, and fat-splitting. We shall take pains,
however, in our detailed discussion to introduce various
equivalent terms. We shall find especially that enzymes
are often named with reference to their sources as well as
to their powers.

Enzymes jare similar in many respects to the catalyzers
of inorganic chemistry. Their presence accelerates reac-
V^ tions which in their absence might not be appreciable.
We do not think of them as contributing either material
or energy to the process. They suggest the oil in a machine
which lessens the resistance of its parts to the driving force.

THE NATURE AND THE MEANS OF DIGESTION 33

The enzyme in a digesting mixture is not forcibly compel-
ling the molecules to disintegrate, but it is removing some
hindrance to their spontaneous rearrangement. It is not
definitely used up in this service. Accordingly, it follows
that a limited quantity of a digestive juice, of course,
containing a still more limited quantity of enzyme, may
be responsible for an amount of digestion practically
unlimited. Unlimited time would be demanded for such
a demonstration. (This form of statement should be
qualified. Enzymes are somewhat unstable and liable
to deteriorate.)

When a process of hydrolysis takes place under the in-
fluence of an enzyme and in a glass vessel, there must be a
rising percentage of the products and a declining per-
centage of the initial substance as the reaction goes on.
The velocity of the transformation is found to diminish
and at last it seems entirely arrested. A mixture now ex-
ists which contains the first and the last members of the
chemical system in proportions which have become con-
stant. It is an instance of chemical equilibrium. The
halting of the reaction does not mean that the enzyme is
exhausted. If any means can be devised by which the
accumulated end-products <;an be removed the hydrolysis
will be continued. It was specified above that the trial
should be made in a glass vessel. The reader will quickly
recognize the important difference between such a con-
tainer, from which nothing can escape, and the alimentary
canal, from which active absorption processes withdraw
the products of digestion. A clear field is thus provided
for the continuance to substantial completion of the reac-
tions which the enzymes are promoting. The contrast
between laboratory conditions and those which prevail
in the body did not escape the acute mind of Spallanzani,
who was a pioneer among students of these matters. As
early as 1777 he recorded that the solution of meat by
gastric juice could be greatly facilitated by letting the
digestive fluid fall drop by drop upon the food and to
trickle away, bearing the dissolved products.

34 NUTRITIONAL PHYSIOLOGY

Enzymes are exceedingly sensitive to varying degrees of
acidity and alkalinity in the medium. Most of them do
not ]$:eep their efficacy if the solution is far from the
neutral point. But they are somewhat individual in this
as in other properties, the acid which is highly favorable
for gastric digestion, for example, being quite prohibitive
of salivary action. They are all destroyed when in solu-
tion by temperatures somewhat short of boiling. Cold
^uspends their activity, but does not prevent its return
upon warming. They are most effective at a temperature
.not far from that of the blood, though in general a few
degrees higher. These relations between the enzymes
and temperature are much like those established in the
case of the simpler living forms. Having this in mind,
one easily adopts the common practice of speaking of the
killing of enzymes by heat. It must not be forgotten that
this is a figurative expression. We are not justified in
thinking of enzymes as living. Living organisms when
they grow and multiply in a nutrient medium may de-
compose it much as suitably assorted enzymes would do,
and, in fact, the organisms in question are probably pro-
ducing their own enzymes for the purpose. Formerly
such living things as the yeasts and the bacteria were de-
scribed as " organized ferments," and the detached en-
zymes, incapable of self-multiplication, were called
" unorganized ferments." These terms are not much used
at the present time. Enzymes are assumed to be products
of living cells and may be very characteristic fragments of
the cell's fabric, but they are not independently living.

The digestive changes to which we pay most attention
are those which occur in the cavity of the alimentary
canal, and which can be observed to take place also when
the same mixtures are placed in the flasks and test-tubes
of the laboratory. But we must not overlook the prob-
able fact that similar changes are constantly occurring
within the boundaries of every active cell. Intracellular
digestion, presumably made possible by intracellular en-
zymes, obviously takes place when a protozoan cell engulfs

THE NATURE AND THE MEANS OF DIGESTION 35

a solid food particle, and is probably just as definite a
process when a muscle-fiber of the human body nourishes
itself at the expense of the surrounding lymph. We owe
to the German chemist, Abderhalden, the clear exposition
of this second digestion, which is an essential feature of the
life-processes of the higher animals. We shall recur to the
subject in treating of protein metabolism.

While the great majority of enzymes are hydrolytic and
favor reactions of the class which we have been discussing,
it must be added that there are other enzymes associated
with other orders of chemical change. Enzymes which
promote oxidations are believed to play a most important
part in the activities of the tissues. When reactions are
hydrolytic they proceed with but little evolution or
absorption of heat. When they are oxidative the release
of energy is a most characteristic attendant condition.

CHAPTER IV
THE WORK OF MUSCLES AND GLANDS

WE cannot enter upon a description of the alimentary
canal and its activities until we have devoted some space
to the physiology of contraction and secretion. Movement
is the most familiar manifestation of animal life. When
visible to the naked eye it is the expression of the shorten-
ing of elongated units — cells or fibers — associated to form
contractile tissues. In the human body there are three
principal kinds of these tissues. The obvious external
movements of the limbs and the features, the act of
breathing, etc., are produced by what we call the skeletal
muscles. Contractile tissue of another order forms the
walls of the heart and furnishes the power for its beating.
A third kind occurs in the walls of the alimentary tract, in
the blood-vessels, and elsewhere.

The term skeletal applied to a type of contractile tissue
implies relationship to the bones. It is easy to see that
external movements are made effective through the connec-
tion of the muscles which produce them with bones acting
as levers. In some instances the term is a misnomer, for
there are some small muscles histologically like the rest
which do not act upon bones. This is clearly the case
with the ring-like band which surrounds the mouth and
by its contraction puckers the lips as in whistling. The
large and conspicuous muscles are attached, usually at
both ends, to the bones. We can generally observe that
one end is more freely movable than the other. The com-
paratively fixed end is called the origin of the muscle, the
end more subject to movement is its insertion.

What is called a skeletal muscle is a bundle in which
we can distinguish an active and a passive part. There are

36

THE WORK OF MUSCLES AND GLANDS 37

the true contractile elements and there is the connective
tissue. The inactive substance forms a sheath enclosing
the rest and also partitions which subdivide the interior.
The arrangement is familiar in the cross-section of a piece
of meat. The subdivision which is apparent to the un-
aided eye is repeated on a microscopic scale until the
finest meshes of the connective tissue enclose the hair-like
individual fibers of the muscle. Each of these slender
fibers is a miniature muscle in principle. The function
of the connective tissue is often overlooked. While
this part of the muscle is entirely passive in character,
and scarcely to be considered alive excepting for a certain
power of renewal after injury, it is quite necessary to the
act of contraction. It may fairly be said to constitute a
harness through which all the numberless, minute contrac-
tile elements are enabled to unite their efforts. As the
end of a muscle is approached the connective tissue in-
creases in quantity at the expense of the typical contractile
material. In most cases there is an extension of the
muscle consisting of connective tissue only, and in a dense
form, which attaches the whole to the bone. This is the
tendon. It may be a long tough cord or it may form a
wide thin sheet. A muscle deprived of its connective tis-
sue would be simply a mass of unattached living fibers
which might slip about among themselves, but which could
not apply their combined tension to accomplish any ex-
ternal effect.

The fiber of skeletal muscle is a modified cell. Its
length is exceptionally great for its width, perhaps a
thousand times as great. When it shortens it conforms
to the general principle laid down in Chapter I, that is, it
does not diminish in volume, but only in surface and,
therefore, in length. How the chemical process which
underlies the forcible shortening is made to contribute
energy to carry it out has proved one of the most difficult
problems of physiology. It cannot be dealt with here.
But the fact is to be emphasized that we are in the presence
of a mechanism somewhat like the steam-engine, inasmuch

38 NUTRITIONAL PHYSIOLOGY

as it produces motion and does physical work at the cost of
fuel destroyed. The resemblance goes farther than this,
for both with the engine and with the muscle the ac-
complishment of a measured amount of work is attended
by seemingly wasteful evolution of heat. An engine is
considered economical if it turns 15 per cent, of the energy
resident in its fuel into horsepower. Muscles sometimes
do better than this, but much of the time they are even less
efficient. It is fair to point out that the heat set free as
an accompaniment of muscle contraction is often of value
to the animal. In our own case the temperature of the
body must be kept above that of its usual surroundings.
By far the largest part of the heat devoted to this main-
tenance of a relatively high body temperature is produced
in connection with muscular activity. Muscles are
thus seen to be organs of heat production as well as
organs to carry out movements. When the external
temperature is high or the degree of muscular contrac-
tion is greatly above the average, the heat evolved
does become distinctly an embarrassment to the organ-
ism.

The source of the energy displayed in muscular activ-
ity is chiefly the disruption of carbohydrate molecules.
Sugar appears to be the preferred fuel of the muscular
machine, though other foods are known to be available also.
When sugar is completely oxidized the only end-products
are carbon dioxid and water, the same substances which
would be formed by the literal burning of the sugar with
an adequate supply of oxygen. These waste-products are
very readily removed from the muscle, when its situation
is normal, by the circulating blood. The carbon dioxid
will almost immediately escape from the blood when it
passes through the lungs. The water becomes part of
the large total volume which is always passing into and out
of the body, and may leave by all the main channels of
excretion — the respiratory passages, the kidneys, and the
skin. It will be noted that the quantity of water leaving
the body is constantly in excess of the income. Ordinar-

THE WORK OF MUSCLES AND GLANDS 39

ily the body excretes all the water which it receives plus
the water which arises within it by oxidation.

A distinction must be borne in mind between the com-
pounds which for the most part make up the muscle and
those substances which it is generally found to use as
sources of energy. The muscle is mainly composed of
proteins. But, as just stated, it is most apt to destroy
carbohydrates when at work. One is reminded of the
fact that a steam-engine is composed chiefly of steel, but
burns coal as its fuel. The comparison is somewhat
faulty, however, for it suggests a more radical difference
between structure and fuel than we can safely infer
for the muscle. Under some conditions muscular work
may involve some destruction of protein material.

All that has been said of contraction up to this time
applies equally to all three classes of muscle. Neverthe-
less each type is adapted to its particular work by peculiar
properties. Skeletal muscle is capable of quick shortening
and prompt relaxation. A contraction may occur and
the return to an extended condition be accomplished in
one-tenth of a second. The trained finger of a pianist may
strike a key ten times in a second. Such movements are
in strong contrast with those executed by the form of
muscle found in the viscera. The contractions of the
stomach develop very slowly, are maintained for some
time, and are correspondingly slow in fading out. Of
course, it is true that skeletal muscles may also make
prolonged contractions, as in keeping the body erect, carry-
ing a suit-case, and in countless other instances. Experi-
mental study has shown that such contractions as these are
really compounded of successive brief twitches occurring
too rapidly to permit relaxation. In view of this the
possibility of having prolonged contractions in skeletal
muscle does not invalidate the statement that it is essen-
tially a quick-acting tissue.

Muscle and Nerve. — The conception that muscular
activity is due to the nervous system is probably suffi-
ciently familiar. Every skeletal muscle has its own strand

40 NUTRITIONAL PHYSIOLOGY

of nerve-fibers placing it in connection with the brain or
the spinal cord and under their control. If its connection
is severed it becomes paralyzed and remains inactive,
unless special local means, like electricity, are employed
to excite it. Ordinarily we are justified in saying that
skeletal muscle is not automatic, meaning that every move-
ment which it makes is an indication of a previous act, or,
as we say, a discharge on the part of the nervous system.

Fig. 4. — The above represents somewhat diagrammatically a
very small fraction of the length of a fiber of skeletal muscle. To
include the entire element with the length proportional to the width
we should have to extend this drawing to a length of several yards.
The fiber is cylindric and enclosed by a more definite membrane than
is usual with animal cells. The cross-marking is not a feature of this
membrane, but stands for a peculiar organization of the protoplasm
inside. Nuclei are seen here and there near the surface. The seg-
ment shown is supposed to be the particular one about in the middle
of the fiber within which falls the connection with the nervous sys-
tem. A nerve-fiber (n.f.) is seen making a junction with the muscle-
fiber (M) through the so-called end-plate (e.p.).

In somewhat sharp contrast is the behavior of the
muscle composing the heart and of the form which is
found in the viscera. These two kinds of contractile
tissue are described as automatic, in the sense that they
show a tendency to rhythmic contraction and relaxation
even when deprived of their nervous connections. The
automatic property of the heart is the cause of its beating.

THE WORK OF MUSCLES AND GLANDS 41

In varying degrees the different portions of the alimentary
canal exhibit the same power, not ceasing to shorten and
to extend when observed entirely outside the body of the
animal. While we emphasize this remarkable tendency
to rhythmic activity, we must hasten to add that tissues
showing such capacities are nevertheless subject to some
nervous control. Thus the heart beats primarily because
of the peculiar nature of its own substance, but varia-
tions of rate and strength are constantly occurring as a
result of the influence of the nervous system. In this
connection it must be pointed out that such influence is not
necessarily so applied as to excite increased activity, but
may be inhibitory, that is, reducing the rate and force of
the spontaneous contractions. A large place is now given
to the inhibitory functions of the nervous system, and we
shall meet with other examples of the restraint which it
imposes upon various organs. A little reflection makes us
realize that much of the highest work of the brain must be
in the line of inhibition. A man is distinguished by the
acts from which he refrains quite as much as by those
which he performs.

Muscular Tone. — It will be well before we go farther to
make clear what is meant by tone (tonus, tonicity) in
connection with the behavior of contractile tissues.
Muscle is said to exhibit tone when it is not completely
relaxed. Tone is thus a mild, sustained contraction. It
seems rarely to be absent altogether, but may vary much
in degree. Tone in the skeletal muscles gives them a cer-
tain firmness and maintains a slight, steady pull upon their
tendons. This is not likely to result in actual movement,
because these muscles usually fall into antagonized
groups, one of which opposes another. A heightened tone
in the muscles of the arm may not change its position,
since the force tending to bend it may be offset by an
equal tension adapted to straighten it. Changes of tone
in the walls of the hollow viscera, as the stomach, have a
much more evident effect, since they alter the size of the
cavity. One must discriminate carefully between stretch-

42 NUTRITIONAL PHYSIOLOGY

ing and tone change in such a case. A non-living, elastic
sac may be distended by increasing its contents, but will
react with a pressure proportional to the distention. A
living organ which adapts itself to increased contents by a
diminution of tone may exert no more pressure when full
than when nearly empty. This principle is well illustrated
by the urinary bladder. At one time this organ may have
a capacity of a pint, and again its cavity may be nearly
obliterated, but there is no strict correspondence between
its size and the internal pressure. Indeed, a strong degree
of tone and a high pressure may exist when the bladder
is quite small.

Glands and Secretion. — We have taken time and space
to deal with the elements of muscular activity, and we
must also give a place to another type of tissue and to its
work. Some appreciation of the physiology of glands is as
much a prerequisite of the study of the alimentary process
as is a knowledge of the mechanism of contraction. Every-
one understands that the nervous system throws the skele-
tal muscles into their orderly activity, but the fact that
the secretions of the body are often produced under
nervous influences is not so familiar. Yet we do not have
to look far for suggestive examples. The flow of tears
as an accompaniment of an emotional experience is clear
evidence that the small organs above the eyeballs which
elaborate the tears are in connection with the brain and
responsive to its changing conditions. A like relationship
can be demonstrated for the glands that produce saliva
and for those which secrete sweat. Secretion and con-
traction are two manifestations of metabolism which are
alike regulated by the nervous system. In fact, it is
doubtful whether we "have any other expression of the
working of the nerve-centers than these two, the phe-
nomena of consciousness being set aside for the present.

What, then, is a gland? The word is used sometimes
to designate a large organ like the liver, the pancreas, or a
kidney. Sometimes it is used with reference to a micro-
scopic affair like an individual sweat-gland or one of the

THE WORK OF MUSCLES AND GLANDS 43

minute pits in the inner surface of the stomach or the in-
testine. The fundamental structure is the same in both
classes. The microscopic gland is a depression of a cel-
lular surface — a pocket, one might say — out of which
when it is active the secretion wells. The cells which

Fig. 5.— The principle of glandular structure. In the upper
figure a simple microscopic gland is supposed to be laid open by a
section along its vertical axis. The cells are seen to surround a recess
into which they discharge their secretion. Below, the same struc-
ture is shown in its entirety, and in addition the encircling blood-
vessels which contribute to make good the losses suffered by the
secreting cells.

bound its cavity are the produ^^ and

are in turn dependent for renewal upon the lymph which
underlies them and the blood which is flowing close by.
A superficial view would suggest that such a gland is a
filtering device adapted for straining off certain portions
of the blood. This is, however, an entirely inadequate con-

44 NUTRITIONAL PHYSIOLOGY

ception. Most secretions contain substances which were
not present in the blood at all, and which must, therefore,
have been elaborated by the cells of the gland. When we
remember that the same blood flows through all the glands
we cannot fail to be impressed by the variety of the
products which are made from the same raw material —
products as unlike as milk and bile, urine and saliva.

A compound gland, like the pancreas, is an aggregate
of numberless units, which are individually like the simple
microscopic glands. Within the meshes of an abundant
supporting tissue which is shot through with blood-
vessels are these small pockets walled around with the
characteristic cells of the gland. These ultimate recesses
are called alveoli or acini. Each has a way open through
which its liquid product may move toward an outlet.
Usually there is a single main duct formed by the union
of all the fine passages from the alveoli and bearing their
combined contributions. A compound gland may have
more than one duct. Glandular secretions may be
discharged directly upon the surface of the skin, as in the
case of the sweat, or they may enter cavities, as happens
with the gastric juice, the pancreatic juice, and the secre-
tion of the intestinal lining. The bile and the urine are
two secretions which accumulate temporarily in special
containers, the gall-bladder and the urinary bladder
respectively, before they reach their final destination.

Internal Secretions. — It may not be premature to add
at this point that any organ may yield some peculiar prod-
uct of its own life process to the lymph or to the blood as
well as to the cavities of the hollow viscera and to the
exterior of the body. A product of this kind which
merges with the circulating medium instead of appearing
distinct and separate from it is called an internal secretion,
One may maintain that every organ has such a secretion,
for inasmuch as each has its unique chemical composition
and its distinctive metabolism, it must give to the blood
compounds which no other organ duplicates. As stated
before, the actual make-up of the blood is the resultant

THE WORK OF MUSCLES AND GLANDS 45

of the action of all the tissues upon it. But we shall find
that internal secretion is a function much more clearly
attributable to certain organs than toothers and most
evident in connection with a few small structures like the
thyroid and the adrenal, to which later reference must
be made. To have an internal secretion an organ need
not be a typical gland. No duct will be required to carry
such materials as its cells turn over to the blood-stream.
In some cases organs believed to work along these lines are
spoken of as ductless glands. A word recently introduced
as an equivalent of the term " internal secretion" should
be given. It is the word " hormone," meaning a chemical
messenger, a very convenient and suggestive expression.

Absorption and Secretion. — Gland-cells have been said
to draw upon the blood or the lymph for their raw mate-
rial and to manufacture their secretions therefrom. In
this process something enters the deeper boundary of the
cell layer and in a more or less transformed state it is
later discharged from the exposed surface. It is helpful
to compare this operation with what takes place in the
intestine when the products of the digestive cleavage are
being removed to the circulation. When absorption is
going on it is the exposed ends of the cells which receive
dissolved substances and their deeper borders which are
discharging to the fluids that underlie them. Such a
process has been well called " reversed secretion," and
there is the same possibility of an extensive making over
of the transferred material in this case as in the other.
In other words, the digestive products which are last
detected in the intestine are not necessarily those which
will be dealt out to the blood by the cells of the absorbing
membrane. Both secretion and absorption are phenom-
ena which can be completely carried out only by living cells.
Each is probably promoted by a definite application of
energy on the part of the cells concerned. In either case
it is possible that there may be some transfer of material
through the clefts between the cells as well as through
the cell bodies.

CHAPTER V
REFLEX ACTION

IN the previous chapter it was pointed out that all the
work done by the skeletal muscles is in response to the
discharges of the central nervous system. For the other
types of muscle — the cardiac and the visceral — it was
shown that there is an inherent tendency to rhythmic
activity, but that over these tissues also the nervous sys-
tem exercises a regulation. Finally, it was stated that the
glands likewise are subject to central government, al-
though not to the same degree in all cases. We must
now proceed to consider how the nerve-centers are them-
selves prompted to throw muscles and glands into action.

As we observe the body at work we cannot fail to be
impressed with the timeliness of its adjustments. It is
constantly meeting with emergencies and adapting itself
to new conditions. If we are inclined to attribute all
these quick adaptations to intelligent choice of courses
to be pursued we shall find that we cannot long defend
such an explanation of the facts as they occur. We can-
not pretend that we think of each inequality of the
pavement as we cross the street, or of each individual in
the crowd through which we make our way. The balan-
cing of our bodies, standing or walking, is not a matter
about which we are given to deliberating. These things
seem to take care of themselves. It is such adjustments
which " seem to take care of themselves " that are called
reflex actions. A reflex is an adaptive change to meet
some new external condition brought about through the
agency of the central nervous system. We may or may
not notice the occurrence of a reflex. If consciousness
is at all involved, it is incidental and not causal. Often

46

REFLEX ACTION 47

the conscious effort is rather to prevent the reflex from
taking place, as is apt to be true when we sneeze. Of
some reflexes we are quite unlikely to be aware; this is the
case with the narrowing of the pupil in response to in-
crease of light.

Let us now go into some detail and analyze carefully
the reflex process. We have seen that the primary cause
is an external change of some sort, the word " external "
meaning outside the central nervous system and not
necessarily outside the body. The change which is at the
root of the reflex is usually referred to as an external
stimulus. It would be easy to give a long list of exam-
ples. A foreign particle comes in contact with the larynx;
its contact is the stimulus which develops the coughing
reflex. Slight drying of the exposed surface of the eyeball
is a common cause of the winking reflex. Irritation of the
lining of the stomach is the most frequent of the many
possible stimuli through which vomiting can be excited.

External stimuli would fail of any extended effect if it
were not for the nervous connections of the parts affected.
In the last chapter the nervous system was spoken of as
sending its impulses out to muscles and glands. But
its work is twofold. It not only acts, but it is acted upon.
Its fibers fall into two classes, those which are concerned
with transmission of effects outward from the brain and the
spinal cord, and those having the opposite function, the
carrying inward of impulses started by external causes.
The first class of conductors are usually called motor; the
second, sensory. Both terms are open to objection, as a
little consideration will show. The effects which the ner-
vous system produces in the tissues of the body are not
solely movements. The word " motor," then, is not in-
clusive enough. It is better to substitute the word
efferent,1 which means simply centrifugal, and which im-
plies nothing whatever about the nature of the responses
evoked. Efferent fibers may be motor, that is, exciting
contraction, but they may also inhibit contraction, and
1 Efferre, to bear away.

48 NUTRITIONAL PHYSIOLOGY

when they end in connection with the cells of glands they
may be secretory. Probably also they may inhibit secretion.

Just as we have found the word " motor " inadequate
and have agreed to replace it by " efferent," so the word
" sensory " does not properly indicate the whole service of
the fibers which bear impulses toward the brain and cord.
Sensory implies " productive of sensation," and we cannot
assign such a property to all the two million fibers which
assail the centers with their communications. In the
great majority of cases we do not feel any consequences of
their activity. The term afferent is free from this objec-
tion and is the logical complement of efferent. If one
hastens to ask what is the significance of afferent fibers
which do not arouse sensation, the answer is simple and
definite: They produce reflexes.

If the first element in the reflex process is the applica-
tion of an external stimulus, it is now clear that the next
element is the afferent transmission of the impulses.
What these impulses are cannot be discussed. It should
be recalled that they are not fluid pulses nor electric cur-
rents in the usual sense of the expression. They repre-
sent energy of some kind in rapid, but not immeasurably
rapid, motion. They pass along the nerves at rates in
excess of 100 feet in a second, so that the longest paths in
the human body are traversed almost instantaneously.
The time used in such transits might be quite appreciable
if we could observe it closely in a whale.

When the afferent impulses reach the central nervous
system the third event in the development of the reflex
act occurs. This is localized in the brain or the spinal
cord, and we may speak of it as "a central process "
without committing ourselves as to its exact character.
What we actually observe is that the arrival of the afferent
impulses is followed by the appearance of efferent ones.
It is not necessary to decide whether these efferent im-
pulses are the same currents which just entered the in-
tricate fabric of the central organ and which have found a
path open through its mazes which has led them out

REFLEX ACTION

49

again. That is one way of picturing the phenomenon.
According to the older and more familiar view the impulses
which come out are not those which went in, but a new
set generated by an energetic metabolic process, a discharge
on the part of cells in the brain or the cord. If this is the
true conception the afferent impusles serve to "touch off"
irritable nervous elements, much as these elements in their
turn may touch off muscle-fibers or gland-cells.

Fig. 6. — The principle of reflex action. The subject touches a hot
object (H). Afferent nerve-impulses travel the route marked by
dots and dashes to the spinal cord (£). Efferent impulses return
promptly along the route marked by little crosses to the muscle (M),
which co-operates with others not shown to withdraw the finger
from the stimulating surface. The situation of the co-ordinating
center is left undetermined, whether in the brain or the cord.

The fourth step in the evolution of the reflex is the
efferent transmission. This may be said always to be more
voluminous than the afferent flow which went before.
Impulses go out by many channels, where but few were
engaged in bringing them in. A great characteristic of
the "central process" is the spreading of the initial stimu-
lation, so that there seems to be no proportion between the

50 NUTRITIONAL PHYSIOLOGY

cause and the response. The number of nerve-fibers
which can be excited by the slender proboscis of a mosquito
as it pierces the skin of a sleeper must be very small. The
reflex movement which results may involve a very large
share of his skeletal muscles.

The fifth and final occurrence completing the reflex is
the reaction on the part of the muscles or the gland-tissue
in which the efferent fibers end. As already indicated, this
may be a movement, an outpouring of secretion, or it may
have a negative character, the suppression of movements
that would naturally have occurred, or possibly the with-
holding of some secretion which would otherwise have been
discharged. The illustrations of reflex action most often
chosen are those in which an immediate, even abrupt,
response is seen. Yet it is quite easy to find examples
of gradual adjustment to the new external condition.
Changes of color, the outward sign of changes in the
blood-supply of the skin, when they occur on account of
warming or cooling of the surface, are reflexes of this
prolonged and gentle order.

If there is any doubt as to whether a certain action is to
be classed as a reflex, it may be tested according to the
foregoing analysis. There must be an assignable stimulus,
external at least as regards the central nervous system,
there must be an afferent flow of the impulses resulting
from the stimulation, a process within the bounds of the
central axis, a return flow of impulses in multiplied volume,
and the action itself. The more one thinks of the common
course of events, the larger the number of actions which
he finds he can place in this class. It becomes appropriate
to ask what kinds of bodily activity are outside this depart-
ment. To this question it can be replied that automatic
actions, such as the beating of the heart, are to be dis-
tinguished from reflexes. The nervous system is not re-
quired to maintain the heart-beat. There are cases also
in which the chemical composition of the blood reaching
the centers modifies their behavior and causes them to send
out certain impulses. Such cases do not fit our descrip-

REFLEX ACTION 51

tion of the reflex, since in them the stimulation is applied
centrally and no afferent nervous mechanism is needed.
Our breathing movements are determined to a great extent
by such chemical conditions, but it is a fact that reflex
disturbances of the breathing are so prevalent that it is
often difficult to give just recognition to the two factors.

We are accustomed to contrast sharply actions which
are reflex with those which we regard as strictly volun-
tary or deliberate. The distinction is a convenient one
and not generally productive of confusion, but sometimes
it becomes quite difficult to draw the line. It may be
urged that all our conscious, intentional acts are performed
in answer to external conditions which have risen to make
an occasion for such new adjustments. So it might be
argued that the writing of a word from a copy should be
considered a reflex in which the retinal image of the copy
furnished the external stimulus. Such images printed
upon the retina of the trained pianist by the notes that are
before him cause his fingers to drop upon the corresponding
keys of the instrument. It may be claimed that this is a
reflex action. Without denying the force of such reasoning,
we shall do well to restrict the term to the class of responses
for which we are quite sure that attention is unnecessary,
and usually to those for which we have an inborn or at
least a very early developed capacity. When we ask
ourselves whether any act is really other than the result
of external circumstances affecting an organism with its
own past history registered in its structure, we find, al-
most with a shock, that we are face to face with philo-
sophic and ethical problems, responsibility and free will.
Most of us like to believe that a place is to be reserved for
a type of action, even though it may be rare and slight,
which is not externally caused.

The great difficulty encountered by the beginner in
physiology lies in the attempt to realize the inevitable
character of reflexes and their structural basis. He finds
it hard not to read conscious purpose into acts which so
constantly prove advantageous to the individual. When a

52 NUTRITIONAL PHYSIOLOGY

frog adroitly catches a fly it is natural to assume a desire
and a design on the part of the frog. Scientific analysis
nevertheless makes it appear far more probable that the
fly is entrapped because the frog is a mechanical device
adapted to do this thing over and over again. The eye
receives the flitting shadow of the insect, the stimulus
excites the brain, and the well-directed fling of the tongue
follows. The reflex is not done away with when the part
of the brain most likely to stand in relation to consciousness
has been destroyed. We have to remember that much of
the service of the eye is subconscious, as when it makes us
turn aside from obstacles in our path. It is in this way
that the eyes of the somnambulist assist in guiding his
movements. Conscious attention is no more essential to
such a use of the eyes in the waking than in the abnormal
sleeping state. In fact, close attention to the balancing
of the body is quite as likely to derange as to promote the
reflex adjustment.

Central Resistance. — Reflexes are not obtainable with
the same ease at all times. We express this fact by saying
that there are variations of resistance in the central ner-
vous system. If reflexes are hard to bring about, we say
that the resistance is high ; if they occur with unusual free-
dom and seem disproportionate to the exciting stimuli, we
say that the resistance is low. Narcotics and anesthetics
are said to raise the resistance, and their effect can be
gaged by observing the degree of difficulty with which
certain reflexes can be produced, or whether, indeed, they
can be produced at all. Drugs of an opposite order, the
true stimulants, make it easy to call out most reflexes.
When one is distinctly under the influence of coffee, a noise
may cause one to start, with a sharp contraction of many
muscles. The auditory stimulus has an undue effect, and
it is natural to assume that the conditions in the brain and
cord are uncommonly favorable to the penetration and to
the multiplication of nervous impulses. In poisoning by
strychnin such an extension of conduction may exist that
some trifling cause may precipitate a terrific and exhaust-

REFLEX ACTION 53

ing convulsion. Clearly, then, a certain degree of central
resistance is the most favorable condition for the activities
of life. Any increase will tend to prevent needed adapta-
tions to external changes, and any great decrease will make
the reflex responses exaggerated, disorderly, and ill suited
to their object. There is reason to suppose that the more
frequently occurring reflexes become easier of production
through a lowering of resistance in their particular path-
ways. This brings us close to the subject of habit for-
mation.

Our emphasis has been constantly upon the advantage
derived by the animal (or by man) from the possession of
reflex capacities. When the environment is the accus-
tomed one and the changes taking place are such as the
species has often experienced, we find that almost every
reflex is obviously beneficial. The reactions are such as
maintain bodily equilibrium, secure nutriment, evade or
defeat enemies, resist changes of temperature, all making
for self-preservation. But it must be noted that an unin-
telligent mechanism will act amiss in any environment
which is sufficiently unlike the accustomed one. It will
hardly be claimed that the reflexes exhibited by the novice
on first going to sea help him in the struggle for existence.
A number of reflex effects can be thought of which can
scarcely be of value. Sneezing when going out into bright
sunlight is one of these. Hiccups following immoderate
laughter do not seem to be of any service, nor does laughter
itself when induced by tickling. These instances, which
on the whole have little importance, are mentioned simply
to enforce the contention that the reflex mechanism, how-
ever refined, is not directed in its routine performances
by intelligence. Its structure determines its conduct. The
finger laid upon hot iron is twitched away before the situa-
tion is reasoned out, in fact, before pain is felt. Central
connections exist which make the movement sure to occur.
If we could rearrange those central connections we can
conceive of a luckless subject who would not remove his
finger from the stove, but would stand violently coughing
while the injury proceeded.

CHAPTER VI
THE ALIMENTARY CANAL

THE single-celled animal digests its food within its own
protoplasm, sometimes holding it for a while in a temporary
cavity filled with fluid, the so-called food vacuole. In
such intracellular cavities true digestive secretions contain-
ing enzymes are doubtless at work. It is probable that
single-celled forms may also secrete enzymes to the exte-
rior and so modify food material which is near-by, but not
yet enclosed. This appears to be the case with bacte-
ria when they dissolve the solid gelatin in which they are
growing.

Among many-celled animals digestion of this second
type, that is, external to the cells, becomes more con-
spicuous. Their bodies are so formed as to contain spaces
in which food may undergo digestion and from which the
hydrolyzed products may be absorbed. In the sea-
anemone a round opening or mouth leads to a cavity which
is very large in proportion to the size of the animal.
This primitive alimentary tract has no other opening.
In the earthworm, a somewhat more highly developed
form, a straight canal in the axis of the body leads from
a mouth near the anterior end to an anus at the posterior.
However much the alimentary systems of the higher ani-
mals may be elaborated, each still represents a more or less
winding passage between a mouth through which food is
received and a vent or anus for the discharge of residues and
excretions. The canal may be greatly lengthened through
coiling. Some sections may be widened and others nar-
rowed; the walls in some places may be thick and elsewhere
thin. Local differentiation of this kind causes us to dis-
tinguish in the human subject the familiar divisions of the

54

THE ALIMENTARY CANAL

55

tract, as the esophagus, the stomach, the small and the
large intestines. The lengthening of the system, it should
be noted, does not merely increase its capacity, but multi-
plies the surface available for the processes of absorption.
A few anatomic expressions may well be denned at this
time. Anterior, as we shall use the word, means toward

Fig. 7. — I represents a protozoan cell — an ameba — which has
enclosed a particle available for food (F). The particle occupies
the center of a clear space or vacuole (V). Undoubtedly it is sur-
rounded by a fluid having digestive powers. II is a diagrammatic
section through the familiar sea-anemone. There is a relatively
huge digestive cavity (S) with a single opening to the exterior (M).
Ill suggests the type of alimentary system found in the earthworm
and in higher animals. Two openings exist, the mouth (M), de-
finitely devoted to the reception of food, and the anus (A), used
exclusively for the discharge of wastes.

the head; posterior, away from the head. Dorsal means
toward the back; ventral, toward the front. Right and
left have their ordinary use. (In most figures, the subject
being viewed from in front, right and left are reversed.)
Reference must often be made to the body cavities. These

56 NUTRITIONAL PHYSIOLOGY

are the thoracic cavity above the diaphragm and within
the cage of the ribs, the abdominal cavity below the dia-
phragm, and the much smaller pelvic cavity bounded by
the bones of the hip girdle. When we speak of these as
cavities we do not mean that they contain any air-filled
space. They are completely filled by the organs which
they enclose plus a small quantity of fluid. Hence they are
only potential cavities in life, becoming actual when their
contents are removed in course of dissection. The thoracic
cavity contains the lungs, nearly surrounding the heart,
and is traversed by the esophagus. The abdominal cavity
is filled almost entirely by the organs of digestion — the
stomach, the small and large intestines, the liver, and the
pancreas. The spleen at the left of the stomach is less
certainly connected in its functions with the alimentary
system. The kidneys lie in the dorsal body wall rather
than in the abdomen. In the small pelvic cavity are the
urinary bladder, the terminal part of the large intestine,
and the reproductive organs.

The mouth, the first division of the alimentary canal,
scarcely calls for detailed description. Above, a bony par-
tition separates it from the intricate spaces of the nasal
passages. At the back this "roof" is prolonged as a
mobile, muscular curtain — the soft palate. Below the
edge of the soft palate a region is reached which is common
to the alimentary and respiratory systems. This segment
of the canal is known as the pharynx, though the term is
extended also to the space behind the soft palate, which is
above the normal course of food. The teeth and the
tongue with its wonderful muscular development are suffi-
ciently obvious. JDucts from the salivary glands open
into the mouth. We are rarely conscious of the situation
of these openings, though in the dentist's chair we may
notice the rapid flow of saliva from one which is opposite
the upper molars. This is the place of entrance of the
secretion of the parotid glandj situated before and below the
ear, the gland usually affected in mumps. Under the
tongue and within the sweep of the lower jaw-bone there

Fig. 8. — The human alimentary canal shown diagrammatically :
0 is the esophagus; S is the stomach; S.I. suggests the small in-
testine; C is the colon (see Fig. 14) ; R is the rectum. The connection
between the stomach and the small intestine occurs behind the
transverse colon, which also hides the pancreas. -

THE ALIMENTARY CANAL 57

are, on either side, two other glands, the submaxillary and
the jiublingual, with ducts opening in the floor of the
mouth.

Below the root of the tongue there is a leaf-like pro-
jection, the epiglottis, which juts backward and guards the
entrance to the larynx. Through the larynx a way is open
to the trachea and the lungs. At this point, therefore,
the courses taken by the food and by the breath part com-
pany. From here the esophagus extends through the neck
and the thorax, lying at first behind the trachea, and lower
down passing back of the heart. Perforating the dia-
phragm slightly to the left of the midline it opens into the
stomach.

The stomach is the most expanded part of the aliment-
ary canal. Its position is higher up than is generally as-
sumed, so that it is well within the embrace of the lower
ribs on the left side. It has a capacity varying greatly
with the degree of its distention and with its variations
of tone. After a full meal it may contain more than a
quart. The form of the stomach also changes consider-
ably from time to time, but we distinguish a large, rounded
portion toward the left and a more conical region tapering
off toward the right and joining the small intestine. The
opening from the esophagus into the stomach is called the
cardia, and that from the stomach to the small intestine is
the pylorus. The pylorus is a trifle to the right of the mid-
line. The upper border from the cardia to the pylorus is
the "lesser curvature" of the stomach; a line drawn from
the cardia around the convex left-hand side and thence
along the lower margin to the pylorus is said to follow the
"greater curvature."

Leading away from the pylorus the small intestine de-
scribes a short turn, within which is the pancreas. This
first curve is called the duodenum. The remainder of the
small intestine is a slender tube almost 20 feet in length,
coiled upon itself in a confusing manner. Two divisions
are recognized, the jejunum, continuous with the duodenum
and the ileum, extending onward to join the large in-

58 NUTRITIONAL PHYSIOLOGY

testine. No sharp line of demarcation exists between
these sections, but the latter is regarded as constituting
somewhat more than half the whole. The ileum finally
arrives at a point not far from the crest of the right hip-
bone and there enters the large intestine.

The large intestine is so called Trom its diameter, which
is two or three times that of the small. It is quite as often

Fig. 10. — The stomach with the pancreas and duodenum: C is
placed at the cardiac opening of the stomach, while P is at the
pylorus. A dotted line is used to complete the form of the pancreas,
which discharges to the intestine near W. The shape of the stomach
is of one moderately filled; with further distention the lower border
of the organ would sag and the pylorus would cease to be the lowest
point. B is the common bile-duct, which reaches the intestine be-
hind at the same point at which the pancreas delivers its secretion.

called the colon. Beginning with a small rounded pouch,
the cecum, it may be followed upward on the right side of
the body to the level of the lower ribs. This part is the
ascending colon. From here it bends sharply to the left
and crosses the full width of the abdominal cavity. This
horizontal segment is known as the transverse colon and
lies close to the ventral body wall. Thus it passes in front
of the duodenum and is in practical contact with the

THE ALIMENTARY CANAL 59

stomach. At the left side of the body and near the spleen
the descending colon begins. Its course is downward
and backward, so that it passes behind the coils of the
small intestine. Following the dorsal boundary of the
cavity around to the middle line, the colon forms the short,
curved region called the sigmoid flexure. The remaining
section is the rectum, situated directly in front of the lower
extremity of the spinal column within the pelvis and ter-
minating at the anus.

Almost everywhere the lining of the alimentary canal is
pitted with microscopic glands. Those in the stomach
furnish the gastric juice; those in the intestine, the intesti-
nal juice. Besides these small glands and the salivary
glands already mentioned, there are the pancreas and the
liver, contributing secretions to the cavity of the digestive
tract. The pancreas has been said to lie in the turn of the
duodenum. It is thus under the pyloric portion of the
stomach and behind the transverse colon. Its main duct
discharges into the small intestine about 3 inches below the
pylorus. A second, but very small, duct opens close by.
The liver, which is the largest gland in the body, is fitted
to the concave under surface of the diaphragm and is
mainly to the right of the midplane. It is cleft into several
lobes, from which ducts converge and unite as they ap-
proach the duodenum. A single duct is finally formed and
it enters the intestine at the same point as the chief pan-
creatic duct. The arrangement serves to blend the two
secretions, and is somewhat suggestive of the devices used
with bath-tubs for mingling hot and cold water.

The liver produces bile, and its channel of discharge to
the duodenum is accordingly known as the bile-duct.
This duct has a side branch which leads to a contractile
sac embedded in the under surface of the liver, this reser-
voir being the gall-bladder. Bile, as it flows down from
the liver, may either find its way directly to the intestine
or it may turn aside into the gall-bladder. The course
taken will depend on the contraction and relaxation of the
muscular walls of the ducts. Active contraction of the

60 NUTRITIONAL PHYSIOLOGY

gall-bladder when it is full may send a considerable amount
of bile at one time into the intestine. The relation of the
gall-bladder to the liver is like that of the urinary bladder
to the kidneys, at least to the extent that its existence
makes possible a continuous production of the secretion
with an intermittent emptying. It has been shown, how-
ever, that the bile is concentrated and otherwise modified

Fig. 11. — This is an entirely schematic section across the human
body in the mid-abdominal region: S indicates the spine; K, the
kidneys; P is the peritoneum, the lining of the abdominal wall.
It is prolonged from the back to form the mesentery (M), which
extends to and around the loop of intestine (/). The large unoc-
cupied space shown does not really exist, for successive portions of
the alimentary canal together with other organs completely fill the
cavity.

during its stay in the gall-bladder. The urine does not
change its character distinctly while it is in storage.

When the abdominal wall of an animal is cut through
and laid back from the organs within, one's first impression
is that the viscera are lying unattached in the cavity.
They are, in fact, not adherent to the ventral or lateral
portions of the wall. But if we take a loop of the_small
intestine at random and attempt to lift it from its resting-
place, we find it attached to the middle of the back by a
tough, transparent membrane, the mesentery. In the

THE ALIMENTARY CANAL 61

mesentery can be seen blood-vessels, lymphatics, and
nerves. This suspending sheet thus serves not merely for
mechanical support, but also establishes connection be-
tween the intestine and the circulatory and nervous sys-
tems. The student is apt to find it hard to visualize the
mesentery in its actual form ; he is to imagine a membrane
which at one edge extends to the entire length of the small
intestine and to much of the large, while its other edge is
condensed to be inserted into the space of a few inches
before the spinal column. What results from these condi-
tions has been described as "a ruffle or flounce." Al-
though the mesentery is thin it is really a doubled sheet
enveloping the intestine. This will be made clear by the
diagram, which also shows how the mesentery is continu-
ous with the exquisitely smooth, lustrous lining of the ab-
dominal cavity, to which is given the name of peritoneum.
Dissection of a small animal will give a comprehension of
these anatomic facts which can scarcely be gained by
reading.

The stomach has a supporting membrane attached to it
along its lesser curvature and uniting it to the liver, which
is, in turn, anchored to the dorsal body wall. This mem-
brane is, in effect, a mesentery for the stomach, but is called
the lesser omentum. An extension of similar tissue hangs
from the greater curvature like an apron over the intestinal
coils and is called the great omentum. It may become
a ponderous appendage from the fat which it sometimes
accumulates. The continuation of the mesentery over
the external surface of the intestine and the identical
covering of the stomach form for these organs what is
spoken of as their serous coat.

The Finer Structure of the Alimentary Organs. — We
have said that the internal surface of the stomach and of
both intestines is provided with glands. The inner layer
of the wall of the canal in which these glands occur is
called the mucous coat or mucous membrane. This is in
reference to the fact that its exposed cells produce the slimy
substance, mucus, more familiarly associated with nasal

62 NUTRITIONAL PHYSIOLOGY

discharges. It probably acts as a lubricant in the digestive
tract and also protects the lining cells from harsh contacts,
both physical and chemical. The mucous membrane is
depressed to form the recesses of the glands, and in the
small intestine is raised into the microscopic prominences
referred to as villi. Underlying it blood-vessels and nerve -
fibers run thickly.

Between the mucous layer and the serous coat on the
outside of the intestine there is a development of muscle
of the order described in a previous chapter as characteristic
of most internal organs, that is to say, slow acting, more or
less automatic, and much given to tone changes. In the
small intestine there are two distinct muscular coats: the
inner and thicker has its fibers at right angles to the axis
of the canal, while the outer has them set parallel to this
axis. The inner coat is hence spoken of as circular and the
outer as longitudinal. There is no doubt of the superior
prominence of the circular coat in the production of intes-
tinal movements. In the stomach the muscular organiza-
tion is less simple and there are oblique elements in addi-
tion to those which can be classed as circular and longi-
tudinal. The colon has the circular coat, but instead of a
complete covering of longitudinal muscle it has three
bands of contractile tissue extending along its wall.

At any point along the course of the intestine temporary
closure may be effected by the contraction of the circular
muscle. But there are certain places where such closure
is far more frequent or, indeed, the usual condition.
Where the esophagus joins the stomach an irritable band,
the cardiac sphincter, is much of the time firmly contracted.
There is, similarly, a pyloric sphincter guarding the open-
ing between the stomach and the duodenum. Where
the ileum enters the cecum a valve exists which is adapted
to prevent the reflux of material from the colon to the small
intestine. This, the ileocecal valve, is probably reinforced
in its mechanical action by muscular support. Finally,
the short anal canal is closed by an inner sphincter which
is essentially a thickening of its own wall, and an external
one composed of skeletal muscle.

CHAPTER VII

THE MOUTH— SWALLOWING; SALIVARY
DIGESTION

Mastication. — The hygienic importance of thorough
mastication is undoubted, but there is little occasion for

ly extended analysis of a process so obvious. It is to be
observed that the lower jaw does not have merely an up-
and-down movement, but that it glides backward and
forward and has some lateral play at the same time. The
teeth, therefore, do not simply chop the food, but rub and
grind it. In the work of mechanical reduction a larger
part is borne by the tongue than is commonly recognized.
The little member seems to be everywhere at once, thrust-
ing food between the teeth, withdrawing it again, bruising
and rasping it against the roof of the mouth. While this

ion is going on an intimate mixture with the saliva
is accomplished. We must now proceed to a discussion
of this the first of the digestive secretions.

Mention has been made of the three pairs of glands which
supply the saliva. Their united product is estimated to
reach an amount of about 3 pints a day, equalling the vol-
ume of the urine. If one finds it hard to credit such a
statement, attention may be called to the copious character
of the flow which is noted when one is interrupted at the
moment of taking food. There is little secretion apart
from eating unless it is excited by chewing sundry things.

(At mealtime a large part of what is swallowed is saliva,
and the proportion must be greatly raised by the practice
of prolonged mastication, so-called Fletcherism. The
formation of saliva is to be regarded as a reflex in which
the primary stimulation is furnished by food in the mouth

64 NUTRITIONAL PHYSIOLOGY

acting upon the endings of nerves excitable by its chemical
ingredients and by its temperature more than by its mere
contact. We have to do with something more than
the typical reflex, however, because it is a familiar fact
that the appeal to consciousness has much influence upon
the flow of saliva. The "watering of the mouth" at the
approach of acceptable food is a hard phenomenon to
classify. It is a reflex, but it is one which would not
occur in an unconscious subject. For such actions the
term psychoreflex is often used.

The saliva is a bland fluid which one would hardly sup-
pose to be endowed with active powers of digestion. In
some animals it does not have any apparent chemical ac-
tion. Still, it has valuable properties which we shall do
well to recognize. Whether it is a digestive juice or not,
its physical effect is useful in mastication, since it softens
the food, makes it cohere into the pellets which are pre-
pared for swallowing, and later lubricates their transit to
the stomach. Moreover, it has a defensive use, protecting
the mouth from injury when food or fluid is taken too hot
or when some corrosive liquid calls for dilution. As it issues
fresh from the glands it is slightly alkaline in reaction.
If it stagnates for a long time in the by-places of the mouth,
as happens during sleep, and if it contains at the same time
traces of carbohydrate food in solution, bacterial fermenta-
tion may make it acid and the effect upon the teeth may
be injurious. The value of an alkaline mouth-wash, like
milk of magnesia, used at bedtime is evident.

Human saliva contains various salts. Attention need
be called only to its lime compounds, which are always de-
posited more or less upon the back surfaces of the teeth,
a process which reminds one of the formation of stalactites
and stalagmites in caverns. The hard crust that results
is the tartar. It is not very unlike the original substance
of the teeth in its chemical composition, and its occurrence
might seem to indicate a mode of making good the wearing
away of the teeth. Unfortunately we cannot regard it in
this favorable light, for the lime salts are always contami-

THE MOUTH— SWALLOWING ; SALIVARY DIGESTION 65

nated with food particles and bacteria. The deposit
should be removed by the dentist at regular intervals.

