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THE PRINCIPLES OF

ANIMAL NUTRITION".

WITH SPECIAL REFERENCE TO THE
NUTRITION OF FARM ANIMALS.

BY

HENRY PRENTISS AEMSBY, Ph.D., LL.D.,

Director of the Institute of Animal Nutrition of The Pennsylvania State College;
Expert in Animal Nutrition, United States Department of Agriculture.

THIRD EDITION, REVISED.
FIRST THOUSAND.

NEW YORK

JOHN WILEY & SONS, Inc.

London: CHAPMAN" & HALL, Limited

1914

Copyright, 1903, 1908,

BY

HENRY P. ARMSBY.

THE SCIENTIFIC PRESS

ROBERT CRilMMOiD AND COMPANY

BROOKLYN, N. Y.

PREFACE.

The past two decades have not only witnessed great activity in
the study of the various problems of animal nutrition, but they are
especially distinguished by the new point of view from which these
problems have come to be regarded. Speaking broadly, it may be
said that to an increasing knowledge of the chemistry of nutrition
has been added a clear and fairly definite general conception of the
vital activities as transformations of energy and of the food as
essentially the vehicle for supplying that energy to the organism.
This conception of the function of nutrition has been a fruitful
one, and in particular has tended to introduce greater simplicity and
unity into thought and discussion. Much exceedingly valuable
work has been done under its guidance, while it points the way
toward even more important results in the future. The following
pages are not a treatise upon stock-feeding, but are an attempt to
present in systematic form to students of that subject a summary of
our present knowledge of some of the fundamental principles of ani-
mal nutrition, particularly from the standpoint of energy relations,
with special reference to their bearings upon the nutrition of farm
animals. Should the attempt at systematization appear in some
instances premature or ill-advised, the writer can only plead that
even a temporary or tentative system, if clearly recognized as such,
may be preferable to unorganized knowledge. The scaffolding
has its uses, even though it form no part of the completed building.

The attentive reader, should there be such, will not fail to note
that the work is limited to those aspects of the subject included
under the more technical term of "The Statistics of Nutrition,"
an.1 that even in this restricted field some important branches of
the subject have been omitted on account of what has seemed to

iv PREFACE.

the writer a lack of sufficient accurate scientific data for their profit-
able discussion. Moreover, many principles which are already
familiar have been considered rather cursorily in order to allow a
more full treatment of less well-known aspects of the subject, even
at the expense of literary proportion.

The substance of this volume was given in the form of lectures
before the Graduate Summer School of Agriculture at the Ohio
State University in 1902, and has been prepared for publication at
the request of instructors and students of that school. In thus
presenting it to a somewhat larger public the author ventures to
hope that it may tend in some degree to promote the rational study
of stock-feeding and to aid and stimulate systematic investigation
into both its principles and practice.

State College, Pa., November, 1902.

CONTENTS.

PAOB

Introdttctton 1

The Statistics of Nutrition 3

PART I.
THE INCOME AND EXPENDITURE OF MATTER.

CHAPTER I.

The Food 5

CHAPTER II.

Metabolism 14

§ 1. Carbohydrate Metabolism 17

§ 2. Fat Metabolism 29

§ 3. Proteid Metabolism 38

Anabolism 3S

Katabolism 41

The Non-proteids 52

CHAPTER III.

Methods of Investigation 59

CHAPTER IV.

The Fasting Metabolism 80

§ 1. The Proteid Metabolism 81

§ 2. The Total Metabolism 83

CHAPTER V.

The Relations of Metabolism to Food-supply 93

§ 1. The Proteid Supply 94

Effects on Proteid Metabolism 94

Effects on Total Metabolism 104

Formation of Fat from Proteids. . „ 107

v

CONTENTS. 0

PAoa

§2. The Non-nitrogenous Nutrients 114

Effects on Proteid Metabolism 114

The Minimum of Proteids 133

Effects on Total Metabolism . 144

Mutual Replacement of Nutrients 148

Utilization of Excess — Sources of Fat 162

CHAPTER VI.

The Influence of Muscular Exertion upon Metabolism 185

§ 1. General Features of Muscular Activity 185

Muscular Contraction 185

►Secondary Effects of Muscular Exertion 191

§ 2. Effects upon Metabolism 193

Upon the Proteid Metabolism 194

Upon the Carbon Metabolism 209

PART II,
THE INCOME AND EXPENDITURE OF ENERGY.

CHAPTER VII.
Force and Energy 226

CHAPTER VIII
Methods of Investigation 234

CHAPTER IX.
The Conservation of Energy in the Animal Body 258

CHAPTER X.

The Food as a Source of Energy — Metabolizable Energy 209

§ 1. Experiments on Carnivora 272

§ 2. Experiments on Man 277

§ 3. Experiments on Herbivora 281

Metabolizable Energy of Organic Matter 284

Total Organic Matter 285

Digestible Organic Matter 297

Energy of Digest ible Nutrients 302

Gross Energy 302

Metabolizable Energy 310

CONTENTS. Vil

CHAPTER XI.

PAQK

Internal, Work 336

§ 1. The Expenditure of Energy by the Body 336

§ 2. The Fasting Metabolism 340

Nature of Demands for Energy 340

Heat Production 344

Influence of Thermal Environment 347

Influence of Size of Animal 359

§ 3. The Expenditure of Energy in Digestion and Assimilation 372

CHAPTER XII.

Net Available Energy — Maintenance 394

§ 1 . Replacement Values 396

§ 2. Modified Conception of Replacement Values 405

§ 3. Net Availability 412

Determinations of Net Availability 413

Discussion of Results 430

Influence of Amount of Food 430

Character of Food 431

The Maintenance Ration 432

CHAPTER XIII.

The Utilization of Energy 444

§ 1. Utilization for Tissue Building 448

Experimental Results 448

Discussion of Results 465

Influence of Amount of Food 466

Influence of Thermal Environment 471

Influence of Character of Food 472

The Expenditure of Energy in Digestion, Assimilation

and Tissue Building „ . 491

§ 2 Utilization for Muscular Work 494

Utilization of Net Available Energy 497

The Efficiency of the Animal as a Motor 498

Conditions determining Efficiency 511

Utilization of Metabolizable Energy 525

Wolff's Investigations 52H

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THE PEINCIPLES OF ANIMAL NUTKITION.

INTRODUCTION.

The body of an animal, regarded from a chemical point of view,
consists of an aggregate of a great variety of substances, of which
water, protein, and the fats, with smaller amounts of certain
carbohydrates, largely predominate. By far the greater portion of
the substance of the body, aside from its water, consists of so-
called "organic" compounds; i.e., compounds of carbon with hydro-
gen, oxygen, nitrogen, and, to a smaller extent, with sulphur and
phosphorus. These compounds are in many cases very complex,
and all of them have this in common, that they contain a con-
siderable store of potential energy.

It is through these complex " organic " compounds that the phe-
nomena of life are manifested. All forms of life with which we are
acquainted are intimately associated with the conversion of com-
plex into simpler compounds by a series of changes which, regarded
as a whole, partake of the nature of oxidations. During this break-
ing down and oxidation more or less of the potential energy of these
compounds is liberated, and it is this liberation of energy which is
the essential end and object of the whole process and which, if not
synonymous with life itself, is the objective manifestation of life.
This is equally true of the plant and the animal, although masked
in the case of green plants by the synthetic activity of the chloro-
phyl in the presence of light. The process is most manifest in the
animal, however, both on account of the inability of the latter to
utilize the radiant energy of the sun and on account of the greater
intensity of the process itself.

Setting aside for the moment any storing up of material, and

4 PRINCIPLES OF ANIMAL NUTRITION.

These topics will be considered in the above order, it being
assumed that the reader is already familiar with the general nature
of the nutritive processes included under the general heads of
digestion, resorption, circulation, respiration, and excretion.

PART I.
THE INCOME AND EXPENDITURE OF MATTER.

CHAPTER I.
THE FOOD.

The supply of matter to the body is; of course, contained in the
food, including water and the oxygen taken up from the air. In
a more limited and familiar sense, the term food is employed to
signify the supply of solid matter, or dry matter, to the animal.
It is proposed here simply to recall certain familiar facts relative
to the composition and digestibility of the food in this narrower
sense, taking up the subject in the barest outline.

Composition. — While a vast number of individual chemical
compounds are found in common feeding-stuffs, the conventional
scheme for their analysis unites these substances into groups and
regards feeding-stuffs as composed, aside from water and mineral
matter, essentially of protein, carbohydrates and related bodies,
and fats. Or, setting aside the mineral ingredients, the "organic"
ingredients may be divided into the nitrogenous, comprised under
the term protein, and the non-nitrogenous, including the fats and
carbohydrates.

Proteix. — The name "protein" originated with Mulder, who
used it to designate what he supposed to be a common ingredient
of all the various proteids, but it has since come to be employed as a
group name for the nitrogenous ingredients both of feeding-stuffs
and of the animal body.

The amount of protein in feeding-stuffs we have at present no

5

6 PRINCIPLES OF ANIMAL NUTRITION.

means of determining directly, but it is commonly estimated from
the amount of nitrogen upon two assumptions: first, that all the
substances of the protein group contain 16 percent, of nitrogen, and
second, that all the nitrogen of feeding-stuffs exists in the proteid
form. On the basis of these assumptions, protein is, of course,
equal to total nitrogen X 6.25.

Although it was never claimed that this method of estimating
protein was strictly accurate, it was for a long time assumed that
the two sources of error involved were not serious. Later investi-
gations, however, have dispelled this pleasing illusion. Further
investigations of the true proteids, notably those of Ritthausen and
of Osborne, have shown a very considerable variation in the per-
centage of nitrogen contained in them, while, on the other hand, the
researches of Scheibler, E. Schulze, Kellner, and others have shown
the presence in many feeding-stuffs of relatively large amounts of
nitrogenous matters of non-proteid nature. The results of these
latter investigations have made it necessary to subdivide the total
nitrogenous matter of feeding-stuffs into two groups, called respec-
tively "proteids" and "non-proteids," while the name "protein"
has been retained in the sense of total nitrogen X 6.25 or other con-
ventional factor. For various classes of human foods, Atwater and
Bryant * propose the following factors, based on the results in-
dicated in the next two paragraphs, for the computation of protein
from nitrogen:

Animal foods 6 . 25

Wheat, rye, barley, and their manufactured products 5 . 70
Maize, oats, buckwheat and rice, and their manufactured

products 6 . 00

Dried seeds of legumes 6 . 25

Vegetables 5 . 65

Fruits 5.80

Proteids. — In the absence of any adequate knowledge regarding
the very complex molecular structure of the proteids, both the
classification and the terminology of these bodies are in a very con-
fused state. For convenience, however, we may adopt here those

* Storrs (Conn.) Ag. Ex. St., Rep. 12, 79.

THE FOOD. 7

tentatively recommended by the Association of American Agri-
cultural Colleges and Experiment Stations,* viz. :

i Albumins,
f Simple -j Globulins,

f Albuminoids \ ( and allies-

Protein. Total { proteM ! . Modified \ derived.

nitrogen com- j L ( Compound,
pounds J [ Collagens or gelatinoids

tvt™ ~~„+„iA, S Extractives,
[ Non-Proteidsi Amides, amido-acids, etc.

It is not necessary for our present purpose to enter into any dis-
cussion either of the properties of the proteids as a whole or of the
differences between the different classes of proteids. One point,
however, is of particular importance, namely, the elementary com-
position of these bodies. As noted above, this has been found to be
more variable than was supposed earlier. In particular the per-
centage of nitrogen has been found to have a somewhat wide range.
"Recent investigations with perfected methods show percentages
of nitrogen in the numerous single proteid substances found in the
grains ranging from 15.25 to 18.78. These are largest in certain
oil seeds and lupines and smallest in some of the winter grains.
Ritthausen,f a prominent German authority, concedes that the
factor 6.25 should be discarded, and suggests the use of 5.7 for the
majority of cereal grains and leguminous seeds, 5.5 for the oil and
lupine seeds, and 6.00 for barley, maize, buckwheat, soja-bean, and
white bean (Phaseolus) rape, and other brassicas. Nothing short
of inability to secure greater accuracy justifies the longer contin-
uance of a method of calculation which is apparently so greatly
erroneous." (Jordan.)

Non~proteids. — This term is used as a convenient designation
for all the nitrogenous materials of feeding-stuffs which are not
proteid in their nature. It is an abbreviated form of non-proteid
nitrogenous bodies. The substances of this class found in plants
are chiefly the organic bases, amides, amido-acids, and similar
bodies which are produced by the cleavage of the proteid molecule
under the action of digestive and other ferments or of hydrating
agents. They appear to exist in the plant partly as intermediate
stages in the synthesis of the proteids and partly as products of

* U. S. Dept. Agr., Office of Experiment Station, Bui., 65, p. 117.
tLandw. Vers. Stat., 47, 391.

PRINCIPLES OF ANIMAL NUTRITION.

their subsequent cleavage in the metabolism of the plant. They
are chiefly soluble, crystalline bodies. The most common of them
is asparagin, which has been, to a certain extent, regarded as
typical of the group.

The non-proteids are commonly determined by determining as
accurately as possible the non-proteid nitrogen and multiplying the
latter by the factor 6.25. In the case of asparagin, however, which
contains 21.2 per cent, of nitrogen, the proper factor obviously
should be 4.7, while the factor would vary for the different forms of
non-proteids which have been observed in plants. It is no simple
matter, therefore, either to determine directly the amount of non-
proteids or to decide upon the proper nitrogen factor in any partic-
ular case. For the present, however, the factor 4.7 would seem
to be at least a closer approximation to the truth than 6.25.

In the animal body the group of non-proteids is represented by
the so-called ''extractives" or "flesh bases" of the muscle, chiefly
creatin and creatinin.

Fats. — The fats of the plant, like those of the animal, consist
chiefly of glycerin compounds of the so-called "fatty acids," or of
similar bodies. These are accompanied in the plant, however, by
other materials — wax, chlorophyl, etc. — which are extracted along
with the fat by the common method of determination and consti-
tute part of the "crude fat" or "ether-extract." The results,
therefore, which have been obtained in feeding experiments with
pure fats cannot be used with safety as a basis for estimating the
nutritive value of the so-called "fat" of feeding-stuffs, particularly
in the case of coarse fodders.

Carbohydrates. — The well - characterized group of carbo-
hydrates makes up a large proportion of the organic matter of our
more common feeding-stuffs. Those members of this group occur-
ring in any considerable quantities in feeding-stuffs may be sub-
divided on the basis of molecular structure into the hexoses,on the
one hand, whose molecules contain six atoms of carbon or a mul-
tiple of that number, and the pentoses, or five carbon series, on the
other. In the grains and other common concentrated feeding-stuffs,
and particularly in the food of man, the hexose group largely pre-
dominates, including starch, dextrin, the common sugars, and more
or less cellulose. In the coarse fodders consumed by our domestic

THE FOOD. 9

herbivorous animals, while the hexose group is also largely repre-
sented it is accompanied by no inconsiderable quantities of carbo-
hydrates belonging to the pentose group. The individual members
of this latter group are both less abundant and less well known
chemically than the hexoses, and at present our knowledge of their
actual nutritive value is somewhat scanty. Since the methods for
their determination are based upon the fact that they yield furfural
upon boiling with dilute hydrochloric acid, some recent analysts
have proposed the term " furfuroids " as a more appropriate desig-
nation of these substances as determined by present methods.

In the conventional scheme for the analysis of feeding-stuffs, the
carbohydrates are subdivided, not upon the basis of their chemical
structure but upon the basis of their solubility. Those members
of the group which can be brought into solution by boiling dilute
acids and alkalies under certain conventional conditions are grouped
together as " Xitrogen-free extract," while those ingredients
which resist solution under these conditions are designated as
"Crude fiber." The more common hexose carbohydrates, such as
starch, sugars, etc., are included in the nitrogen-free extract, while
the larger part, although not all, of the cellulose is included under
the crude fiber. At the same time, more or less of the pentose carl »o-
hydrates or "furfuroids" are found in both these groups, while the
crude fiber of coarse fodders contains also a variety of other ill-
known compounds, somewhat roughly grouped together under the
general designation of ligneous material.

Digestibility. — A part of nearly all common food materials is
incapable of digestion and is rejected in the feces. In the food of
man and that of carnivorous animals this indigestible portion is
usually small and may disappear entirely. In the food of herbivora,
on the other hand, there are contained relatively large amounts of
substances which are incapable of solution in the digestive tract,
while varying proportions of materials which in themselves are
capable of being digested may escape actual digestion under some
circumstances. In the latter animals, therefore, it becomes par-
ticulary important to determine the digestible portion of the food.
The digestibility of a feeding-stuff is estimated indirectly by deter-
mining as accurately as possible the undigested matter eliminated
from the body in the feces and subtracting it from the total amount

io PRINCIPLES OF ANIMAL NUTRITION.

contained in the food. This method may of course be applied
either to the dry matter or the organic matter of the food as a
whole or to any single_ determinable ingredient.

Metabolic Products. — The digestive tract of an animal, how-
ever, not only serves as a mechanism for the digestion of food but
has excretory functions as well, and the rejected matter contains,
besides the undigested portion of the food, these excreta and the
metabolic products of intestinal action. In the case of food largely
or completely digestible, these substances may make up the larger
portion or even the whole of the feces, while, on the other hand,
they constitute but a small proportion of the bulky excreta of
herbivora.

It is obvious that these products must be taken account of if it is
desired to learn the actual digestibility of the food. Unfortunately,
however, we have at present no trustworthy method for their deter-
mination. In the past it has been customary to designate the
difference between food and feces as digestible and, in the case of
domestic animals at least, to assume that the error involved is not
serious.

Apparent Digestibility — Availability. — Even with herbivo-
rous animals, however, the presence of the so-called metabolic
products in the feces may give rise to serious errors in the deter-
mination of the real digestibility of some ingredients of the food,
notably fat and protein. With carnivora, or with the human
subject, the case is for obvious reasons still worse, and it is
scarcely possible to determine the digestibility of the food in the
strict sense of the word.

The difference between food and feces does represent, however,
the net gain of matter to the organism resulting from the digestion
of the food. To express this conception, the use of the word avail-
able has been proposed by Atwater.* The "available nutrients" of
a food, according to him, are the actually digestible nutrients minus
the metabolic products contained in the feces and which may be
regarded as representing the expenditure of matter, in the form of
residues of digestive fluids, intestinal mucus, epithelium, etc.,
necessarily incident to the digestion of the food. The term has been

* Storrs (Conn.) Agr'] Expt. St., Rep., 12, 69.

THE FOOD. II

used chiefly in connection with human nutrition. In discussions of
animal nutrition the terms digestible and digestibility have become
so firmly established that it may be questioned whether the intro-
duction now of a new term would not create more confusion than it
would prevent, and whether it is not preferable, when strict accuracy
of expression is required, to attach a modifjdng word and designate
the difference between food and feces as apparently digestible, in
distinction from the real digestibility, which we cannot as yet deter-
mine.

Determination of Apparent Digestibility. — The determi-
nation of the apparent digestibility of the nutrients of a feeding-
stuff in the above sense, or of their " digestibility " in the older sense,
consists simply in determining the amount of the feces or of their
separate ingredients and comparing them with the correspond-
ing amounts in the food.

Aside from ordinary analytical precautions, the chief condition
of accurate results is that the feces correspond to the food consumed.
In animals with a comparatively simple digestive canal, like man
and the carnivora, this is readily brought about by the ingestion of
a small amount of some substance like powdered charcoal or infu-
sorial earth, which is in itself indigestible and which serves to sepa-
rate the feces of two successive periods. In the case of herbivora,
on the other hand, the undigested residues of the food become mixed
to a large extent with those of the previous period. In this case,
therefore, it is essential that a preliminary feeding be continued for
a sufficient length of time to remove the residues of previous foods
from the digestive organs, and further that the experiment itself
extend through a number of days in order to eliminate the
influence of irregularity of excretion.

Significance of Results. — It is plain from what has just
been said that what the results of such an experiment actually
show is that a certain amount of material has disappeared from
the food during its transit through the alimentary canal. This
fact of itself, however, does not necessarily show that the missing
material has been digested in any true sense. In the case of animals
possessing a relatively short and simple digestive apparatus, we are
probably justified in assuming that the difference between food and
undigested matter represents material that has actually been

12 PRINCIPLES OF ANIMAL NUTRITION.

digested. In the long and complicated digestive apparatus of
herbivora, however, there is the possibility that a variety of proc-
esses may go on aside from a simple solution of nutrients by the
digestive fluids. In particular, it has been shown, as will appear in
greater detail later, that extensive fermentations, particularly of the
carbohydrates, occur, and that relatively large amounts of these
bodies may be destroyed in this way.

Furthermore, with our present conventional scheme for fodder
analysis, we have to take account of the possibility of the conversion
of members of one group of nutrients into those of another. For
example, it seems not improbable that a portion of the crude fiber
of feeding-stuffs may be so modified in the digestive tract, without
being actually dissolved, that, in the feces, it is determined as
nitrogen-free extract, thus diminishing the apparent digestibility
of the latter group and increasing that of the crude fiber.*

Composition of Digested Food. — The proteids during the
process of digestion are largely converted into proteoses and pep-
tones, while the trypsin of the pancreatic juice, at least outside the
body, carries the cleavage of the proteid molecule still further and
gives rise to comparatively simple, crystalline bodies. It is not
altogether clear to what extent this degradation of the proteids
occurs in natural digestion, but the probability appears to be that
it does not play a large part, and it has been generally believed that
the proteids are resorbed chiefly as proteoses and peptones. f

The non-proteids being largely crystalline bodies and readily
soluble, we may presume that they are resorbed without material
change except so far as they may serve as nitrogenous food for the
micro-organisms of the digestive tract.

The fat of the food does not undergo any profound change in
digestion, but is claimed to be resorbed largely in the form of an
emulsion. A part of it, however, is undoubtedly saponified by
the bile, although the extent to which this process takes place is a
disputed point, while in some cases at least a cleavage into glycerin
and free fatty acids appears to take place.

The carbohydrates, particularly the easily soluble members of
the hexose group, are in the case of man and the carnivora, and

* Cf. Fraps, Jour. Am. Chem. Soc, 22, 543. t See note, p. 58.

THE FOOD. 13

probably also to a large extent in the swine and horse, converted
into sugars and resorbed in that form.

Ft rmt ntatiojis. — Reference has already been made to the fermen-
tations taking place in the digestive tract. In the herbivora, and
especially in ruminants, these fermentations play an important
part in the solution of the carbohydrates which make up so large
a portion of the food of these animals. These bodies undergo a
fermentation which was first studied by Tappeiner * in the case of
cellulose, but which has since been shown by G. Kiihn f to extend
also to the more soluble carbohydrates. The products of this
fermentation appear to be methane, carbon dioxide, and organic
acids, chiefly, according to Tappeiner, acetic and butyric. Of these
products, only the organic acids at best can be supposed to be of
any value to the animal organism, and obviously it makes a very
serious difference in our estimate of the nutritive value of starch,
for example, whether it is resorbed chiefly or entirely in the form of
sugar or whether in a ruminant more than half of it, as in some of
Kuhn's experiments, is fermented.

* Zeit. f. Biol., 20, 52. f Landw. Vers. Stat., 44, 569.

CHAPTER II.
METABOLISM.

General Conception. — By the various processes of digestion
and resorption the epithelium of the alimentary canal extracts from
the crude materials eaten those ingredients which are fitted to
nourish the animal and transmits them more or less directly to the
general circulation which carries them to all the tissues of the body.
While these ingredients are many in number and diverse in charac-
ter, yet the vast mass of them, aside from the water in which most
of them are dissolved, may be grouped under six heads, viz., ash
ingredients, albuminoids or bodies related to the albuminoids,
amides and other crystalline nitrogenous substances, fats, carbo-
hydrates, and organic acids, and these, together with relatively
small amounts of other materials, may be regarded as constituting
the real food of the organism.

As was pointed out in the Introduction, the cells of which the
living tissues of the animal body are composed are the seat of con-
tinual chemical change. On the one hand, the digested ingredients
of the food which are brought to them by the circulation are being
built up into the structure of the body. On the other hand, the
material of the cells is undergoing a continual process of breaking
down and oxidation, uniting with the oxygen supplied by the blood
to form the waste products which are removed from the body
through the organs of excretion. These excretory products are
substantially carbon dioxide, water, and urea and similar nitroge-
nous substances.

The general term Metabolism is commonly used to designate the
totality of the chemical and physical changes which the materials
of the resorbed food, or of the tissues formed from them, undergo in
being converted into the excretory products. Similarly, we may
speak in a more restricted sense of the metabolism of a single ingrc

14

METABOLISM. 15

client of the food, as of the proteids, carbohydrates, or fats. Thus
proteid metabolism signifies the chemical changes undergone by the
proteids of the food in their conversion into the corresponding
excretory products. In ordinary usage the chemical reactions
undergone by the ash ingredients of the food are not included, the
word metabolism being practically used to designate the chemical
changes in the organic matter of food or tissue.

Metabolism a Process of Oxidation. — The process of met-
abolism as a whole is one of oxidation. While we must beware
of being misled by analogy into regarding as a simple burning of
food-materials that which is in reality a highly complex action of
the living cells of the organism, still the final result is much the
same in both cases. Starting with more or less complex organic
substances and oxygen, we end either with the completely oxidized
compounds carbon dioxide and water or with nitrogenous sub-
stances like urea more highly oxidized than the protein from which
they are derived.

The oxidative character of the total metabolism is most simply-
illustrated by a comparison of the percentage of oxygen contained
in the most prominent ingredients of the food, on the one hand, and
in the chief excretory products, on the other hand, as in the follow-
ing statement :

Percentage of Oxygen.
In food:

Protein (average) 23.00

Fats 11.50

Dextrose 53 . 33

In excreta :

Urea 26.67

Carbon dioxide 72 . 72

Water 88.89

Metabolism ax Analytic Process. — From a slightly different
point of view, metabolism may be described as an analytic process.
Tho molecules of the food constituents are highly complex. The
molecule of dextrose or IedvuIosc, the forms in which the carbo-
hydrates are chiefly resorbed, contains 24 atoms; the molecules of

1 6 PRINCIPLES OF ANIMAL NUTRITION.

the three most common fats, respectively 155, 167, and 173 atoms.
The molecular structure of the proteids has not yet been made out,
but it is highly complex.* The molecules of the excretory prod-
ucts, on the contrary, are comparatively simple, those of carbon
dioxide and water containing but three atoms each, that of urea
eight, and even that of hippuric acid but twenty-two.

In metabolism, in other words, the complex molecules of the
carbohydrates, fats, proteids, etc., which have been built up in the
plant, by means of the energy contained in the sun's rays, out of
carbon dioxide, water, and nitric acid or ammonia, gradually break
down again into simpler compounds, their atoms reuniting with
the oxygen from which they were separated in the plant.

Metabolism a Gradual Process. — The chemical changes in-
cluded under the term metabolism take place gradually. As has
already been indicated, metabolism is not a simple oxidation of
nutrients, like the burning of fuel in a stove, but the nutrients enter,
to a large extent at least, into the structure of the cells of which the
various tissues are composed. Metabolism is really the sum of the
chemical actions through which the nutrition and life of these cells
is manifested. These actions, however, differ from tissue to tissue
and from cell to cell, and even in the same cell from time to time,
and the resulting metabolic products are correspondingly varied.
Between the nutrients supplied to the cells by the blood and the
final products of metabolism as excreted from the body there are
innumerable intermediate products, a few of which we know but
concerning most of which we are still ignorant. We know the first
and last terms of the series and thus are able to measure, as it
were, the algebraic sum of the changes, but of the single factors
making up this sum. as well as of the specific tissues concerned in
the changes, we are largely ignorant, although we know that they
are numerous.

Anabolism and Katabolism. — While the process of metab-
olism as a whole is one of analysis and oxidation, with liberation
of energy, it must not be supposed that each single step in the
process is of this nature. As has been already pointed out, the
chemical activities of the tissues possess a dual character. By the

* Osborne (Zeit. physiol. Chem., 33, 240) has recently obtained the
number 14,500 as the approximate molecular weight of edestin.

METABOLISM. 17

various processes of nutrition, ingredients of the food are first incor-
porated into the tissues of the body, to be subsequently broken
down and oxidized. In this building-up process changes undoubt-
edly occur in the direction of greater complexity of molecular struc-
ture, involving the temporary absorption of energy. Thus it is
known that fats ma}* be formed from carbohydrates in the body.
Many physiologists hold that the metabolism in the quiescent muscle
results in the building up of a complex "contractile substance,"
whose breaking down furnishes the energy for muscular work. In
general, we may regard it as highly probable that the molecules of
the living substance of the body are much more complex than those
of the nutrients of the food, and that the former are built up out of
the latter by synthetic processes, carried on at the expense of energy
derived from the breaking down of other molecules. Such changes
as this are called anabolic and the process anabolism, w*hile the
changes in the direction of greater simplicity of molecular structure
are called katabolic, and the process katabolism. The metabolism
of the living body, then, consists of both anabolism and katabolism.
By the former the food nutrients are built up into body material;
by the latter they are broken down, yielding finally the compara-
tively simple excretory products. On the wrhole, however, the
katabolism prevails over the anabolism, so that metabolism as a
whole is, as already stated, an analytic and oxidative process.
Neither the anabolism of tissue production nor the minor anabolic
changes which seem to occur in various tissues alter the main direc-
tion of the metabolic changes in the body, but, from the standpoint
of the statistics of nutrition, are simply eddies in the main current.

§ 1. Carbohydrate Metabolism.

HEXOSE CARBOHYDRATES.

The hexose carbohydrates of the food appear to be resorbed
chiefly by the capillary blood-vessels of the intestines. For the
most part, they reach the blood in the form of dextrose, with smaller
amounts of lsevulose and galactose, and with greater or less
quantities of acetic, butyric, lactic, and other acids derived chiefly
from the fermentation of the carbohydrates in the digestive
tract.

1 8 PRINCIPLES OF ANIMAL NUTRITION.

The percentage of dextrose in the blood is small, but remarkably-
constant, the limits of variation being from about 0.11 to about 0.20
per cent., and the average about 0.15 per cent. Its amount
varies but slightly in different regions of the body, and in different
classes of animals, and is scarcely at all affected by the nature or
amount of the food. Not only so, but any excess of dextrose in the
blood is promptly gotten rid of. It is a striking fact that if any con-
siderable amount of this substance, which forms so large a part of
the resorbed nutriment, be injected directly into the blood it is
treated as an intruder and at once excreted through the kidneys.
Evidently it is of the greatest importance to the organism that the
supply of this substance to the tissues shall be constant.

Under ordinary conditions, however, the influx of sugar from the
digestive tract is more or less intermittent. After a meal rich in
easily digestible carbohydrates, an abundant supply of it is taken
up by the intestinal capillaries, while on a diet poor in carbohydrates
or in prolonged fasting, the supply sinks to a minimum. This is, of
course, especially true of animals like man and the carnivora in
which the process of digestion is comparatively rapid, but even in
herbivorous animals, with their more complicated digestive appara-
tus, the rate of resorption of dextrose, and still more its absolute
amount, must be more or less fluctuating. Evidently there must be
some regulative apparatus which holds back from the general circu-
ation any excess of dextrose, on the one hand, and prevents its
being excreted unused, and on the other, supplements any lack
resulting from a deficiency of the food in carbohydrates. This
regulation is accomplished by the liver.

Functions of the Liver.

The functions of the liver in this regard appear to be twofold:
First, it manufactures dextrose and supplies it to the general circu-
lation; and second, it serves as a reservoir, or a place of deposit, for
any excess of carbohydrates supplied by the digestive apparatus.

The Liver as a Source of Dextrose. — The blood as it
comes from the intestinal capillaries, bearing the digested carbo-
hydrates and proteids, enters the liver through the portal vein and
is distributed by means of the capillary blood-vessels into which this
vein divides through all parts of that organ, reaching the general

METABOLISM. 19

circulation again through the hepatic vein. In its passage through
the capillaries of the liver, the blood is subjected to the action of the
cells of the liver (hepatic cells). Our knowledge of the exact nature
of this action is still more or less conjectural, in spite of n vast
amount of experimental investigation, but certain general facts are
pretty clearly established.

In the first place, the hepatic cells appear to serve as a source of
dextrose when no carbohydrates are supplied in the food. If a
carnivorous animal be given a diet as free as possible from carbo-
hydrates, as. for instance, prepared lean meat, consisting substan-
tially of proteids, its blood still contains a normal amount of dex-
trose and the blood in the hepatic vein is found to be richer in
dextrose than that of the portal vein, showing that this substance
is being formed in the liver. Moreover, while the percentage of
dextrose in the blood is small, the total amount thus manufactured
is very considerable. Seegen * estimates it at about one per cent,
of the weight of the body in twenty -four hours. This is regarded
by many physiologists as an overestimate, the considerable differ-
ences in sugar content between the portal and hepatic blood found
by Seegen being regarded as in part the effect of the necessary
operation. Indeed, it is questioned by some whether any actual
difference in sugar content between the portal and hepatie blood
under normal conditions has been satisfactorily established analyti-
cally, but the indirect evidence at least seems strongly in its favor.

In the second place, the same outflow of dextrose from the liver
appears to take place when the animal consumes a mixed diet con-
taining carbohydrates. In this case also, except shortly after a
meal containing much carbohydrates, the blood of the hepatic vein
shows an excess of dextrose over that of the portal vein. The
amount of dextrose thus introduced into the circulation is sub-
stantially the same as in the first case, and its percentage in the
blood is not perceptibly altered. The source of this dextrose, how-
ever, is not so simple a question, since it is possible that all or a
considerable portion of it may be supplied directly or indirectly
by the dextrose resorbed by the intestinal capillaries.

Granting the continual production of sugar by the liver, sub-

* Die Zuckerbildung im Thierkorper, p. 115.

20 PRINCIPLES OF ANIMAL NUTRITION.

stantially two suppositions are open: On the one hand, we may
consider that the resorbed carbohydrates of the food, after being
temporarily stored up in the liver, as described below, are given off
again without radical change and that the sugar-forming power of
the hepatic cells is limited to the transformation of the proteids and
perhaps the fats of the food. Or, on the other hand, we may sup-
pose that the nutrients brought to the liver by the portal blood
enter into the constitution of the protoplasm of the hepatic cells,
and that the vital activity of this protoplasm gives rise to the dex-
trose found in the blood, to the glycogen found in the liver, and to
other products of whose nature we are largely ignorant. The
evidence at hand is doubtless insufficient for a final decision between
these alternatives, but the latter hypothesis would seem more in
accord with our general knowledge of cell activity. As relates to
the carbohydrates, it is supported by the fact that while various
sugars besides dextrose (lsevulose, mannose, galactose, sorbinose,
and, as Munch * has shown, certain artificial hexoses) may be con-
verted into glycogen, the resulting glycogen is always the same and
the product of its hydration is always dextrose. f In other words,
the molecular structure of these sugars is altered in a manner sug-
gesting an assimilation by the hepatic cells rather than anything
resembling an enzyme action. The subject can be more intelli-
gently considered, however, in the light of a discussion of the second
function of the liver.

The Liver as a Reservoir of Carbohydrates. — When the
food is rich in carbohydrates, the supply of dextrose to the blood
through the intestinal capillaries is more or less intermittent. As
a means of regulating this intermittent supply, the hepatic cells
have the power of arresting the dextrose brought to them
by the portal vein and converting it into a polysaccharide called
" glycogen " or " animal starch " which is stored up in the liver. On
the other hand, when the supply of carbohydrate food is cut off,
and especially if all food be withdrawn, the glycogen of the liver
rapidly diminishes, being apparently reconverted into dextrose.
This latter phenomenon may be readily observed in the liver of a
freshly killed animal. If the fresh liver, after removal from the

*Zeit. physiol. Chem . 29, 493.

f Compare Neumeister, Physiologische Chemie, p. 326

METABOLISM. 21

body, be washed out by water injected through the portal vein till
all sugar is removed, and if then, after standing for a time, the wash-
ing be renewed, the first portions of water that pass contain sugar.
The same process may be repeated several times.