The three glands furnish slightly different varieties of
saliva. Mucin, the essential compound in mucus, is pres-
ent in thfTsecretion of the two lower glands and not in
that of the parotid. It gives to the saliva from the sub-
maxillary and sublingual glands a ropy, mucilaginous
character, which, of course, becomes more apparent when
evaporation has concentrated the solution. This is illus-
trated when the mouth is dried by rapid breathing during
exercise and becomes furred with the residue of salivary
mucin. This constituent of the saliva probably makes it
superior to water as an agent for molding the food into
pellets. The most interesting property of the secretion,
its power to hydrolyze^tarch^may be discussed to more
advantage aTEerwtf have followed the food to the stomach.
Clearly, there is not time enough for much digestive change
in the mouth of a person of average habits.

Swallowing. — The transfer to the stomach is a more
complex matter than is likely to be realized. It involves
an interruption of breathing and the protection of the nasal
passages and the larynx against the intrusion of food. The
first purpose is effected by the swinging back of the soft
palate against the back of the pharynx. The second is
accomplished by the drawing forward of the larynx toward
the chin, a movement which can be plainly felt. By it
the larynx is tucked under the root of the tongue and over-
laid by the epiglottis. The same motion serves to widen
the upper part of the esophagus, which is not usually ap-
preciably open. With the parts in this position the bolus
of food is crowded back from its original seat upon the
tongue and urged through the pharynx by the successive
contraction of the bands of muscle which surround it.
As soon as it is fairly within the esophagus the soft palate
is lowered, the larynx is allowed to emerge from its covert,
and the breathing can be resumed. Such quick and well-
ordered adjustments give evidence of co-ordinated reflex
action, the contact of the food morsel with one spot after

5

66

NUTRITIONAL PHYSIOLOGY

another furnishing the requisite stimuli. We cannot go
through the series of movements unless there is at least
a little saliva to be swallowed, and we cannot arrest the
march of events when it is once begun.

The contractile tissue in the upper part of the esophagus
is._ol a skeletal variety. Lower down this gives place to
typical visceral muscle. Hence it is not strange to find that
the advance of the bolus becomes progressively slower as
it descends. The movement which is here taking place

i JL m

Fig. 12. — An exaggerated representation of peristalsis. I and II
are successive views of the same portion of the alimentary tube:
P is the zone of contraction shifting downward and always pre-
ceded by the zone of unusual relaxation (N). Ill is an imaginary
section through II, showing the food bolus (b) slipping along in
advance of the contracting region, its advance being facilitated
by the relaxation below.

is what is known as a peristalsis, and it is highly important
that its principle should be understood. The most ob-
vious feature is a ring of contraction setting in above the
enclosed pellet, causing it to slide onward, and following
it down by involving in succession each level of the tube.
The mechanical application can be simply illustrated by
propelling a glass bead through a soft-rubber tube by
pinching repeatedly behind it with thumb and finger. A
strict analysis of what occurs in the esophagus obliges us to
recognize that the process is not so simple as it first appears.

THE MOUTH — SWALLOWING.; SALIVARY DIGESTION 67

There seem to be two phases in what is called the peristal-
tic wave. The eye detects chiefly the traveling contrac-
tion, but this is apparently preceded by a zone of unusual
relaxation, a region of inhibition.

The peristaltic wave which is necessary for the propul-
sion of solid food does not seem to be required to send liquid
to the stomach. A swallow of water is shot swiftly from
the mouth to the cardiac sphincter and arrives there dis-
tinctly in advance of the plodding peristalsis. When one
drinks a glass of water, the swallows following in rapid suc-
cession, a single peristaltic wave ends the series. Of
course, when fluid is carried up-grade in the esophagus, as
when a horse is drinking from a pool at his feet, active
peristalsis is as necessary as though solid food were being
moved, and one may plainly see the passing of each swallow
along the extended neck. We shall find that the small
intestine exhibits movements which are approximately
the same in principle as those of the esophagus, but far
slower and usually less energetic.

Salivary Digestion. — Within the stomach the accumu-
lated food with a large admixture of saliva lies for some time
with little motion. Here then salivary digestion must take
place. The statement has been made that in some animals
the saliva has only mechanical and protective functions.
More frequently, however, it has the power to hydrolyze
starch, forming malt-sugar as the chief end-product. This
seems to justify the assumption that an enzyme is present,
and it is variously named ptyalin, salivary amylase, or
salivary diastase. Such an enzyme probably plays an im-
portant part in the digestive processes of ruminants, ani-
mals which chew the cud. Human saliva acts upon starch
with surprising energy. A simple demonstration of the
fact may be had by holding a bread-crumb in the mouth
longer than is habitual, when it will gradually develop a
mildly sweet taste.

The prevailing opinion in regard to the amount of diges-
tion accomplished by the saliva in man has undergone a
change during the last few years. It is allowed a larger

68 NUTRITIONAL PHYSIOLOGY

place than was formerly granted to it. The enzyme is
extremely sensitive to acid. Inasmuch as the gastric juice
is decidedly acid, it used to be claimed that salivary diges-
tion could not proceed in the stomach. But it has come
to be recognized that when a large mass of food is intro-
duced into the stomach within a short time the gastric juice
penetrates it rather slowly. A few minutes after the com-
pletion of a meal we may picture tKe stomach-contents as
being acidified near the surface, the acid slowly making its
way inward, but having a neutral or even alkaline central
portion. Salivary digestion will be continued in the stead-
ily diminishing region not yet reached by the acid, and will
cease only when the gastric secretion from one wall of the
stomach meets that from the other. Any rotation of the
contents would probably bring about an earlier distribution
of the acid and arrest of starch digestion. No such rota-
tion seems normally to occur. A factor which operates to
postpone the destruction of ptyalin is the power of the
proteins of the diet to engage hydrochloric acid in com-
bination. Since proteins are almost always present, the
gastric glands must secrete acid enough to satisfy their
capacity before there can be the excess of strictly free acid
which will put an end to salivary digestion.

If the mixed food is quite acid at the outset, it is hard to
see how there can be any hydrolysis of starch brought
about by the saliva. Yet we constantly eat acid fruits
before our breakfast cereal and notice no ill effects.
Starch which escapes digestion at this stage is destined to
be acted upon by the pancreatic juice, and the final result
may be entirely satisfactory. Still it is reasonable to as-
sume that the greater the work done by the saliva, the
lighter will be the task remaining for the other secretions
and the greater the probability of its complete accomplish-
ment. The power of saliva to convert raw starch to sugar
is almost incomparably smaller than its capacity to digest
starch which has been cooked. Raw starch exists in very
dense grains which have to be dissolved from the surface
inward. Cooking, especially boiling, utterly destroys these

THE MOUTH— SWALLOWING; SALIVARY DIGESTION 69

grains, and permits a reaction between the enzyme and
the separated molecules of the carbohydrate.

The change from starch to sugar seems not to be effected"
by a single reaction, but by stages. Physiologic chemists
have studied extensively the numerous intermediate bodies
which have a fugitive existence in the process. Most of I
these are covered by the term dextrins. It is sufficient for<
our present purpose to regard them as carbohydrates,
simpler in their molecular structure than the original stare]
but complex as compared with the familiar sugars. We
have said that the chief product oLsalivar^y^rolysisJs
malt-sugar^ or maltose. This is one of several sugars
classed asjiisaccharixls. It can be hydrolyzed further to
form dextrose (or glucose), a sugar of the simplest type,
and one which is ready to be absorbed and to minister
to the living tissues. Some dextrose is said to be formed
in prolonged salivary digestion, but the cleavage lags
when the maltose stage has been reached.

CHAPTER VIII
THE MOVEMENTS OF THE STOMACH

IT will be recalled (Chapter VI) that the stomach consists
of a main rounded portion from which a much smaller
conical segment extends to the right to join the duodenum
at the pylorus. The larger part is called the fundus, the
tapering region is the antrum. The muscular coats of the
antrum are somewhat thicker than those of the fundus and
show an especially conspicuous development of the circu-
lar elements. There is no such contrast between the two
parts of the human stomach as between the thin-walled
crop and massive gizzard of the bird, but there is a faint
suggestion of an analogous difference. The fundus is,
indeed, primarily a place for the storage of food; the an-
trum, while not a crushing mechanism, has distinctly
greater motor properties.

The antrum is considered to be set off from the fun-
dus by the so-called transverse band. This is an irritable
ring of the circular muscle which is often contracted enough
to indent the outline of the stomach at this point, and which
may occasionally create a temporary division of the gas-
tric cavity into two parts. It has been called the sphinc-
ter of the antrum, but it cannot fairly be compared with
the cardiac and the pyloric sphincters, since these are
habitually closed, while closure at the transverse band is
rare.

Regulation of the Cardiac Sphincter. — Food and drink
entering the stomach pass the cardiac sphincter. The
guardian muscle is, usually more or less contracted. It
relaxes upon the arrival of the peristaltic wave in the
esophagus. If it is recalled that the peristalsis consists of
a wave of inhibition running before a contraction, it is

70

Fig. 13. — To suggest the probable appearance of the distended
and active human stomach. Three marked waves of contraction
are seen in the antral region. These are to be conceived of as pass-
ing onward toward the pylorus.

THE MOVEMENTS OF THE STOMACH 71

easy to see how the cardia may be opened at the moment
when its muscular walls fall under the influence of the
phase of relaxation. Closure will follow immediately, as
the second or positive part of the peristalsis involves the
sphincter. Attention has been called to the fact that liq-
uids may outrun a pursuing peristalsis and arrive several
seconds in advance of it at the cardiac opening. Under
such circumstances it is said that the fluid remains at
the bottom of the esophagus until overtaken by the wave,
when the relaxation occurs which permits it to pass into
the stomach. A person drinking with ill-advised haste
may have the disagreeable experience of filling the esoph-
agus enough to produce a painful distention. Relief
comes abruptly when the peristalsis has made its way to
the cardiac sphincter and secured an entrance for the
liquid.

While this has been the generally accepted description
of the facts, it has been shown recently that the cardia is
not always so firmly contracted. Observations by x-ray
methods, to be described presently, have shown that for
some time after a meal there may be a reflux from the
stomach of a cat into the esophagus. Each escape of food
evokes a local peristaltic wave which returns it to the
stomach. Such an incident does not entail any movement
of the throat muscles and is probably subconscious. As
the period of digestion continues the sphincter becomes
more tightly set and no longer allows any such return of
stomach-contents. The increased tension has a simple
explanation, which, like many another point about the
stomach, we owe toW. B. Cannon. He has shown that
the tension is developed in response to the rise of acidity
in the liquid just within the cardia. Since the acid appears
normally after each filling of the stomach, we have here
an automatic provision for the establishment of the requi-
site guard over this opening. The influence of the ner-
vous system seems capable of nullifying the local effect of
acid, since the sphincter may be relaxed to permit vomit-
ing at times when the gastric contents are excessively acid.

72 NUTRITIONAL PHYSIOLOGY

The Fundus. — An important service of the stomach is
to store food in relatively large quantities at mealtimes
and to deliver it gradually to the intestine. A person who
has been deprived of the stomach, or of most of it, by sur-
gery is made aware of this when he finds it impossible to
eat "a square meal," and is compelled to take small por-
tions of food at short intervals. He is then serving his
intestines somewhat as they are normally treated by the
stomach. Storage is not the sole function of the stomach,
but we do well to emphasize it. The fundus accommodates
itself to its contents by tone changes, relaxing when food
is being swallowed and afterward exerting a steady, mod-
erate pressure which insures the filling of the antrum after
every discharge at the pylorus. A lack of this tonic re-
action may be a cause of serious disorders.

The earlier writers often claimed that there is a definite
and regular overturning of the contents of the fundus.
In the light of more recent observations this does not seem
to be usual. A German investigator fed to a rat three
courses of food of contrasted colors. The animal was then
killed and frozen. A section made through the hardened
mass within the stomach showed distinct stratification.
The food first taken was in the antrum and the lower part
of the fundus, the second instalment was above the first,
and the third was just under the cardia. It seems hardly
probable that entirely liquid food could remain thus strati-
fied when one considers the extent to which the stomach is
subjected to the influence of bodily movements.

«-Ray Studies of the Stomach. — While much can be
learned of the behavior of the stomach through experi-
ments involving its exposure by surgical procedures, the
ideal method is clearly one which leaves the animal in its
normal condition. Such a method became available when
the x-ray was first turned to account to observe visceral
movements. The image of any part of the body projected
by means of the x-ray shows the bones in clear contrast
with the softer parts, but scarcely outlines the organs. If,
however, any harmless substance opaque to the x-ray is

THE MOVEMENTS OF THE STOMACH 73

introduced into the contents of the alimentary canal, it
becomes possible to recognize the situation of this sub-
stance so long as it remains sufficiently concentrated.
More than this, if the cavity is well filled its outline is, of
course, identical with that of the included material. The
z-ray picture will then show the changing contour of the
organ in silhouette. The compounds most used to secure
opacity to the #-ray are the salts of bismuth, generally the
subnitrate or the subcarbonate.

The most numerous experiments of this sort are those of
Cannon, and the cat has been the favorite subject. When
the animal has had a full meal of bread-crumbs and milk
with the bismuth salt evenly mixed in the mass the x-ray
shows the entire form of the stomach. The fundus has an
even outline and preserves it unchanged from hour to
hour, except that a very gradual contraction takes place.
The antrum is traversed by deep but slow-moving peris-
taltic waves, which originate near the transverse band and
pass to the pylorus. The tendency of such waves must be
to force successive portions of the food into the intestine,
but in the great majority of cases the waves bear down
upon a tightly closed pyloric sphincter. The only possible
result is then an eddying movement, the contents advanc-
ing only to rebound from what is, for the time, a blind
pouch. This favors the reduction of the larger morsels and
helps to secure at length the formation of the smooth,
creamy "chyme." But it is probable that most people
have an exaggerated notion of the mechanical powers of
the stomach.

The waves which pass over the antrum arise in the cat
with strange regularity at intervals of ten seconds. Each
wave takes about half a minute to make its way to the
pylorus, so there are commonly three creases to be seen,
all shifting with a motion of clock-like slowness toward the
outlet. During the prolonged period required for the
emptying of the stomach of the cat — eight hours or more —
it is evident that the total number of the waves may be
over two thousand. When the pyloric sphincter momen-

74 NUTRITIONAL PHYSIOLOGY

tarily relaxes, under influences to be discussed presently,
the peristalsis of the antrum naturally drives more or less
of its contents into the duodenum.

Nervous Control of the Gastric Movements. — Muscular
elements of the order found in the stomach have been
said to have an automatic property. We have insisted,
however, that this fact does not exclude the influence of the
central nervous system. There is abundant evidence so
far as the stomach is concerned that the musculature of
the organ is played upon by efferent impulses. If it is
separated from the central nervous system many of its
reactions take place in a nearly normal manner, but we
cannot assume that its adjustments are as well timed and
decisive as they were before. Laboratory trials show that
the impulses which are sent to the stomach may either
accentuate or abate its spontaneous movements. In
other words, they may either excite or inhibit the contrac-
tile elements.

Of the two types of nervous control, the inhibitory
seems to be of particular significance. Complete arrest of
the peristalsis of the antrum may be brought about. This
happens in the cat when the animal is enraged or terrified,
and, indeed, when it seems merely to be restless. Cannon
has again and again seen the peristaltic notches fading
away from the x-ray profile of the antrum when the animal
has wearied of being kept under restraint, and is manifest-
ing its feeling by switching its tail and struggling. He has
seen the regular activity resumed when the cat has been
pacified. Similar facts have been demonstrated for the
rabbit. Since we generally believe that the higher the
grade of an animal's development, the more extensive the
command of the nervous system over its organs, we have
every reason to think that unpleasant emotions may be
accompanied in man also by inhibition of the gastric move-
ments. We shall have occasion to enlarge upon this matter
in connection with the Hygiene of Nutrition.

The Pyloric Sphincter. — The regulation of the escape
of the stomach-contents to the intestine has long been a

THE MOVEMENTS OF THE STOMACH 75

subject of interest. Nearly eighty years ago William
Beaumont published his observations upon the stomach of
Alexis St. Martin, a young Canadian trapper, who had
suffered a gunshot wound in the left side, in consequence
of which he had a permanent gastric fistula. The impres-
sion which went abroad from this celebrated case has
seemed to convey to the less scientific writers the idea that
the pylorus has a power almost akin to intelligent inspec-
tion whereby it permits the passage of certain portions of
the chyme and refuses egress to other portions. This
notion recalls amusingly the teaching of Van Helmont,
in the seventeenth century, that the soul of man resides in
the pylorus. It cannot be said that all the conditions
affecting the discharge from the stomach are entirely clear,
but much progress has been made in this direction.

The sphincter is influenced both by the physical consist-
ency and by the chemical reaction of the gastric contents.
The contact of coarse, angular particles with the adjacent
mucous membrane seems to reinforce its contraction, so
that such material tends to be kept longer in the stomach.
A more important factor with this sphincter, as with the
cardiac, is the acidity of the chyme. It was stated above
that when the stomach-contents becomes distinctly acid-
ified, the tone of the cardiac sphincter is increased. The
acid in this case is acting upon the lining below the irritable
ring. Comparison of the two sphincters shows it to be a
principle applicable to both — that acid acting immediately
above favors their relaxation, while acid below causes them
to tighten. If this is the main factor in regulating the
pylorus, the first opening will occur when the acidity in the
antrum has reached a certain point. The next peristaltic
wave will transfer a little chyme to the'duodenum. At this
instant there will be acid material both above and below
the sphincter. The action from below appears to predom-
inate, so that closure will be established and maintained
until the acid in the duodenum is either neutralized or re-
moved somewhat from the pylorus. When the stimula-
tion from below is no longer effective the acid above will

76 NUTRITIONAL PHYSIOLOGY

cause a second gaping of the sphincter, followed as before
by prompt and decisive contraction. A more efficient
mechanism to insure gradual delivery to the intestine
without distending it locally can scarcely be conceived.
When the latest portions of a meal are leaving the stomach,
the first which went out may have reached the colon, and
intermediate fractions may be undergoing digestion in
numerous loops of the intervening small intestine.

Our meals are usually of a mixed character, including
proteins, fats, and carbohydrates. For purposes of experi-
ment single food-stuffs may be fed to an animal and the
rate of departure from the stomach noted for each. The
x-ray has been employed for this purpose. Carbohydrates
have a striking tendency to escape rapidly to the intestine.
The discharge of proteins and of fats is relatively much de-
layed. We must be content with stating the fact without
undertaking to discuss its somewhat complex causes.

Vomiting. — The occasional expulsion of the stomach-
contents through the cardia and esophagus is accomplished
as the result of a reflex movement in which the chief mus-
cles involved are not the coats of the stomach, but the
contractile tissue of the diaphragm and abdominal wall.
When these contract simultaneously a high pressure is
thrown upon the stomach. Such a pressure may accom-
pany the act of straining or bracing the body for lifting,
but does not ordinarily result in vomiting, it would appear,
because of the resistance offered by the cardiac sphincter.
In the crisis of nausea, however, convulsive movements
of this kind take place with inhibition of the sphincter.
With the passage open, each application of intense pressure
to the stomach may drive a portion of its contents to the
exterior. The palate meanwhile has assumed the same
position as for swallowing; the larynx is drawn forward and
is shielded by the root of the tongue, which is depressed and
grooved. At every descent of the diaphragm the capacity
of the thorax is increased, and as no air is permitted to
enter the lungs the esophagus is dilated.

During the act of vomiting the fundus is said to contract

THE MOVEMENTS OF THE STOMACH 77

steadily upon its diminishing contents. This is not a
movement powerful enough to secure the emptying of the
stomach, but adapts it to be gripped effectively by the
muscles of the body wall. The transverse band is at the
same time strongly contracted and the antrum has a very
small volume. The pyloric sphincter is said to be closed,
but it is a familiar fact that bile from the duodenum may
be pressed backward into the stomach under the stress of
violent vomiting. Profuse salivation precedes and accom-
panies the act. Such a reflex has a manifest value when
it serves to remove from the stomach material which might
prove poisonous. It occurs, however, under many circum-
stances when it seems not to have any advantageous re-
sult. Its apparent uselessness in seasickness has already
been alluded to. It appears equally illogical when, in
pregnancy, it is excited by irritation of the pelvic nerves.

CHAPTER IX
GASTRIC SECRETION AND DIGESTION

IT was in the eighteenth century that the chemical
factors in digestion were first clearly separated from the
mechanical. The accounts which have been preserved
of the experiments of Reaumur (1752) and of Spallanzani
(1777) are of extraordinary interest. An entertaining
summary is to be found in Foster's "Lectures on the His-
tory of Physiology," Chapter VIII. These ingenious
investigators were the first to show that digestive changes
may be caused to take place outside the body and in the
absence of any mechanical process whatever. They
obtained small quantities of gastric juice from various
animals, mixed it with food in flasks and test-tubes, and
watched for signs of alteration. Spallanzani, in particular,
succeeded in bringing about considerable solution of his
samples. .

From that time to the present studies of digestion have
continued. Where the pioneers were forced to be content
with observing the dissolving of solid food, their successors
are drawing inferences regarding the transformations of
unseen molecules. A deeper insight into the meanings
of digestion became possible as the new science of organic
chemistry was swiftly advanced by the researches of
Wohler, Berzelius, and Liebig. As was pointed out in
Chapter III, it is the molecular change which is significant
and not the physical. Experiments in which material is
introduced intp the alimentary canals of animals and later
withdrawn for analysis are constantly compared with
trials in which the digestion takes place throughout in the
thermostats of the laboratory.

The stomach is popularly supposed to have a very large
share in the total work of digestion. It cannot, however,

78

GASTRIC SECRETION AND DIGESTION 79

be claimed that it is indispensable to man. It forms, as
already indicated, a convenient place of deposit for food
and a gradual feeder of the small intestine, but it is mean-
while the seat of preliminary digestive changes which
greatly facilitate the further advance of the process.
Attention has been called to the fact that salivary diges-
tion is continued for a time in the almost stationary con-
tents of the fundus. When this is stopped by the penetra-
tion of the acid gastric juice it is superseded by a new type
of digestion in which the proteins are the food-stuffs acted
upon. The gastric hydrolysis of proteins is generally re-
ferred to as peptic digestion. Before we discuss it in
detail we must consider the nature and the circumstances
of formation of the gastric juice.

This secretion is the product of the numerous, relatively
simple glands with which the mucous coat of the stomach is
provided. Beaumont has vividly described the appearance
presented by the lining of St. Martin's stomach directly
after a meal. The surface, usually of a pale gray, flushed
deeply, and the gastric juice welled up in glistening beads
from the invisible mouths of the glands. Its manner of
breaking out resembled the rising of perspiration from the
pores of the skin. The empty stomach may have a well-
marked film of mucus upon its walls, but its active secre-
tion is limpid and free flowing. The volume which the
human stomach produces in twenty-four hours is ap-
parently very large; if we have a right to judge from what
is known of the dog, it may be 3 or 4 quarts. A dog, being
carnivorous, probably secretes a disproportionate quan-
tity, and our estimate for man should very likely be re-
duced.

The Acid of the Gastric Juice. — Repeated reference has
been made to the acidity of the stomach-contents. The
early investigators were surprised and puzzled when they
were forced to recognize that the acid in question is largely
free hydrochloric. When we consider that this acid can-
not be made industrially except by decomposing a chlorid
with the still stronger and more corrosive sulphuric acid

80 NUTRITIONAL PHYSIOLOGY

and in an earthen container, we can appreciate their
feeling. For here is what we call a strong mineral acid
proceeding from delicate living cells and from fluids of
neutral reaction. The formation of hydrochloric acid by
the cells of the gastric glands has become much more
intelligible in the light of modern chemical teachings.
The current theories cannot be presented here. It is
evident that when the elements of an acid are withdrawn
from a neutral fluid it must, theoretically at least, be
rendered alkaline. We have a capital illustration of the
refinement of the mechanism by which the body preserves
uniform internal conditions in the fact that when gastric
juice is being secreted, the urine, usually acid to common
indicators, becomes alkaline. Thus the normal chemical
equilibrium of the blood is maintained sacred from disturb-
ance. The juice secreted into the antrum is said not to be
acid.

Mention has been made of some ways in which the acid
of the stomach modifies local conditions. We have seen
that it gradually checks salivary digestion. It is the most
important controlling factor for the two sphincters.
Other features of its action must now be presented.
Among these is its distinct antiseptic infleunce. Spallan-
zani noticed that pieces of meat undergoing digestion in
gastric juice passed into solution without putrefying.
Similar pieces kept for an equal time in water had radically
spoiled. This was a significant observation "at the time,
because digestion and putrefaction had been regarded by
many as identical. We know now that putrefactive de-
composition of protein is due to the influence of swarming
micro-organisms, and that hydrochloric acid in the con-
centration usual in the gastric juice restrains the develop-
ment of such forms.

The average strength of the acid in man is given as
0.2 to 0.3 per cent. Such a concentration by no means
suffices to sterilize the stomach-contents, but in all prob-
ability it destroys many kinds of bacteria, including some
which might become the cause of disease. Others it un-

GASTRIC SECRETION AND DIGESTION 81

doubtedly weakens, so that they multiply less rapidly
when the chyme passes on to the small intestine, where the
conditions for bacterial growth are more favorable. It
is not surprising to find that the species of organisms which
thrive most in the stomach are those which themselves
produce acid. The " sour stomach " commonly referred to
is a stomach in which the generation of lactic acid from
sugars is actively taking place. This is a process very like
the familiar souring of milk. Indeed, milk is one source of
such acid fermentation in the stomach. Excessive acidity,
whether due to the native juice or to the activity of bac-
teria, may be a cause of discomfort and a hindrance to
digestion. Cannon has lately shown that acidity above
a certain degree delays the departure of food from the
stomach, and this is easily comprehensible when it is re-
called that after each brief relaxation the pylorus remains
closed until the acid which has just passed has been
neutralized or dispersed. Acidity within limits is a neces-
sary condition of gastric digestion, and this will be dis-
cussed later.

The Secretion of the Gastric Juice. — The glands of the
empty stomach seem to be quite inactive. The natural
supposition that they begin to secrete when food comes in
contact with the mucous membrane is not borne out by the
results of experiments. It is most important to note that
the juice may start somewhat in advance of the arrival of
food. The stomach as well as the mouth may be said to
water at the contemplation of a meal. Abundant evi-
dence of this fact has been furnished by the extraordinary
experiments of the Russian physiologist Pawlow upon
dogs. When a permanent opening has been made to the
interior of a dog's stomach a little gastric juice may issue
when the dog is merely shown food which he likes.

That it is unnecessary to have actual contact with the
stomach wall is still better shown in the case now to be
described. A dog having a gastric fistula is subjected to a
second operation, by which the esophagus is severed and
the portion connected with the pharynx made to open
6

82 NUTRITIONAL PHYSIOLOGY

through the skin of the neck. Whatever is swallowed by
the dog is now returned to the exterior. The pleasure of
eating is not impaired. To maintain nutrition suitable
food may be introduced directly into the stomach. When
the dog chews and swallows a meal he is quaintly said to
have a "Scheinfutterung" — a fictitious feeding. This
proceeding is accompanied by a steady flow of the secre-
tion from the gastric fistula.

The secretion obtained under circumstances like the
above is called the psychic secretion. This term serves to
emphasize the fact that a mental element is a necessary
incident of the reaction. The conditions governing gastric
secretion seem to be quite parallel with those which regu-
late the movements of the stomach, and their importance
in hygiene is equally evident. There are a number of
individuals who have in various ways lost the power to
swallow food, commonly because of the closure of the
esophagus. Their lives are preserved by feeding through
gastric fistulas established by surgery. These unfortu-
nates find it to their advantage to attend to the idea of
eating, and to taste and chew portions of their food at the
time when it is being introduced into their stomachs.

After a brief period of fictitious feeding the flow of
gastric juice into the stomach of a dog may continue for
two or three hours. In view of this we must conclude
that, however important the mental state may be for the
initiation of the process, it need not be its accompaniment
throughout. We cannot pretend that a meal receives our
constant attention during any such interval. Neverthe-
less we are hardly likely to overestimate the necessity of
securing a normal start. If the initial circumstances are
not favorable the secretion may be long delayed or even
lacking.

Now that we have reserved a due place for the psychic
element, we must pass on to consider the other factors
which modify the activity of the glands of the stomach.
Much has been learned from the surprising operations of
Pawlow and others, who have succeeded in dividing the

GASTRIC SECRETION AND DIGESTION 83

stomach of the dog into two parts without robbing either
of its connection with the circulatory and nervous systems.
When the dog has recovered from the immediate effects
it may be said to have two stomachs. Either or both may
communicate with the exterior by a fistula, while one still
retains its normal relations with the esophagus and small
intestine. This arrangement makes it possible to place
food in one stomach and to obtain and measure the un-
mixed juice secreted into the other. The evidence goes
to show that when the glands in the lining of one sac are
active there is corresponding activity on the part of those
in the other.

A most significant fact is at once noted when such a dog
is under observation. There are kinds of food, perfectly
appropriate for the animal's nutrition, which may lie in
the stomach without exciting any flow of the juice. This
is said to be true of bread, white of egg, starch, and some
sugars. The same articles of diet would be met by an
abundant gastric secretion if they had been eaten with
enjoyment by the hungry dog. It makes a radical differ-
ence, then, whether these materials enter the stomach
through the mouth and attract the favorable notice of
the animal, or whether they are slipped through a fistula,
a proceeding which would probably not be recognized by
him as a mode of feeding.

On the other hand, there are some things which do cause
an outbreak of gastric juice by their mere presence in the
stomach and in the absence of any psychic factor. To a
somewhat limited extent this is the case with water,
though only, it appears, when there is enough of it to dis-
tend the stomach slightly. The best-known excitant of
the secretion is meat, and the property is said to belong to
the extractives or minor substances in this food, and not to
the proteins of which it is chiefly composed. Meat causes
a considerable flow of the juice, but there is a much longer
initial delay than when the psychic element has its normal
place. In fact, the total quantity produced when meat is
introduced into a dog's stomach without attracting his

84 NUTRITIONAL PHYSIOLOGY

attention is decidedly less than when it is eaten in the
natural way, and when the psychic and chemical agencies
are combined.

It is"unfortunate that our knowledge of this matter has
been drawn so largely from a carnivorous animal. Meat
might be expected to stimulate the stomach of the dog
more surely than other foods. How far the secretion may
be elicited by placing other compounds in the stomach is
imperfectly known. Milk is credited with some power to
call it forth, but its superiority to water seems doubtful.
Alcohol is said to have a positive action of the same kind.

It is claimed that the dextrins, the intermediate bodies
produced in salivary digestion of starch, have the property
of starting the gastric flow. If this is true it is interesting
as establishing a connecting link between the two successive
processes, and making it apparent how one may tend to
insure the setting in of the other in due time. A link of
this sort exists between gastric digestion and pancreatic
secretion, as we shall have occasion to point out. The
condiments, such as pepper and spices, have a reputation
for stimulating the discharge of gastric juice, and un-
doubtedly do so when they favorably affect the flavor of
the food. They are known to increase the blood flow in the
lining of the stomach, which would perhaps help to con-
tinue the secretion process when once under way, but
whether they can actually initiate it apart from their
psychic effect remains uncertain.

When gastric secretion is well started there is provision
for its maintenance as long as the stomach contains food.
It appears that some of the early products of digestion
act after the manner of the extractive substances of meat
and excite the glands to continued activity. The acid
itself has been found to be absorbed by the cells lining the
antrum and to set in motion a train of events leading to
the same result. This form of stimulation will evidently
cease only with the departure of the last portions of the
acid chyme. The flow of gastric juice is retarded by fats
and by alkaline mixtures.

GASTRIC SECRETION AND DIGESTION 85

Digestion in the Stomach. — The gastric juice is usually
said to contain two enzymes. Recent work indicates the
presence of a third. The two familiar ones are pepsin and
rennin. The third is the gastric lipase. Certain writers
have questioned whether we ought to speak of pepsin and
rennin as distinct individuals, suggesting rather that there
is in the secretion a single body having two sets of proper-
ties. We need not enter into such a discussion; we shall
for the present continue the convenient usage of speaking
of pepsin and rennin as two substances.

Rennin. — The fact that extracts of the stomach wall
cause the curdling or coagulation of milk has been known
from very early times. Such extracts, usually derived
from the stomach of the calf, have long been in use in the
manufacture of cheese. Rennet is the industrial term for
an extract with this property; rennin is the scientific term
for the supposed enzyme contained in it. Cheese curd
consists of the bulk of the protein of milk which has
undergone an obscure chemical change and has passed into
an insoluble form. From a physical standpoint this is an
anomaly among the digestive processes. We look to see
solids becoming liquids, while in this curious instance a
liquid becomes a solid. No very convincing explanation
of this occurrence has been offered. It may be suggested
that it prevents an unduly rapid passage to the small in-
testine, but so far as we know the mechanism of the
pylorus is entirely competent, even for liquid food. The
curd when formed has to undergo solution like any other
solid.

The action of rennin becomes the more enigmatic when
it is noted that it is found in the stomachs of animals
which do not have milk in their normal diets. Milk is
curdled by extracts of various organs other than the digest-
ive glands and by some vegetable juices. In the human
stomach a very firm curd may be formed when a large
quantity of cows' milk is taken at one time. The dense
mass may be slow to digest. Human milk is said never to
set into such a tenacious coagulum, and this is natural,

86 NUTRITIONAL PHYSIOLOGY

since, regarded as a solution of proteins, it is much more
dilute than the milk of the cow.

Peptic Digestion. — The chief enzyme of the gastric juice
is the one commonly called pepsin. Its relations with the
acid of the stomach are so close that many writers urge
that we should speak rather of "pepsin-hydrochloric acid/*
the term suggesting the existence of a compound of the
two which is responsible for the action on the food. The
power to digest proteins is manifested only with an acid
reaction, and is permanently lost when the mixture is made
distinctly alkaline. The conditions which permit peptic
digestion to take place are, therefore, precisely those which
exclude the action of the saliva.

When protein in solid form, such as boiled white of
egg, is subjected to the influence of gastric juice the pieces
swell and become softened. Later they are dissolved.
When the trial is made with protein which is originally in
solution, such as unboiled white of egg, there is no visible
evidence of change. But there are physical and chemical
tests which can be employed to show that digestion is as
definite a change in this case as in the other. An early
manifestation of this fact is the loss of the property of
coagulation on heating. Later there are indications that
the molecules are undergoing cleavage. At each successive
stage there is a gain in the power of diffusion, a reduction
of viscosity, and a diminution in the number of precipitants
which can be employed to throw the protein out of solution.

Physiologic chemists have studied minutely the charac-
teristics of the hydrolytic products during the advance of
peptic digestion. They have attempted to identify nu-
merous compounds, each of which has a transient existence
and is then itself hydrolyzed. For our present purposes it
would be unprofitable to dwell upon such questions of detail.
Certain of the earlier cleavage products are included under
the general name of proteases or albwnoses; others, arising
later and of a simpler character, are called peptones.
Roughly speaking, there is a parallel between salivary and
peptic digestion. In either case, molecules of great size —

GASTRIC SECRETION AND DIGESTION 87

of starch and protein respectively — are subdivided pro-
gressively, first with the formation of somewhat complex
bodies — dextrins and proteoses — later to form maltose in
the first instance and peptones in the second. The corre-
spondence is imperfect in several ways; for example, mal-
tose is a single substance, while there appear to be a number
of peptones. It will be remembered that maltose itself
is slowly transformed by long-continued action of saliva.
Quite similarly, the gastric juice will in time effect a fur-
ther digestion of the peptones, but this advanced diges-
tion seems normally to be postponed until the intestine is
reached.

Gastric Lipase. — Down to a recent time it was held that
fat underwent no true digestion in the stomach. It was
recognized that some forms of fat, butter, for example,
must be melted, and that the fat in the adipose tissue of
meat might be released from the enclosing cells when their
protein portion was dissolved. These, however, are mere
physical changes. Attentive study has brought out the
fact that there is, after all, some hydrolysis of fat in the
stomach, though it is probably slight. An enzyme with
this action is accordingly assumed and is spoken of as
gastric lipase. It is only the most finely divided (emulsi-
fied) fats which seem to be appreciably affected. The
products of this decomposition as well as of the later fat
digestion in the intestine are glycerin and free fatty acids.

Summary. — The material passing the pylorus is com-
paratively dilute and normally free from coarse particles.
It is acid in reaction, both the native hydrochloric acid and
the acids formed by fermentation contributing. Much
of the food is as yet practically undigested. On the other
hand, some progress has been made in the transformation
of cooked starch into sugar. The proteins are partially
peptonized. If milk has formed a part of the diet, it will
have been curdled and redissolved. Fats may have been
liquefied and scattered, but are not likely to have been
extensively hydrolyzed. On the whole, gastric digestion
may fairly be described as preliminary in character.

CHAPTER X

THE SMALL INTESTINE: ITS MOVEMENTS,
SECRETIONS, AND DIGESTIVE PROCESSES

THE intestinal content is propelled on its winding way
by peristaltic movements. These are similar in their
mechanical principle to the waves of muscular contraction
passing down the esophagus in the act of swallowing.
They are, however, much slower and gentler in character.
As a rule they do not run an uninterrupted course from the
pylorus to the ileocecal valve, though such a phenomenon
is an occasional possibility. More commonly a wave will
travel a limited distance and then fade out, leaving the
material which it was moving to rest for a time in some
depending loop. If this is true, the progress of the food is
intermittent; x-ray observations upon human subjects
have led to the estimate that it takes four or five hours for
the passage through the whole length of the small intestine.
This time may be assumed to vary widely with the indi-
vidual and the diet. Reduced to an average rate the data
quoted above give us about one inch per minute.

Intestinal peristalsis, like the movements of the stom-
ach, is governed mainly by local mechanisms. Yet in this
case as in the other the central nervous system may exert
an influence tending to accelerate or to suppress the ac-
tivity of the muscular coats. The second or inhibitory
action is the more marked. A question much discussed
is whether the direction of peristalsis in the small intestine
is at all subject to reversal. The sum of the evidence at
present supports the view that such a reversal (antiperis-
talsis) is possible, but only under conditions which are
clearly abnormal. Ordinarily it is plain that the intestine
is distinctly specialized to act in one way rather than the
other.

THE SMALL INTESTINE 89

Not all the muscular contractions exhibited by the small
intestine have a progressive character. Frequently a loop
which contains food will become creased at short intervals
by rings of constriction which do not shift their position,
but remain stationary for a time. The internal effect is to
create a series of small pouches holding separate portions
of the chyme. After this condition has persisted for a
time the regions originally contracted become relaxed, and
new contractions set in at points midway between them.
Under the influence of such movements the food is con-
stantly shifted about and subdivided, but it is not driven
steadily in one direction. This "marking time" on the
part of the small intestine is referred to as rhythmic seg-
mentation. Inasmuch as it serves to alter the contact
relations between the intestinal contents and the lining,
it probably favors absorption. Some writers have made
much of the effect which these contractions may be as-
sumed to have upon the flow of blood and lymph in the
walls of the canal. When pressure is applied at brief
intervals to tissue containing these fluids the result may be
described as a massaging action, hastening the circulation
and crowding out some of the lymph. This again must
tend to promote the absorption process.

A longitudinal movement of individual loops is often
described. Two neighboring turns of the intestine may be
seen to glide the one upon the other, coming to rest after
slipping a short distance, and presently reversing the direc-
tion of their travel. This form of activity cannot defi-
nitely further the progress of the contents, but when it
affects segments enclosing liquid material we may suppose
that the food and juices are tilted about and brought into
relation with the largest possible area of absorbing surface.

The Secretions Entering the Small Intestine. — In
Chapter VI it was stated that this part of the canal re-
ceives the contributions of the liver and the pancreas, as
well as of the microscopic glands in its own mucous mem-
brane. The bile and the pancreatic juice, it will be re-
membered, enter just below the pylorus. The intestinal

90 NUTRITIONAL PHYSIOLOGY

juice is produced by all parts of the extensive lining, but
more abundantly in the upper than in the lower segments.
The three secretions have some characters in common.
They are all alkaline in reaction, owing to the presence in
them of sodium carbonate. This confers upon them in a
considerable degree the power to neutralize acids. As
the acid chyme from the stomach meets the alkaline secre-
tions in the duodenum there must be more or less carbonic
acid gas evolved. This may be helpful to the digestive
process, since the tendency will be to lighten the texture
of the food particles, much as dough is lightened by the
agency of yeast.

It is not merely the acid from the stomach which may be
combated by the alkali of the juices below; there are two
other sources of acid to be taken into account. One of
these is found in the bacterial fermentation, chiefly of
sugars, which goes on in the intestine. The second is the
entirely normal formation of free acids occurring in the
course of fat digestion. So far as the first class of acids are
neutralized the products are mainly lactates and buty-
rates; the fatty acids may be converted into soaps. There
is no guarantee of an exact proportionality between the
acids and the alkali, and it is impossible to say which will
be in excess in a particular part of the canal. Generally,
however, the resulting reaction of the mixture is not far
from the neutral point.

The united volume of the three secretions is held to be
very large, but any estimate tends to mislead, since
throughout the length of the intestine we have the with-
drawal of water keeping pace approximately with its in-
flow. In a certain section selected for observation the
bulk of the contents may show little change during a long
period, and yet there may have been profuse secretion
entirely disguised by counterbalancing absorption. In
this consideration we see indicated an important service
shared by all the digestive secretions, that of supplying
liberal quantities of water to act as the solvent and carrier
of food-stuffs destined for absorption. If absorption were

THE SMALL INTESTINE 91

to continue without compensatory secretion, a final stage
might be reached in which the cavity of the intestine would
be drained of all fluid, while the walls would be crusted
with a dry residue suggestive of boiler scale.

The Pancreatic Juice. — In considering the causes of
gastric secretion the importance of the central nervous
system called for emphasis. This is not true to the same
extent of the government of the pancreas, though here
also it is held that the nervous system plays a part. A
chemical means of control is better known. As has been
hinted before, there is an intimate relation between the
gastric activity and the later awakening of the pancreas
from its usual state of repose. The arrival of acid from
the stomach in the duodenum causes a timely outflow of
pancreatic juice.

This might be supposed to be an instance of reflex action,
like the production of saliva when acid is taken into the
mouth. It has been shown, however, that it has another
explanation. The acid, striking into the lining membrane
of the duodenum, initiates a series of reactions which have
been studied in detail, and which lead at last to the for-
mation of a substance of quite definite chemical properties
called secretin. This finds its way into the circulation,
and, like a drug, is swept far and wide. It stimulates the
pancreas to produce its secretion, augments the formation
of bile by the liver, and probably excites the glands in the
wall of the intestine to greater activity. In the light of
these facts it becomes clear that a vigorous gas'tric digestion
with the strongly acid chyme which results goes far to
insure a normal intestinal process.

The pancreatic juice in man is an abundant secretion —
a pint or more daily — and, in marked contrast with the
saliva and the gastric juice, it has relations with all three
principal classes of food-stuffs.. It contains an enzyme
which some have thought to be identical with the ptyalin
of the saliva, but generally called amylopsin or pancreatic
amylase. Thus the progress of starch digestion, inter-
rupted for a time in the stomach, is now renewed. The

92 NUTRITIONAL PHYSIOLOGY

slight acidity which may exist in the intestine is not likely
to prevent this type of digestion from going on to com-
pletion.

Beside acting on starches, the pancreatic juice continues
and greatly accelerates the hydrolysis of fats which has
been barely begun in the stomach. The enzyme concerned
is called steapsin in the older books, but by more recent
writers lipase. The immediate products are glycerin and
fatty acids. A secondary formation of soaps is a possibil-
ity already indicated. When an oil undergoes digestion
it breaks up at an early stage into microscopic drops and is
said to be emulsified. This subdivision clearly multiplies
the surface of contact between the food and the digestive
juice, and has an effect corresponding to that of mastication
upon solids. It is, thereftire, most helpful to digestion,
but it is not to be confused with digestion itself.

The statement is commonly made that the pancreatic
juice continues the work of pepsin upon proteins by virtue
of an enzyme called trypsin. While this is approximately
true, it calls for a certain qualification. If the juice is
carefully collected as it comes from the main duct of the
pancreas without being allowed to mingle with other se-
cretions or even to touch the lining of the intestine, it is
reported that it is usually incapable of hydrolyzing pro-
teins. This power it gains in a striking degree when it has
been mixed with ever so little of the intestinal juice, the
succus entericus, as it is sometimes called. The interaction
of the two secretions is described as resulting in the
"activating" of the pancreatic juice. The natural infer-
ence is that the inactive fluid contains in solution a body,
not yet deserving the name of enzyme, but ready to be-
come one by a quick transformation. An inactive ante-
cedent body of this kind is termed ^.zymogen; in this specific
instance, trypsinogen. Acting on the assumption that
there is a definite substance in the intestinal juice capable
of changing trypsinogen to trypsin, physiologists have
given the name of enterokinase to the agent concerned.

Tryptic digestion differs in characteristic ways from the

THE SMALL INTESTINE 93

peptic process which has been described. Acid which is
essential to digestion in the stomach is antagonistic to the
pancreatic type. Trypsin does its work best in a nearly
neutral mixture. In the normal course of events it acts
upon material already partly hydrolyzed, but it has the
power to carry through all its stages the digestion of native
protein. If the food is a solid, mere inspection reveals a
difference between peptic and tryptic solution. In the
former case there is marked swelling, as previously stated ;
in the latter there is progressive corrosion, a shredding or
honeycombing of the specimen.

When chemical methods are employed in the study of
tryptic digestion, it is found to run a course roughly parallel
with that of the gastric digestion of proteins. Correspond-
ing intermediate bodies — proteoses — are described in both
instances, though it is said that some stages in the tryptic
process are passed so rapidly as to seem almost to be omit-
ted. What distinguishes the pancreatic proteolysis most
radically from the peptic is the facility with which the
peptones are broken down into still simpler bodies. We
have made the statement that these late cleavages take
place only very slowly under the influence of gastric juice.
So it becomes clear that trypsin is distinctly adapted to
follow after pepsin in the accomplishment of protein di-
gestion.

Peptones are compounds which are simple by comparison
with standard proteins, but which are still too complex
to be given precise chemical formulas. When they are
hydrolyzed, most of the products come within the knowl-
edge of the organic chemist so definitely that their molecu-
lar structure can be confidently expressed. To the student
it must be admitted that such formulas do not appear
simple, but if he is disposed to resent the use of the word
he is to reflect that these molecules stand in some such rela-
tion to the original protein complex as the bits of the mosaic
bear to the whole design. It is with some such an idea
that the Germans have called them Bausteine, that is,
the building-stones, from which a new architecture can be

94 NUTRITIONAL PHYSIOLOGY

constructed. The simplest products of the tryptic process
are conveniently called amino-acids.

The Intestinal Juice. — This copious secretion was for-
merly regarded as having little to do with digestion. The
present disposition is to credit it with a very considerable
share. When the pancreatic juice is prevented from enter-
ing the intestine it remains possible to keep up the nutri-
tion of the animal, and one must conclude that the intes-
tinal juice is successfully preparing more than one kind
of food for absorption. Samples of the secretion have often
been obtained from loops of the intestine disconnected from
the remainder of the canal. Different workers give vary-
ing accounts of its properties.

The feature of its digestive action concerning which there
is the most general agreement is the hydrolysis of the more
complex sugars, the disaccharids. Of these sugars, three
are commonly present, and there appear to be three en-
zymes adapted to act upon them. Maltose arises princi-
pally from the salivary and pancreatic digestion of starch.
It is hydrolyzed to dextrose (glucose) by an enzyme, which
is best called maltase. Lactose, or milk-sugar, is similarly
converted into equal parts of dextrose, and the less familiar
sugar galactose by the enzyme lactase. _Saccharose, or
cane-sugar, gives rise to dextrose, and a sugar of different
properties, levulose (fructose), under the influence of the
enzyme, invertase.

When an extract is prepared from the thoroughly minced
lining of the small intestine it can be shown to have the
power to cause proteoses and peptones to undergo hydro-
lysis, though it is said not to act upon the original unmodi-
fied protein. This is equivalent to saying that such an
extract can parallel the later work of trypsin, though lack-
ing its power to initiate digestion. The enzyme implied
has been named erepsin. It is regarded as an open ques-
tion whether this enzyme normally enters the cavity of the
intestine or does its work within the confines of the cells
from which it can be extracted. We may conceive that
when an animal is deprived of its pancreatic secretion it is

THE SMALL INTESTINE 95

still able to digest proteins, pepsin beginning the digestion
and carrying it to a stage at which the products are suscep-
tible to the action of erepsin. The presence of enteroki-
nase in the intestinal juice has just been noted.

The Bile. — The secretion of the liver cannot be regarded
in the same light as the digestive juices mentioned hereto-
fore. It is not secreted merely after meals, but is always
flowing through the ducts which converge from the several
lobes of the liver. It is not necessarily entering the in-
testine at all times, since the gall-bladder provides a place
for its temporary storage, as described in Chapter VI.
While the production of bile never ceases, it does show an
acceleration during the digestive periods, and this is be-
lieved to be in response to the stimulating effect of secretin.

Bile attracted the attention of physicians in very early
times, its bright color and intensely bitter taste giving it
a certain distinction. It entered largely into ancient
theories of disease and of medicine. We have traces of
these facts in the root-meaning of such words as bilious,
choleric, and melancholy. In popular estimation bile is
a poison arising now and then in the system and causing
digestive disturbances. Patient study has shown that the
bile is a complex mixture, and that it numbers among its
constituents some which are waste-products and others
which have a favorable effect upon the progress of diges-
tion and absorption. It stands, therefore, in a position
intermediate between that of the gastric juice, which is
formed solely to advance digestion, and that of the urine,
which is composed of material useless to the body.

The pigments of the bile are counted as waste substances.
A red one predominates in carnivorous animals. The bile
of the herbivora is green, as is also the case with human bile.
These pigments show in their chemical nature a close rela-
tionship with the red coloring-matter of the blood, the
important compound hemoglobin. All the evidence goes
to show that the bile-pigments are modified fractions of the
great hemoglobin molecule, and that their abundance is
an indication of the amount of destruction suffered by the
red corpuscles of the blood. These pigments are relatively

96 NUTRITIONAL PHYSIOLOGY

insoluble and are not always successfully carried to the
intestine by the bile. When they deposit in solid form in
the gall-bladder they contribute to the formation of "gall-
stones," aggregations in which another waste-product,
the waxy cholesterin, may be included. The pigments are
usually more or less altered by bacterial action in the
course of their journey through the canal, and eventually
become the chief coloring-matter of the feces.

The bitter taste of bile is due to. two organic salts of
high molecular weight. These seem to have a totally dif-
ferent significance from that of the pigments and choles-
terin. They are not lost to the body, but are absorbed
from the lower part of the small intestine and are presum-
ably secreted again and again. This phenomenon has
been spoken of as the "circulation of the bile-salts." The
withholding of these bodies from the alimentary tract tends
to derange digestion and, in particular, to diminish the
absorption of fats. When bile is tested by itself it shows
only the feeblest digestive powers, yet pancreatic digestion
is greatly promoted by its presence, and it may be likewise
an ally of the intestinal juice.

Light is thrown on the properties of the bile by observing
the condition of jaundice. This disorder is commonly
caused by the more or less general plugging of the bile-
ducts with mucus. The secretion cannot make its escape
from the liver in the normal way and some of it enters the
circulation. Bile-pigments make their appearance in the
white of the eye and in the skin. The urinary pigment,
which is always closely related to the pigments of the bile,
is much increased. The ill feeling which usually attends
the condition may be due in part to the mildly poisonous
effect of the abnormally retained bile constituents. It is
likely to be aggravated by indigestion. The bile-salts are
lacking and the capacity to digest the food and promptly
absorb the end-products is greatly reduced. Bacterial
action in the intestine may become pronounced. This
last fact has suggested that the bile may be an antiseptic.
It cannot be shown to have anything like a universal action
of this kind, but it is very probable that it has a selective

THE SMALL INTESTINE 97

one, favoring one type of organism and restraining another.
Even though it had no such influence, the intestinal
bacteria might be expected to multiply in its absence, for
the simple fact of delayed absorption would suffice to bring
this about. Our best defense against excessive fermenta-
tion and putrefaction is in the early and complete removal
of the food from the sphere of action of micro-organisms.

Summary. — The secretions flowing into the small intes-
tine supply enzymes in sufficient number and variety to
accomplish the digestion of all common foods. The trans-
formation of starch to maltose begun in the stomach is
completed by the pancreatic amylase. The resolution of
proteins into the simple structural units from which their
molecules are built is carried out under the influence of
trypsin and perhaps of erepsin. The last-named enzyme
is supposed by many to work upon the tryptic products as
they pass through the lining cells on their way to the blood.
Fats are hydrolyzed by the pancreatic lipase, with the
formation of glycerin and fatty acids, the latter being
in some measure converted to soaps. . The disaccharids are
changed to monosaccharids, a work attributed to the in-
testinal juice. Fermentation caused by bacteria has been
taking place along with the strictly normal processes.
The most evident products are organic acids, which may or
may not be fully neutralized by the alkaline secretions.

Protection Against Self-digestion. — There has been
much discussion of the fact that the proteolytic enzymes
in the digestive tract do not ordinarily attack its mucous
membrane. They may do so after death and when the
tissues are in an abnormal condition, as in case of gastric
ulcer, the juices may strongly antagonize the healing pro-
cess. It is often asserted that there is a definite chemical
difference between the proteins of living and of lifeless
matter. A recent explanation of the resistance which
living cells offer to digestion is based on the apparent fact
that such cells form bodies which have the capacity to neu-
tralize enzymes in fixed proportions. The name of anti-
enzymes has been applied to such protective substances.
7

CHAPTER XI
THE LARGE INTESTINE

THE material passing into the colon is dilute and much
reduced in volume as compared with the chyme passing out
of the stomach. In the lower part of the small intestine
the absorption of water more than counterbalances the
secretion, hence the shrinkage of the contents. But
since the end-products of digestion are being absorbed also,
there is no tendency toward extreme concentration. In
the large intestine there is but little secretion, and the con-
tinuation of the absorption of water reduces the contents
at last to a nearly solid consistency.

The colon appears to be of very unequal value to animals
of different classes. In the carnivora the work of diges-
tion and absorption is so nearly finished by the small in-
testine that very little remains to be done. The small
quantity of matter having a potential food value may be
accompanied into the large intestine by enzymes, which
may there carry further their digestive action. Such food,
however, is likely to be a negligible amount, and the digest-
ive powers of the mixture at this point are unreliable.
In the herbivora, the food being bulky and refractory,
a considerable portion may arrive undigested in the colon.
These animals have very capacious ceca, in which great
masses of contents seem to be held for long intervals.
The digestion occurring there may be partly effected by
the native secretions, but it is believed to be largely the
work of bacteria.

The average human being resembles the carnivorous
type rather than the other. Numerous cases have been
observed in which no use was made of the colon, and it was
never difficult to maintain nutrition. When the large in-

98

THE LARGE INTESTINE

99

testine is no longer traversed the discharges are watery and
rather voluminous, but they contain only small percentages

Fig. 14. — The colon with special reference to its movements:
T is placed near the seat of frequent sustained or tonic contraction.
From here backward to the cecum (C) antiperistalsis is of common
occurrence, but this segment is swept also by occasional waves in
the opposite direction ; V indicates the position of the ileocecal valve,
which prevents reflux to the small intestine under the influence of
antiperistalsis; beyond T the infrequent movements which take place
have always a progressive character; S and Sl are regions often
found to contain stationary contents; K is the kink referred to in
the text; R is the rectum, and Sph is the double sphincter at the
anus.

of valuable nutriment. The fluid escaping from the end of
the small intestine or the beginning of the large is described
as entirely inoffensive, which indicates that putrefactive

100 NUTRITIONAL PHYSIOLOGY

decomposition of protein is usually confined to the colon.
Protein putrefaction is nowadays held accountable for
many ailments, and from this view has arisen the common
teaching that man would be better off without a large in-
testine. Comment upon this opinion may well be re-
served for the chapter on the Hygiene of Nutrition.

The Movements of the Colon. — The average rate of
progress in the large intestine is low. A factor tending to
make it so has recently been brought to notice through the
studies of Cannon and others. The ascending colon has a
property not duplicated elsewhere in the canal, that of
habitual antiperistalsis. As repeated instalments pass the
ileocecal valve the cecum is filled and the accumulating
contents reach higher and higher levels, but instead of
thrusting onward this gathering mass, the circular mus-
cular coat most of the time urges it backward. The result
is said to be much like what is seen in the antrum of the
stomach, although here the direction of the movement is
reversed, that is, the waves of contraction press the mix-
ture downward into a pouch, from which the only escape
is by an eddying reflux through the crawling ring. It is
assumed that the ileocecal valve prevents any return to the
small intestine. A deliberate hindrance to progress at this
point may be of service so far as it secures a more perfect
absorption of digestive products and the retrieving of sur-
plus water. The antiperistaltic waves are described as
starting from a zone in the transverse colon near the mid-
line of the body.

From time to time contractions sweep over the ascending
colon in what we should call the normal direction, and send
onward more or less of its contents. Beyond the midpoint
referred to above those movements which occur are invari-
ably progressive, but are separated by long periods of re-
pose. If we adopt a recent estimate we can make the fol-
lowing general statement: The foremost portion of a meal,
that which was first to pass the pylorus and the ileocecal
valve, may be expected to be near the spleen at the end of
the ninth hour. When defecation occurs all the colon be-

THE LARGE iNfeklNE 101

yond this point may be freed of its contents. If this hap-
pens once in twenty-four hours the age of the food residues
discharged will evidently vary from thirty-three hours, for
the matter longest retained, down to nine hours, in the case
of that which barely came within the section evacuated.
There must be extremely wide departures from these fig-
ures in individual subjects.

The descending colon is reported to be found empty, as a
rule, when observed with the #-ray. This is taken to mean
that it is an irritable segment which is stimulated to ad-
vance all that enters it to the sigmoid flexure without de-
lay. The sigmoid flexure, on the contrary, permits a rela-
tively large accumulation before it is excited to contract.
This region of the colon is interesting from a biologic stand-
point, because it seems to be an adaptation to the erect posi-
tion of the body. Quadrupeds hold the large intestine,
roughly speaking, in a horizontal plane, and with them
the effect of gravity on the contents is immaterial. These
animals have no sigmoid flexure, the descending colon in-
clining toward the midline, and joining the rectum without
the S-shaped connection. When the erect position is as-
sumed there will naturally be a tendency on the part of the
fecal material to settle toward the anus. The develop-
ment of the sigmoid in the apes and in man provides a
place of lodgment for the burden and spares the rectum
from a constant distention.

The sharp bend between the sigmoid and the rectum,
amounting almost to a kink, such as one may see in a rub-
ber tube that has been doubled, is not easily passed by the
feces. It is. only when the quantity has become consider-
able that a vigorous peristalsis overcomes the resistance
and fills the rectum. Here for the first time the pressure
arouses distinct sensations. If defecation is postponed the
tone of the rectum may be lowered and these sensations
cease to be felt. If the occasion favors, the anal sphincters
are inhibited and the rectum is emptied by its own peris-
talsis, reinforced by external pressure developed through
contractions of the abdominal muscles and the diaphragm.

102 NUTKlYIOfrAV PHYSIOLOGY

The action of the skeletal muscles at this time resembles
that in vomiting, but is, of course, much more largely
voluntary and more sustained in character.

The Feces. — Two classes of material may be mingled in
the contents of the colon: the residues of the diet and the
excretions of the alimentary tract including its glands.
The proportion existing between these two is a variable
one. Physiologists have lately come to regard the food
residues in the feces, under average conditions, as forming
a less prominent element than was formerly supposed.
A correspondingly increased importance is assumed by the
excretions. The feces passed during long fasting are evi-
dently made up exclusively of bodies of the second class.
Such feces, while small in amount, may have practically
the same composition as those formed upon a moderate
and digestible diet. This suggests that the increased
quantity which results from eating does not imply a greater
residue so much as a greater production of secretions along
the active canal. A mass very much like normal feces
may gather in an isolated loop of the intestine to which no
food is admitted. Among the substances originating from
the tract rather than from the food are the modified bile-
pigments, cholesterin or its derivatives, mucus, and de-
tached cells.

Bacteria, living and dead, intimately mixed with the
numerous products of their own life activity, are promi-
nent in the bowel discharges. These organisms can in one
sense be said to have their origin in the diet, since that was
the source of the primary seeding or infection. From
another point of view they are largely developed at the
expense of the body itself, for they have multiplied in the
intestine and may have lived in part upon its secretions as
well as upon the food. The gases of the colon are due to
them.

True food residues include the indigestible matter of the
diet and some undigested food — normally but little of the
latter. Absorption is strikingly efficient with most sub-
jects and reasonable diets. Not more than 5 per cent, of

THE LARGE INTESTINE 103

most foods goes to waste. The principal indigestible com-
ponent is the cellulose of vegetable tissues. When it is
abundant, as in a diet containing coarse, woody matter,
such as is eaten by the rabbit, the feces are given an im-
mense bulk. The same result is secured in a measure
when man adopts a ration in which fruits and vegetables
enter freely. Much cellulose is an impediment to com-
plete digestion and absorption of the proteins and carbo-
hydrates which it envelops, but the foods in which it
occurs are cheap and the waste involved does not open
an economic question of moment. Some bacterial decom-
position of cellulose is said to take place in the colon and
may be of slight advantage to us, not that we profit by the
products of such fermentation, but that a freer exposure
of proteins and starch may be insured.

Most writers have held that a moderate amount of in-
digestible material is a desirable feature of the diet. This
teaching was emphasized by the celebrated Sylvester
Graham early in the last century. Lowell has referred to
him as an "apostle of bran." Later authorities have not
recommended such wholesale loading of the tract with
husks and fibers, but an admixture of such elements has
been generally advocated. The favorable effects include
the well-recognized stimulation of peristalsis and probably
a better distribution of food along the canal. The indi-
gestible fraction of the food has been spoken of as ballast
and also by the expressive name of "roughage."

Under abnormal circumstances the loss of valuable food
through the stools may become extensive. This will be
the case when absorption is interfered with either by an
acceleration of peristalsis or in other ways. Different
cathartics bring this about in a varying manner, some by
hastening the rate of propulsion, and others, especially the
salines, giving the intestinal contents a concentration and
a chemical character which forbids absorption. A mild
disturbance of this nature is more apt to result in a waste
of fats and fatty acids than of other forms of food.

Rectal Feeding. — While the large intestine of man is not

104 NUTRITIONAL PHYSIOLOGY

called upon in normal conditions to absorb much nutriment,
it has a marked reserve power to do so. This is often of
the greatest value in sickness, when it may be possible
to tide over a critical time and to keep up a measure of
strength by introducing suitable foods into the colon.
Fluid mixtures used in this way must be of low osmotic
pressure, a fact which, unfortunately, rules out the sugars
excepting in small amounts. Milk and eggs are much em-
ployed. Experience has shown that the body makes a
partial use of the proteins offered, even though they have
not undergone digestion. The results are better when arti-
ficial digestion has been brought about in advance. So far
as we know there is no flow of efficient digestive secretions
in response to the introduction of food through the rectum.
It is believed that nutritive enemata may in part at least
enter the region of prevailing antiperistalsis, and that this
must favor their long retention and better utilization.
Stimulants may be given through the large intestine and
water to aUay thirst.

CHAPTER XII
THE BLOOD

THE intestinal lining is a barrier between the mixture
of food and secretions within the canal and the blood
which flows through the neighboring vessels. The main
problem to be dealt with in treating of absorption is the
transfer of portions of the intestinal contents to the blood.
We shall find that a certain fraction of the incoming nutri-
ment is carried for a time in the lymph, but this is destined
before long to blend with the main current of the circula-
tion. It will be for our advantage to become somewhat
familiar with the make-up and service of the blood before
we discuss the entrance into it of the products of digestion.

Blood is a carrier. Regarding it as such, we can con-
veniently subdivide its functions according to the classes
of material which it conveys. Most people will think of it
as, first of all, the bearer of food to the tissues. This is,
indeed, a matter of prime importance. The food is added
to the blood mainly as it flows through the capillaries of the
wall of the alimentary canalx and taken from it by the
various parts of the living body in proportions correspond-
ing with the degree of the local activity. The largest tax
is that levied by the skeletal muscles. An unfailing sup-
ply of oxygen is a need even more urgent from moment to
moment than the presentation of food. Oxygen is added
to the blood in the lungs, and is withdrawn from it as it
makes its round through the body, the quantity consumed
in different organs being an even better measure of their
individual metabolism than is their appropriation of food.
Where oxygen is taken from the blood, carbon dioxid is
returned to it. This indicates that the blood is a bearer of
wastes. The excess of carbon dioxid makes its escape dur-

105

106 NUTRITIONAL PHYSIOLOGY

ing the next transit of the blood through the lungs. Other
waste substances are gathered by the blood as it flows near
the active cells. The major part of these is destined for
excretion by the kidneys, and these organs receive so large
a share of blood that the entire volume must come under
their influence within a short space of time. A minor
fraction of the waste finds an outlet in the bile, the sweat,
and through the intestinal wall.

It has already been pointed out that every organ has a
chemical constitution and a metabolism peculiar to itself.
Therefore each organ must make certain demands upon the
blood not exactly duplicated elsewhere. What is more
important, each organ gives rise to products unlike those
formed by any other part. In a number of cases these
products can be shown to have far-reaching effects. Their
existence has been mentioned in Chapter IV. Recognizing
the large part which they play in the economy of the organ-
ism, we may state at this point that an essential service
of the blood is the transportation of internal secretions.

Another function of the blood, and one often overlooked,
is that of equalizing the temperature of different regions.
One is reminded of the arrangement of a heating system in
a house where a fire burns in a furnace and a circulation
of air, hot water, or steam disperses the heat through the
rooms above. Interruption of this circulation will allow
the upper stories of the house to cool off, while the base-
ment is overheated. In the living body heat production
is a function of all active tissues. The ancients believed
that it was the particular duty of the heart. This organ is,
in fact, distinguished for its rapid evolution of energy, but
it must be remembered that it is of small bulk. A much
larger share of the aggregate heat production is borne by
the skeletal muscles. These are supplemented to some
extent by the liver and the other great glands of the body.
Blood passing through a tissue which is undergoing lively
metabolism will have a higher temperature as it leaves than
it had when it entered. As it flows on it will communicate
some of this surplus heat to resting structures in which

THE BLOOD 107

little or no heat production is going on. This is the case
with the connective tissues and with the skin.

At the surface the blood loses heat, at least under all
conditions which can be called normal. The cooler blood
then returns to the large vessels, merges with the heated
blood from the muscles and glands, and brings the
temperature of the mixture to an average which seldom
varies materially. The means by which this standard tem-
perature is maintained will be discussed at length in a
later chapter. It will be well to point out even at this
time that the skin is subject to considerable changes of
temperature, and that our sense of being warm or cold
depends entirely on the condition prevailing at the sur-
face. If we turn again to our illustration of a house heated
by a "circulation" of hot air or water, we shall recognize that
here, as in the living body, there is a constant escape of heat
to the environment. The temperature of a pane of glass
in a window of the house will be influenced both by the
internal and the external state of affairs. The glass may be
warmed as hot air is wafted against it from within or cooled
by a gust from without. So the skin may be warmed by a
waxing of the blood-current close beneath it or chilled by a
passing draft.

Blood may be described as a red fluid, but inspection
with the microscope shows at a glance that its redness is
Hot due to a coloring-matter in solution and uniformly dis-
tributed, but to the presence of minute solid bodies in sus-
pension. These are the red corpuscles. The liquid in
which they are swept about is the plasma. The corpuscles
make up something less than one-half the total volume.
In view of this, it is surprising that the blood can have such
a free-flowing character and find its way through the cap-
illaries so readily. One would anticipate that such a
mingling of solid particles with fluid would result in the
formation of a highly viscid mass. That the actual condi-
tion is so different must be due to the absolute smoothness
and the great pliability of the corpuscles.

The Red Corpuscles. — The individual corpuscle is usu-

108 NUTRITIONAL PHYSIOLOGY

ally seen in the form of a disk slightly hollowed on its sur-
faces. It is about five times as wide as it is thick. A very
slight unbalanced pressure acting from one side will con-
vert it into a saucer- or even a cup-shaped form. It is re-
markably elastic, and springs back into its original shape
as soon as there is no longer a force acting to distort it.
Most cells of the body are variable in outline and in size,
but one red corpuscle is as much like another as though all
had been made in the same mold. The diameter does not
vary perceptibly from ^TTTT inch. This is the figure for
human blood; there is a standard size of corpuscle for each
animal species.

The red corpuscles are often spoken of as cells, but their
claim to rank as such is questionable. Regarding them

Fig. 15. — Red blood-corpuscles. Several are shown in different
positions. The hollow centers are evident. In one case two cor-
puscles are overlapped to show that they are transparent. They
tend to run into piles, as shown at the right. A saucer-shaped form
is represented.

from an anatomic standpoint, they lack a feature which is
reckoned an essential of the cell; namely, the nucleus.
On the physiologic side there is no good reason for suppos-
ing them to be alive. The fairest view, probably, is to
look upon each red corpuscle as a modified and, in a sense,
degenerate cell. A single substance has come to constitute
a very large percentage of its make-up. This is the
peculiar, iron-containing protein, hemoglobin. We shall
not be far wrong if we consider the corpuscle to be a packet
of hemoglobin moving passively at the mercy of the cur-
rent. Its function is clear and definite. Hemoglobin has
the property of combining with oxygen whenever the gas
is freely present in the surrounding medium. It has also
the property of releasing this oxygen when it comes into a
situation where there is a relative scarcity of the element.

THE BLOOD 109

The hemoglobin of the blood, as can be seen from what
has been said, is ever passing from one state to another, as
it alternately adds oxygen to itself and parts with it.
Evidently it can be described as existing in two varieties:
a form fully charged with detachable oxygen (oxyhemoglo-
bin), and a form from which this loosely engaged oxygen
has been removed (reduced hemoglobin) . The first is the
prevailing variety in arterial blood fresh from the lungs,
and it gives to such blood a bright scarlet color. The
venous blood, returning from the tissues, still contains a
goodly proportion of oxyhemoglobin, but mingled with it
there is now -a variable amount of the reduced compound.
Reduced hemoglobin is of a dark color, and venous blood,
in consequence, inclines toward a purple. The sharply
contrasting red and blue so commonly used in diagrams to
distinguish arteries from veins greatly exaggerate the
actual difference.

The red corpuscle, it will now be clear, is a bearer of
oxygen. A service like this must be dependent on the
extent of surface exposed for the absorption and the dis-
charge of the gas. The disk is obviously more efficient
than a sphere would be, for it has more surface in pro-
portion to its mass. Small corpuscles likewise must be
quicker to load and to unload than large ones, and this
helps us to understand why the largest corpuscles in nature
are not found in large animals, but in those like the frog
and the fish, which do not have a very intense metabolism.
There is an additional reason why small corpuscles are
better fitted to meet the demand for a great oxygen supply:
the capillaries must be of a size to admit the corpuscles,
and an animal with large corpuscles cannot have the
closely woven capillary net which becomes a possibility
when the diameter of the corpuscles is reduced and which
brings the oxygen into closer relations with the cells which
require it.

The red corpuscles originate, generally speaking, by the
transformation of cells in an unexpected locality. This is
the so-called "red marrow" which fills the small spaces that

110 NUTRITIONAL PHYSIOLOGY

abound in the enlarged extremities of the long bones.
Here there is always going on a progressive change in the
composition of the cells, in course of which hemoglobin
replaces most of their original substance. The cell nuclei
are eventually lost and the newly formed corpuscles de-
tach themselves and drift away in the blood-stream. Their
hollow centers strongly suggest the deficiency due to the
loss of the nuclei. It is an interesting fact that when re-
covery is taking place after hemorrhage, red corpuscles
containing nuclei are often to be found in samples of the
blood. This makes it seem as though in meeting the emer-
gency corpuscles in an incomplete stage of their devel-
opment had been pressed into service.

The first glance at a specimen of blood under the micro-
scope leaves the impression that the corpuscles are all of one
unvarying type. Closer observation shows here and there
among the host of colored elements bodies of another or-
der, the white or colorless corpuscles. There is but one of
these to 500 or 1000 red. The white corpuscles are not
flattened, but more nearly spheric. They are, however,
of no fixed form, and many of them have the property of
ameboid movement. This is good evidence that they are
to be considered living, and it can be shown further that
they have nuclei and conform to our conception of com-
plete cells. Much that is of interest is known of them, and
the relation which they bear to the checking of bacterial
infection is of the utmost importance. We shall not enter
upon a discussion of this fascinating subject.

The Plasma. — Aside from the transportation of oxygen,
all the chief functions of the blood could apparently be
fulfilled by the plasma. The standard foods of the tissues
are here. So also in much smaller amounts are the non-
gaseous wastes. Carbon dioxid, the most abundant of all
waste-products, is carried jointly by the plasma and the
corpuscles. It is evident that we must expect a solution
meeting such manifold requirements to be of a highly
complex nature. This is eminently the case.

More than nine-tenths of the plasma is water. The

THE BLOOD 111

proportion is little increased by drinking and little reduced
by thirst. Its constancy is secured by kidney activity
coupled with rapid exchanges of fluid which take place be-
tween the blood and the lymph and tissues. When a great
draft is made upon the water of the blood, as is the case in
profuse sweating, the volume is restored by taking in water
from outside the vessels, and this may mean a marked
loss of body weight. The blood itself is not likely to share
appreciably in such a loss. Its total volume in the adult
of average build has been variously estimated at from 3 to
5 quarts. The earlier calculations led to the higher figures;
4 quarts (8 pounds or -^ of the weight) is a reasonable
standard for us to adopt.

Among the substances in the plasma which can be classed
under the general head of foods, proteins take the first
place. They form about four-fifths of the total solids.
This high proportion does not correspond at all with the
make-up of an average diet, in which, as we have seen,
proteins take the second or even the third place. It is
usual to distinguish three varieties of protein in the plasma,
serum-albumin, serum-globulin, and fibrinogen, but indica-
tions are multiplying which support the belief that the ac-
tual number of proteins with clearly individual characters
is much greater. Our knowledge of these bodies, their
mode of origin, place of formation, and the particular
value of each one in the system, is in a highly unsatisfac-
tory state.

Carbohydrates, which occupy the leading position in the
rations ordinarily chosen by civilized man, are very scantily
represented in the plasma. The principal one present is
the monosaccharid, dextrose, known also as glucose, or
rape-sugar. The percentage of dextrose figured for the
rhole blood is between 0.1 and 0.2. This means from 1
2 grams of sugar in a liter of blood, and limits the total
imount in circulation to 10 grams at most. Such a quan-
tity seems insignificant when it is remembered that 100
rams of sugar may easily be formed in the digestion of a
loderate meal. The sugar is not readily increased by

112 NUTRITIONAL PHYSIOLOGY

free feeding of carbohydrates, nor does it noticeably di-
minish during long fasting. The explanation of this sin-
gular constancy will be given later.

Fat, or its derivatives, is found in the plasma in a pro-
portion of a similar order to that of sugar. It is much more
subject to variation, rising notably after a meal in which
there was much fat. Plasma obtained from a dog at such
a time exhibit the phenomenon of developing a true cream,
the fat gathering at the surface. During starvation the
blood is not necessarily poor in fat, since it is likely to
be engaged in carrying this form of food from places of
storage — the adipose tissue — to the muscles and elsewhere
to be oxidized.

Those compounds in the plasma which we can confi-
dently designate as waste-products occur only in the small-
est quantities. Carbon dioxid is, of course, an exception,
being very abundant. Among the non-gaseous wastes
destined to be dealt with by the kidneys, urea is the only
one which is easily detected. This is the compound in
which seven-eighths of the nitrogen is carried from the
body.. The fact that the waste substances are kept down
to such a low level in the blood is evidence of the remark-
able efficiency of the kidneys and the supplementary or-
gans of excretion. It is also a reminder of how rapid and
copious is the circulation. No portion of the blood can
long escape passage through the glands which have this
striking power to hold it to a standard composition.

If the mixed solids from a sample of plasma are inciner-
ated, we have left a small mass of ash or mineral matter
amounting to about 1 per cent, of the whole blood. By
far the largest component is sodium chlorid, the chief salt
of the diet, and the only one which we deliberately add to
our food. Other bases represented are potassium, calcium,
and magnesium. Besides chlorids, we find carbonates and
phosphates, the former having greatly involved relations
with the carbon dioxid of the blood. To the list which has
been given might be added many minor constituents, some
of which are judged to be present because of certain prop-

THE BLOOD 113

erties displayed by the blood rather than because they can
be chemically identified. Illustrations of such are afforded
by the internal secretions, enzymes, and the immune bodies.
Individual peculiarities of metabolism, susceptibility, and
resistance must depend on substances of this obscure
class.

Coagulation. — Blood when shed shows the familiar
property of clotting. This is a most valuable quality,
since it provides for the automatic checking of ordinary
bleeding and also forms a protective shield, the scab, be-
neath which the healing process may go on. The im-
portance of coagulability is emphasized by the rather fre-
quent observation of cases in which it is lacking, and in
which serious or even fatal hemorrhages follow trifling
injuries. The essence of the process is a chemical change
affecting one of the minor proteins of the plasma in such a
way that it passes into a solid form and cements together
the red corpuscles. The original protein is the one already
named as fibrinogen. The modified form after coagulation
is called fibrin. The actual amount of fibrin is extremely
small (perhaps 2 to 4 parts in 1000 of blood), but it is not
difficult, when we consider its gummy character, to under-
stand how it can convert a liquid medium into a stiff jelly
by knitting together the suspended corpuscles.

The entirely natural impression that coagulation is the
result of exposure to the air is erroneous. The matter has
been the subject of the most painstaking studies, of which
we can give only the briefest summary. The formation of
fibrin is an instance of enzyme action, and forcibly recalls
the curdling of milk in the stomach under the influence of
rennin. The resemblance is not complete in every re-
spect, but is still suggestive. If we are to assume that an
enzyme exists in the blood at the time of clotting and not
before, we must establish its origin. Reducing the facts
to the barest essentials, we may make the following state-
ment: normal blood contains in great numbers minute and
extremely perishable bodies known as the blood-plates.
/ These are much smaller than the red corpuscles. When

' I 8

114 NUTRITIONAL PHYSIOLOGY

the surroundings are normal, as is presumably the case
within the vessels, they keep their integrity, or at least do
not disintegrate en masse. When they are brought in con-
tact with foreign surfaces they do undergo prompt decom-
position and, of course, their constituent material is dis-
solved in the plasma. Something derived from the blood-
plates sets in motion the series of chemical reactions which
leads at last to the perfecting of an enzyme, thrombin,

Fig. 16. — The origin of the lymphatics. This is an extension
of Fig. 3. The course of the blood among the cells is shown as be-
fore (B-B). A detail of the lymphatic system is added (Ly.).
The drawing is severely conventional. Two branches of the lym-
phatic are represented as beginning with open mouths so situated as
to receive the surplus fluid directly from the interstices of the tissue.
A third is connected with a blind sac. There is at present some dis-
agreement as to which is the more typical mode of origin.

capable of transforming the soluble fibrinogen into the
insoluble fibrin.

Lymph. — This term is usually made to stand for the
fluid outside the blood-vessels. Used in this way, it
includes the liquid filling the microscopic intervals between
neighboring cells, the larger spaces which often occur in
the loosely woven connective tissues, and also the contents
of an inconspicuous system of tubular channels known as
lymphatics. A recent writer, Starling, has suggested

THE BLOOD 115

that a distinction should be made between the fluid in the
lymphatics and that which is not enclosed in vessels of any
sort. He would restrict the term lymph to the first appli-
cation, and would speak of the other as tissue-fluid. This
usage appears highly desirable, but is not as yet widely
current.

Lymph, in the sense of tissue-fluid, cannot be collected
for analysis. Considering the delicacy of the capillary
wall and the probable freedom with which exchanges take
place through it, there is reason to believe that this fluid
must closely resemble the blood-plasma. When a tissue
is the seat of an active metabolism the income and outgo
of its cells must tend to alter the composition of the adja-
cent lymph and to make it less like the plasma. The
general effect will be to reduce the oxygen to a low level, to
raise the carbon dioxid content correspondingly, to con-
sume a fraction of the organic food, and to add miscella-
neous waste-products. The lymph which can be obtained
by cutting one of the larger lymphatics of the body has
these characters. This lymph may not be precisely like
the mixture which exists in close contact with the living
cells, but it is probable that the differences are not great.
According to a view formerly universal and still held by
many, lymph can work its way from any interstice of an
organ into the branches of the lymphatics, and so to larger
vessels of the same class. Many believe, however, that
there is a definite separation between the unwalled spaces
of the tissues and the interior of the true lymphatic system.
If this is the correct conception, we have good reason to
differentiate lymph from tissue-fluid. We may adopt
either view provisionally without being seriously misled.
Lymph contains white corpuscles and blood-plates.
Since it has in it some fibrinogen it may coagulate, but
owing to the absence of red corpuscles the clot is frail and
tremulous.

CHAPTER XIII
THE CIRCULATION

WE must postpone still further our account of the ab-
sorption of the products of digestion until we shall have
made clear the general course followed by the circulating
blood. We have quoted the estimate that the body con-
tains 8 or 10 pounds of blood. At a given moment about
one-fourth of this may be assumed to be in the thorax
(the heart, the lungs, and the great blood-vessels), a
fourth in the skeletal muscles, a fourth in the liver, and the
remaining fourth elsewhere. The ceaseless movement of
this large volume of liquid is maintained by the beating of
the heart.

This organ consists of two halves, right and left, com-
pletely separated, so far as their cavities are concerned, by
a middle partition. Regarded as a mass of muscle, the
heart is single; considered as a pump, it is double. Each
half is, in a literal sense, a force-pump. Each side shows
us two communicating chambers, an auricle above and a
ventricle below. The auricles receive the blood, which the
corresponding ventricles will shortly discharge. The
vessels leading to the auricles are called veins; those which
convey the blood from the ventricles are called arteries.
The auricles have thin walls, while the ventricles are
fitted for their task by heavy muscular development.
The left ventricle has much more power than its fellow,
and the necessity for this will soon be evident.

A single great artery, the aorta, springs from the left
ventricle. Its branches reach all parts of the body. Sub-
dividing repeatedly, they introduce the blood at last into
the capillaries, the innumerable vessels of the smallest
order through whose exquisitely thin walls take place the

116

THE CIRCULATION

117

exchanges with the lymph already described. The word
"capillary" means hair-like, but the description falls
far short of indicating the actual slenderness of these mi-
croscopic tubes. They are so narrow that the corpuscles
pass through them practically in single file. The capillar-

Fig. 17. — In this diagram, as is usual in such cases, right and left
are reversed. This is as though the observer were looking at an-
other subject. The short pulmonary path is to be traced from the
right ventricle to the left auricle (P. C.). Alternative routes are
suggested for the passage of the blood through the greater* circula-
tion from the left ventricle to the right auricle. The blood which
traverses the digestive tract (D) passes through a second set of
capillaries in the liver (L) before it can return to the heart. Note
that the liver has in addition a separate arterial supply of blood.

ies are short and soon unite to form the smallest veins.
These lead the blood back toward the heart, joining as
they go to form larger and less numerous channels, until at
the last there are but two great veins. These enter the
right auricle, one from above and one from below. The
sweep of the blood from the left ventricle through the body

118 NUTRITIONAL PHYSIOLOGY

at large and back to the right auricle is called the systemic
circulation.

From the right auricle the blood descends to the ven-
tricle of the same side, and is sent forth again, this time
through the vessel known as the pulmonary artery.
This immediately forks into branches which plunge into the
two lungs. The smaller pulmonary arteries lead to rich
capillary networks which are wrapped around the number-
less air-sacs of the lungs. Here the corpuscles are re-
charged with oxygen and the carbon dioxid of the blood is
reduced to a standard amount. Pulmonary veins return
the blood to the left auricle. The relatively short journey
of the blood from the right ventricle to the left auricle
by way of the lungs is called the pulmonary or lesser cir-
culation.

It is necessary to call attention to the fact that the ad-
jectives "arterial" and "venous" are not used in a sense
which exactly corresponds with the meanings of the nouns
"artery" and "vein." The adjectives have a chemical
significance; the nouns, an anatomic one. Arterial blood
is blood fully oxygenated; venous blood is blood more or
less deficient in oxygen. But an artery, as has been said,
is merely a vessel carrying blood away from the heart.
The blood within will be arterial if we are observing the
systemic circulation, and venous if it is in the pulmonary.
So the systemic veins are filled with venous blood, while the
pulmonary veins carry that which has just been brought
up to the arterial standard through coming into relation
with the air in the lungs.

From what was stated above with respect to the distri-
bution of the blood, it is evident that less than one-fourth
of the whole volume is in the pulmonary circulation at one
time, but the student must be cautioned against the con-
clusion that the pulmonary circuit is traversed by less
blood than passes through the systemic pathways. The
quantities passing along the two routes are inevitably
equal, for it is, after all, the same blood which runs through
each in turn. If a chain is traveling over two wheels, as

THE CIRCULATION 119

shown in the diagram (Fig. 18), the links pass the two in
equal numbers, and yet there are constantly more links on
their way from L to R than from R to L.

When we speak of a heart-beat we mean a co-ordinated
contraction of the heart's peculiar muscle. The phe-
nomenon includes two phases: a brief and not forcible
contraction of the auricles followed by a longer and much
more powerful closing in of the ventricles upon their cavi-
ties. The ventricular contraction is the essential factor in
propelling the blood. Valves between the auricles and the
ventricles permit the latter to fill during their period of

Fig. 18. — For explanation of figure see text.

relaxation, and forbid the backward flow from the ventricles
when they are emptying themselves. Other valves at the
commencement of the aorta and the pulmonary artery
allow blood to pass out during each contraction of the
ventricles, but not to return from either artery into the
heart when the resting phase ensues. The action of the
two sets of valves is precisely on the principle of the two
which make a syringe-bulb an effective force-pump. The
right and left halves of the heart act practically at the
same time.

Either ventricle when full may contain 5 or 6 fluidounces
of blood. As it contracts it reduces its capacity and dis-

120 NUTRITIONAL PHYSIOLOGY

charges blood in like measure. At the conclusion of the
active period it may have nearly obliterated its cavity, or
the effort may fail while there is still considerable blood
within. Estimating an average output from one ventricle
to be about 4 ounces, or 100 c.c., it is apparent that forty
beats will suffice to discharge from one side of the heart
an amount of blood equivalent to the entire quantity of
circulation (40 x 100 = 4000 c.c., or 4 liters). In other
words, it will require less than one minute for all the blood
to pass successively through both sides of the heart. When
this statement is made it should be remembered, that
certain routes in the systemic circuit are many times longer
than others (compare the path to and from the feet with
that to and from the esophagus), and some corpuscles may
revisit the heart two or three times while others are making
one prolonged journey.

It would be out of place to enter here upon a discussion
of the value of the auricles. From a mechanical point of
view they contribute but little of the energy required for
driving the blood. They serve to accommodate the
gathering volume of blood which the veins bring in during
the rather prolonged contraction of the ventricles, and they
secure a more efficient filling of the lower chambers of the
heart. While their muscular development is slight, their
automatic power is peculiarly marked, and it is a fair
statement that the ventricle, under normal conditions, is
stimulated to perform each beat by an influence radiating
to it from the auricle. If there is an interruption of certain
strands of tissue which normally unite the two, they cease
to maintain the same rhythm, the auricle continuing at
the accustomed rate, while the ventricle beats much less
frequently. When the heart is dying the last signs of
pulsation are in the auricles.

The Portal Circulation. — In our general description of
the course followed by the blood on its way through the
body it was stated that when it has passed through a set
of capillaries it is gathered up into veins to be returned
to the right auricle. An important departure from this

THE CIRCULATION 121

order is now to be indicated. The blood which has trav-
ersed the small vessels of the digestive tract (including the
stomach, both intestines, the pancreas, and the spleen)
would be expected to make its way back to the heart
through branches of the systemic veins. The actual ar-
rangement is as follows: A large vessel receives all the
blood from this region and carries it into the liver, where
a second set of capillaries is entered. When these unite
it is to form short veins discharging into the chief vein of the
body, just below the diaphragm and practically within the
boundaries of the liver. The channel leading from the
organs of digestion to the liver occupies a unique position.
Inasmuch as it is formed from a capillary system it seems
to be a vein, but since it also supplies a capillary system
it may be contended that it is an artery. Its structure
favors the view that it is a vein and it is so called. The
vessels which gather the blood from the digestive tract
and distribute it inside the liver are said to form the portal
system; the chief conductor described above is called the
portal vein. An important consequence of this arrange-
ment is that the absorbed products of digestion, so far as
they are in the blood rather than the lymph, are brought
under the influence of the liver before they go elsewhere.

The question will naturally be raised whether the liver
is supplied exclusively with venous blood. Provision is
made for a supplementary supply through what is known
as the hepatic artery, a vessel which is an offshoot of the
general arterial system, and which, of course, introduces
into the liver blood rich in oxygen. An analogous condi-
tion may be noted in the lungs, where the main currents
are the venous streams coming from the right side of the
heart, but where there are also interwoven in the tissue
much smaller vessels which take their rise in the arterial
tree springing from the left ventricle.

The problems of the circulation fall into two classes:
There are those which are purely physical and which can
be approximately reproduced and more or less successfully
studied in lifeless models. There are also those which have

122 NUTRITIONAL PHYSIOLOGY

to do with the behavior of the organs concerned when they
are viewed as living structures. Under the first class are
included the facts of blood-pressure, velocity of flow, the
resistance overcome, etc. The interpretation of such data
is almost a science in itself and is known as hemodynamics.
Among the questions of the second sort are the mysterious
automaticity of the heart and the nervous government of
the quantity and the distribution of blood flow. These
difficult matters may well be left to be briefly dealt with
toward the end of the book, when the general work of the
central nervous system will be presented. We must,
however, devote some space at this time to the fundamen-
tals of hemodynamics.

The Character of the Blood Flow in Vessels of Different
Classes. — When an artery is cut the blood escapes in a
forcible jet which may spring to a distance of several feet.
The stream is not steady, but mounts and declines in a
rhythm corresponding with the heart-beat. A good deal
of pressure must be applied to restrain the bleeding. If a
vein is severed the flow of blood is rapid and copious, but
easily repressed. It is uniform so far as can be judged.
These observations lead to the conclusion that the blood
in the arteries is under a high average pressure, with large
fluctuations from moment to moment. The pressure in
the veins is evidently very low.

The diminution of pressure between the arteries and the
veins can be simply explained. When the blood is started
on its course through the systemic circulation the high
pressure which it exerts against the elastic wall may be
regarded as a measure of the energy which the ventricle
has impressed upon it. When it draws near the right side
of the heart, the goal of its journey, its abated pressure is
the sign that the initial energy has been spent. How has
it been consumed? There is but one possible answer: It
has been transformed into heat in overcoming the friction
encountered in the vessels. Analogous conditions can be
demonstrated for any tubular system through which liquid
is driven. In the mains which carry a city water-supply

THE CIRCULATION 123

from a pumping-station to distant points there is a similar
decline in pressure along each line as it is followed farther
from the fountain head. (The assumption is made that
we have to do with pipes which are on the same level
throughout.)

In any such system the cutting down of the pressure will
be abrupt at any point on the route where there is an
unusual impediment to the flow. This is the case in the
body where the blood-steam is so extensively subdivided
to enter the smallest vessels. Subdivision of channel
means multiplied surface for friction. Thus there occurs
a radical drop in pressure between the smallest arteries in
which we can measure it and the veins of similar size.
The highest pressure developed anywhere must be in the
left ventricle when it is discharging the blood, for it is
then to be regarded as the starting-point of the circulation.
The lowest pressure which is ever registered is probably in
the same ventricle when it is beginning to fill, for now it is
the terminus of one of the two circuits.

The swelling of the arteries which promptly follows each
ventricular contraction is what we recognize as the pulse.
It is the sign of a newly introduced portion of blood dis-
tributing itself in the vessels. We have said that the veins
show no such rhythmic enlargement. We have, therefore,
to account for the conversion of the intermittent flow in the
arteries into the constant flow in the veins. The principal
factor concerned is the marked elasticity of the arterial
trunks. It will be helpful to refer to the artificial devices
which serve the same purpose in connection with force-
pumps. A familiar one is the air-chamber. This is a
large container communicating with the outflow-pipe.
Whenever a stroke of the pump drives water along this
pipe a certain share proceeds directly toward the outlet,
while another fraction turns aside into the air-chamber.
During the return stroke when no water is issuing from the
pump barrel the air which a moment before underwent
compression in the chamber tends to regain its original
volume, and in so doing forces water through the lateral

124 NUTRITIONAL PHYSIOLOGY

branch and thence along the main pipe. The intermittent
delivery through the valve is transformed more or less
successfully into a continuous flow through the remote
outlet.

The neutralizing of intermittency in the blood system is
referable to the same principle, but the elastic compensator
is not to be found as a single localized feature; it is discov-
ered in the universal capacity of the arteries to stretch and
to regain their former size. At the beginning of the aorta
or of the pulmonary artery we have complete intermittency,
the blood alternately forging ahead and halting. But each
portion of blood ejected from the heart finds room for
itself partly by distending the arteries and not altogether
by driving forward the blood which is before it. Hence,
when the ventricles relax and the outpouring of blood
ceases there is still an onward movement in the smaller and
more distant arteries, because the larger ones, which were
momentarily overdistended, are now contracting and send-
ing along part of their accumulation. The farther we go
from the heart the more largely the driving of the blood is
to be attributed to the reaction of the stretched arterial
walls and the more nearly uniform it becomes. This does
not mean that the whole power keeping up the circulation
is not to be sought in the heart-beat ; it merely means that
this energy may be stored temporarily by these elastic
structures and rendered back again.

The facts we have been treating may be expressed in
another way. The arterial tree forms a reservoir of con-
siderable capacity. Within it is an amount of blood so
large that the single contribution of the ventricle makes
a rather small addition to it. The escape of the blood
through the terminal twigs cannot cease while there is
so much stored under a high pressure in the aorta and its
branches. The heart may omit or "drop" a beat without
noticeably diminishing the flow through the capillaries.
A standstill will be reached only when the arteries have
attained a degree of contraction such that the internal
pressure is no higher than that in the veins. The homely

THE CIRCULATION

125

illustration (Fig. 19) which accompanies this may be help-
ful. The pump delivers water intermittently to the leaky
trough, keeping it filled to a level which is nearly constant,
though fluctuating a little in the rhythm of the strokes.
Meanwhile the escape of water through the cracks is
all but uniform in its rate. One important difference be-
tween the pump and trough, on the one hand, and the cir-
culatory system, on the other, lies in the fact that in the

Fig. 19. — At 7 the discharge is in gushes with pauses between —
the type of the expulsion of blood from the heart At C the escape
through the cracks is at a practically constant rate; this is true of the
blood flow through the capillaries.

first case the driving force is gravity; in the second, it is
the reaction of the enclosing elastic walls.

A set of facts which it is well to separate in one's thought
as completely as possible from considerations of pressure
is the body of data respecting the linear velocity of the
blood. By this is meant the rate of advance of the aver-
age corpuscle. In any vessel the stream runs more swiftly
in the central axis and lags along the walls. The velocity
in the arteries rises and falls as does the pressure, but, on

126 NUTRITIONAL PHYSIOLOGY

the whole, is relatively high. The aorta is passed at a
speed of at least a foot in a second. The veins also are
traversed at a high velocity, though the figures are some-
what lower than for the arteries. The movement in the
capillaries is in sharp contrast to that in both arteries and
veins, being exceedingly slow, perhaps ?V mch m a second.
It appears that a corpuscle may take as long to go through
a capillary link which would scarcely span the breadth of a
pin-head as to travel from the heart to the brain.

When it is remembered that the actual service of the
blood to the tissues is rendered in the capillaries (since all
other vessels have walls too thick to permit free diffusion),
the value of the slow passage is obvious. At the same time,
one recognizes the desirability of the rapid transit to and
from this department of the system. The heart itself and
all the main vessels may be thought of as accessory to the
capillaries. The explanation of the slowing of the stream
in the small channels is entirely simple, yet often mis-
apprehended. Whenever an artery forks to form two
branches, these are individually of smaller cross-section
than the parent stem, but their combined cross-section is
greater. The result is the same that is seen when a river
widens or deepens — the current slackens. If the river
broadens into a lake the current may become impercep-
tible, yet we know that the water is still setting forward
toward the outlet.

Are we then to believe that the capillary system is many
times wider than the aorta or the great veins? There is no
escape from this conclusion: their number is so vast that,
despite their infinitesimal size as single conveyors of the
blood, collectively they form the broadest division of the
entire path. If they fail to suggest a lake, an analogy may
be found in the tangled swamp in which a stream loses
itself, breaking into many sluggish arms, from which at
last the waters converge to resume a rapid course over a
narrow bed. The acceleration noticed as the veins are fol-
lowed toward the heart is merely a sign that as their num-
ber grows less their combined cross-section also contracts.

THE CIRCULATION 127

The Movement of the Lymph. — Just as we find veins
returning from every part of the body, we can make out,
though with much greater difficulty, small vessels bringing
lymph in the same general direction, that is, toward the
thorax. Like the veins, they unite as they draw near the
heart, and the great majority eventually contribute to a
trunk known as the thoracic duct. This springs from the
union of many branches in the abdominal cavity, pierces
the diaphragm, and can be traced upward in front of
the spinal column until it empties into a great vein which
is bringing the blood from the left shoulder toward the
right auricle. A comparatively insignificant group of
lymphatics centers at an outlet in the corresponding posi-
tion on the right.