What is known as the glycogenic function of the liver was dis-
covered by Claude Bernard in 1853, and has been the subject of a
bewildering amount of discussion and controversy, both as to the
origin of glycogen, its finai fate, and its relations to the production
of dextrose by the liver. Certain facts, however, may be regarded
as established with at least a high degree of probability :

First — The liver produces glycogen from dextrose and other
(not all) carbohydrates, as above described.

Second — The liver seems also to form glycogen from proteids,
since this substance is found in considerable quantity in the livers
of animals fed exclusively on meat.

Third — Glycogen largely disappears from the liver during fast-
ing, and to a considerable degree also in the absence of carbo-
hydrates from the food.

Fourth — The liver produces dextrose at an approximately con-
stant rate, largely independent of the food-supply or the variations
in the store of glycogen.

These facts seem to point unmistakably to the sugar-producing
function of the fiver as the primary factor in the whole matter. The
general metabolism of the body requires a constant proportion of
dextrose in the blood, and as this dextrose is consumed the liver
furnishes a fresh supply. This supply it manufactures from the
materials brought to it by the blood of the portal vein. When
carbohydrates are lacking in this blood, it apparently has the power
of breaking down the proteids and perhaps the fats, thus supplying
the needful dextrose. Some authorities claim that the same process
goes on when carbohydrates are present, and it seems not unlikely
that this is true, but when the food-supply consists so largely of
carbohydrates as it does in the case of our domestic herbivorous
animals, the conclusion seems unavoidable that at least a consider-
able part of the dextrose consumed in the body must be derived
from these substances. As already suggested, a very plausible view
of the matter is to regard the resorbed nutrients of the portal blood
as serving to feed the protoplasm of the hepatic cells and to look

22 PRINCIPLES OF ANIMAL NUTRITION.

upon the dextrose as one of the products of the metabolism of
those cells.

Since, however, the demands of the organism for dextrose and
the supply of it, or of the materials for its manufacture, in the food
do not keep pace with each other, sometimes one and sometimes the
other being in excess, the liver has a second function. When the
food-supply, of whatever kind, is in excess, instead of continuing to
produce dextrose the metabolism in the liver takes a slightly differ-
ent form and produces the insoluble glycogen, or perhaps the dex-
trose of the portal blood is simply converted into glycogen without
entering into the structure of the hepatic urotaplasm. When, on
the other hand, the food-supply is deficient, the stored-up glyco-
gen is converted into dextrose; whether by some sort of enzyme
action or by again serving as food for the hepatic protoplasm is
uncertain.

Fate of the Dextrose of the Blood.

The fact that the proportion of dextrose in the blood is approxi-
mately constant, notwithstanding the continual supply which is
received from the liver, shows that there must be a continual abstrac-
tion of dextrose from the blood, which is as continually made good
by the activity of the hepatic cells. In fact, the dextrose of the
blood appears to play a very prominent part in the animal economy,
and the function of the liver in preparing it from other ingredients
of the food is a most important one.

Consumption in the Muscles.— From the point where it leaves
the liver, our knowledge of the metabolism of the dextrose of the
blood is scanty, but a large proportion of it undoubtedly takes
place in the muscles. It was early shown by Chauveau that
the proportion of dextrose in the blood diminishes in its passage
through the capillaries of the body, so that the arterial blood con-
tains more of this substance than the venous. In conjunction with
Kaufmann * he has subsequently shown more specifically that in its
passage through the muscular capillaries and through those of the
parotid gland the blood is impoverished in dextrose, and to a much
greater extent in the active than in the quiescent muscle. Coin-

*Comptes rend., 103, 974 and 1057; 104, 1126 and 1352.

METABOLISM. 23

cident with this disappearance of dextrose, there is an increase in
the carbon dioxide of the blood and a decrease of its oxygen.

The relations of the dextrose of the blood to the evolution of
heat and work in the muscles and other tissues, so far as they are at
present understood, will be considered in a subsequent chapter.
For our present purpose it suffices to note the fact that it disappears
in the capillaries with the ultimate production of carbon dioxide
and water. That the dextrose is immediately oxidized to carbon
dioxide and water, however, is extremely unlikely. It has been
suggested that the lactic acid which is found in the muscle after
muscular contraction is one of the intermediate products of the
oxidation. Several considerations, however, seem to render it
more probable that the dextrose first enters in some way into the
constitution of the muscles, or in other words, that a synthetic or
anabolic process precedes the katabolic one.

Muscular Glycogen. — Another fact, of much interest in this
connection, is that the muscles (and other tissues also), as well as
the liver, contain glycogen. Moreover, the muscular glycogen
diminishes or disappears during work and reappears again after
rest. It would appear, then, that the muscular tissue shares with
the liver the ability to form glycogen. As in the case of the former
organ, the simplest supposition is that this glycogen is produced
from the dextrose supplied in the blood, and Kultz * and others
have shown that subcutaneous injections of sugar give rise to a
formation of muscular glycogen in frogs whose livers have been
removed. On the other hand, of course, the considerations pre-
sented above relative to the sources of the liver glycogen apply,
ceteris paribus, to the formation of glycogen in the muscles. Neither
the source nor the exact functions of the muscular glycogen are
yet beyond controversy, but the facts just stated strongly sug
a storing up of reserve carbohydrates during rest to be drawn upon
when there is a sudden demand for rapid metabolism.

Fat Production. — In addition to its important relation to the
muscles, the dextrose of the blood likewise supplies nourishment
for the fat tissues of the body. Hitherto we have spoken as if the
supply of dextrose to the blood were determined substantially by

*Neumeistfr, Physiologische Chemie, p. 322.

24 PRINCIPLES OF ANIMAL NUTRITION.

the demands of the general metabolism for material to produce heat
and motion. Plainly, however, the capacity of the muscles and
the liver to store up carbohydrates is limited, and if the food-supply
is permanently greater than the demands of the organism, some
other provision must be made for the excess. Under these circum-
stances the superfluous dextrose which finds its way into the blood
gives rise to a production of fat, which is stored up as a reserve in
special tissues and apparently does not enter again into the general
metabolism until a permanent deficiency in the food-supply occurs.
The experimental evidence of the production of fat from carbo-
hydrates, as well as the quantitative relations of the process so far as
they are known, will be considered subsequently. In its relations
to the economy of the organism the process is analogous to the
formation of glycogen in the liver, except that the storage capacity
of the fat tissues is vastly greater, but as compared with the forma-
tion of glycogen it is distinctively an anabolic process, the fat
molecule being more complex and containing more potential energy
than that of dextrose. Hanriot,* assuming the formation of olein,
stearin, and palmitin in molecular proportions, represents the
process by the equation:

13C6H1206 = C55H104Oe + 23C02 + 26H20.

PENTOSE CARBOHYDRATES.

The facts of the foregoing paragraphs relate primarily to the
hexose carbohydrates, particularly starch and sugar, and to a con-
siderable extent to the metabolism of carnivorous animals. The
food of herbivora, however, contains a great variety of carbohy-
drates and especially considerable quantities of the pentose or five-
carbon carbohydrates. That these substances are in part digest-
ible, or that at least a considerable proportion of them disappears
from the food during its transit through the alimentary canal . was
first shown by Stone,f and has since been fully confirmed by tlm
investigations of Stone & Jones J and of Lindsey & Holland, :<
but of their further fate in the body relatively little is known.

* Archives de Physiol. , 1893, 248. % Agricultural Science, 5, 6.

t Amer. Chem. Jour., 14, 9. %Ibid., 8, 172.

METABOLISM. 25

Ebstein,* who was the first to investigate this subject, showed
qualitatively the presence of pentose carbohydrates in the urb?e of
man after the ingestion of arabinose and xylose even in very small
doses, and concluded that these sugars are not assimilable.

Salkowski f shortly afterward observed the appearance of pen-
toses in the urine of rabbits given arabinose after five or six days of
fasting. He found in the urine, however, only about one-fifth of
the amount ingested, together with small amounts in the blood and
larger ones in the muscles, but there was a considerable increase of
the glycogen of the liver. From the latter fact Salkowski con-
cludes that arabinose may be, either directly or indirectly, a source
of glycogen. The glycogen found in his experiment was the ordi-
nary six-carbon glycogen.

Subsequent investigations by Cremer, J Munk,§ FrentzelJ Linde-
mann & May,!" Fr. Voit,** Jacksch,tf Miinch,JJ Salkowski,§§
and others have been directed largely to two questions, viz.,
whether the pentose carbohydrates are oxidized in the body and
whether they serve as a source of glycogen.

Pentoses Oxidized ix the Body. — As the general result of
these investigations, it may be stated that pentoses (in particular
arabinose and xylose), whether administered by the stomach or
injected into the blood, are at least partially oxidized in the "body.
In the human organism the power of oxidizing the pentoses, which
do not normally constitute any considerable portion of its food,
appears to be quite limited, and even when they are given in small
quantities a portion (not aU) is excreted in the urine. In the rabbit
the pentoses seem to be more vigorously oxidized, only about
twenty per cent, being excreted unaltered, even when compara-
tively large doses are given.

In these experiments the pentose sugars were administered in
considerable amounts at once, and the excretion of a portion unal-
tered would seem to be a phenomenon similar to the temporary

* Virohow's Archiv, 129, 401 ; 132, 368. ^ Arch. klin. Med., 56, 283.

tCentralbl. med. Wiss., 1893, p. 193. ** Ibid., 58, 524.

% Zeit, f. Biol., 29, 536; 42, 428. ft Zeit. f. Heilk., 20, 195.

§Centralbl. med. Wiss., 1894, p. 83. It Zeit physiol. Chem., 29, 493.

|| Arch. ges. Physiol., 56 , 273. %%Toid., 32, 393.

2 6 PRINCIPLES OF ANIMAL NUTRITION.

glycosuria caused by large doses of the common sugars. The pen-
tose carbohydrates in the ordinary food of herbivora, however, are
largely or entirely the comparatively insoluble pentosans. As
already stated, these bodies are partially digested — that is, they do
not reappear in the feces. As to the manner of their digestion we
are ignorant. If we are justified in assuming that the digested
portion is converted, wholly or partially, into pentoses, then the
conditions differ from those of the experiments above mentioned
in that the production and assimilation of the pentoses is gradual.
Under these circumstances we might be justified in anticipating
a more complete oxidation of these bodies. To what extent this
is true it is at present impossible to say. Weiske,* in connection
with his investigations upon the digestibility of the pentosans, states
that the urine of the sheep and rabbits experimented upon gave
only a slight reaction for pentoses. The writer has not been able
to find any records of other tests of the urine of domestic animals
for pentoses.

Pentoses as a Source of Glycogen. — Most, although not
all, investigators have found an increase in the glycogen of the liver
consequent upon the ingestion of pentoses, but in every case it has
been the ordinary six-carbon glycogen. This has been commonly
and most naturally interpreted as showing that the pentoses are not
themselves converted into glycogen in the body, but are simply
oxidized in the place of some other material which is the true source
of the observed gain of glycogen. In the light of known facts
regarding the apparent power of the liver to produce glycogen from
very diverse hexoses (see p. 20) it would seem, however, that the
possibility of an actual assimilation of the pentoses by the hepatic
cells should at least be borne in mind.

THE ORGANIC ACIDS.

In addition to such quantities of the organic acids, free and com-
bined, as are contained in their food, relatively large amounts of
these substances are, in the case of herbivorous animals and par-
ticularly of ruminants, produced by the fermentation of the carbo-
hydrates in the alimentary canal. For this reason their meta-

*Zeit. physiol. Chem., 20. 489.

METABOLISM. 27

bolism may properly be considered in connection with that of the
carbohydrates themselves.

But little is known of the metabolism of the organic acids, how-
ever, beyond the fact that they are oxidized in the body, a portion
of the resulting carbon dioxide appearing in the urine, in combina-
tion with sodium and potassium, rendering that fluid alkaline.
Wilsing * and v. Knieriem f have shown that organic acids such as
result from the fermentation of carbohydrates are not found to any
appreciable extent in the excreta, while the researches of Munk J
and Mallevre,§ which will be considered more particularly in
another connection, have shown that the sodium salts of butyric
and acetic acids when injected into the blood are promptly oxi-
dized, and Nencki & Sieber || have shown that lactic acid is
readily oxidized, even by a diabetic patient.

XOX-XITROGENOUS MATTER OF THE URINE.

It has been implied in the foregoing pages that the digested
carbohydrates of the food, whatever the intermediate stages through
which they may pass, are ultimately oxidized to carbon dioxide and
water. Of the ordinary hexose carbohydrates this is doubtless
true, but with some of the large variety of substances ordinarily
grouped together, by the conventional scheme of feeding-stuffs analy-
sis, as "carbohydrates and related bodies," or as " crude fiber ' '
and "nitrogen-free extract," the case appears to be otherwise.

It has been shown that the urine, in addition to the nitrogenous
products of proteid metabolism which will be considered in a
subsequent section, contains also non-nitrogenous materials, pre-
sumably metabolic in their nature. In the urine of man and of the
carnivora these non-nitrogenous substances are chiefly or wholly
such as might be derived from the metabolism of proteids (phenols
and other compounds of the aromatic series), and their amount is
comparatively small. In the urine of herbivora, particularly of
ruminants, however, their quantity is relatively very considerable,
and it seems impossible to regard any large portion of them as
derived from the proteid metabolism.

*Zeit, f. Biol., 21, 625. 1 Arch. ges. Physiol., 46, 322.

ilbid., 21, 139. S Ibid., 49, 460.*

|| Jour. pr. Chem., X. F.. 26, 32.

28 PRINCIPLES OF ANIMAL NUTRITION.

Henneberg * found that from 26.7 to 30.0 per cent, of the organic
matter of sheep urine was neither urea nor hippuric acid, while from
95 to 100 per cent, of the total nitrogen was contained in these two
substances. G. Kiihn in his extensive respiration experiments on
oxen, as reported by Kellner,f assuming that all the nitrogen of
the urine was in the form either of hippuric acid or urea, found that
from 40.05 to 67.64 per cent, of the total carbon of the urine was
present in non-nitrogenous substances. The more recent investi-
gations of Kellner,J as well as those of Jordan § and of the writer, |j
have fully confirmed this fact.

Apparently these non-nitrogenous organic substances are de-
rived in some way largely from the coarse fodders. Their propor-
tion in the urine is relatively large when the ration consists exclu-
sively of coarse fodder, and the addition of such fodders to a basal
ration causes a marked increase in their amount, while, on the
other hand, such concentrated feeding-stuffs as have been inves-
tigated do not produce this effect in any very marked degree.
Furthermore, their amount seems to bear no fixed relation to the
protein of the coarse fodder. When the amount of the latter
ingredient is small, the total organic matter of the urine has in
some cases exceeded the maximum amount that could have been
derived from the protein of the food, thus demonstrating that a
portion at least of the non-nitrogenous urinary constituents must
have had some other source. As the proportion of protein in the
food increases, the amount of nitrogenous products in the urine
likewise increases, while that of the non-nitrogenous products
appears to be more constant, so that the ratio of urinary nitrogen
to carbon increases. The most plausible explanation of these facts
seems to be that the substances in question are derived from some of
the non-nitrogenous ingredients of the coarse fodders, but from what
ones, or what is the nature of the products, we are still ignorant. ^[

*Neue Beitriige, etc., p. 119.
■fLandw. Vers. Stat., 44, 34S, 404, 474, 520.
$ Ibid., 47, 275; 50,245; 53, 1.
§New York State Expt. Station, Bull. 197, p. 27.
]| Penna. Expt. Station, Bull. 42, p. 150.

If A further discussion of this subject in its relations to the energy of
the food will be found in Part II.

METABOLISM. 29

§ 2. Fat Metabolism.

Scarcely a tissue or portion of the animal body can be named
in which more or less fat is not found. The muscular fibers, the
epithelium, the nerves and ganglia, etc., all contain cells in which
globules of fat may be recognized, so that the capacity to produce
or store up fat seems to be common to almost all the cells of the
body. It is particularly in certain cells of the connective tissue,
however, that the large accumulations of visible fat in the body
take place. At the outset these cells present no special characters,
but in a well-nourished animal globules of fat begin to accumulate
in them, the cells enlarge, the globules of fat coalesce into larger
ones, and finally the cell substance is reduced to a mere envelope,
the nucleus being pushed to one side and almost the whole volume
of the cell occupied by fat. Masses of connective tissue thus loaded
with fat constitute what is called adipose tissue. Large deposits
of adipose tissue are met with surrounding various organs, particu-
larly the kidneys, but the largest deposit of fat is usually in the
connective tissue underlying the skin. In milk production, too,
large amounts of fat appear in the epithelial cells of the milk glands.

Fat Manufactured in the Body. — The older physiologists held
that all the ingredients of the body pre-existed in the food. Specifi-
cally, animal fat was regarded as simply vegetable fat which had
escaped oxidation in the body and been deposited in the tissues.
But while there is no doubt that the fat of the food can contribute
to the fat supply of the body, the food of herbivorous animals
usually contains a relatively small quantity of fat and the amount
produced by a rapidly fattening animal or by a good dairy cow is
usually much greater than that consumed in the food.

Deferring to subsequent pages a discussion of the sources of
animal fat,* we may content ourselves here with anticipating the
general results of the great amount of experimental inquiry which
has been expended upon this question. These results may be
briefly summarized in the following statements:

* For a very complete review of the literature of fat production up to 1894,
see Soskin, Journ. f. Landw., 42, 157.

30 PRINCIPLES OF ANIMAL NUTRITION.

1. The animal body produces fat from other ingredients of its

food.

2. The carbohydrates and related bodies of the food serve as

sources of fat.

3. It is probable that the proteids also serve as sources of fat.

So far, then, as that portion of the fat which is actually pro-
duced in the body from other substances is concerned, we may most
readily conceive of its formation as consisting essentially of a
manufacture of fat by the protoplasm of the fat cells, which are
nourished by the carbohydrates, proteids, and other materials
brought to them by the circulation.

Functions of the Food Fat. — The fat which is manufactured
in the body from other ingredients of the food, however, often con-
stitutes the larger portion of the total fat production, while but
a relatively small proportion at most can be derived from the fat
of the food. The question naturally arises whether this smaller
portion contained in the food is simply deposited mechanically, so
to speak, in the fat cells, or whether it too, like the carbohydrates
and proteids, serves to nourish the fat cells and supply raw material
out of which they may manufacture fat.

At first thought the former alternative might seem more prob-
able. The fat of the food, so far as we are able to trace it, does not
undergo any considerable chemical changes, such as the proteids
do, e.g., in the process of digestion, but is largely resorbed in the
form of only slightly altered fat. Moreover, resorption of fat takes
place largely through the lacteals and the resorbed fat reaches the
general circulation without being subjected like the carbohydrates
to the action of the liver.

Deposition of Foreign Fats. — The view just indicated is
supported to a considerable extent by the results of experiments
upon the fate of foreign fats introduced into the body.

Experiments by Radziejewsky * and Subbotin f were indecisive,
but Lebedeff J was later successful in obtaining posit ive re-
sults. Two dogs, after prolonged fasting, received small amounts
of almost fat-free meat together with, in the one case, linseed oil,

*Virehow's Archiv., 56, 211 ; 43. 268 fZeit. f. B'ol .. 6, 73.

JThier. Chera. Ber.. 12,425; Zeit. Physiol. Chem.. 6. 149; Centralbl. med.
Wiss.. 1882.. 129.

METABOLISM. 3r

and in the other, mutton tallow. After three weeks, during which
the animals recovered their original weights, the adipose tissue was
found to contain, in the one case, fat fluid at 0° C. and agreeing very
closely with linseed oil in its chemical behavior, while in the other
case the fat had a melting-point of over 50° C, and was almost
identical with mutton fat. On the other hand, the same author
in experiments with tributyrine failed to obtain any noteworthy
deposition of this substance.

Munk * fed large amounts of rape oil to a previously fasted dog
for seventeen days and found in the body considerable amounts
of fat differing markedly in appearance and properties and in the
proportion of olein to solid fats from normal dog fat. He likewise
succeeded in isolating from the fat eruic acid, the characteristic
ingredient of rape oil. In a second experiment f the fatty acids
prepared from mutton tallow were fed with similar results, the
proportion of stearin and palmitin to olein being approximately
reversed as compared with normal dog fat. The latter experiment
is also of interest as showing that the fatty acids may be synthesized
to fat in the body, the change taking place, according to Munk, in
the process of resorption.

More recently Winternitz \ has experimented with the iodine
addition products of fats. He observed the retention of a con-
siderable proportion of iodine in the body (of hens and dogs) in
organic form and also found iodine in the fat of the body at the close
of the experiment. Similar experiments on a milking goat § showed
that at least 6 per cent, of the fat fed passed into the milk.

Henriques and Hansen || fed two three-months-old pigs for about
nine months with ground barley, to which was added, in one case
linseed oil and in the other cocoanut oil, while in the succeeding
three months the rations were exchanged. Samples of the sub-
cutaneous fat of the back were taken (with the aid of cocaine) at
four different times and the fat of the carcasses at the close of the
experiment was also examined. The results showed an abundant
deposition of the linseed oil (and cocoanut oil?). On the other

*Thier. Chem. Ber., 14, 411; Virchow's Archiv., 95, 407.
t Archiv. f. (Anat. u.) Physiol., 1883, p. 273.
% Zeit. phvsiol. Chem . 24, 425.
§Thier. Chem. Ber., 27, 293.
li Ibid., 29. 68.

32 PRINCIPLES OF ANIMAL NUTRITION.

hand, experiments with cows failed to show any passage of linseed
ril as such into the milk.

Leube * made subcutaneous injections of melted butter on two
dogs and found an abundant deposit of butter fat especially under
the skin of the abdomen, the Reichert-Meissl number of the fat
being 20.46 in the first case and 15.3 in the second. Rosenfelt \
fed fasted dogs with mutton fat and observed a large deposit
of this fat in all parts of the body.

Influence of Feeding on Composition of Fat. — In addition
to the more purely physiological experiments just cited, there are
on record a not inconsiderable number of feeding experiments,
especially upon swine, in which the feeding appears to have
sensibly influenced the appearance, firmness, melting-point, or
composition of the body fat.

While it is not impossible, however, that in some cases the
peculiar fats of the food (e.g., the fat of maize or of the oil-meals)
may have been deposited in the adipose tissue unchanged, it must
be borne in mind that these experiments were made on mixed rations
and that undoubtedly there was a considerable production of fat. in
the body from other ingredients of the food. This being the case,
we are left in doubt as to whether the effect observed is due directly
to the fat of the food or is to be explained as an effect of the food as
a whole, or of some unknown ingredients of it, in modifying the
nature of the metabolism in the fat cells. That such an explana-
1 i< hi is at least possible would seem to be indicated by the well-
established fact that marked changes of food do modify the
metabolism in the milk gland sufficiently to materially affect the
proportion of volatile fatty acids in butter fat.

A striking example of the possibility of such an effect upon the
metabolism of the fat cells is afforded by the recent investigations < >f
Shutt \ into the causes of "soft" pork. On the average of a con-
siderable number of animals, he finds that the shoulder and loin fat
of pigs fed exclusively on maize shows a very low melting-point
and a high iodine absorption number, indicating a large percentage
of olein, and inclines to attribute this effect to the oil of the maize.
When, however, he fed skim milk with the maize, he obtained pork

* Thior. Chem. Bcr., 25, 45. f Ibid., 25, 44.

\ Canada: Dominion Experiment Station, Bull. 38.

METABOLISM. 33

of good quality, the fat having a melting-point and iodine number
not widely different from those obtained with the most approved
rations. While it is possible that part of this effect was due to a
reduced consumption of maize oil, so that more fat was produced
from the other ingredients of the food, the conclusion seems justified
that the principal factor was the influence of the skim milk upon
the nutrition of the fat cells. This influence may with some degree
of probability be ascribed to its protein, and it is worthy of notice
that in Shutt's experiments the rations which produced the highest
grade of pork were composed of materials rich in protein.

Another fact warns us to be cautious in our interpretation of
the results of this class of feeding experiments. Such experiments
in most cases involve a comparison of the composition of the fat
from animals differently fed. Albert * has found that both with
swine and sheep the composition of the body fat is subject to very
considerable individual variations as to melting-point, refractive
index, and iodine number, the differences being, in his experiments,
greater than the average differences which could be ascribed to the
feeding.

Moreover, the fat of the same individual has not the same com-
position in different parts of the body. This point has recently
been the subject of an. elaborate investigation by Henriques &
Hansen, f whose results show a higher melting-point and a lower
iodine number in the inner as compared with the outer layers of
fat. This difference they ascribe to the difference in the tempera-
ture of the tissues and support this view by an experiment with
three pigs. One animal was kept in a stall heated to about 30° C.
for two months, while the others were exposed to a temperature ( \
0° C. one unprotected and the other partially enveloped in a sheep-
pelt. At the close of the experiment the fat immediately under

the skin gave the following figures:'

Iodine Solidifying

Number. Point.

Kept at 30°-35° C 69.4 24.6° C.

Kept at 0°, in sheep pelt:

Part under the pelt 67.0 25.4° C.

Part exposed 69.4 24.1° C.

Kept at 0°, unprotected 72.3 23.3° C.

* Landw. Jahrb., 28, 961, 98fi.

t Bied. Centr. Blatt. Ag. Ch., 30, 182.

34

PRINCIPLES OF ANIMAL NUTRITION.

Towards the interior of the body the differences became grad-
ually less.

It is evident; then, that the sources of possible error in ex-
periments upon the influence of food on the composition of body
fat are considerable, and that not only is great care necessary to
secure representative samples of fat for examination, but the effect
of individuality must be eliminated so far as possible by the use
of a considerable number of animals. When we add to this the
other fact that the fat production of herbivorous animals is
largely at the expense of other nutrients than fat, we shall hardly
incline to give the results of such investigations much weight as
regards the question of the functions of food fat.

Quantitative Relations. — Some further light upon the point
under discussion may perhaps be obtained from a consideration of
the quantitative relations of food fat to fat production shown by
respiration experiments and which will be considered more fully
on subsequent pages (compare Chapter V). In scarcely any of these
experiments has the food fat been deposited quantitatively in
the tissue. In three out of five experiments by Rubner in which
fat was given to a previously fasting animal, from 65.82 to 91.89
per cent, of the fat supplied in excess of the amount metabolized
during fasting was stored up in the body. Similarly, in the ex-
periments of Pettenkofer & Yoit, in which the fat was added to a
ration already more than sufficient for maintenance, on the average
87.86 per cent, of the fat of the food was deposited in the tissues.

Kellner,* among his extensive respiration experiments upon
cattle, reports the results of three in which peanut oil was added to a
basal ration more than sufficient for maintenance. The amounts
of fat consumed in excess of the basal ration and the resulting gains
by the animals were as follows, the slight variations in the amounts
of the other nutrients being neglected:

Additional Fat

Digested,

Grams.

Gain by Animal.

Gain of Fat

Animal.

Protein,
Grams.

Fat,
Grams.

in Per Cent, of Fat
Digested.

D

F
G

677
542
458

8
86
44

239
205
279

35.30
37.83
60.91

* Landw. Vers. Stat.. 53, 112, 124. 199. 214.

METABOLISM. 35

Computations of the proportion of the energy of the added fat
which was recovered in the total gain of flesh and fat (compare
Chapter XIII, § 1) showed, according to the method of computa-
tion employed, a loss of from 31 to 48 per cent.

The comparatively small losses observed in Rubner's and in
Pettenkofer & Yoit's experiments may well be ascribed to a con-
sumption of energy in the work of digestion (compare Chapter
XI), but it hardly seems possible to account in this way for
the large losses observed by Kellner. Apparently the peanut
oil in these experiments, after its digestion and resorption, must
have been subjected to extensive molecular changes involving a
considerable expenditure of potential energy, and if this be true,
the suggestion of an assimilation by the fat cells and a construction
of animal fat from the oil is obvious.

Constancy of Composition of Fats. — The relatively constant
and characteristic composition of the fat of the same species of
animal, notwithstanding differences in the food, has been urged in
favor of the view that the fat of the animal is a product of the
protoplasmic activity of the fat cells. "The fat of a man differs
from the fat of a dog, even if both feed on the same food, fatty or
otherwise" (M. Foster). The steer produces beef fat and the sheep
mutton fat on identical rations. Unless, however, we are prepared
to discredit the experimental results above cited, it would appear
that this general and approximate uniformity of composition is
largely due to a general uniformity of food, and that marked changes-
in the nature of the latter may result in altering the former. To this
must be added, as already insisted upon, the fact that much of the
fat found in the body, especially in the herbivora, is undoubtedly
produced in the organism. We may fairly presume that this fat
will be the characteristic fat of the species. If we may suppose
further that a considerable share of the food fat is oxidized directly,
and if we take into consideration the general uniformity of diet of
our domestic animals and the relatively small total amount of fat
which it often contains, we have at least a plausible explanation of
the observed facts and one which does not preclude a direct deposi-
tion of food fat in the body and a consequent effect upon the com-
position of the body fat.

The Katabolism of Fat. — The proportion of the food fat which

3& PRINCIPLES OF ANIMAL NUTRITION.

serves to increase the store of fat in the body depends largely upon
the total food-supply. When the latter is more than sufficient to
balance the total metabolism of the organism, the excess may give
rise to a storage of fat, and under these circumstances the food fat
or a part of it may, as we have seen, contribute to the increase
of adipose tissue. On the other hand, when the food-supply is in-
sufficient, not only is its fat in common with its other ingredients
in effect consumed to support the vital processes, but the fat pre-
viously stored in the adipose tissue is drawn upon to make up the
deficiency. Under these circumstances the fat disappears more or
less rapidly from the fat cells, passing away gradually either into
the lymphatics or the blood-vessels in some manner not as yet fully
understood.

Fat, then, whether derived immediately from the food or drawn
in the first instance from the adipose tissue of the body, passes into
the circulation and serves to supply the demands of the body
for oxidizable material and energy, the final products of its oxida-
tion being carbon dioxide and water. Of the intermediate steps
in this katabolic process we are comparatively ignorant, but one
hypothesis regarding it has acquired so much importance in its
bearings on the availability of the potential energy of the food as to
require mention here.

Formation of Dextrose from Fat. — This hypothesis is, in
brief, that the first step in the katabolism of fat takes place in the
liver and consists in its conversion into sugar. In other words, it is
held that the fat of the food or that drawn from the adipose tissue
of the body supplies the liver with part of the material for its func-
tion of sugar production described in the previous section.

This hypothesis is advocated especially by those physiologists
who, like Seegen in Vienna and Chauveau and his associates in Paris,
look upon the carbohydrates, and particularly dextrose, as the im-
mediate source of the energy exerted in muscular contraction or
in the various other forms of physiological work. The evidence
upon which this view is based will be considered in subsequent
chapters. For the present it suffices to point out that, if we admit
its truth, then the general metabolism of the body is essentially a
carbohydrate metabolism. Whether we consider the case of a
fasting animal, living upon its store of protein and fat, or that of an

METABOLISM. 37

animal receiving food, the liver breaks down the proteids and fat
supplied to the blood either by the food or from the tissues, pro-
ducing dextrose. This dextrose, like that derived from the carbo-
hydrates of the food, is then, as indicated in the previous section,
oxidized in the tissues either directly or with previous conversion
into glycogen.

As regards the katabolism of fat, in particular, Nasse * has
brought forward reasons for believing that the liver is concerned in
it. Seegen f submitted fat to the action of finely chopped, freshly
excised liver suspended in defibrinated blood at a temperature of
35-40° C, in a current of air and observed a considerable formation
of sugar in five to six hours as compared with a control experiment
without the fat. He likewise found \ in experiments upon dogs
fed on fat with little or no meat that the blood of the hepatic vein
was much richer in sugar than that of the portal vein. On the basis
of the probable amount of blood circulating through the liver, he
computes that the total amount of sugar thus produced was much
greater than could have been supplied by the glycogen stored in
the liver and the amount of proteids metabolized (as measured
by the urinary nitrogen), and hence concludes that at least the
difference was produced from fat. As was pointed out in the
preceding section, however, many physiologists regard the large
differences between the dextrose content of the portal and the
hepatic blood observed by Seegen as being in large part the result of
the necessary operation and thus abnormal, and the production of
glycogen or dextrose from fat is not regarded as proven by the
majority of physiologists. § Thus Girard || and Panormow •[ found
the post-mortem formation of sugar in the liver to be strictly pro-
portional to the disappearance of glycogen, and similar results
were obtained by Cavazzani and Butte.**

Kaufmann,ft who has developed this hypothesis in considerable

* v. Xoorden, Pathologie des Stoffweehsels, p. 85.

t Die Zuckerbildung im Thierkorper, p. 151.

%Ibid., p. 171.

§ Cf. Neumeister, Physiologische Chemie, p. 368.

|| Arch. ges. Physiol., 41, 294.

1[ Thier. Chem. Per., 17. 304.

** Ibid., 24,391 and 394.

ft Archives de Physiol., 1896, p. 331.

3$ PRINCIPLES OF ANIMAL NUTRITION.

detail, represents the two supposed stages in the katabolism of fat
by the two following equations, proposed by Chauveau:*

First Stage : 2(C57H110O6) + 6702 = 16(C6H1206) + 18C02+ 14H20 .

Second Stage: 16(C6H1206) + 9602 = 96C02 + 96H20.

Even, however, if we admit the formation of dextrose from fat
in the body, it may fairly be doubted whether the process is as
simple as these equations, even if regarded as simply schematic,
would imply.

§ 3. Proteid Metabolism.

ANABOLISM.

Digestive Cleavage. — The digestion of the proteids is essen-
tially a process of cleavage and hydration under the influence of
certain enzyms. By this process the complex proteid molecules
are partially broken up into simpler ones. By the action of pepsin
in acid solution we obtain albumoses and peptones, while the
trypsin of the pancreatic juice, at least outside the body, carries
the cleavage still further, producing crystalline nitrogenous bodies of
comparatively simple constitution. Opinions are still more or less
divided as to how far these processes of cleavage and hydration are
carried in the actual process of digestion, where the products of the
action are constantly being resorbed, but there are not wanting in-
dications that it is both less extensive and less rapid than in arti-
ficial digestion. It likewise seems to have been demonstrated that
some soluble proteids are capable of direct resorption without
change, while others are not and some, notably casein, are promptly
coagulated by the rennet ferment, apparently expressly in order
that they may be subjected to the action of the digestive ferments.
In a general way, the statement appears to be justified that the
larger share of the proteid material of the food is resorbed as
albumoses and peptones. (See note, p. 58.)

Purpose of the Cleavage. — The fact just mentioned that,
on the one hand, some soluble proteids appear capable of direct re-
sorption, while, on the other hand, some, like casein, are at once
rendered insoluble as the first step in digestion, plainly necessitates
a material modification of the old view that the object of the cleav-
age and hydration of the proteids in digestion is to render them
* La Vie et l'Energie chez l'Animale.

METABOLISM. 39

soluble. Undoubtedly this is an important function of the
digestive fluids, but the fundamental object lies deeper and is
found in the constitution of the proteids themselves.

Constitution of the Proteids. — When acted upon by the various
digestive ferments, or by strong acids or alkalies, the proteids
readily undergo hydrolysis and yield a series of products of
decreasing molecular complexity and increasing solubility, rang-
ing from very slightly modified proteids through the so-called
proteoses and peptones to still simpler substances. When the
hydrolysis, especially acid hydrolysis, is pushed as far as possible
there result a number of comparatively simple crystalline
products, which are qualitatively the same for all the simple
proteids, with a few exceptions. The known primary cleavage
products of the simple proteids are all amino-acids. One of the
first of these to be isolated was glycocol or amino-acetic acid.
The other cleavage products of the simple proteids may be
regarded as derived from glycocol by the replacement of one
atom of hydrogen by various atomic groupings. In all of them,
the amino group occupies the same position in the molecule
relatively to the carboxyl group as in glycocol, viz., the so-called
alpha position.

The amino-acids derived from the proteids may be sub-
divided into mon-amino and di-amino acids, the larger number of
those at present identified belonging to the first of these groups.
Some of them are derived from the simple fatty acids, others
contain aromatic and other radicals, and a few the element
sulphur. Altogether, some seventeen different cleavage products
of this sort have been identified. These amino-acids account
for from 60 to 70 per cent, of the proteid molecule and appear
to be its characteristic ingredients, the constitution of the rest of
the proteid molecule being unknown.

The amino-acids which are obtained by the hydrolysis of
proteids may be caused to combine with each other, the amino-
group of one uniting with the carboxyl group of the next, with
the elimination of one molecule of water, forming the so-called
peptids. As many as seven amino-acids have been thus linked
up into peptids, the more complex of which resemble in many
respects the proteids. The latter are, indeed, believed to be
substantially very complex "poly peptids," which are split up by

40 PRINCIPLES OF ANIMAL NUTRITION.

the action of the digestive ferments into the comparatively simple
atomic groupings of which they are composed — the so-called
"building-stones" of the proteids.

Differences in Proteids. — In a few proteids, certain of these
atomic groupings are not found at all. For example, no glycocol
is produced by the hydrolysis of casein or albumin and no lysin
from gliadin or zein. Furthermore, while most of the simple
proteids yield qualitatively the same cleavage products, the
relative proportions of these several products vary widely in
proteids from different sources. One of the most striking in-
stances of this is glutaminic acid, which ranges from about 30
per cent, on the average of the wheat proteids to 9 per cent, on
the average of the four common animal proteids, casein, egg
albumin, serum albumin, and scrum globulin.

Food Proteids and Body Proteids. — What is especially to be
noted in this connection is that the food proteids are not identical
with the body proteids. This is especially true of the vegetable
proteids in the food of the herbivora, and of the casein of milk, but is
measurably true in all cases. A simple resorption of unaltered
proteid, therefore, would not serve the purposes of the organism.
The food proteids must be changed to body proteids. This means,
h( >wever, that the proportions of those molecular groupings which
have just been spoken of must be changed — that is, the molecules
of the food proteid must be so far broken down into their constituent
atomic groupings as to permit of a rearrangement and repropor-
tioning of the latter into molecules of body proteid.

Such a partial breaking down of proteid material takes place in
digestion. The products of proteid digestion, as they are pre-
sented to the resorbent organs of the'digestive tract, are no longer
proteids, but the constituent atomic groupings out of which
body proteids may be built up.

Rebuilding of Proteids.— The simple proteids are resorbed, as
we have just seen, in the form of comparatively simple cleavage
products, in part as amino-acids and in part probably as more
or lev-: complex polypeptids. Out of these substances the body
. '-uilds up the great variety of proteids peculiar to itself and
which differ in chemical structure and properties from those of
the vegetable world.

Since it has not been possible to identify any of the amino-

METABOLISM. 4 1

acids in the blood, the current view has been that these "building-
stones" of the proteids are synthesized in the epithelial cells of
the resorbent organs, and that the resulting proteids — in par-
ticular serum albumin — are passed on to the blood to serve as
nourishment to the proteid tissues of the body. If this be the
case, however, it is evident that the blood proteids must sub-
sequently undergo an extensive hydrolysis and that their con-
stituent atomic groupings must be rearranged on a different
plan to produce the various tissue proteids. In other words, if
the molecular debris of the food proteids is promptly recon-
verted into blood proteids in the epithelial cells, the splitting up
effected in digestion must be to a considerable extent repeated
in the nutrition of each tissue and even, perhaps, of each cell.
This fact has led to the belief that the real seat of the synthesis
is not, or at least not exclusively, the epithelial cells of the intes-
tine, but that every living cell, each in its own measure, builds
up its own proteids from the fragments supplied by the digestive
process. The failure to detect individual cleavage products in
the blood is plausibly explained by the relatively small amounts
resorbed in ordinary cases and by the fact that when not synthe-
sized to proteids they seem to undergo rapid katabolism.

Whatever may finally prove to be the case in this respect,
however, there is no dispute as to the general facts that the food
proteids undergo more or less complete cleavage in the digestive
process and that the resulting fragments are subsequently built
up again into body proteids and that, therefore, the first step in
proteid metabolism is an anabolic process.

KATABOLISM.

Final Products. — The anabolic processes which have just been
indicated might be characterized in general terms as a preparation
of the food proteids for their diverse functions in the body. In the
performance of those functions they, like all the organic ingredients
of the body, undergo katabolic changes, liberating the energy whiqh
was originally contained in them or which may have been tem-
porarily added in the preliminary anabolic changes. We have
every reason to believe that the katabolism of proteids is a gradual
process, passing through many intermediate stages, but we have
very little actual knowledge of the steps which intervene between

42 PRINCIPLES OF ANIMAL NUTRITION.

the proteids and bodies which are either excretory products
themselves or closely related to them. Such information as has
thus far been acquired upon this subject has resulted chiefly from
attempts to trace back the excretory products to their antecedents.

The products of the complete breaking down and oxidation of
proteids in the body are carbon dioxide and water, excreted through
the lungs, skin, and kidneys, and urea and a number of other com-
paratively simple crystalline nitrogenous compounds found in the
urine. To these are to be added the nitrogenous metabolic prod-
ucts of the feces, the sulphuric and phosphoric acids resulting from
the oxidation of the sulphur of the proteids and the phosphorus of
the nucleo-proteids, and the relatively minute amounts of nitroge-
nous matter found in the perspiration.

Excretion of Free Nitrogen. — The question whether any
portion of the nitrogen of the proteids is excreted as free gaseous
nitrogen is one which has been the subject of no little investigation
and controversy in the past, the especial champions being, on the
affirmative, Seegen in Vienna and, on the negative, Voit in Munich.
It would lead us too far aside from our present purpose, however, to
attempt even to outline the evidence, and it must suffice to say that
the great majority of physiologists regard it as established that there
is no excretion of gaseous nitrogen as a result of the katabolism of
proteids, but that all the proteid nitrogen is excreted in the urine
and feces with the exception of small amounts in the perspiration.
In accordance with this view, we shall assume in subsequent pages
that the urinary nitrogen (together with, strictly speaking, the
metabolic nitrogen- of the feces and perspiration) furnishes a meas-
ure of the total proteid katabolism of the body.

A brief consideration of some of the principal nitrogenous
products of proteid katabolism will serve to indicate some of the
main features of the process, so far as they have been made out.

Upt:a. — Urea, or dicarbamid, CON2H4, is the chief nitrogenous
product of proteid metabolism in the carnivora and omnivora. In
the urine of man, e.g., from 82 to 86 per cent, of the nitrogen is in
the form of urea.*

Antecedents of Urea. — A vast amount of study has been expended
upon this question without as yet leading to any general unanimity
of views. It appears, however, to be fairly well made out that at
* v. Noorden, Pathologie des St.offwechsels, p. 45.

METABOLISM. 43

least a considerable part if not all of the urea is formed in the liver,
and that its immediate antecedent is ammonium carbonate, to
which it is closely related chemically. This theory of Schmiede-
berg's is supported by the facts:

1st. That ammonium salts, and also the amid radicle NH2 in
the amino acids of the fatty series, when administered in the food
are converted into urea.

2d. That ammonium carbonate or formiate injected into the
portal vein is converted in the liver into urea which appears in the
blood of the hepatic vein.

3d. That the administration of inorganic acids to the dog and to
man results in the excretion of ammonium salts in the urine, it
being supposed that the acid displaces the weaker carbonic acid
and that the resulting ammonium salt is incapable of conversion
into urea in the liver.

4th. Severe disease of the liver has been observed to result in
a decreased production of urea and an excretion of ammonium salts
in the urine.

Later investigations by Minkowski * and others have followed
the process of the formation of urea one step further back and ren-
dered it highly probable that the ammonium salts out of which urea
is formed reach the liver in the form of ammonium lactate. It has
been shown that sarcolactic acid is one of the products of the meta-
bolism of the muscles. It would appear that this acid unites with
the ammonium radicle derived from the proteids to form ammonium
lactate, and that the latter on reaching the liver is first oxidized to
the carbonate, which is then converted into urea. If, by disease
or surgical interference, this action of the liver is prevented, ammo-
nium lactate appears in the urine, and the same effect may even be
produced by excessive stimulation of the proteid metabolism, so
that the production of ammonium lactate exceeds the capacity of
the liver to convert it.

Uric Acid. — Uric acid is contained in small amounts in the
urine of mammals. With birds it constitutes the chief nitrogenous
product of the proteid metabolism. In mammals it must be
regarded as essentially a product of the katabolism of thenucleo-
proteids of the food or of the body tissue, although a portion of

* Cf. Neumeister, Physiologische Chemie, pp. 313-318.

44 PRINCIPLES OF ANIMAL NUTRITION.

the uric acid thus produced is further katabolized and yields
urea. In birds there is an extensive synthetic production of uric
acid from simpler katabolic products.

Hippuric Acid. — This substance is a normal ingredient of the
urine of mammals, but in that of man and the carnivora is found in
but very small amounts. In the urine of herbivora, on the other
hand, it occurs abundantly.

Light was thrown upon its origin by the well-known discovery
by Wohler, in 1824, that it is also found in large amount in the urine
of man or of carnivora after the administration of benzoic acid.
Chemically, hippuric acid is benzamido-acetic acid, or benzoyl
glycocol. When the food contains benzoic acid the latter unites
with glycocol resulting from the metabolism of the proteids and
forms hippuric acid, while otherwise the glycocol would be further
oxidized to simpler nitrogenous products. The synthesis of hip-
puric acid has been shown to occur only in the kidneys in the dog,
but in the case of the rabbit and frog they appear to share this
capacity with other organs.

In this action of benzoic acid we have the most familiar demon-
stration of the formation of metabolic products intermediate be-
tween the proteids and the comparatively simple nitrogenous sub-
stances found in the urine. Glycocol has never been detected in the
body, obviously because as fast as it is formed it is again decom-
posed. Benzoic acid reveals its presence by seizing upon it and
converting it into a compound which is incapable of further oxida-
tion, and is therefore excreted. Other less familiar examples of
the same fact might be cited did space permit,

The normal presence of small quantities of hippuric acid in the
urine, even when no benzoic acid is contained in the food, arises
from the fact that the putrefaction of the proteids in the intestines
yields aromatic compounds, containing the benzoyl radicle, which
are resorbed and combine with glycocol to form hippuric acid.
The origin of the large quantities of hippuric acid ordinarily ex-
creted by herbivora, however, or rather of its benzoyl radicle,
is still more or less of a puzzle, notwithstanding the consider-
able amount of investigation which has been devoted to its
study. The most natural supposition would be that the food of

METABOLISM. 45

these animals contains substances of the aromatic series capable
of yielding benzoic acid or its equivalent in the body, but in none
of the feeding-stuffs known to be efficient in causing an excretion
of hippuric acid have such compounds been discovered in quantity
even remotely sufficient to account -for the hippuric acid produced.

On the other hand, the hypothesis that the benzoyl radicle of
the hippuric acid is derived to any large extent from the proteids
of the food appears to be decisively negatived by several facts:
First, the quantity of proteids in the ordinary rations of herbivora
is relatively small, and even if it all underwent putrefaction the
amount of aromatic products which could be formed, on any reason-
able estimate, would account for only a small fraction of the hip-
puric acid actually found.* Second, in several instances it has
been observed that variations in the extent of the putrefactive
processes in the intestines, as measured by the amount of con-
jugated sulphuric acid in the urine (compare p. 46), bore no rela-
tion to the variations in the production of hippuric acid. Third,
the addition of pure proteids or of foods very rich in proteids to a
ration does not increase the production of hippuric acid, and in at
least one case | was found to diminish it and even stop it alto-
gether.

Apparently we must regard the non-nitrogenous ingredients of
feeding-stuffs as the chief source of hippuric acid formation, but be-
yond this our knowledge is rather vague. It is well established
that the coarse fodders are the chief producers of hippuric acid,
while the concentrated feeding-stuffs give rise to little or none, and
may even reduce the amount previously produced on coarse fodder,
as may also starch. Among the coarse fodders, the gramineae give
rise to a markedly greater production of hippuric acid than the
leguminosse. This effect of the coarse fodders naturally led to the
suspicion that the crude fiber contained in them in large amounts
might be the source of the hippuric acid, and in fact numerous
experiments seem to show that some relation exists between the
two. although the results of various investigators are far from con-
cordant.

Finally, the investigations of Goetze & Pfeiffer, \ and of

* Compare Salkowski, Zeit. physiol. Chem., 9, 234.
f Henneberg and Pfeiffer, Jour. f. Landw., 38, 239.
% Landw. Vers. Stat., 47, 59.

46 PRINCIPLES OF ANIMAL NUTRITION.

Pfeiffer & Eber.* have shown with a high degree of probability
that the pentose carbohydrates of the feed have some connection
with the production of hippuric acid.f The former investigators
observed a marked increase in the production of hippuric acid by
a sheep after the administration of cherry gum (impure araban) and
of arabinose, and the latter obtained the same effect, although in a
less marked degree, by feeding cherry gum to a horse. They also
call attention to the differences in the behavior of the pentose carbo-
hydrates in the organism of the herbivora and in that of man and
the carnivora, but do not attempt to give a final solution of the
problem of the origin of the hippuric acid in the former case, while
they freely admit that it is difficult, if not impossible, to explain
some of the facts already on record on the hypothesis that the pen-
toses are the chief source of hippuric acid.

Creatin and Creatinin. — Among other nitrogenous constit-
uents of the urine of man and the carnivora may be mentioned
creatinin. This body is the anhydride of creatin, and the two
together constitute the principal part of the so-called flesh bases
which are contained in considerable quantity in muscular tissue.
When meat is consumed, its creatin is converted into creatinin and
excreted quantitatively in the urine, the creatinin content of which
may be thus considerably increased. As to the physiological signifi-
cance of the creatin of muscular tissue opinions are divided, but
good authorities are inclined to regard it as an intermediate
product of the metabolism of the proteids which is ultimately con-
verted into urea, and to urge that the fate of creatin taken into the
stomach is not necessarily the same as that of the creatin produced
in the muscles.

Aromatic Compounds. — Besides the benzol radicle of hippuric
acid, small amounts of other aromatic compounds are also found in
the urine. These bodies, belonging chiefly to the phenol and indol
groups, owe their origin exclusively to the putrefactive processes
already mentioned as taking place in the intestines, and are found in
the urine almost entirely in combination with sulphuric acid as the
so-called conjugated sulphuric acids, so that the amount of the
latter is employed as a measure of the extent of these putrefactive
processes.

* Landw. Vers. Stat., 49, 97.

t Later results by the same authors, however, throw doubt on this con-
clusion.

METABOLISM. 47

Metabolic Products in Feces. — As already stated in Chapter
I, the feces contain, in addition to undigested residues of the food,
certain materials derived from the body of the animal. This fact
was early recognized as true of both carnivora * and herbivora.f
Of more recent investigations may be noted especially those of
Muller,J Rieder,§ and Tsuboi || on carnivora, those of Prausnitz* and
his associates on man, and those of Kellner,** Stutzer.ft Pfeiffer, \\
and Jordan §§ on herbivora.

These "metabolic products" appear to consist of unresorbed
or altered residues of the digestive fluids and of mucus and other
materials excreted or otherwise thrown off by the walls of the intes-
tines. Their production goes on even when the digestive tract is
void of food, producing the so-called fasting feces which constitute
a true excretory product. The consumption of highly digestible
food— e.g., lean meat — does not seem to materially increase their
amount, but when food containing indigestible matter is eaten it is
believed that they increase in quantity.

It is presumed that these substances are largely nitrogenous in
character, and it is known at any rate that not inconsiderable
amounts of nitrogen may leave the body by this channel. In other
words, these nitrogenous substances, derived from the proteids
of the body, instead of undergoing complete conversion into the
ordinary crystalline products have their katabolism interrupted
as it were at an intermediate stage.

Many attempts have been made to determine the amount of
these metabolic products, or of their nitrogen, in the feces, but
without much success, and it may fairly be said that at present
we have no method which can be depended upon to distinguish
sharply between the nitrogen of undigested-food residues and that
of metabolic products.

* Bisehoff and Voit, Die Ernahrung des Fleischfressers, p. 291,

t Henneberg, Beitriige, etc., 1864, p. 7.

JZeit. f. Biol., 20, 327.

§ Ibid., 20, 378.

II Ibid., 35, 68.

f Ibid., 35, 287; 39, 277; 42, 377.

**Landw. Vers. Stat., 24, 434; Bied. Ccntralbl., 9. 763.

ffZeit. physiol. Chem., 9,211.

it Jour. f. Landw., 31, 221 ; 33, 149; Zeit. physiol. Chem., 10, 561.

§§ Maine Expt. Station Rep., 1888, p. 196.

48 PRINCIPLES OF ANIMAL NUTRITION.

Nitrogen in Perspiration. — The perspiration of such animals
as secrete this fluid must be regarded as one of the minor channels by
■which nitrogen is excreted. In human perspiration there have been
found, in addition to small amounts of proteids, urea, uric acid,
creatinin, and other nitrogenous products of the proteid meta-
bolism. In a recent investigation, Camerer * found about 34 per
cent, of the total nitrogen of human perspiration to be in the form
of urea, about 7.5 per cent, existed as ammonium salts, and the
remainder in undetermined forms, including uric acid and traces
of albumen.

The total quantity of nitrogen excreted in the insensible perspi-
ration appears to be insignificant. At water & Benedict f found
it to amount to 0.048 gram per day for an adult man in a state
of rest. Rubner & Heubner J obtained from the clothing of an
infant 2.83 mgrs. of ammonia and 0.0205 mgr. of urea per day
and estimated the total nitrogen of the perspiration at 39 mgrs.

When the secretion of sweat is stimulated by work or a high
external temperature the amount of nitrogen excreted may be con-
siderably increased as compared with a state of rest, although its
absolute amount is still small. Atwater & Benedict, § in a work ex-
periment, observed an excretion of 0.220 gram of nitrogen per day
in the perspiration of man.

The Non-nitrogenous Residue of the Proteids. — The various
nitrogenous products found in the urine and other excreta, the most
important of which have been noticed above, are believed to con-
tain all the nitrogen of the metabolized proteids. This does not
imply, however, that a quantity of proteids equivalent to this nitro-
gen, or even to that of the urine, has been completely oxidized to the
final products of metabolism, viz., carbon dioxide, water, and urea
and its congeners.

A comparison of the ultimate composition of the proteids with
that of the nitrogenous products of their metabolism reveals the
fact that an amount of the latter sufficient to account for all the
nitrogen of the proteids contain but a relatively small part of their
carbon, hydrogen, and oxygen. Taking urea as the chief and
*Zeit. f. Biol., 41, 271.

fU. S. Dept. Agr., Office of Expt. Stations, Bull. 69, 73.
tZeit. f. Biol., 36,34.
§Loc. cit:, p. 53.

METABOLISM. 49

typical metabolic product, and using average figures for the com-
position of animal proteids, we have omitting the sulphur of the
proteids, the following:

Proteids. Urea. Residue.

Carbon 53.0 6.86 46.14

Hydrogen 7.0 2.29 4.71

Oxygen 24.0 9.14 14.86

Nitrogen . . 16.0 16.00

100.0 34.29 65.71

After abstracting the elements of urea, we have left considerably
over half the hydrogen and oxygen of the proteid and the larger
part of its carbon. A substantially similar result is reached in case
of the other nitrogenous metabolic products. The splitting off of
these products from the proteids leaves a non-nitrogenous residue.

Fate of the Non-xitrogenous Residue. — The foregoing
statements and comparison must not be understood to mean that
the proteids split up in the body into two parts, viz., urea, etc., on
the one hand, and an unknown non-nitrogenous substance or sub-
stances on the other. As we have already seen, the processes of
proteid metabolism are far more complicated than such a simple
cleavage. Neither are we to assume that any substance or group
of substances corresponding in composition to the "residue" of the
above computation exists. The figures mean simply that while
the nitrogenous bodies of the urine contain all the nitrogen of the
proteids they do not account for all of the other elements, but that
part of the latter must be sought elsewhere.

Ultimately, of course, the elements of this non-nitrogenous
residue are converted into carbon dioxide and water. The conver-
sion into these final products, however, is necessarily a process of
oxidation, presumably yielding energy to the organism. It is a
matter of some interest, then, to trace the steps of the transforma-
tion bo far as this is at present possible.

Formation of Sugar. — In discussing the functions of the liver in
§ 1 of this chapter, we have seen reason to believe that this organ
continues to produce sugar when the diet c< nsists largely or exclu-
sively of proteids. In this case we arc forced to the conclusion that
this sugar is manufactured from the elements of the n on -nitrogenous
residue.

■5° PRINCIPLES OF ANIMAL NUTRITION.

This conclusion, based on what appears to be the normal func-
tion of the liver, is further strengthened by a large number of ex-
periments and observations upon the metabolism in diabetes.
This disease, whether arising spontaneously or provoked artificially,
is characterized by the presence of large amounts of sugar in the
urine. It has been shown that this production of sugar continues
when all carbohydrates are withdrawn from the diet, and further-
more, that the amount of sugar excreted bears a quite constant
relation to the amount of proteids metabolized, thus clearly in-
dicating the latter as the source of the sugar. It is true that the
formation of sugar from proteids is denied by some physiologists,*
but by the majority it seems to be accepted as a well-established
fact that sugar is one of the intermediate products of proteid
metabolism.

Of the steps of the process, as well as of its quantitative rela-
tions, we are ignorant. In effect, it is a process of oxidation and
hydration, since a residue of the composition computed above
would require the addition of both hydrogen and oxygen to con-
vert it into sugar, but that it is as simple a process as this state-
ment would make it appear, or that the conversion is a quantitative
one, may well be doubted.

In conclusion it may be stated that while recent investigations
have shown the presence of a carbohydrate radicle in numerous
(although by no means all) proteids, it does not appear that this
fact stands in any direct relation to the physiological production of
sugar from these substances. In the first place, the carbohydrate
radicle constitutes a much smaller proportion of these proteids than
corresponds to the amount of sugar which they are apparently
capable of yielding in the body, and in the second place it appears
to be a well-established (although not undisputed) fact that the
organism can produce sugar from proteids which do not contain
the carbohydrate radicle.

Formation of Fat. — Whether fat is formed from the elements
of proteids in the animal body is at present a subject of controversy,
but this question will be more profitably considered in a subsequent
chapter. It is sufficient to remark here that while much of the
earlier evidence bearing upon this point has been shown to be

*Cf. Schondorf, Arch. ges. Physiol., 82, 60.

METABOLISM. 51

inconclusive, the formation of fat from proteids has not yet been
disproved and has weighty direct evidence in its favor, while the
facts that sugar may be formed from proteids, and that carbohy-
drates are certainly a source of fat to the animal organism are
strong additional arguments in favor of its possibility.

Schematic Equations. — Chauveau and his associates * whose
views regarding the functions of the carbohydrates in the body
have already been mentioned, regard the katabolism of the proteids
as taking place in three stages. The first consists of the splitting
off of urea with production of carbon dioxide, water, and fat, accord-
ing to the equation :

4(C72H112N18022S) + 13902

(Stearin)

= 2(C57HU0O6) + 36CON2H4 + 138C02 + 42H20 + 2S2.

The resulting fat is then, according to Chauveau, further oxi-
dized in the liver, yielding dextrose, in accordance with the equation
already given on p. 38, viz.,

2C57HU0O6 + 67O2= 16C6H1206+ 1SC02 + 14H20,

and the dextrose is finally oxidized to carbon dioxide and water.
Another equation representing the katabolism of proteids is that
proposed by Gautier, which regards the first step in the process as a
combined hydration and cleavage with the production of urea, fat,
dextrose, and carbon dioxide, as follows :

2(C72H112N18022S)+28H20

(Tripalmitin)

= 18CON2H4 + 2C51H9808 + C6H1206 + 18C02 + S2.

It may be assumed that these authors regard the above equa-
tions simply as schematic representations of the general course of
proteid metabolism and do not intend to imply that there are no
intermediate stages in the process. Interpreting them in this
sense, we have good reasons for believing that the facts which they
represent are qualitatively true. A chemical equation however,
expresses not merely qualitative but quantitative results. If the
above equations have any significance beyond that of the mere
verbal statement that fat and sugar are products of proteid meta-

*Cf. Kaufmann, Archives de Physiol., 1896, p. 341.

52 PRINCIPLES OF ANIMAL NUTRITION.

bolism, they mean that from 100 grams of proteids there is pro-
duced, according to the first scheme, 27.61 grams of fat, and that
from this, by the addition of oxygen, 44.67 grams of sugar are
formed. Some of the evidence by which these equations are sup-
ported will be considered in another connection, but may be antici-
pated here in the statement that, in the judgment of the writer, it
is far from sufficient to establish them as quantitative statements.

THE NON-PROTEIDS.

Under this comprehensive but somewhat vague term have been
grouped all those numerous nitrogenous constituents of the food
which are not proteid in their nature, the name being a contraction
of non-proteid nitrogenous substances. It includes the extractives
of meat, and in vegetable foods several groups of substances, of
which, however, the amides and amido-acids are most abundant.
Various substances of this class are produced by the splitting up of
the reserve proteids in the germination of seeds and apparently
also to some extent in the translocation of proteids in the growing
plant, while some at least of them appear to be produced syntheti-
cally from inorganic materials and to be the forerunners of pro-
teids. In young plants a considerable proportion of the so-called
crude protein (N X 6.25) often consists of these non-proteids, and
considerable interest, therefore, attaches to their transformations
in the body.

Amides Oxidized in the Body. — It has been shown by numer-
ous investigators that various amides and amido-acids when added
to the food are oxidized, giving rise to a production of urea.
Shultzen & Nencki * found that glycocol, leucin, and tyrosin were
thus oxidized, while acetamid apparently was not. So far as
glycocol is concerned, this result is what would have been expected,
since, as we have seen (p. 44), this body appears to be normally
formed in the body as an intermediate product of proteid meta-
bolism. Similar results were obtained by v. Knieriem f from
trials with asparagin, aspartic acid, glycocol, and leucin. Munk \
likewise found that the ingestion of asparagin increased the pro-

*Zeit. f. Biol., 8, 124.

t Ibid., 10, 277 ; , 36.

JVirchow's Archiv. f. path. Anat., 94, 441.

METABOLISM. 53

duction of urea in the dog, all the nitrogen of the asparagin together
with an excess over that previously found in the urine being ex-
creted. The sulphur in the urine also increased. Hagemann *
has more recently fully confirmed this result. Salkowski f found
that glycocol, sarkosin, and alanin were oxidized to urea and caused
no gain of proteids. Apparently, then, this class of bodies, like
ammonia, furnish material out of which the organism can con-
struct urea.

Can Amides Replace Proteids? — Since the amides yield the
same end products of metabolism as the proteids, it is natural to
inquire whether they can perform any of the functions of those
substances.

Amides not Synthesized to Proteids. — We have already seen that
the albumoses and peptones resulting from the cleavage of the
proteids during digestion are built up again into proteids in tlie
process of resorption. The amides commonly found in vegetable
feeding-stuffs are likewise simpler cleavage products of the proteids,
and some of them are also formed in digestion by the proteolytic
action of trypsin. Can proteids be regenerated from these simpler
cleavage products?

If this is the case, then it should be possible, under suitable con-
ditions, to cause a gain of proteids, or at least to maintain the
stock of proteids in the tissues, on a food free from proteids but
containing amides. Up to the present time, however, all attempts
of this sort have failed. With the most abundant supply of non-
nitrogenous nutrients and ash, the animals perished when supplied
with amides (asparagin) but not with proteids.]; What has thus
been found to be true of asparagin we may regard as probably true
of other amides and say that there is no evidence that the animal
body can build proteids from amides.

Partial Replacement of Proteids. — But even if the amides can-
not serve as a source of proteids to the animal, it seems not impos-
sible that they may by their oxidation perform a part of the func-
tions of the proteids, thus protecting a portion of the latter from
oxidation and rendering it available for tissue production.

*Landvv. Jahrb., 20, 264.

f Zeit. physiol. Chem., 4, 55.

\ Compare Politis. Zeit. f. Biol., 28, 492, and Gabriel, lb., 29, 115.

54 PRINCIPLES OF ANIMAL NUTRITION.

The earliest investigations upon this point are those of Weiske *
and his associates upon the nutritive value of asparagin. The
experiments were made upon rabbits, hens, geese, sheep, and goats,
and in the case of the two latter species included experiments on
milk production. While the experiments are open to criticism in
some respects, as a whole they seemed to show that asparagin,
especially when added to a ration poor in proteids, caused a gain of
proteids by the body. Weiske accordingly concluded that aspara-
gin, while not capable of conversion into proteids, was capable of
partially performing their functions and thus acting indirectly as a
source of proteids, and this view has been somewhat generally
accepted. Subsequent experiments by Bahlmann,f Schrodt,£
Potthast,§ Meyer, || and Chomsky^ upon milch-cows, rabbits, and
sheep gave results which tended to confirm Wciske's conclusions.

Not all of Weiske's experiments, however, gave positive results
in favor of asparagin, and experiments upon carnivorous and omniv-
orous animals have failed to show any such effect. In addition
to the experiments of Politis and of Gabriel, referred to above,
Mauthner,** Munk,ff and Hagemann \\ have failed to observe any
gain of proteids by the body as a result of the ingestion of asparagin,
but found simply an increase in the apparent proteid metabolism
as measured by the urinary nitrogen.

Influence on Digestion.- — It can hardly be assumed that the
actual processes of metabolism in the body tissues are fundamen-
tally different in different species of mammals, and investigators
have therefore been led to seek an explanation of the striking differ-
ence in the effects of asparagin on herbivora and carnivora in the
differences in the digestive processes of the two classes of animals.

Digestion in herbivora is a relatively slow process and, as pointed
out in Chapter I, is accompanied by extensive fermentations par-

*Zeit, f. Biol., 15, 261- 17, -415; 30, 254.

t Reported by Zuntz, Arch. f. (Anat. u.) Physiol., 1882, 424.

t Jahresb. Agr. Chem., 26, 426.

§Arch. gcs. Physiol., 32, 288.

|| Cf. Kellner, Zeit. f. Biol., 39, 324.

t Ber physiol. Lab. Landw. Inst. Halle, 1898, Heft 13, p. 1.

**Zeit. f. Biol., 28, 507.

ff Virchow's Arch. f. path. Anat., 94, 441.

# Landw Jahrb., 20, 264.

METABOLISM. 55

ticularly of the carbohydrates of the food, as is shown by the large
amounts of gaseous hydrocarbons produced by these animals. In
carnivora, on the contrary, digestion is relatively rapid and the
dog, as a representative of this class, excretes, according to Voit &
Pettenkofer,* but traces of .hydrocarbons, and according to Tap-
peiner,t none.

Zuntz \ has therefore suggested that soluble amides introduced
into the digestive canal of herbivora may be used as nitrogenous
food by the micro-organisms there present in preference to the less
soluble proteids, so that the latter are to a certain extent protected,
and that it is even possible that the amides are synthesized to
proteids by the organisms. Hagemann § has added the suggestion
that the proteids possibly thus formed may be digested in another
part of the alimentary canal and thus actually increase the pro-
teid supply of the body.

If this explanation is correct. Ave should expect the effect of
asparagin to be more marked when the proportion of proteids in
the food is small, and precisely this appears to be the case. In
Weiske's first experiments, which gave the most decided results,
the nutritive ratio of the ration without asparagin was 1 : 19-20,
while a later experiment with a nutritive ratio of 1:9.4 showed no
effect of the asparagin upon the gain of protein. Chomsky's results,
too, were obtained with rations poor in protein and rich in carbo-
hydrates.

Later experiments on lambs by Kellner || have fully confirmed
this anticipation. In his first experiment two yearling lambs were
fed with a mixture of hay, starch, and cane-sugar, having a nutri-
tive ratio of 1:28, until nitrogen equilibrium was reached, when
fifty grams of the starch was replaced by asparagin. The result
was a gain of protein by both animals as compared with a loss in
the first period. In the third experiment asparagin was substi-
tuted for starch in a ration having a nutritive ratio of 1 : 7.9, and
caused with one animal a slight gain and with the other a slight
loss of protein. In the fourth experiment it was added to a ration

*Zeit. f. Biol., 7, 433; 9, 2 and 438.

flbid., 19, 318.

JArch. ges. Physiol., 49, 483.

§ Landw. Jahrb., 20, 264.

|| Zeit. f. Biol., 39, 313.

56 PRINCIPLES OF ANIMAL NUTRITION.

having a nutritive ratio of 1 : 7.7, and caused neither a gain nor
a loss of any consequence.

Particular interest attaches to Kellner's second experiment in
which ammonium acetate was added to a ration poor in protein
(1:19), followed in a third period by a quantity of asparagin con-
taining the same amount of nitrogen. The average amounts of
protein (N X 6.25) gained per day and head by the two lambs
were as follows:

Basal ration 4. 12 grms.

" " + ammonium acetate 15.56 "

" + asparagin 15.69 "

Although it is impossible to suppose that the ammonium acetate
is capable of performing any of the functions of proteids in the
body, it nevertheless caused as great a gain of protein by the body
as did the asparagin. The only obvious explanation is that both
these substances acted in the manner suggested by Zuntz to protect
the small amount of protein in the food from the attacks of the
organized ferments of the digestive tract. Accepting this explana-
tion, we must suppose that when the contents of the alimentary
canal contain a normal amount of proteids the micro-organisms
find an abundant supply of nitrogenous food in their cleavage
products and reach their normal development, so that an addition
of soluble nitrogenous substances is a matter of indifference. When,
on the other hand, the amount of protein present is abnormally
low, as in Weiske's and Kellner's experiments, the organisms are
limited in their food-supply and attack the food proteids them-
selves.

Kellner's results stand in apparent contradiction to the earlier
(Hies of Weiske and Flechsig,* who report no gain of proteids as re-
sulting from the addition on three days of a mixture of ammonium
carbonate and acetate to a ration poor in protein. The excretion
of sulphur in the urine was likewise unaffected. They assume,
however, a long-continued. after effect of the ammonium salts on the
nitrogen excretion. If the comparison be limited to the three days
on which the ammonium salts were given and the next following
day, a gain of 1.15 grams of nitrogen per day results, but, as just
stated, there was no corresponding gain of sulphur.
*Journ. f. Landw., 38, 137.

METABOLISM.

57

Kellner's experiments afford indirect evidence that both the
asparagin and the ammonium acetate actually did stimulate the
development of the ferment organisms, in the fact that the apparent
digestibility of the carbohydrates of the food was increased. On
the basal rations starch could be readily recognized in the feces,
but under the influence of the two substances mentioned it dis-
appeared. In the second experiment the increase in the amounts
of crude fiber and of nitrogen-free extract digested was as follows :

Nitrogen -free
Crude Fiber. Extract.