The onward movement of the lymph in its channels is
parallel with that of the venous blood, but is incomparably
slower. It is sometimes almost entirely arrested. In the
last chapter it was stated that we cannot confidently say
whether the lymphatics drain all the microscopic spaces
in the tissues or whether they rise in small definite enclos-
ures. In either case they are bearing away a certain over-
flow of fluid and may be regarded as supplementing the
service of the veins. As to the cause of the halting move-
ment of their contents, the simplest statement that can
be made is somewhat as follows: The formation of new
lymph in all the organs crowds away the lymph previously
in the beginnings of the lymphatics, and this is the central
fact to be considered. The energy required is derived
partly from the heart, since liquid may be forced out of
the capillaries by its transmitted pressure, and partly from
other sources too obscure to be discussed. The lymph
is generally referred to as a carrier of waste, but a partial
exception must be made in favor of the lymph coming
from the intestinal area during digestion. This lymph
may contain absorbed food, principally fat. It was in the
mesentery that lymphatics distended with milky liquid
were first seen. They were called lacteals because of their
appearance, and this term is still used in a local sense.

CHAPTER XIV
THE ABSORPTION OF THE FOOD-STUFFS

IF two unlike solutions are separated by a membrane,
such as a sheet of parchment or some artificial substitute,
they will usually tend to equalize both in composition and
concentration. When, for example, potassium chlorid and
sodium chlorid solutions are placed on opposite sides of
such a partition, each salt proves its ability to pass through
the barrier, and in the course of time there will be uniform
mixtures in both compartments. This is said to show that
the salts are diffusible and that the membrane is permeable.
Different membranes are permeable in very different
degrees, and the freedom with which various salts pass
through a particular membrane is also far from constant.
Some substances may appear freely soluble and may go
readily through ordinary filters, but may hardly diffuse at
all. This, as a rule, is the case with the proteins. If a
mixture of unboiled white of egg and sodium chlorid is on
one side of a membrane and the other side is washed with
running water, nearly all the salt will escape, leaving the
protein practically undiminished. The process by which
diffusible salts are encouraged to separate themselves from
substances which cannot accompany them through the
membrane into the water beyond it is called dialysis.

The products of digestion are, in general, much more
diffusible than the food-stuffs from which they are derived.
Starch, even when boiled for a long time, does not make
its way through ordinary membranes; the sugars do so
with relative ease. Fats are not even soluble in water;
soaps and glycerin are diffusible compounds. Peptones
arising from the hydrolysis of proteins have some power
to penetrate membranes, and the simpler amino-acids pass

128

THE ABSORPTION OF THE FOOD-STUFFS 129

still more freely. We might picture the situation in the
intestine somewhat as follows: The blood flows steadily
beneath a rather complex cellular wall, the other surface
of which is bathed by the mixed products of digestion and
the digestive secretions. Peptones and amino-acids, su-^
gars, soaps, and glycerin, being formed in relative abun-
dance in the intestine, diffuse into the blood, which contains
little of these bodies. If the blood were to stand still the
small volume in direct relation with the absorbing surface
would accumulate digestive products until it held them in
the same concentration in which they exist in the canal.
Then a state of equilibrium would be established and no
further transfer to the blood would occur.

Detailed observation shows that the facts of absorption
cannot be expressed in this simple manner. It has already
been hinted that large allowance must be made in such a
case for the fact that the membrane under examination is
alive. Its cells may discharge material at one surface
quite unlike that which they receive at the other. They
probably have a considerable metabolism. This means
that energy is set free within their borders and a share of it
may be applied to the moving of the absorbed food. We
have previously called attention to the parallelism be-
tween secretion and absorption.

Before we can go further with this discussion something
must be said concerning the place of absorption and the
minute anatomy of the structures involved. There is a
measure of absorption from the stomach. Until rather
recently this organ was not credited with any marked
powers of this kind, unless it were in the case of alcohol.
This is a highly diffusible compound and its prompt
entrance into the circulation is noteworthy. When one
says of a glass of wine, "This goes to my head," the state-
ment is literally true. The alcohol strikes through the
walls of the stomach at once and is borne to all parts of the
body, including the brain. On the other hand, water is
not at all freely absorbed when it is taken into an empty
stomach. It is known to pass the pylorus in practically

130 NUTRITIONAL PHYSIOLOGY

unchanged volume. Some poisons produce no positive
effects while in the stomach, but exert their action promptly
when they pass to the small intestine.

As regards the sugars and the products of peptic diges-
tion, it is now believed that there is a considerable absorp-
tion of these substances from the stomach. They disap-
pear most rapidly when they are present in high concentra-
tion, and the process is promoted by condiments which
increase the blood flow under the gastric mucous mem-
brane. When all reasonable allowance is made for the
part played by the stomach in absorption, the fact remains
undisputed that the major part of the work is done below
the pylorus. Furthermore, since, as has been said, there
is usually little valuable material left to be recovered by the
colon, it becomes evident that the small intestine is of
central importance.

A striking peculiarity of the small intestine is the _e_x-
tension of its internal surface. This is effected, first, by
the numerous cross-folds which cut into its cavity, and,
second, by the microscopic projections which stud its
lining. These are the villi. An individual villus may be
described as a minute finger-shaped process. It rises
above the general surface in a contrast to the glands, which
sink below; the villus is a peg, as the gland is a pit. Ob-
viously the existence of the villi increases many times over
the number of cells in contact with the intestinal contents.
These cells are described as columnar; they are prisms
standing side by side, with their larger surfaces in contact
and their smaller ends presented to the interior of the in-
testine and to the loose internal tissue of the villi. A
certain share of absorption may take place through the
crevices between the cells, but the main transfer of material
seems to be through their own protoplasmic bodies.

The interior of a villus is filled by a confusing assortment
of cells, some of which have been thought to be contractile.
There are probably intervals between these cells containing
lymph, and near the central axis of the villus is a rather
definite lymphatic channel. It is a small branch of the

THE ABSORPTION OF THE FOOD-STUFFS

131

general lymphatic system and opens one way, by which
food can pass from the seat of absorption to the veins in
the thorax, there to mingle with the blood. Between
the exposed cells of the villus and the lymphatic at its

X

.

A

Fig. 20. — This is a conventionalized drawing to show the essen-
tials in the structure of a villus. The lining cells of the intestine are
shown as in section. Within is seen a tangle of capillaries, and at the
very core of the villus a lymphatic (L). The loose tissue, which in
reality exists inside the villus, has been ignored for the sake of sim-
plicity.

core is interposed a net of capillaries carrying blood which
has come from the neighboring aorta, and which will
flow through the liver before it returns to the heart. It
may be said at once that, of the two possible routes for
absorption, the portal system is the more important.

132 NUTRITIONAL PHYSIOLOGY

The statement has been made that the cells lining the
intestine do not act in a way that can be imitated by life-
less models. The fact is covered by the expression that
they exercise selective powers. Too much might easily be
inferred from this phrase; there is the same danger which
was pointed out in connection with the pyloric sphincter,
the inclination to credit the cells with something more like
intelligence than it is right to assume for them. It is
reasonable to believe that however complex and unex-
pected may be the behavior of the cells concerned, a
mechanistic explanation would be apparent if our knowl-
edge were sufficiently full. What is meant by selective
action can be readily illustrated.

If a comparison is made in the laboratory to determine
the relative rates at which glucose and magnesium sul-
phate make their way through an ordinary membrane, the
sugar will lag behind the salt, although both pass with
relative freedom. If a mixture of the two is introduced into
the living intestine the impression is totally different.
The sugar is absorbed, while the magnesium sulphate is
kept back. We say that the mucous membrane is imper-
meable to this salt, though we can hardly picture the
peculiarity of structure which makes it so. A salt which
is refused absorption by the intestinal wall will act as a
laxative, for it will hold back from absorption a large quan-
tity of water and this will be swept through by the peris-
talsis. An investigator has called attention to the fact
that all the common precipitants of calcium are denied
passage into the blood, and may, therefore, be reckoned
as cathartics. These include, beside the sulphates, the
phosphates, the citrates, and the tartrates.

Again and again we find as we pursue the subject that
laboratory tests give us little indication of what may be
expected of the intestine as an absorbing mechanism.
Some substances usually held to be indiffusible pass into
the circulation with comparative readiness. Even egg-
albumen, a protein of enormous molecule, may enter the
blood. This was recently proved by the observation that

THE ABSORPTION OF THE FOOD-STUFFS 133

after eating a number of raw eggs the albumin may be
found in the urine. It evidently reaches the capillaries to
some extent before it can be hydrolyzed by the digestive
enzymes, even at a time when these are presumably pres-
ent and active. The application of mild poisons to the
lining of the intestine causes it to behave much more like
the typical indifferent membrane. Thus a weak solution
of sodium fluorid (which does not visibly disorganize the
cells) wipes out the selective properties which have been
instanced. This makes it seem the more probable that
the normal processes require the application of energy, and
that accordingly they cannot continue after the death of
the cells.

A mosaic surface formed of cells, each one of which is a
living body, is far from comparable with a homogeneous
partition. Even if we leave out of account the crevices
between the cells which may bear a part in absorption, as
already noted, we must consider the individual cell to be
an elaborate structure. Its surface layer is undoubtedly
different in its chemical nature from its interior. There is
good reason to believe that the exposed border differs
entirely from the end which abuts on the connective tissue.
A result of this complex organization is the existence of a
property that might be called "polarity," that is, a capacity
to act in one direction rather than the other. A somewhat
ponderous expression for the same idea is found in the
phrase "irreciprocal permeability." This means special-
ization for absorption, and strongly suggests that if the
cells could be reversed in their relation to the interior of the
intestine they would begin to absorb from the lymph and
secrete into the canal. Gland-cells may be said to have
irreciprocal permeability in this reversed sense.

The departure of the intestinal lining from the behavior
of a common membrane is still more obvious when we note
that absorption may be accompanied by chemical trans-
formation. The food-stuffs which leave the interior of the
canal do not of necessity reappear in the circulation in the
same form. Two possibilities exist: either the process of

134 NUTRITIONAL PHYSIOLOGY

digestive cleavage may be continued during the passage
through the wall, or a recombining of the products of diges-
tion may be accomplished. The first action is thought to
occur to some extent when the peptones formed in the
cleavage of proteins are moving toward the blood. It will
be recalled that the enzyme erepsin, having the power to
decompose peptones still farther, is obtainable from
these cells, and it is quite possible that its action is intra-
cellular.

The opposite property, that of synthesizing, is illustrated
by the behavior of the products of fat digestion. We have
seen that the pancreatic enzyme hydrolyzes fats, with the
formation of fatty acids and glycerin as the first result of
the cleavage, and that the fatty acid may be changed to
soaps, though we do not know how extensively they under-
go this second change. It follows that it is these bodies
which disappear from the intestine, but they are not to be
found at all freely in the contents of the lymphatics.
In their place we have neutral fat evidently reconstructed
during the transfer. Microscopic study of the cells
through which the material has been passing shows that
the border adjoining the cavity of the intestine is without
fat droplets, but that these occur deeper down, increasing
in size toward the other border. The appearance strongly
indicates that some compounds other than neutral fats
entered the cells and underwent a transformation while
progressing through the protoplasm.

CHAPTER XV

THE METABOLISM OF FATS AND CARBOHY-
DRATES

IT was pointed out early in our treatment of the subject
that foods serve a twofold purpose : to some extent they
are built into the body as relatively permanent parts of its
structure, while in a much larger degree they are steadily
oxidized, yielding their energy to maintain its activities.
The proportion between the two divisions of the supply
cannot be constant. Clearly, the fraction of the diet
devoted to construction must be larger in childhood than
in adult life. There may be other periods during which the
constructive work is notably prominent, for example, re-
covery from illness or from fasting, pregnancy, and perhaps
athletic training. Apart from these times we must assume
that the actual building of tissue is a very small item. In
other words, the living matter of the body is comparatively
stable and needs only slight though perfectly definite
contributions to insure its up-keep from day to day.

The food-stuffs entering the circulation may be destined
for immediate destruction or for storage. Throughout
long terms of our lives a fair balance is preserved between
the income and the consumption of these compounds.
If we receive in one day certain quantities of proteins,
fats, and carbohydrates, and there is evidence of an exactly
equal decomposition of the three classes of material, we
cannot say with precision whether the oxidation affected
the particular food eaten or corresponding matter stored
previously, but in either case the condition of the system
at the beginning and at the end of the twenty-four hours
is the same. We must now proceed to discuss the possibil-
ities of transformation and retention of the different food-

135

136 NUTRITIONAL PHYSIOLOGY

stuffs, and we shall find that the story is most simple in the
case of the fats.

Fat Metabolism. — Broadly speaking, we can say that
fat never becomes anything else until it is decomposed with
release of energy. This statement may be too sweeping
to cover all the conditions which may arise in disease,
but it is substantially true in health. We have traced the
fat from the walls of the intestine into the lymphatics.
From the smaller branches it must find its way to the
thoracic duct, and through this vessel it goes to merge with
the general blood-stream. In the blood, the lymph, and
the tissues at large fat is present in a small percentage.
We must now give attention to the special provision made
for the storage of fat in what is called adipose tissue.

The word "fat" is used in two senses. In its strict
chemical meaning it describes a certain type of compound,
and it is this usage which we have thus far employed.
But when we speak of the fat of meat we include something
more. We mean a form of connective tissue in which the
cells hold a large accumulation of fat in the chemical sense.
Under the microscope this tissue is seen to be composed of
a fibrous network holding within its meshes these distended
cells. The fat which they contain is in drops of such a size
that the protoplasmic portion of each cell seems a mere
envelope, while the nucleus is crowded to one side. So it
happens that while the fat is really an inclusion, it forms a
very large percentage of the whole mass. When a piece
of adipose tissue is subjected to the action of gastric juice
the fibers and protein of the cells are rapidly digested, and
the actual fat, -being liberated, rises to the top and floats
as a clear layer of oil.

Of course, the amount of adipose tissue varies widely
with the individual. Still it is more abundant in subjects
of spare build than is usually supposed. The hollow shafts
of the long bones, such as those of the arms and legs, con-
tain what is called the white marrow. This is typical
adipose tissue. A large deposit is to be found at the back
of the abdominal cavity, where it closes round the upper

THE METABOLISM OF FATS AND CARBOHYDRATES 137

parts of the kidneys. Flakes of it occur in the mesentery
and on the surface of the heart. It is developed in the
deep eye-sockets. ^subjects better nourished there will
be more or less of this tissue widely distributed over the
body occupying a position between the skin and the
underlying muscles — the so-called subcutaneous fat.
This may be indefinitely increased in the obese. Another
characteristic of obesity is the gathering of adipose tissue
in the great omentum, the sheet of membrane hanging
from the lower border of the stomach. When much fat
is present in this situation the ventral wall of the body has
a double burden, one layer of this reserve material out-
side and a second within the abdominal muscles.

We shall postpone to a later time any discussion of the
factors which influence the accumulation of fat in the sys-
tem. A point previously made is to be insisted upon, that
the fat of the body is not derived solely, nor, indeed, chiefly,
from the fat of the food_. We shall presently consider to
what extent it is formed from the other food-stuffs.
Whether there is a great or only a moderate amount, it
will serve to maintain the activities of the muscles during
periods of insufficient feeding or absolute fasting. When
an animal has died from starvation but little fat can be
found in its tissues. The reduction of the fat presumably
present at the beginning of inanition has been estimated to
have reached 97 per cent, of the supply when death finally
supervenes. The power to endure starvation is naturally
greater for an animal or a man having a large initial store.

For each species the body fat has a nearly constant
character. There is a certain ratio maintained between
the several fatty acids, and, as a result of this, a definite
melting-point. There cannot be such a diversity here, as is
the case with the proteins of different animals, but there
is a similar appearance of individuality. Accordingly,
when one animal preys upon another of a different species
and is nourished at the expense of its victim, it .does not
store precisely the form of fat which it has eaten, but
^modifies it to conform to its own standard. The making

138 NUTRITIONAL PHYSIOLOGY

over is a still more marked phenomenon when a vegetable
oil is eaten and transformed into animal fat.

The Metabolism of Carbohydrates. — The sugar of the
blood is usually called dextrose or glucose. As a matter of
fact, it cannot be strictly correct to speak of a single sugar
in the plasma, for there are probably three at least.
Glucose, however, is undoubtedly the principal one, and
so far as we know the possible services to the body are prac-
tically the same for all. Reference has already been made
to the fact that the quantity of sugar in the blood is small,
but singularly constant. It is now time to explain how this
constancy is maintained.

It has been stated that the body contains relatively
little carbohydrate in spite of its large supply. When it
is considered that the entire volume of blood contains less
than 10 grams of sugar, though the amount absorbed after
a single meal may be as much as 100 grams, it appears
strange that there should be, as a rule, no significant in-
crease in the percentage circulating during the period of
digestion. The solution of this problem was achieved in
great measure by the French physiologist Bernard, near
the middle of the last century. Knowing that the incom-
ing sugar passes to the liver, he anticipated that this organ
might have the power to take the surplus from the passing
stream and store it temporarily in some form.

Glycogen. — Investigation showed that there could be
obtained from the liver of a well-fed animal (rabbit) con-
siderable quantities of a carbohydrate resembling starch.
This substance is called glycogen. Its presence within the
cells of the liver can be demonstrated in microscopic prep-
arations. Its molecule is of unknown size, and it is
capable of undergoing digestion in the same manner as
vegetable starch with the formation of sugar. The amount
may be strikingly large, reaching, in the rabbit, one-fourth
the total weight of the liver, deducting the weight of the
blood usually contained in it. In the human liver it does
not attain to such a high percentage, but may still equal
something like 10 per cent, of the net weight of the organ.

THE METABOLISM OF FATS AND CARBOHYDRATES 139

This means that a full-sized liver may hold 150 grams of
glycogen.

Bernard's interpretation of his discovery was somewhat
as follows : The liver is the carbohydrate bank of the body.
Like any bank, it is subject by turns to deposit and with-
drawal. Its hoard is increasing when much sugar is ar-
riving from the intestine, for it is then diverting the surplus
from the blood of the portal circulation. The change by
which sugar is made into glycogen is clearly just the re-
verse of the digestive process- a dehydration and a con-
densation to form larger molecules as contrasted with the
familiar hydrolytic cleavage. The liver cells seem to be
stimulated to make this change by the rise of the per-
centage of sugar in the portal blood. When absorption
ceases it may be assumed that the sugar of the blood in
general sinks slightly in amount. This condition appears
to cause a reversal of the prevailing reaction in the liver,
the stored glycogen is gradually transformed to sugar,
and this passes out to renew the supply in the circulation.
The approximate constancy of the sugar in the blood is thus
accounted for in the main by the power which the liver
possesses to remove or return it, according to the shifting
conditions.

The making of glycogen from sugars occurs only during
life. The converse change from glycogen to sugar takes
place freely after death, and is doubtless due to an enzyme.
It might be anticipated that a comparatively short period
of fasting would suffice to exhaust the glycogen of the liver.
As a matter of fact, there is a large reduction in the first
day, but the removal then proceeds slowly and is scarcely
ever completed. The disappearance is greatly hastened by
muscular activity, most effectually by the intense con-
vulsions produced by strychnin-poisoning. For human
(subjects it has been shown that glycogen is consumed
rapidly under the influence of iced baths. (How we are
able to judge of the abundance or scarcity of glycogen in
living men will be explained in another connection.)

For some time after Bernard first called attention

140 NUTRITIONAL PHYSIOLOGY

to the "glycogenic function" of the liver, the fact that gly-
cogen is deposited also in the skeletal muscles was over-
looked. In these tissues it does not reach any such per-
centage as may be found in the liver, but inasmuch as the
muscles form nearly half the entire mass of the body, a
small percentage means a large aggregate. Collectively,
the muscles have commonly been estimated to hold an
amount equal to that in the liver, and there is a growing
impression that they contain even more. The total gly-
cogen in the system may probably be as much as 400
grams, or nearly a pound. The question which now calls
for consideration concerns the importance of maintaining
such a strict constancy in the sugar-content of the blood.
Some light is thrown on this matter when we observe
the result of artificial increase of sugar concentration.
This may be brought about by injecting sugar solution
into the blood-vessels. If this is done freely there is
excessive lymph-formation and other evidence of deranged
conditions in the circulatory system. A symptom to
which especial attention must be called is the appearance
of sugar in the urine under such circumstances. The
kidneys are so organized that any distinct rise of sugar in
the blood leads to the excretion of the excess. Thus the
composition of the blood is restored to the standard, while
potential food is lost to the tissues. Such a waste of sugar
is less likely to follow abundant feeding of foods rich in it
than to occur after the experimental procedure just de-
scribed, but it may nevertheless result from the selection
of peculiar diets. It is then called alimentary glycosuria.
This escape of surplus sugar is not to be confused with
diabetes. Alimentary glycosuria is induced somewhat
readily by eating sugar, but hardly ever by eating starch.
The differing reaction is presumably explained by the fact
that sugar requires a brief digestive treatment and is then
rapidly absorbed. Starch, on the other hand, has to pass
through serial stages of digestion,- and the absorption of
the resulting sugar is extended over a longer period. In
the first case we may suppose that the inrush of sugar over-

1

•

THE METABOLISM OF FATS AND CARBOHYDRATES 141

whelms the liver, which is unable to arrest all of it. -That
which goes by raises the sugar content of the blood above
the level at which it begins to pass into the urine.

If for any reason much of the glycogen in the liver or
the muscles is quickly resolved into sugar the blood must
be affected quite as though the added sugar had come from
the intestine. Glycosuria will ensue. Certain changes in
the circulation are known to cause such a flooding of the
system with sugar and the appearance of a part of it in the
urine. A most interesting instance of such glycosuria is
that following an experience of strong emotion. It
seems that one of the results of 'the disturbance in the
central nervous system is the conversion of a large amount
of glycogen into dextrose. With acute insight a physiolo-
gist has pointed out that this release of sugar is a helpful
reaction under the circumstances. The occasion of emo-
tion is usually an occasion for strenuous action, perhaps for
flight or for giving battle, and the muscles may be re-
inforced by the increased supply of their preferred fuel
brought to them.

The regulating action of the liver and the muscles upon
the carbohydrate distribution may be paralleled, in part at
least, by an analogy. Let us compare the active tissues to
a mill turned by the waters of a stream. The water-
supply to the mill is to be compared with the sugar-supply
to the cells which derive their energy from it. A meal is to
the body as a storm is to the mill-stream — it adds to the
volume of the power-producing element. The dam by the
mill is like the kidney in its relation to the accumulated
store; if the water rises above the crest of the dam it flows
over and passes on down the stream without having con-
tributed its energy to the turning of the machinery; if
the sugar rises above a certain level it begins to escape,
with its potency for work lost to the organism. More-
over, the capacity of the liver and the muscles to hold back
carbohydrate suggests the function of a broad mill-pond.
The larger the pond above the dam the more successfully
the irregularities due to alternating rain and drought will

142 NUTRITIONAL PHYSIOLOGY

be offset and the less likely will be a wasteful overflow when
a storm follows a term of low water. The conversion of an
intermittent supply into a constant one is the function of
the mill-pond, and it is equally the service of the tissues
holding glycogen.

While a relatively sudden rise of sugar in the circulation
may produce glycosuria, a slight chronic excess over the
normal may have an entirely different effect. If the diet
is supplying day by day a little more carbohydrate than
the body is oxidizing, the surplus may be transformed to
fat. The opinion that starchy and saccharin foods are
fattening has scientific support as well as common observa-
tion in its favor. Glycogen formation is limited and we
may suppose that fat-building takes place when the gly-
cogen reserve is at its maximum and still more sugar is
offering. An important advance was made in physiologic
knowledge when Liebig called attention to the fact that a
cow's milk contains an amount of fat utterly out of pro-
portion to the scanty supply furnished by the food of the
animal. It had been believed that no such transformation
could be accomplished by animal tissues, and that all fat
found in the body or in the secretions must have been re-
ceived as fat. Liebig fell into error when he stated that
the milk-fat had been made solely from proteins and not at
all from carbohydrates, but he had taken a notable step in
recognizing the possibility of changing one food-stuff into
another. Quantitative experiments soon showed that car-
bohydrates must be given a very prominent place among
fat-forming materials.

The steps through which sugar is transformed into fat
are little understood. A comparison of the composition
of the two makes it evident that a great deal of oxygen has
to be removed in the reactions. This element is never set
free in animal metabolism; in the present instance it is
separated in the form of carbon dioxid. This is not merely
a theoretic consideration, but a condition which can be
demonstrated by experiments upon an animal rapidly
gaining fat. A wood-chuck, for example, when it is eating

THE METABOLISM OF FATS AND CARBOHYDRATES 143

voraciously of starchy food and gaining steadily in fat,
breathes out more carbon dioxid than can be accounted
for by the oxygen consumed. The excess is a by-product
of the process in which sugar with its high percentage of
oxygen is made over into fat with a much lower per-
centage.1

Recourse may be had once more to analogy. Bernard
compared the liver to a bank, but we may extend the com-
parison to the whole body. The products of digestion are
the daily deposits; the oxidations stand for the daily
payments. The bank will have a convenient cash balance
on hand from which to meet current demands. This is
the function of glycogen. The. cash in the bank will
be but a small fraction of its total resources, and its varia-
tions from hour to hour will signify but little as regards the
stability of the institution. So the glycogen of the body
is a small reserve and may vary by 50 per cent, within
twenty-four hours. The body-fat, like the securities held
by the bank, is a large accumulation and less subject to
change. If for some time the deposits are in excess of the
withdrawals, the officials of the bank will, of course, make
new investments. The parallel is clear : the body receiving
more carbohydrate than it is expending will not allow it to
go on increasing in the liver and muscles, but will begin to
convert it to fat. Unhappily, the correspondence fails
at one point: a bank which is subject to a "run" may sell
its bonds and other holdings that it may have cash to
pay its depositors. The obvious suggestion is that the
fat of the body will be reconverted to sugar when there are
demands to be met and no incoming food. This is not
known to occur ; the oxidation of fat during starvation ap-
pears to proceed without any such previous change.

Using another metaphor, though still a financial one, it

1 Without assuming that the process is fully understood, French
authorities have suggested that the principles involved may be shown
in the following equation:

13(C6H1206) = C55H10406 -f 23(C02) + 26(H2O).
(Shafer, " Text-book of Physiology," vol. i, p. 933.)

144 NUTRITIONAL PHYSIOLOGY

may be said that the glycogen is like a checking account
which a man uses to pay his routine expenses, drawing
upon it often and recruiting it at longer intervals. Such
an account is sometimes nearly wiped out and then at one
stroke largely increased. The fat of the body is like a
savings bank deposit, gathered slowly, drawn upon only
in emergencies, and, it may be added, gaining by com-
pound interest in many cases.

Two facts readily suggest themselves which may be
used to explain the low limit of glycogen storage. For
one thing, its physical properties would probably make a
high percentage of it undesirable. In the second place,
there is a distinct economy in substituting fat for glycogen
because fat represents more energy in proportion to its
mass. An individual who carries 20 pounds of adipose
tissue may wish to be rid of a part of it, but if he were com-
pelled to bear a load of glycogen equivalent in energy his
burden would amount to about 45 pounds.

The Pancreas and Carbohydrate Metabolism. — We
have repeatedly compared the carbohydrate of the body
to money. Just as it is the eventual function of money to
be spent, so it is the destiny of carbohydrate to be oxidized
that its latent energy may be turned to account. This
oxidation takes place chiefly in the muscles, to some extent,
doubtless, in the glands, the gray matter of the nervous
system, and the absorbing cells of the intestine. The in-
tensity of the local process will in every case be proportional
to the heat and other forms of energy developed. Consid-
ering the wide distribution of this purposeful destruction
of sugar it is surprising to find it all dependent upon an
obscure action of the pancreas.

This organ has been dwelt upon previously as a most im-
portant contributor of digestive juice to the intestine. It
would be anticipated that the removal of the pancreas
from an animal would be followed by defects of digestion
and assimilation. This is probably justified by the results
of a trial, but the effect upon digestion is overshadowed by
a consequence hardly to be foreseen. This is the almost

THE METABOLISM OF FATS AND CARBOHYDRATES 145

complete loss on the animal's part of the power to oxidize
dextrose. In other words, a process not occurring in the
pancreas at all, but in the tissues at large, is arrested by the
removal of this gland. What is the natural explanation of
this condition? Evidently that something proceeds from
the pancreas through the circulation to other parts of the
body, without which the cells in general are incapable of
decomposing the sugar.

This imperfectly known product of the pancreas is an
example of what has been referred to as a hormone. One
is tempted to think of it as an enzyme, but it cannot be
accurately described by this word. If it were a typical
enzyme we might expect it to cause the destruction of
sugar in solutions to which it has been added. No marked
disappearance of sugar occurs when the experiment is
made. In view of this it is wiser not to commit ourselves
as to the precise character of the body in question. The
fact seems to be that neither the pancreatic extract by itself
nor an extract of muscle will particularly promote the
oxidation of sugar, but that a combination of the two is
necessary.

Diabetes. — If, owing to a lack of the pancreatic hormone,
the ability to utilize sugar is lost, the continued absorption
of carbohydrate will raise the percentage in the blood, with
the result which always ensues under this condition. The
kidneys will steadily excrete the surplus sugar. Unlike
alimentary glycosuria, this overflow of sugar will not be
limited to times of free feeding with carbohydrates, but
will attend the ingestion of the most moderate amounts.
(As a matter of fact, sugar will still be excreted when
carbohydrates are excluded from the diet and in fasting.
The explanation of this fact must be deferred to the next
chapter.) The difficulty of keeping up nutrition when the
cells can make no use of glucose will be evident in a measure
even at this point, and we shall find additional reasons
later.

Whether human diabetes is necessarily associated with
some disorder of the pancreas cannot be stated with cer-
10

146 NUTRITIONAL PHYSIOLOGY

tainty. It is known to be so in an increasing number of the
cases studied. Often when the pancreas shows no defect
to the naked eye, some abnormality is revealed by the
microscope. Assuming that the trouble centers in the lack
of the hormone, physicians have frequently undertaken
to treat diabetes by administering preparations of pancre-
atic tissue or extracts. The results have been generally
disappointing, though a solitary case recently reported
showed marked improvement for a time. The peculiarity
of the subject in this instance was an uncommon capacity
for eating nearly raw meat. This made it possible for him
to be fed incredible quantities of pancreatic tissue (sweet-
breads) from calves. The helpful treatment was inter-
rupted when, after an attack of indigestion, he found him-
self unable to eat any more sweetbreads. To say that
he could not do so to save his life is to express the literal
truth, for he fell into a rapid decline and soon died.

CHAPTER XVI
NITROGENOUS METABOLISM

OUR knowledge of the history of the products of protein
digestion has been much extended in the past few years, but
there are still many uncertain passages. Any account
which can be given must be held subject to revision.
Nevertheless the course of nitrogenous metabolism, in its
broader aspects, is tolerably clear. The present interpre-
tation is most easily appreciated when earlier conceptions
have been briefly reviewed.

Not long ago it was held that the earlier products of
tryptic digestion were promptly absorbed, and that the
formation of the late products, the amino-acids, was rather
an accidental and, possibly, an unfortunate occurrence.
It was believed that the simplest bodies could not serve all
the purposes of nutrition. Such compounds were known
to arise in the laboratory experiments, but their formation
in the intestine under strictly normal conditions was ques-
tioned. The opinion prevailed also that the peptones
which disappeared from the canal were reconstructed at
the time of absorption and represented thereafter by the
protein of the blood. The old impression that but little
in the line of synthesis could be expected of the animal
tissues was distinctly influential.

About 1901 it was shown that a dog can be nourished
when the nitrogenous food which it receives is in the form
of the most advanced products of tryptic hydrolysis. A
quantity of lean meat had been digesting with pancreatic
juice for a period of months. The resulting mixture
contained only bodies of a simpler order than the ones
on which nutrition had hitherto been supposed to depend.
Food so prepared is not attractive, but it can maintain an

147

148 NUTRITIONAL PHYSIOLOGY

animal in fair condition. Physiologists accepted the
evidence and granted that the proteins of the organism
could be synthesized — sometimes at least — from amino-
acids. Shortly after this change in our conceptions the
discovery of erepsin was announced. It was recognized
that the existence of this enzyme made it more probable
that digestion should normally run its full course rather
than it should be terminated at an early stage by the in-
tervention of the absorbing cells. It became apparent
that even though the material leaving the intestine might
have 'the relatively complex character which we associate
with the peptone stage of digestion, the products delivered
to the interior of the villi by the cells might have under-
gone further cleavage. The ability of the animal to turn
to account such simple bodies in synthesizing its own
proteins became clear.

In the chapter on Intestinal Digestion (Chapter X) the
statement was made that the various amino-acids have
been called the "building-stones" of metabolism. In the
course of digestion they are separated, and after absorption
or during the very act of absorption, they are to some
extent assembled again. Many facts bearing upon this
process have been brought to our attention by the recent
studies of physiologic chemists concerning the constitu-
tion of protein molecules. All that has been done of late
in this direction has served to emphasize the variety of in-
dividual structure comprehended under the term protein.
When such compounds from various sources are subjected
to decomposition, either by digestion or by other means,
the assortment of amino-acids obtained differs with the
particular protein under investigation.

Some, which one is tempted to regard as perfect proteins,
yield the full list of amino-acids as at present known.
Others, seemingly defective when judged by this rather
arbitrary standard, fail to yield certain members of the
series. This variation has an important bearing on mat-
ters of nutrition. It can no longer be maintained that all
substances characterized as proteins are equivalent in

NITROGENOUS METABOLISM 149

their power to minister to growth and repair. Among
an increasing number of defective proteins now recognized
gelatin is the best known. Its behavior in the system calls
for exposition.

Gelatin. — This familiar compound belongs to the ill-
defined group often spoken of as the albuminoids. These
may fairly be regarded as proteins which have undergone
a more or less definite degeneration both in function and
in chemical structure. They are found as intercellular
substance in the connective tissues and also in the dead
and dry surface layer of the skin. They make the chief
substance of the hair and nails. Gelatin itself is derived
by boiling certain varieties of connective tissue, including
bone and tendon. It gives nearly average percentages of
the five elements present in ordinary proteins. When this
fact was discovered in the early days of organic chemistry
it was urged that gelatin must have a value in nutrition
quite equal to that of any nitrogenous food.

Trials were made on a large scale in the pauper institu-
tions of France. It was found very definitely that the
free use of gelatin led to indigestion and that it soon
became repugnant to the subjects, however hungry they
might be. These effects were later found to be correlated
with inadequacy to maintain the weight and strength of
animals. An animal cannot be said to be perfectly nour-
ished if it is losing more of any element day by day than it
is receiving. This is eminently true of the nitrogen in the
income and the outgo. "Nitrogenous equilibrium,"
expressing the equality of the two, is a phrase we shall use
freely. Now nitrogenous equilibrium cannot be estab-
lished by feeding gelatin in place of all other protein, no
matter how skilfully the experiment is conducted.

For many years the insufficiency of gelatin to make good
the losses from the tissues remained a mystery. It has
been amply explained by the findings of chemists in our
own time. Gelatin yields most of the building-stones re-
quired for the new construction, but it does not furnish
them all. Therefore it is impossible to make blood-pro-

150 NUTRITIONAL PHYSIOLOGY

teins or the proteins of the living cells from its cleavage
products. It is useless to increase the quantity of the
amino-acids if the variety is not great enough to supply the
details of the molecular pattern to be wrought. What is
true of gelatin is true of a number of proteins from vege-
table sources. They do not give all the groupings needed
in the constitution of the more elaborate animal proto-
plasm.

In some cases a single protein may be adequate for nutri-
tion, supplying a complete assortment of amino-acids.
But it will be seen that successful nutrition is more cer-
tainly to be secured by using proteins from various foods.
This is our practice, save in the important case of the milk-
fed infant. Milk actually contains proteins of more than
one order, so that the exclusive use of this food does not
narrow the selection of building units so greatly as might
be supposed. The chief protein of milk contains the ele-
ment phosphorus and is perhaps of somewhat . unusual
complexity.

In an introductory chapter the protein molecule was
likened to a watch with its many dissimilar parts associ-
ated in the one possible way to secure a desired result.
One or more of these parts might be missing without their
absence being apparent to the untrained person as he
looked into the works. He could nevertheless observe the
fact that the watch would not go. This is quite parallel
with our progress toward an understanding of the failure
of gelatin and other proteins to serve all purposes in nutri-
tion. Just as the watch-maker, with his special knowledge,
easily detects what is wanting, so the physiologic chemist
is now able to say with much accuracy what particular
amino-acids are lacking in his feeding experiments. As
the defective watch may be made serviceable by the addi-
tion of certain bits of mechanism, so in a measure an in-
sufficient diet may be made adequate when extra amino-
acids are supplied.

It is necessary now to approach a subject of some
difficulty. We must attempt to show why a given quan-

NITROGENOUS METABOLISM 151

tity of protein fed — say 100 grams — cannot contribute an
equal quantity to the protein supply of the body. When
protein of one kind undergoes complete hydrolysis and
protein of a new kind is to be made from the resulting
cleavage products, certain of the building-stones will be
needlessly plentiful, while others will be relatively scarce.
We have seen that if a single one of these structural units
is not furnished, there is complete failure to synthesize
the new compound. Similarly, if the second body is to
contain a large percentage of an amino-acid which is but
scantily represented in the first, the possible formation is
definitely limited. The principle is easily illustrated.
Suppose that in a club of 100 members there are 25 Demo-
crats. It is desired to elect for purposes of debate the
largest possible body, consisting of Democrats and Re-
publicans in equal numbers. Evidently, this body will
comprise 25 men of each party. There will be 50 men un-
related to the new organization. We may change the
comparison: A house is pulled down and another is to be
erected from the timbers. If the second house is of an
architecture entirely unlike that of the first, there will be
many unavailable pieces to discard and the new building
will be smaller than the old. It is not at all unlikely that
the misfit fragments of building material will go into the
cellar of the new house, later to be used as fuel. This is
just what the body does with the misfit amino-acids. So
far as they do not find place in the mosaic which is put
together they serve as producers of energy.

Again, a better analogy suggests itself : The structure of
a molecule of food-protein, previously compared with that
of a watch or a house, may be likened to the type set up to
print a page. The letters, some of which occur frequently
and some rarely, stand for the amino-acids. The type is
allowed to fall apart, the symbol of digestion. It is to be
set up again to print different matter. If the language and
vocabulary are much as at first, it may be possible to com-
pose nearly a whole page before the lack of some letter
brings the proceeding to a standstill. But some shrinkage

152 NUTRITIONAL PHYSIOLOGY

will be inevitable and when the type-setting has to halt
there will be some unused letters. The shrinkage will be
much greater if the second page is to be printed in a lan-
guage other than the original. Suppose, for example, the
type used in English composition is next devoted to Ger-
man. The resulting difficulty is readily foreseen — the
letter z is uncommon in English, but frequent in German.
Hardly a line can be perfectly set up before this -letter will
be vainly sought. Almost the whole collection of type will
be useless for composing. This is analogous to the attempt
to minister to animal growth with some isolated vegetable
protein of exceptional constitution. Offering the cleavage
products of gelatin to the cells is like giving the compositor
incomplete fonts of type. He cannot set up connected
matter if some of the letters are not to be found.

It seems natural to assume that the closer the structural
resemblance between the proteins digested and those to be
synthesized, . the more economically the making over can
be accomplished. It is permissible to infer that nutri-
tion can be subserved by a smaller quantity of proteins
when they are derived from animal sources than when
they are of vegetable origin. One cannot, however, use
this as a strong argument against vegetarianism. The
quantity of proteins which one takes when following the
dictates of the appetite is apparently so liberal that all
constructive requirements are easily met, even though the
difference between the composition of the food-proteins
and those of the body is a wide one. Too rigorous logic
applied in this connection might lead to the recommending
of cannibalism.

Our knowledge of the place and the manner of protein
synthesis is incomplete. The cells which line the intestine
and receive the digestive products are generally held to
bear a large part in the work. The fact that these cells
contain erepsin, an enzyme capable of breaking down the
more complex nitrogenous bodies, does not exclude the
possibility that dehydrations and condensations may still
take place within them. The enzyme may be modified un-

NITROGENOUS METABOLISM 153

der some conditions so as to be inactive. It is even con-
ceivable that it may facilitate the combining of the build-
ing-stones. Some enzymes, like other catalysts, may favor
the progress of reactions either in one direction or the
reverse, according to the proportions of the substances
present at the moment.

It is not easy to estimate the extent of protein synthesis
which normally takes place. Of course, it is more promi-
nent during growth than during adult life. The present
impression is to the effect that only a very small fraction
of the usual protein income is thus used. Most people
can materially reduce the quantity of protein in the diet
and still remain in nitrogenous equilibrium. When the
lowest level at which this is possible has been attained it
is still true, as we have just pointed out, that the amount
of new protein constructed is but a fraction of that sup-
plied. It is, therefore, certain that under average con-
ditions by far the larger part of the nitrogenous food eaten
never exists in the form of proteins after absorption. We
must now consider the destiny and value of the uncombined
building-stones.

The surplus amino-acids, either free or in simple com-
binations, are borne away from the intestine in the portal
blood. Accordingly, these digestive products, like the
sugars, are brought under the influence of the liver-cells.
They undergo a transformation in this organ — and very
probably elsewhere — which has important consequences.
This is the process described as "deaminization." To
deaminize an amino-acid is to remove from it the group
to which it owes its name, the radicle NH2. One of the
products of this reaction will be non-nitrogenous, the other
will contain nitrogen in a greatly increased percentage.
We cannot claim to know all the steps which are gone
through in this connection, and we shall not discuss those
which are known. We shall emphasize simply the final re-
sults.

The chief nitrogenous compound which issues from the
series of reactions occurring in the liver is urea. This is

154 NUTRITIONAL PHYSIOLOGY

apparently of no further use in the system. It is destined
to circulate until it shall find its way into the urine. The
efficiency of the kidneys is so remarkable and the whole
blood volume is carried through them at such short in-
tervals that the percentage of urea in normal blood is
kept very low. Urea seems to be a most convenient form
for nitrogen elimination; it is highly soluble and diffusible,
inert, and, comparatively speaking, non-poisonous. If
a man eats 100 grams of protein in twenty-four hours he
will excrete some 30 grams of urea, an amount which rep-
resents about seven-eighths of the total nitrogen passing
into and out of the body.__ This urea is not all made by the
liver, but a large share of it proceeds from the deaminizing
activities of this organ.

What, then, is the principal non-nitrogenous compound
produced from the amino-acids in the liver? There seems
to be no doubt that it is dextrose. Evidence in support of
this belief has been derived from the study of diabetes.
Whether this condition is experimentally induced or devel-
ops spontaneously, it is found that, if the case is one of full
severity, sugar excretion goes on even when no carbohy-
drate is fed, and, indeed, throughout long periods of fast-
ing. This sugar might be assumed to have come from the
fat of the body, but it can be more surely attributed to the
protein which is being decomposed. The following con-
sideration shows this: the nitrogen and the sugar in the
urine of the fasting diabetic patient maintain a singularly
constant ratio; 1 gram of nitrogen is accompanied by 3.6
grams of dextrose. They rise and fall together. This
seems to prove that the two must have a common source,
which can only be protein. Again, the feeding of amino-
acids to the diabetic increases his loss of sugar; the feeding
of fat does not have this effect. A simple calculation
shows that 100 grams of protein fed may give rise within
the body to about 57 grams of sugar. The seriousness of
diabetes will now be better appreciated than has been
possible up to this time. The organism loses not merely
the support normally secured from the chief carbohydrates

NITROGENOUS METABOLISM 155

of the diet, but, in great part, that ordinarily furnished by
protein food.

From all this it appears that much of the protein which
we eat serves only to supply the tissues with carbohydrate.
The impression is likely to be that this is a roundabout
and not an economical way to provide sugar, which might
have been taken as such at the outset. The facts may
fairly be employed to support the modern teaching that
excess of protein is to be avoided, but we have already
shown the necessity for allowing more than is actually to
be reconstructed after the digestive dismembering. Recog-
nizing as inevitable the discarding of amino-acids, we can
see the desirability of having them made to furnish a
simple standard food like sugar, valuable for its store of
energy. The possibility of glycogen formation from pro-
tein naturally follows. The glycogen of carnivorous
animals presumably has such an origin. The maintenance
of the sugar of the blood during long fasting is also ascribed
to the disintegrating protein of the tissues. Given dex-
trose in such quantities, the production of fat from protein
becomes at least theoretically possible. Broadly speaking,
we may claim for protein that it can do all that any form
of organic food can do for the system. Yet this does not
impair the statement, equally to be recognized, that car-
bohydrates and fats should form much the larger part of
the income.

After the constructive requirement has been met, all ad-
ditional protein seems to entail unprofitable labor on the
part of the liver in deaminizing the cleavage products, the
presence of various substances of a possibly detrimental
nature in the circulation, and an activity on the part of the
kidneys which may amount to an abuse of these important
excretory organs. There is this general contrast between
the behavior of proteins and non-proteins in the body:
the former give rise to rather complex waste-products im-
posing a task upon the liver and kidneys; the latter are
oxidized cleanly to carbon dioxid and water, two com-
pounds which are eliminated with ease. A fuller discus-

156 NUTRITIONAL PHYSIOLOGY

sion of these facts will be undertaken in the chapter on The
Hygiene of Nutrition (Chapter XXII).

Folin, of the Harvard Medical School, has classified the
facts of protein metabolism in a particularly clear and
helpful form. He distinguishes two lines of transforma-
tion, the endogenous and the exogenous. Under the head
of endogenous metabolism he traces the various steps in
the history of those building-stones which are erected into
the proteins of the blood and other tissues. The narrative
is continued in the account of the rather gradual and steady
crumbling which such tissue-proteins undergo. What is
loosely called the wear and tear of the cells gives rise to
definite end-products, one of which Folin holds to be of
particular value in estimating the extent of such decompo-
sition. This is the substance creatinin, which accompanies
urea out of the body and which is far less subject to fluctua-
tion. The urea excreted rises and falls with the protein
ration, but the creatinin is not markedly responsive to
dietetic variations. It is believed, therefore, that endog-
enous metabolism, a necessary feature of animal life, is
relatively independent of feeding, at least while nutrition
is satisfactory.

Exogenous metabolism is an expression to cover all the
reactions affecting the uncombined amino-acids. Hence
it includes the formation of urea, dextrose, and whatever
substances may be formed from the nitrogenous cleavage
products in the liver. It may be extended to take in the
secondary production of glycogen or of fat from surplus
sugar originating in this way. In contrast with endogen-
ous metabolism its amount varies widely. With a low
protein diet the exogenous changes will be but a fraction
of what they will become with abundant protein. One
may be tempted to conclude that in fasting the metabolism
will be wholly endogenous. The insight of a German writer
has served to show us that this is not so. We have spoken
at length of the assembling of amino-acids fresh from the
intestine to form the standard proteins of the blood. Now
there are differences of constitution between the blood-

NITROGENOUS METABOLISM 157

proteins and those of the various organs much as there are
between these same .blood-proteins and those of the food.
When a particular tissue, muscle, for example, is to be
nourished at the expense of the blood (or the lymph), a
true local digestion is necessary, and once more there must
be the selecting and rejecting of amino-acids. Those not
available may reach the liver and be deaminized there
quite as if they had come from the seat of the original
digestion.

Note. — The outlines of nitrogenous metabolism given
in this chapter follow closely Abderhalden's presentation
of the subject. It is impossible to predict how far the
prevailing views may be modified as the result of work now
in progress. Mendel's researches seem tending to support
the belief that more remodeling of the amino-acids can be
carried on than has been generally supposed. A recent
suggestion of his must be considered. The food which lies
in the intestine is not used solely by the animal, but serves
also to nourish swarms of bacteria. These are plants and
are known to possess well-marked synthetic powers. It is
certain that the multiplying bacteria construct new pro-
teins from the nitrogenous cleavage products placed at
their disposal. The proteins thus formed may be radically
different in molecular pattern from any in the diet. Great
numbers of these intestinal bacteria are always perishing,
and when they die they must yield their substance to the
digestive juices for resolution into its component groupings.
Thus there may be a hidden supply of amino-acids to the
system which is more or less independent of the ration
which the experimenter has furnished.

Another possible divergence from the account given
above must be indicated. As we have pictured the suc-
cession of events, the amino-acids used for synthesis have
been combined to form the characteristic and abundant
proteins of the blood, these to be locally digested and made
to supply the necessary building units to the various
tissues. But the impression seems to be gaining in favor

158 NUTRITIONAL PHYSIOLOGY

that amino-acids uncombined may escape the influence of
the liver, and be taken to all parts of the body and offered
in free condition to organs which may require them. If
this is the usual procedure the suggestion is that the blood
proteins form a rather stable reserve mass of food material
not much subject to depletion and renewal under ordinary
conditions of feeding. In starvation we know that the pro-
teins of certain organs are used to keep up the nutrition of
others more essential to survival. It would be interesting
to know whether such transferred material is carried in the
form of amino-acids or organized temporarily into proteins
of the type found in the plasma. But there is no part of
physiologic chemistry where our knowledge is so much at
fault as just this section.

A SUMMARY OF METABOLISM

Fats are hydrolyzed in the alimentary canal to fatty
acids and glycerin. To an uncertain extent there is soap
formation. These products are largely recombined to
form fat during the passage through the lining cells of the
intestine. Fat is stored chiefly in adipose tissue. Its
eventual service is to be oxidized to carbon dioxid and
water with release of its energy.

Carbohydrates enter the circulation in the form of simple
sugars, mainly glucose. There is little sugar in the blood
at any one time. Much is dehydrated by the cells of the
liver and muscles to make glycogen, subject to reconversion
to sugar when required. A surplus may be converted to
fat. The possibility of the converse change from fat to
sugar is generally held to be unproved. The final value
of carbohydrate is like that of fat; its energy is set free
through the respiratory oxidation, and the end-products
again are carbon dioxid and water. The internal secretion
of the pancreas is necessary to bring about this destruction.

Proteins are hydrolyzed to simple compounds (amino-
acids or combinations of these), and these are used for the
synthesis of the new proteins — of types peculiar to the
species — which can be utilized for growth or repair. A

NITROGENOUS METABOLISM 159

large surplus of uncombined amino-acids is usual ; these are
dealt with by the liver, and the best-known resulting pro-
ducts are urea — destined to be excreted — and glucose,
for which all the possibilities exist that have been men-
tioned above. The amount of wasting suffered by nitrog-
enous tissues as a part of their life process is believed to
to be indicated by the extent to which the product creatinin
is eliminated. All proteins yield sulphur compounds
among their decomposition products. Some yield phos-
phorus compounds also, and it is these proteins which
give rise to much of the uric acid which often tends to
be retained and to cause trouble.

CHAPTER -XVII

THE REMOVAL OF THE END-PRODUCTS OF
METABOLISM

THE statement has repeatedly been made in varying
form that the bulk of the food is taken for the sake of its
potential energy. Either at once or after storage it is
oxidized, and the energy turned to account for tempera-
ture maintenance and for the performance of muscular
work. The main products are carbon dioxid and water.
These are likewise the chief products formed when familiar
fuels are burned outside the body. Wood, coal, and gas
yield the two in great quantity and only small amounts of
other compounds. Hence the primary problems of excre-
tion concern the manner of elimination of carbon dioxid
and water.

The water leaving the body during twenty-four hours
may be 2 or 3 kilograms. The carbon dioxid discharged
in the same period is not often in excess of 1 kilogram.
Nevertheless, we say that carbon dioxid is the leading
waste-product of animal life. This is justified by the con-
sideration that by far the larger part of the water which we
measure is merely water previously received in the same
state. To this large volume the tissues have added a mod-
erate quantity of water — say 250 grams — which is a true
metabolic product. This has been formed by the oxidation
of compounds containing hydrogen. The water output
of the body is inevitably greater in the long run than the
water income. This fact may be disguised on single days
by water retention.

Carbon Dioxid Elimination. — Respiration has been
defined as the process within the living cells in course of
which complex organic molecules are decomposed and

160

REMOVAL OF THE END-PRODUCTS OF METABOLISM 161

more or less completely oxidized. Carbon dioxid is the
most conspicuous product. The respiratory exchanges
occur in the different tissues in a measure corresponding
with the extent to which they severally evolve energy.
The skeletal muscles lead in amount of respiration (and
of carbon dioxid set free), both because of their great mass
and their activity. The glands, especially the liver and
the kidneys, contribute largely to the total. So does the

Cap

Fig. 21. — I is intended to suggest the form of an air-sac of the
lung overlaid with a network of capillaries belonging to the pul-
monary system. II is an imaginary section through such an air-
sac. B in both I and II is the minute bronchial tube through which
the air is renewed. Ill is a bit of detail from II, still more en-
larged, showing two capillaries (Cap.) conveying corpuscles (Corp.).
The air is close by (Al), yet two partitions intervene, the capillary
wall and the wall of the air-sac.

heart. Other tissues have a minor part in the general res-
piration. Some which are passive and stable, like cartilage,
can have but little.

The carbon dioxid formed by the cells is first transferred
to the lymph. The concentration of the gas in the lymph
leads to its passage into the blood. A gas will always pass
from a higher to a lower concentration when the two solu-
tions are placed in communication. The delicate capillary
11

162 NUTRITIONAL PHYSIOLOGY

wall between the lymph and the passing blood offers prac-
tically no impediment to the movement. It will be re-
membered that the blood which enters upon the very short
journey through the capillaries is arterial; that which
enters the minute veins only a fraction of an inch away is
reckoned venous. It has parted with a large share of its
oxygen and has received carbon dioxid. It is swept on
without significant change to the right side of the heart
and thence to the lungs. The development of these organs
is such as to multiply the surface of contact between the
blood and the air within them. The capillaries of the
pulmonary circulation are wrapped about innumerable
elastic sacs, the walls of which are as thin as those of the
capillaries themselves. There is, accordingly, a double
partition between the blood and the air, but it is of a nature
to permit free gaseous exchange.

If the air in the sacs were not renewed it would accumu-
late carbon dioxid in increasing amount, while its oxygen
would progressively diminish. This tendency is normally
counteracted through the effects of breathing. The lungs
have no power to move of themselves; the changes which
they undergo are due to the widening and the return of the
thoracic walls. This is not the place to analyze the breath-
ing movements. The muscles employed are of the skeletal
order. Being so, they are not automatic, and it follows
that every breath taken is the expression of a separate and
distinct act on the part of the central nervous system.
Each time the chest is made larger the air presses in along
the breathing passages to fill the space created for it in
the host of widened sacs. The return of the chest walls
to their first position reduces the capacity of the sacs, and
air is pressed out along the same channels by which it
entered. The action is that of a pair of bellows not pro-
vided with the usual inlet valve.

It is not to be conceived that we empty and refill the
air spaces of the lungs with each breath. We usually expel
something like one-fifth or one-sixth of the air contained
and replace that fraction with fresh air. When allowance

REMOVAL OF THE END-PRODUCTS OF METABOLISM 163

is made for the rather long and capacious passages between
the air-sacs and the nostrils the impression that out breath-
ing is rather ineffective becomes strengthened. To offset
this idea we must remember that the movements occur
fifteen or eighteen times a minute, providing thus for at
least two fairly complete renewals of the whole volume of
air within that time. Still it is a fact that when the breath-
ing is rarely deepened beyond the constant habit, some
portions of the lungs, notably their upper extremities, are
but little subject to extension and contraction. By the
deepest possible breathing we can increase the proportion
of the air removed and replaced to perhaps three-fourths of
the total at a single movement. A quiet breath, unmodi-
fied by the influence of attention, muscular activity, or
any other temporary condition, is said to amount to about
500 c.c. (30 cubic inches or 1 pint). Reckoning sixteen
breaths in a minute, this will mean 8 liters of air breathed
in that interval, about 500 in an hour, or 12,000 in a day.
Fresh air has but a small content (0.03 to 0.04 per cent.) of
carbon dioxid. The air expired has 4 per cent., more or
less; 4 per cent, of 12,000 liters is 480 liters, a fair average
volume to represent the daily output of this gas. This
quantity, changed from volume to weight with correction
for temperature, is about 800 grams.

The air to which the blood is exposed in the lungs is at
least as rich in carbon dioxid as that which we breathe out.
Coming into relation with air of such a composition the
blood by no means frees itself of its large carbon dioxid
content. It carries on to the left side of the heart and so to
the general arterial system some five-sixths of the carbon
dioxid which it contained when it entered the lungs. The
actual amount in venous blood is in the neighborhood of
45 in 100 c.c. of blood. As much as 38 c.c. in 100 will
usually remain in blood which is counted arterial and which
carries a maximum of oxygen. Carbon dioxid does not
interfere with the capacity of the blood to carry oxygen,
and the converse is equally true. It may be well to state
that the color of blood varies with the extent to which the

164 NUTRITIONAL PHYSIOLOGY

corpuscles are charged with oxygen and is independent
of the carbon dioxid present.

While there is no question of the propriety of calling
carbon dioxid a waste-product, it does not follow that the
system would be benefited by its complete removal.
How far this is from being the case has been shown by the
important experiments of Yandell Henderson. He has
demonstrated that any considerable lowering of the carbon
dioxid below the high standard noted above as character-
istic of arterial blood results in marked prostration, often
involving the suspension of breathing and perhaps result-
ing fatally. The inference is that a certain concentration
of carbon dioxid is a desirable source of stimulation to the
nervous system and especially to the respiratory center.
The intimate connection between this gas and breathing is
manifest when its percentage in the blood is ever so little
increased. A noteworthy deepening of the respiration
promptly results. Since this is true it is not strange that
a reduction of the carbon dioxid should cause inhibition of
the breathing movements.

Water Elimination. — Water leaves "the body by all the
possible excretory routes. Statements regarding the
proportion taking this or that course can have but little
value, so great is the variation under different circum-
stances. If we exclude the effects of exercise and of un-
usual temperatures we may expect to find somewhat more
than half the whole amount to be removed by the kidneys.
The daily volume of the urine is customarily set down at
1200 to 1500 c.c. The remaining excretion of water will
be almost wholly accounted for by the perspiration and by
evaporation from the breathing passages. Of these two,
the former is commonly more considerable. Some loss of
water will occur in the feces, but normally this is not to be
compared with the Quantities discharged in the three ways
just mentioned.

When the extern'al temperature is high the water passes
in increased amounts through the skin and the perspira-
tion may greatly exceed the urine. The urinary secretion

Fig. 22. — The kidneys and the urinary bladder. The two kidneys
are shown within an outline which suggests the body cavity. Their
advantageous connections with the chief artery and vein of the sys-
tem are indicated. Below is the bladder reached by the two ureters.
These vessels enter the bladder low down and behind — not at the
level, where they disappear from the figure.

REMOVAL OF THE END-PRODUCTS OF METABOLISM 165

may shrink somewhat or may be kept near the standard as
a result of the water drinking which is stimulated. The
output of water from the skin during the hot weather and
also during muscular activity does not serve primarily as a
vehicle of waste, but rather as a means of ridding the body
of heat. This matter will be discussed at length at an-
other time. The evaporation from the respiratory tract
probably varies less widely than the perspiration. The
body seems bound to saturate all the air that is breathed,
so that this loss increases with the volume of breathing
and is greater when the air is dry than when it is humid.

The Kidneys and the Urine. — The chief significance of
the kidneys is their function of excreting the distinctive
products of protein metabolism,. A secondary service is
the disposal of inorganic salts. The two glands are placed
to the right and left of the spinal column at the level of the
lower ribs. Each kidney receives a large short artery from
the aorta which passes between them. Each returns a
large vein, not to the portal system, but to the chief venous
trunk of the body. The kidneys, in consequence of this
arrangement, constitute short cuts or "shunts" in the cir-
culation, and are perfused by an exceptionally large quan-
tity of blood. No portion of the blood can long escape
their influence. The urine discharged by the many tubu-
lar units of the kidneys is conveyed through the ureters,
contractile vessels which lead to the bladder. This is a
saccular organ capable of accommodating much urine when
dilated, and of contracting again to nearly complete expul-
sion of its contents. Its walls of muscle (the same type
found in the alimentary canal) are obviously under ner-
vous control and much subject to reflexes.

Urine of average composition is a complex solution con-
taining some 3 or 4 per cent, of dissolved solids. The
leading substance is urea, the chief nitrogenous waste of
the system, and the index, according to Folin, of the
exogenous metabolism. Its origin has been discussed.
Evidently the duty of the kidney is less to manufacture
urea than to select and remove from the blood the urea

166 NUTRITIONAL PHYSIOLOGY

originating in the liver and elsewhere. Second in abun-
^lance among the urinary constituents we ordinarily find
the mineral salts. The quantity of these depends in a
large measure upon the amount in the diet, and as sodium
chlorid is the one taken most freely, so it will generally be
the principal inorganic compound in the urine. The
chlorids of the mixture are accompanied by phosphates and
sulphates. These are not to any extent salts which have
been eaten, but, like the urea, they represent modified
fragments of protein molecules. The phosphates come
from a limited class of proteins, largely from those of the
cell-nuclei; the sulphates arise from all proteins.

The minor ingredients of the urine are very numerous.
Those of most interest are the bodies which carry the ni-
trogenous waste over and above that handled as urea.
To one unfamiliar with organic chemistry a list of their
names can have little meaning. The substance_creatinin,
already mentioned, is provisionally regarded as indicative
of the rate of true endogenous metabolism, the inevitable
gradual wasting of the nitrogenous tissues. Another com-
pound which has attracted much attention on account of its
apparent relation to several pathologic conditions is uric
acid. A certain amount of this is produced during fasting
and is not increased by the taking of many kinds of food.
The addition to the diet of meats leads to a larger formation
of uric acid. A maximum quantity is elaborated when
glandular tissues, such as liver, kidneys, and sweetbreads,
are eaten. These articles contain an exceptional propor-
tion of jiuclear material rich in the proteins just referred
to as sources of phosphates. The same proteins are evi-
dently uric-acid formers. The chief peculiarity of uric
acid is its slight solubility, which renders its complete
excretion difficult and uncertain. Retention of this crys-
talline substance has been held accountable for the pain-
ful symptoms of gout and of a good deal that goes by the
inclusive name of rheumatism.

We have by no means exhausted the list of normal urin-
ary constituents, but the student must be referred to other

REMOVAL OF THE END-PRODUCTS OF METABOLISM 167

sources for details. The most commonly occurring
compounds of an abnormal character are sugar and albu-
min. The significance of the sugar should be clear in the
light of what has been said. Its transient appearance as
a result of free consumption does not indicate a diseased
condition, but only dietetic indiscretion. Continuous elim-
ination shows that the body has not the usual power to
oxidize its sugar. The kidneys are not generally at fault;
the defect is in the metabolism of the pancreas or elsewhere.
An abundant escape of albumin (presumably drawing
upon the protein mass of the blood) is commonly due to a
disordered condition of the kidneys. It takes place, for
example, in Bright's disease.

Urine when freshly secreted is ordinarily acid to litmus.
It may be alkaline when much vegetable food is eaten, and
becomes so on standing in any case. The change is due
to a bacterial fermentation whereby urea is transformed
into ammonium carbonate. An ammoniacal odor develops
in connection with this alteration and the liquid is likely
to become turbid. The deposition of a sediment under
such conditions is no cause for anxiety. It has often been
represented by unscrupulous quacks to be a serious symp-
tom. The urine of the herbivora, which is normally al-
kaline, becomes acid when the animals are fasting, and it
may be pointed out that they are then carnivorous — living
upon their own flesh and fat.

How much urine is secreted depends largely on the quan-
tity of water taken, so far at least as this is in excess of the
perspiration. Kidney activity is stimulated by almost
any dissolved substance foreign to the standard composi-
tion of the blood. The nitrates, for example, are absorbed
rather freely from the intestine and afterward removed by
the kidneys in a large volume of water. They, therefore,
belong to the class of bodies known as diuretics. The
active principle of coffee and tea has the same action. In
securing their own elimination such compounds may
promote the excretion of others. Diuretics bring about
their effect partly through modifying the circulation, and

168 NUTRITIONAL PHYSIOLOGY

partly, it is believed, through direct influence upon the
kidney cells. As regards the first mode of working, it may
be said that the kidney is readily responsive to increased
blood-flow, especially if it is attended with high arterial
pressure. Difficulties with the heart, if they entail re-
tarded circulation and lowered pressure, frequently lead to
deficient output.

Other Factors in Excretion. — The lungs and the kidneys
perform so large a share in the disposal of metabolic waste
as to leave relatively little of the work undone. The
feces, however, include small quantities of miscellaneous
excretions, and it is assumed that the precise part borne by
the intestine and the liver (in the separation of bile) could
not be taken by the kidneys. The modified bile-pigments
and cholesterin of the feces illustrate this specific action.
The share of the skin in the removal of waste is popularly
overestimated. The belief that "the pores must be kept
open" lest poisons gather in the system is so fruitful of
wholesome practices that one is reluctant to question it.
Candor requires, however, that the physiologist repudiate
the moral in the story of the Italian boy, who died because
the surface of his body had been sealed with gold paint
for a few hours. If the case is authentic, he must have
died because of the character of the application and not
from toxic products of his own evolving. Volunteers have
submitted to have the skin shellacked and have not suf-
fered any other ill effects than sensitiveness to heat and cold.

Perspiration is almost purely a mineral solution and the
salts it carries could doubtless be cared for by the kidneys.
When made profuse by severe exercise, it contains in small
amounts some of the organic constituents of the urine,
but its highest possible rating as a vehicle of nitrogen ex-
cretion is not impressive. The same may be said of the
assistance rendered by the skin to the lungs. Carbon
dioxid passes from the skin in measurable quantities
when there is abundant perspiration, but the largest loss
which can occur in this way seems insignificant when com-
pared with the discharge from the lungs.

CHAPTER XVIII
THE ESTIMATION OF METABOLISM

IT is about fifty years since the first well-equipped
laboratory for the quantitative study of human nutrition
was opened in Munich. Before that time much had been
accomplished in the analysis of foods, the measurement of
rations, and the examination of urine, but no satisfactory
knowledge of the general metabolism could be had until
means should be devised to entrap and measure the gas-
eous outgo of the body. This difficult task was accom-
plished by the construction of the first respiration chamber,
now one of several in various centers of scientific research.

In the long run there must be a correspondence between
the food and the metabolism, the income and the outgo,
but on a single day there is no necessary agreement be-
tween them. This is radically demonstrated on a day of
fasting, when the income is nil and the outgo is consider-
able. It is to the excreta that we attend, therefore, when
we wish to judge to what extent various materials have
been broken down in the body. Studies of the food may
be valuable, but in our first discussion we shall limit our-
selves to the simple case of the subject without income.
A great deal can be learned about the metabolism by deter-
mining two chemical elements — the carbon and the nitro-

in of the waste-products. Other facts can be ascertained
when the quantity of oxygen consumed is noted. Water
excretion is frequently measured also.

Nitrogen Elimination. — The fact is already familiar
that the nitrogen leaving the system is found almost
wholly in the urine. An additional fraction is in the
feces. Concerning this latter item it will be remembered
that we cannot easily say how largely it is a residue signi-

169

170 NUTRITIONAL PHYSIOLOGY

fying incomplete absorption and how far it is a true waste-
product. If feces are discharged during long fasting the
nitrogen contained must be the body's own contribution.
On the whole, the fecal nitrogen is nowadays regarded as
an excretion unless it is clearly excessive. The nitrogen
of the perspiration can usually be ignored.

When we assume all the nitrogen eliminated to have
come from the decomposition of proteins, a certain error \
always exists, but it is not great enough to be considered'/
in the present elementary treatment of the subject.
Nitrogen constitutes about 16 per cent, of protein. Ac-
cordingly, the excretion of 16 grams is taken to stand for
the destruction of 100 grams of protein. This is not far
from an average amount when the diet is freely chosen.
In the opinion of an increasing number of authorities it
is higher than it should be for the best nutritional condi-
tion. To arrive at an estimate of the protein metabolized
we multiply the quantity of nitrogen in the outgo by 6.25
(an operation which is equivalent to dividing by 16 to
find 1 per cent, and multiplying by 100 to obtain the total).
This was a familiar procedure before the erection of res-
piration chambers made possible a complete survey of the
metabolism.

Carbon Elimination. — A subject excreting 16 grams of
nitrogen may be expected to excrete something like 200 or:
250 grams of carbon in the same period. This will be in
the respiratory carbon dioxid so largely as to make the.
urinary and fecal carbon appear insignificant. Figures:
from an actual experiment are:

In the respiration '. . . . 208 grams. 1

In the urine 6 "

In the feces 11 "

Total 225 "

This carbon may have been furnished by all three types of
body substance — the proteins, fats, and carbohydrates —
in numberless possible combinations. The amount ot
protein decomposition has already been fixed at 100 grams.

THE ESTIMATION OF METABOLISM 171

Such an amount of protein must have yielded in its falling
apart a quantity of carbon represented by the percentage
of that element in the compound. The actual percentage
is about 53, so that in this instance 53 grams of the 225
may be ascribed to protein as a source. The remaining
carbon, 172 grams, must have been derived from non-
nitrogenous material. How far it has come from fat and
how far from carbohydrate we cannot exactly determine
without additional data.

It is of value to know all the circumstances of such a
trial. If the twenty-four hours under consideration is the
first fasting day and the diet of the day before has been the
ordinary one, we may assume that the subject entered
upon the experimental period with a fair stock of glycogen.
This will be used rather freely at the outset, but more
and more slowly as the hours pass. Carbohydrate, accord-
ingly, contributes largely to the support of the organism
during the first day of abstinence, and thereafter bears but
a very small part. To say that the glycogen of the body
is used up in a single day of fasting would not be correct;
the fact is rather that the rate of consumption diminishes
sharply. In proportion to this diminution in the use of
carbohydrate the fat is called upon increasingly. For any
day of hunger after the first it is substantially true that
the individual is living on protein and fat.

Let us continue the discussion of our numerical illus-
tration with the added statement that the day is the second
rather than the first in a fast. The 172 grams of carbon
from non-protein material may now be attributed to fat.
The percentage of carbon in fat is about 77. A simple cal-
culation (dividing by 77 and multiplying by 100, or, what
is the same thing, multiplying our first quantity by 1.3)
gives 223.6 grams of fat as the amount destroyed in the
body during twenty-four hours. The total metabolism
is then 100 grams of protein and 223.6 grams of fat. It is
not safe to conclude from this that the loss of weight
will prove to be j ust equal to the sum of the two items. It
may be found to be either more or less, the result depend-

172 NUTRITIONAL PHYSIOLOGY

ing chiefly on the relation of the income and outgo of
water.

Could there be conditions under which all the non-pro-
tein carbon could confidently be assigned to carbohydrate
sources? Not in the fasting state nor commonly on a
mixed diet. The case might be approximated by giving
ample rations with minimal fat and maximal carbohydrate
for days together. This is nearly equivalent to the nutri-
tion of the herbivora. If our supposed human subject
yielded the amounts of carbon and nitrogen already
quoted while adequately fed upon protein and carbo-
hydrate, we should not be much in error in assuming
that the carbon from non-protein had been evolved from
starches and sugars metabolized. Carbon forms about 40
per cent, of carbohydrate, and, if we reckon according to
the same principle as before, we find that 172 grams of
carbon could have come from 430 grams of carbohydrate,
more or less. Under more ordinary conditions of feeding
— and on the first day of a fast — both carbohydrate and
fat would share with protein the sustaining of the body's
activities.

The Respiratory Quotient. — The modern respiration
chamber is a small room with impervious walls and care-
fully controlled ventilation. The carbon dioxid of the
air drawn off is either determined directly or estimated
from measured samples bearing a known relation to the
total volume. In chambers of the best type the oxygen
consumption is also ascertained. If we know both the
carbon dioxid production and the oxygen absorption we
can, of course, compute the ratio between the two. The
value of this ratio, based on the volumes and not the
weights of the two gases, is known as the respiratory quo-
tient. It is figured by dividing the volume of carbon dioxid
by the volume of oxygen. So determined it has most of the
time the character of a proper fraction, or, rendered as a
decimal, it is less than one. This is a way of saying that
the carbon dioxid discharged is generally less than the
oxygen which has disappeared in the exchange.

THE ESTIMATION OF METABOLISM 173

Every molecule of carbon dioxid holds combined the
equivalent of a molecule of oxygen. It follows that if all
the oxygen were devoted to the formation of carbon dioxid
the two volumes would be equal and the ratio between
them would be unity. The failure p| apart Lof_the oxygen
to reappear as carbon dioxid indicates that it has been
combined in some other way. It has actually gone to form
the second great respiratory product, the water of the
metabolism. The interest which physiologists feel in the
respiratory quotient springs from the fact that it varies
with the prevailing employment of one kind of material
or another in the general oxidation which is going on.
The decimal value of the ratio is elevated in proportion
to the prominence of carbohydrate in the process. It is
lowest when fat is bearing a principal part. Since, at the
beginning of a fast, carbohydrate is called upon to meet
the requirement, while its place is taken by fat a few hours
later, the respiratory quotient will show a decline which
marks with exactness the shifting' of the current!

It would not be easy to show how the respiratory quotient
can be made the basis of equations which determine
how much fat and how much carbohydrate are broken
down to give a certain output of carbon dioxid. Suffice it
to say that the possibility exists and is highly fruitful of
results in the quantitative studies of nutrition laboratories.
The meaning of the respiratory quotient is sometimes al-
tered by temporary conditions. Perhaps the most in-
teresting of these is the peculiar increase in carbon dioxid
outgo exhibited by an animal which is rapidly fattening on
a diet rich in carbohydrates. Such an animal may show
for days together a respiratory quotient in excess of unity,
that is, it produces carbon dioxid not accounted for by the
observed oxygen intake. This extra carbon dioxid is ex-
plained satisfactorily as having come from the carbo-
hydrate undergoing transformation to fat. (See also
Chapter XV, page 143.)

Equilibrium. — Our nearly uniform weight, maintained
for periods of years, suggests that income and outgo are

174 NUTRITIONAL PHYSIOLOGY

often nicely balanced. Complete equilibrium demands
strict equality between the income and outgo of water, of
mineral matter, of nitrogen, and of carbon. The realiza-
tion of these conditions is not likely, though it is often
closely approached. Partial equilibrium, that is, equality
between intake and output for one class of compounds with
inequality for another, is more common. The most fre-
quent striking of a balance is between the nitrogen of the
food and that of the excreta. Nitrogenous equilibrium is
the rule rather than the exception. The tendency of the
body to establish this correspondence, in spite of wide
variations in the diet, was noted long ago. It was said
that the organism refused to store protein when supplied
with large amounts of this kind of food. This is true in the
narrow sense that the body does not add freely to the adult
measure of its living tissues when offered extra protein.
Yet, as we have seen, the return of all the nitrogen fed
does not of necessity mean that the body has retained no
part of the protein supplied to it. The chief reason why
there is such a marked disposition to make the output of
nitrogen equal the income is found in the fact that all the
amino-acids beyond the small quantity used for protein
synthesis are deaminized. So long as this is the case,
raising the nitrogen of the food must result merely in
adding to the urea excreted. The non-nitrogenous resi-
dues may find more or less permanent lodgment in the
tissues in the form of glycogen or fat.

Nitrogen retention is to be expected during growth when
the protein syntheses are more extensive than the endogen-
ous decomposition. The recovery from illness or from a fast
is another instance when the body protein must definitely
increase. This is really only a special case of growth.
Change of climate or the pursuit of athletic training may
encourage some degree of protein storage. And without
seriously qualifying what has just been said it may be
stated that abundant nitrogenous food may have the same
effect, but the nitrogen retention secured by forced feeding is
always limited to a very small fraction of the protein given.

THE ESTIMATION OF METABOLISM 175

The roughly maintained equilibrium, which is, after all,
a striking example of the adjustments of the organism, is
to be traced to the singular reliability of the appetite.
This is the agent which prompts so surely to the taking of
extra food when one exchanges an inactive life for one of
bodily activity. The most radical changes in the total
metabolism are unlikely to lead to lasting variations in
body weight beyond slight gains and losses, which, by the
way, are often the reverse of what was anticipated. Ex-
ercise, which is supported by large oxidation, may even
result in some increase of weight, showing that the appe-
tite has rather more than met the precise need of the body.

Carbon Retention. — When a quantitative comparison
is made between the compounds in the diet and those ex-
creted it is not infrequently found that carbon is being
stored, though the nitrogen of income and outgo may be
balanced. What can be inferred as to the nature of the
substance added to the tissues? Just as in the previous
case where we desired to interpret the meaning of the
carbon loss during fasting, we have to consider the re-
spective share taken by carbohydrate and by fat. As be-
fore, it is important to know the condition of the subject
prior to the trial. If the day is the first of feeding after
a fast there will be some recruiting of the glycogen in the
body, and a part of the carbon retention may be attributed
to a gain of this material. Otherwise, when carbon is
stored in the midst of a period of liberal feeding, the prob-
ability is that fat rather than carbohydrate has been de-
posited.

Applying the same factors as in the earlier instance, we
multiply the retained carbon by 1.3 if the circumstances
point to its having been held as fat. An excess of 10
grams of carbon in the income over the outgo would be
assumed to indicate the addition of 13 grams of fat to the
supply in the body. An amount of this magnitude would
not show itself decisively in the weight, being easily dis-
guised by the temporary gain or loss of water that might
occur at the same time.

CHAPTER XIX
THE ENERGY OF THE METABOLISM

THE initial statement in this book — that living things
are transformers of matter and energy — is a text to which
we have closely adhered. In recent chapters the emphasis
has been placed upon transformations of matter. We
shall now pass on to speak of the energy evolved by animals
and particularly by the human body. The fundamental
facts are presumably clear. The energy of the income is
potential in the complex molecules of the food. It is
released in the oxidative decomposition processes of life
and made kinetic. It appears chiefly — often solely—
in the form of heat. Measurements of the heat production
of living organisms are generally to be accepted as indic-
ative of the total energy production. Certain exceptions
to the rule will soon be noted.

Fuel Values. — Since energy can be transmuted from one
form to another it is possible to make the units which stand
primarily for one kind do duty for all. It is our constant
practice to use the units of heat to measure all the energy
of metabolism. The unit which we shall employ is the
large Calorie, approximately defined as the amount of
energy required to raise the temperature of 1 kilogram of
water one centigrade degree. The large Calorie is invari-
ably distinguished from the small by the capital C. The
small calorie is ygVfr °f the large; no further reference to
it will be made. When a combustible organic substance
of a standard composition is completely oxidized a definite
quantity of heat is evolved. The heat produced by
oxidizing 1 gram of any compound is its fuel value.

The highest fuel value recorded is that of hydrogen,
about 34 Cal. This is the amount of heat produced when

176

THE ENERGY OF THE METABOLISM 177

a gram of hydrogen gas (11 liters) is oxidized to. water.
One gram of carbon oxidized to carbon dioxid gives nearly
8 Cal. These two illustrations do not bear directly on our
physiologic inquiry, for the body does not use the free
elements for oxidation, but their compounds. It is, there-
fore, of more interest to turn to the fuel values of carbo-
lydrates, fats, alcohol, and proteins, since these are the
actual sources of heat and kindred energy. The calorific
value of a compound is not precisely that of the carbon and
the hydrogen contained in it and not yet bonded to oxygen,
although some early work of a useful kind was based upon
that assumption. It is to be noted that the oxygen in the
physiologic compounds reduces their potency; the less they
contain the more largely they will consist of elements sub-
ject to oxidation. This is the main reason why fats have
fuel values greatly in excess of those of carbohydrates.
A gram of fat contains nearly twice as much carbon as a
gram of sugar. It also contains much hydrogen with un-
satisfied affinities for oxygen.

The actual heat production observed when a gram of
starch is burned is a trifle more than 4 Cal. Sugars, which
are slightly richer in oxygen than starch is, have a little
lower fuel value. The figure (4 Cal.) is fairly representa-
tive of carbohydrates as a class. The oxidation of 1
gram of fat liberates about 9.3 Cal., or 2J times as much as
starch. A gram of grain alcohol fully oxidized gives an
intermediate quantity, about 7 Cal. All these non-nitrog-
enous compounds are made to yield the same simple prod-
ucts, namely, carbon dioxid and water, whether they are
destroyed by literal burning outside the body or by the
metabolic processes. We shall see shortly that the energy
which is found to be set free in their oxidation can be
proved to be equal in the two cases.

Protein stands somewhat apart in its behavior. A gram
of dried protein burned in oxygen gives nearly 6 Cal.
But some of the products generated in such a laboratory
test are not those which the body forms from protein and
excretes. Urea, for example, is not found after the actual

178 NUTRITIONAL PHYSIOLOGY

burning of protein. Urea itself has a certain capacity for
oxidation and a low but distinct fuel value, something
like 2.5 Cal. per gram. The residual fuel value in urea is a
sign that some portion of the energy latent in protein is
constantly lost to the animal economy. Bacteria may
profit by it, but it seems not to be available for the higher
living forms. There are certain products other than urea
which remain after the decomposition of protein molecules
and which likewise represent unused energy. Some of
these minor products accompany the urea in the urine,
while others are mingled with the feces. To make ac-
curate allowance for the energy lost with these incompletely
oxidized compounds is a difficult matter. The estimate
arrived at credits to a gram of protein 4 Cal. or a little
more. It is a coincidence with very convenient results
that this is almost exactly the same as the figure for car-
bohydrate.

The Total Daily Metabolism.— The widest limits be-
tween which the metabolism of an adult may vary may be
set down as 1000 and 10,000 Cal. per day. The lowest
level will be approached when there is complete rest and
protection from cold during the twenty-four hours. The
maximum will be reached, if ever, when a large, powerful
man performs the heaviest muscular work while under ob-
servation. The food taken on a single day influences
the result less than would be anticipated. (As already
pointed out, the appetite follows the metabolism rather
than precedes it.) High protein feeding has an effect
which will be discussed later.

Of course, the average for an individual will generally
fluctuate much less than is suggested above. It will
rarely fall below 1500 or rise above 3000 under the con-
ditions of city life. Until recently it would have appeared
reasonable to fix upon 2400 as a mean. This value has
lately come to be regarded as higher than the actual energy
output of most people. Perhaps 2000 may be adopted as
an ordinary amount. For sixteen hours of waking we
may allow 100 Cal. per hour, and for eight hours of sleep

THE ENERGY OF THE METABOLISM 179

60 per hour, an estimate adding up to 2080. It will be
suggestive if we consider how much of a single food-stuff
would be required to furnish a total of this order.

Two thousand Calories could be obtained by the oxida-
tion of about 500 grams of starch or sugar. The meeting
;Of the energy requirement is not the only qualification to
be demanded of a day's dietary. A ration of pure sugar
is obviously not to be recommended, though the consump-
tion of candy to such an extent may be entirely possible
for some subjects. The quantity of fat needed to afford
the same heat value will be in the vicinity of 216 grams.
One can hardly conceive of eating clear fat to this amount.
Making a similar calculation for alcohol, we find that a little
less than 300 grams of this compound will theoretically
meet the need. A palpable absurdity is apparent. While
the attempt to use alcohol exclusively as a fuel for the
body would evidently be disastrous, it is interesting to
consider that a lamp burning 300 grams of absolute alcohol
in a day would equal a human being as a source of energy.
The flame would be a very small one and the comparison
gives one a feeling of dissatisfaction and disappointment.

Since protein is nearly equivalent to carbohydrate as a
source of energy, the theoretic ration of protein would be
the same as that of sugar, namely, 500 grams. The prac-
tical impossibility of eating so much protein is evident
when it is remembered that protein is acceptable only when
softened and expanded with liberal quantities of water.
Lean meat, which is not strictly a straight protein food,
but which approaches that composition, is three-fourths
water. White of egg, in which the solid portion is almost
all protein, is seven-eighths water. The attempt to dry
such material and so to reduce it to the smallest bulk
produces a mass resembling hardened glue or shellac.
To subsist on the proteins of meat alone, the fat having
been removed, one would be compelled to eat some five
pounds daily. Travelers have described certain peoples as
living almost wholly on meat or fish, but this does not mean
a pure protein diet, the fat undoubtedly figuring largely.

180 NUTRITIONAL PHYSIOLOGY

The Eskimos necessarily eat but little carbohydrate, for
they can obtain no vegetable food of importance; it seems
plain, however, that they have increased both fat and pro-
tein consumption and not protein alone.

The foregoing suggestions make clear the inconveniences
and the unhygienic aspect of any attempt to live on a
single type of food. It is a fact, furthermore, that one
could not under any conditions continue indefinitely to eat
only non-nitrogenous food. Protein metabolism never
ceases and a certain nitrogenous income must be provided.
We cannot, therefore, judge the fitness of a diet solely by
its heat value. It must measure up to a reasonable stand-
ard in this respect, but it must also include a suitable pro-
portion of protein. Another criterion, not so commonly
insisted upon, is that a sufficient quantity and variety of
mineral compounds shall be supplied. Of course, these
scientific characterizations of the diet are inadequate unless
attention is paid also to attractiveness and digestibility.

When a subject freely chooses his food, unprejudiced
by chemical knowledge, he is apt to make use of all three
classes of food-stuffs to a considerable extent. Carbo-
hydrate will usually amount to more than half the total
solids of the ration, while protein and fat show a curious
tendency to be taken in nearly equal weights. An 'ex-
ample of such a selection, perhaps the one most frequently
quoted, is the following:

Protein 100 grams (410 Calories).

Fat 100 " 930 "

Carbohydrate 250 " 1025 "

2365 "

Of late years, observation having been extended to large
numbers of people, it has become evident that Americans
of student and professional classes rarely choose to eat as
much as 100 grams of protein.

Calorimetry. — We have been speaking of the amounts
of heat set free in the oxidation of various food materials,
and of the energy liberated by animals and men as an ac •

THE ENERGY OF THE METABOLISM 181

companiment of their metabolism. The determination of
such data requires the use of calorimeters. - These are of
various forms, but have the same underlying principle. In
case a food sample is to be burned it is enclosed, in a
small chamber, preferably in an atmosphere of oxygen, and
its combustion is initiated by means of an electric spark.
The heat is imparted to a large mass of water surrounding
the chamber and can then be estimated by the elevation of
temperature observed in this water. This is an elementary
and somewhat sweeping statement; numerous corrections
would be necessary in practice.

One of the earliest attempts to gage the heat production
of an animal consisted in confining it within a chamber
having double walls and ice between. The amount of ice
melted could thus be made the basis for the estimate de-
sired. The employment of a water calorimeter evidently
does away with the unnatural chilling which must have
been entailed in this primitive trial. The large and costly
calorimeters in modern laboratories for the study of human
nutrition are of the water type. No adequate idea of the
difficulty involved in such work is likely to be grasped by
reading this condensed presentation. Ventilation must be
maintained and allowance made for the warming of the
circulating air. The evaporation of water must be ac-
curately measured, for if this were ignored a great deal of
heat produced in the metabolism would escape measure-
ment. (In the evaporation of 1 kilogram of water heat to
the amount of 536.5 Cal. will be made latent.) Changes
in the body temperature cannot be overlooked. If, for
example, a human subject equivalent in heat capacity to
50 kilograms of water should experience a rise of tempera-
ture amounting to 0.5° C. during an experiment, 25 Cal.
would have been retained in his body. If, instead, there
were a fall of 0.5 degrees there would have been a discharge
of 25 Cal., and the observed heat output would have been
greater than that due to the current metabolism.

Now the question may be asked whether we can be sure
that the observed heat production (corrected for body

182 NUTRITIONAL PHYSIOLOGY

temperature changes and supplemented by the heat loss
through evaporation) is a just representation of the total
energy set free. May there not be other forms of energy
than heat? What are the facts concerning muscular
work? We cannot say positively that no energy eludes
the calorimeter, but the impression is constantly being
confirmed that any such escape must be insignificant. As
to the energy of movement it can be shown that under most
conditions this will eventuate in heat. This is an import-
ant matter and deserves to be carefully illustrated.

Let us take for an example the case of the heart. This
organ does a great deal of work in forcing the blood through
the vessels. Does this work appear as heat so as to be
arrested and recorded by the calorimeter? There is no
doubt that it does. The conversion is effected as the re-
sistance to the blood-flow is encountered and overcome.
The heart produces some heat within itself, and some addi-
tional heat due to its metabolism is made to appear in all
parts of the body. Wherever the blood is driven there is
friction, which is a means of transforming the energy of
motion into heat. The same statement applies to the
breathing movements. Work is done, in the physical sense
of the term, each time the ribs are lifted, but with their
return to the expiratory position this work is reconverted
into heat. So, too, all ordinary forms of exercise may be
shown to result in heat production.

Nevertheless it is possible to devise conditions under
which a part of the metabolic energy will not be given to
the calorimeter. Suppose, for example, that the subject
within the apparatus is employed in taking books from the
floor and placing them upon shelves. As long as he pur-
sues this form of activity a share of his evolved energy will
become potential and be lost to direct observation. It
may be said to exist as "energy of position" in the mass
which he has elevated. In other words, it has been stored.
If, after a day of this labor, he should occupy himself in
removing the books from the shelves and laying them upon
the floor, he would be giving to the calorimeter more heat

THE ENERGY OF THE METABOLISM 183

than that actually produced as a result of his metabolism.
When the books had all been returned to the original level
the sum of the calories for the two periods would justly
represent the energy production of his body. One could
conceive of two calorimeters placed side by side, in one of
which a man might turn a crank operating a shaft which
should pass into the second chamber and there revolve a
wheel against the resistance of a brake. Most of his energy
would be registered by the first calorimeter, but a fair pro-
portion of it, standing for most of his muscular work, would
be apparent in the second.

Direct and Indirect Calorimetry. — We have said else-
where that the proof of the validity of the principle of the
conservation of energy for living things was a great achieve-
ment of the nineteenth century physiologists. The
method of this proof may now be outlined. We have just
seen that the total energy production of animals, including
man, may be satisfactorily measured in calorimeters,
provided that the amount of evaporation is known and
any changes of body temperature considered. This pro-
cedure is direct calorimetry. Now, of course, it is possible,
during the same period to collect the excreta and to deter-
mine the character and the amount of the metabolism
according to the principles explained in the last chapter.
Knowing the metabolism — so many grams of protein, of
fat, and of glycogen destroyed — we may credit to each of
these its respective fuel value and calculate the number of
calories which could theoretically result from just such
oxidation. We may then compare the energy as deter-
mined from the living organism and the energy which
should have been liberated in the formation of the measured
wastes.

To make the matter plain, we may refer once more to the
case on which the numerical estimation of metabolism
was previously based. The subject was credited with a
metabolism of 100 grams of protein and 223.6 grams of fat.
(This, it may be recalled, is the usual assumption when
the day is one of fasting and does not closely follow carbo-

184 NUTRITIONAL PHYSIOLOGY

hydrate feeding.) The apparent heat value of the calcu-
lated metabolism will be as follows:

100 grams of protein at 4.1 calories = 410 Calories.
223.6 grams of fat at 9.3 " = 2079
Total "2489

This bit of reckoning is an instance of indirect calorimetry.
It is found in practice that direct and indirect calorimetry
lead to results so nearly identical as to make it certain that
the discrepancy between them is accidental. The two
figures differ in modern work by less than 1 per cent.

The correspondence between the observed and the cal-
culated calories means this: that the animal displays no
powers of movement or heat production which cannot be
referred to the oxidation of organic compounds in its body,
and so eventually to the stores of chemical energy in its food.
It is an engine, a transformer, and not a creator of energy.
A glimpse of this conception came within the vision of the
brilliant Count Rumford more than one hundred years ago.
He had become convinced that heat must be a form of
energy rather than a substance. His conclusions had been
drawn in part from the observation that there seemed to be
no limit to the quantity of heat obtainable from small
masses of metal in the process of drilling. With a flash
of insight he looked at the horse which was walking in a
circle to move the drill, and queried whether the heat in the
iron turnings might not be a derivative of the muscular
work of the animal, and whether the same energy had not
in some sense pre-existed in the food given to it. The
verification of his induction was long deferred, but has at
length been made entirely conclusive. The credit for
this demonstration is shared by many workers, but most
conspicuously by Rubner, in Germany, and Atwater, in
this country.

Let us return once more to our quantitative illustration.
If the day of the experiment had been one of ample feeding
instead of fasting, and the diet had consisted of protein
and carbohydrate, the metabolism might have been held

THE ENERGY OF THE METABOLISM 185

to have involved these materials rather than protein and
fat. The supposition has already been entertained and
may now be made a basis for indirect calorimetry. From
p. 180 we copy the amounts of protein and carbohydrate,
multiplying by 4.1, the value common to both:

100 grams protein 410 Calories.

430 carbohydrate J_763 "

Total 2173 . "

Comparing this sum with the 2489 Cal. previously de-
termined, we see that for a given production of carbon di-
oxid the evolution of heat will be greatest when fat is the
exclusive non-protein source, and least when the metabo-
lism is as nearly as possible on a carbohydrate footing.
The difference is some 15 per cent.

If both carbohydrate and fat had participated in the
decomposition underlying the observed excretion, the heat
of the metabolism might have had any value intermediate
between 2173 and 2489 Cal. The figure obtained by
direct calorimetry might in such a case of mixed metabolism
be used to determine the part borne by carbohydrate and
fat respectively. How this could be done, roughly at least,
may be shown here. Suppose the total heat production to
be 2300 Cal. (The data regarding excreta are still as-
sumed to be as before, 16 grams of nitrogen and 225 grams
of carbon.) We can first deduct from 2300 the 410 Cal.
necessarily assigned to the protein metabolism. The
remainder, 1890 Cal., stands for the heat released in the
oxidation of glycogen and fat. We can now write simul-
taneous equations as below:

Let x = the carbon from fat.

y — " carbohydrate.

Then x + y = 172 (carbon from non-protein sources, page 171).
Also 1.3 X 9.3z -f- 2.5^ X 4.1 = 1890 (remembering that by mul-
tiplying the carbon in
fat by 1.3 we obtain the
fat represented, and that
by multiplying this re-
sult in turn by 9.3 we
get the calories. The
case of the carbohydrate
is parallel).

186 NUTRITIONAL PHYSIOLOGY

Solving the equations we obtain the following (decimals
disregarded) :

x = 69 90 grams of fat, heat value, 837 Calories.

y = 103 227 " carbohydrate, " " 1054

1891 "

Thus we see that when the character of the metabolism is
known we can quite accurately predict the associated heat
production (indirect calorimetry), and when the heat pro-
duction and the excreta are known we can deduce equa-
tions to find the relative shares of fat and carbohydrate
in the general decomposition.

CHAPTER XX
THE FACTORS WHICH MODIFY METABOLISM

THE circumstances which radically affect the quantity
and kind of decomposition going on in the body must al-
ready be evident. There can be no hesitation in placing
first among these conditions muscular activity. Minimal
metabolism attends the most nearly complete state of
rest which can be secured. It is somewhat lower during
sleep than during a like period of lying awake, doubtless
because the waking subject cannot so successfully abolish
muscular contraction. When one is sleeping metabolism
obviously continues in certain of the breathing muscles
and the beating heart. We cannot doubt that it proceeds
also in the glands, the muscular coats of the alimentary
canal, the nerve-centers, and many other localities. The
lowest rate at which the metabolism of a healthy adult is
likely to go on during sleep is in the vicinity of 50 Cal. per
hour.

Very moderate activity suffices to double this rate of heat
production. More vigorous exercise raises the figure in
many cases to 200 or 300 Cal. per hour, while we have records
of as much as 600 Cal. per hour. This strikingly high level
was reached by a professional bicycle rider exerting himself
to the utmost upon a stationary bicycle inside a calor-
imeter. The total for a day observed upon an athlete
working to the limit of endurance may reach 9000 Cal.
Changes so extreme as these could hardly fail to attract
early attention. Lavoisier in the eighteenth century
noticed that exercise led to increased consumption of oxy-
gen. Everyday experience of the heating and the "wind-
ing" effect of activity would lead to the clear impression
of heightened metabolism.

187

188 NUTRITIONAL PHYSIOLOGY

When it had become plain that muscular contractions
must involve increased destruction of material, the ques-
tion arose as to the nature of the substance sacrificed.
Chemical teaching in the middle of the nineteenth century
was dominated by the influence of Liebig. It was his view
that the performance of muscular work could be supported
only by the consumption of protein. The impression
was a natural one, since muscle is so largely composed of
protein. He recognized that carbohydrates and fats were
oxidized in the respiratory process, but held that heat
alone and not movement resulted from their metabolism.
The distinction which Liebig attempted to draw between
the service of protein and non-protein material was des-
tined to be wiped out in consequence of a certain memorable
trial. The reference is to the so-called "Faulhorn experi-
ment" of Fick and Wislicenus.

By the year 1860 the conception of the convertibility
of energy from one form to another had become familiar.
A valuable datum, the mechanical equivalent of heat, had
become available. (One calorie is equal to the energy
consumed in raising 426.5 kilograms 1 meter.) The fuel
values of a number of substances were known. Two young
scientists, Fick and Wislicenus, conceived a project for
testing the prevailing belief in the unique service of pro-
tein as the support of muscular activity. They knew that
the nitrogen of the urine would afford the approximate
measure of the protein undergoing destruction in a given
interval. If a known 'amount of work were done it would
be possible to find out whether the protein consumed in
the same time would account for all of it.

They ascended the Faulhorn, a mountain rising 1956
meters above the lake at its foot. In reaching the top
each experimenter must have done an amount of work
represented by the product of his weight by the vertical
height attained. (Much additional work must have been
done also — by the heart, the breathing muscles, and in the
execution of other movements — for which no credit is
given in the calculation.) The figured work, therefore,

THE FACTORS WHICH MODIFY THE METABOLISM 189

was a minimum quantity. For Fick it was 129,096 kilo-
grammeters; for Wislicenus, a heavier man, it was 148,656.
The investigators collected their urine while on the path
and for some hours after completing the ascent. They ate
only non-nitrogenous food, that the excretion might not be
increased merely as a result of diet. The nitrogen found
in the urine indicated in each case the destruction of be-
tween 30 and 40 grams of protein. The highest theoretic
contribution which such a quantity of protein could
have made to the work done would have been only half
the known total, and, of course, a smaller part of the actual
performance. The inference could not be escaped; some-
thing other than protein had been used as a source of
muscular energy.