With ammonium acetate . . . 10.7 grms. 20.4 grms.

With asparagin 10.0 " 20.0 "

Since we know that large amounts of the nitrogen-free extract
are attacked and decomposed by organized ferments in the her-
bivora, and that this is the chief if not the only method by which
crude fiber is digested, we are justified in interpreting the above
figures as demonstrating an increased activity of these organisms
as a result of the more abundant supply of nitrogenous food. The
bearing of this result upon the so-called depression of digestibility
by starch and other carbohydrates is obvious, but is aside from
our present discussion.

Tryniszewsky * experimented upon a calf weighing about 175
kgs., using in the second and fourth periods (the first period being
preliminary) a ration of barley straw, sesame cake, starch and sugar,
containing a minimum of non-proteids. In the third period one-
third of the sesame cake was replaced by a mixture of asparagin,
starch and sesame oil, computed to contain an equivalent amount
of nitrogen, carbohydrates, and fat. Owing to differences in digest-
ibility, however, the amounts of digested nutrients, and particu-
larly of nitrogen, varied more or less. The results of the nitrogen
balance per 100 kgs. live weight were:

Nitrogen Digested.

Nitrogen

Metabolism,

Grms.

Gain of

Proteid,

Grms.

Non-proteid,
Grins.

Total,
Grms,

Nitrogen,
Grms.

Period II

72.16
67.05
90.86

72.16
90.73
90.86

56.86
78.43
76.36

15 3

" III

IV

23.68

12.3
14 5

Jahresb. Ag. Ch., 43, 513.

5 8 PRINCIPLES OF ANIMAL NUTRITION.

From the smaller gain in Period III, the conclusion is drawn
that the asparagin has a lower nutritive value than the proteids.
In this period the percentage digestibility of the crude fiber
of the ration was found to be 64.88, as compared with 43.96 and
33.33 in the second and fourth periods, an effect corresponding to
that observed by Kellner, and which Tryniszewsky also ascribes
to an increase in the micro-organisms of the digestive tract.

The results of the experiments which have been cited are, of
course, valid, in the first instance, only for the particular non-
proteids experimented with. If, however, the above interpretation
of the results is correct, it is to be anticipated that other soluble
nitrogenous substances in the food will be found to produce similar
effects. If this anticipation proves to be correct, then we shall
reach the following conclusions regarding the amides and similar
bodies in feeding-stuffs.

1. That they do not serve as sources of proteids.

2. That in rations very poor in protein they have, in the her-
bivora, an indirect effect in protecting part of the food protein
from fermentation in the digestive tract.

3. That in carnivora, and in herbivora on normal rations, they
probably have no effect on the production of nitrogenous tissue.

Note. — Since the foregoing lines were put in type the investiga-
tions of Cohnheim,* Loewi,f Kutscher&Seemann.| Abderhalden,§
Strusiewicz,! and others, seem to have shown that the proteid
cleavage in digestion is more complete than had been previously
believed. Cohnheim finds an enzym, ercp^in, in the small intestine
which acts energetically upon the peptones, forming crystalline
cleavage products, while Loewi and others state that the mixture
of "amides" thus produced is synthesized to proteids in the intes-
tinal epithelium and may completely replace proteids in the food.
The negative results of earlier experiments are ascribed to the fact
that usually a single amide was experimented upon and that con-
sequently but one out of the several molecular groupings necessary
for the reconstruction of the proteid molecule was present.

* Zeit, physiol. Chem., 33, 451. § Ibid., 44, 199.

t Centbl. f. Physiol., 15. 590. || Zeit. f. Biol., 47, 143.

% Zeit. physiol. Chem., 34, 528.

CHAPTER III.
METHODS OF INVESTIGATION.

Ax essential prerequisite for an intelligent study of the income
and expenditure of matter by the animal body is a knowledge of
the general nature of the current methods of investigation and of
the significance of the results attained by means of them. It is
not the purpose here to enter into technical details; this is not
a treatise upon analytical or physiological methods. The present
chapter will be confined to outlining the general principles upon
which those methods are based and to pointing out the logical
value of their results. It wall be confined, moreover, mainly to
those general methods by which the balance of income and ex-
penditure of matter is determined.

Tissue. — The animal body has already been characterized as
consisting, from the chemical point of view, of an aggregate of
various substances, chiefly organic, representing a certain capital
of matter and energy. These various substances are grouped
together in the body to form the organized structures known as
tissues. For the sake of brevity, then, it may be permissible to use
the word tissue as a convenient general designation for the aggre-
gate of all the organic matter contained in the tissues of the body,
including both their organized elements and any materials present
in the fluids of the body or in solution in the protoplasm of the
cells. In this sense tissue is equivalent to the whole capital or
store of organic matter in the body.

Gains and Losses. — The tissue of the body, as thus defined, is
in a constant state of flux, the processes through which the vital
functions are carried on constantly breaking it down and oxidizing
it (katabolism), while the processes of nutrition are as constantly
building it up again (anabolism). If the activity of nutrition

59

60 PRINCIPLES OF ANIMAL NUTRITION.

exceeds that of destruction, material of one sort or another is stored
up in the body, and such an addition to its capital of matter and
energy we may speak of as a gain of tissue. Conversely, if the
katabolic processes consume more material than the processes of
nutrition can supply, the store of matter and energy in the body
is diminished and a loss of tissue occurs. A simple comparison of
the amount of matter supplied in the food (including, of course,
the oxygen of the air) with that given off in the solid, liquid and
gaseous excreta, therefore, will show whether the body is gaining
or losing tissue.

The mere fact of a gain or loss of matter by the body, however,
conveys but little useful information unless we know the nature of
the material gained or lost. This we have no means of determining
directly. The processes of growth or decrease are not accessible
to immediate observation, while changes in the weight of the animal
(even aside from the great uncertainties introduced, especially in
the herbivora, by variations in the contents of the alimentary
canal) represent simply the algebraic sum of the gains and losses
of water, ash protein, fats, and other materials, and so give but a
very slight clue if any to the real nature of the tissue-building. We
are compelled, therefore, to have recourse to indirect methods, and
to base our conclusions as to tissue-building upon a comparison
of the income and outgo of the chemical elements of which the body
is composed, particularly of nitrogen and carbon.

The Schematic Body. — The basis of this method of compari-
son is the conception of the schematic body, first introduced by
Henneberg.* This conception regards the dry matter of the body
of the animal as composed essentially of three groups of substances,
viz., ash, fat, and protein, with at most comparatively small amounts
of carbohydrates (glycogen), and assumes that the vast number of
other compounds which it actually contains are present in such
small and relatively constant proportions as not to materially
affect the truth of this view. A knowledge of the ultimate compo-
sition of these three groups then affords the basis for a computation
of the gain or loss of each from the income and outgo of their ele-
ments.

Ash. — The ash ingredients of the body form a well-defined

*Neue Beitrage, etc., p. vii.

METHODS OF INVESTIGATION.

61

group, and the determination of the gain or loss of each ingredient
from a comparison of income and outgo is in principle a relatively-
simple matter and calls for no special consideration here.

Fat. — The elementary composition of the fat of the body has
been shown to be remarkably similar not only in different animals
of the same species, but likewise in different species. The classic
investigations of Schulze & Reinecke* upon the composition of
animal fat gave the following results :

Beef fat. . .
Pork fat .,
Mutton fat

Average

Dog

Cat

Horse

Mail

No. of
Sam-
ples.

Carbon.

10

6

12

28

Aver-

Maxi-

age

mum

Per

Per

Cent.

Cent.

76.50

76.74

76.54

76 7s

76.61

76.85

76.50

76.63

76 . 56

77.07

76.62

Hydrogen.

Mini-
mum
Per
Cent.

Aver-
age
Per

Cent.

76.2711.91
76.29,11.94

76.27 12.03

12.00

12.05
1 1 . 90
11.69
11.94

Maxi-
mum
Per

Cent.

12.11
12.07
12.16

Mini-
mum
Per
Lent.

Oxygen.

Aver-
age
Per

Cent.

11.7611.59
11.8611.52
11.87 11.36

11.50

1 1 . 32
11.44
11.24
11.44

Maxi-
mum
Per
Cent.

Mini-
mum
Per
Cent.

11.

11.83

11.56

8611.15
11.15
11.00

Benedict and Osterberg f obtained the following results for the
composition of human fat :

Carbon,

Per Cent.

Hydrogen,
Per Cent.

" " 2

" 3

" 4

" 6

" 7

" 8

Average

76.29
76.36
75.85
75.95
75.94
76.07
76.13
76.05

11.80
11.72
11.87
11.85
11.74
11.69
11 84
11.81

76.08

11.78

The fat of the body has been commonly regarded as containing
76.5 per cent, of carbon. A gain of 100 parts of fat by the body

*Landw. Vers. Stat., 9, 97.

fAmer. Jour. Physiol., 4, 69.

62

PRINCIPLES OF ANIMAL NUTRITION.

is accordingly equivalent to a gain of 76.5 parts of carbon, and con-
versely, if it be shown that the body has gained one part of carbon
in the form of fat, this is equivalent to again of 1-f- 0.765 =1.307,
or, in round numbers, 1.3 parts of fat. Benedict & Osterberg's
average corresponds to the factor 1.314.

Protein. — As in the case of the food, the term protein is used
to signify the whole mass of nitrogenous material in the body, in-
cluding, besides the true albuminoids, the collagens or gelatinoids,
the keratin-like bodies, the nitrogenous extractives, etc.

Neumeister * gives the following figures for the elementary
composition of the simple albuminoids:

Minimum,
Per Cent.

Maximum,
Per Cent.

Average,
Per Cent.

50
6.5

15
19
0.3

55

7.3
17.6
24

2.4

52

7

16

23

o

100

Some of the compound albuminoids, particularly the nucleo-
albuminoids, do not vary greatly in composition from the above
figures, while others notably the mucins, which contain a carbo-
hydrate group, show a higher percentage of oxygen and less carbon
and nitrogen.

The gelatinoids, likewise, do not differ greatly in composition
from the albuminoids. For collagen, Hofmeister f found the fol-
lowing averages:

Carbon. 50. 75

Hydrogen 6 . 47

Nitrogen 17.86

Oxygen j 24 91

Sulphur )

100.00

Keratin is distinguished by a relatively high proportion of
sulphur (3 to 5 per cent.), but otherwise, according to Neumeister, %
does not differ materially in composition from the true albuminoids.
*Lehrbuch der Physiol. Chem., p. 22. \ZqH. physiol. Chem., 2, 322.
{Loc. cit., p. 493.

METHODS OF INVESTIGATION.

63

Hoppe-Seyler * quotes the following figures for the composition
of epidermis and some of the tissues derived from it :

Epidermis
of Man.

Hair.

Nails.

Horn of
Cow.

Hoof of
Horse.

50.28

6.76

17.21

25.01

0.74

50.65

6.36

17.14

20.85

5.00

51.00

6.94

17.51

21.75

2.80

51.03

6.80

16.24

22.51

3.42

51.41
6 96

17.46

19.49

4.23

100.00

100.00

100.00

100.00

99.55 (?)

flenneberg f obtained the following figures for the composition of
two samples of pure and dry wool, calculated ash-free :

I. IT.

Carbon 49.67 49.89

Hydrogen 7.26 7.36

Nitrogen 16.01 16.08

Oxygen 23.65 23.10

Sulphur 3.41 3.57

100.00 100.00

The following analyses by Rubner,J Stohmann & Langbein,§
and Argutinsky|| show the ultimate composition of ash-free muscular
tissue after prolonged extraction with ether : 1"

Carbon,
Per Cent.

Hydro-
gen,
Per Cent.

Nitrogen,
Per Cent.

Sulphur,
Per Cent.

Oxyeren.
Per Cent.

Heat of
Com-
bustion
peiGiam.
Cals.

53.40

7.30
7.30

16.30
16.36
16.15

5.6561

Stohmann and Langbcin.

52.02
52.33

24
24

.32
.22

5.6409

f Neue Beitrage, etc., p. 98.

§Jour. f. prakt. Chem., N. F., 44, 364.

* Physiol. Chem., p. 90.

JZeit. f. Biol., 21, 310.

]Arch. ges. Physiol., 55, 345.

^f It has since been shown by Dornmeyer (Arch. ges. Physiol , 65, 90)

that such material is not fat- free.

64

PRINCIPLES OF ANIMAL NUTRITION.

Kohler * has investigated the elementary composition of the
muscular tissue of cattle, sheep, swine, horses, rabbits and
hens. The material was prepared with much care, the fat being,
removed as fully as possible by prolonged extraction with ether.
The residual fat which cannot be removed in this way was deter-
mined by Dornmeyer's digestion method,! and a corresponding
correction made in the analytical results. The following are his
averages for the fat- and ash-free substance :

No. of

Carbon,

Hydrogen,

Nitrogen,

Samples.

Per Cent.

Per Cent.

Per Cent.

Cattle

4

52.54

7.14

16 67

2

52.53

7.19

16.64

2

52.71

7.17

16 60

3

52.64

7.10

15.55

2

52.83

7.10

16.90

Hen

2

52.36

6.99

16.88

Sulphur,
Per Cent.

Oxygen,
Per Cent.

0.52
0.69
0.59
0.64

0.50

23.12
22.96
22.95

24.08

23.28

Heat of

Combustion

per Gram,

Cals.

5.6776
5.6387
5.6758
5.5990
5 6166
5.6173

All the samples were tested for glycogen, but only traces were
found, except in the horseflesh, for the two samples of which an
average of 3.65 per cent, was obtained, a result which accounts for
the low figure for nitrogen.

In the classic investigation by Lawes & Gilbert X into the com-
position of the whole bodies of animals, determinations were made
of the total dry matter, the ash, the fat, and the total nitrogen.
From these data Henneberg § has compared the total amount of
dry matter other than ash and fat with the total amount of nitro-
gen. His results in a slightly altered form are given in the table
opposite.

The average nitrogen content is 16.21 per cent. Lawes & Gil-
bert extracted the fat with ether and hence, as above noted, the
residue was not absolutely fat-free. Kohler's average results for the

* Zeit, phvsiol. Chem., 31, 479.
f Arch. ges. Physiol., 65, 102.
t Phil. Trans , 1859, II, 493.
§Neue Beitrage, etc., p. x.

METHODS OF INVESTIGATION.

65

Water

Dry matter

In the dry matter :

Ash

Fat

Other organic matter
by difference

Total nitrogen ,

Per cent, of nitrogen in
"other organic matter"

Ox.

Sheep.

Swi

Half Fat.
Per Cent.

Fat,

Per Cent.

Lean,
Per Cent.

Fat,
Per Cent.

Lean,
Per Cent.

56.1
43.9

48.6

51.4

61.0
39.0

46.2
53.8

58.2
41.8

100T00

100.00

100.00

100.00

100.00

5.1
20.7

4.1
31.9

3.4
19.9

2.9
37.9

2.8
24.6

18.1

15.4

15.7

13.0

14.4

43.9

51.4

39.0

53.8

41.8

3.0

2.4

2.55

2.1

2.3

16.58

15.59

16.24

16.19

15.97

Fat,
Per Cent.

42.9
57.1

"lOO.OO

1.7
44.0

11.4

57.1

1.9

16.66

flesh of cattle, sheep, and swine, after extraction with ether for

480 hours, computed ash-free, were:

Carbon 52.84

Hydrogen 7.22

Nitrogen 16.46

Oxygen 22.89

Sulphur 0.59

100.00

Considering the indirect method by which Henneberg's result
was reached, the agreement as regards nitrogen, both with Kohler's
results and with those of Rubner, Stohmann, and Argutinsky just
cited, is remarkably close. Henneberg assumed the following
round numbers to represent the average composition of the total
protein of the body, and his example has been generally followed
by subsequent investigators:

Carbon 53 per cent.

Hydrogen 7 " "

Nitrogen 16 " " ~"

Oxygen 23 " "

Sulphur ... 1 " "

100

66 PRINCIPLES OF ANIMAL NUTRITION.

Kohler's averages for dry, fat-free flesh are:

Carbon 52 . 60 per cent.

Nitrogen 16.54 " "

Glycogen. — Of the substances other than ash, fat and protein,
which are found in the animal body, only glycogen calls for special
mention here. This body, as we have seen, may be stored up in
considerable amounts in the liver, and is found also in the muscles,
although not in large proportion, except in case of the horse. In
the aggregate, however, the store of glycogen in the body is not
inconsiderable, having been estimated to be in the neighborhood
of 300 grams in the human body. Moreover, changes of food or
conditions, as well as muscular activity, may materially alter the
store of glycogen and thus, perhaps, appreciably affect the make-
up of the schematic body.

So far as appears, however, the capacity of the body to store up
glycogen is limited, as is indicated by the relatively small amount
of it formed after even the most abundant feeding, and we may
fairly assume that, at least on a ration equal to or exceeding the
maintenance requirements, no long-continued change in the amount
of glycogen in the body is likely to occur.

Summary. — We may sum up the foregoing paragraphs in the
brief statement that for the purpose of investigating the statistics
of nutrition we may consider the organic part of the animal body
as composed essentially of fat and protein, with small amounts of
glycogen, and that we may regard the permanent effect of a ration
upon the body as consisting (aside from its effect on the ash ingre-
dients) in an increase or decrease of its stores of fat and protein,
these substances having the average compositon indicated above.
The Gain or Loss of Protein. — Since the term protein as here
used is synonymous with total nitrogenous matter, the gain or loss
of protein by the body is necessarily indicated by its gain or loss of
nitrogen.

The supply of nitrogen to the body is contained in the pro-
tein of the food. The losses of nitrogen from the body are
contained —

First, in that part of the protein of the food which fails of
digestion and is excreted in the feces.

METHODS OF INVESTIGATION. 67

Second, in the nitrogenous products of the proteid metabolism,
contained chiefly in the urine but including also the small quanti-
ties of nitrogenous metabolic products contained in the feces and
perspiration.

The nitrogen of urine and perspiration, then, together with the
metabolic nitrogen of the feces, will indicate the extent of proteid
katabolism, while the difference between total income and total
outgo of nitrogen will show whether the body is gaining or losing
protein. Finally, since the losses of metabolic nitrogen in feces and
perspiration are relatively small, and often not readily determinable,
in cases where the greatest accuracy is not required, and particularly
in comparative experiments, we may regard the total urinary nitro-
gen as representing with a fair degree of accuracy the amount of
protein broken down by the organism.

In the foregoing statements, however, it has been tacitly assumed
that the protein of the food consists of true proteids. If, how-
ever, the latter are accompanied by amides and other non-proteid
nitrogenous bodies, which do not appear to contribute to the forma-
tion of proteid tissue (compare p. 53), the corresponding amount
of nitrogen will appear in the urine and be added to that derived
from the actual katabolism of body or food proteids. This, how-
ever, does not, of course, affect any conclusions as to the gain or loss
of protein by the body.

Factor for Protein. — It is thus comparatively easy to deter-
mine in terms of nitrogen both the proteid katabolism and the
gain or loss of protein, the principal precaution necessary, aside
from technical details, being that the experiment shall extend over
a sufficient length of time to eliminate the influences of irregulari-
ties in ingestion and excretion.

Knowing approximately the ultimate composition of the pro-
tein of the body, we may take a step further and infer from the
amounts of nitrogen determined the corresponding amounts of
protein, the accuracy of the result depending, of course, upon the
accuracy of the factor on which it is based. The composition
commonly assumed for the body protein has been that given on
page 65, and the same conventional factor, 6.25, has been used
to convert nitrogen into protein which has been employed in case
of feeding-stuffs. Kohler's investigations (p. 64) show that the

68 PRINCIPLES OF ANIMAL NUTRITION.

nitrogenous organic matter of muscular tissue has a materially
higher percentage of nitrogen, viz., about 16.67 per cent. This
would reduce the factor for protein from 6.25 to 6.00. Kohler's
samples, after extraction with ether for 480 hours, still contained
from 0.27 to 1.61 per cent, of fat. If we assume the ash and
fat-free substance of Lawes & Gilbert's experiments (p. 65) to
have still contained 1 per cent, of fat, the average nitrogen con-
tent of the fat-free substances would be 16.38 per cent, and the
corresponding protein factor 6.11, while the factor 6.00 would re-
quire the assumption of a fat-content of 2.7 per cent.

The factor 6.0 has been used by Kellner in computing the results
of his extensive investigations upon cattle at Mockern. Strictly
speaking, this assumes that all the gain of nitrogen takes place
either in the form of muscular tissue or of material of the same
average composition. To what extent such an assumption is
justified it is difficult to say. Certainly a part of the protein of the
food is applied to the production of epidermis, hair, horns, hoofs,
etc., consisting largely of keratins. The data regarding the com-
position of these tissues given on p. 63 would seem to show-
that they are, on the average, richer in nitrogen than muscular
tissue, a fact which would tend to lower the protein factor, but on
the other hand, the amount of this growth is small as compared
with the usual protein supply. On the whole, Kohler's factor
would seem to afford the most trustworthy basis of computation
which is at present available, especially in view of its close agree-
ment with Lawes & Gilbert's results.

Urea as a Measure of Proteid Metabolism. — In the earlier
investigations upon this subject, the urea of the urine, as deter-
mined by Liebig's titration method, was commonly taken as the
measure of proteid metabolism, one part of urea equaling 2.9 parts
of protein, while in many cases the metabolism was also expressed
in terms of "flesh" (muscular tissue) with its normal water con-
tent and an average of 3.4 per cent, of nitrogen. The errors inci-
dent to the use of this method are now generally recognized, while
its inapplicability to herbivora was obvious from the first, and with
the improvements in the methods of nitrogen determination, the
latter has almost entirely replaced the old urea determination and

METHODS OF INVESTIGATION. 69

the proteid metabolism is now almost exclusively expressed in
terms of either nitrogen or protein.

The Gain or Loss of Fat. — As the balance between income
and outgo of nitrogen serves to measure the gain or loss of protein
by the schematic body, so the balance between income and outgo
of carbon furnishes the means for estimating the gain or loss of fat.

The income of carbon is, of course, the carbon of the food.
The outgo of carbon consists of —

First, the carbon of the undigested food contained in the feces.

Second, the carbon of the products of metabolism contained
in feces, urine, and perspiration.

Third, the carbon of the gaseous excreta, including the carbon
dioxide given off by the lungs and skin and the carbon dioxide and
hydrocarbons resulting from fermentations in the digestive tract.

Respiration Apparatus. — The carbon of the visible excreta
is readily determined by the ordinary analytical methods. The
determination of the carbon of the gaseous excreta requires the use
of a special apparatus, commonly called a respiration apparatus.

In early experiments upon respiration the animal was simply
placed in a known confined volume of air which was analyzed before
and after the experiment. By this method, however, the oxygen
of the air is progressively diminished, while the respiratory products
accumulate, both of which conditions are liable to disturb the
normal respiratory exchange, although Kaufmann,* who has re-
cently reverted to this primitive method, claims to have secured
accurate results in rather short experiments.

The obvious desirability of renewing the oxygen and removing
the products of respiration soon led to the construction of more
complicated forms of apparatus of which three principal types
may be distinguished.

The Regnault Apparatus. — The oldest of these is the Rcgnault f
or closed circuit respiration apparatus. In this type of apparatus
the subject breathes in a confined volume of air, the carbon dioxide
being removed by suitable absorbents and weighed, while the oxy-
gen consumed is replaced from a receiver containing pure oxygen,
the amount admitted to the apparatus being measured. These

* Archives do Physiol., 1896, p. 329.

f Regnault & Reiset, Ann. de Chim. et de Physique, 3d series, 26, 299.

70 PRINCIPLES OF ANIMAL NUTRITION.

data, with the addition of analyses of the known volume of air
contained in the apparatus at the beginning and end of the
experiment, afford the means of computing both the carbon dioxide
and other gases given off and the oxygen cousumed.*

In theory this is the most complete and satisfactory type of
respiration apparatus, since it permits a determination of the total
gaseous exchange. Serious practical difficulties have been found
in its use, however, especially for the larger animals, among them
the difficulty of maintaining the air reasonably pure, the difficulty
of securing a uniform temperature and mixture of the gases in a
large and complicated apparatus, and the liability to contamination
of the oxygen used. Seegen & Nowak f used an apparatus of
this type for their experiments upon the excretion of gaseous nitro-
gen by animals (see p. 42). Laulanie \ has described a Regnault
apparatus for small animals in which a continuous graphic measure-
ment of the oxygen admitted to the apparatus is made, Hoppe-
Seyler § has constructed at Strasburg an apparatus of this type
large enough to contain a man, Bleibtreu | has recently made use
of a small one to investigate the formation of fat in geese, and
Atwater & Benedict have perfected one for experiments on man.lf

The Pettenkofer Apparatus. — The second type of respiration
apparatus is that of v. Pettenkofer. In this type the subject
breathes in a closed chamber through which a measured current
of air is maintained.

Scharling ** appears to have been the first to construct an appa-
ratus of this sort. The ingoing air was freed from carbon dioxide
by passing- through potash solution, while the outcoming air, after
drying, gave up its carbon dioxide to a weighed potash bulb. Vari-
ous similar forms of apparatus were constructed, but it was found

* For a description of the apparatus, see also Hoppe-Seyler, Physiol.
Chem., pp. 526 and 536.

f Sitzungsbcr. Wiener Akad., Math.-Naturwiss. Classe, 71, Til, 329;
Arch. ges. Physiol., 19, 349.

JArchives de Physiol., 1890, p. 571.

gZeit. physiol. Chem., 19, 574.

|| Arch. ges. Physiol., 85, 366.

IF See also Pflilger and Colasanti (Arch. ges. Physiol., 14, 93) and Schulz
(/&., p. 78).

**Ann. Chem. Pharm., 45, 214.

METHODS OF INCEST/CATION. 7 1

to be impossible to secure complete absorption of the carbon dioxide
and at the same time maintain adequate ventilation.

In 1S62 v. Pettenkofer* introduced the important improve-
ment of diverting a known aliquot of both the ingoing and outcom-
ing air for analysis. The results of these analyses, calculated upon
the whole volume of air used, show the amounts of carbon dioxide
and other gases added by the subject of the experiment.

The Pettenkofer apparatus has the advantage of placing the
subject under unquestionably normal conditions as to purity of
air, of maintaining a practically uniform temperature and mixture
of gases throughout the apparatus, and of dispensing with the ex-
treme care necessary in the Regnault apparatus to prevent gaseous
diffusion between the air outside and that inside the apparatus.
Its great drawback is that it does not in practice permit the deter-
mination of the amount of oxygen consumed.! To this is to be
added the magnification of experimental errors involved in com-
puting the results obtained by the analysis of small samples upon
the whole volume of air used.

Despite these drawbacks, however, the Pettenkofer apparatus
in various forms has been widely used, especially in experiments
upon domestic animals, and has shown itself capable of yielding
very accurate results within its scope. Laulanie,} by largely re-
ducing the rate of ventilation, has been able to make determinations
of the oxygen consumed which he regards as satisfactory, while
Haldane § has constructed an apparatus for small animals, in which
the entire air current is passed over absorbents before entering
and after leaving the apparatus, which also permits of a satisfac-
tory indirect determination of the oxygen consumed. Sonden and
Tigerstedt | have also constructed a modified Pettenkofer respira-

* Ann. Chem. Pharm., Suppl. Bd. II, p. 1. See also Atwater, U. S. Dep.
Agr.. Office of Experiment Stations, Bull. 21, p. 106.

fSuch a determination is theoretically possible from a comparison of the
oxygen content of ingoing and out coming air, but the delicacy of the measure-
ments and analyses required is so great as to render the method impracti-
cable, while the determination by difference concentrates all the errors in
this one quantity.

X Archives de Physiologie, 1895, p. 619.

§Jour. Physiol., IS, 419.

flSkand. Arch. Physiol., 6, 1.

72

PRINCIPLES OF ANIMAL NUTRITION.

tion apparatus of very large dimensions. Recently Atwater &
Rosa * have constructed a form of Pettenkofer apparatus for use
as an animal calorimeter in which the method of measuring and
sampling the air current has been materially improved and rendered
more accurate.

When the Pettenkofer apparatus is employed for experiments
upon herbivora, special provision is necessary for the determination
of the gaseous hydrocarbons excreted in considerable quantities
by these animals. This is accomplished by passing a sample of the
air coming from the apparatus through a combustion- tube contain-
ing copper oxide, or preferably spongy platinum (platinized kaolin),
heated to redness. The hydrocarbons are thus oxidized and the
resulting carbon dioxide determined.

Pettenkofer & Voit,f in their earlier investigations, deter-
mined the excretion of combustible gases by a dog, with the follow-
ing results per day:

Food.

Hydrogen,

G rains.

Methane,
Grams.

Carbon

Meat,
Grams.

Fat,
Grams.

Starch,
Grams.

Dioxide,
Grams.

500

200
200
200

7.2
5.2
7.2
6.4
4.3

4.1
6.3
4.7
3.7
4.5

416.0

500

420.6

500

428.2

500
500

200
200

417.3

427.8

According to the above figures, a trifle less than 3 per cent.,
on the average, of the total carbon excretion was in the form of
methane. No similar determinations seem to have been made by
Pettenkofer & Voit in their later experiments, and it appears to
be generally assumed that they are unnecessary in investigations
upon man and the carnivora.

The Zuntz Apparatus. — Both the Regnault and the Pettenkofer
types of apparatus are calculated for the determination of the
total gaseous excreta of lungs, skin, and digestive tract through
considerable periods of time, and their use enables us to compare
the total income and outgo of carbon.

* U. S. Dept. Agr., Office of Experiment Stations, Bulletins 44 and 63.
t Ann. Chem. Pharm., Supp. Bd. II, p. 66.

METHODS OF INVESTIGATION. 73

The third type of respiration apparatus is best known by the
name of Zuntz,* from the extensive development given it by this
investigator, although it has assumed various forms in the hands
of different experimenters. This apparatus is radically different
from the other two types in that it is intended simply for the deter-
mination of the respiratory exchange in the lungs. For this pur-
pose the expired air is collected, either by means of a mask or a
tracheal cannula, its volume measured, and its content of carbon
dioxide and of oxygen determined in an aliquot sample, the com-
position of the inspired air being assumed to be that of the normal
atmosphere. The fundamental principle is really that of the Petten-
kofer apparatus, but, owing to the fact that the excretory gases
are not diluted with many times their volume of air, the results are
much sharper and it is possible to determine the amount of oxygen
consumed as well as of the carbon dioxide given off. In addition
to this advantage, it permits the experimenter to follow the varia-
tions in the respiratory exchange in comparatively short periods.
It is thus especially adapted for investigating such questions as the
influence of muscular work upon metabolism, and it is in the study
of this question that it has found its chief application. On the
other hand, it is impracticable to continue its use through long
periods — a day, e.g. — and it takes no account of the excretion
through the skin and the alimentary canal. Only by indirect
methods, therefore, is it possible to compute the total income and
outgo of carbon by its use.

But while the Zuntz form of respiration apparatus is especially
adapted for investigating the carbon metabolism during short
periods, it is important that these periods be not made too short.
What is actually determined by the use of any form of respiration
apparatus is the excretion or absorption of carbon dioxide or oxy-
gen. In .an experiment extending over several hours, we may
fairly assume that this is substantially a measure of the actual pro-
duction or consumption of these gases going on in the tissues. In
periods of a few minutes, however, there is always a possibility of
an accumulation of oxygen or a partial retention of the products
of metabolism in the tissues or the blood, while, on the other hand,

*Rohrig & Zuntz, Arch. ges. Physiol., 4, 57; v. Mehring & Zuntz, ib., 32,
17:5 ; Ceppert & Zuntz, ib., 42, 189.

74 PRINCIPLES OF ANIMAL NUTRITION.

the products of previous metabolism may be added to those formed
during the experiment. This is especially true of the carbon diox-
ide, particularly in work experiments, where the rate and volume
of respiration are largely affected. During severe work, there may
be more or less accumulation of this gas in the blood, while, on the
other hand, the increased respiration in an immediately following
period of rest may reduce the proportion in the blood below the
normal. The oxygen is thought to be far less subject to this error
than the carbon dioxide, and therefore to be a more accurate indi-
cator of the total metabolism.

The Respiratory Quotient. — This name was given by Pfliiger
to the ratio of the volume of carbon dioxide excreted to the volume
of oxygen consumed in the same time. It is frequently represented

CO

by the abbreviation R.Q., or by the symbol -~r *•

It is obvious that this ratio will vary with the nature of the
material metabolized. Thus the oxidation of a carbohydrate, e.g.
dextrose, will give rise to a volume of carbon dioxide equal to that of
the oxygen consumed, since, as the following equation shows, each
molecule of oxygen gives rise to a molecule of carbon dioxide:

C6H1206 + 602 = 6C02 + 6H20.

In this case the respiratory quotient is equal to unity. On the
other hand, when fat is oxidized, a portion of the oxygen combines
with the hydrogen of the fat to form water, and the volume of car-
bon dioxide produced is less than that of the oxygen employed.
Representing the process by the equation used by Chauveau,* viz.,

2C57H110O6+ 1630,= 114C02+ 110H2O,

114

the respiratory quotient is -^,5 = 0.6993. Computed from -the aver-
age percentage composition of animal fat as given on p. 61, it
equals 0.7069.

The proteids of the food, as we have seen, are not completely
oxidized in the body, a portion of their carbon, along with all their
nitrogen, being excreted in the form of urea and other organic

*La Vie et l'Energie chez l'Animale.

METHODS OF INVESTIGATION. 75

compounds in the urine. Chauveau & Kaufmann,* starting with
an empirical formula for albumin, represent its complete meta-
bolism in the body by the equation

2C7,H112X18022&, + 1 5 1 02 = 18CH4X20 + 1 26C02 + 76H20 + S2,

126
thus obtaining the respiratory quotient —=0.8344, neglecting

the oxygen required to oxidize the sulphur.

The urine, however, always contains greater or less quantities
of nitrogenous compounds richer in carbon than urea, and in herbiv-
orous animals in particular such compounds are abundant. The
respiratory quotient of the proteids is therefore variable, depend-
ing upon the extent to which their carbon is completely oxidized.
Thus Zuntz and Hagemann f in an experiment upon the horse
in which approximately 15 per cent, of the total nitrogen of the
urine was contained in hippuric acid, compute it at 0.765.

Deductions from Respiratory Quotient.- — The value of a determi-
nation of the respiratory quotient lies in the clue which it affords to
the nature of the substances which are being oxidized in the body.
Assuming that the materials available for oxidation in the schematic
body are substantially proteids, carbohydrates and fat it is evi-
dent that when the quotient approaches 1.0 the material consumed
must consist largely of carbohydrates, while if it falls to the neigh-
borhood of 0.7 it is clear that the oxygen is combining chiefly with
fat. An intermediate value, on the other hand, would be more am-
biguous, since it might result from the oxidation of proteids, carbo-
hydrates and fat in several proportions.

Tf, however, the amounts of oxygen consumed and of carbon
dioxide produced in the oxidation of any one of the three groups be
known, it is a simple matter to compute the proportion in which
the other two enter into the reaction. For the amount of proteids
metabolized, we have an approximate measure in the total urinary
nitrogen. If we can also determine the amounts of carbon, hydro-
gen and oxygen contained in these nitrogenous urinary products,
we can compute the quantity of oxygen required to oxidize the non-
nitrogenous residue of the proteids and the amount of carbon diox-
ide resulting from it upon the assumption of complete oxidation.

♦Compare p. 51. fLandw. Jahrb., 27, Supp. Ill, 240.

76 PRINCIPLES OF ANIMAL NUTRITION.

As a matter of fact, however, it is not easy to determine satis-
factorily the proportion of the respiratory exchange due to the
proteids, both because the nitrogenous products of their meta-
bolism are numerous and occur in varying proportions in the urine,
and because we may not always be justified in assuming complete
oxidation of the non-nitrogenous residue. Computations of the
nature indicated above, therefore, must be accepted with some
reserve.

A simpler case, and one which has been extensively investigated,
is the nature of the increased metabolism arising from muscular
exertion. As we shall see in a succeeding chapter, such exertion
causes a marked increase in the respiratory exchange while pro-
ducing at most but a slight effect upon the proteid metabolism.
If we neglect altogether this latter effect, the ratio between the in-
crements of carbon dioxide and oxygen will indicate whether the
additional material consumed during the performance of the work
consisted of fat or carbohydrates or a mixture of the two, of course
on the same assumption as before, viz., that substantially only
these two classes of substances are available in the schematic body.

For example, in an investigation by Zuntz, cited on a subsequent
page, the performance of one kilogram-meter of work of draft
by a dog caused the following increments in the respiratory ex-
change :

Oxygen 1.6704 c.c.

Carbon dioxide 1 . 4670 "

Respiratory quotient 0 . 878

Assuming, as above, that these amounts arise from the oxida-
tion of fat and carbohydrates only, let x equal the amount of oxy-
gen consumed in the oxidation of fat and 1.6704 — a; the amount
consumed in the oxidation of carbohydrates. Since the respira-
tory quotient of fat is 0.7069, the x cubic centimeters of oxygen
would yield 0.7069a; cubic centimeters of carbon dioxide, while the
1.6704 — z cubic centimeters of oxygen used to oxidize the carbohy-
drates would yield an equal volume of carbon dioxide. We there-
fore have —

0 . 7069a; + ( 1 . 6704 - x) = 1 . 4670,
whence x = 0. 6939.

METHODS OF INVESTIGATION. 77

The division of the increments of the respiratory gases was accord-
ingly—

Oxygen Carbon Dioxide

Consumed. Produced.

By fat 0.6939 c.c. 0.4905 c.c.

By carbohydrates 0 . 9765 " 0 . 9765 "

_____ it

Total 1.6704 " 1.4670 "

From these data the actual amounts of fat and carbohydrates
metabolized can be readily computed, one gram of fat requiring for
its oxidation 2.S875 grams (2.028 liters) of oxygen and producing
1.434 liters of carbon dioxide, while one gram of a carbohydrate of
the composition of starch requires 1.185 grams (0.832 liter) of
oxygen and produces the same volume of carbon dioxide.

Computation of Fat from Carbon Balance. — While the use
of the Zuntz type of respiration apparatus may afford invaluable
information regarding the nature of the chemical changes going
on in the body, a satisfactory determination of the gain or loss of
carbon by the body usually requires the employment of one of the
other types of apparatus.* Having by such means added a deter-
mination of the carbon balance to that of the nitrogen balance, we
have the data necessary for computing the gain or loss of fat as well
as of protein by the schematic body.

For this purpose we first compute the gain or loss of protein
in the manner already described. Using KOhler's factor for pro-
tein (p. 67), a gain of 16.67 grams of nitrogen is equivalent to a
gain of 100 grams of protein. This 100 grams of protein will
contain, according to Henneberg, 53 grams, or according to Kohler,
52.6 grams of carbon. Any gain of carbon in excess of this amount
must therefore be in the form of non-nitrogenous organic matter,
while if less than this amount of carbon has been gained the non-
nitrogenous matter of the body must have been drawn upon to
supply the difference. The only non-nitrogenous organic substance
assumed to be present in the schematic body, however, is fat, con-
taining on the average 76.5 per cent, of carbon (p. 61). Neces-

*For a direct comparison of results obtained upon the horse by the Zuntz
and the Pettenkofer forms of apparatus, see Lehmann, Zuntz, & Hagemann,
Landw. Jahrb., 23, 125.

78 PRINCIPLES OF ANIMAL NUTRITION.

sarily, then, on this assumption, each gram of carbon gained in
excess of that stored in the form of protein will represent 1.3 grams
of fat stored.

Formation of Glycogen. — Granting the substantial accuracy of
the computation of the gain or loss of protein, the only serious
criticism to which the above method of computing the gain or loss
of fat is subject is that it does not take account of the possible stor-
age of carbon in other forms, and particularly as glycogen. In
other words, it may be contended that the schematic body should
be regarded as consisting of water, ash, fat, and carbohydrates.
There is undoubtedly a certain degree of justification for this con-
tention, and the significance of small gains of carbon, or of gains
observed during short periods, is by no means unambiguous. But
when such a gain is observed to continue day after day for weeks
on an unchanged ration, as in some of the experiments cited on
subsequent pages, the objection loses all force.

Computation of Total Metabolism.— The same principle
may be applied to the computation of the total amount of protein
and fat metabolized. From the urinary nitrogen (plus that of the
feces if the latter be regarded as a metabolic product) by multipli-
cation by the conventional factor we obtain, as already explained,
the total proteid metabolism. Subtracting the amount of carbon
corresponding to this quantity of protein from the total carbon
excretion leaves a remainder which must have been derived from
non-nitrogenous material. If carbohydrates are absent from the
food, this material, in an experiment of any length, must be
substantially fat, and the amount of the latter can be computed
from the carbon by the use of the factor 1.3. In the presence of
any considerable amount of carbohydrates, however, the results
are ambiguous unless we know also the quantity of oxygen con-
sumed.

Other Determinations. — The great majority of investigations
upon the metabolism of matter have been confined to determi-
nations of the nitrogen and carbon balance. Occasionally, how-
ever, other determinations have been made.

Hydrogen Balance. — Determinations of water and of hydrogen
in organic combination in food and excreta enable us, after making

METHODS OF INVESTIGATION. 79

allowances for the hydrogen gained or lost in protein and fat, to
compute the gain or loss of water by the body.

With the earlier forms of respiration apparatus, great diffi-
culty was experienced in obtaining satisfactory results for the
water,* and Stohmann f has traced the difficulty to an invisible
condensation of water on the walls of the chamber and connections.
More recently Rubner % has been able to make satisfactory deter-
minations of water with a Pettenkofer apparatus by avoiding as
much as possible differences of temperature between different parts
of the apparatus and by taking the sample of the outcoming air for
analysis as close to the respiration chamber as possible. Atwater
& Rosa have shown that their form of Pettenkofer apparatus
(p. 72) permits of very accurate determinations of water.

Oxygen Balance. — Owing to the technical difficulties already
indicated in considering the different types of respiration apparatus,
direct determinations of the oxygen balance have rarely been made.
This is the more to be regretted since such a determination would
-erve to check those of nitrogen, carbon, and hydrogen, and would
be a test of the accuracy of our deductions from those determina-
tions as to the nature of the material gained or lost by the body.

Ash Ingredients. — The gain or loss of ash ingredients can of
course be readily determined, but the subject as yet has hardly
received the attention which it deserves.

Sulphur and Phosphorus. — Sulphur forms an essential con-
stituent of the proteids, while phosphorus enters into the composi-
tion of the nucleins and also of lecithin. The determination of the
income and outgo of these two elements is often of value in rela-
tion to special physiological questions, but from the somewhat
general standpoint of this work may be considered as of rather
minor importance.

*Zeit. f. Biol., 11, 126.
fLandw. Vers. Stat., 19, 81.
% Arch. f. Hygiene, 11, 160.

CHAPTER IV.
THE FASTING METABOLISM.

The matter which the animal organism derives from its food is
applied substantially in three general directions : first, to the main-
tenance of those vital activities, such as circulation, respiration,
secretion, the metabolic activity of the various tissues, etc., and
probably to some extent the direct production of heat, which in
their entirety make up the physical life of the organism ; second,
to the support of those functions by which the crude materials
ingested are prepared to nourish the body, that is, to the work of
digestion and assimilation; third, to the production of external
mechanical work or to the storage of surplus material in the form
of growth of tissue.

Of these three general functions of the food, the one first named
is obviously of fundamental significance, and a determination of
the nature and amount of its demands constitutes the natural first
step in a study of the laws of nutrition. For this purpose we can
eliminate the influence of the other two factors by keeping the ani-
mal as nearly as possible in a state of absolute rest and by with-
holding food. Under these circumstances the expenditure of matter
from the tissues of the body may be taken as representing the
miminum demands of the vital functions. It will therefore be both
logical and convenient to consider first, in the present chapter, the
fasting metabolism of the quiescent animal, while in succeeding chap-
ters we take up the influence respectively of the food-supply and of
external work upon metabolism. The protein of the food has such
peculiar and distinct functions in the animal economy that it will
be a matter of practical convenience to follow the historical order
of investigation and consider first the proteid metabolism by itself

80

THE FASTING METABOLISM. 81

and subsequently the total metabolism as shown by the combined
nitrogen and carbon balance.

§ i. The Proteid Metabolism.

Tends to Become Constant. — When food is withheld from a
wrell-nourished animal, particularly a carnivorous animal, the proteid
metabolism usually diminishes, at first rapidly and more slowly later,
until within a few days it reaches a minimum value which may then
remain nearly unchanged for a considerable time. This was first
shown by the investigations of Carl Voit, in conjunction with
Bischoff and later with v. Pettenkofer, and has been fully confirmed
by later results.

The following table shows the results obtained by Voit * in
several experiments upon a dog weighing about 35 kgs., the pro-
teid metabolism being expressed in grams of urea per day. As
noted in Chapter III, such results are not absolutely accurate and do
not represent the total proteid metabolism, but the fact that they
are comparable is sufficient for our present purpose.

Previous Food per Day.

1800 Grms.

2500 Orms.

Meat;

1500 Grms.

1500 Grms.

Nothing.

Meat.

250 Grms.

Meat.

Meat.

Fat.

Urea per day:

Grms.

Grms.

Grms.

Grms.

Grms.

Last day of feeding

180.8

130.0

110.8

110.8

24.7

1st

60.1

37.5

29.7

26 5

19.6

2d '

' '«

24.9

23.3

18.2

18.6

15.6

3d '

' " ....

19.1

16.7

17.5

15.7

14.9

4th •

• "

17.3

14.8

14.9

14.9

13.2

5th '

' "

12.3

12.6

14.2

14.8

12.7

6th «

• ««

13.3

12.8

13.0

12.8

13.0

7th '

' ««

12.5

12.0

12.1

12.9

8th '

t <<

10.1

12.9

12.1
11.9
11.4

9th '

10th '

< ><

Two Factors of Proteid Metabolism. — In these, as in many
similar experiments, the proteid metabolism was quite unequal on
the last day of the feeding and on the first fasting day, but in a

*Zeit. f. Biol., 2, 311.

82

PRINCIPLES OF ANIMAL NUTRITION.

comparatively short time it sank to a minimum which was practi-
cally the same in all the experiments upon this particular animal,
viz., the equivalent of about 12 grams of urea per clay. This mini-
mum we may fairly regard as representing the necessary and inev-
itable destruction of proteids involved in the vital processes of
the organism, and therefore may consider as taking place also
when the animal was fed. If, now, we subtract from the total
urea excreted the 12 grams corresponding to the minimum de-
mand of the body, there is revealed the second and variable factor
of the proteid metabolism, which is large in the well-fed animal but
rapidly disappears during fasting, as the following table shows:

Previous Food per Day.

1800 Grms.

•2500 Grms.

Meat and

1500 Grms.

1500 Grms.

Nothing.

Meat.

^50 Grms.

Meat.

Meat.

Fat.

Urea per day:

Grms.

Grms.

Grms.

Grms.

Grms.

Last day of feeding

168.8

118.0

98.8

98.8

12.7

1st " " fasting

48.1

25.5

17.7

14.5

7.6

2d " " "

12.9

11.3

6.2

6.6

3.6

3d " " "

7.1

4.7

5.5

3.7

2.9

4th " " "

5.3

2.8

2.9

2.9

1.2

5th " " "

0.3

0.6

2.2

2.8

0.7

6th " " "

1.3

0.8

1.0

0.8

1.0

7th " " "

0.5

0.0

0.1

0.9

8th " " "

-1.9

0.9

0.1

9th " " "

- 0.1
-0.6

10th " " "

Organized and Circulatory Protein. — It is evident from
the above results, and will appear still more clearly when we come
to consider the influence of the food-supply upon proteid meta-
bolism, that in addition to the great mass of proteid tissue in the
body, whose metabolism results in the excretion of a relatively
small and constant amount of nitrogenous products, the well-
nourished organism may also contain variable amounts of nitrogen-
ous matter which is subject to rapid metabolism and winch speed-
ily disappears during fasting. Voit employed the term circula-
tory protein (Zirkulaiionseiiveiss) to designate this variable store
of rapidly metabolized nitrogenous matter, which he regards as
being substantially the dissolved protein which penetrates from

THE FASTING METABOLISM. 83

ihe blood and lymph into the cells of the tissues, while he termed
the protein of the organized tissues, which is relatively stable and
but slowly metabolized, organized protein (Organeiweiss). The
amount of the circulatory protein is small in all cases as compared
with that of the organized protein, its absolute amount being de-
pendent, as the above tables indicate and as will appear more
clearly in the next chapter, upon the supply of proteids in the
food. Owing to its rapid metabolism, however, it furnishes by
far the larger part of the nitrogenous waste products in the liberally
fed animal.

That the anatomical distinctions implied in the terms used by
Voit correspond to the actual facts of the case has been disputed
and may be open to question, but for our present purpose this does
not particularly concern us. The fact of the existence of the two
factors of pr.oteid metabolism, viz., a variable one, depending upon
the previous food-supply and a relative!}' constant one independ-
ent of the latter is fully established, by whatever names we may
choose to call them.

A Minimum of Protein Indispensable. — "While the proteid
metabolism of the fasting animal is speedily reduced to relatively
small proportions, it is never entirely suspended as long as the
animal lives. Moreover, to anticipate a portion of the follow-
ing chapter, even the most liberal supply of non-nitrogenous
nutrients is powerless to suspend or very greatly reduce the pro-
teid metabolism of a fasting animal. A certain amount of proteid
metabolism is indissolubly associated with the continuance of life,
and neither the fat of the body nor the non-nitrogenous ingredients
supplied in the food can perform these special functions of protein
in the body.

§ 2. Total Metabolism.

Constant Loss of Tissue. — Common observation, no less than
scientific investigation, teaches that a fasting animal suffers a con-
tinual loss of tissue. Such an animal derives the energy required
for its vital activities from the metabolism of its store of proteids
and of fat. As regards the former, we have just seen that in a
short time, or as soon as the influence of the previous supply of

84

PRINCIPLES OF ANIMAL NUTRITION.

proteids in the food is exhausted, the proteid metabolism reaches
a minimum and thereafter remains nearly constant for a consider-
able time, and subsequent investigations have shown that this
constancy is still more marked when the proteid metabolism is
computed per unit of live weight.

What has thus been found to be true of the proteid metabolism
has also been shown to hold good of the total metabolism of pro-
teids plus body fat. As soon as the influence of the previous food
has disappeared, the rate of metabolism of both proteids and fat
shows but slight variations throughout a considerable time. Of
the early experiments of Pettenkofer and Voit, the following * may
be cited as illustrating approximately this constancy:

Series a, 1862.

Series b, 1861.

March 10,
0th Day.

March 14,
10th Day.

April 5,
2<1 Day.

April 8,
5th Day.

April 11,
8th Day.

Kgs.
31.21

Grms.
104.1
5.95

37.18
107.

1.19
3.43

Kgs.
30.05

Grms.
82.4
5.23

32.69
83.

1.09

2.76

KKs.
32.87

Grms.

108.7

11.6

72.51

Kgs.

31.67

Grms.
100.0

5.7
35.63

Kgs.
30.54

Carbon of excreta

Total loss:

Proteids

Grms.
93.2

4.7

29.38

Fat

86. 103.

99.2

Loss per Kg. live weight:
Proteids

2.21
2.62

1.13
3.25

0.96

Fat

3.25

Finkeler f determined the respiratory exchange of fasting
guinea-pigs in two-hour periods. Upon the highly probable assump-
tion that their proteid metabolism was relatively small and con-
stant, the results of such experiments would furnish a measure of
the relative intensity of the total metabolism. Finkeler 's average
results are contained in the table on the opposite page.

But a slight decrease in the amount of oxygen consumed is
observed in the different stages of the fasting, while there is a
marked decrease in the amount of carbon dioxide produced. The
relation between these two quantities, as expressed by the respira-
tory quotient,^ shows us that at the beginning of the fasting the
metabolism was largely at the expense of the carbohydrates of the
* Zeit. f. Biol., 5, 369. f Arch, ges Physiol., 23, 175. X Compare p 74.

THE FASTING METABOLISM.

85

Per Hour and Kg. Live Weight.

Length of Fasting,
Minutes.

Oxygen Consumed,
c.c.

Carbon Dioxide

Excreled,

c.c.

Respiratory
Quotient.

0
146S
2950

1202.19
1154.53
1146.76

1111.80
923.75
811.12

0.93
0.80
0.71

0
1575
3543
5940

1250.28
1226.18
1241.78
1192.50

334

1712
3233

1959.45
1850 . 02
1809.85

1494.68
1318.19
1289.63

0.76
0.71
0.71

body, while as the experiment progressed the store of carbohydrates
(glycogen) in the body was gradually exhausted and the meta-
bolism finally became a fat metabolism. Since, nowr, as will be
shown in Chapter VIII, the consumption of equal amounts of oxygen
results in the liberation of approximately equal amounts of energy
whether that oxygen is employed to oxidize carbohydrates or fats,
Finkeler concludes that the total metabolism, as measured in terms
of energy, was nearly constant.

Lehman 1 1 and Zuntz * have observed a similar constancy of
the respiratory exchange per unit of weight in the case of two
men fasting for eleven and six days respectively, while Munk f
found their urinary nitrogen to be also approximately constant.
Magnus-Levy j has likewise observed a similar constancy in the
respiratory exchange of the dog and of man during fasting, as have
also Johansson, Landgren, Sonelen, & Tigerstedt § for man.

Rubner,| as a preliminary to his investigations upon the re-
placement values of the nutrients, discusses this question at some
length and gives the results of experiments upon dogs, rabbits,
guinea-pigs and fowls, in which the excretion of nitrogen and car-
bon per unit weight shows a marked degree of constancy through
considerable periods.

* Yircliow's Arc-hiv, 131, Supp. J Arch. ges. Physiol., 55, 1.

■\Ibid. §Skand. Archiv. f. Physiol., 7, 29.

||Zeit. f. Biol., 17, 214; 19, 313; Biologische Gesetze, p. 15.

S6 PRINCIPLES OF ANIMAL NUTRITION.

Metabolism Proportional to Active Tissue. — In a critical
discussion of these and other results on fasting animals, to which
we. shall have occasion to refer again in Part II, E. Voit * shows
that a still more constant relation is obtained when either the pro-
teid or the total metabolism is compared with the total mass of
proteid tissue estimated to be contained in the body on the several
days of the experiment. The total protein of the body, however,
may be regarded as at least an approximate measure of the active
cell mass, as distinguished from the relatively inactive cells of
adipose tissue. It is the vital activities of the former, in the fast-
ing animal, that mainly determine the amount of the total meta-
bolism, the energy liberated being supplied in part by the relatively
small amount of proteid metabolism which gees on in the cells of
the fasting animal, but largely by the metabolism of fat supplied
to the active cells from the adipose tissue.

Ratio of Proteid to Total Metabolism. — In the preceding
paragraph it was implied that the proteid metabolism constitutes
but a small portion of the total metabolism of the fasting animal,
the remainder of the necessary energy being supplied, after the
small store of glycogen in the body is exhausted, by the metabo-
lism of body fat. Rubner f appears to have been the first to call
specific attention to this aspect of the question. In his investiga-
tions upon the relation of size of animal to total metabolism he
adduces experimental results to prove that this ratio is not mate-
rially different in large and in small animals. The question has,
however, been more recently discussed by E. VoitJ from a general
point of view, the results of numerous investigators being summa-
rized. In discussing these results, Voit has computed from the
nitrogen and carbon balance, when these data were available, in
substantially the manner described in Chapter VIII, the amount of
energy liberated by the metabolism of the protein and fat lost by
the body. In those instances in which only the nitrogen balance
was determined, he estimates the amount of energy liberated in
the body from the computed surface on the basis of average results
with similar animals. (Compare Chapter XI, § 2.) Taking this
amount, expressed in calories, as the measure of the total meta-
bolism, and including only experiments in which the animals
* Zeit. f. Biol., 41, 113. t Ibid., 19, 557. \ Ibid., 41, 167.

THE FASTING METABOLISM.

87

are believed to have been in good bodily condition (well nourished)
at the beginning of the trials, he obtains the following average
results :

Live Weight,
Kgs.

Nitrogen Excretion per Day.

Total,
Grms.

Per Kg.

Live Weight,

Grms.

Proteid
Metabolism
in % of Total
Metabolism.

Swine
Man

Dog

Rabbit . . .
Guinea pig
Goose
Hen

115.0

63.7

28.6

18.7

7.2

2.7

0.6

3.3

2.1

6.8
12.6
5.1
3.8
2.2
1.2
0.4
0.8
0.7

0.06
0.20
0.18
0.20
0.30
0.46
0.65
0.23
0.34

7.3
15.6
13.2
10.7
13.5
16.5
10.8

7.4
10.0

As will appear later, the total metabolism of a small animal is
greater per unit of weight than that of a large animal. The above
ilgures show that the same thing is true of the proteid metabolism.
When, however, the proteid metabolism is computed as a percent-
age of the total metabolism, as in the last column of the table, this
dependence upon the live weight disappears. While the figures
still show considerable variations, these are much reduced and
show no connection with the live weight . In other words, the proteid
metabolism tends to be a somewhat uniform percentage of the
total metabolism, ranging in these experiments, aside from two
apparently exceptional results, between 10 and 16 per cent.

The individual experiments cited by Voit show a similar general
uniformity, both in the same animal on successive days of fasting
and in case of different animals. Thus twenty-seven experiments
on the dog gave the following :

Range of Proteid Metabolism in Pt-r tent.

Number of Cases.

of Total Metabolism.

Absolute.

Per Cent.

Less than 10

4

15

5

3

27

14.8

10-14

55.6

14-17

18.5

More than 17

11.1

100.0

88 PRINCIPLES OF ANIMAL NUTRITION.

The great majority of cases gave values lying between 10 and
17 per cent.

Effect of Body Fat. — Both from the summary on p. 87 and
from the individual results cited by Voit, it is evident that while
the proportion of energy supplied by the metabolism of pro-
teids in the fasting animal is normally small and varies only within
rather narrow limits, it is still subject to relatively considerable
variations. The most important cause of these variations in the
fasting animal under uniform external conditions appears to be
the ratio of fat to protein in the body.

C. Voit * appears to have first noted that when fasting is pro-
longed sufficiently to nearly exhaust the reserve of visible fat in
the body, the proteid metabolism, after remaining nearly constant
or decreasing slightly for some clays, as in the examples just given,
begins to increase somewhat rapidly. This increase Voit attrib-
uted to the exhaustion of the fat, the oxidation of which had hith-
erto partially protected the organized proteids of the body. Sub-
sequent investigations, particularly Rubner's,f have in general
confirmed Voit's observation, while giving it a somewhat more
general form.

E. Voit \ has recently reviewed the available experiments upon
fasting metabolism in their bearing on this question. From the
experimental data he computes or estimates, first the ratio of pro-
teid to total metabolism (expressed in terms of energy), and second
the ratio of proteids to fat in the body on the several days of each
experiment. A comparison of these ratios shows a very marked
correspondence, a high ratio of proteids to fat in the body coin-
ciding with a large proteid metabolism compared with that of fat,
and vice versa. The graphic representations of the relations as
given by Voit are especially convincing. Moreover, the results
show that the extent of the proteid metabolism does not depend
directly upon the duration of the fasting. With different animals,
or with the same animal under different conditions, a certain ratio
of proteid to total metabolism is attained whenever the correspond-
ing ratio of proteid tissue to fat in the body is reached, whether this
be early or late in the experiment.

The growing ratio of proteid to total metabolism in the fasting
* Zeit. f. Biol., 2, 326. f Loc. cit. See p. 86. t zeit. f. Biol., 41, 502.

THE FASTING METABOLISM.

89

animal is explained by Voit to be due to an increasing difficulty in
transferring tne reserve fat from the adipose tissues, thus resulting
in a diminution of the amount of fat (or its cleavage products?)
circulating in the organism. If the body is well supplied with fat
at the outset this phenomenon does not at first appear, and the
ratio of proteid to total metabolism remains nearly constant for a
time.

With continued fasting the store of body fat is, as has just been
shown, drawn upon much more rapidly than that of protein, while
at the same time the total amount of the former present at the
beginning of fasting is often less than that of the latter. As a
necessary result, the ratio of fat to protein in the body decreases.
When this decrease passes a certain point, the fat of the adipose
tissue is drawn upon with more and more difficulty for material to
supply the demand for energy, and as a result additional protein is
metabolized to make good the deficiency of available fat. From
this time on, the ratio of proteid to total metabolism shows a -con-
tinually accelerated increase. The time when the increase in the
proteid metabolism becomes marked depends upon the original
condition of the body. If the animal is well nourished, and espe-
cially if it contains large reserves of fat, the increase may be long
deferred or even fail to appear at all. If, on the other hand, it is
poorly nourished and contains little fat, an increase of the proteid
metabolism may take place almost from the outset. The following
three examples, cited by E. Voit from Rubner's experiments, may
serve to illustrate these three types of fasting metabolism:

Ou

nea Pig.

Dog.

Rabbit.

Proteid

Proteid

Proteid

Day of

Metabolism

Dav of

Metabolism

Day of

Metabolism

Fasting.

in % of Total

Fasting.

in % of Total

Fasting.

in % of Total

Metabolism.

Metabolism.

Metabolism.

2

10.4

2-A

16.3

3

16.5

3

11.1

10-11

13.1

5-7

23.6

4

11.0

12

15.5

9-12

26.5

5

11.9

13

17.4

13-15

29.8

6

11.8

14

20.0

10

50.1

7

6.9

17-18

96.4

8 ,..

11.2

9

10.9

90 PRINCIPLES OF ANIMAL NUTRITION.

Schulze * claims that this increase in the proteid metabolism of
the fasting animal is not, in all cases at least, due to lack of fat or
other non-nitrogenous material to protect the protein from destruc-
tion. He advances the hypothesis that the loss of protein incident
to the fasting so injures the cells that finally many of them die
and the protein of their protoplasm becomes part of the circula-
tory protein of the body and is rapidly decomposed, thus giving
rise to an increased excretion of nitrogen.

While it is not impossible that this ingenious hypothesis has
some basis of fact, Kaufmann;f in a quite full review of the litera-
ture of the subject, together with original experiments, shows
that it can by no means supplant Voit's explanation. He points
out in particular that the time when the increase in the proteid
metabolism begins seems to bear no relation to the loss of protein
which the body has sustained, while, on the other hand, it coin-
cides quite closely with the time when the supply of visible fat is
nearly exhausted. J

Summary. — In the light of the facts set forth in the foregoing
paragraphs we may sketch the general outlines of the fasting meta-
bolism somewhat as follows:

In the early stages of fasting, particularly if the previous food
has contained an abundance of proteids, the proteid metabolism
may be considerable. As the effect of the previous food disappears,
however, and the store of " circulatory protein " in the body is ex-
hausted, the proteid metabolism speedily falls to the minimum
amount required for the vital activities of the protoplasm, and the
remaining demands of the body for energy are supplied by the
metabolism of the stored-up fat. If the latter is fairly abundant,
this stage may last several days, the total metabolism remaining
nearly constant and the proteids supplying a nearly constant pro-
portion of the necessary energy (according to E. Voit about 15-16
per cent.). Sooner or later, however (unless in a very fat animal),
the supply of fat from the adipose tissue begins to flag. The de-
mand for energy, however, remains unabated, and as the fat-supply
falls off, more and more protein is metabolized in its place, until at

* Arch. ges. Physiol., 76, 379. f Zeit. f. Biol., 41, 75.

% Compare also E. Voit's critique of Schulze's investigations. (Zeit. f.
Biol., 41, 550.)

THE FASTING' METABOLISM. 91

last the metabolism may even become almost entirely proteid in
its character. We have in these facts the first of the numerous
illustrations which we shall meet in the course of this discussion
of the plasticity of the organism in adapting itself to differences
in the food-supply, and of the controlling influence exerted upon
the course of it^ metabolism by the demand for energy.

The Intermediary Metabolism— The prime object of the
metabolism of the quiescent fasting animal is, as already pointed
out, to supply energy for the performance of the vital functions.

.Mention has already been made in Chapter II of the hypothesis
that the immediate source of energy to the cells of both muscles
and glands is the metabolism of carbohydrate material. This
hypothesis in effect regards the metabolism of the fasting animal as
divisible into three processes: first, the splitting up of the proteids,
yielding urea and fat; second, the partial oxidation of fat, whether
derived from the proteids or from the adipose tissue, yielding dex-
trose; third, the oxidation of the resulting dextrose in the tissues.

So far as the kind and amount of excretory products are con-
cerned, it of course makes no difference whether the metabolism
takes place in accordance with this hypothesis or whether the
proteids and fat are oxidized directly in the tissues. In either
case the fasting animal lives upon its store of proteids and fat, and
the resulting excretory products, as well as the amount of heat
produced, are qualitatively and quantitatively the same, so that
the coincidence observed by Kaufmann * between the observed
results and those computed from his equations is without special
significance in this case.

There is, nevertheless, an important and essential difference in
the two views. If we regard the proteids and fat as yielding up
their energy directly for the vital activities, then all the energy
thus liberated is available for this purpose. If, on the contrary,
we suppose these substances to be first partially metabolized in the
liver <>r elsewhere in the organism, then only that portion of their
potential energy which is contained in the resulting dextrose is
available directly for the general purposes of the body. The re-
mainder of their energy is liberated as heat during the preliminary
♦Archives do Physiologie, 1896, pp. 329 and 352.

92 PRINCIPLES OF ANIMAL NUTRITION.

metabolism, and while contributing its quota towards maintaining
the normal temperature of the body is not directly available for
other purposes. In other words, the question is not one as to the
total energy liberated, but as to its form and distribution. As
regards the fasting animal itself, the question is of minor impor-
tance; but, as will appear in subsequent chapters, it materially
affects our views as to the relative values of the several nutrients
of the food.

CHAPTER V.
THE RELATIONS OF METABOLISM TO FOOD-SUPPLY.

The metabolism of the fasting animal was regarded in the pre-
ceding chapter as representing the essential demands of the vital
functions for a supply of matter as a vehicle of potential energy.
Under these conditions, as we have seen, the total metabolism bears
a close relation to the mass of active tissue, while the qualitative
character of the metabolism, that is the ratio of proteid to non-
proteid matter consumed, appears to be likewise constant for any
given condition of the body, depending upon the relative supply
of proteids and non-nitrogenous matters to the active cells. When
food is given to such an animal the conditions are modified in essen-
tially three ways:

First, to the metabolism incident to the fasting state is added
that required to supply the energy consumed in the digestion and
assimilation of the food.

Second, the food-supply may alter the proportions in which the
various nutrients are supplied to the active cells, and thus affect
the metabolism qualitatively, giving rise to a relatively greater
or less metabolism of proteids, fats, carbohydrates, etc.

Third, the food-supply may be in excess of the requirements of
the body and lead to a storage of matter of one sort or another.

The quantitative relations of the food-supply to the total
metabolism and to the storage of matter and energy in the body
may be most satisfactorily considered upon the basis of the amount »
of energy involved. Accordingly we may content ourselves here
with a simple mention of this side of the question, deferring a dis-
cussion of it to Part II and confining the present chapter largely
to a study of the qualitative changes in the metabolism brought
about by variations in the food-supply. As in the previous chapter,
it will be convenient to consider the relations of the proteids of the

93

94 PRINCIPLES OF ANIMAL NUTRITION.

food and of the body separately from those of the non-nitrogenous
nutrients.

§ i. The Proteid Supply.
The effects of the proteid supply upon metabolism may be most
readily and clearly traced in experiments in which the food consists
solely, or nearly so, of proteids, deferring to the next section a
consideration of the modifications introduced by the presence of
non-nitrogenous nutrients in the food.

Effects on Proteid Metabolism.

Our knowledge of the relations between proteid supply and
proteid metabolism in the animal body is based upon the funda-
mental investigations of Bischoff & Voit,* Carl Voit,f and Petten-
kofer & Voit,f at Munich. The results of these researches have
been so fully confirmed by subsequent investigators and have
become so much the common property of science that it is unneces-
sary to do more than summarize them here, with the addition of
such examples as may seem best adapted to illustrate them.

Amount Required to Reach Nitrogen Equilibrium. — As
we have seen, the proteid metabolism of a fasting animal speedily
reaches a minimum which we may probably regard as representing,
at least approximately, the amount of proteids necessarily broken
down and oxidized in the vital activities of the tissues of the body.
If we supply proteid food to such an animal, we might naturally
be inclined to expect that the first use to which the proteids of
the food would be put would be to stop the loss of proteid tissue,
and that if as much proteid was supplied in the food as was being
metabolized in the body, nitrogen equilibrium would be reached.

Experiment shows, however, that this is very far from beinj;
the case. Even the least amount of proteids causes a prompt
increase in the urinary nitrogen, and each successive addition of
proteids results in a further increase, so that it is not until the food
proteids largely exceed the amount metabolized during fasting
that nitrogen equilibrium is reached. Thus Bischoff & Voit,J

* Gesetze der Ernahrung des Fleischfressers, I860.

f Published chiefly in the Annalen der Chemie und Pharmaoie and the
Zeitschrift fi'ir Biologie. See also Voit, " Physiologie des Stoffwechsels," in
Herman's Handbuch der Physiologie.

% Zeit. f. Biol., 3, 29 and 33.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY.

95

in a scries of experiments upon a dog fed exclusively on lean meat,
obtained the results shown in the following table, the proteid
metabolism being expressed in terms of flesh with its normal water
content (N X 29.4) instead of dry proteids:

Date.

Meat Fed.

" Flesh "
Metabolized.

Gain or Loss
of Flesh.

1858.
Aug. 25

" 26

" 27 and 28.

" 29-Sept, 1.
Sept. 2 and 3. ..

" 4 " 5. ..

" 6 " 7. ..

0

0

300

600

900

1200

1500

223
190
379
665
941
1180
1446

-223

-190

- 79

- 65

- 41
+ 20
+ 54

Nov. 16

1800

1500

1200

900

600

300

176

0

1764
1510
1234
945
682
453
368
226

+ 36

•• 17 and is

- 10

" 18 " 19

- 34

" 20 " 21

- 45

" 22 " 23

- 82

" 24 '• 25. . . .

— 153

" 26 " 27

-192

" 28-Dec. 1

-226

A much later series by E. Voit & Korkunoff,* in which the
results were determined in terms of nitrogen, may be cited to illus-
trate the same point. The food was lean meat from which the
extractives had been removed by treatment with cold water. It
contained 1.25 to 1.96 per cent, of fat.

Food.

Nitrogen in

Food,

Grms.

Fecesari'l Urine,
Grms.

Gain or Loss,
Grms.

Nol iiinji

0

4.10

5.74

6.77

7 . 59

8.20

10.24

11.99

15.58

13.68

3.996

5 . 558

6.495

7.217

7.804

8.726

10.579

12.052

14.31 t

13.622

— 3 996

100 grms. extracted meat

140 " " "

-1.458

-0 755

165 " " "

-0.447

185 " " "

-0 214

200 " " " .