The figures obtained by Fick and Wislicenus have been
revised and corrected by critics in view of later discoveries,
but their chief significance has never been modified. At
the time of this celebrated experiment the respiration ap-
paratus at Munich had not been built. As soon as this
plant was in operation under the direction of Voit and
Pettenkofer it became possible to learn much more about
the effects of exercise upon metabolism. It was soon es-
tablished that instead of being the sole fuel employed to
h muscular energy, protein is not even the principal
The nitrogen excretion of a man on a mixed diet
not increase notably in a period of work as compared
with what it is during rest. On the other hand, his out-
put of carbon dioxid, like the calories, is a reliable index of
the degree of his activity. The contracting muscles evi-
dently employ non-nitrogenous material for oxidation when
the usual conditions of supply are maintained.

The question which naturally follows is as to whether
carbohydrate or fat is the preferred fuel. This cannot be
discussed at length, but it seems fair to say that both may
be used to good purpose and with nearly equal economy.
The herbivorous animals may be assumed to work most of
the time at the expense of carbohydrate oxidized. The
carnivora use much more fat, though it is to be borne in

190 NUTRITIONAL PHYSIOLOGY

mind that the large quantities of protein which they con-
sume are a source of sugar within the body. Gram for
gram, it must be remembered, sugar and fat are not equiv-
alent. One gram of fat has the energy of approximately
2i grams of carbohydrate. Quantities of the two food-
stuffs which are in this proportion (and hence equal in
fuel value) are said to be isodynamic. The principle may
be illustrated as f ollows : 50 grams of fat is withdrawn from
a day's ration and 113 grams of starch substituted. A
simple computation will show that the heat value of the
diet has neither been increased nor diminished. The
calories lost through the removal of the fat are 465, while
those introduced with the starch are practically the same.
Of course, the possibility of such substitutions is much
limited by considerations of palatability and individual
variations of digestive power.

The fact that carbohydrate and fat are freely used to
yield energy for muscular movement does not exclude pro-
tein from such service. A carnivorous animal can be kept
for a long time strong and active upon a diet composed
almost wholly of protein. Yet it remains possible that
carbohydrate is the chosen fuel during feeding of this
kind. When enough protein is given to secure nitrogen
equilibrium and to furnish energy to the full extent nqeded
for the work done, the uncombined amino-acids must give
rise to abundant sugar, and it may well be that this isfne
chief factor in the muscular metabolism. An engine is
made of iron and steel, but its regular fuel is coal ; a muscle
is made essentially of protein, but protein is not its usual
fuel. Protein food must be needed in definite amounts for
the original development of muscles and also for the in-
crease in their substance under conditions of training. It
is not clear that any considerable supply is needed for their
best working when the desired level of development has
been reached. This distinction between growth and
operation has important bearings upon theories of hygiene.

Mental States and Metabolism. — After hearing of the
striking correspondence between the degree of muscular

THE FACTORS WHICH MODIFY THE METABOLISM 191

work and the extent of metabolism, one is disposed to ask
what are the facts regarding mental work. When it is
said that mental states have no distinct influence, apart
from that which is secondary to the changes in tissue
activity which may attend them, a feeling akin to disap-
pointment is often manifested. Yet a little reflection will
convince one that positive effects are scarcely to be ex-
pected. The central nervous system, important as it is,
constitutes, after all, only 3 or 4 per cent, of the body.
Most of its substance is stable and relatively passive in its
nature. If its really active cells were to double their
average metabolism the addition to the heat production
and carbon dioxid elimination of the subject would not
be significant in the totals observed. A slight change in
the state of the muscles would suffice to offset and dis-
guise it.

An emotional experience is much more than a cerebral
phenomenon. In times of excitement the skeletal muscles
are played upon by the nervous system and metabolism
in consequence may be largely increased. But this is not
the direct result of the brain process; it is merely a special
case of muscular activity. An emotion, pleasurable or the
reverse, is a kind of exercise and often one of marked inten-
sity. This is recognizable in the increased heart action,
in the quickened breathing, and in the contractions which
bring about characteristic attitudes and expressions.

Mental application of a kind which can be more or less
successfully separated from these muscular accompani-
ments cannot be shown positively to affect the metabolism.
A short time ago a trial was made at Middletown, Con-
necticut, in which a group of students in Wesleyan Uni-
versity took bona-fide examinations in a respiration cham-
ber which was at the same time a calorimeter. Each man
took his turn, spending three hours over his paper and
experiencing the usual anxiety and strain attendant on such
a proceeding. On other days each subject spent a like
period in the chamber engaged in copying printed matter.
Thus it was possible to compare in about twenty cases the

192 NUTRITIONAL PHYSIOLOGY

metabolism of a period of brain work with the metabolism
during a time in which similar muscular movements were
made, but in the absence of conscious effort. There was no
distinct difference.

Brain cells undoubtedly have peculiar metabolic prod-
ucts and make demands upon the blood for supplies of a
somewhat different order from those required by any
other tissues. But their distinguishing wastes can hardly
be recognized when mingled with the outgo from so many
other organs, and it is equally difficult to discover just
what they appropriate for their nutrition. The notion
that certain articles of diet are brain "foods" rests on very
unsafe assumptions. The popular association of phospho-
rus with the brain and its activity has no more justifica-
tion than could be claimed for sulphur or any element
present in the proteins.

From what has been said it will be evident that the diet
must be increased for the support of muscular work, but
that no more food is needed for the student occupied with
his books than for the same man at leisure. The chances
are that his leisure days will be spent in less sedentary
fashion than his days of application, and that his appetite
will lead to a larger consumption in his so-called resting
time.

Feeding and Metabolism. — What can be said at this
time about the influence of food upon metabolism must
be in the nature of a summary of points already made.
Any effect which food may have is generally so much less
in degree than the effect of activity as to be readily con-
cealed by changes in the exercise taken. Thus, while it is
broadly correct to say that a fasting man metabolizes less
material than a man who has abundant food, the relation
will be reversed if the starving subject is compelled to
work more actively than his fellow. An animal which is
denied food is economical in its metabolism, but the econ-
omy is secured chiefly through its marked tendency to be
quiet. When the influence of muscular activity is elimi-
nated as far as possible we can discover some suggestive

THE FACTORS WHICH MODIFY THE METABOLISM 193

facts with reference to the varying properties of the differ-
ent food-stuffs.

Suppose that a man remains for two days in a calori-
meter, the first being a day of fasting and the second a day
of feeding. His occupation on the two days is made as
nearly identical as may be. We will assume that the
metabolism of the first day is found to be 2000 Calories.
The food value of the ration allowed for the second day may
be also 2000 Calories. It can be predicted that the metabo-
lism will rise somewhat in consequence of the taking of
food, but the increase will not be striking. The new total
will perhaps be something like 2300 Calories. A far greater
increment would have resulted from the prescribing of
moderate muscular work. The precise extent of the ad-
vance will be conditioned largely upon the make-up of the
diet. If protein is given quite freely the stimulation of
the metabolism will be decidedly more evident than if the
food furnished is almost wholly non-nitrogenous. Protein
is said to exert a specific dynamic effect upon the decom-
position processes of the body.

The divergence between protein and other types of food
in the matter of increasing the metabolism has been care-
fully estimated by Rubner. He has given us some helpful
figures. If the fasting heat production of an animal is
represented by the number 100, the requirement will not
be exactly met by a supply of any food having this value,
but must be met by larger supplies. ^Carbohydrate is the
most economical of the three kinds; a fasting" metabolism
of 100 Calories may be compensated and equilibrium of
income and outgo established by giving 106 Calories in the
form of starch or sugar. (The quantity must be that ac-
tually assimilated, not merely what is swallowed.) In
other words, feeding an animal for one day on carbohydrate
exclusively, and liberally enough to make the energy given
equal that evolved, may be expected to raise its meta-
bolism some 6 per cent, above the previous fasting level.

With fat there is a slightly more positive effect in the
same direction. If fat is given to an animal after an in-

13

194 NUTRITIONAL PHYSIOLOGY

terval of fasting the metabolism responds by rising more
than in the first case. Instead of the 6 per cent, increase
about 14 per cent, is to be anticipated. Hence, to insure
equality of income and outgo from the energy standpoint
114 Calories must be furnished in the form of fat to corre-
spond with a fasting production of 100 Calories. The
specific dynamic effect of protein is much more decisive.
If pure protein is the only food the metabolism may in-
crease by as much as 40 per cent, over the fasting standard.
The selection of much protein at a meal will be followed,
during the next few hours, by a definite rise in heat pro-
duction which may be a really wasteful operation. A
quantity of heat may be generated entirely apart from the
performance of muscular movements and have to be re-
moved as a useless excess.

Foods rich in protein have the name of being "heating,"
and here as in many another instance the popular impres-
sion has been supported by scientific findings. We can
see that the eating of much meat in warm weather must
add to the discomfort of the eater. The effect is some-
thing like the opening of a furnace draft when the house is
already too hot for the pleasure of its tenant. A distinc-
tion must be made between the heating influence of protein
and the high fuel value of fat. It is true that fat, gram
for gram, can liberate more energy in the body than can
protein. But it is less likely to be destroyed when there is
no useful application to be made of its energy content.

Age and Sex as Influencing Metabolism. — Young and
growing individuals whether human or otherwise have
relatively more heat production than is the case with
adults. Thus an infant weighing 10 kilograms (22 pounds)
may be expected to have a metabolism of 600 or 700 Cal-
ories. That is to say, that with a weight only one-sixth
or one-seventh of that of a grown man, it has a heat pro-
duction one-third or one-fourth as great. It has been
shown, however, that if the metabolism of both the infant
and the adult is calculated for the surface of the body there
is a reasonable proportionality between them. The area

THE FACTORS WHICH MODIFY THE METABOLISM 195

of the skin of an average man is something less than 2
square meters. The minimum metabolism may be said to
be in the vicinity of 1000 Calories per square meter.

The smaller the body, if it is of a certain standard shape,
the larger its surface in proportion to the weight. This
is an important biologic principle. Mathematically
stated it runs as follows : If two animals of similar build are
compared with reference to a given dimension, such as
length, their weights will vary as the cube and their sur-
faces as the square of this measurement. That is, if one
animal is twice as long as another it will weigh eight times
as much and have four times the surface. Since the body
loses heat in proportion to the extension of its surface it is
not strange that this is the determining factor for the
metabolism. It is surprising nevertheless that animals
as unlike as man, mouse, and fowl should evolve heat in
nearly equal quantities for a unit of superficial area.

Broadly speaking, it may be said that men have a greater
metabolism than women. When the larger average stature
of men is taken into consideration the difference is dimin-
ished, but does not wholly disappear. We must recognize
that when a man and a woman are equal in weight there
are still characteristic features of organization which modify
the comparison. Of these the most evident is the greater
prominence of subcutaneous adipose tissue in the female
figure. When allowance is made for this inactive material,
it is plain that for equal weights the woman has less truly
living substance to be the seat of metabolism. Her skeletal
muscles in particular are very unlikely to make as large
a mass as those of a man who weighs the same. The
adipose tissue may have a secondary effect also. While
its presence means that the metabolism is confined to a
more limited system in the woman than in the man, it
tends at the same time to economize her heat loss and so to
lessen the need of internal oxidation. Such a conception
will be more readily entertained when attention shall have
been given to the subject matter of the next chapter.

CHAPTER XXI

THE MAINTENANCE OF THE BODY
TEMPERATURE

ALL animals are producers of heat, and must, therefore,
maintain temperatures above those of their surroundings
unless the evaporation of water more than compensates
for the heat of metabolism. We speak, however, of warm-
blooded and cold-blooded animals as though some were
far more liberal than others in their calorific output.
This is actually the case. But a better distinction be-
tween the two classes may be found in the fact that some
animals allow themselves to be warmed and cooled readily,
while others keep near to a constant standard of internal
temperature. We call a frog a cold-blooded animal, but
what we really mean is that it accepts for its own the tem-
perature of its environment. On a very hot day the frog
may be as warm as a man; in winter the frog is chilled
through and through, while the tissues of a human being
everywhere save at the surface are kept at the same tem-
perature as in summer.

Animals which have a fixed standard to which they ad-
here under all ordinary conditions are spoken of as homo-
thermous. This trait is shared by birds and mammals.
Our interest, of course, centers in man's remarkable capac-
ity to keep his deeper tissues at the same level in spite of
radical external changes and equally sweeping changes in
his own metabolism. The means by which this result is
attained differ somewhat with different animals of the
homothermous order. We shall confine our attention
almost entirely to the human problem.

The common clinical thermometer bears the mark
"Normal" opposite the point on its scale corresponding to

196

THE MAINTENANCE OF THE BODY TEMPERATURE 197

98.6° F. (In all the following discussion the familiar
Fahrenheit system will be used.) The standard just
quoted is for the mouth. The body temperature is often
estimated by placing the thermometer elsewhere, as in the
armpit or the rectum. The rectal temperature is the most
reliable, and is usually higher by half a degree, or a little
more, than that of the mouth. It is clear that the tem-
perature of the skin cannot be a constant one ; it is affected
both by the external conditions and by the variations
of blood-flow in the superficial vessels.

There are small but distinct changes of body tempera-
ture during each day. The lowest figures are noted early
in the morning before one has become active and when the
sense of prostration is apt to be overpowering. A gentle
upgrade is maintained until the maximum is reached in the
late afternoon or early evening. The average extent of the
rise is about 1° F. It coincides suggestively with the
temperamental change, which is to be observed from a
prosaic and literal frame of mind to a condition of emotional
instability. Compared with the morning, the evening is
a feverish period. When there is actual fever the diurnal
ascent of the temperature often adds considerably to the
restlessness and discomfort of the patient as night ap-
proaches.

After we have made allowance for such irregularities it
remains true that the approximate constancy of the tem-
perature is one of the most wonderful facts which the
physiologist has to explain. Uniformity of temperature
implies equality of heat production and heat loss. Our
task, then, is to show how this equality is continued in the
face of variable factors tending to disturb it. Artificial
contrivances for keeping constant temperatures in cer-
tain chambers may be considered with advantage before we
attempt to analyze a mechanism so much more intricate
than they. There are two principles on which incubators
or thermostats may be operated: (1) The heat given to the
system may be increased or diminished to keep pace
with the heat lost; (2) a constant supply of heat may be

198 NUTRITIONAL PHYSIOLOGY

provided and adjustments made to favor its escape or to
conserve it, according as the tendency is toward a rise or
fall in temperature.

The first principle is illustrated by those thermostats in
which gas flames are automatically caused to rise when the
apparatus begins to cool off and are cut down when it
begins to be warmed above the intended level. The
second way of securing the same result is exemplified by
the common incubator used for hatching eggs, in which the
heat is furnished by a lamp kept burning at the same height
at all times, while a ventilator, opening and closing, dissi-
pates or retains the heat as required. In the living
body we can recognize the working of both these princi-
ples, but it is rather surprising to find how much is ac-
complished by the second without the aid of the first.
In other words, the constant internal temperature is
maintained much of the time by the promotion or the
retarding of heat loss without any appeal to the tissues for
metabolic support.

Our practice of adapting our clothing to the season and
to in-door and out-door life is an extension of the means
which the organism itself employs for the same purpose.
Extra clothing hinders the escape of heat from the body
and makes possible the maintenance of the normal state
with no increase of oxidation in spite of some degree of
external cold. A race of naked savages must certainly
vary the amount of their metabolism much more positively
from summer to winter than we do with the habits of our
civilization. Animals may enjoy the protection of heavier
coats in cold weather, and they show, moreover, an instinct
to assume those positions that reduce to a minimum their
surface exposure.

To explain in detail how changing circumstances are
met, let us imagine a man placed successively in atmo-
spheres of different temperatures. We will begin with a
room at 68° F., a condition which we regard as agreeable
and which we aim to produce when artificial heating is in
use. If our subject has spent an hour in this room he has

THE MAINTENANCE OF THE BODY TEMPERATURE 199

very likely had a metabolism of 100 Calories, and he has in
the same time discharged by radiation, conduction, and
evaporation a similar amount of heat. His blood is some
30 degrees warmer than the air around him. Let him now
take his place in a room where the thermometer stands at
84° F. Suppose him to remain for an hour in this disagree-
ably warm apartment. His temperature will be found to
rise but little, perhaps not at all. Yet the change has re-
duced the difference between his own temperature and that
of his surroundings from about 30 degrees to about 15.
The tendency of his environment to withdraw heat from
his body must have been halved. What, then, has
happened? Has his metabolism fallen to 50 Calories or has
there been a readjustment to facilitate the removal of the
full 100 Calories? The latter is found to be the case.

The withdrawal of heat from the body is favored by two
reflex changes. One of these consists in an increase in the
amount of blood flowing through the skin and thus exposed
to the cooling influence of the outside air. The other is
seen in the breaking out of perspiration. The evaporation
of the water thus brought to the surface cools the skin and
the blood beneath it. The blood then mingles with that
which has been passing through the deeper tissues and the
rising temperature is checked. It is well to insist just here
that the appearance of the skin gives little indication of the
rate at which the sweat is being secreted. So long as evap-
oration keeps up with the arrival of the water at the pores
there will be no visible moisture. We notice the perspira-
tion most when it is failing to accomplish its object, that is,
when it accumulates instead of being vaporized. The
evaporation of water from the respiratory passages is, of
course, one means of removing heat, but with human
beings this is not a quantity which increases with external
warming. It does enter into the adaptive reaction in the
case of animals which pant.

Let us now transfer our subject to the unusual tempera-
ture of 105° F. The radiation and conduction effects will
now be reversed; the blood will tend to be warmed rather

200 NUTRITIONAL PHYSIOLOGY

than cooled as it approaches the surface. The metabolism
will continue unabated. The body is thus exposed to
warming from without, while it does not cease to heat itself
from within. How can it escape a steady rise of its in-
ternal temperature leading to prostration and death?
Benjamin Franklin answered this question when he pointed
out that the sole resource of the organism in such a situa-
tion must be its power to evaporate water. To dispose of
100 Calories in an hour by evaporation alone demands the
secretion of about 200 grams of water in the same time, an
amount which can readily be produced.

When the air is warm the humidity has much to do with
human power to endure the condition. If there is full
saturation and a temperature as high as that of the blood
the heat of metabolism will be pent in the body and heat-
stroke is inevitable if there is not a prompt relief; 100 Calor-
ies added to the average human body in an hour will raise
its temperature by nearly 4° F. A second hour of such
an upgrade could hardly be survived. Men may live and
work for several hours on a stretch in dry air with the tem-
perature around them as high as 130° F., but they cannot
be active in saturated air at 90° F. The first of these
conditions is realized in the stoke-holds of ocean steamers ;
the second, in certain deep mines. Everyone knows that
the most trying weather we have to put up with is not
that which makes the record for the mercury, but those
days which are less warm, but which we describe as muggy
or sticky. We are acceding to a correct instinct when we
are relaxed and indolent under such circumstances.

We may now return to the starting-point and submit our
imaginary victim to temperatures lower than 68° F. If
he is taken to a room where the thermometer is at 60° F.
he will probably feel chilly. We are affected more by a
slight change in the vicinity of 65° F. than in any other
part of the scale. The fact is, apparently, that when the
external temperature is cut down from 68° to 60° F. the
skin temperature, on which our sensation depends, is re-
duced by a good deal more than 8 degrees. This is due to

THE MAINTENANCE OF THE BODY TEMPERATURE 201

the reduction in the volume of blood in the cutaneous
vessels. It is the expression of the endeavor of the organ-
ism to economize to the utmost its outgo of heat. Dis-
comfort is permitted to develop as an incident of the adap-
tation. The metabolism still remains about as it has been
throughout the series of trials.

If the room temperature is now lowered decidedly and
no wraps are provided the body can no longer maintain
itself by mere economy of heat loss. It must shift from
the method of the incubator with a constant flame and
an adjustable ventilator to the other form of regulation —
that of the thermostat with variable flame. That is to
say, the metabolism must be stimulated. A sign of this
rallying on the part of the heat-producing tissues is seen
in the onset of shivering. This is obviously a form of
muscular exercise, and as such is attended with increase of
metabolism. When we resist the impulse to shiver, as
we sometimes do, we merely adopt another kind of con-
tractile activity with its accompanying contribution of heat
to the body. If we analyze the experience of being cold
we find that we can recognize a disposition to muscular
tenseness, while it is familiar enough that when the cold
is severe we cannot keep from moving briskly and so sup-
plying the necessary heat. It is desirable to reiterate that
the muscles are not merely organs of movement, but our
main reliance for heat production. Cold weather generally
means large metabolism, but the connection is indirect; the
increase is only a special case of the rise always associated
with exercise. So, too, the increase of appetite which is usual
in winter is secondary to the greater use of the muscles.

Humidity makes for discomfort in cold as well as in
warm weather. It seems at first unreasonable to say that
moisture can make us more sensitive to heat in summer
and also to cold in winter, yet this is true. The climate of
our Atlantic coast is notorious for the "penetrating" char-
acter of its cold, and this in spite of the fact that the
thermometer does not fall so low as it does a short distance
inland. The paradox is easily explained. We have said

202 NUTRITIONAL PHYSIOLOGY

that high humidity in hot weather interferes with our com-
fort and efficiency by hindering free evaporation. In
winter it has no such influence, because even though the
cold air is fully saturated it ceases to be so when it has been
warmed by contact with the skin. Evaporation can,
therefore, never be retarded seriously by moisture in cool
air. What we notice in cold weather is the increased con-
ducting power of air containing water vapor. Damp air
may fairly be said to partake of the nature of the water
that is in it; water feels colder to the hand than does air of
the same temperature because it abstracts the heat more
rapidly. Similarly, moist cold air takes heat more rap-
idly than does dry cold air. This property is present
just as surely in warm humid air, but it can affect us only
when there is a wide difference in temperature between the
skin and the surroundings.

Temperature Maintenance During Exercise.— We have
been discussing the ways in which the human body guards
itself against changes of temperature which tend to impress
themselves upon it from the outside. Another question
is in regard to how it escapes the tendency to overheat
itself when, during exercise, its metabolism is doubled,
trebled, or even more strikingly augmented. Reflection
shows that it employs the two reflexes on which it relies
for defense against the heat of the warm room, namely,
increased surface blood-flow and increased perspiration.
These are rendered more efficient by the hastened circula-
tion, a condition not produced in any great degree by ex-
ternal heat without the activity.

A supplementary factor exists in the deepened breathing
which takes heat from the system, both in the act of warm-
ing the respired air and in the process of saturating it.
Still another factor can be recognized in the fanning effect
of the movements of parts of the body or of the body as a
whole. A man running brings the exposed portions of his
skin constantly into contact with fresh volumes of air and
slips away from the air which he has just warmed and
saturated. The cooling of his blood is in this way con-
siderably facilitated. If he is not progressing through

THE MAINTENANCE OF THE BODY TEMPERATURE 203

space, but carrying on his activities in one place — for
example, in sawing wood — his arms and trunk still make
short excursions and exchange old air for new. A breeze
makes a considerable difference with the heat output of a
man's body.

Fever. — When the body temperature is found to rise
above its normal level and to persist at an elevation which
brings many ill consequences upon the subject, what shall
we name as the cause of the disorder? Shall we say that
the metabolism is excessive? Studies made upon fever
patients show that their nitrogenous metabolism is often
surprisingly high, an indication of rapid destruction of the
tissues, but the total heat production is not impressively
large. We shall more nearly express the facts if we say
that there is interference with heat loss. Frequently we
can see evidence of a withholding of the perspiration.
Perhaps it is best to say that the central fact in fever is the
setting up of a false standard by the nervous system to
which for a time there is an adherence as strict as that
obtaining in health for the normal one. This perverted
action of the nervous system is brought about by the
poisons in the circulation at such times. Transient fever
may be brought on by very severe exercise ; it is experienced
by men who run Marathon races. In these cases the con-
trolling centers are probably acting in the normal way, but
cannot secure the complete removal of the extraordinary
quantities of heat which are produced.

Summary. — The maintenance of a nearly uniform body
temperature is the result of a balance between heat evolved
and heat dissipated. So long as the external conditions
are not such as to cause shivering, muscular tension, or
instinctive activity of some other form the organism regu-
lates its temperature almost wholly by making adjust-
ments to promote or to restrict the loss of heat. The
wearing of clothing adapted to the season makes it possible
to minimize the demands made upon the muscles for extra
heat. Decidedly low temperatures and exceptional ex-
posure can be withstood only by calling upon the contrac-
tile tissues for an increased heat production.

CHAPTER XXII
THE HYGIENE OF NUTRITION

IT is convenient to make a division under this general
heading between those factors not directly connected with
the diet, which none the less deserve consideration, and
those which do concern the choice of food. Probably too
little attention is paid to the fact that disorders of digestion
and nutrition frequently arise when the food eaten is above
criticism, both as to quantity and kind.

Nervous Conditions Affecting Digestion. — Enough has
already been said to make it plain that the processes oc-
curring in the alimentary canal are greatly subject to
influences radiating from the brain. It is especially strik-
ing that both the movements of the stomach and the secre-
tion of the gastric juice may be inhibited as a result of
disturbing circumstances. Intestinal movements may be
modified in similar fashion. Emphasis has been placed on
the dependence of the whole digestive process upon a good
start. This can hardly be too strongly enforced. The
good start can scarcely be secured if the mental state of the
subject is not favorable to the enjoyment of his meal.

Cannon has collected various instances of the suspension
of digestion in consequence of disagreeable experiences,
and it would be easy for almost anyone to add to his list.
He tells us, for example, of the case of a woman whose
stomach was emptied under the direction of a specialist
in order to ascertain the degree of digestion undergone by a
prescribed breakfast. The dinner of the night before was
recovered and was found almost unaltered. Inquiry led
to the discovery that the woman had passed a night of
intense agitation as the result of misconduct on the part of

204

THE HYGIENE OF NUTRITION 205

her husband. People who are seasick some hours after a
meal often vomit undigested food. Apprehension of
being sick has probably inhibited the gastric activities.

Just as a single occasion of painful emotion may lead to
a passing digestive disturbance, so continued mental de-
pression, worry, or grief may permanently impair the
working of the tract and undermine the vigor and capacity
of the sufferer. Homesickness is not to be regarded lightly
as a cause of malnutrition. Companionship is a powerful
promoter of assimilation. The attractive serving of food,
a pleasant room, and good ventilation are of high import-
ance. The lack of all these, so commonly faced by the
lonely student or the young man making a start in a strange
city, may be to some extent counteracted by the cultiva-
tion of optimism and the mental discipline which makes it
possible to detach one's self from sordid surroundings.
Alcohol works to the same end, but is a perilous resource
under these circumstances.

Children are very often the victims of sharp attacks of
indigestion. Their sicknesses, which are accepted with
little surprise in many families, are almost always held to be
due to injudicious eating. While this is a reasonable belief
in many cases, it may be asked whether emotional causes
of indigestion in children are considered as much as they
should be. How common it is to see children made to cry
while at the table by ill-timed rebukes. The quick temper
and thoughtlessness of parents destroys the happiness of
many a meal. Granting that instruction in table-manners
ought to be given at suitable times, one may still protest
against the downright rudeness of elders toward children
when it is shown in the infliction of ridicule and humilia-
tion. Consideration of others' feelings is the finest element
in deportment.

Moreover, it is probably fair to claim that children may
be injured by being forced to eat food which they dislike.
The aversions of early life are singularly strong. What
passes for a foolish whim may be an instinctive loathing.
There is an element of hypocrisy in the attitude of parents

206 NUTRITIONAL PHYSIOLOGY

who are selecting precisely what they please to eat, while
compelling little children to swallow food which repels.
To oblige a child to finish a plateful of food against its
inclination may be crass brutality. Of course, children
cannot be humored in the selection of eccentric diets, but
they need not be made to eat when they would rather go
hungry. There is little likelihood that they will refuse the
staple articles.

Unhappiness may give rise to digestive difficulties which
do not disappear with the removal of the first cause. It
is not hard to show how this may be. Suppose that the
power to digest and absorb food is lessened by central in-
hibitions. A consequence is likely to be the accumulation
of unabsorbed organic material in the colon and perhaps
higher up as well. Bacterial decomposition will be fos-
tered. Some of the products of such a process may be
sufficiently like the normal products of enzyme action to
play a part in nutrition, but others will probably prove
distinctly detrimental. With the entrance into the circu-
lation of such bodies there is originated what is known as
auto-intoxication.

Long ago it was recognized that the reception into the
system of bacterial products might be a cause of general
ill health, of headache, and of somnolence. Within a few
years the impression has gained ground that poisons from
the colon have a much larger and more definite share in the
development of disease. Much that goes by the name of
rheumatism appears traceable to this source. Some of the
toxic compounds seem to have the property of dissolving
the red corpuscles of the blood, leading thus to anemia and
the serious crippling of the energies which accompanies it.
Nervous symptoms are among the most frequent nowadays
referred to this condition, seemingly so remote from the
brain.

Physicians apply the term "vicious cycle" to a set of
conditions in which the establishment of one tends to ac-
centuate the others, and these, in their turn, add to the
intensity of the first. We can see how a vicious cycle may

THE HYGIENE OF NUTRITION 207

become operative in the case of a person whose digestive
abilities have been once reduced by mental depression.
Auto-intoxication may be induced and one of the clearest
results is a further depression of spirits. In this way a
lasting injury may be done and the indigestion which be-
gan as the effect of temporary unhappiness may be per-
petuated in spite of the return of favorable circumstances.
Auto-intoxication may come from errors of diet as well as
from emotional causes, and a further discussion of it will
be postponed.

Physical fatigue as well as mental may interfere with the
progress of digestion. It is well that the appetite usually
flags in times of exhaustion, so that one is in a measure
insured against the tendency to overtax weakened organs.
But when the fatigue is habitual there is an unfortunate
dilemma; the body must have abundant food to support
the heavy labor and it is not well able to care for the food
eaten. Loss of weight is common when a man is so situ-
ated. Deprivation of sleep emphasizes this state of affairs,
and by dulling both the appetite and the digestive capacity
it proves for many people the surest means of reducing
adipose tissue.

Other possible causes of indigestion may be mentioned
briefly. Taking cold is one of these. While the congestion
and inflammation which so often follow exposure to drafts
and dampness are most frequently centered in the mucous
membranes of the nose and throat, a corresponding involve-
ment of the alimentary canal is not rare. Diarrheal at-
tacks are common in the spring and fall when the weather
changes are erratic. They are probably to be classed as
intestinal "colds." Another source of alimentary trouble
is to be found in uncorrected defects of vision. While
headaches are the most persistent symptoms of astig-
matism and other ocular imperfections, indigestion is not
uncommon, and its disappearance when glasses are worn
ms sometimes almost magical.

Quantity of Food. — Is overeating the prevalent error
of the race? Is undereating easily possible? These are

208 NUTRITIONAL PHYSIOLOGY

questions which call for open-minded treatment and re-
garding which it is hard to exclude personal bias. A
writer's love for the pleasures of the table, or his ascetic
superiority to them, must tincture his expressed views.
The present author once committed himself to the state-
ment that overeating is the rule with men and the excep-
tion with women. A pupil — a girl — rendered the teaching
in her examination as follows: " Women rarely overeat,
men do constantly, and are, as a result, bulky and stupid."
The assertion, though radical, is exceedingly well worth
considering.

The claim is often made that the average practice of
mankind must be the expression of a correct biologic in-
stinct. Against this it is urged that our own generation
may inherit appetites which were adapted to spur our
ancestors to find food when the quest was difficult. Such
appetites may be false guides when no effort is required
to obtain the means of satisfaction. Variety of food
may lead to overconsumption. Modern conditions make
it possible to have many kinds of food and interesting
contrasts of flavor which encourage eating for sensuous
gratification. The primitive diet was monotonous and
unseasoned.

Economic factors are tending to lessen the individual
ration, and a great mass of published instruction influences
people in the same direction. It is probable that the
American breakfast is a much less substantial meal to-
day than it was twenty years ago. The use of fruit has in-
creased and food of greater fuel value has been displaced.
Odd patent cereals, small quantities of which exhaust the
appetite, have been widely substituted for the reliable oat-
meal. Meat and potato are not demanded. Lighter
lunches at noon are the rule, and though the late dinner
may be a heavier meal than the old-time supper, the day's
ration seems to have definitely diminished. This is par-
ticularly true of protein. The choice of 100 grams of
nitrogenous food, an amount once described as average, is
now found to be exceptional. If any one of the three main

THE HYGIENE OF NUTRITION 209

classes of food-stuffs is used more freely than formerly,
it is the carbohydrate group.

Protests against overeating and the advocacy of few
and simple foods have been heard from time to time all
along the course of history. More than one recent com-
mentator on these movements has cited the story of the
captive Israelites in Babylon. The four young men who
excelled all other members of the king's household were
those who had substituted a diet of cereals for the meat and
wine that were urged upon them. They were the proto-
types of Sylvester Graham and Horace Fletcher. The
general teaching that less food should be eaten is very
often coupled with an approach to vegetarianism, if not
its downright adoption.

An earlier reference has been made to Graham. He was
a gifted and well-educated New Englander, born in 1794
and dying in 1851. Fitted for the ministry, he employed
his great talents as a public speaker, chiefly in the temper-
ance cause and in support of his dietetic doctrines. His
"Lectures on the Science of Human Life," published in
1839, constitute an impressive work. He had a large
following, and is said to have injured the trade of the
butchers in some localities to such an extent that he was in
danger of mob violence at their hands. Aside from his
extreme recommendation of the inclusion of husks and
bran with food, he taught practically what we are continu-
ally hearing at the present time : that the diet should be
limited in quantity and variety, that it should be unstimu-
lating, and that it ought to be eaten "slowly and cheer-
fully."

In our own day we have become used to the claim that
most men would attain to better health and greater effici-
ency if they would reduce their rations by 25 per cent, or
more. Conditions of life which were formerly held to war-
rant individual allowances of 2500 or 3000 Calories are
now said to be met by the supply of 2000 or even of 1800
Calories. The most authoritative statements to this effect
have come from the physiologists of the Sheffield Scientific

14

210 NUTRITIONAL PHYSIOLOGY

School. In a more popular form the same ideas have been
attractively presented by Mr. Fletcher.

The experience and the principles of this gentleman are
somewhat familiar to the American public. By his prac-
tice of protracted mastication he contrives to satisfy the
appetite while taking an exceptionally small amount of
food. Salivary digestion is favored and the mechanical
subdivision of the food is carried to an extreme point.
Remarkably complete digestion and absorption follow.
By faithfully pursuing this system Mr. Fletcher has vastly
bettered his general health, and is a rare example of muscu-
lar and mental power for a man above sixty years of age.
He is a vigorous pedestrian and mountain-climber and
holds surprising records for endurance tests in the gym-
nasium.

The chief gain observed in his case, as in others which
are more or less parallel, is the acquiring of immunity to
fatigue, both muscular and central. It is not claimed that
the sparing diet confers great strength for momentary
efforts — "explosive strength," as the term goes — but that
moderate muscular contractions may be repeated many
times with far less discomfort than before. The inference
appears to be that the subject who eats more than is best
has in his circulation and his tissues by-products which act
like the muscular waste which is normally responsible for
fatigue. According to this conception he is never really
fresh for his .task, but is obliged to start with a handicap.
When he reduces his diet the cells and fluids of his body
free themselves of these by-products and he realizes a
capacity quite unguessed in the past.

The same assumption explains the fact mentioned by
Mr. Fletcher, that the hours of sleep can be reduced de-
cidedly when the diet is cut down. It would seem as
though a part of our sleep might often be due to avoidable
auto-intoxication. If one can shorten his nightly sleep
without feeling the worse for it this is an important gain.
The small ration decreases the contents of the colon in
two ways : First, the food residues themselves are minimal,

THE HYGIENE OF NUTRITION 211

and, second, the secretions of the digestive tract are less
voluminous when there is only a light task for them to
perform. Well-marked constipation is established, the
Fletcherite having only one or two evacuations in a week,
but the material retained is apparently innocuous.

Many of the ills referable to auto-intoxication often dis-
appear when the practice of prolonged chewing is followed.
This could be explained as resulting from the limitation
of colon accumulations, but seems in part to proceed from
the strange fact that under the system the consumption
of protein invariably shrinks even more than in proportion
to the general restriction of the food. This occurs when the
subjects are guided entirely by instinct and have no the-
ories in regard to the matter. Indifference to meat or even
an antipathy to it may be developed.

One does not have to look far to see examples of the
practical working of the Fletcher system, though the per-
sons who illustrate it may have no associations with the
name. Middle-aged and elderly housewives are to be
found in large numbers who are slow and frugal eaters, who
care little for meat, who are constipated, and who are mar-
vels of endurance in the execution of their hard tasks.
They sleep lightly and rise early — habits diagnostic of
Fletcherism. They live to great ages, restlessly active to
the last. They show the strong points of the regimen; do
they display any drawbacks attributable to it?

It is certainly possible to undereat. There can be no
reasonable doubt that the condition of the very poor would
be bettered if they could have more food, even though it
were of no finer quality than that to which they are
accustomed. Crichton-Browne, an English writer, has
acutely pointed out that the allowance of the poorest classes
comes near to the ideal of the New Haven School, and that
it is also almost identical with the "punishment diet" of
British prisons. It may well be that a diet which is suc-
cessful in connection with the best housing conditions and
a life abounding in stimuli to the intellect and the feelings
may not support cheerfulness and vigor in those whose

212 NUTRITIONAL PHYSIOLOGY

surroundings are wretched. Underfeeding produces gloom
and moroseness unless other circumstances are more in-
fluential and oppose it.

Previous experience is something to be considered when
seeking to judge whether an individual ought to reduce his
food-supply. If he has lived in luxury and under every
temptation to indulgence, if he is overweight as a result
of his habits, short of breath and drowsy, he can probably
profit by the expedient. If he has been restrained from
satisfying his appetite by sheer poverty or by voluntary
sacrifices, ' if he is thin, sensitive to cold, and subject to
insomnia, it is quite likely that an increase of food will make
him stronger and more energetic. Undereating is often
coupled with indigestion in such a way that it is impossible
to say which is the cause and which the effect. Within
wide limits the digestive glands accommodate themselves
to the tax levied upon them, providing adequate amounts
of their juices for large or small meals. Low diet, there-
fore, may weaken the adaptibility of the organs it is
designed to spare.

Benedict, of the Carnegie Nutrition Laboratory, has
proved himself an exceedingly shrewd critic of those writers
who go to extreme lengths in their championship of re-
duced feeding. He has shown that when the food supply
is very small the absorption is often less complete than with
a more liberal income, a clear sign that the digestive sys-
tem is not in such good working order as it might be with
more to do. Professor Chittenden, the foremost advocate
of careful restriction, agrees with his critic to this extent
at least: he believes that an occasional banquet is a
valuable means of testing the canal to see whether it has
retained the reserve power which it is desirable to have.
Many people of the economical type characterized just now
have lost the faculty of enjoying a large meal and cannot
easily increase their income when the physician recom-
mends it.

The Peculiarities of Protein. — While the advice to
limit all forms of food is constantly heard, it is the use of

THE HYGIENE OF NUTRITION 213

much protein which is most vigorously condemned.
The decisive effect of a large protein allowance upon the
metabolism — an effect in the direction of a seemingly use-
less increase — has been mentioned. Other peculiar prop-
erties call for discussion. We may conveniently distin-
guish the influences proceeding from an excess of protein
in the intestine from those which are associated with an
excess of nitrogenous metabolism. In regard to the first
it may be said that protein far more than the non-
nitrogenous foods is capable of generating toxic sub-
stances and so of becoming a cause of true auto-intoxi-
cation. An unabsorbed surplus of protein is, therefore,
to be avoided.

If, however, the absorption is as complete as can be
desired, reasons can still be given for keeping the protein
income of the body relatively low and depending largely
upon carbohydrates and fats in preference to nitrogenous
food. Emphasis has been placed elsewhere on the sim-
plicity of the normal oxidation products formed from sugar
and fats in contrast to the numerous and rather complex
bodies which arise from the working over of the protein
derivatives. Carbon dioxid and water are disposed of by
the healthy system with apparent ease. The nitrogenous
wastes require preliminary treatment by the liver and per-
haps by other organs, and must then be removed from the
blood by the kidneys. The principal one, urea, is not
commonly a source of trouble, but the minor attendants,
such as uric acid, are viewed with disfavor.

One is naturally led to the opinion that high protein
feeding must lay a burden upon the liver and the kidneys,
but it is probably just to assume that what is a severe
tax for one person may not be so for another. The native
efficiency of these organs is doubtless as variable as many
other inherited qualities. Nitrogenous by- or end-pro-
ducts, whether having their origin in the decompositions
within the colon or in the metabolism, are presumably
responsible for the premature fatigue of which we have
spoken. An additional injury which may be laid to their

214 NUTRITIONAL PHYSIOLOGY

charge must now be considered. This is the production in
later life of arteriosclerosis.

The saying is current among physicians that "a man is
as old as his arteries." It is certainly a fact that the
stiffening of these vessels is a chief cause of malnutrition
and waning power in the tissues of the aged. If arterio-
sclerosis sets in prematurely, other features of a senile
decline may be expected. Metchnikoff has sought to
establish a connection between the overconsumption of
proteins and early loss of elasticity and adaptability in the
human circulatory apparatus. Such a connection has
long been granted to hold for alcohol We seem brought
to admit that this serious impairment of efficiency may
spring from intemperance in eating as well as in drinking.

The postponement of old age by frugality in feeding
seems in a measure possible. Yet we must remember that
self-denial in this respect may defeat its own end, since it
may too greatly weaken the digestive capacity. An al-
ternative to low diet, it has been suggested, may be found
in the deliberate regulation of the bacterial conditions in
the intestine. The claim is made, and seemingly with good
reason, that a harmless type of fermentation may be en-
couraged with the result that the undesirable decompo-
sition may be prevented. This is the theory underlying
the various modes of sour-milk treatment so much in
vogue during the last five years. Lactic acid in moderate
amounts appears to be quite devoid of danger to one's
health. Its presence in the canal is hostile to the develop-
ment of the organisms, which cause radical putrefaction
of the proteins and definite auto-intoxication. A domi-
nant acid fermentation may be secured either by the
taking of sour milk (kephir, koumiss, matzoon, etc.) or by
swallowing from time to time cultures of the lactic organ-
isms, together with sugar for them to work on.

The comparative harmlessness of even extreme constipa-
tion when associated with sparing feeding and active
absorption has been granted. Such inaction of the intes-
tine when it accompanies more liberal indulgence in food

THE HYGIENE OF NUTRITION 215

must be unfortunate and must favor some phases of auto-
intoxication. Many people resort periodically to the use
of cathartics when they feel slightly "under the weather,"
and enjoy a buoyant recovery of energy and ambition
when the disturbance is over. It is natural to interpret
such an experience as showing, first, the existence of a
source of poisoning, and, second, its successful removal.
The wise man, however, will not be satisfied with knowing
a way out of such disorders; he will aim to prevent their
recurrence. The habit of depending upon laxatives in-
stead of general hygiene is to be deplored. Irrigation of
the colon to relieve from auto-intoxication is often resorted
to as a part of medical treatment with good effects, but
is not to be advised so long as attention to diet and ex-
ercise can be made to serve the need.

Obesity. — The accumulation of adipose tissue in burden-
some and disfiguring deposits is regarded as mirth-provok-
ing, but should be viewed with due appreciation of its
seriousness. In the light of what has gone before, there
is no escape from the conclusion that during the period
of increasing weight the diet must have been in excess of
the requirement. Yet when a person has once become
notably stout he is often observed to be a light eater. It
is likely to be found that he cannot reduce his allowance of
food without feeling quite uncomfortable. Various means
may be resorted to in the attempt to abate the unwelcome
condition, but frequently without success. Thus out-
door exercise, which, of course, increases the metabolism
and should destroy the body-fat, may stimulate the ap-
petite to an extent which fully corresponds with the oxida-
tion, and so defeats its own purpose,

The gathering of adipose tissue under the skin has an
effect somewhat like that of extra clothing. It is an
obstacle to heat loss and makes it possible for the possessor
to maintain himself with less expenditure of fuel. Hence,
when the protecting layer is once established it becomes
more and more easy to add to it. Its absence from the
lean subject makes him a more prodigal dispenser of heat

216 NUTRITIONAL PHYSIOLOGY

to his environment and he must presumably consume more
food to secure an equilibrium. It is, therefore, much
harder to begin the deposit than to nurture it when it
exists. A fat man is like a house with double windows —
the arrangement will save coal.

Reduction of adipose tissue by fasting is possible, but
not popular. Several other methods have been tried.
The Banting system, formerly much in vogue, consisted
essentially in a diet containing a maximum of protein.
It was held that such a diet would keep the muscles and
glands from losing substance, while not promoting the
formation of fat. We have, seen, however, that fat forma-
tion from protein is at least theoretically possible. The
success of the Banting treatment probably depended upon
two factors: first, foods rich in protein are satiating, and
an unconscious cutting down of income is therefore likely;
second, the specific dynamic effect of so much protein
would be expected to increase the metabolism. Restric-
tion of water drinking is often recommended. Laxatives
are sometimes used, presumably to hinder absorption.
A procedure which seems rational is the selection of a
rather bulky but not highly nutritious diet, including fruit
and the coarser vegetables. By this means hunger can be
fairly appeased, while the actual quantity of food entering
the circulation is not sufficient to maintain carbon equi-
librium.

CHAPTER XXIII
THE HYGIENE OF NUTRITION (Continued)

WATER; MEAT; SUGAR

Water. — There are particular foods which call for indi-
vidual notice; one of these is water. The fact is already
familiar that this compound so abundant in nature is also
the largest item in the income of the human body. It is
an essential part of all the tissues, and its percentage in
their make-up cannot be materially reduced while life
continues. It has been described before as an important
vehicle of excretion, and we have seen that when it evapo-
rates it often provides for the removal of heat, which would
otherwise accumulate in the body to its hurt. When the
discharge of water is unusually great the feeling of thirst
is roused and dictates a renewal of the stored supply.

We can vary considerably our practice of water drinking.
Hence this is a matter often dealt with by writers on
hygiene. The common teaching is to the effect that one
can hardly drink too much water unless it be at mealtimes.
The beneficial results supposed to accrue from free drink-
ing are assumed to include the avoidance of constipation
and the promotion of the elimination of dissolved waste by
the kidneys and possibly by the liver. There seem to be
no reasons for doubting the soundness of these popular
ideas. The drinking of a great deal of water is an excel-
lent habit, and consumption of tea, coffee, and other
beverages in which water is the principal constituent has
this in its favor — it leads people to take much more water
than they would otherwise.

It is probably correct to say that the duties of the kid-
neys are made lighter when we give them more water to

217

218 NUTRITIONAL PHYSIOLOGY

excrete. This may be contrary to the general impression,
for the temptation is to judge the work of a gland by the
volume of its output. But in the light of facts which
cannot be discussed here we are led to believe that con-
centration rather than sheer amount of secretion is what
puts the tubules of the kidneys to the severest test. They
act at the greatest disadvantage when required to excrete
a maximum of solids in a minimum of water. The urine
almost always has a concentration much higher than that
of the blood from which it is derived, and it is fair to assume
that the separation of the two fluids would be made easier
if the difference of concentration could be lessened.
Water drinking is the natural way to secure this result.
"Water," says Osier, "is, after all, the great diuretic."

While teachers of hygiene are well agreed upon the value
of water in liberal quantities as conducing to health,
there has been much said against its free use with meals.
We can hardly question the impression that many cases of
indigestion have been benefited by the prohibition of water
at the table. An approach to Fletcherism is favored
when the saliva is not aided by swallows of water. Slower
eating is likely, and slower eating may be expected to
satisfy the appetite with a smaller actual intake. The
idea that the digestive juices are seriously diluted by water
taken with the food does not seem to be well founded.
Laboratory experiments show that dilution of the fresh
secretions is at least as likely to increase as to diminish
their activity. It must be borne in mind that the dilution
of a liquid containing a fixed amount of enzyme does not
reduce the quantity of the enzyme, but only makes it act
in a larger volume of the mixture.

Very cold water swallowed rapidly may chill the mucous
membrane of the stomach and possibly retard the prog-
gress of gastric secretion. Digestion itself may also be
slowed if the contents of the stomach are cooled. But
bacterial fermentation should apparently be delayed just
as definitely as the normal hydrolysis, and we ought not
to make too much of these possibilities. Very recently

THE HYGIENE OF NUTRITION 219

Hawk, of the University of Illinois, has made a number of
trials which are entirely favorable to the practice of drink-
ing all the water that one chooses with meals. He has
shown that the fecal nitrogen is lower when water is taken in
large volume than when it is forbidden. This fact he holds
to indicate more complete digestion and more thorough ab-
sorption. His subjects were in the best of health, and his
results do not contradict those of physicians who have found
the restriction of water beneficial in particular cases.