-0.526

230 " " "

-0.339

360 " " "

-0.062

410 " " "

+ 1.266

360 " " "

+ 0.058

* Zeit. f. Biol., 32, 67.

96 PRINCIPLES OF ANIMAL NUTRITION.

The proteid supply gradually overtakes the proteid metabolism,
but when only proteids are fed the supply must largely exceed the
fasting metabolism in order to attain nitrogen equilibrium. E.
Voit has endeavored to obtain a numerical expression for this
relation by taking as the basis of comparison the fasting meta-
bolism. He estimates (loc. cit., p. 101) that of the total nitro-
gen excretion of a fasting animal 81.55 per cent, is derived from
true proteids and 18.45 per cent, from the extractives of the
muscles. Since the food in his experiments consisted substan-
tially of true proteids, he compares its nitrogen with 81.55 per
cent, of the nitrogen of the excreta and thus finds that the mini-
mum supply of proteid nitrogen required to reach nitrogen equi-
librium was between o.67 and 4.18 times that metabolized during
fasting, the true value being estimated at o.6S. Five other less
exact experiments gave confirmatory results and similar confirma-
tion is found in the experimental results of C. Voit.

Effect of Excess of Proteids. — If the supply of proteids to
a mature animal be still further increased after nitrogen equilib-
rium is reached, the excess of proteids is promptly metabolized,
its nitrogen reappearing in the excreta. In other words, the ex-
cretory nitrogen keeps pace with the supply of nitrogen in the food.
The experiments by Bischoff & Voit just cited serve to illustrate
this fact also. Approximate nitrogen equilibrium was reached on
1200-1500 grams of meat, but in other trials even double this
supply caused but a slight apparent gain of nitrogen, and it is
probable that if the total urinary nitrogen had been determined
instead of the urea, and account taken of the nitrogen of the
feces, even this small difference would have disappeared.

It is needless to multiply examples of this perfectly well-estab-
lished fact. The animal body puts itself very promptly into equi-
librium with its nitrogen supply and no considerable or long-con-
tinued gain of proteid tissue can be produced in the mature animal
by even the most liberal supply of proteid food.

Transitory Storage of Proteids. — But while no continued
gain of protein by the body can be brought about by additions to
the proteid food, nevertheless, during the first few days following
such an increase in the proteid supply a transitory storage of nitro-
gen takes place. Conversely, too, a decrease in the proteid supply

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY.

97

causes at first a loss of nitrogen from the body, which, however,
unless the new supply of proteids falls below a certain minimum is
as transitory as the gain in the other case. In other words, while
the nitrogen excretion of the mature animal is in the long run
equal to the supply in the food, when the amount of the latter
is changed the full effect on the excretion is not realized at once.
This fact is well illustrated by the following selection from
C. Yoit's investigations upon the dog,* the results being expressed
in terms of " flesh " :

New

" Flesh " Metabolized per Day.

Previous

Ration.

Ration.
Grms.

On
Previous

Grms.

l ]

Meat.

Meat.

Ration.

1st Day.

2d Day. 3d Day. 4thDay.l5thDay. 6th Day.

7th Day.

Grms.

Grms.

Grms. Grms. i Grms. Grms.

Grms.

Grms.

1800

2500

1800

2153

24S0 2532

500

1500

547

1222

1310 j 1390

1410

1440

1450

1500

0

1500

176

1267

1393 1404

2500

2000

2^00

2229

1970

1500

1000

1500

1153

1086 | 1088

1080

1027

1000

500

1000

706

610 623

560

An example of the same fact is found in the experiments cited
on p. 81, in which »l. proteid food was withdrawn, the nitrogen
excretion falling rapidly, but reaching its minimum only after three
or four days.

Voit explained the facts just adduced as the consequence of
the difference between organized and circulatory proteids already
noted on p. 82. According to this hypothesis, the amount of
the proteid metabolism is chiefly determined by the store of circu-
latory proteids in the body. The ingestion of additional proteids
increases the amount of these circulatory proteids in the body, and
as a consequence the proteid metabolism increases until the nitrogen
excretion overtakes the supply. Similarly, a decrease in the pro-
teid food has the converse effect.

Proteid Metabolism and Nitrogen Excretion. — Up to this
point, following common usage, the terms nitrogen excretion and
proteid metabolism have been employed as practically synony-
mous. In one sense this usage is correct, but it is liable to give
* E. v. Wolff, Ernuhrung Landw. Nulzthiere, p 271.

98 PRINCIPLES OF ANIMAL NUTRITION.

rise to a misconception. It is perfectly true that the presence of
one gram, e.g., of nitrogen in the urine, implies that about six grams
of protein have yielded up their nitrogen in the form of urea or
other metabolic products and therefore have ceased to exist as pro-
tein. It by no means follows from this, however, that this protein
has been completely oxidized to carbon dioxide and water. We
have already seen (Chapter II, p. 48) that the abstraction of the
elements of urea from protein leaves a non-nitrogenous residue
equal to nearly two-thirds of the protein, and that there is reason
to believe that this residue may, according to circumstances, be
oxidized to supply energy or give rise to a production of glycogen
or of fat. In other words, the separation of its nitrogen from pro-
tein and the complete oxidation of its carbon and hydrogen are
two distinct things. When, therefore, we assert, on the basis of
the evidence noted above, that the proteid metabolism of the mature
animal is determined by the supply of proteids in the food, what
we really mean is that the cleavage of proteids and the excretion
of their nitrogen is so determined.

Rate of Nitrogen Excretion. — A consideration of the course
of the nitrogen excretion after a meal of proteids is calculated to
throw light upon the relations of nitrogen cleavage to the total
metabolism of the proteids. The early investigations of Becher,
Voit, Panum, Forster, and Falck showed that when proteids are
given to a fasting animal the rate of nitrogen excretion shows a
rapid increase, reaching a maximum within a few hours.

Feder * observed the maximum rate of nitrogen excretion by
dogs in different experiments between the fifth and eighth hour
after a meal of meat. From this point the rate of excretion de-
creased less rapidly than it had increased and continued to decrease
until about thirty-six hours after the meal.

Graffenberger,t experimenting upon himself, obtained similar
results after the consumption of fibrin, gelatin, and asparagin, while
the results with a commercial "meat peptone" were markedly
different; and Rosemann,J in studies upon the rate of nitrogen
excretion by man, traces clearly a similar influence of the ingestion
of nitrogenous food, while Krummacher's § results on dogs fully

* Zeit. f. Biol., 17, 531 ; Thier Chem. Ber., 12, 402. 1 Zeit. f. Biol., 28, 318.
t Arch. ges. Physiol., 65, 343. § Zeit. f . Biol., 35, 48L

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY.

99

confirm those of Feder. Sherman and Hawk * have likewise found
the curve of nitrogen excretion by man after the ingestion of lean
meat to show the same general form observed by Feder and by ( iraf-
fenberger.

Nitrogen Cleavage Independent of Total Metabolism. —
Kaufmann,f by the method outlined in Chapter VIII, has made
a series of determinations of the nitrogen excretion, respiratory
exchange, and heat production of dogs during the time when nitro-
gen cleavage is most active, i.e., from the second to the seventh hour
after a full meal of meat. From his theoretical equations for the
complete metabolism of proteids (pp. 51 & 75) he computes the
respiratory exchange and heat production corresponding to the
observed excretion of urinary nitrogen and compares them with
the actual results per hour as follows:

Proteid

Computed.

Observed.

bolism.J
Grms.

CO,

Excreted.

Liters.

oQ

Consumed.
Liters.

Heat Pro-

duction.

Cals.

CO,
.Excreted.

Liters.

oa

Consumed.

Liters.

Heat Pro.
duction.

Cals.

No. 1 . . .

9.329

8.132

9.745

45.0

5.953

6.767

30.6

" 2...

9.926

8.565

10.373

48.0

7.064

7.972

34.6

" 3...

9.350

8.153

9.771

45.4

7.161

8.236

34.0

" 4. .

9.540

8.231

9.864

45.8

7.398

8 . 673

34 . 0

" 5...

6.632

5.783

6.930

32.0

5.228

6.596

27.7

" 6...

9.491

8.276

9.918

46.1

6.393

7.813

29.7

" 7...

S ISS5

7.573

9 . 075

42.2

6.325

7.730

29.0

" 8...

9.958

8.683

10.406

48.4

6 . 702

7.903

33.6

" 9...

8.928

7.785

9.235

43.0

6.062

7.916

35.3

" 10...

10.553

9.202

11.027

51.0

7.125

8.589

32.7

But a glance is needed to show that the total metabolism,
whether measured by the gaseous exchange or by the heat produc-
tion, is much less than that computed, which is equivalent to saying
that the non-nitrogenous residue of the proteids was not completely
oxidized. Gruber,§ whose experimental results upon the rate of
nitrogen excretion fully confirm those above cited, has shown very
clearly the bearing of these facts. He points out that if we

* Amer. Jour. Physiol., 4, 25.
t Archives de Physiologie, 1896, pp. 346 and 768.

X Kaufmann's factor for proteids, derived from the formula C72H112N180,,S,
is 6.39.

§ Zeit. f. Biol., 42, 407.

PRINCIPLES OF ANIMAL NUTRITION.

regard the nitrogen excretion as denoting the complete metabo-
lism to carbon dioxide, water, urea, etc., of a corresponding
amount of proteids, we get figures for the total evolution of
energy (heat) in the organism which are entirely incompatible
with those derived from other considerations. For example, a
daily diet of 1500 grams of lean meat given to a dog not only suf-
ficed to supply the demands for energy but produced a storage of fat
in the body. The total daily production of heat, computed from
the results of respiration experiments (see Chapter VIII), was
1060.2 Cals., equivalent to S8.3 Cals. in two hours, which must have
been derived essentially from the metabolism of proteids. If, how-
ever, we compute the evolution of energy from the results of the
nitrogen excretion as determined in two-hour periods, we get strik-
inciv variable results.

Hour.

Urinary Nitrogen,
Grams.

Equivalent Energy,*
Cals.

9 11

3.11
5.71
6.62
6.98
6.35
6.04
5.08
2.65
1.24

80.6

11 1

1 IS. 2

13

171.6

3 5

181.2

5 7 •

165.1

7 9

156.0

9 11

132.6

68.9

7 9

32.5

The heat production as thus computed varies from over twice
the average two-hour rate to an amount equal to scarcely more than
one half of the average fasting metabolism of the same animal
(62 Cals. per two hours). Such fluctuations are entirely inconsistent
with all data as to the heat production of the body, which, as we
shall see later, appears to go on with a remarkable degree of Uni-
formity under uniform conditions. The only reasonable conclu-
sion, then, appears to be that the nitrogen cleavage and the total
oxidation of the proteids are distinct and at least largely inde-
pendent processes.

('.ruber's explanation of these facts is substantially as follows:
It is well established that a relatively constant composition of the
blood and of the fluids of the body generally is an essential condi-
* One grain N equivalent to 26 Cals. See Chapter VIII.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. ioi

tion of normal physiological activity. It has been repeatedly
demonstrated, however, that when the period of growth is past, the

animal body has not the ability to produce any material amount of
proteid tissue. A large supply of proteid food, then, necessarily
tends to alter the composition of the blood and other fluids of the
body, and the nitrogen cleavage is evidently an effort on the part
of the organism to counteract this effect by splitting off from the
proteids a nitrogenous group which can be rapidly excreted, leav-
ing a non-nitrogenous residue which, so far as it is not immediately
needed to supply energy, is capable of storage in the relatively inert
and insoluble forms of glycogen and of fat.

According to Rosemann,* the rapid increase in the nitrogen ex-
cretion after a meal arises from two concurrent causes: first, a
direct stimulus to the proteid metabolism, due to the rapid increase
of proteids and their digestion products hi the blood, which is
somewhat transitory in character; and, second, the effect of a
larger relative supply of proteids in causing, according to well-
known physico-chemical laws, a relatively larger number of mole-
cules of these substances to enter into reactions with the cell proto-
plasm. The accompanying graphic representation by Gruber f of
the course of the nitrogen excretion of a dog on the second day of

3RMG

N

1

,

3

t

1

\

.

RATE OF NITROGEN EXCRETION PER TWO HOURS.

feeding with 1000 grams of lean meat and on the three following

fasting days shows plainly the sudden stimulation of the excretion

* hoc. tit. f Loc. tit., p. 421.

102 PRINCIPLES OF ANIMAL NUTRITION.

at first and the fall, rapid at first, and then very gradual, until the
minimum of the fasting excretion is reached about the third day.

On the other hand, Rjasantzeff * and Shepski f ascribe the in-
crease in the nitrogen cleavage after a meal to the increase in the
digestive work rather than to the proteids as such. They find it
possible, by stimulating the activity of the digestive organs without
introducing food, to considerably increase the nitrogen excretion
in the urine, while, on the other hand, the introduction of proteid
food through a gastric fistula produced little or no effect. They
also find the increase with the same amount of food nitrogen to be
proportional to the (estimated?) amount of digestive work, but seem
to offer no explanation of the equality of nitrogen cleavage and
nitrogen supply.

Cause of Transitory Storage. — As already noted (p. 96),
any change in the rate of proteid supply in the food, while resulting
ultimately in a corresponding change in the rate of nitrogen excre-
tion, gives rise to a transitory gain or loss of nitrogen by the body,
which was interpreted by Voit as consisting in a corresponding
change in the stock of "circulatory protein" in the body. The
facts which we have just been considering permit us to trace some-
what more fully the details of the phenomenon. Gruber points
out that while the larger part of the nitrogen cleavage consequent
upon a single meal of proteids takes place within a few hours, the
remainder is prolonged over two or three days, as in the case illus-
trated above, while he likewise shows experimentally that this
effect is not due to a retention of the nitrogenous metabolic prod-
ucts, but represents the actual course of nitrogen cleavage.

Such being the case, the transitory gain or loss incident to a
change in the rate of proteid supply is most simply explained as
the result of a superposition of the daily curves. Let it be assumed,
for example, that 80 per cent, of the nitrogen cleavage incident to
a single meal of proteids takes place on the first day, 13 per cent,
on the second, 5 per cent, on the third, and 2 per cent, on the fourth.
Then if we give to a fasting animal an amount of proteids contain-
ing 100 grams of nitrogen for five successive days and then with-
draw the food, the food nitrogen will be excreted as follows on the
several days:

* Jahr. Thier Chem., 26, 349. | Ibid , 30. 711.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 103

Feeding.

Fasting.

1st
Day.

2d
Day.

3d
Day.

4th
Day.

5th
Day.

1st
Day.

2d
Day.

3d
Day.

From food of 1 st day of feeding . .
" " " 2d " " "

it u «3d u u u

" "4th " " " ..
" " "5th " " "

Total

80
80

13

80
93

5
13

80

98

2

5

13

80

100

2

5

13

SO

100

2

5

13

20

2
5

7

2
2

On the above assumptions, there remained in the body at the
end of the first day 20 grams of nitrogen in the form of unmeta-
bolized proteids. At the end of the second day this had increased
to 27 grams, and at the end of the third day had reached the maxi-
mum of 29 grams. At the end of the first day's fasting it had fallen
to 9 grams, at the end of the second day to 2 grams, and at the end
of the third day to zero. In other words, the transitory storage
of proteids observed by Voit and others is explained by Gruber as
due to the fact that the nitrogen cleavage extends over more than
a single day.

In reality, of course, the excretion does not take place with any
such mathematical exactness as in this schematic example, and
after long fasting in particular a certain rebuilding of proteid tissue
may occur, but the assumed figures may serve to give a general
notion of the relations of food-supply and excretion.

In brief, then, we may suppose that when proteid food is given
to a fasting animal the stimulating effect upon the nitrogen cleavage
anticipates the use of the proteids for constructive purposes and
that a large proportion of them is thus destroyed as proteids before
it can be used to make good the loss of proteids by the organized
tissues. In other words, the proteids actually available for the
tissues are much less than the amount supplied in the food. In
this view of the matter we can readily see why the proteid supply
overtakes the nitrogen excretion so slowly and why two or three
times the amount metabolized in fasting is necessary to make good
the loss from the body and ensure nitrogen equilibrium.

T04

PRINCIPLES OF ANIMAL NUTRITION.

Effects on Total Metabolism.

In the preceding paragraphs the effects of an exclusive proteid
diet upon the proteid metabolism have been discussed. There
remain to be considered its effects upon the metabolism of fat.

Proteids Substituted for Body Fat. — When proteids are
given to a fasting animal the proteid metabolism is increased, as we
have seen, but at the same time the loss of body fat is diminished.

Pettenkofer & Yoit * fed a dog with varying amounts of lean
meat, which may be regarded as consisting chiefly of proteids
together with small amounts of fat, with the following average
results in terms of nitrogen:

Meat Fed,
Grms.

Nitrogen

of Food,

Grms.

Nitrogen Gain or Loss
Metabolized, of Nitrogen,
Grms. Grins.

Gain or Loss
of Fat,

Grms.

ot

500
1000
1500 \

0
17.0
34.0
51.0

5.6
20.4
36.7
51.0

-5.6
-3.4

-2.7
0

-95

-47
-19
+ 4

Rubner § has obtained a similar result by the use of the proteid
mixture resulting from the extraction of lean meat with water, and
which still contained some fat. As compared with the fasting state,
the consumption of 740 grams of the moist material (containing
72.2 per cent, of water) produced the following effect:

Nitrogen of Food,
Grms.

Nitrogen

Metabolized,

Grms.

Fat

Metabolized,

Grms.

Fasting

Fed

0
35.22

5.25
20 . 37

84.39
28.37

Difference

+ 21.12

-56.02

The increased nitrogen cleavage resulting from an increase in
the proteid supply liberates a certain amount of energy for the vital
activities of the body, while the non-nitrogenous residue of the cleav-

* Zeit f. Biol., 7, 489.

t Average of first two experiments, p 84, Chapter rV.

% Scries I only. The others showed a greater gain of fat and of nitrogen

§ Zeit. f. Biol., 22, 51.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 105

age becomes available also as a source of energy to the organism,
and the metabolism of fat is correspondingly diminished. In
effect, then, the protcids are simpry substituted for more or less of
the body fat as a source of energy, and Rubner, in a series of experi-
ments which will be considered in Part II, has shown that the sub-
stitution takes place, under the condition of these experiments, ap-
proximately in proportion to the amount of available potential
energy contained in the proteids and fats respectively. That is,
if the extra proteids metabolized can supply a certain amount,
100 Cals., e.g., of energy to the organism, the fat metabolism is
diminished by a corresponding amount, so that the total expend-
iture of energy 1>\ the body remains unchanged, being simply
drawn from different sources in the two cases.

Amount Required to Produce Carbon Equilibrium. — In
the experiments by Pettenkofer & Voit cited above, the quantity
of food proteids which resulted on the average in nitrogen equili-
brium produced substantially an equilibrium also between the
supply and excretion of carbon. The earlier experiments of Bidder
& Schmidt * gave similar results. Later experiments, however,
have given divergent results, nitrogen equilibrium appearing to
be reached with an amount of proteids which is far from supplying
sufficient energy for the organism, so that while the stock of pro-
teids in the body is maintained, its store of fat is still drawn upon.

We have seen that the proteid metabolism in the normal fast"
ing animal amounts to 10-14 per cent, of the total metabolism,
while according to E. Voit (p. 96) the food proteids required for
nitrogen equilibrium are, roughly, 2\ to 3 times the fasting proteid
metabolism. It follows, then, that an amount of proteids con-
taining from 25 to 42 per cent, of the total available energy expended
by the fasting organism will maintain its store of proteids, and this
being so, the remaining 58-75 per cent, must necessarily !><■ sup-
plied by the metabolism of body fat. Thus with the dog on which
E. Voit's main experiment was made, nitrogen equilibrium was
approximately reached with 12.05 grams of nitrogen in the food,f
equivalent to 75.31 grams of protein (NX6.25) and containing

♦Compare At water & Langworthy; Digest of Metabolism Experiments;
U. S. Dept. of Agr., Office of Experiment Stations, Bui. 45, 3S8.
t hoc. cit., p. 69.

io6

PRINCIPLES OF ANIMAL NUTRITION.

approximately, according to Rubncr (see Chapter X), 321 Cals. of
available energy. The actual expenditure of energy by the animal
was not determined, but is estimated by the author on the basis
of Rubner's investigations at about 1280 Cals.

Several experiments by Rubner * lead to the same conclusion.
In these experiments the carbon and nitrogen of the excreta were
determined and the nitrogen of the food estimated from average
figures. The proteid metabolism having been computed from the
total excretory nitrogen, the corresponding amount of carbon is
computed from the average composition of the proteids and any ex-
cess in the excreta is assumed to be derived from the metabolism
of fat. (Compare p. 78.) The following are the results in brief,
including the one cited above (p. 104) :

Food.

Nitrogen

of Food,

Grins.

Nitrogen

of
Excreta,

(Inns.

Fat
Metab-
olized,

Grms.

Remarks.

Nothing

415 grms. lean meat ... 14 . 1 1

Nothing

740 grms. lean meat

25.16

Nothing

740 grms. extracted lean

meat 35 . 22

Nothing

390 grms. lean meat . .

Nothing

350 grms. lean meat . .

Nothing

580 grms. lean meat .

13.26

ii ^90

19 '72

4.38
13.72

2.80
20.63

5.25

26.37

1.08
8.53

1.08

10.10

3.50
18.47

49.33
25.44

79.94
30.73

84.36

28.37

22.88
11.42

22.88
11.79

37.24
21.45

Average of several days.
1st two days of feeding.

1st to 4th day of feeding
1st day of feeding.
3d to 6th day of feeding.
1st to 7th day of feeding.

While some of the experiments were hardly continued long
enough to absolutely establish the sufficiency of the proteid supply,
nevertheless we see in all cases a material loss of fat on rations which
apparently are sufficient to prevent a loss of nitrogen from the
body.

It should perhaps be noted that in Pettenkofer & Voit's ex-
periments 1000 grams of meat nearly prevented a loss of nitrogen
*Zeit. f. Biol., 22, 43-48; 30, 122-134.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 107

from the body. It appears possible, then, that nitrogen equilib-
rium might have been reached with a less amount than 1500 grams,
and that with this less amount there might still have been a loss of
fat from the body. Whether this possibility is sufficient to explain
the apparent discrepancy between these and later results must,
however, remain a matter of conjecture.

Utilization of Excess of Proteids. — We have seen that no
very considerable or long-continued storage of protein takes place
in the body of the mature animal. However large the supply of
food proteids, the body very soon reaches the condition of nitrogen
equilibrium, the outgo of this element in the excreta equaling the
supply in the food. This fact, as has been pointed out, does not
necessarily prove that the elements of the food proteids are com-
pletely oxidized in the organism. As was shown in Chapter II,
the abstraction from proteid matter of the elements of urea (or,
more strictly speaking, of the elements found in the urine) leaves
a very considerable non-nitrogenous residue available for the pur-
poses of the organism. It was there stated that this residue could
serve as a source of energy, and likewise that there was good reason
to believe that sugar was formed from it, while finally the question
of its ability to serve as a source of fat was reserved for discussion
in the present connection.

For hia Hon of Fat from Proteids.

Mention has already been made in Chapter II (p. 29) of the fact,
first asserted by Liebig,* that the animal body manufactures fat
from other ingredients of its food. As a result of the investiga-
tions incited by the publication of his views regarding the origin
of animal fat. Liebig's classification of the nutrients into " plastic "
and "respiratory" was generally accepted. The proteids were
regarded as the material for the growth and repair of the muscles
and the force exerted by the latter was considered to arise from
their oxidation, while the non-nitrogenous ingredients of the food,
especially the carbohydrates, were the source of the animal heat,
and when present in excess gave rise to a production of fat.

As time went on, however, observations began to accumulate

* Compare p. 1G3.

108 PRINCIPLES OF ANIMAL NUTRITION.

tending to show that the proteids were not without influence on
fat-production.

As early as 1745 R. Thomson,* in experiments on jrtilch cows,
noted an apparent connection between the supply of proteids in
the food and the production of butter.

Hoppe f in 1856 interpreted the results of an experiment in
which a dog was fed lean meat with and without the addition of
sugar as showing a formation of fat from proteids. The same
author % in 1859 claimed to have shown a slight formation of fat
from casein in milk exposed to the air, and this was confirmed later
bySzubotin.§ The latter author, and also KemmerichJ and later
Voit,1[ experimented upon the production of milk-fat by dogs.
Their results, while indicating the possibility of a formation of fat
from proteids, were indecisive.

Pettenkofer & Voit's Experiments. — Carl Voit, however,
was the first to distinctly champion the new theory, and aside from
certain confirmatory facts,** such as the formation of fatty acids in
the oxidation of proteids, the formation of adipocere, the alleged
formation of fat from proteids in the ripening of cheese and in the
fatty degeneration of muscular tissue, especially in cases of phos-
phorous poisoning, — facts not all of which are fully established and
whose importance in this connection has probably been over-
estimated,— the evidence bearing on the question of the formation of
fat from proteids has been until recently largely that supplied by
the famous researches of Pettenkofer & Voit ft at Munich.

In these experiments a dog weighing about 30 kgs. was fed
varying amounts of prepared lean meat from which fat , connective
tissue, etc., had been removed as completely as was possible by
mechanical means. The material thus prepared, while still con-
taining small amounts of fat, etc., was as near an approach to an
exclusively proteid diet as was practicable, it having been found
impossible to successfully carry out feeding experiments with pure

* Ann. ('hem. Pharm., 61, 228. % Virchow's Archiv, 17, 417.

t Yin-how's Archiv, 10, 144 %Ibid., 36, 561.

|| Wolff, Erniihrung Landw. Nutzthiere, p. 351.

If Zeit. f. Biol., 5, 136.

** Compare Voit's summary in 1869, Zeit. f. Biol., 5, 79-169.

ft Am. Chem. Pharm., II, Suppl. Bd., pp. 52 and 361 ; Zeit. f. Biol., 5,
106; 7, 433.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 1 09

proteids. The experiments were conducted with the aid of a respi-
ration apparatus, the gain or loss of proteids and fat being com-
puted from the nitrogen and carbon balance in the manner described
in Chapter III.

The following is a condensed summary of the average results
of these experiments, as given by the authors,* but includes also
the average of ah the experiments with 1500 grams of meat. On

Meat Eaten per Day,
Grins.

Gain (+) or Loss (-) by Animal.

Experiments.

Flesh.
Grms.

Fat.
Gims.

3t
22

1
2

1

0

500
1000
1500
1500
1800
2000
2500

-165

- 99

- 79

0
+ 18
+ 43

- 44

- 12

-95

-47
-19

+ 4
+ 9

+ 1
+ 58

+ 57

the smaller rations, which were obviously insufficient for main-
tenance, the animal lost both flesh and fat. A ration of 1500
grams of meat per day sufficed approximately to maintain the ani-
mal as regards flesh and to cause a small gain of fat. On the
heavier rations the excretion of nitrogen kept pace with the supply
in the food in the manner illustrated on pp. 94-96 but the excretion
of carbon fell considerably below the supply, indicating a produc-
tion of fat.

It is to be noted that only the last three experiments in the above
table actually show any very considerable production of fat. The
insufficient rations naturally do not, and while among the twenty-
two trials with 1500 grams of meat the majority appear to show a
formation of fat, the amount is usually comparatively small, and
in two cases a loss was observed. On the whole, however, the evi-
dence of this series of experiments has been generally accepted as
conclusive in favor of the formation of fat from proteids.

Pfldger's Recalculations. — One very important point, how-
ever, has until recently been overlooked. The evidence is based on

* Zeitschr. f. Biol., 7, 489.
t Series 1 only.

no PRINCIPLES OF ANIMAL NUTRITION.

a comparison of the income and outgo of carbon and nitrogen.
Pfliiger,* however, has called attention to the fact that while Pet-
tenkofer & Voit made direct determinations of the outgo of these
elements, or at least of the principal factors of it, the income is not
computed from actual analyses of the meat used, but upon the
assumption of average composition. According to Pfliiger, not
only are the possible variations from the average in individual
experiments a serious source of error, but the average itself is
erroneous, the percentage of carbon assumed in the meat being
too high. Pettenkofer & Yoit estimate the ratio of nitrogen to
carbon in lean meat f as 1 : 3.684, while according to Pfliiger it is
not higher than 1 : 3. 28, and probably lower. Moreover, Petten-
kofer & Voit failed to take due account of the fact that a part of
the gain of carbon which they observed could be ascribed to the fat
still contained in the prepared "lean" meat. Another, although
slight, source of error, according to Pfliiger, lies in the fact that
the carbon in the urine was estimated from the amount of nitrogen
found by analysis on the assumption of a ratio of 1 : 0.60, while it
should be 1 : 0.67.

Using the above corrections, Pfliiger has recalculated twenty-
four of the experiments by Pettenkofer & Voit, which have been
generally accepted as demonstrating the formation of fat from pro-
teids, with the results shown on the opposite page.J

In the great majority of cases the experiments as recalculated
show a loss instead of a gain of fat, and in three of the four cases in
which a gain still appears it is small in amount, and, as Pfliiger
believes, within the limits of experimental error. Naturally such
calculations as the above can neither prove nor disprove the hypoth-
esis that the proteids serve as a source of fat. They simply show
that the experiments which have served as the principal support
for that hypothesis do not demonstrate what they were supposed
to. The question turns largely upon the elementary composition
of the meat used by Pettenkofer & Voit, which they failed to
determine. It is manifestly impossible to repair this error now,

♦Arch. ges. Physiol., 51, 229.

t Including such fat as cannot be removed by mechanical means.
%Loc. cit., p. 267. The experiments which showed a loss of fat as origi-
nally computed are omitted.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. nr

Meat Eaten

per Day,

Grins.

Gain (+) or Loss (— ) of Fat.

Date of Experiment.

According to Pet-

lenkoff-r & Volt.
Grms.

According to
Pfliiger.
Grins.

Feb. 19, 1S61

1S00
2500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
2000
2000
1500
1500

+ 1.4

+ 56.7
+ 3.4
+ 7.3
+ 34.4
+ 20.7
+ 35.9
+ 22.9
+ 8.7
+ 17.4

0.0
+ 9.9
+ 2.1
+ 14.3

0.0
+ 13.8
+ 9.0
+ 12.7
+ 26.3
+ 29.1
+ 55.9
+ 58.5
-11.9
+ 9.4

-35.8

Apr. 3, "

+ 3.93

Mch. 4, 1862. .

—29.3

" 21, " .

—23.4

+ 3.7

-12, "

-11.1

"14, "

+ 3.8

"16, "

- 8.4

Aug 6, "

-13.5

" 8, " .

- 8.3

Feb. 20, 1S63

-31.6

-22.1

"27, "

-24.4

Mch. 4, "

-16.9

Apr. 1 , " ...

-31.8

" 7, "

-17.5

"10, "

-22.0

June 1, "

-13.0

" 8, "

- 5.4

" 12, " .

- 2.9

"21, "

+ 13.6

" 26 * "

+ 1.6

July 3, "

-20.6

" 6, "

-23.7

and since Pfluecr's estimates seem to be at least as trustworthy as
Pcttcnkofer & Yoit's, and lead to exactly the opposite results, the
only verdict possible, so far as these experiments are concerned, is
"Not proven."

Later Experiments. — Shortly after the publication of Pfli'iger's
critique, E. Yoit.t in a preliminary communication, presented the
results of investigations upon this question undertaken in the
Munich laboratory. So far as the writer is aware, no complete
account of these experiments has yet appeared, but the data given
by Yoit in the preliminary account show a retention in the body
of 8 to 10 per cent, of the carbon of the metabolized proteids, and
to litis extent confirm the earlier results obtained by his father.
He believes the observed gain of carbon to be too great to be
accounted for by the storage of glycogen and interprets it as
showing a production of fat from proteids.

* Includes a correction of Pettenkofer & Voit's figures for the urinary
nitrogen, hoc. cit. p. 263.

f Thier. Chem. Ber., 22, 34.

112 PRINCIPLES OF ANIMAL NUTRITION.

Kaufmann likewise interprets the results of the respiration ex-
periments cited in another connection on p. 99 as demonstrating
the production of fat from proteids, but in view of the brevity of
the experiments (five hours), and the fact that they covered the
period of most active nitrogen cleavage, this conclusion seems
hardly justified.

Cremer * has reported the results of an experiment upon a cat
which he regards as showing a formation of fat from proteids. The
animal, weighing 3.7 kgs., passed eight days continuously in the
respiration apparatus and received per day 450 grams of lean
meat. Xo complete nitrogen and carbon balance is reported. The
average daily excretion of nitrogen was 13 grains. Assuming the
ratio of nitrogen to carbon in fat and glycogen-free flesh to be 1 :3.2,f
this corresponds to 41.6 grams of carbon in the form of proteids,
while the total excretion of this element was only 34.3 grams, thus
showing a retention by the organism of 7.3 grams per day. The
body of the animal at the close of the experiment was found to con-
tain not more than 35 grams of glycogen and sugar, while the ob-
served gain of carbon during the eight days was equivalent to
about 130 grams of glycogen. It is therefore concluded that fat
was formed from proteids. In three other experiments, with an
abundant meat diet, it is computed that from 12.6 to 17.0 per cent.
of the carbon of the metabolized proteids was stored in the body.

Gruber \ has recently reported two experiments, dating from
the year 1882, in which a clog was fed 1500 grams per day of lean
meat. The nitrogen of feces and urine were determined daily
for six and eight days respectively and the carbon dioxide of the
respiration on five days in each experiment; the carbon of urine
and feces and of the metabolized proteids was computed from tbo
nitrogen, using for the carbon of the proteids the factor 3.28. The
excretion of nitrogen approximately equaled the supply, especially
on the later days of the experiments, but from 10 to 15 per cent,
of the carbon was unaccounted for in the excreta. The total reten-
tion of carbon during the experiments, together with the equivalent
quantities of glycogen, were:

*Jahresb. Agr. Chem., 40. 538; Zeit. f. Biol., 38, 309.

t Kohler (Zeit. f physiol. Chem., 31, 479) found an average of 1:3.16 for
the fat -free flesh of cattle, swine, sheep, rabbits, and hens. See p. 64.

% Zeit f Biol., 42, 409.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 113

Carbon.

Equivalent Glycogen.

Experiment 1, seven days 113.9 Grms. 256.3 Grms.

2, eight " 195.9 " 441.0 "

These amounts of glycogen are much greater than have ever
been found in the body of a dog of this weight (about 20 kgs.), and
the larger part of the storage of carbon must therefore have been
in the form of fat.

In addition to the above results on normal animals, Polimanti *
has reported experiments apparently showing a formation of fat
from proteids in phosphorous poisoning. The latter investigation,
as well as those of Cremer and of E. Voit, have been the subjects of
searching criticism by Pfluger,t who claims to have shown the in-
sufficiency of the experimental evidence adduced, but it is impos-
sible here to enter into the details of these controversial articles.
Negative results have also been reported by Kumagawa &
Kaneda.^ Rosenfelt,§ TaylorJ Athanasiu,!" and Lindemann,** but
in a matter of this sort negative evidence naturally carries much
less weight than positive, and on the whole there would appear
to be good reason for still regarding the proteids as a possible
source of fat.

Difficulty of Proof. — A serious difficulty in the way of an
unquestionable demonstration of this possibility lies in the limited
amount of proteids which an animal can consume. As we have
seen, a relatively large supply of them is necessary even to produce
nitrogen equilibrium, and a still further large addition is required to
supply the demands of the organism for energy. Only the proteids
supplied in excess of this latter amount are available for fat produc-
tion, and thus it comes about that the limit of consumption and
digestion by the animal is reached before any very large produc-
tion of fat can take place.

On the other hand, if non-nitrogenous nutrients (carbohydrates

* Arch. ges. Physiol., 70, 349.

■\Ibid, 68. 176; 71, 318.

t U. S. Dept. Agr., Expt Station Record, 8, 71.

§Jahresb. Physiol., 6, 260.

II Ibid.. 8, 249, Jour Ex per. Medicine, 4, 399.

•jf Arch. ges. Physiol., 74, 511.

**Zeit. f. Biol., 39, 1.

H4 PRINCIPLES OF ANIMAL NUTRITION.

for example) are employed to supply a part of the necessary energy,
a more abundant fat production may be caused but the results
are ambiguous, since it is possible that the non-nitrogenous residue
of the proteids may be metabolized to furnish energy otherwise
supplied by the non-nitrogenous nutrients and that the actual
material for the formation of fat may come from the latter.

That proteids added to a mixed ration may give rise to a large
amount of fat has been strikingly shown by Kellner * in experi-
ments on oxen in which wheat gluten was added to a fattening
ration. Approximately 198 grams of fat were produced for each
kilogram of protein fed, but to the writer the reasoning by which
Kellner seeks to prove that this fat must have been derived directly
from the proteids seems inconclusive.

Finally, as was indicated in Chapter II (p. 50), the apparently
well-established fact that the metabolism of proteids in the body
gives rise to the formation of carbohydrates (or at least may do so),
together with the further fact that fat is undoubtedly formed from
carbohydrates, renders it difficult to assign any reason why the non-
nit rogenous residue of the proteids should not supply material to
the cells of the adipose tissue for the production of fat.
§ 2. The Non-nitrogenous Nutrients.
Effects on the ProU id Metabolism.

The relations between proteid metabolism and proteid supply
which have been outlined in the preceding section, while deduced
mainly from experiments in which the food consisted substantially
of proteids only, are of general applicability, yet are subject to im-
portant modifications in the presence of non-nitrogenous nutrients.

Tend to Diminish Proteid Metabolism. —As was first shown
by C. Voit, the addition of non-nitrogenous nutrients to a ration
consisting of proteids tends to render the proteid metabolism less
than it otherwise would be. The effect is common to the fats and
carbohydrates, although with some differences in details.

Fats. — The following example, taken from Voit's experiments,!
illustrates in a somewhat marked way the influence of the addition
of fat to proteid food upon the excretion of nitrogen. A dog con-
suming daily 1000 grams of lean meat received in addition on two
days 100 and 300 grams of fat, with the following results:
* Landw. Vers. Stat., 53, 456.
| Zeit. f. Biol., 5, 334.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. u

July 31

Aug. 1

" 2

" 3

Food per Day.

Meat,

Grtns.

1000
1000
1000
1000

Fat,
Gnus.

0
100
300

0

Urea per Day,
Grins.

81.7
74.5
69.3

SI. 2

In the whole series of eight experiments with varying amounts
of meat and fat the decrease in the excretion of urea ranged from 1
per cent, to 15 per cent, of the amount supplied in the food, averag-
ing about 7 per cent. With the same amount of fat in the food
the decrease in the excretion of urea was not, as a rule, greater with
large than with moderate rations of meat. On the other hand,
with a small proteid supply in the food the production of urea was
sometimes increased slightly by the addition of much fat, and the
same result was observed to a more marked extent when fat alone
was given to fasting animals. With medium rations of meat, in-
creasing the fat supply had usually little effect, but with heavy
meat rations it tended to further diminish the excretion of urea.

Subsequent investigation has fully established this tendency
of fat to diminish the proteid metabolism, and the fact is too well
known to require extended illustration here. As a recent instance
may be cited the following results obtained by Kellner * in experi-
ments upon oxen, in which oil was added to a basal ration:

Nitrogen Digested.

Nitroge

n in Urine.

Basal Ration,

Grms.

Basal Ration + Oil,
G-rms.

Basal Ration,

( JrlllS.

Basal Ration + Oil,

(inns.

Ox I)

Ox F

135.30
111.67

86.27

134 . 55
109.17

87.08

122.54

106.03

86.30

120.38
89.27

OxG

79.83

Carbohydrates. — The effects of the readily soluble hexose
carbohydrates (starch and the sugars) have been quite fully inves-
tigated, while as to those of the less soluble carbohydrates, particu-
larly of the five-carbon series, considerable diversity of opinion
still prevails.

* Landw. Vers. Stat., 53, 121 and 210.

n6

PRINCIPLES OF ANIMAL NUTRITION.

Starch and Sugars. — The investigations of C. Voit * show that
starch or sugar added to a proteid diet causes, as does fat, a decrease
in the elimination of urea. Yoit found an average decrease of
about 9 per cent, in the proteid metabolism, the extremes being 5
and 15 per cent, with varying amounts of carbohydrates. An in-
crease in the carbohydrates, the proteid food remaining the same,
tended to further diminish the excretion of urea. The following
examples illustrate this effect of the carbohydrates. When given
to a fasting animal, carbohydrates did not, as in the case of fat,
cause an increase in the proteid metabolism.

Food.

Urea

Meat,
Grins.

Carbohydrates,
Grins.

per Day,
Grms.

June 23-July 2, 1859

July 2-5, "

500
500

300-100
0

35.4
39.9

July 4-10, 1864

800
800
800

0

100-400

0

59.1

" 10-19, "

54.5

" 19-20, "

63.8

July 23-26, 1864

1000
1000
1000

0

100-400

0

73.5

" 26-28, "

64.4

" 28-Aug. 1, 1864

79.6

June 29-July 8, 1863

1500
1500

0
200

114.9

July 8-13, "

103.3

Jan 6. 1859

2000
2000

0
200-300

143.7

" 7-11, 1859

131.0

This effect of the carbohydrates, like that of fat, has. been abun-
dantly confirmed by later investigators and is one of the well-estab-
lished facts of physiology. Weiske f in particular has investigated
the effect of the non-nitrogenous nutrients upon the metabolism
of sheep, while Miura J and Lusk § have shown that the abstraction
of carbohydrates from the diet of a man results in a marked increase
in the proteid metabolism. The following data, taken from Kell-
ner's extensive respiration experiments at Mockern, illustrate the
same effect of starch in the case of cattle:

* Zeit. f. Biol., 5, 434.

t Zeit. physiol. Chem. 21, 42; 22, 137 and 265.

% v. Noorden, Pathologie des Stoffwechsels, p. 117.

§ Zeit. f. Biol., 27, 459.

THE RELATIONS OF METABOLISM TO FOOD SUPPLY. 117

Nitrogen Digested.

Nitrogen in Urine.

l!;isal Ration,
Grins.

Basal Ration

-+- Starch,

Grnis.

Basal Ration

Gnus.

Basal Ration
-t- Starch,

( Irms

Ox D

135 . 30
111.67
86.27
116.51
128.11

118.40

107.55

80.92

94 . 66

118.18

122.54
106.03
86.30
109.28
122.62

104 69

Ox F

81 18

Ox G

63 83

Ox H

81.71

Ox J

103.13

Since the addition of starch to the basal ration diminished the
apparent digestibility of the protein, the effect is most clearly seen
by comparing the daily gains of nitrogen by the animals on the
two rations, as follows:

On Basal Ration,
Grins.

Wi.ii Addition
of Starch,

Grins.

Difference.

Ox. I)

12.76

5.64

-0.03

7.23

5.49

13.71

26.37
17.09
12.95
15.05

+ 0.95

Ox F

+ 20.73

Ox G

+ 17.12

Ox H . . .

+ 5.72

Ox J

+ 9.56

Cellulose. — The peculiar position occupied by cellulose, as the
essential constituent of the "crude fiber" of feeding-stuffs, in the
nutrition of domestic animals causes much interest to attach to the
stud}- of its effects upon metabolism. We shall consider here only
its effects upon the proteid metabolism.

The first to take up this subject appears to have been v. Knie-
riem,* who experimented upon rabbits. In a preliminary experi-
ment the addition of prepared "crude fiber" to a basal fiber-free
ration in which the necessary bulk was obtained by the use of horn-
dust f gave the following results for the urinary nitrogen per day:.

I. Without fiber 0.9034 grams

II. With 9.284 grams fiber 0.7618 "

III. Without fiber 0. 7500 "

The low figure for the third period is ascribed to the effect of
the crude fiber still remaining in the digestive tract. In a follow-
ing series, in which respiration experiments were also made, the
following results per day were obtained for the nitrogen :

* Zeit. f. Biol., 21, 67. f Shown to have been entirely indigestible.

n8

PRINCIPLES OF ANIMAL NUTRITION.

Period.

Food per Day.

Nitrogen

of Food, *

Orms.

Nitrogen

of Excreta *

Grms.

Gain of

Nitrogen,
Grms.

I. 9 da\ s.

II. 10 dais.
Ill 5 davs.

Same + 22 grms. crude fiber . . .
Milk and horn dust

2.75
2.75
2.70
2.70
2.70

3.35
2.65
3.03
3.02
2.73

-0.60
4-0.10
-0.33

IV. 4 davs.

V. 3 davs.t

Same 4-11 grms. cane sugar . . .
" 4- 33 " " " ...

-0.32
-0.03

Weiske \ disputes v. Knieriem's conclusion that cellulose dimin-
ishes the proteid metabolism. He experimented upon a sheep,
which was fed in a first period exclusively on beans. In succeeding
periods the effect upon the proteid metabolism of adding to this
ration, first, inferior oat straw, and second, starch was tested, the
bean ration being diminished slightly in these periods in order
to keep the total digestible protein of the ration as nearly uniform
as possible. On the basis of a preliminary digestion trial with the
straw, the quantity of starch was so adjusted as to supply, in Period
III, according to computation, an amount of digestible carbohy-
drates equal to the digested fiber and nitrogen-free extract of the
straw of Period II, while in Period V it equalled the digested
nitrogen-free extract only. Actual determinations of the digesti-
bility of the mixed rations showed that this equality was approxi-
mately, although not exactly, reached, the amount of digested
starch being rather less than the computed amount.

The results as regards the proteid metabolism as originally re-
ported by Weiske are given in the first portion of the table on
the opposite page. v. Knieriem,§ having criticised the results on
the ground that the metabolic nitrogen of the feces was not taken
into account, and that when this was done the experiments made a
more favorable showing for the digested crude fiber, Weiske || has
recalculated his results on the assumption that the feces contained
0.4 grams of metabolic nitrogen for each 100 grams of dry matter
digested,1! with the results shown in the second half of the table.

♦Not including that of the horn dust,
f Results regarded by the author as of doubtful value
% Zeit. f. Biol., 22, 373.
%Ibid., 24, 293.
\\Ibid., 24, 553.

^ Compare Kellner, Landw. Vers. Stat., 24, 434; and Pfeiffer, Jour. f.
Landw., 33, 149.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 119

L

ucorrected.

Corrected.

0

Nitro-

Com-

S

Ration.

gen

Nitro-

puted

Nitro-

Ph

Appar-
ently

gen
of

Gain.

Nitro-
gen

gen
of

Gain.

Di-

Urine.

Di-

Urine.

gested.

gested.

Onus.

firms.

(inns.

Grms.

Orms.

• ;■ ms.

I.

500 grms. beans ....

20.51

20.93

-0.42

22.02

20.93

+ 1.09

II.

( 490 " beans )
j 515 " straw (" ' "

19.58

16.82

+ 2.76

21.78

16.82

+ 4.96

IV.

j 490 " beans )
( 515 " straw \ m m

Average of II. and IV .

18.81

17.26

+ 1.55

21.09

17.26

+ 3.83

+ 2.16

+ 4.40

(510 grms beans )

III.

■^ 180 " starch V ..
( 20 " sugar )
( 500 " beans )

20.03

14.94

+ 5.09

22.16

14.94

+ 7.22

V.

\ 90 " starch V . .

( 10 " sugar )

20.64

17.75

+ 2.89

22.43

17.75

+ 4.68

It will be seen that the experiments make substantially the
same showing for the relative effects of cellulose and starch whether
we take the uncorrected results or eliminate so far as possible the
effects of the greater amount of food in the later periods upon the
excretion of metabolic products in the feces. The addition of starch
and sugar in Period III produced about twice as great an effect in
reducing the proteid metabolism as did a somewhat larger amount
of digestible fiber and nitrogen-free extract from straw in Periods
II and IV. In Period V the starch added was only equal to the
digested nitrogen-free extract of the straw in Periods II and IV.
Since the effect upon the proteid metabolism is substantially the
same. Weiske concludes that the nitrogen-free extract of the
straw, which has the elementary composition of starch, is equal to
it in its effect upon the proteid metabolism, and that the digested
crude fiber is valueless in this respect. It must be said, however,
that this latter conclusion is not warranted by the facts, since it
rests upon the unproved assumption of equality of nutritive value
(in respect of the proteid metabolism, at least) of starch and the
nitrogen-free extract of the straw. Weiske also experimented
with rabbits, finding in one case no effect upon the proteid metab-
olism and in the second an increase of it, as a result of adding crude
fiber to a fiber-free ration.

120 PRINCIPLES OF ANIMAL NUTRITION.

Lehmann * experimented upon a sheep by adding respectively
crude fiber, prepared from wheat straw, and starch to a basal ration.
The results were not entirely sharp but showed plainly a decrease
of the proteid metabolism on the crude fiber ration which was
equal approximately to 61 per cent, of that secured by the use of
starch. In a second series of experiments, Lehmann and Vogel |
compared the effects upon the proteid metabolism of sheep of cane-
sugar and of the digestible non-nitrogenous matters of oat straw.
On the basis of a very careful discussion of the experimental errors,
they show that the latter substances have a marked effect in diminish-
ing the proteid metabolism, and in particular that if we ascribe this
effect exclusively to the digested nitrogen-free extract, as Weiske
does, we must admit that the latter produced an effect from two
to nine times as great as that of cane-sugar. They therefore con-
clude that their results show qualitatively an effect of the digested
cellulose upon the proteid metabolism. Reckoning the digested
nitrogen-free extract of the straw as equivalent to sugar, they com-
pute from the average of all their experiments that the cellulose
produced 75.7 per cent, as great an effect as the sugar, but they do
not regard this quantitative result as well established.

Holdefleiss \ experimented upon two sheep, feeding in a first
period meadow hay exclusively. In the second period one half of the
hay was replaced by a mixture of peanut cake, starch, and a little
sugar, while in the third period the starch was replaced by paper
pulp. In one case a fourth period was added in which the paper
pulp and sugar were simply omitted from the ration. The digested
nutrients and the proteid metabolism per day are tabulated on p. 121.

Converting the small differences in the amount of crude fat
digested into their equivalent in nitrogen-free extract by multipli-
cation by the factor 2.5, Holdefleiss computes from a comparison
of the second and third periods that the digested crude fiber pro-
duced on the first animal 80.1 per cent, and on the second animal
84.2 per cent, of the effect of the starch. A somewhat higher value
would be obtained from a comparison of the first and second periods
in the case of Sheep II, while on the other hand a comparison of the
corresponding periods with Sheep I gives a much lower value, and is

* Jour, f . Landw., 37, 267. t Kid., 37, 281.

% Bied. Centr. Bl. Ag. Chem., 25, 372.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 121

Apparently Digested.

Crude

Fat.

Grnis.

( Irude

Filiei',

Grnis.

N. Cr.

Ext tact.
Grins.

Nitro-
gen,

(inns.

Nit ro-

K'ell Of

Urine,

(Inns.

Gain

of
Nitro-
gen,
Gi ms.

Sheep I.

Period I

" 2

" 3

" 4
Sheep II.
Period 1

" 2

Hay only 113.55315. 72

Hay, peanut cake, sugar,!

and starch 15-. 27 II

Hay, peanut cake, sugar,]

and paper pulp 13 . 67 439 . 32

Hay and peanut cake il8. 57 171.12

Hay only 11.76

Hay, peanut cake, sugar,

and starch

Hay, peanut cake, sugar,

and paper pulp

13.07
15.14

171.92

77. 4S

235.31

470.85 15.02

560.71 13.55

320.21 13.76
345.92 16.25

276.42 10.

336.62, 9.54

198.46 8.82

13.83 1 19

LI. 31 2.24

11.26 2.50

14.45 1.80

8.45
7.85

2.43
1.69

7.62 1.20

even consistent with the view that cellulose has no effect upon
the proteid metabolism. In other words, the results on Sheep I, in
the first period, appear inconsistent with the other results.

Kellner * has experimented with rye straw extracted with an
alkaline liquid under pressure in the same manner as in paper-
making and containing 76.78 per cent, of "crude fiber" and 19.96
per cent, of nitrogen-free extract. The results as regards the pro-
teid metabolism, compared with those on starch, are given in the
upper table of p. 122.

Taking the figures as they stand, and attempting no correction
for the marked depression in the apparent digestibility of the nitro-
gen resulting from the addition of the extracted straw or starch,
they show a considerable effect by both in diminishing the proteid
metabolism relatively to the supply in the food and thus causing
an increased gain of nitrogen by the body. Any correction for the
metabolic nitrogen of the feces, as in Weiskc's experiments, would,
of course, tend to make the effect appear still greater. With the
first animal, after taking account as well as possible of the slight
differences in the fat digested in both periods and of the slight
effect of the starch upon the digestibility of the fiber of the basal
ration, the digestible matter of the extracted straw, five sixths of
which was cellulose, appears to have produced more than twice
as great an effect as an equal amount of starch. With the second
* Landw. Vers. Stat., 53, 278.

122

PRINCIPLES OF ANIMAL NUTRITION.

Apparently Digested.

Nitrogen

of Urine,

Grms.

Crude
Fat,
Grins.

Crude
Fiber,
Grms.

N. fr.

Extract,

Grins.

Nitrogen,
Grins.

Nitrogen
Grms.

Ox H.
Period 5
" 4

Extracted straw. .
Basal ration

Difference

Starch

116
101

3129
1083

3351
2912

102.47

116.51

76.31
109.28

26.16
7.23

" 3

15

92
101

2047

1057
1083

439

4773
2912

-14.04

94.66
116.51

-32.97

81.71
109.28

18.93
12 95

« 4

Basal ration

Difference

Extracted straw . .
Basal ration

Difference

Starch

7.23

Ox J.
Period 5
« 4

-9

110

107

-26

3101
1114

1861

3344

2895

-21.85

112.19
128.11

-27.57

95.80
122.62

5.72

16.39
5.49

" 3

3

85
107

1987

1105
1114

449

4396
2895

-15.92

118.18
128.11

-26.82

103.13
122.62

10.90
15 05

« 4

Basal ration

Difference

5.49

-22

-9

1501

-9.93

-19.49

9.56

animal, on the contrary, the effect of the digested matter of the ex-
tracted straw was but little more than two thirds that of the starch.
Ustjantzen * has recently reported the results of an experiment
upon a sheep substantially like those of Weiske (p. 118), a basal
ration of beans receiving, in succeeding periods, additions of meadow
hay, rice, or sugar, the two latter being computed to supply an
amount of digestible carbohydrates equal to the digestible nitrogen-
free extract supplied by the hay. The increased amounts of crude
fiber and nitrogen-free extract digested and the resulting increases
in the gain of nitrogen by the animal were as follows :

Crude Fiber,
Grms.

Nitrogen-free
- Extract,
Grms.

Gain of

Nitrogen,

Grins.

From hav ration

108 . 60

-2.53

5.07

95.55
107.15
109.20

3.33

" rice "

2.90

" sugar "

2.59

It appears that, as in Weiske's experiments, the carbohydrates
of the rice and sugar produced nearly as great an effect upon the
* Landw. Vers. Stat., 56, 463.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 123

gain of nitrogen as the total non-nitrogenous matter digested from the
hay, and the author follows Weiske in concluding that the digested
crude fiber producd but little effect on the proteid metabolism.

The same author also reports experiments upon a rabbit similar
to those of v. Knieriem, crude fiber prepared from hay being added
to a basal ration of peas, with the following results, which show
practically no effect of the crude fiber upon the proteid metabolism :

Nitrogen

of Food,

Grms.

Nitrogen Excreted.

Gain of

Urine,
Grms.

Feces,
Grms.

Total,

Grms.

Nitrogen.
Grms.

Peas

0.845
0.857

3.845
0.860

0.855
0.821
0.701
0.899

0.016
0.120
0.080
0.170

0.871
0.941
0.781
1.069

— 0 026

" and 5 grams, crude fiber... .

" " 5 " sugar

" " 6.5" crude fiber....

-0.084
+ 0.064
-0.209

While it is obviously unsafe to draw any positive conclusions
regarding the relative effect of cellulose and the more soluble carbo-
hydrates from the various experiments cited above, the balance
of evidence seems clearly to show that their influence upon the pro-
teid metabolism is qualitatively the same, while it appears on the
whole probable that digested cellulose is at least not greatly in-
ferior quantitatively to digested starch.

Organic Acids. — Certain methods of preparing or preserving
fodder, notably ensilage, result in the formation of not inconsider-
able amounts of organic acids. Moreover, it appears that these acids
are normally produced in considerable quantities in the herbivora
by the fermentation of cellulose and other carbohydrates, and that
fact naturally leads to a consideration of their effects upon meta-
bolism as compared with the latter substances.

We have seen (p. 27) that the organic acids are oxidized in the
body, and it therefore seems natural to suppose that they may
influence the proteid metabolism. This question has been investi-
gated by Weiske & Flechsig.* After some only partially success-
ful experiments on a rabbit, they fed a sheep with a basal ration
(of hay, starch, cane-sugar and peanut cake) containing a liberal
supply of protein and having a nutritive ratio of 1:3.4. To this
ration there was added in succeeding periods lactic acid as calcium
lactate, acetic acid as sodium acetate, and for comparison dextrose.
*Jour. f. Landw., 37, 199.

124

PRINCIPLES OF ANIMAL NUTRITION.

Disregarding for our present purpose the slight effect of these sub-
stances upon the digestibility of the non-nitrogenous ingredients
of the ration the results were:

Nitrogen
Digested,

(inns.

Nitrogen

of Urine,

Grms.

Gain of

Nitrogen,
Grms.

18.06
17.83

IS. 03

18.69
17.69

17.93

18.70

[IS. 70*]

17.56
15.60

15.72

16.85
15.29

12.86

16.54

17.04

0.50

" +120 " " " )
(Three days only.) j

2.23
2.31
1.84

2.40

" +120 " " )
(1 hree days only.) f

5.07
2.16

" " +60 grms. acetic acid j
(Three days only.) j

1.66

The smaller amount of lactic acid seems to have produced as
great an effect in reducing the proteid metabolism as an equal
weight of dextrose, but no further effect was noted from an increase
in its amount, as was the case with the dextrose. The acetic acid,
on the contrary, seems to have had a tendency to increase rather
than to diminish the proteid metabolism, and the same effect was
indicated in one of the experiments on a rabbit. It is to be re-
marked, however, that the sodium acetate appeared to be particu-
larly obnoxious to the animals. In the case of the sheep it was in-
troduced into the stomach in solution by means of a funnel, and
besides causing the animal considerable discomfort had a very
marked diuretic action. It may perhaps be questioned whether
the results obtained under such conditions represent the normal
effects of acetic acid.

Pentose Carbohydrates. — While the fate of the pentose carbo~
hydrates in the body has been the subject of considerable research,
(compare Chapter II, p. 24), their effect upon the proteid meta-
bolism does not seem to have been specifically investigated, although
Pfeiffer & Eber,f in the course of experiments upon the origin of
hippuric acid, observed that after the consumption of 500 grams of

* Assumed to be the same as with the basal ration. The actual nitrogen
of the feces for these three days was 4.78 grms., making the apparently
digested nitrogen 19.33 grms.

f Landw. Vers. Stat., 49, 137.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 125

cherry gum, containing 41. 9S per cent, of pentose carbohydrates,
the urinary nitrogen of a horse decreased by over 6 per cent. They
leave it uncertain, however, whether the effect was due to the
pentosans or to other ingredients of the gum.

Among the early experiments of Grouven * are also four in which
gum arabic, added to an exclusive straw ration, materially reduced
the proteid metabolism, but the methods of these early experiments
were naturally somewhat defective. On the other hand, Cremer's
experiments f with rhammose on rabbits showed no marked effect
of tliis substance upon the proteid metabolism.

Total A on-nitrogenous Matter of Feeding-stuffs. — The digestible
non-nitrogenous matters of feeding-stuffs, aside from a small pro-
portion of fat, are commonly although loosely grouped together as
carbohydrates. The}7 include both hexose and pentose carbohy-
drates, such organic acids as may be present or as are formed during
digestion, and a variety of other less well-known substances.

As has already appeared in discussing the effect of crude fiber, the
mixture of material included in the digestible crude fiber and nitro-
gen-free extract shows the same tendency as starch and sugar to
diminish the proteid metabolism. In other words, while the com-
mon designation of digestible carbohydrates may be of questionable
accuracy from a chemical point of view, nevertheless the some-
what heterogeneous mixture to which it is applied behaves, in this
respect at least, qualitatively like the pure hexose carbohydrates.
Numerous instances of this are cited by v. Wolff % in his discus-
sion of the data prior to 1876. Of more recent results, attention
may be specially called to those of Kellner, some of which have
been cited above. The results upon coarse fodders are those which
are of particular interest, since it is these whose ingredients are
least known chemically. The}' are presented on the following
page in the same form as those upon extracted straw above.

Although the addition of hay or straw to the basal ration in-
creased the supply of digestible nitrogenous matter, the; proteid
metabolism was not proportionately increased, but in every instance

* Wolff, Emahrung Landw. Nutzthiere, p. 289.

fZeit. f. Biol., 42,451.

% Erniihrung Landw. Nutzthiere, pp. 288-309.

126

PRINCIPLES OF ANIMAL NUTRITION.

5

<B

0*

Apparently Digested.

Nitrogen
of

Urine.

M
O

la
fa

<D

■a

3

o

u
0

s

0

■a

3
u.
D

§ 8

O » .5

is » h

o

ttuh

ES 0) u

i = §

a
o
be
o
h

2

Gain
of

Nitro-
gen.

F
F

1

3

2
3

2

4

7
4

2

4

2
3

1
3

1
4

i

i

Meadow Hay.
Basal ration + hay . .

it n

Difference

Basal ration + hay . .

a it

Difference

Basal ration + hay . .
it it

Difference

Basal ration + hay . .
it n

Difference

Basal ration + hay . .

Difference

Oat Straw.
Basal ration + straw

Difference

Basal ration + straw-
it it

Differecne

Wheat Straw.

Basal ration + straw
u a

Difference

Basal ration + straw
it a

Grnis.

123

90

Grms.
1553
1007

Grms.

3850
3014

Grms. Grins.
1383 133.84
1069 111.67

Grms.
97.19
106.03

Grms.
36.65
5.64

G
G

33

58
20

546

1675
1137

836

4006
3120

314 22.17

1498 108.96
1143 86.27

-8.84

91.30
86.30

31.01

17.66
-0.03

a

H

38

150
101

538

1786
1083

886

4037
2912

355 22.69

1487 145.94
1071 116.51

5.00

122.19
109.28

17.69

23.75
7.23

H
H

49

165
101

703

1822
1083

1125

4148
2912

416 29.43

1531 146.84
1071 116.51

12.91

130.78
109.28

16.52

16.06
7.23

J
J

64

141
107

739

1797
1114

1236

4108
2895

460 30.33

1542 163.37
1059 128.11

21.50

137.97
122.62

8.83

25.40
5.49

F
F

34

139

90

683

1701
1007

1213

3735
3014

483 35.26

1553 119.15
1069 111.67

15.35

99.40
106.03

19.91

19.75
5.64

G
G

49

86
20

694

1732
1137

721

3S04
3120

484

1574
1143

7.48

91.77
86.27

-6.63

71.36
86.30

14.11

20.41

-0.03

B
H

66

115

101

595

1904
1083

684

3436
2912

431

1485
1071

5.50

110.80
116.51

-14.94

106.32
109.28

20.44

4.48
7.23

J
J

14

111

107

821

1943
1114

524

3511

2895

414

1555
1059

-5.71

128.94
128.11

-2.96

119.89
122.62

-2.75

9.05
5.49

4

829

616 496

0.83

-2.73

3.56

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 127

save one the gain of nitrogen by the body showed a marked increase,
and this, it is to be noted, after the feeding had been continued for
a considerable time. The one exceptional case, on wheat straw,
id readily explained by the obvious effect of this material in increas-
ing the metabolic nitrogen of the feces and thus diminishing the
apparent digestibility of the protein of the ration. Had account
been taken of these metabolic products, the increased gain of
nitrogen by the animals would doubtless have been more marked
in all cases. This gain, it would seem, may fairly be ascribed to the
large additions of digestible non-nitrogenous matter derived from
the hay or straw added.

Comparative Effects of Fat and Carbohydrates. — C. Voit *
found the hexose carbohydrates to be superior to fat in diminishing
the proteid metabolism. He gives the following comparisons:

Date.

Food per Day.

Meat,
Grms.

Carbohydrates or Fat,
Grms.

Urea

per Day,

Grins.

Nov. 16-22, 1857

" 22-Dec. 2, 1857

Oct. 28-Nov. 8, 1857
Nov. 8-15,

Feb. 23-25, 1861

" 25-28 "

" 28-Mc'h. 3, 1861

June 19-23, 1859

" 23-26, "

" 26-29 "

" 29-July 2, 1859'

Feb. 17-22, 1865

" 22-25, "

July 23-26, 1S64

" 26, " . . . .

" 27, " ...

" 27-Aug. 1, 1864
Aug. 1, 1864

" 2, "

" 3, "

Jan 7-12, 1859

" 12-15, "

150
150

176
176

400
400
400

500
500
500
500

800
800

1000
1000
1000
1000
1000
1000
1000

2000
2000

150-350 sugar
250 fat

100-364 starch
250 fat

200 fat
250 starch
250 sugar

250 fat
300 sugar
200 "
100 "

250 starch
200 fat

0
100 starch
400 "

0
100 fat
300 "

0

200-300 starch
250 fat

13.4
15.6

15.1
16.2

31.9
30.5
30.3

38.5
32.7
35.6
37.9

52 . 8
54.7

73.5

68.5
60.2
79.6
74.5
69.3
80.2

128.4
135.9

*Zeit. f. Biol., 5, 447.

128 PRINCIPLES OF ANIMAL NUTRITION.

Subsequent investigations have substantially confirmed this
conclusion. Thus Kayser * in an experiment upon himself found
that the replacements of the carbohydrates of his diet by an amount
of fat equivalent to them in heat value caused a marked increase
in the urinary nitrogen, resulting in a loss of this element by the
body in place of the previous small gain. The possible effect upon
the apparent digestibility of the proteids of the food does not appear
to have been considered.

Wicke & Weiske \ report two series of experiments upon sheep
in which equivalent (" isodynamic ' ') quantities of fat and of starch
were added to a basal ration. In the first series the basal ration
was comparatively poor in proteids and fat, having a nutritive ratio
of about 1 : 8.3 ; in the second series it was richer in both these
substances and had a nutritive ratio of 1 :5.1 and 1 : 6.3 for the
two animals respectively. As is usually the case, the starch dimin-
ished the apparent digestibility of the protein of the basal ration,
while the fat produced but a slight effect in this direction. Not-
withstanding tins complication, however, the effect of the starch
in diminishing the proteid metabolism was clearly greater than
that of the fat, and if the results were corrected for the increase
in the nitrogenous metabolic products in the feces they would be
still more decisive.

The investigations of E. Voit & Korkunoff upon the minimum
of proteids, which will be considered in a subsequent paragraph,
also show a superiority in this respect of the carbohydrates over
the fats which these authors ascribe to the greater lability of
their molecular structure which enables them to enter into reactions
in the body more readily than the fats.

Magnitude and Duration of the Effect. — The pre-eminent
position of the proteids in nutrition has perhaps led investigators
to attach undue importance to this power of the non-nitrogenous
nutrients to diminish the proteid metabolism. It is well to note
that it is relatively small. C. Voit, as already stated, found an
average decrease of about 7 per cent, with fats and about 9 per
cent, with carbohydrates, and subsequent investigators have ob-
tained results entirely comparable with these.

Proteid Metabolism Determined bySttpply.— In the presence
* v. Noorden, Pathologie des Stoffwechsels, p. 117.
+ Zeit. physiol. Chem., 21, 42; 22, 137.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 129

of non-nitrogenous nutrients it is still true that the proteid meta-
bolism, or more exactly the excretion of nitrogen, is mainly deter-
mined by the supply of it in the food just as it is upon an exclusive
proteid diet. Fat or carbohydrates simply produce a relatively
small, and probably more or less transitory, diminution of it with-
out affecting the substantial truth of the above statement.

Lawes & Gilbert,* in discussing the results of their fattening
experiments upon sheep and pigs, called attention to the very wide
variations in the amount of protein consumed, both per unit of
weight and especially per unit of gain, and concluded that the ap-
parent excess of protein in some cases must have served substan-
tially for respiratory purposes. The subsequent investigations of
Bischoff, Voit, and v. Pettenkofer upon the proteid metabolism of
carnivora showed clearly that the dependence of the latter upon
the proteid supply, which is so marked upon a purely proteid diet,
is equally evident upon a mixed diet, and thus supplied a scientific
explanation of the facts observed by Lawes & Gilbert. The
effect of the proteid supply upon the nitrogen excretion is clearly
shown by the following summary of Yoit's experiments: f

Food.

Fat.
Grms.

Lean Meat.
Grms.

<ii ms,

250
300
250
200
200
250

150
176
250
500
800
1500

17.0
18.9
19.7
36.6
56.7
100.7

Since Yoit's researches, very many experiments, among the
earliest of which were those of Henneberg & Stohmami $ upon
cattle, have confirmed his results, both for carnivora, herbivore
and omnivora. A somewhat striking example is afforded byStoh-
mann'sj? experiments upon milch goats which are summarized in
the following table:

*Rep. Brit Asso Adv. Sci., 1852; Rothamsted Memoirs, Vol. II.

t Zeit. f. Biol , 5, 329.

% Beitritge, etc., Heft 2, p 412.

§BioIopi~rli<- Studien, 121.

i3°

PRINCIPLES OF ANIMAL NUTRITION.

Date.

Eaten per Day.

Hay,
Grms.

Linseed
Meal,
Grms.

Protein

Digested

per Day,

Grins.

Protein

Metabolized

per Day,*

Grms.

1
2
3
4
5
6
7
8
9
10

May 23-29
June 6-12

" 20-26
Julv 4-10

" 25-31
Aug. 8-14

" 22-28
Sept 5-11

" 19-25
Oct. 3-9.

1500
1450
1400
1350
1250
1100
950
800
1600
1600

100
150
200
250
350
500
650
800
0
0

111.6
125.0
132.2
150.9
170.5
193.8
221.4
257.2
92.9
74.1

66.6

79.4

90.6

90.1

101.6

117.9

143.1

173.7

56.3

41.9

A full compilation of these earlier results has been made by
v. Wolff,f and the fact is now so well established that further cita-
tions would be superfluous.

Rate of Nitrogen Excretion. — Some interesting hints as to
the manner in which the non-nitrogenous nutrients produce the
effect upon the proteid metabolism which has just been described
are afforded by a consideration of the rate of nitrogen excretion
under their influence.

It was shown in the preceding section that the effect of a meal
of proteids was a sudden, almost explosive, increase in the nitrogen
cleavage and excretion, reaching its maximum within a few hours
after the meal. If, however, non-nitrogenous nutrients are given
along with the proteids, the character of the curve is essentially
altered, the maximum rate of excretion being less and being reached
somewhat later, while the fall from this maximum is less rapid.
In other words, the rate of excretion becomes more uniform — the
curve is flattened out. The influence of fat in this respect is clearly
shown in the experiments of Panum \ and of Feder \ cited pre-
viously, and appears evident also in those of Craffenbergor. § In
the latter experiments the nitrogenous substances to be tested were
added to a mixed diet. The results show a distinct maximum, but
the rate of decrease after the maximum was reached was not rapid,

* Exclusive of the protein of the milk.

f Ernilhrung Landw. Nutzthiere, pp. 285-309

% Thier. Chem. Ber., 12, 402.