The opinion is commonly held that drinking much water
favors increase of weight. To a limited extent such in-
crease of weight may result from actual retention of water.
We can see that a definite addition to the adipose
tissue may proceed from the tendency to eat more food
when water is taken freely. Perhaps the converse of this
form of statement is more accurate; namely, that weight
is lost when water is restricted and the quantity of solid
food unconsciously diminished. Sometimes an erroneous
inference may have been drawn from the fact that stout
people drink a great deal of water. This is in part a
consequence rather than a cause of their condition. Sub-
cutaneous fat is a hindrance to the escape of heat from the
body, and its presence during warm weather necessitates
an unusual amount of perspiration. This, in turn, pro-
duces thirst.

Meat. — Much that is written tends to create the im-
pression that meat is entirely unlike any other food. Its
peculiarities are constantly exaggerated. When Erasmus
Darwin excused himself from Lenten abstinence on the
ground that "all flesh is grass" he perverted Scripture, but
uttered a physiologic truth. Meat has two distinctive
characters: it is very rich in proteins and in extractives.
It is not any richer in protein than are peas and beans, but
while these vegetables contain a large proportion of carbo-
hydrates, lean meat contains these bodies very scantily.
A diet of lean meat, therefore, comes near to being a
straight protein diet. A plate of beans is equivalent in
composition to a plate of meat and potato.

220 NUTRITIONAL PHYSIOLOGY

Vegetarianism has sometimes been advocated upon
humanitarian grounds and sometimes because of its sup-
posed favorable effect upon health. The idea that the
destruction of animal life for human nutrition is morally
wrong cannot be discussed here. It is true that not many
people would kill animals for their own use if there were no
other way to obtain meat. It is equally true that so many
of the noblest and gentlest people that ever lived have
enjoyed meat and fish that we cannot well credit the claim
that flesh foods cause deterioration of character.

A diet containing much meat is naturally a high protein
diet, and is, accordingly, subject to the drawbacks already
mentioned in this connection. These have been seen to
include an unprofitable spurring of the metabolism — more
particularly objectionable in warm weather — and the
menace of auto-intoxication. The typical proteins of meat
are probably not better nor worse than other proteins in
these relations. There is, however, greater, likelihood of
overconsumption of proteins when meat is the source be-
cause of its very attractiveness. Men, especially those who
spend money freely, are certainly prone to such indiscre-
tions, while women set an example of temperance in this as
in so many other practice's. So far as the proteins eaten
are destined to be reconstructed into those of the blood
and tissues, it may be asserted that meat proteins are pecu-
liarly well suited to the purpose. Their molecular corre-
spondence with the pattern to be imitated is close. But
the demand for amino-acids for this use seems to be
small.

The real individuality of meat is owing to the presence
in it of the secondary substance which we call extractives.
These give it its odor and, in conjunction with mineral
salts, its flavor. Their absolute amount is not large, but
they are of much interest. They have been described by
writers holding meat in disfavor as waste-products of
animal life. The characterization seems broadly to be a
justifiable one, though our disgust at the notion is not
necessarily well founded. The extractives of meat cer-

THE HYGIENE OF NUTRITION 221

tainly originated during the lifetime of the animal through
the breaking down of its proteins, and were destined for
ultimate excretion, either unchanged or after some altera-
tion. It is hard to escape the conclusion that our pleasure
in the taste of meat is due to compounds nearly akin to
those of the urine.

Furthermore, it is plain that the opponents of meat are
correct in making the point that when we receive these ex-
tractives we are simply adding to the duties of our own kid-
neys. When we eat 50 grams of protein in beans we have
later to excrete about 8 grams of nitrogen in urea and other
forms. When we eat 50 grams of protein in beefsteak we
must subsequently excrete the same quantity of nitrogen
plus that of the extractive bodies. The addition is not a
large one, but it is partly in the form of uric acid and per-
haps other bodies less tractable than urea. The advisa-
bility of limiting meat in rheumatism and related conditions
has long been recognized.

While the extractives may be held to account for some of
the possible ill effects of meat, they are also the source of its
especial virtue. Palat ability itself is of great hygienic
worth, and these substances confer qualities which for
most people cannot be equaled apart from meat. The
promotion of gastric secretion in the normal subject and
its establishment in the invalid are most surely secured by
means of these same extractives. Aside from their favor-
able influence upon the stomach, they are probably mild
stimulants in the same sense in which coffee and tea can
be called so.

Some kinds of meat are well known to occasion indiges-
tion. Pork and veal are particularly feared. While we
may not know the reason why these foods so often disagree
with people, it seems probable that texture is an important
consideration. In both these meats the fiber is fine, and
fat is intimately mingled with the lean. A close blending
of fat with nitrogenous matter appears to give a fabric
which it is hard to digest. The same principle is illus-
trated by fat-soaked fried foods. Under the cover of the

222 NUTRITIONAL PHYSIOLOGY

fat thorough-going bacterial decomposition of the proteins
may be accomplished with the final release of highly poi-
sonous products. Attacks of acute indigestion resulting
from this cause are much like the so-called ptomain-
poisoning. But it is best to reserve the term for those
cases in which the harmful bacterial change had taken
place in the food before it was eaten.

Texture is an exceedingly weighty factor in determining
the ease or difficulty of digestion for any food. It is this
which makes the recognized difference between new and
old bread and between the upper and the under crust of a
pie. Osier has said, with his usual picturesqueness, that
"pie north of Mason and Dixon's Line and hot bread south
of it have done more harm than alcohol." Fats are best
cared for when emulsified if liquid, and when of a flaky
or crystalline character if solid. This last quality is real-
ized in good bacon.

Sugar. — The reader will have noted that the starch,
which is usually the most abundant compound in the daily
income of man, is converted to a sugar before it is admitted
to the circulation. The question arises why it is not
equally well to eat sugar altogether in place of starch. This
is the actual habit during the period of milk feeding in in-
fancy. The enzymes that act upon starch are not freely
furnished until some months after birth. In later life,
however, starch comes to be the main reliance of the race.
Experience has shown that cane-sugar is often productive
of indigestion. Can we find definite reasons for the
superiority of starch to sugar?

The fact has previously been mentioned that much sugar
causes alimentary glycosuria, while this is never produced
by the freest eating of starch. Here we have a clue to the
difference between the two classes of carbohydrates.
The glycosuria is not in itself a serious matter, but it
shows that the solubility of sugar and the slight changes
requisite for its digestion lead to its very rapid absorption.
The digestion of starch is a more gradual and protracted
process, and the resulting glucose is not formed so swiftly

THE HYGIENE OF NUTRITION 223

as to raise the concentration of the intestinal contents
appreciably, nor delivered to the blood so abruptly as to
increase markedly the percentage of sugar in circulation.
It seems safe to assume that the storage of glycogen is
effected more smoothly and easily after the ingestion of
starch than after the taking at one time of a large quantity
of sugar.

Highly soluble bodies of low or moderate molecular weight
are said to confer on their concentrated solutions the prop-
erty of high osmotic pressure. This is not the place to
discuss what is meant by the expression. For practical
purposes it can be said that concentrated solutions take
water from tissues with which they may be brought in
contact. Hence an irritant effect is to be expected. This
is exemplified by the action of strong salt solutions in
producing vomiting. Everyone who is fond of candy
knows that it can be eaten until a point is reached at
which an uneasy sensation of satiety verging on nausea
is developed. This is relieved by drinking water, which, of
course, lowers the concentration of the syrupy gastric
contents and so lessens the irritation.

Cane-sugar is much sweeter than sugar of milk or the
other sugars which arise in the course of digestion. It is,
therefore, cloying and its free use may blunt the appetite
for other foods. It is said also to be more disturbing to the
stomach than the others. These two properties perhaps
account sufficiently for the ill effects which are attributed
to the increasing consumption of candy in this country.
Still it is probable that the evils resulting have been much
exaggerated. There is, however, no question that the
constant eating of candy threatens to damage the teeth,
and the indirect impairment of the digestive powers may be
serious.

The relation between sugar and the decay of the teeth
is apparently quite simple. Sugar is prone to ferment,
a change brought about by bacteria which are inevitably
present in the mouth. The chief product of such bacterial
decomposition is lactic acid. This acid attacks the lime-

224 NUTRITIONAL PHYSIOLOGY

salts of the teeth at points where the enamel has been
chipped or worn away, or where it fails to meet the gum.
The dissolving of the lime-salts leaves a soft and perishable
organic structure which readily undergoes true decay.
Every tiny deposit of sugar in the crevices of the teeth may
soon become a focus of acid production and a center of
disintegration. The popular impression that plain sugar
is not so hurtful as sugar mingled with other substances in
candy has this basis : pure sugar is so readily dissolved by
the saliva that it is not likely to remain long clinging to the
teeth. On the other hand, sugar which is mixed with fatty
materials like chocolate may be sealed into crannies or
retained under the edge of the gum with unfortunate effect.
One who is bound to eat much candy should be willing to
exercise unusual care to free the teeth from the remains,
and may, in spite of his pains, have periodical days of
reckoning at the dentist's.

Food Accessories. — It is perhaps unnecessary to enlarge
upon the service of those compounds which are classed
under this head. The double value of the extractives of
meat which favor digestion both because of the flavors
which they develop and because of their direct action upon
the stomach wall has been sufficiently emphasized. The
various condiments are believed to have a similar signifi-
cance. So far as they season the food so as to make it
more acceptable, they must evidently promote secretion.
Their power to call forth the gastric juice by a purely local
effect is not so well established, but they are known to in-
crease the blood-flow in the mucous membrane, which
must help to sustain the local activities.

Tea and Coffee. — These beverages owe what limited
food value they have to the cream and sugar usually
mixed with them. They give pleasure by their aroma, but
they are given a peculiar position among articles of diet by
the presence in them of the compound caffein, which is
distinctly a drug. It is a stimulant to the heart, the kid-
neys, and the central nervous system. It is chemically
related to uric acid, but is not known to yield this incon-

THE HYGIENE OF NUTRITION 225

venient waste-product in the human body. Individual
susceptibility to the action of caffein varies greatly.
Where one person notices little or no reaction after a cup of
coffee, another is exhilarated to a marked degree and hours
later may find himself lying sleepless with tense or trem-
bling muscles, a dry, burning skin, and a mind feverishly
active. Often it is found that a more protracted disturb-
ance follows the taking of coffee with cream than is caused
by black coffee.

It is too much to claim that the use of tea and coffee is
altogether to be condemned. Many people, nevertheless,
are better without them. For all who find themselves
strongly stimulated it is the part of wisdom to limit the
employment of these decoctions to real emergencies when
uncommon demands are made upon the endurance and
when for a time hygienic considerations have to be ig-
nored. If young people will postpone the formation of the
habit they will have one more resource when the pressure
of mature life becomes severe.

Chocolate and its derivative, cocoa, may be regarded as
having somewhat similar properties. There is a measure
of drug action, though it is less pronounced than in the
case of coffee. Here the active principle is theobromin,
nearly related to caffein. Chocolate is more than a food
accessory, being exceedingly nutritious. A chocolate
habit is easily formed, but, aside from threatening damage
to the teeth, it is comparatively innocent.

Mineral Salts. — These compounds have been referred
to in an earlier chapter as forming an essential part of the
tissues. Hence they must be supplied in sufficient quan-
tity and variety during the period of growth. There is no
real danger of failing to do this, though there are certain
cases of malnutrition in which the central difficulty seems
to be the lack of such constituents in the body of the
child. The actual trouble is probably with the absorp-
tion- rather than with the diet. Thus, in rickets there
is an evident deficiency in the quantity of lime-salts in-
corporated into the developing skeleton, but it is not

15

226 NUTRITIONAL PHYSIOLOGY

often true that there is any shortage in the amount offered
in the food.

When the full stature is reached the need of a continued
salt income is less marked, though there is reason to believe
that the demand always exists. A certain loss in the urine
and through the skin seems bound to occur, though the
usual excretion of salts is far greater than the bare mini-
mum, and appears to indicate a needless excess in the in-
come. The salts of the diet have much to do with its
palatability and so deserve a place with the organic condi-
ments. An unusual quantity of saline matter may be sup-
posed to impose a hard task upon the kidneys, and is
known to aggravate any dropsical tendency that may be
present.

Sodium chlorid is the one salt which we take pains to
secure. We are inclined to think of it as a mere relish,
but it has been shown that it has a deeper significance.
The Austrian physiologist Bunge has found that it is
sought both by animals and men whose food is largely or
entirely vegetable. It is repugnant to those that are ap-
proximately carnivorous. Bunge points out that vegetable
foods are generally rich in compounds of potassium and
relatively deficient in those of sodium. He has demon-
strated that when an excess of potassium salts is eaten
the kidneys discharge the foreign material promptly, and
in doing so let slip a good deal of the sodium chlorid from
the blood. Accordingly, it is inevitable that the steady
consumption of foods rich in potassium should create a
demand for sodium. The seeking of common salt to meet
the need is a singular illustration of the almost unerring
working of instinct.

CHAPTER XXIV
ALCOHOL

ALCOHOL occupies a peculiar position among the con-
stituents of the diet of mankind. A perfectly dispassionate
estimate of its values and its drawbacks is arrived at with
difficulty. Most discussions of the subject are frankly
partisan and, therefore, partial. Alcohol affects the
human system in many ways, and it is possible to select for
emphasis either those aspects of its action which are detri-
mental or those which are favorable. In this way a writer
may be entirely accurate in all his affirmations and yet fail
to be just to a complex question because of what he leaves
unsaid. One cannot properly approach such an analysis
of the effects of alcohol without first learning the extent
of its use the world over. In our own environment there
is a feeling of hostility toward the intoxicant which would
excite wonder in many countries enjoying an advanced
civilization. Intemperance is deplored by thoughtful
people everywhere, but the demand for total abstinence
is sectional, though probably extending steadily.

The older literature abounds in the praise of wine. The
Bible itself has many appreciative references to its potency
as a comforter. It has also eloquent passages which con-
demn its abuse and records of abstinence on the part of
certain sects or guilds among the Hebrews. No better
physiologic distinction has ever been drawn than in the
verse which places "wine which maketh glad the heart of
man" in comparison with "bread which strengthened
man's heart." To "make glad" is to minister to feeling,
to "strengthen" is to confer power which can be demon-
strated to an observer — an objective instead of a subjective

227

228 NUTRITIONAL PHYSIOLOGY

result. We cannot point to many great men in the his-
tory of nations who have entirely avoided the use of al-
cohol.

Here in America there was little concerted protest
against the use of alcoholic drinks until the nineteenth
century. The Puritans, with all their restriction of recre-
ation and self-indulgence, were singularly tolerant of hard
cider and Jamaica rum. This laxity extended to all classes
of society. About one hundred years ago Lyman Beecher
described the immoderate drinking which was a feature of
an ordination to the ministry in a Connecticut parsonage,
host and guests being clergymen. ' Beecher himself be-
came a vigorous leader of the movement for temperance,
which was not until some years later an agitation for total
abstinence.

Edward Everett Hale has told us in his "A New Eng-
land Boyhood" of the common practice of serving wine
at children's parties about the year 1830. He also
tells us that when the "Franklin Medals" were annually
awarded to Boston schoolboys, an entertainment and ban-
quet was provided from which the youths departed in a
tipsy condition. The utter impropriety of these proceed-
ings shows us in an impressive manner how far we have
moved from the standpoint of that age. The offenses of
the time appear the more aggravated when we reflect that
the liquors used were largely of the strongest type.

The world has long had one conspicuous example of con-
sistent abstinence on the part of a great population. This
is afforded by the Mahometan peoples. It is said that the
Lascar sailors who visit our ports can be allowed shore
leave with implicit confidence that they will return to the
ship as sober as when they left it. A seaman who erred
in this respect and was remonstrated with by his captain is
reported to have excused himself on the ground that he had
embraced Christianity.

Something will be said of alcohol under each of five
heads. We shall proceed to consider it as a relish, a food,
a drug, a cerebral alterative, and as a poison. While

ALCOHOL 229

such a treatment is most convenient, it must be recognized
that alcohol can scarcely exert a single influence unmixed
with the others. One of the five aspects named may be
particularly prominent for the moment, but traces at least
of the others are to be looked for. It is this intricacy of
action which makes it so hard to weigh the facts with
equity. Another disturbing consideration is found in the
very unequal susceptibility of different persons to the
temporary effect of alcohol as well as to its habit-forming
property.

Alcohol as a Relish. — It must be sufficiently clear from
what has gone before that anything that adds to the zest
and pleasure of a meal may be expected to promote diges-
tion. The only exception to this rule may be looked
for when the relish in some way interferes with the di-
gestive process. Alcohol or, more correctly, alcoholic
beverages may certainly be held to enhance the enjoy-
ment of dining, and must, therefore, favor the digestion
and absorption of food. It is often claimed in rebuttal
that alcohol retards the action of enzymes. This is doubt-
less true of high concentrations, but no such mixtures can
possibly be made to exist in the stomach. Rapid dilu-
tion by the juices and rapid absorption of the alcohol
through the lining membrane combine to bring down its
percentage to a level which cannot hinder the normal
hydrolysis.

The appeal of wines and cocktails may be said to be
due quite as much to the minor substances which they
contain as to the alcohol. Pure dilute alcohol would
not be attractive to many people. At the same time the
extractives conferring flavor and fragrance do not seem
to make a complete beverage when the alcohol is removed.
With the frank admission that the finer alcoholic drinks
may be promoters of digestion, we may fairly couple
certain qualifying statements. First, such stimulation
is unnecessary for those whose health is what it should be.
Second, the employment of such means to spur the ap-
petite leads readily to overeating. It is also to be ob-

230 NUTRITIONAL PHYSIOLOGY

served that the use of alcoholic relishes gives no sanction
to drinking apart from meals.

Alcohol in the stomach has the effect of increasing the
local blood-flow, and it is well established that absorp-
tion is thereby promoted. Whether it takes its place
with the extractives or meat as a chemical agent to
excite gastric secretion is not so certain. It is hard to
discriminate between the influence which it exercises
through the central nervous system and that which it may
exert upon the stomach wall in a more direct manner.
The maximum favorable effect upon the digestion is pro-
duced by a small quantity of alcohol. Larger quantities
are notoriously apt to nauseate and to precipitate dis-
graceful scenes, all too common in connection with elab-
orate banquets.

Alcohol as a Food. — There has been a great deal of
opposition to the claim that alcohol can be reckoned a
food. It has seemed to many of its antagonists that the
admission weakens their position, but this is not neces-
sarily the case. ' To say that alcohol may be a food is not
to deny that it is a dangerous one. Professor Atwater
was roundly censured by leaders of the total abstinence
movement for a publication which is really a powerful
tract in favor of their position. He had said that alcohol
might serve as a food, and all his earnest warnings against
it were regarded as discounted. His experimental work
on the subject is entirely conclusive.

The fact has been reiterated that the chief use of food
is to undergo oxidation with release of energy. Alcohol
in considerable amounts may be oxidized to carbon dioxid
and water in the human system. When it is thus oxidized
the heat value of each gram is about 7 Calories. There
seems to be no provision for the retention of alcohol against
a future time of need nor for its conversion into glycogen
or fat. Its oxidation is bound to take place with little
delay, and it is in this respect not so adaptable to the
changing demands of the organism as is carbohydrate.
Nevertheless we must grant that it may take its place

ALCOHOL 231

in the diet as a substitute for other non-nitrogenous
foods. Atwater's experiments, as well as many parallel
studies, have made this evident.

A subject was brought into equilibrium on a ration
without alcohol. It was then found possible to withdraw
carbohydrates and fats to a certain extent and to replace
with alcohol in isodynamic amounts without disturbing
the equilibrium. The well-known fattening effect of
moderate drinking may be explained as due in part to the
whetting of appetite and in part to the sparing of car-
bohydrate and fat by the alcohol which is utilized in their
stead. Fifty years ago the same fact was recognized by
George Henry Lewes, in his interesting "Physiology
of Common Life." He tells us how a convention of total
abstainers once gathered in the city of Frankfort, in Ger-
many, and how the cooks in the hotel in which the delegates V
lodged were put to it as never before to supply the pastry '
and pudding ordered by these unfamiliar patrons. The
guests for whom the table had heretofore been set were
accustomed to supply with alcohol a want which the tee-
totalers met with carbohydrate.

How far the substitution of alcohol for other non-
nitrogenous foods may be carried has been much debated.
If it is given too freely its oxidation is incomplete and,
what is of more practical importance, the cerebral effect
becomes prominent. To make the utmost use of alcohol
as a nutrient it must be taken in small quantities and
frequently. There seems to be a general agreement that
from 50 to 75 grams of alcohol in twenty-four hours is
about as much as can be allowed to an adult without un-
toward reactions. If we assume the larger amount to be
permissible, it follows that alcohol may furnish as much as
500 calories, or about one-fifth of the day's total. The
requirement may be translated into terms of various bev-
erages: whisky, something less than \ pint; sherry or port,
1 pint; champagne, 1 quart; beer, 2 quarts.

Meltzer has said that "alcohol in health is mostly a
curse and in sickness mostly a blessing." Its peculiar

232 NUTRITIONAL PHYSIOLOGY

merits in illness are connected with the fact of its appetizing
character and with the circumstance that it requires no
digestion. In this it resembles glucose. When alcohol is
given to a patient no call is made upon the digestive glands.
Hence it may be tolerated and absorbed when most foods
would remain undigested. So far as it can be introduced
into the circulation at such a time it becomes a source of
heat and spares the dwindling stores of the system, but
there is an obvious tendency on the part of physicians to
restrict the use of alcohol in sickness. Statistics from rep-
resentative hospitals show a marked shrinkage in the con-
sumption of liquors during the last few years.

Alcohol as a Drug. — Certain properties of alcohol may be
conveniently brought under this head, although no sharp
line of demarcation can be drawn between these qualities
and others to be dealt with later. The drug effect is
obtainable from rather large single doses in contrast to the
nutritive effect which was said above to be best secured
by small amounts given at intervals. The most striking
reaction of the system to alcohol in doses which may be
regarded as having the drug effect is manifested by the cir-
culatory apparatus. The taking of a glass or two of wine,
especially on an empty stomach, will usually cause increased
heart action and a flushing of the skin, accompanied by the
subjective impression of warmth. This bringing of more
blood to the surface of the body may be expected to lessen
the volume of blood passing through the internal organs.

The chief value of alcohol as a drug is connected with
this tendency to abate internal congestions. The central
fact in the process which we call ''taking cold" is an excess
of blood in the mucous membranes. This may be con-
tinued for a time without ill consequences, but is always a
menacing condition, lowering the local resistance to in-
fection and so inviting disease. Alcohol has been much
used to "break up" incipient colds and with very good
success. Its influence upon the distribution of the blood
is not unlike that of quinin and some other drugs; it is also
similar to the action of hot applications to the skin.

ALCOHOL 233

The supposed warming power of alcohol needs critical
examination. It is a fact not generally recognized that we
have no reliable sensations indicative of the true body tem-
perature. We realize only the surface conditions. A
man who is out in the cold may produce a sense of comfort
by taking alcohol, which will send more blood into the
cutaneous vessels, but this pleasant glow is the sign that
heat is passing from the body to its surroundings. It may
be purchased at the cost of an expenditure which is im-
prudent. Arctic explorers seem well agreed that depend-
ence on alcohol during exposure to intense cold is unwise,
and that it is better to suffer a greater degree of discomfort
.than to rely upon its delusive support. This must be most
distinctly the case when the hardships are to be borne for
an indefinite period. It is more rational to give alcohol
after exposure to wet and cold than during the trial.
Afterward it may help to readjust the circulation and to
ward off possible evil results.

Alcohol as a Cerebral Alterative. — After all, the main
reason why humanity clings to alcohol and is with so
much difficulty won over to abstinence, is found in its
singular influence upon temperament. This is at the
root of its social employment. The rather awkward
term, "cerebral alterative," has been chosen to avoid the
more familiar but questionable name of stimulant. Much
discussion has been carried on concerning the right of al-
cohol to this designation. Stimulant and narcotic are
opposing terms. Alcohol in large quantities is clearly a
narcotic; whether it is invariably so is a subject of lively
debate. It might be supposed that there would be no
difficulty in deciding. Observation of men slightly af-
fected by wine shows them to be animated and talkative;
the natural verdict would be that they exemplify stimula-
tion. Yet all the results of taking alcohol can be explained
upon the theory that it is a narcotic. To show how this
may be it is only necessary to point out that many opera-
tions of the nervous system are normally inhibitory in
nature. When a reticent man becomes garrulous, it need

234 NUTRITIONAL PHYSIOLOGY

not be inferred that he is stimulated in the best sense of
the word; it may be more accurate to say that an agent
which is essentially depressing in its influence has attacked
first of all the inhibitory centers. Loss of self-conscious-
ness is an obvious feature of the reaction, and loss of self-
respect is reached by an easy transition.

A genuine stimulant should be an aid to application.
Alcohol, on the contrary, is hostile to perseverance. Our
word dissipation, which we use for intemperance, is a very
suggestive one. Scattering rather than concentration is
the essence of the mental state produced. A keen thinker
has said that we tacitly contrast alcohol with coffee, a
recognized stimulant, when we acknowledge the difference
between the feelings with which we should view the use of
one and the other by the engineer of a limited train. We
instinctively feel that coffee will favor his unswerving at-
tention to duty and that alcohol will make him less reli-
able. Alcohol in small quantities leads to inconsequence
in thinking and is a handicap to any steady pursuit.
On the other hand, the facile changes in the currents of
mental life, the lightness and the unexpectedness of one's
remarks, may promote social ease and the power to be
entertaining. If alcohol is the foe of application it is also
the foe of prosiness.

Too much emphasis can scarcely be brought to bear
on the wide disparity between what a man thinks that
alcohol does for him and what an impartial study shows
that it really does. That "wine is a mocker" is a shrewd
observation. The subjective impression is often of an ex-
altation of capacity which objective testing fails utterly
to confirm. A subject does certain problems before taking
a drink of whisky and comparable ones afterward. He
says that the second task was done with greater speed and
with a nonchalant confidence in his results. The watch
says that he was slower, and checking up the work shows
that the errors were more numerous. It is evident that his
judgment of his own performances is unreliable. This is
true also of manual operations. The recent tests made

ALCOHOL 235

upon German type-setters have shown that speed and
accuracy are both made to suffer when alcohol has been
taken even in very limited amounts. Here, again, the
subjects have an impression of their own superior accom-
plishment under alcohol, which turns out to be erroneous.

It has been freely admitted above that a little alcohol
may promote sociability, but there can be no question
that the reputation which alcohol possesses of bringing out
the best wit and humor of which men are capable is largely
unfounded. The reason is not far to seek. This reputation
rests on the reports of men who were themselves influenced
by the same agent which was working upon the nervous
systems of the speakers to whom they were listening. Their
reminiscences are to be taken with more than a grain of
salt. The auditor who is "vinously exalted" — to use a
phrase of Holmes — is an exceedingly lenient critic. He
applauds with delight sallies which a neighbor, who has
turned down his glass, perceives to be inane, if not in bad
taste.

The justification of the social use of alcohol must be
based on its power to produce this singular state of mind.
It removes the consciousness of fatigue and the feeling
of care. The attention is limited to the present moment
and immediate interests. The faculty of discrimination
is dulled, and with the consequent lowering of esthetic
and intellectual ideals there comes a bland self-satisfaction
and a naive admiration of one's fellows. A vigorous
writer has called this process "drugging for delectation."
Can such an artifice be defended? It is most difficult to
answer this question with entire justice to both sides.
Perhaps it may be impossible to answer it in sweeping
fashion for all men. One who is cynical and pessimistic
by nature may really view his affairs more justly and judge
his neighbors more equitably while under the influence of
wine. This may be true of other temperaments, the neu-
rasthenic, for instance. But the optimist — and may we
not say the normal individual? — is not likely to be bettered
by such an agent. Increased buoyancy and good humor

236 NUTRITIONAL PHYSIOLOGY

in such subjects means silliness. If it is true, as is claimed,
that gatherings of total abstainers are comparatively dole-
ful, the lesson may be, not that alcohol is necessary to good
fellowship, but rather that the average nervous system
is below par. We doubt whether a man ought to rest
content with any lower measure of health than that which
will insure the social virtues without chemical aid.

With advancing age it may be unreasonable to demand
so high a standard. As infirmities increase there must
usually come a time when comfort rather than efficiency
is to be sought. When it is clear that this time has
arrived there is much to be said in favor of the more or
less regular, moderate use of alcohol. It is a great
anodyne. Granting this, we may also point out that the
beneficent effect in age will be more surely obtained by
those who have not exhausted the consolations of alcohol
in earlier years.

Alcohol as a Poison. — It seems hardly necessary to en-
large upon the poisonous properties of alcohol. That these
have been ridiculously exaggerated is obvious; that they
are very real is equally clear. The spectacle of drunkenness
and the shame and misery that attend it are too familiar.
No one who begins to use alcohol can be quite sure that he
will continue within bounds. The temperate lives of his
relatives cannot be held to prove him secure. Suscepti-
bility to the tendency to increase the indulgence is found
again and again in young men of clean heredity and fine
gifts. Hence the only absolute safety is in total abstinence.
Yet the chances do not favor the ruin of the average man
who adds alcohol to his diet.

Aside from the habit-forming property, it is becoming
more and more widely recognized that alcohol often im-
pairs the health of men who cannot be charged with in-
temperance. It predisposes to diseases of the heart,
liver, and kidneys. It notoriously lessens the chance of
survival when the user contracts pneumonia. It makes
him an unfavorable subject for surgical operations. By
hastening the development of arteriosclerosis it shortens

ALCOHOL 237

the period of active and effective life. Insurance exam-
iners are glad when they can record of an applicant that
he is a total abstainer.

Enough has been said to show how various are the
aspects of alcohol. It has been easy to treat them sep-
arately in the preceding paragraphs, but no such separa-
tion is possible in practice. The undoubted value of the
alcoholic relish, its occasional merit as a significant part
of the ration, and even its virtue as a drug cannot be util-
ized without some experience of its cerebral effect and the
risk, not always remote, of forming a habit. The hygienic
ideal to be striven for is a robustness of life which shall
make alcohol superfluous as relish, food, or drug, and a
cheerful, active mind which needs no artificial aid to keep
it hopeful and sympathetic. The attainment may not be
an easy task. Grief and worry and overwork may be
added to an original depression of temperament, but the
use of alcohol is never more unsafe than when sorrows are
the excuse, and never so selfish and cowardly as when the
motive is to shun responsibilities that ought to be faced.
Men do not often see the sinister suggestion in the high
spirits of one who has forgotten his cares for an evening by
the most moderate indulgence. They fail to see that the
banishment of the sense of pressing duties is the very
characteristic of the drunkard when, developed to a logical
extreme, it makes him indifferent to every obligation of
conscience and of love.

CHAPTER XXV
INTERNAL SECRETION

IN an early chapter of this book it was stated that there
are two ways in which one organ of the body may exert
an influence upon another. The more familiar and more
studied method has been through the nervous connections
which are maintained between all organs and the brain
and cord. The existence of such ties provides for the
possibility of reflex action. By this means any part of the
organism may modify the behavior of any other part, not
by affecting it directly, but by stimulating or inhibiting
the centers which preside over the second organ. Some
account of the work of the central nervous system is to
be given hereafter. Before this is undertaken it is well to
pay some attention to the other way in which co-ordination
is promoted; namely, by the transfer of chemical products
from place to place through the agency of the circulation.

This is the subject usually covered by the term internal
secretion. The word hormone has been used elsewhere to
denote an active substance generated in one place, but
destined to take effect in another. The secretin produced
in the walls of the upper part of the small intestine and
carried thence to the pancreas and the other digestive
glands to excite them to pour out their juices is an example.
So also is the contribution made by the pancreas to the
blood, which proves to be so important to the utilization of
sugar. In this case it seems to be chiefly in the muscles
that the hormone is valuable. Other instances of similar
interaction can now be given, and there is reason to an-
ticipate that the list of internal secretions will soon be
made longer than it is at present.

238

INTERNAL SECRETION 239

There are several organs once regarded as insignificant
which are now recognized as vital to the welfare of the
whole system. Among these the thyroid gland has at-
tracted particular interest. This is a small bilobed mass of
tissue situated in front of the trachea below the larynx.
It is one of several organs sometimes called ductless glands.
The term is more appropriate in this case than in some
others, since the microscope shows that the arrangement
of the cells is distinctly glandular. They surround small
recesses such as in typical glands would be in communica-
tion with an outlet or duct. Here, however, these cavities
are blind. They are seen to contain a viscid material, the
so-called colloid substance, which is evidently the product
of the secreting cells. Since there is no channel leading to
the exterior, the only possibility is that the distinctive
secretion shall enter the circulation either directly or by
way of the lymph.

The thyroid gland is frequently enlarged and then gives
rise to the disfiguring swelling known as a goiter. Such
enlargements are not necessarily attended with general
disturbances of health, yet in many cases there are symp-
toms which can be referred to an excess of the active
product. Palpitation, breathlessness, extreme nervous-
ness, and marked loss of weight are likely to be observed.
The same manifestations follow the giving of overdoses of
thyroid extract. This has been employed for the correc-
tion of obesity, but it seems unwise to resort to a drug
so powerful and far reaching in its action for the treat-
ment of this condition.

Just as there may be too much of the thyroid material
for the good of the subject, so there may be a serious de-
ficiency. The relative failure of the gland to function as
it should is the cause of a definite disease in human sub-
jects, and its removal from dogs is followed by a decline
if not by death. Young and growing animals are most
seriously affected. All the facts which have been gathered
support the belief that development and general well-
being depend to a considerable extent on the normal

240 NUTRITIONAL PHYSIOLOGY

functioning of the thyroid. Very recently it has been
shown that what has been called the thyroid is, in reality,
a compound structure. In addition to the type of tissue
which forms the main mass of the organ in man, there are
four nodules of a different sort, the parathyroids. These
undoubtedly have a chemistry of their own and a distinc-
tive relation to the economy of the body. For a statement
of their peculiar importance reference must be had to
larger works on physiology.

The full measure of the influence which radiates from
the thyroid can be appreciated by considering the condi-
tion known as cretinism. This is the term used to de-
scribe the state of individuals in whom the thyroid has
never performed its proper work. These subjects remain
for years in a condition of arrested progress, both physical
and mental. They are uncouth dwarfs with large heads,
slack-walled abdomens, and feeble limbs. If they survive
to the age of twenty or thirty they will scarcely have ad-
vanced beyond the stage reached at four or five. That
the lack of the thyroid is actually responsible for these
shocking cases is now abundantly proved. The demonstra-
tion is found in the happy circumstance that great im-
provement follows the judicious feeding of thyroid prepa-
rations to cretins. The material is obtained from calves
or sheep. Its administration for a few months often re-
sults in transforming a repulsive cretin into a presentable
child with a prospect of at least a moderate mental de-
velopment.

In the dog it is possible to graft an extra thyroid into
the abdominal cavity and then to remove the original
gland, when the second organ will assume the functions
necessary to the preservation of health. This was an
important discovery, since it removed the ground for a
prevalent opinion that the service of the thyroid was
limited to the reflexes which it was supposed to originate.
It had been thought that the gland affected the nutrition
and general health by sending impulses along the nerves
leading from it to the centers. A gland substituted for the

INTERNAL SECRETION 241

native one and placed in a remote part of the body could
not be in connection with the old afferent pathways, and
whatever favorable effect it might have must be chemical
in its origin.

There has been the same discussion as to whether the
reproductive organs exercise their well-recognized influence
on growth by nervous or by chemical means. The strange
modifications of the type which are produced as a result
of castration are familiar. The differences in build and
temperament between the unruly bull and the tranquil ox
illustrate the consequences. Such peculiarities might be
referred to reflex changes due to the removal of sources
of stimuli, but the present tendency is to regard them as
due chiefly to the loss of active internal secretions. The
case of the pancreas reminds us that an organ may well give
rise simultaneously to products which are permanently sep-
arated from the blood and to others which return to it.
Twenty years ago Brown-Se*quard created a furore among
people given to premature acceptance of extravagant
claims when he announced that great rejuvenating virtues
could be demonstrated to exist in the extracts of animal
reproductive glands. His so-called "Elixir of Life" was a
pulp formed from crushed testes of sheep. At some peril
of infection he injected this unsterilized mixture under his
skin and into the bodies of other aged volunteers. A cer-
tain degree of stimulation was noted, but it has usually
been referred to suggestion.

Since animals and men are at their best physically and
intellectually when the reproductive organs are active,
the expectation entertained was not wholly unreasonable.
Yet it was not to be supposed that their decline was re-
sponsible for all the losses incident to age. No extensive
use of such extracts has followed the pioneer work of
Brown-Sequard. The corresponding preparations from
the female — ovarian extracts — have a better standing in
modern medicine. Given after surgical removal of the
ovaries they greatly relieve many of the symptoms which
commonly annoy the patient.

16

242 NUTRITIONAL PHYSIOLOGY

The adrenal bodies are also numbered among the organs
which help to maintain the system in normal working order.
These are two inconspicuous structures placed one above
each kidney. Their microscopic features are obscure and
do not indicate a glandular organization. From the ad-
renal bodies can be obtained a powerful drug-like com-
pound, adrenalin, which the living cells of these organs may
be supposed to deliver continually to the passing blood.
Its presence in the circulation is evidently a necessity.
Destruction of the adrenals — which sometimes results
from local tuberculosis — produces the singular fatal dis-
turbance known as "Addison's bronze disease." An odd
feature of this condition is the dark pigmentation of the
skin, occurring sometimes uniformly and sometimes in
patches. More important, however, is the steady loss
of vigor affecting all the contractile tissues and leading to
death.

Adrenalin injected into the blood-stream of an animal
shows its most marked effect in the intense contraction of
the small blood-vessels which is produced. This property
has made the substance valuable to surgeons, since it can
be used to check bleeding from cut surfaces. In like man-
ner it can reduce congestion in inflamed tissues, for example,
in a blood-shot eye. These artificial uses of adrenalin can
all be connected with its natural service, which is largely
in the direction of a reinforcement of contractility. Can-
non has recently called attention to this fact in an unex-
pected and interesting relation.

He has proved by delicate tests that the blood of an
animal receives additional adrenalin during a terrifying
experience. For example, he has found the active com-
pound increased in the blood of a cat which has been kept
under restraint in the presence of a barking dog. There-
fore it must be concluded that one of the many bodily
accompaniments of an emotional outbreak is a stimulation
of the adrenal bodies to unusual activity. The value of
this reaction has just been made clear through the further
researches of the same investigator. It appears that

INTERNAL SECRETION 243

extra resistance to fatigue is conferred upon the skeletal
muscles when adrenalin is sent to them. Thus, in an
emergency that might call either for flight or conflict the
animal is prepared for a maximum output of energy. We
have here a scientific conception of the "strength of des-
peration."

The influence of other organs than those mentioned upon
the welfare of the organism as a whole is constantly being
studied. A great deal of attention is being given to the
very small but distinctly compound structure, called the
hypophysis, which is lodged beneath the brain in a hollow
of one of the cranial bones. Its relation to the course of the
metabolism is far from simple, but it seems to be clear that
it is an indispensable contributor to the circulating medium.
A much larger and more conspicuous affair, anatomically
speaking, is the thymus of young animals, a mass of cells
lying back of the breast-bone. It is the "neck sweetbread"
of the market. Since it is prominent during growth the
inference is natural that it ministers in some way to the
process of development. It almost vanishes at maturity.
It is not yet possible to make very definite statements
about the thymus. Removal does not seriously hinder the
progress of the growing animal.

Here and there in the body are firm kernels of tissue,
spoken of as lymphatic glands or, better, as lymph-nodes.
These may be regarded as producers of internal secretion,
though it is probable that this description does not cover
all their activities. They are found especially in the neck,
the armpits, the groins, and in the mesentery. Wherever
placed, each is set upon the route of the lymph as it comes
from some region toward the chest. Thus the lymph-nodes
of the neck must be passed by the lymph that has had its
origin in the head, while those of the mesentery intercept
that which has come from the intestine. Microscopic
study shows that the lymph has to take a tortuous course
among the cells of the nodes. Looked at from a mechan-
ical standpoint these bodies are obstructions in the path.
They are also suggestive of filters. Their own cells are

244 NUTRITIONAL PHYSIOLOGY

continually becoming detached and drifting away in the
lymph, a phenomenon which is a type of internal secre-
tion made visible. They are believed to add substances in
solution as well as cells to the passing fluid.

There is good ground for the view that the lymph-
nodes are a defense against the spread of infection. When
a boil exists upon the arm the nodes above the place where
the bacteria are working so destructively are usually ob-
served to be enlarged and tender. The lymph which is
returning from the seat of the trouble is bearing the pro-
ducts of the suppuration, if not the infecting organisms
themselves. Apparently this polluted lymph is more or
less successfully disinfected before it is allowed to pass on
into the thorax to merge with the blood in the veins. The
lymph-nodes of the mesentery stand as outworks of the
bodily fortifications against the entrance of microbic
invaders from the intestine. When overpowered in their
struggle the lymphatic glands themselves become foci of
infection, as in the familiar form of tuberculosis known as
scrofula.

One large organ, which from its anatomic relations has
been called a ductless gland and which might be expected
to have an internal secretion, has failed to give satisfac-
tory evidence of such a function. This is the spleen, which
is placed below the diaphragm to the left of the stomach.
It remains an enigma to physiologists. Its blood-supply
is large and its frequent changes of volume, contractions,
and dilations alternating in a slow rhythm, give a strong
suggestion of some well-marked action in progress. But
it has never been shown that an animal is affected in any
characteristic way by the loss of the spleen, provided the
immediate effects of the severe operation are survived.
Certain enzymes have been extracted from the tissue of the
spleen, which may have to do with the metabolism of those
peculiar proteins which yield uric acid. We may not be
warranted in asserting that the spleen has no useful ser-
vice to perform, but we can say that other organs appear
to be able to make good its deficiency. .

CHAPTER XXVI
THE NERVOUS SYSTEM

IN the introductory chapter it was said in substance
that the one word which most nearly covers the work of the
nervous system is the word co-ordination. This state-
ment arouses in one the impulse to protest that it leaves
out of account the relations subsisting between the nervous
system and the states of consciousness which are of the
most immediate interest to us all. The physiologist, being
human, sympathizes with such a protest, but he must
continue to treat his material for the most part from an
external point of observation. This is not for want of
respect for the psychologic method; it is rather with frank
recognition of the vastness of the realm in which that
method is applicable. It is because he must defer to
experts in that field that he will not enter upon it as an
amateur.

Most readers need to be told with the utmost emphasis
and with frequent reiteration that consciousness is the
accompaniment of an excessively small share of the mani-
fold reactions of the nervous system. In the words of
President Hall, it is "a little candle burning in one room of
the mind's museum." All that occurs from first to last in
the life history of a fish or a frog can be explained as reflex
adjustment, without the assumption of self-knowledge or
conscious purpose on the part of the animal. That was a
weird and fascinating picture which duBois Reymond once
drew of a world precisely like our own, save that its in-
habitants were unconscious. In such a world an artist
without will or pleasure in his work might create a faultless
statue because his inherited nervous mechanism and the

245

246 NUTRITIONAL PHYSIOLOGY

existence of materials, tools, and a model, made the result
inevitable.

In the previous discussion of reflex action (Chapter V)
the conception was developed that the nervous system con-
sists of pathways capable of transmitting energy in the
form of "nerve-impulses" to and from its central portion.
That central part is represented in the higher forms by
the brain and the spinal cord. The afferent or incoming
paths begin in localities where external influences or stim-
uli can be brought to bear. The nerve-endings which lie
thus exposed are called "receptors." They may be simple
terminal twigs or bulbs subject to stimulation by pressure
or temperature. They may be connected with elaborate
organs like the eye and the ear. The eye is a device for
converting the energy of certain ether-waves into the im-
pulses that traverse the optic nerve. The ear is, at least
in part, a device for transforming the energy of certain
air-waves into impulses that run along the auditory
nerve.

The impulses which arrive within the brain or the cord
may cause the immediate or delayed return flow of im-
pulses along the efferent or centrifugal paths of the system.
These lead in most cases to the contractile tissues which
give objective expression to animal life. It must not be
forgotten that the efferent branches of the nervous system
extend also to various glands and may modify secretion as
well as contraction. It will be recalled that efferent im-
pulses may initiate the flow of tears, of the saliva, the
gastric juice, and the adrenal principle. Other instances
are almost equally clear.

In animals of all grades responses of the reflex type are
the constant duty of the nervous equipment. Those forms
which we regard as highest in the scale are distinguished by
a second property, that of the modification of the re-
sponses as a result of individual experience. By this is
meant the capacity for training, forming habits, or learn-
ing to profit by the past. It is what makes different mem-
bers of the same species vary in their behavior. It is what

THE NERVOUS SYSTEM 247

makes an older member superior to a younger one in his
powers of adjustment and maintenance. This is, of course,
exemplified in the fullest degree by human beings. The
prolonged period of growth and progress must indicate
a plasticity in the arrangements of the nervous elements
that is hardly ever entirely outlived.

Descartes in the seventeenth century grasped the two
properties with his usual keenness. He pictured the reflex
process with quaint symbolism, but with essential correct-
ness. He likened a path of afferent transmission to a cord
that is pulled. A valve is opened in the central organ and
a fluid released to flow back along the nerve. Reaching
a group of muscles this fluid wakes them to action. Des-
cartes had used different analogies for the afferent and the
efferent impulses, making one a mechanical twitch and the
other a spurt of liquid; we now believe them to be nearly
or quite identical in character. The same writer recog-
nized the ability of the higher nervous systems to retain
impressions, and likened the registration of a memory to
the imprint of a seal upon wax.

Physiologically, the particular mark of the highly de-
veloped nervous axis is this capacity for recording indi-
vidual and not merely racial experience. Anatomically,
the advance in organization is signalized by an increasing
predominance of the brain over the cord and of the
"f ore-brain" over the parts behind. A frog will carry out
many complex reactions when the entire brain has been
destroyed. That is to say that in the frog the cord is ade-
quate for a great deal of reflex action. In a human para-
lytic whose injury has removed the lower half of the spinal
cord from functional union with the brain, the reflexes
which can be obtained from the lower extremities are few
and slight. In other words, the cord in man has become
less important as an organ for correlating afferent with
efferent impulses and more important as the largest of all
nerves, the highway to and from the brain. We shall now
set forth in some detail certain of the adaptive changes
which are constantly occurring through the .mediation of

248 NUTRITIONAL PHYSIOLOGY

the brain quite apart from attention or desire on the part
of the subject.

This kind of subconscious control of organs is well illus-
trated in the case of the respiratory center. About a
hundred years ago it was found by French workers that
cutting across the nervous axis at the point where it leaves
the skull — that is, where the brain passes into the cord —
instantly stops the breathing. A similar cut a little higher
up does not have this immediately fatal effect. It was in-
ferred that the part of the brain next adjoining the cord
must contain the special cells which stand in charge of the
respiratory muscles. This section of the brain, just within
the skull, is called the medulla. Later experiments have
confirmed the early belief that the breathing is governed
from this region. The muscles employed are not in them-
selves automatic. Every contraction which they make is
referable to a metabolic change in the cells of the medulla.
So there is a fundamental difference between the breathing
movements and the beating of the heart. The former
are due to central causes; the latter, to an innate quality
of the contractile substance.

The respiratory center is frequently involved in reflex
action. In fact, it is easy to convince one's self that hardly
any considerable reflex occurs without some disturbance of
the breathing as an incident. Any sort of shock will in-
fallibly change the depth, regularity, or some other feature
of the movements. Outcries following such shocks are
merely respiratory reflexes, but the center is not prompted
to each successive discharge by afferent impulses; it shows
us the possibility of another means of regulation. This
is through the influence upon the nerve-cells of the
chemical composition of the blood and lymph in the
vicinity. When exercise is taken the breathing is involun-
tarily deepened. The cause of this adjustment is found in
the increase of carbon dioxid in the circulation. The
center is remarkably sensitive to any rise in the percentage
of this gas. Conversely, it is temporarily paralyzed by a
reduction of .the circulating carbon dioxid to an unusually

THE NERVOUS SYSTEM 249

low level. Variations of the oxygen of the blood affect its
action surprisingly little.

An important work of the nervous system and one which
is quite overlooked by the layman consists in the regula-
tion of blood-flow. In our previous description of the
plan of the circulation only its mechanical principles were
discussed : the heart was spoken of as a force-pump and the
vessels as elastic tubes. This is a proper presentation so
far as it goes, but we must now proceed to show that the
whole circulatory apparatus is living, and being so has the
biologic characteristic of adaptation. The changes l>y
which the ever-varying requirements of the body are met
are of two kinds, changes in the force and frequency of the
heart-beat and changes in the caliber of the small arteries
and veins. The former are brought about by the cardiac
nerves; the latter, by a department of the nervous mechan-
ism which we call the vasomotor system.

It is a matter of familiar experience that the heart-rate
is subject to striking variations. From an average of per-
haps 65 per minute, when one is lying in comfortable re-
laxation, it can be driven to 150 or more, when one is
hurrying up a grade. Such a change under the influence of
muscular activity is evidently purposeful. The working
tissues need a swifter current of blood to supply them with
oxygen and to bear away their waste. The lungs must be
visited at shorter intervals to keep normal the gaseous com-
position of the blood. Muscular activity is not to be sup-
ported by increased breathing alone ; it is equally necessary
that there shall be acceleration of the circulation. As the
quick, deep breathing of one who is taking exercise be-
speaks a governing center for the muscles employed, so
the rapid beating of his heart suggests a central control of
that organ.

Experimental proof of the central regulation of the heart
is ample and detailed. Branches of the nervous system
reach it from two distinct sources and are contrasted in
their effect upon it. One set of fibers is said to be inhib-
itory, the other to have an accelerator action. The terms

250 NUTRITIONAL PHYSIOLOGY

would seem to explain themselves. The inhibitory fibers
restrain the heart much of the time from beating at the
rate which it would exhibit if nervous regulation were
entirely withdrawn. An exaggerated inhibitory influence
may weaken its working to the point where the circulation
becomes quite inadequate and faintness is produced. In
laboratory trials actual arrest of the heart of a dog may be
caused, though the beat is always resumed within a minute,
so that death cannot be made to result. We ought not to
lay much stress upon these extreme possibilities. It
would be absurd to say that the function of the inhibitory
cardiac nerves is to stop the heart; that could never be for
the best good of the animal. We can say with more reason
that their service is to economize the strength of the heart
and to provide a reserve for emergencies. This has been
described as a "brake action."

The accelerator nerves of the heart have a stimulating
effect which extends to the vigor as well as to the rate of
its beating. When we observe a specific increase of heart
action we can hardly say whether it has been produced by
positive accelerator influence or by the abatement of the
habitual inhibitory control. It may, of course, represent
the combined result of both. The government of organs
by means of the balanced action of two opposing sets of
nerves is repeatedly met with in the body. It can be de-
monstrated for the stomach and for other sections of the
alimentary canal. A singular example is afforded by the
nervous relations of the iris, the colored ring surrounding
the black pupil of the eye. Stimulation of one nerve causes
contraction of the pupil, while widening or dilatation fol-
lows the application of stimuli to an entirely different
strand of fibers.

The heart may quicken the circulation by beating at a
more rapid rate and with increased power, but it cannot
send the blood to a particular part of the body at the ex-
pense of another part. The total blood-flow thus depends
upon the heart acting under the balanced sway of inhib-
itory and accelerator nerves, but the distribution of blood,

THE NERVOUS SYSTEM 251

the favoring of organs which need it, is the work of the
vasomotor system. The walls of the microscopic vessels,
those which adjoin the capillaries in particular, are provided
with muscular elements of the same general order as those
which produce the movements of the digestive tract.
These contractile elements are connected with the terminal
branches of certain nerves. Hence the anatomic study of
the tissues involved, even when unsupported by physio-
logic experiments, makes clear the possibility that the
centers when acting reflexly or otherwise may influence
the diameter of the blood-vessels and the volume of the
circulation in any or all regions of the body.

Physiology reinforces anatomy at this point. For sixty
years it has been certainly known that nervous regulation
of the blood-flow is a fact. Much earlier than this the
power was assumed to exist, "Ubi stimulus, ibi affluxus,"
they said, meaning that where there is activity there is an
increase of blood. The paling and the flushing of the skin,
occurring either in response to external changes of tempera-
ture or emotional conditions, are strongly suggestive of a
central command of the vessels. This has been taken for
granted in our own treatment of the matter of the main-
tenance of a normal body temperature. Much as the heart
is reached by impulses which inhibit it and by others which
spur it to a greater expenditure of energy, the small arteries
and veins are subject to antagonistic influences. We speak
of a vasoconstrictor effect when we mean a reinforcement
of the existing tone, and we use the word vasodilator in
the opposite sense, that is, to characterize changes in
which the degree of contraction is lessened, with the result
that the blood finds its way in greater quantity through the
affected vessels.

The difference between the cardiac and the vasomotor
factors in the control of the circulation can be made plain
by means of an illustration borrowed from the laundry.
Suppose that a supply-pipe runs along above a row of
tubs and bears a faucet for each. Suppose, also, that there
is a stop-cock on the pipe before it reaches the tubs. Fi-

252

NUTRITIONAL PHYSIOLOGY

nally, to make the correspondence closer, assume that the
faucets can never be shut tight (for it is not likely that the
blood-vessels can be). Now, by manipulating the cock
(H) we can increase or diminish the outflow into all the
tubs, but we cannot in this way hasten the filling of one
more than another. This is obviously the type of cardiac
regulation in the living system. The heart is appealed to

Fig. 23. — The supply, to all the tubs is controlled at the " shut
off " ( H), while the share flowing into each compartment is regulated
by its own faucet (F).

when there is a general and widespread demand for more
blood per unit of time, muscular activity being by far the
most important occasion.

The faucets over the several tubs 'are the symbols of
vasomotor equipment. By using them we can change the
share of the total stream which shall enter any compart-
ment. If our only desire is to fill one tub, we can open
widely that faucet and close the others as far as possible.
The vasomotor system actually operates in that way to the

THE NERVOUS SYSTEM 253

extent that a dilation in one large area seems often to be
offset by a compensatory constriction in another. We
count on this reciprocal relation in our medical and hy-
gienic practice. For instance, we think that if we can en-
courage blood-flow at the surface we shall reduce it in the
deeper parts. This is often an object, since congestions
are important features of many disorders. We antici-
pate this balanced reaction when we make use of hot ap-
plications and when we give alcohol for a cold.

No one is likely to overestimate the services rendered
by the vasomotor system. It has both a general and a
local action. In the first sense it maintains a certain
average state of contration in the arteries — the arterial
tone — which keeps up to a desirable level the pressure in
the arterial trunks. It is this pressure which guarantees
the prompt and sufficient supply of blood wherever the
paths are opened. The principle is the same that is ob-
served in the mains distributing water through the streets
of a city. The pressure must be great enough to keep up
the supply on the high ground as well as the low. Leakage
or wasteful use of water will lower the pressure and the
failure will be noted first on the hills. An unusual dila-
tion of many blood-vessels, such as probably accompan-
ies an attack of indigestion where the abdominal organs are
engorged, may lessen the pressure and the volume of blood
passing through the brain with the consequence that there
is faintness. Partial relief is found in lying down because
the factor of gravity is eliminated and the head given an
equal chance with the rest of the body.

We notice the failure of the vasomotor system to do its
duty in cases like the above. We ought also to appreciate
how remarkable it is that the adjustments usually occur
with such smoothness and success. It is really a wonder-
ful thing that we can rear up the elongated human form
from a horizontal to a vertical position without entirely
deranging the circulatory mechanism. That the blood
does not distend the vessels below the heart and forsake
those above must be due in a great measure to vasomotor

254 NUTRITIONAL PHYSIOLOGY

correction. Doubtless, when one rises to his feet in the
morning one is saved from falling in a faint by the prompt-
ness with which the arterial tone in the lower extremities
is raised by the nervous system to fight back the threat-
ened excess of blood. It is easy to see what would happen
in a lifeless model under similar conditions.

While our vasomotor reactions are usually executed
without our conscious attention, it has always to be
borne in mind that close ties exist between the higher and
the lower regions of the nervous fabric. Emotions register
themselves in changes of blood-flow. The obedience of
the vasomotor center to hypnotic suggestion is almost
unlimited. It would appear that this division of the ner-
vous system is the chief mediator in the correction of the
many ills which unquestionably yield to Christian Science
and kindred ministrations. A normal circulation goes
far to insure normal nutrition and irritability in each part,
and these states are the fundamentals of health. Fur-
thermore, the feeling of health — which is not always the
same thing as the possession of health — must depend, in
the absence of more positive sensations, on the character
of the cerebral circulation. Assuming the blood to be
chemically normal a moderate change in the amount
passing through the brain may make all the difference
between the sense of power and the feeling of utter help-
lessness.

Beside dictating the character of the breathing and
determining the volume and distribution of the blood-
flow, the lower part of the brain exercises control of the
alimentary canal and, in varying degree, of the glands.
To mention these facts is merely to recapitulate what has
already been pointed out. It must be recalled that the
government of the glands is partly through direct com-
munication with their secreting cells and partly through
vasomotor regulation of their blood-supply. In many
cases of sustained glandular activity both features exist
at the same time.

CHAPTER XXVII
THE NERVOUS SYSTEM— ITS HIGHER WORK

" Our wills are ours, we know not how. ..."

IN the last chapter emphasis was constantly placed on
the subconscious nature of the multifarious adjustments
which are each moment secured through the action of the
central nervous system. We are accustomed to think
that the use of our sense-organs and the employment of
our skeletal muscles are activities with which consciousness
is far more closely concerned. While this is broadly
true, we need to recognize that a great part of these re-
actions also goes on without our notice and beyond the
reach of our intervention. This is readily admitted of the
breathing. It will be found almost equally characteristic
of the maintenance of the balance at rest and during
locomotion.

The ability to stand is dependent on the occurrence of
inconspicuous but indispensable reflexes which check each
swaying movement of the body as it threatens a fall.
When one walks the attention seems for the most part to
be detached from the elaborate muscular performance and
to be given to other matters. The contact of the feet
with the ground, the gliding of one joint surface on another,
the shifting of stresses from one muscle or tendon to a
neighbor — these local changes become in a regular se-
quence the source of impulses which ascend to the brain
and evoke appropriate responses. It has been generally
believed that the division known as the cerebellum has a
peculiar importance in this connection.

The position of the thinker himself with reference to the
great afferent and efferent departments of his nervous

255

256 NUTRITIONAL PHYSIOLOGY

system may be likened to that of the general in command
of a great army. From his headquarters he can see but
a small proportion of his troops. Their line of battle
stretches for miles beyond his sight. He issues the order
for a general advance. His aides ride to the right and left,
bearing the word to the commanders of corps and divisions.
The simple act of the general has been followed by a train
of events which becomes each moment more difficult to
trace. The original order is transmitted to officers of
lower rank and interpreted by them in conformity with
local needs. When the private soldiers are at last put in
motion it is at the word of colonels and captains. The
impossibility of having the voice of the commander-in-
chief the source of guidance in each company is perfectly
evident. It is not merely that he is too far removed from
most of his men, but that there are too many problems
arising at one time. The minor ones must be solved at
the discretion of his subordinates.

So in the living body the will to walk — which seems as
simple as the dictation of the first order from headquarters
— is promptly followed by the action of many nervous
mechanisms whose function is to distribute impulses and to
apply them in helpful sequence. We have no sense of the
subdivision which is involved. We cannot analyze the
groups of muscular movements which take place. Yet
it is plain that there is an apportionment of stimulation,
more to this muscle and less to that, without which the
effective result could not be secured. How utterly we
should fail in the attempt to regulate the part taken by
each of a hundred co-operating muscles by giving attention
to each in its turn! The efforts necessitated would be like
those of the general deprived of his staff and messengers
who should seek to ride swiftly from point to point in the
endeavor to direct all his soldiers by his spoken word.
The organization of the highly developed nervous system
is that of a disciplined hierarchy.

The comparison we have been using can be made to
serve even further. We can find in it a place for the af-

THE NERVOUS SYSTEM — ITS HIGHER WORK 257

ferent side of nervous action. We have said that reflex
elements can be found in almost any elaborate movement.
In walking, for example, the assumption of a given
position by the body insures the return to the centers of
impulses which cause the next appropriate change. We
do not consciously resolve to take each step. The very
idea is painful. One step accomplished makes the taking
of another the natural sequel. Turning to our analogy,
we readily see that when the grand advance is under way
the continuance of the march will probably be intrusted
to the direction of lower officers. Difficulties encountered
they will report to the general if this seems necessary;
minor obstacles they will meet upon their own responsi-
bility. Thus when we walk we may be aware of gutters
to be crossed, but we do not notice at all the incessant
slight adjustments which are made for the lesser inequal-
ities of the path.

The human brain contains at birth connections which
make possible the execution of a moderate number of
useful reflexes. Among these are sucking, winking, cough-
ing, sneezing, and vomiting. Of course, there are also the
requisite mechanisms for the government of breathing,
the circulation, and the digestive tract. Such a brain
appears to differ from that of one of the lower animals
chiefly in its capacity for continued development. Loeb
has shown that all that is acquired in the experiences which
come to the child may be covered by the term "associative
memory." The expression as used by him does not refer
to the power to review past experience as a conscious
process, but only to the power to acquire new reactions to
environmental conditions. An early gain in this direction
is the attainment of the ability to reach after and grasp an
object which has stimulated the visual department of the
nervous system. This seems to signify the opening of a
pathway in the brain from the region receiving impulses
from the retina to the region from which impulses go out to
the muscles controlling the hand.

In similar fashion we can picture the acquirement of one

17

258 NUTRITIONAL PHYSIOLOGY

accomplishment after another. All imitative action must
be made possible by the establishment of bonds between
receiving and discharging stations in the surface gray
matter of the cerebrum. As these connections are formed
the conduct of the individual under given circumstances
becomes predictable; in other words, we have here the
physical basis of mannerisms and habits. It is even per-
missible to say that the foundations of character are denned
in the direction which these association ties are found to
take. All of education, so long as it is viewed from with-
out and not from within, consists in the cultivation of
response to varied influences. There can be no doubt that
the anatomic accompaniment consists in the multipli-
cation of brain pathways.

During the last hundred years the question has been
much debated whether particular powers have definitely
localized registration in the structure of the brain. In the
first half of the nineteenth century the phrenologists at-
tracted much favorable attention by their claims of very
precise subdivision of the brain into "organs of different
faculties." The notion of "bumps" is still current, though
it is usually referred to in a jocular spirit. The literature
of phrenology is very large, and it is probable that it would
repay a critical review, but the advocates of the cult went
much farther than the facts warranted and their concep-
tions fell into disrepute. By 1850 the reaction had carried
scientific opinion to the belief that there is little localiza-
tion in the brain.

Since then a great many experimental studies, together
with observations of the consequences of injuries suffered
by human brains, have united to encourage the view that
there is some degree of localization. The new doctrines
have not followed the old traditions at all. Where the
phrenologists sought to refer mental functions to particular
regions of the brain surface, modern students have looked
with more success for the central representation of bodily
processes. They have shown that a certain area stands in
definite relation with the skeletal muscles, that another

THE NERVOUS SYSTEM — ITS HIGHER WORK 259

area is the place for the reception of the visual impulses,
and so on. In most respects the human brain has been
found to be organized in a manner closely corresponding
with what is traceable in the brain of the ape and hinted
at in the brain of the dog.

Yet the distinctive accomplishments in which man ex-
cels the lower animals must be coupled in some way with
his cerebral equipment. One or two of these distinguishing
features seem to have quite definite positions. This is
especially clear in the case of the well-recognized "speech
center. " The belief is commonly held that the mechanisms
which are correlated with such acquired powers as speech,
reading, and writing are restricted to one-half of the
brain, usually the left. The right hand, which in most
subjects is so much superior in skill, is governed from the
left side of the cerebrum. A charming account of the
apparent relations between the human brain and human
capacities is given in W. H. Thomson's "Brain and Per-
sonality." The claims made for precise localization in
that book are regarded by many conservative writers as
extreme. It is certainly fair to say that at present the
most learned interpreters of brain physiology are placing
emphasis on the development of paths between centers
rather than on the organization of the centers themselves.

The characteristic of early life is the ease and freedom
with which these paths are opened and the comparative
frequency with which they are changed. During the long
period of mature efficiency they are less subject to multi-
plication and are used with increasing regularity and with
rarer deviations. The man is becoming a creature of
habit and acquiring "ruts." In old age, as has been finely
said, the nervous system, instead of holding a prophecy
of what may be, contains a record of the past. It is a
fascinating fancy — though it is nothing more — that a
physician of surpassing insight might look upon the warp
and woof of fibers in a dead brain and tell us of the tastes,
talents, and pursuits of its former possessor. When we
walk through the rooms of a deserted house we can tell

260 NUTRITIONAL PHYSIOLOGY

by the worn places on the floors and thresholds and by the
grimy edges of the doors just where the tenants came and
went most often. The lifeless brain must bear a more
subtle registry of the same order.

Afferent fibers reach the brain from all parts of the body.
Many of these have had their origin within its tissues,
where they are normally stimulated by conditions that
belong to the organism itself rather than to its environ-
ment. The impulses that enter the brain from such
sources are most of the time serving their purpose in pro-
moting subconscious adaptive reflexes. When they affect
consciousness it is to bring to the attention the so-called
"general sensations" — those feelings which we refer to
states of the organs. Such are hunger and thirst, many
kinds of pain, satiety, nausea, faintness, fatigue, and the
like. The majority of these general sensations seem to
signify conditions that need to be rectified and they are
mostly unpleasant.

Contrasted with these are the "special sensations,"
which are referred to causes acting upon the afferent ap-
paratus from outside. Various terminal structures are
developed at or near the surface of the body which serve
to transmute different forms of environmental energy
into nerve-impulses. Such structures are called sense
organs or "receptors." A very suggestive distinction has
been made between the "proprio-receptors," which are
affected only by the literal contact of the stimulating sub-
stance, and the "distance-receptors," which respond to
forms of energy radiating from places more or less remote.
The nerve-endings in the skin which are acted upon by
pressure and by temperature changes are proprio-recep-
tors. So are those in the tongue on which various dis-
solved substances take effect, giving the conscious sub-
ject sensations of taste. The olfactory endings high up in
the nasal cavities are reckoned by most writers to be in the
same class.

The ear is a distance receptor. The energy which sets
its intricate mechanism into vibration may have originated
as far away as the thunder-cloud hanging near the horizon.

THE NERVOUS SYSTEM — ITS HIGHER WORK 261

The possession of an ear greatly extends the compass of the
environment which can exert directing influences on the
conduct of an animal. If this is true of the ear, how shall
we estimate the widening of environment that comes
with the addition to the receptor system of an eye! Our
ability to see the stars means nothing less than this: that
the reactions of the organism so endowed may be modified
by energy proceeding from the incomprehensible distances
of the stellar universe.

Throughout this book we have held steadily to the point
of view defined in its very first paragraphs : that all living
things are transformers of energy, and engaged so long as
they live in reacting according to the principles of mechan-
ics and chemistry in response to external changes. A pres-
entation in this spirit provokes resentment and protest
from many readers. It seerns to leave out of account all
that is instinctively held to be highest and finest in human
life. To this remonstrance we are glad to give place.
The scientist is, after all, a man, and no scientist was ever
so ruthlessly logical as to convince himself that his friend
was no more than a reflex mechanism. The impression
that the study of science deprives one of the philosophic
outlook and of the conviction of moral responsibility
belongs to the earlier stages of the student's experience.
Later it is seen that no incentive to right conduct and no
worthy consolation is to be taken away.

A few years ago a brilliant astronomer was concluding
a course of lectures in which he had traced the long story
of planetary evolution. He had pictured the ages of forma-
tive process, the slow condensation and cooling of the
globe, the gradual approach to conditions suitable for or-
ganic life. He had sketched the brief flourishing of that
life, the remorseless chilling of the planet, and its frigid
and sterile old age. His hearers were weighed down with
the appalling sense of futility and insignificance. At the
very last he asked abruptly: Which, after all, is the greater
— these awful ranges of time and reaches of space or the
mind of man which comprehends and ponders them?

262 NUTRITIONAL PHYSIOLOGY

REFERENCES

Chapter I: Sedgwick and Wilson, " General Biology," Holt, New
York, 1895, chaps, i, ii, iii. — Bunge, " Text-book of Physio-
logical and Pathological Chemistry," Kegan Paul, Trench,
Triibner & Co., London, 1890, chap. i.

Chapter II: Sedgwick and Wilson, chap. xvii. — Bunge, chaps, ii, iii.

Chapter III: Howell, "A Text-book of Physiology," 4th ed.,
Saunders, Philadelphia, 1911, chap. xl. — Fischer, " Physiology
of Alimentation," Wiley, New York, 1907, chap. iv. — Starling,
" Recent Advances in the Physiology of Digestion," Keener,
Chicago, 1906, chaps, i, ii. — Bayliss, " The Nature of Enzyme
Action," Longmans, London, New York, Bombay, and Cal-
cutta, 1908.

Chapter IV: Hough and Sedgwick, " The Human Mechanism,"
Ginn, Boston, 1906, chaps, iii, iv.

Chapter V: Hough and Sedgwick, chap. vii.

Chapter VI: Kimber, "Anatomy and Physiology for Nurses,"
3d ed., Macmillan, New York, 1910, chap. xiy.

Chapter VII: Howell, chap. xli. — Fischer, chaps, i, iii, v, x.- — Starling,
chap. iii.

Chapter VIII: Howell, chap, xxxix. — Cannon, "The Mechanical
Factors of Digestion," Longmans, New York, 1911, chaps, iv,
v, vi, ix, x. — For Alexis St. Martin, see Osier, " An Alabama
Student and Other Biographical Essays," Oxford University
Press, American Branch, New York, 1909, pp. 159-188.

Chapter IX: Howell, chap. xlii. — Starling, chap. iv. — Fischer,
chaps, v, vi, xi.

Chapter X: Howell, chap, xliii. — Cannon, chap. xi. — Starling,
chap. v. — Fischer, chaps, vi, viii, xii, xiii.

Chapter XI: Cannon, chap. xii. — Howell, chap, xliii.

Chapter XII: Martin, "The Human Body," 9th ed., Holt, New
York, 1910.

Chapter XVII: Howell, chap. xxiv.

Chapter XIII : Martin, chaps, xix to xxi.

Chapter XIV: Howell, chaps, xlii, xliii.- — Fischer, chaps, xiv to xvii.
—Starling, " The Fluids of the Body," Keener, Chicago, 1909,
chaps, ii, iii.

Chapter XV: Howell, chap, xlviii. — Lusk, " Elements of the Science
of Nutrition," 2d ed., Saunders, Philadelphia, 1909, chap. vii.

Chapter XVI: Howell, chap, xlvii. — Lusk, chaps, iv, v. — Abder-
halden, " Text-book of Physiological Chemistry," Wiley,
New York, 1908, chaps, x to xiv.

Chapter XVII: Howell, chaps, xxxvi, xiv. — Martin, chaps, xxiii,
xxiv, xxxi.

Chapter XVIII: Howell, chap, xlvii. — Lusk, chap, i.— Schafer,
" Text-book of Physiology," Macmillan, New York, 1898, vol.
i, article " Metabolism," p. 868. — Tiegerstedt, " A Text-book
of Human Physiology," Appleton, New York, 1906, chap. iv.

Chapter XIX: Howell, chaps. 1, Ii. — Lusk, chap. i. — For Count
Rumford, see Jordan, " American Men of Science," Holt,
New York, 1910.

THE NERVOUS SYSTEM — ITS HIGHER WORK 263

Chapter XX : Howell, chaps. 1, li, — Lusk, many sections, especially
chaps, vi, viii.

Chapter XXI: Hough and Sedgwick, chap. xii. — Lusk, chap. iii.

Chapters XXII, XXIII: Hough and Sedgwick, chap. xix. — Lusk,
chap. ix. — Chittenden, " Physiological Economy in Nutrition,"
Stokes, New York, 1904.— Fletcher, " The A-B-Z of our own
Nutrition," A, B, C Life Series, New York, 1903.— Benedict,
" American Journal of Physiology," August, 1906. — Crichton-
Browne, " Journal of the Royal Institute of Public Health,"
1908, vol. xvi, pp. 471, 527— Meltzer, " The Factors of Safety
in the Animal Structure and Animal Economy," Jour, of Amer.
Med. Assoc., 1907, vol. xlviii, p. 655.— Pyle, " Personal Hy-
giene," 4th ed., Saunders, Philadelphia, 1910, pp. 9 to 51.—
Herter, " Common Bacterial Infections of the Digestive Tract,"
etc., Macmillan, New York, 1907.— Metchnikoff, " The Nature
of Man," Putnam, New York, 1903.

Chapter XXIV: Billings, "Physiological Aspects of the Liquor
Problem," Houghton, Mifflin & Co., Boston, 1903.— Kelynack,
" The Drink Problem," etc., Methuen & Co., London, 1909.
— Hough and Sedgwick, chap. xx.

Chapter XXV: Howell, chap. xlvi. — Tigerstedt, chap. xi.

Chapters XXVI, XXVII: Tigerstedt, chaps, xxii to xxiy.— Howell,
chaps, ix, x, xiii. — Thomson, " Brain and Personality," Dodd,
Mead & Co., New York, 1907— Miinsterberg, " Psycho-
therapy," Moffat, Yard & Co., New York, 1909, chap. iii.
—James, " Psychology," Holt & Co., New York, 1892.— Pyle,
pp. 275 to 314.

ADDITIONAL REFERENCE BOOKS ON FOODS AND NUTRITION

Hutchison, " Food and Dietetics," Wood, London, 1911.
Thompson, " Practical Dietetics," Appleton, New York, 1902.
Pattee, " Practical Dietetics," 6th ed., published by the author,

Mount Vernon, New York, 1910.
Locke, " Food Values," Appleton, New York, 1911.
Sadler, " The Science of Living," McClurg, Chicago, 1910.
Sherman, " Chemistry of Food and Nutrition," Macmillan, New

York, 1910.
Jordan, " Principles of Nutrition," Macmillan, New York, 1911.

INDEX

ABSORPTION, cell activities and,
129

chemical changes during, 133,
134

definition of, 45

dialysis and, 128

effect of concentration upon,

130
of condiments upon, 130

in small intestine, 130-133

of alcohol, 129
Adipose tissue, 136
Adrenal bodies, internal secre-
tion of, 242
Alcohol, 30, 227

absorption of, 129

as cerebral alterative, 233

as drug, 232

as food, 230

as poison, 236

as relish, 229

historical, 227, 228
Alimentary canal, 54

glycosuria, 140
Alveoli of glands, 44
Amino-acids, " deaminization " of,
153

from gelatin, 149

from proteins, 94, 147, 148

surplus acids, 151
Amylopsin in pancreatic juice, 91
Animals as transformers of en-
ergy, 176

Antiperistalsis in colon, 100
Antrum, 70, 73
Assimilation, definition of, 13
Auricle, 116, 120
Auto-intoxication, 206, 213

BILE, description of, 95
pigments in, 95
salts in, 96
waste products of, 95
Blood as carrier, 105
as equalizer of temperature,

106

coagulation of, 113
corpuscles in, 108
plasma of, 110
Blood-flow, character of, 122
in arteries, 122
in veins, 122

intermittency of, 123-125
pressure, 122, 123
regulation of, 249-254
velocity of, 125

Body cavity, definition of, 55, 56
composition of, 22-27
temperature, changes in, 197,

203

of metabolism in relation
to perspiration and,
198-200, 201
humidity and, 200, 201
maintenance of, 196-203
during exercise, 202
265

266

INDEX

Brain, localization of function in,

258, 259
Breathing, 162, 163

CALORIE, definition of, 176
Calorimetry, 180
direct, 183
indirect, 184
Capillaries, 19, 116
Carbohydrates in blood, 111, 140
in body, 25
in diet, 24
in fasting, 171, 193
in liver, 138
metabolism of, 138-146
Carbon dioxid, elimination of,

160-164

in expired air, 163, 164
from protein, carbohydrate,

and fat, 172
as stimulus to respiratory

center, 164, 248
retention, 175
Cardia, 57
Cardiac sphincter, effect of acid

upon, 71
regulation of, 70
Cecum, 58, 98
Cells, 12, 15
Central resistance, 52
Chocolate, value of, in diet, 225
Cholesterin in bile, 95
Chyme, 73, 75
Circulation, portal, 120
pulmonary, 118
systemic, 116
Coagulation of blood, 113
fibrin and, 113
thrombin and, 113
Coffee, food value of, 224
Colon, 58

Colon, absorption in, 130-133

antiperistalsis in, 100

movements of, 100

peristalsis in,, 88, 100, 101
Connective tissue, 37, 149
Contraction, 14, 36^2
Co-ordination, 17, 245
Corpuscles in blood, 108

red, 107

white, 110
Creatinin from protein, 156

DEAMINIZATION, 153

Dextrose from amino-acids, 154

Diabetes, cause of, 145

light thrown on protein metab-
olism by, 154
Dialysis and absorption, 128

definition of, 128
Diastatic enzymes, 32, 67, 91
Digestion, children and, 205
cooking an aid in, 30
definition of, 28
effect of fatigue upon, 206
of nervous condition upon,

204

gastric, 85
intracellular, 34
peptic, 86

quantity of food and, 212
salivary, 67
tryptic, 92, 93

Digestive juices, action of, 31-35
gastric juice, 78-87
intestinal, 93
pancreatic juice, 91-93
saliva, 63-65, 67
Disaccharids, 26, 94
Diuretics, action of, 167
Ductless glands, 45, 238
Duodenum, 57

INDEX

267

ELIMINATION of carbon dioxid,
160-164

of nitrogen, 165-168
Endogenous metabolism, 156
Energy, animals as transformers
of, 176

from various foods, 176-178

of metabolism, 178

of muscular contraction, 37, 38

plants and, 20-22

source of, for work, 187-191

transformation of, 181-183
Enterokinase in intestinal juice,

92
Enzyme action, energy change

in, 35

equilibrium, 33

Enzymes, action of acids and al-
kalies upon, 34

amylopsin, 91

definition of, 32

diastatic, 32, 67, 91

erepsin, 94, 134

gastric lipase, 87

invertase, 94

lactase, 94

lipase (pancreatic), 92

lipolytic, 87, 92

maltase, 94

pepsin, 86

proteolytic, 86

ptyalin, 67

relation to temperature, 34

rennin, 85

steapsin, 92

thrombin, 114

trypsin, 92
Equilibrium in weight, 175

nitrogen, 174

Erepsin in intestinal juice, 94
Esophagus, peristalsis in, 66
Exogenous metabolism, 156

Extractives, drawbacks of, in

diet, 220, 221

exciters of gastric secretion, 83
in diet, 26

FASTING, carbohydrate in, 171,

193

fat in, 171
Fat, definition of, 25

derived from carbohydrates

and fats, 136, 142
distribution of, in body, 25, 136
in diet, 26
in plasma, 112
substituted for glycogen, 144
"Faulhorn experiment," 188
Feces as bearers of wastes, 168,

169

composition of, 102
Feeding and metabolism, 192
Fever, 203

Fibrin in clotting, 113
Fletcherism, 210, 211
Food accessories in diet, 224
action of bacteria upon, 31
purpose of, 13, 135
Fuel value, definition of, 176
of carbohydrates, 177
of fats, 177
of hydrogen and carbon, 176,

177

of proteins, 177, 178
Fundus, definition of, 70
function of, 72
movements in, 72, 76

GASTRIC digestion, 85
juice, 78-87

acid of, 79, 80

Spallanzani and, 78
lipase, 87

268

INDEX

Gastric secretion excited by dif-
ferent means, 81-84

Gelatin, defective protein, 149
nutritive value, 149, 150

Glands, ductless, 45, 238
nervous control of, 42, 81
work of, 42, 44

Glycogen from protein, 155
from sugar in blood, 138

Glycosuria, alimentary, 140

HABIT, 53, 258

Heart, anatomy of, 116

beat of, 119

nervous control of, 249, 250

valves in, 119
Hormone, action of, 145, 146

definition of, 45

secreted by pancreas, 145, 146
Humidity and body temperature,

200, 201

Hydrolysis. See Digestion.
Hydrolytic cleavages, 30

ILEOCECAL valve, 62, 100
Ileum, 57
Income, 13
Inhibition, 62, 100
Internal secretion, definition of,
44, 238

of adrenal bodies, 242

of pancreas, 145

of reproductive organs, 241

of thyroid, 239, 240
Intestinal juices, 93

enterokinase in, 92

enzymes in, 94
Intestine. See SmaU Intestine or

Colon.

Invertase in intestinal juice, 94
Irreciprocal permeability, 133

JEJUNUM, 57

KIDNEYS as organs of excretion,

165-168

blood supply of, 165
work of, 217

LACTASE in intestinal juice, 94
Large intestine, bacterial action

in, 102, 206

effect of cellulose upon, 103
Larynx, 65

Lipase, definition of, 32
in gastric juice, 87
in pancreatic juice, 92
Lipolytic enzymes, 87, 92
Liver, deaminization of proteins

by, 153

secretion of bile by, 95
storage of glycogen in, 138
Lymph, composition and use of,

19

in villi, 130, 131
movement of, 127
Lymph-nodes, action of, 243

MALTASE in intestinal juice, 94

Maltose, 69

Mastication, 63, 210

Meat, drawbacks of, as food, 220,

221

in diet, 219-222
Mechanism, 11
Mesentery, 60, 61
Metabolism and body tempera-
ture, 207

calculation of, 169-175
carbohydrate, 138-146

relation of pancreas to, 145
endogenous, 156

INDEX

269

Metabolism, energy of, 176-186
exogenous, 156
fats, 136-138
feeding and, 192
modifications of, by age and

sex, 194, 195
by mental state, 190-192
by muscular work, 187-190
total daily, 178
Milk, acid fermentations of, in

stomach, 81
proteins in, 150
sour, 214

Mineral salts in diet, 225, 226
Mouth, 54, 64
Mucin, 65
Muscles, connection of, with

nerves, 39, 40
efficiency, 38
properties, 36-42
skeletal, 36

Muscular contraction, 14, 36-42
tone, 41

NERVOUS system, 17, 18, 46-53,

245-262
Nitrogen, calculation of, 170

elimination of, 165-168

from protein, 170

in feces, 102

in urine, 166

retention of, 174
Nitrogenous equilibrium, 174

metabolism, 147-159
Nutrition, bacteria and, 157, 206,
214

hygiene of, 204-226

OUTGO, 169-175

Oxygen carried by blood, 109, 162

PANCREAS, internal secretion of,
145

relation to carbohydrate met-
abolism and diabetes, 145
Pancreatic juice, 91-93
cause of secretion, 91
enzymes in, 91-93
Parotid gland, 56
Pepsin in gastric secretion, 86
Peptic digestion, 85-87
Peptones, 86, 93
Peristalsis in esophagus, 66

in intestine, 88, 100, 101
Peritoneum, 60, 61
Permeable membranes, 128
Perspiration and body tempera-
ture, 198-200, 202

as carrier of waste, 168

composition of, 168
Pharynx, 56

Plants, energy and, 20-22
Plasma, blood, 110

carbohydrates in, 111

composition of, 110-113

fats in, 112

proteins in, 111

urea in, 112
Portal circulation, 120
Protein, 23, 24

defective, 148-150

dextrose from, 154

extent of synthesis, 153

in body, 23

in diet, 26

in plasma, 111

peculiarities of, 212, 214

perfect, 148-150

place of synthesis, 152, 157

synthesized by animals, 147,

148

from amino-acids, 150-153
Proteolytic enzymes, 86, 92, 94

270

INDEX

Protoplasm, 13

Psychic secretion, 81, 82

Ptomains, 31

Ptyalin, 67

Pulmonary circulation, 118

Pyloric sphincter, effect of acid

upon, 74-76
regulation of, 74-76
Pylorus, 57

RATIONS, changes in, 192
effect of sex, 194, 195
reduction of, 207-209
Receptors, 246, 260, 261
Rectal feeding, 103, 104
Rectum, 101
Red corpuscles, 107

description of, 107-109

function of, 109

hemoglobin, 108

origin, 109, 110
Reflex action, 46-53
Rennin in gastric secretion, 85
Reproductive organs, internal

secretion of, 241
Respiration, 13, 14, 160-165
Respiratory center, 248
quotient, definition of, 172

variations in, 173
Rhythmic segmentation, 89

SALIVA, 63-65, 67

mucin in, 65

ptyalin in, 67

Salivary digestion in stomach,
67-69

glands, 56, 57
Secretin, 91
Secretions, 42-44
Secretory nerves, 42 •

Sensations, general, 260

special, 260
Shivering, 201
Skeletal muscles, 36
Skin as organ of excretion, 168
Small intestine, absorption in,

130-133
digestion in, 97
peristalsis in, 88
rhythmic segmentation in,

89

structure, 57, 58, 62
villi, 130
Sour milk, 214
Specific dynamic effect of protein,

193, 194

Spleen, function of, 244
Starch, hydrolysis of, 67-69, 91
Steapsin in pancreatic juice,

92
Stomach, absorption from, 129,

130

antrum of, 70, 73
cardia of, 57
fundus of, 70-72, 76
glands of, 79
movements of, 70-77
nervous control of, 74
peristalsis in, 73
pylorus of, 57

salivary digestion in, 67-69
share of digestion, 86, 87
z-ray studies of, 72-76
Sublingual gland, 57
Submaxillary gland, 57
Sugar absorbed from stomach,

130

and teeth, 223
in diet, 222
in urine, 140
Swallowing, 65-67
Systemic circulation, 116

INDEX

271

TEA, food value of, 224

Tendon, 37

Thrombin in coagulation, 113

Thyroid gland, 239
and cretinism, 240
and goiter, 239
secretion of, 239-241

Transformation of energy, 181-
183

Transverse band, 70

Trypsin, activation of, 92
in pancreatic juice, 92

Trypsinogen, 92

Tryptic digestion, 92, 93

UREA from amino-acids, 154, 165

in plasma, 112

in urine, 165
Uric acid in urine, 166

properties of, 166, 221
Urine, action of salts upon, 226

albumin in, 167

composition of, 165-167

secretion of, by kidneys, 165-
168

Urine, sugar in, 140
urea in, 165
uric acid in, 166

VENTRICLE, 116, 119, 120
"Vicious cycle," 206, 207
Villi, 130

in absorption, 130
Vitalism, 11
Vomiting, 76, 77

WASTE products in bile, 95
in blood, 112
in feces, 102, 168, 169
Water as metabolic product, 160
drinking, excretion and, 218
meals and, 218, 219
nutrition and, 217-219
effect of temperature upon

elimination of, 199, 200
elimination of, by kidneys, 164
White corpuscles, 110

X-RAY studies of stomach, 72-76

TEXT-

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I2mo of 213 pages. Cloth, $1.25 net. New (zd) Edition.

Professor Drew's work gives the student a working knowledge of com-
parative anatomy and leads him to an appreciation of the adaptation
of the animals to their environments. It is a practical work, express-
ing the practical knowledge gained through experience. The type
method of study has been followed.

Prof . John M. Tyler, Amherst College: "It covers the ground well^
is clear and very compact. The table of definitions is excellent."

American Pocket Medical Dictionary. Edited by W. A. NEW-
MAN DORLAND, M. D. 677 pages. Flexible leather, $1.00 net;
thumb index, $1.25 net. Eighth Edition.

A dictionary must be full enough to give the student the information
he seeks, clearly and simply, yet it must not confuse him with detail.
The editor has kept this in mind in compiling this Pocket Dictionary,
and he has produced a work of great value to the student. There are
over 100,000 American Medical Dictionaries now in use.
I. V. S. Stanislaus, M.D., Medico-Chirurgical College: "We have
been strongly recommending this little book as being the very best."

Saunders' College Text-Books

Veterinary Bacteriology. By ROBERT E. BUCHANAN, Ph. D.,
Professor of Bacteriology in the Iowa State College of Agriculture
and Mechanic Arts. Octavo of 516 pages, 214 illustrations.
Cloth, $3.00 net.

Professor Buchanan's new work goes minutely into the consideration
of immunity, opsonic index, reproduction, sterilization, antiseptics,
biochemic tests, culture media, isolation of cultures, the manufacture
of the various toxins, antitoxins, tuberculins, and vaccines.
B. F. Kaupp, D. V. S., State Agricultural College, Fort Collins: " It is
the best in print on the subject. What pleases me most is that it con-
tains all the late results of research."

issosa's V®t®ras»ry Anatomy

Veterinary Anatomy. By SEPTIMUS SISSON, S. B., V. S., Pro-
fessor of Comparative Anatomy, Ohio State University. Octavo
of 826 pages, 588 illustrations. Cloth, $7.00 net. The Standard.

Here is a work of the greatest usefulness in the study and pursuit of
the veterinary sciences. This is a clear and concise statement of the
structure of the principal domesticated animals — an exhaustive gross
anatomy of the horse, ox, pig, and dog, including the splanchnology of
the sheep, presented in a form never before approached for practical
usefulness.

Prof. E. D. Harris, North Dakota Agricultural College: " It is the best
of its kind in the English language. It is quite free from errors."

Ophthalmology for Veterinarians. By WALTER N. SHARP, M. D.,
Professor of Ophthalmology, Indiana Veterinary College. i2mo
of 210 pages, illustrated. Cloth, $2.00 net.

This new work covers a much neglected but important field of veter-
inary practice. Dr. Sharp has presented his subject in a concise, crisp
way, so that you can pick up his book and get to " the point " quickly.
He first gives you the anatomy of the eye, then examination, followed
by the various diseases, including injuries, parasites, errors of refrac-
tion, and medicines used in ophthalmic therapeutics. The text is
illustrated.

Saunders' College Text-Books

Personal Hygiene. Edited by WALTER L. PYLE, M. D., Fellow
of the American Academy of Medicine. 12010 of 515 pages, illus-
trated. Cloth, $1.50 net. Fifth Edition.

Dr. Pyle's work sets forth the best means of preventing disease — the best
means to perfect health. It tells you how to care for the teeth, skin,
complexion, and hair. It takes up mouth breathing, catching cold,
care of the vocal cords, care of the eyes, school hygiene, body posture,
ventilation, house-cleaning, etc. There are chapters on food adulter-
ation (by Dr. Harvey W. Wiley), domestic hygiene, and home gymnastics.
Canadian Teacher: "Such a complete and authoritative treatise
should be in the hands of every teacher."

Personal Hygiene and Physical Training for Women By
ANNA M. GALBRAITH, M. D., Fellow New York Academy of
Medicine. i2mo of 371 pages, illustrated. Cloth, $2.00 net.

Dr. Galbraith's book meets a need long existing — a need for a simple
manual of personal hygiene and physical training for women along sci-
entific lines. There are chapters on hair, hands and feet, dress, devel-
opment of the form, and the attainment of good carriage by dancing,
walking, running, swimming, rowing, etc.

Dr. Harry B. Boice, Trenton State Normal School: "It is intensely
interesting and is the finest work of the kind of which I know."

Exercise in Education and Medicine. By R. TAIT McKnNZiE,
M. D., Professor of Physical Education, University of Pennsyl-
vania. Octavo of 406 pages, with 346 illustrations. Cloth, $3.50
net. Adopted by U. S. Army.

Chapters of special value in college work are those on exercise by the
different systems: play-grounds, physical education in school, college,
and university.

D. A. Sargent, M. D., Hemenway Gymnasium: "It should be in the
hands of every physical educator."

Saunders* College Text-Books

9nm

Normal Histology and Organograpky. By CHARLES HILL, M. D.
i2mo of 468 pages, 337 illustrations. Flexible leather, $2.00 net.

Second Edition.

Dr. Hill's work is characterized by a brevity of style, yet a complete-
ness of discussion, rarely met in a book of this size. The entire field
is covered, beginning with the preparation of material, the cell, the
various tissues, on through the different organs and regions, and end-
ing with fixing and staining solutions.

Dr. E. P. Porterfield, St. Louis University: " I am very much gratified
to find so handy a work. It is so full and complete that it meets all
requirements."

Histology. By A. A. B6HM, M. D., and M. VON DAVIDOFF,
M. D., of Munich. Edited by G. CARL HUBER, M. D., Professor
of Embryology at the Wistar Institute, University of Pennsyl-
vania. Octavo of 528 pages, 377 illustrations. Flexible cloth, $3.50
net. Second Edition.

This work is conceded to be the most complete text-book on human
histology published. Particularly full on microscopic technic and
staining, it is especially serviceable in the laboratory. Every step in
technic is clearly and precisely detailed. It is a work you can depend
upon always.

New York Medical Journal: "There can be nothing but praise for
this model text-book and laboratory guide."

Embryology. By J. C. HEISLER, M. D., Professor of Anatomy,
Medico-Chirurgical College of Philadelphia. Octavo of 432 pages,
205 illustrations. Cloth, $3.00 net. Third Edition.

A book of the greatest teaching value. The subject is taken up sys-
tematically, treating the development of each tissue, each organ, each
region and system in a most thorough way. There are frequent allu-
sions to certain facts of comparative embryology.

Journal American Medical Association : " The text is concise, and
yet sufficiently full for a text-book."

Saunders' College Text-Books

General Bacteriology. By EDWIN O. JORDAN, Ph. D., Professor
of Bacteriology, University of Chicago. Octavo of 623 pages,
Dlustrated. Cloth, $3.00 net. Third Edition.

This work treats fully of the bacteriology of plants, milk and milk
products, dairying, agriculture, water, food preservation; of leather
tanning, vinegar making, tobacco curing; of household administration
and sanitary engineering. A chapter of prime importance to all stu-
dents of botany, horticulture, and agriculture is that on the bacterial
diseases of plants.

Prof. T. J. Burrill, University of Illinois: "I am using Jordan's Bac-
teriology for class work and am convinced that it is the best text in
existence."

acft®rk>Iogic

Bacteriologic Technic. By J. W. H. EYRE, M. D., Bacteriologist
to Guy's Hospital, London. Octavo of 525 pages, illustrated.
Cloth, $3.00 net. Second Edition.

Dr. Eyre gives clearly the technic for the bacteriologic examination of
water, sewage, air, soil, milk and its products, meats, etc. It is a work
of much value in the laboratory. The illustrations are practical and
serve well to clarify the text. The book has been greatly enlarged.
The London Lancet : " It is a work for all technical students, whether
of brewing, dairying, or agriculture."

Laboratory Bacteriology. By FREDERIC P. GORHAM, A. M.,
Associate Professor of Biology, Brown University, Providence,
lamo of 192 pages, illustrated. C'oth. $1.25 net.

The subjects of special interest to scientific students are the identifica-
tion of bacteria of water, milk, air, and soil. Professor Gorham has
succeeded in making his instructions clear and easily grasped by the
student. The text is illustrated.

Science: " The author has described small points of technic usually
left for the student to learn himself."

Saunders* College Text-Books

s Efl®m<iinifts off

Elements of Nutrition. By GRAHAM LUSK, Ph. D., Professor of
Physiology, Cornell Medical School. Octavo of 402 pages, illus
trated. Cloth, $3.00 net. Second Edition.

The clear and practical presentation of starvation, regulation of tem-
perature, the influence of protein food, the specific dynamic action
of food-stuffs, the influence of fat and carbohydrate ingestion and of
mechanical work render the work unusually valuable. It will prove
extremely helpful to students of animal dietetics and of metabolism
generally.

Dr. A. P. Brubaker, Jefferson Medical College: " It is undoubtedly the
best presentation of the subject in English. The work is indispensable."

Physiology. By WILLIAM H. HOWHLL, M. D., Ph. D., Professor
of Physiology, Johns Hopkins University. Octavo of 1020 pages,
illustrated. Cloth, $4.00 net. Fifth Edition.

Dr. Howell's work on human physiology has been aptly termed a
"storehouse of physiologic fact and scientific theory." You will at
once be impressed with the fact that you are in touch with an expe-
rienced teacher and investigator.

Prof. G. H. Caldwell, University of North Dakota: "Of all the text-
books on physiology which I have examined, Howell's is the best."

Hygiene. By D. H. BERGHY, M. D., Assistant Professor of Bac-
teriology, University of Pennsylvania. Octavo of 530 pages, illus-
trated. Cloth, $3.00 net. Fourth Edition.

Dr. Bergey gives first place to ventilation, water-supply, sewage, indus-
trial and school hygiene, etc. His long experience in teaching this sub-
ject has made him familiar with teaching needs.

J. N. Hurty, M. D., Indiana University: "It is one of the best books
with which I am acquainted."

Saunders* College Text-Books

Immediate Care of the Injured, By ALBERT S. MORROW, M. D.,
Adjunct Professor of Surgery, New York Polyclinic. Octavo of
360 pages, 242 illustrations. Cloth, $2.50 net. Second Edition.

Dr. Morrow's book tells you just what to do in any emergency, and it
is illustrated in such a practical way taat the idea is caught at once.
There are chapters on bandaging, practical remedies, first-aid outfit,
hypodermic injections, antiseptics and disinfectants, accidents and
emergencies, hemorrhages, inflammation, contusions and wounds, burns
and scalds, the injurious effects of cold, fractures and dislocations,
sprains, removal of foreign bodies from the eye, ear, nose, etc., poisons,
and their antidotes. There is no book better adapted to first-aid class
work.

Health : " Here is a book that should find a place in every workshop
and factory and should be made a text-book in our schools."

American Illustrated Medical Dictionary. By W.A.NEWMAN
DORLAND, M. D., Member of Committee on Nomenclature and
Classification of Diseases, American Medical Association. Octavo
of 1107 pages, with 323 illustrations, 119 in colors. Flexible
leather, $4.50 net ; thumb indexed, $5.00 net. Seventh Edition

If you want an unabridged medical dictionary, this is the one you
want. It is down to the minute; its definitions are concise, yet accu-
rate and clear; it is extremely easy to consult; it defines all the newest
terms in medicine and the allied subjects; it is profusely illustrated.
This new edition alone defines over 5000 new terms not defined in any
other medical dictionary — bar none. There is no other medical dic-
tionary that will meet your needs as well as The American Illustrated.
Why not then get the best ?

John B. Murphy, M. D., Northwestern University: "It is unquestion-
ably the best lexicon on medical topics in the English language, and,
with all that, it is so compact for ready reference."

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