§Zeit. f. Biol., 28,318.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 131

and only a part of the nitrogen appeared during the twenty-four

hours following its ingestion, viz.:

AYith fibrin 49 . 2 per cent.

With gelatin 37.6 " "

With peptone 67 . 6 " "

With asparagin 79 . 0 " "

Rosemann's* results upon the rate of nitrogen excretion by-
man, likewise cited above, indicate a similar effect of the non-
nitrogenous nutrients, the fluctuations due to the ingestion of mixed
food being much less sharp than those found by other experi-
menters with proteids alone.

If we accept Rosemann's view (p. 101), that the sudden increase
in the nitrogen cleavage is due, in part at least, to a direct stimulus
to the metabolic activity of the cells, arising from the presence in
the fluids of the body of an increased percentage of proteids, we
may perhaps suppose that the simultaneous resorption of non-
nitrogenous matter renders this stimulus less and so reduces the
maximum rate of nitrogen cleavage. This conjecture possibly
receives some support also from the results of Krummacher,f who,
contrary to Adrian and Munk, finds that the division of the
proteid ration into several meals not only renders the rate of nitro-
gen excretion more uniform, but reduces somewhat the total amount
excreted. Gebhardt \ has also obtained similiar results.

There is also the possibility, however, that the non-nitrogenous
nutrients may modify the rate at which the proteids are resorbed,
or perhaps, as has been suggested by various investigators, the
extent to which the proteids are converted into amide-like bodies
by the pancreatic juice or the extent of proteid putrefaction in the
intestines. Suggestive in this regard is the fact found by Gruber §
that common salt, which acts as a stimulant to thesecretion of hydro-
chloric acid by the stomach, and would thus tend to favor gastric
as compared with intestinal digestion of the proteids, produces an
effect on the nitrogen excretion similar to that of the non-nitroge-
nous nutrients.

♦Arch. ges. Physiol., 65, 343.
tZeit. f. Biol., 35, 481.
X Arch. ges. Physiol., 65, 611.
§Zeit. f. Biol., 42, 425.

I32 PRINCIPLES OF ANIMAL NUTRITION.

Extent of Protein Storage. — Whatever may be the expla-
nation of the action of the non-nitrogenous nutrients, its effect is-
obvious. Attention has already been called (p. 102) to Gruber's
hypothesis that the transitory storage of nitrogen following an
increase in the proteid supply is the result of a superposition of the
daily curves of nitrogen excretion. The effect of the non-nitroge-
nous nutrients appears to be to diminish the rate of nitrogen cleavage
and to protract it, in the case of a single meal of proteids, over a
longer time. Evidently, then, an increase of the proteid supply in
a mixed diet, or the addition of non-nitrogenous nutrients to a pro-
teid diet, will extend its effect over a considerably longer period
than in case of an exclusive proteid diet — that is, nitrogen equi-
librium will be reached more slowly, and there will be a longer or
shorter time after the change during which the nitrogen excretion
will be less than in the absence of the non-nitrogenous matters.

This explanation also implies, however, that the storage of
nitrogenous matter in the body of the mature animal is of limited
duration and that no long-continued gain of protein can occur; in
other words, that it is impossible to materially increase the proteid
tissue (lean meat) of a mature animal.

Numerous comparative fattening experiments with domestic
animals, notably those of Henneberg, Kern, & Wattenberg* upon
sheep, fully sustain this conclusion. On the other hand, metabo-
lism experiments with domestic animals rarely showr an equality
between the income of nitrogen and its outgo in feces and urine,
but almost always indicate a gain of nitrogenous matter by the
body. As regards the significance of this fact, however, several
considerations must be borne in mind.

Eirst, the normal growth of the epidermal tissues — hair or wool,
hoofs, horns, etc. — as pointed out in Chapter III, consumes a por-
tion of the nitrogen of the food and contributes its share to the
storage of nitrogen in the body.

Second, the adipose tissue itself contains a small percentage of
proteid matter, and a storage of fat in considerable amounts in-
volves the production of new adipose tissue in which to store it.

Third, in many cases the metabolism experiments which show
a storage of nitrogen have been made within a rather short time
*Jour. f. Landw., 26, 549.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 133

after a change in the ration, and can therefore be interpreted as
showing simply that sufficient time had not elapsed to reach nitro-
gen equilibrium.

If we consider also the somewhat indefinite nature of the term
mature, and likewise the possibilities of error due to mechanical
losses of excreta and to escape of nitrogen from the hitler by fermen-
tation and decomposition, we can readily see why the results of a
short metabolism experiment may not agree with those of a long
fattening experiment; yet, nevertheless, it must be confessed that
the impression left by a comparison of the whole mass of evidence
is that the discrepancy is as yet but partially explained.

In conclusion, we may anticipate a discussion in Chapter VI,
and call attention to the fact that muscular exertion may, to a
limited extent at least, stimulate those constructive processes which
result in a storage of protein in the body.

The Minimum of Proteids. — In the preceding section it ap-
peared that the administration of proteid food to a previously fast-
ing animal caused a prompt and large increase in the nitrogen
cleavage and excretion, while but a comparatively small portion
of the proteids was applied to constructive purposes, the result
being that two to three times as much proteids must be given as
are metabolized during fasting before nitrogen equilibrium is
reached. This effect was there ascribed to the stimulating effect
of the rapid digestion and resorption of the proteids upon the nitro-
gen cleavage, much of the proteids being apparently destroyed
as such before they can serve for tissue-building.

We have just seen that the effect of the non-nitrogenous nutri-
ents is to diminish somewhat the nitrogen cleavage, apparently
by moderating this stimulating effect. The necessary result is
that, as the nitrogen supply is increased, it and the nitrogen excretion
will start more nearly together and approach each other more rapidly
upon a mixed diet than upon one consisting of proteids only. Conse-
quently, while the percentage decrease in the proteid metabolism
is, as we have seen, relatively small, nitrogen equilibrium may be
reached with a much smaller supply of proteids than is the case in
the absence of the non-nitrogenous nutrients. Indeed, it is con-
ceivable that a sufficient supply of carbohydrates or fats in the diet
should practically destroy the stimulative effects of the proteids. in

J34

PRINCIPLES OF ANIMAL NUTRITION.

which case we might expect a proteid supply equal to the fasting
proteid metabolism to be sufficient to produce nitrogen equilibrium.
Seen in this light, the apparently insignificant effect of the non-
nitrogenous nutrients becomes a very important factor in nutrition.
The effect of the non-nitrogenous nutrients in largely diminish-
ing the necessary proteid supply was pointed out by C. Yoit * and
appears clearly in many of his experimental results. Thus from the
summary on p. 95 it appears that from 1200 to 1500 grams of lean
meat per day was required to maintain the animal experimented
upon in nitrogen equilibrium. When fat or carbohydrates were
added to the ration, however, strikingly different results were
reached, as appears from the following comparative statement,
the results being expressed as " flesh " with 3.4 per cent, of nitrogen :

Food.

Flesh

Meta-
bolized.

Gain of

Meat.

Fat or Carbo-
hydrates.

Flesh.

Meat only (average of both series) A
Meat and fat <

300

600

900

1200

1500

500

800

1000

500

800
1000

250
200
250 .

300-100
100-400
1 00-100

416
674
943

1207
1478

444
720

875

502
763
902

-116

- 74

- 43

- 7
+ 22

+ 56
+ 80

Meat and carbohydrates (compare \
p. 116) 1

+ 125

- 2

+ 37

+ 88

In the presence of non-nitrogenous nutrients, nitrogen equi-
librium was reached with quantities of proteids from one third to
one half as great as the amount required when fed alone. In other
words, the non-nitrogenous nutrients materially reduced the mini-
mum of food proteids required to maintain the proteid tissues of
the body.

In view of the peculiar importance of the proteids in nutri-
tion, as well as of their relative scarcity and high cost, particu-
larly in the food of our domestic animals, great interest attaches
to a determination of the least amount required to sustain a mature

* Zeit. f. Biol., 5.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 1 35

animal. The results obtained by E. Voit & Korkunoff * regard-
ing the minimum requirement upon an exclusive proteid diet have
already been stated in the first section of this chapter (p. 95). The
same investigators have also studied the more interesting question
of how far the necessary proteid supply can be reduced in the
presence of non-nitrogenous nutrients.

Proteids and Fat. — The experiments were upon the same
general plan as those just referred to on proteids alone. Beginning
wnth an insufficient quantity of proteids, the amount was gradually
increased, that of the fat remaining constant, until nitrogen equi-
librium was reached. As in those experiments, too, the nitrogen
of the food was practically all in the proteid form, and its amount
is compared with the proteid nitrogen excreted, it being assumed
that 18.45 per cent, of the urinary nitrogen was derived from the
extractives of the flesh metabolized in the body. To the writer it
would seem that a more suitable unit would be the total excretory
nitrogen, since the proteids of the food had to make good the loss
of extractives as well as of true proteids from the body, and the
former loss is as unavoidable as the latter. Accordingly, the results
have been stated in the table below in both ways.

Two series of experiments were made: one in which the total
food-supply was less than was required to supply the estimated
demands of the body for energy, and one in which it considerably
exceeded that demand, with the following results:

Series I:

Experiment 1
2

Series II:

Experiment 3
4
5

Total
Nitrogen
Excretion,

Fasting,

Grms.

4.85
4.22

4.98
4.01
3.86

Per Cent, of Energy
Demand Supplied by

Fat,
Per Cent.

72

73

116
127
137

Total

Food,

Per Cent.

90
86

128
140
150

Minimum of Food Nitrogen.

Amount,
Grms.

7.63
>5.61

>6.61
5.12
5.07

Per Cent of Fasting
Metabolism.

Total, Proteid.
Per Cent. Per Cent

157
>133

>133
128
131

193
>163

>162
157
161

The authors also compute from experiments by C. Voit and by
Rubner percentages lying between 162 and 207, and state as their
* Zeit. f. Biol., 32, 58.

136 PRINCIPLES OF ANIMAL NUTRITION.

final result that the minimum of proteid nitrogen on a diet of pro-
teids and fat lies between 160 and 200 per cent, of the proteid nitro-
gen excreted during fasting. These figures when computed on the
total excretory nitrogen would become 131 per cent, and 1G3 per
cent, respectively.

Proteids and Carbohydrates. — We have seen (p. 127) that
the carbohydrates diminish the proteid metabolism to a greater
extent than the fats. The results which have been reached as to
their effect in lowering the minimum demand for proteids are on
the whole in accord with this fact. With a liberal supply of carbo-
hydrates in the food, a much smaller quantity of proteids would
seem to suffice to maintain nitrogen equilibrium than when the
non-nitrogenous matter of the ration consists of fat. Indeed, ac-
cording to some investigators, the proteid metabolism may evei.
be thus reduced much below that during fasting.

Munk * appears to have been the first to advance the view last
mentioned. In an investigation upon the formation of fat from
carbohydrates a dog was fasted for thirty-one days and then re-
ceived a diet consisting of a little meat with large amounts of
carbohydrates (starch and sugar) and also, during the first twelve
days, gelatin. Omitting these twelve days and also the earlier days
of the fasting period, the average daily excretion of nitrogen in the
urine was

Twelfth to thirty-first days of fasting 5.38 grams

Thirteenth to twenty-fourth days of feeding (200

grams meat, 500 grams carbohydrates) 5 . 79 "

On the seventeenth day of the feeding the urinary nitrogen
reached, the minimum of 4.133 grams, and Munk regards this as
showing the possibility of a reduction of the proteid metabolism
considerable below the fasting level. It is to be noted, however,
that the nitrogen excretion varied considerably from day to clay,
and a selection of a single day for comparison seems hardly justified.

Hirschfeld f and Kumagawa \ found that the nitrogen equili-

* Arch. path. Anat. u. Physiol., 101, 91.
t Ibid., 114, 301.
% Ibid., 116, 370.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 137

brium of man could be maintained on a diet containing little nitro-
gen but abundance of non-nitrogenous nutrients. Under these
conditions the urinary nitrogen was reduced to 5.87 grams and 6.07
grams per day respectively, and the total nitrogen excretion to
7.45 grams and 8.10 grams, amounts much lower than have been
observed for fasting men. Thus in the extensive investigations by
Lehmann, Midler, Munk, Senator, & Zuntz * of the metabolism
of two fasting men, much higher figures than the above were ob-
tained for the urinary nitrogen, and Munk (loc. cit., p. 225) calls
attention to the fact that in one case the urinary nitrogen on the
secontl day succeeding the fasting period was materially less than
on the last day of the fasting, viz., 8.26 grams as compared with
9.88 grams.

In a subsequent series of experiments upon dogs, Munk | showed
that by very liberal feeding with food poor in proteids (rice with
small amounts of meat) the nitrogen balance could be maintained
for a considerable time at an amount very much lower than pre-
vious observers had found for the proteid metabolism of fasting
dogs of similar weight.

Length
of Exper-
iment,
Days.

Average
Live

Weight,
Kgs.

Food per Day.

Urinary

Fat.
Grms.

Starch,
Grms.

Nitrogen,

Grms.

Nitro-
gen,
Grms.

r 1

With food: < jJJ

LIV

Fast.ng: ] Fa,ck

5
5
4
4

11.20

10.21

9.88

8.25

14.4
8.9

55
38
53
47

116

96

108

100

2.63
2.48
2.66
2.60

2.61
2.40
2.67
2.62

3.65
5.10

Munk also cites in support of his conclusions Rubner's results
on a dog fed exclusively on carbohydrates. A reference to these
results as tabulated on a subsequent page does in fact show in most
cases a decrease in the proteid metabolism as compared with the
fast in": values, but how much of this is due to the normal decrease
during the first few days of abstinence from proteid food it is dif-

*Arch. path. Anat. u. Physiol., 131, Supp.
ilbid., 132,91.

13«

PRINCIPLES OF ANIMAL NUTRITION.

ficult to decide. Munk also cites results obtained by Salkowski,*
who observed the nitrogen excretion of a dog on a light ration con-
taining but little proteids to be scarcely greater than in the absence
of all food.

E. Voit & Korkunoff {loc. cit.) also included the carbohydrates
in their investigation upon this subject, following the same general
method as in the experiments with fat. The following are their
results compared with the fasting proteid metabolism exactly as
in the former case:

Total
Nitrogen

Per Cent, of

Energy Demand

Supplied by

Minimum

of Food Nitrogen.

Per Cent. of Fasting

Weight,

Excre-
tion,

Amount,
Grms.

Metabolism.

1 Total

Kgs.

Fasting,

Carbo- Food.

Grms.

hydrates,! per

Total,

Proteid,

PerCent. (.ent

Per < Vnt. PerCent.

I

Series I;

>

Experiment

3a

24.0

4.93

78

91

>5.43

' >\V)

>133

"

2

24.6

4.94

79

92

5.00

101

124

Series II:

Experiment

5

27.7

4.98

111

122

5.11

103

126

«

1

24.1

5.25

115

126

>4.91

>94

>123

u

2

24.7

4.94

118

131

<4.35

<88

<108

It

4

30.0

4.08

122

136

<4.47

<110

<134

It

3b

24.0

4.93

155

168

<4.48

<91

<111

The authors also compute from a few experiments by C. Voit
and by Rubner values not inconsistent with the above.

When compared with the total nitrogen excretion, the results of
Voit & Korkunoff .show in but a single case a minimum unmistak-
ably greater than the fasting proteid metabolism. In three cases
the minimum falls below this amount, while in the remaining cases
it is either substantially equal to it or doubtful. Regarded in this
way, they seem on the whole in accord with Munk's claim that the
proteid metabolism may be reduced below the fasting limit. Voit
& Korkunoff, however, dispute this and subject Munk's experi-
ments to a detailed criticism, the principal points of which are that
in the earlier experiments, as noted above, the nitrogen excre-
tion was irregular and that the result of a single day is arbi-
trarily selected for comparison, while in the later experiments no
* Zeit. physiol. Chem., 1, 44.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 139

determinations of the fasting metabolism of the animals actually
used for the experiments were made. By a re-computation of
Munk's experiments they obtain results varying but little from
100 per cent. A computation from the average figures given on
p. 136, assuming 3.4 per cent, of nitrogen in the meat and 0.51
grams of nitrogen per day in the feces, shows that the minimum is
probably less than 107 per cent, of the fasting nitrogen excretion.

Much depends, however, upon whether we take as the unit of
comparison the total nitrogen excretion or, like Yoit & Korkunoff,
eliminate that portion derived from the extractives. If we select
the former, then it appears that with a liberal supply of carbohy-
drates in the food the supply of proteids certainly need not exceed
the fasting metabolism in order to maintain nitrogen equilibrium,
and perhaps may be reduced materially below it.

Finally, it must be remembered that the fasting proteid meta-
bolism itself is not a constant. In Chapter IV it was shown that
as the store of fat in the body of a fasting animal becomes depleted
the body proteids are drawn upon to an increasing extent to supply
energy to the animal. It is not possible to show that the experi-
mental results which have been cited are materially affected by this
variability of the fasting proteid metabolism — indeed, it seems
doubtful whether they are — but the fact that the demands of the
organism for energy may affect the proteid metabolism is of itself
sufficient to show that our unit of comparison, while practically
convenient and perhaps sufficiently accurate, is not invariable.

Amount of Non-nitrogenous Nutrients Required.— In
most of the experiments which have been cited, the very low figures
for the necessary proteid supply have been obtained by the em-
ployment of an amount of non-nitrogenous nutrients materially
in excess of the estimated requirements of the animal for energy,
although in no case was this hitter factor actually d< termined.

Riven,* however, experimenting upon himself with a diet equal
in amount to that ordinarily required to maintain his weight, was
able to gradually reduce the total nitrogen of his food to 4.52 grams
and maintain nitrogen equilibrium. He did not determine his fast-
ing metabolism, but the above figure? which is equivalent to 0.08
gram of nitrogen per kilogram live weight, is lower than the low-

* Skand. Arch. f. Physiol., 10, 91.

140

PRINCIPLES OF ANIMAL NUTRITION.

est fasting values previously obtained, Moreover, much of the
nitrogen of his food was in the non-proteid form, the proteid nitro-
gen being estimated at only 0.03 gram per kilogram live weight.

Cremer & Henderson * have attempted to reproduce Siven's
results in two experiments upon a dog, the total amount of food
being equal to or slightly less than the estimated requirements of
the animal. Under these conditions they were unable to reach
even as low a minimum as did Voit & Korkunoff. On the other
hand, Jaffaj in a dietary study of a child on a diet of fruits and nuts
(so-called frutarian diet), observed a gain of nitrogen by the sub-
ject with only 0.041 gram of food nitrogen per kilogram body weight.

The Minimum for Herbivora. — The ordinary food of our
domestic herbivora contains an abundance of non-nitrogenous
matter and relatively little protein. It is impossible, for obvious
reasons, to determine the fasting metabolism of ruminants, and
the basis for comparisons like those made above is therefore
largely lacking. There is, however, abundant evidence to show that
only a comparatively small amount of proteids is necessary to
maintain the nitrogen equilibrium of cattle in particular, although
exact data as to the least amount required are still lacking.

The early experiments of Henneberg & Stohmann % upon the
maintenance ration of cattle furnish the following examples of the
sufficiency of a very small proteid supply, the results being com-
puted per 500 kgs. live weight per day :

Digested.

Gain of

Protein,
Grins.

Non -nitrogenous

Nutrients,

Grms.

Nitrogen

by Animal,

Grms.

Ox I:

Period 1

178
259
209

278

4247
3546
3926

3607

4.0

" 2

21.0

" 3

11.0

Ox II:

Period 2

19.5

* Zeit. f. Biol., 42, 612.

f U. S. Dept Agr., Office of Expt. Stations, Bull. 107, 21.
% Beitrage zur Begrlindung einer rationellen Futterung der Wiederkauer,
Heft I.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 141

The following figures, obtained by the same investigators * in
later experiments, are taken from Wolff's compilation : f

+

Live

Weight,

Kgs.

Digested.

Gain of

Protein,
Grms.

Non- nitrogenous

Nutrients,

Grms.

Nitrogen

by Animal,

Grms.

1860-61.
Ox I. Period 5

514

315

405
375
280
435

395
410
400
445
390

2435
4090
4980
3060
4590

4995
3610
3620
5540
6060

0

" 14

531
533
625
643

638
643
661
701
715

+ 96

" 16

+ 24 8

Ox II. Period 6

+ 9.6
+ 14.4

" 15

1865.

Ox I. Experiment 1

2

tt . It q

Ox II. " 5

6

+ 0.5

- 0.8
+ 4.0

- 6.4
+ 3.2

G. Kuhn's extensive investigations at Mockern.J together wdth
subsequent ones by Kellner,§ afford the following data for the
periods in which the ration was approximately a maintenance
ration:

Live

Weight,
Kgs.

In Digested Food.

Gain of

Protein,
Grms.

Metabolizable

Energy,

Cals.

Nitrogen

by Animal,

Grms.

Kuhn's Experiments:

Ox II. Period 1

" III. " 1

632
632
631
623
602
644
672

620
612

748
750
858

413
338
339
320
451
458
540

440
213
343
696
665

16388
17986
18077
17125
15072
15872
17416

16322
15447
13716
18655
24558

+ 0.1
- 2.6

"IV. " la

- 0.5

" IV. " 16....

- 5.7

" V. " 1

" VI. " 1

+ 8.5
+ 6.3

" XX. " 1

KeUner's Experiments:

Ox A

+ 3.3

+ 6.2

" B

-14.6

" I..

-13.8

" II

- 2.8

" III

+ 5.1

♦Beitrage, etc., Befi II., and Xeue Beitriige, etc.
fErnahrung landw. Nutzthiere, pp. 406-410.
% Landw. Vers, Stat., 44, 257.
§ Ibid., 47, 275; 50, 245.

142

PRINCIPLES OF ANIMAL NUTRITION.

Experiments by the writer * have shown that nitrogen equi-
librium may be maintained, for a time at least, on even smaller
amounts of protein than the above figures would indicate. The
figures in the first column of the following table signify the proteid
nitrogen only of the food multiplied by 6.25:

Digested Pro-
teids pt- r Day
and 500 Kgs.
Live Weight,
Grms.

Meta-
bolizable

Energy

of Food,

Cals.

Average
Live

Weight,
Kgs.

Gain or Loss

of Nitrogen

by Body,

Grins.

Nutritive

Ratio

1: .

Experiment I:

Steer 1

129
113
133

192
202
209

297

277
314

156
131
152

258
242
275

7956

7588
7191

8144
9590
8084

11130
11318
11324

11955
11904
11557

11634
12976
12030

420
450
400

420
450
400

450
490
430

450
490
430

543
629
516

-2.51

-0.39
-1.08

+ 1.76
+ 4.23

+ 4.62

+ 4.67
+ 6.47
+ 2.65

+ 5.68
+ 3.98
+ 4.15

+ 0.26
-0.20
-2.31

20 1

" 2

" 3

20.4
18.6

Experiment II:
Steer 1

13 4

" 2

13 6

" 3

12.8

Experiment VI:
Steer 1

10 9

" 2

10.9

" 3

10.6

Experiment VII:
Si eer 1 .

23.0

" 2..

25.3

" 3

Experiment VIII:
Steer 1

23.9
10.4

" 2

10.7

"3

10.6

While the above data are hardly sufficient to fix absolutely the
minimum of proteids for cattle on a maintenance ration, they indi-
cate clearly that from 200 to 300 grams of digestible protein per day
is at least sufficient for a steer weighing 500 kgs., and there is a
possibility that the amount may be somewhat further reduced.
Although we are unable to compare this with the fasting meta-
bolism, a comparison on the basis of live weight with sonic of the
results previously cited shows that the minimum demand for pro-
teids on the part of cattle is relatively much less than on the part
of carnivora. Thus the results obtained by Lclnnann et. al. and
Munk (p. 137), and by Voit & Korkunoff (p. 138), computed in
*Penna. Expt Station, Bull. 42, 165.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 143

grams of food nitrogen per kilogram live weight, give the following
figures for the minimum nitrogen requirements of the dog and of
man as compared with cattle :

Experiments on Dogs.

Munk

0 . 235 gram.
0.243 "
0.269 "
10.315 "

Average 0.266

Voit & Korkunoff .

Experiments on Man.

I.ehmann et al.

>0.226
0.203
0.185
>0.204
<0.176
<0.149
<0.187

r 0.190

J 0.180
0.090
0.180

Experiments on Cattle.
Range of experiments cited 0.064-0.098 gram.

Only one of the results on man, together with the very low
figure obtained by Siven (p. 139), is comparable with those reached
with cattle. Whether we are to ascribe the small demand of the
latter for proteids to a specific difference in their rate of meta-
bolism or to the large amounts of carbohydrate material which
they habitually consume does not clearly appear.

Effects upox Health. — Munk, in his experiments with rations
very poor in proteids, made the observation that while such rations
were adecmate to maintain the nitrogen balance of the body they
nevertheless appeared to produce, in time, profound functional dis-
turbances, sometimes ending in death. Similar observations have
also been made by Rosenheim.* These experimenters ascribe

*Arch. ges. Physiol., 54, 61.

144

PRINCIPLES OF ANIMAL NUTRITION.

the ill effects directly to the small supply of proteids, but some other
writers appear inclined to explain them as due to the long continu-
ance of a uniform and rather artificial diet. The writer's experi-
ments, cited above, showed no evidence of any ill effect in the case of
cattle upon a ration containing but about 200 grams digestible pro-
tein per day and continued for seventy days, and subsequent obser-
vations, as well as the common experience of farmers in wintering
cattle upon such feeding-stuffs as inferior hay, straw, etc., fully
confirm this result.

Effects on Total Metabolism.

Substitution for Body Fat. — We have seen in the preceding
section that proteid food, or rather the non-nitrogenous residue
arising from its cleavage in the body, may be utilized as a source of
energy in place of the body fat which would otherwise be meta-
bolized. Similarly, the non-nitrogenous nutrients supplied in the
food may be thus substituted for body fat in the metabolism of the
animal. The substitution is shown most clearly in experiments
upon fasting animals, although it appears also in those in which
these nutrients are added to an insufficient ration.

Fat. — The following averages of Pettenkofer & Yoit's experi-
ments,* computed from Atwater & Langworthy's digest,! illustrate
this substitution of food fat for body fat :

Food,
Grins.

Number of
Experiments.

Gain or Loss by Body.

Nitrogen,
Grms.

Fat,
Grms.

Nothing
100 fat
350 "

5
2
1

-6.64
-4.90

-7.70

- 97.76

- 16.25
+ 113.60

The smaller amount of fat not only diminished the proteid meta-
bolism but also largely reduced the loss of fat from the body. The
larger amount of fat showed the tendency noted on p. 1 15 to increase
the proteid metabolism, but at the same time it not only suspended
the loss of body fat but caused a storage of fat in the organism. Of
course we have no means of distinguishing in such a case betwe en

* Zeit. f. Biol., 5, 370.

f U. S. Dept. Agr., Office of Experiment Stations, Bull. 45.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 145

food fat and body fat, but it is most natural to suppose that the re-
sorbed fat of the food, being already in circulation in the body, is
more easily accessible to the active cells than the stored-up fat of
the adipose tissue and is, therefore, metabolized in preference to the
latter.

Paibner,* in his study of the replacement values of the several
nutrients, has demonstrated the same effect of food fat. Fat
supplied in the food is utilized as a source of energy to the body and
a corresponding quantity of body fat escapes oxidation, while if
supplied in excess fat is stored up in the body. The experiments
were made in the same manner and are computed on the same
assumptions as those upon proteids recorded on p. 106. All were
on dogs except the third, which was on a rabbit.

Food.

Total Nitrogen

of Excreta,

Grms.

Fat

Metabolized,

Grms.

Gain or Loss
of Fat,
Grins.

Nothing

200 grms. bacon

Nothing

39 . 75 grms. of butter fat

Nothing

26 . 1 grms. bacon j

Nothing

100 grms. fat

Nothing

40 grms. bacon

1.69
1.68

2.14
2.44

0.778
1.045

1.08
1.32

60.47
71.80

33.78
33.48

7.18
6.44

42.40
47.73

22.88
28.73

- 60.47
+ 128.20

- 33.78
+ 6.27

- 7.18
+ 19.63

- 42.40
+ 52.27

- 22.88
+ 11.27

In nearly every case there was a slight increase in the proteid
metabolism, as in Pettenkofer & Voit's experiments, and a some-
what greater, although still not very considerable, increase in the
fat metabolism. In the main,. however, the food fat was metabolized
in place of the body fat.

In those of Pettenkofer & Voit's experiments in which fat was
added to an insufficient ration of meat the same effect was pro-
duced, as appears when we compare the results upon a ration of meat

* Zeit. f. Biol., 19, 328-334; 30, 123.
| Results approximate only.

146

PRINCIPLES OF ANIMAL NUTRITION.

and fat given on p. 150 with those upon the same ration of meat
without the fat, as in the table below:

Number

of
Experi-
ments.

Food per Day.

Gain or Loss by Body.

Meat,
Grms.

Fat,

Gnus.

Nitrogen,

Grms

^Carbon,
Grms.

" and fat

6
1
5

500
500
500

ioo
200

-3.4
+ 0.3
-0.6

-49.1
+ 27.1

it it a

+ 67.3

Carbohydrates. — The more soluble hexose carbohydrates
when given to a fasting animal serve, like the fats, as a source of
energy for the organism in place of the body fat which would other-
wise be oxidized.

The following is a summary of the average results obtained by
Pettenkofer & Voit * by feeding starch with a small amount of
fat, the fasting metabolism being the same as that just given on
p. 144. The averages are computed as before from Atwater &
Langworthy's digest (loc. cit.) :

Number

of
Experi-
ments.

Food per Day.

Gain or Loss by Body.

Starch.
Grms.

Fat,
Grms.

Nitrogen,
Grms.

Carbon,

Grms.

Fasting

5

450
597
700

ie'9

21.2
20.2

-6.64
-7.20
-9.40
-6.20

-97.76

Starch

+ 19.40
-28.50

+ 61.30

The fasting metabolism in this case represents a series of expcvi-
ments antedating by a year or two that upon starch. In only one
case were the respiratory products of the fasting animal determined
during the latter series. That determination immediately fol-
lowed a day on which a large amount of starch was consumed,
and the results are believed by the authors to be affected thereby.
No very strict comparison is therefore possible, but the general
effect of the starch in diminishing the loss of body fat is evident.

*Zeit. f. Biol., 9,485.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 14 7

The experiments by Rubner,* which have been already several
times referred to. include trials in which sugar or starch was fed
alone. The results are computed as previously described, with the
additional assumption that all the carbohydrates digested (with the
exception of small amounts of sugar found in the urine in some
cases) were completely oxidized in the body. The gain or loss of
<?arbon as fat is therefore computed by subtracting from the total
excretory carbon, first, the carbon due to the protein metabolized,
-and second, that assumed to be derived from the carbohydrates.
On this basis the results are as follows, the amounts of carbohydrates
given in the table being those believed to have been actually oxi-
dized :

Food.

Total Nitrogen

of Excreta,

Grms.

Total Carbon

of Excreta,t

Grms.

Gain or Loss
of Fat,
Grms

Nothing; . 1.94

76 . 12 grms. cane-sugar 1 45

104.97 " 1.07

Nothing 1.86

97 . 3 grms. cane-sugar 1 . 92

17.0 " " " 1.41

143.0 " " " 1.22

Nothing 1.32

34 grms. cane-sugar 1.41

45 " " " 1.25

50 " " " 1.57

Nothing 1.39

Nothing 1.42

42 . 96 grms. starch 1 . 53

Nothing 2.00

57 . 38 grms. starch 1 . 52

Nothing (second day) 2.64

94.30 trrt t is. cane-sugar )

67.96 " starch - 1.23

4.70 " fat

38.18
43.19
47.78

39.22
50.69
39.52
46 . 45

21.36

26.18
29.14
27.68
25.79

26.47
33.28

31.53

39.67

27.86
38.94

- 40.99

- 8.41
+ 0.51

- 42.72

- 2.95

- 35.80
+ 23.32

- 21 .88

- 9.10

- 7.4C>

- 1.64

- 27.86

- 28.10

- 10.54

- 32.10

- 10.74

- 24.97
+ 116. 35J

In place of the slight increase in the proteid metabolism fre-
quently noticed when fat is consumed, the tendency of the carbo-

*Zoit. f. Biol., 19, 357-379; 22, 273.

fNot including the carbon of the carbohydrates found in feces and urine.

X Total gain of carbon, computed as fat. Compare, loc. cit., 22, 279.

148

PRINCIPLES OF ANIMAL NUTRITION.

hydrates seems to be to cause a slight decrease, but the chief effect
is upon the carbon metabolism, increasing rations of carbohydrates
resulting in a progressive reduction of the amount of body fat meta-
bolized.

The effect of starch or sugar when added to an insufficient pro-
teid diet may be illustrated, as in the case of fat, by a comparison
of Pettenkofer & Voit's results, cited on p. 150, with those on pro-
teids alone:

Number

of
Experi-
ments.

Food per Day.

Gain or Loss
by Body.

Meat,
Grms.

Fat,
Grins

Starch,
Grms.

Dextrose,
Grms.

Nitrogen,
Grms.

Carbon,
Grms.

Proteids alone

" and starch. . .
" " dextrose .

6

8
3

500
500
500

5~3

200

200

-3.4

-1.8
-1.3

-49.1
+ 9.0

+ 7.2

Mutual Replacement of Nutrients. — The facts which have been
considered in the foregoing pages show a remarkable degree of
flexibility in the animal organism as regards the nature of the mate-
rial consumed in its vital processes. The amount of proteid mate-
rial necessarily required for the metabolism of the mature animal
we have seen to -be relatively small. Aside from this minimum, the
metabolic activities of the body may be supported, now at the ex-
pense of the stored body fat, now by the body proteids, and again
by the proteids. the fats, or the carbohydrates of the food. What-
ever may be true economically, physiologically the welfare of the
mature animal is not conditioned upon any fixed relation between
the classes of nutrients in its food-supply, apart from the minimum
requirement for proteids.. The possibility of a mutual replacement
of the several classes of nutrients in the food follows almost neces-
sarily from the power of the organism to utilize them all indiffer-
ently (in a qualitative sense at least).

Replacement of Proteids. — It has been shown that proteids
in excess of the minimum demand can be used by the organism to
take the place of body fat previously metabolized. Furthermore,
as we have just seen, the non-nitrogenous nutrients of the food may
likewise be substituted for body fat. It is natural to suppose, there-
fore, that that portion of the proteid supply which serves substan-

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 149

tially as a source of energy only may be replaced cither by body fat
or by other food nutrients, and this supposition is borne out by the
observed facts.

By Body Fat. — In considering the total metabolism of the fast-
ing animal in Chapter IV, we saw that the fat of the body has a
marked effect in protecting the body proteids from metabolism, and
that with the progressive impoverishment of the body in fat, more
and more of the proteids are substituted for the latter as a source of
energy. In § 1 of the present chapter it was further shown that
the food proteids, or their non-nitrogenous residue, may be oxi-
dized in the organism in place of the stored fat of the bod}'.

It is clear, however, that the same experiments may equally
well be regarded from the converse point of view as showing that
the body fat may be oxidized and serve as a source of energy in
place of the proteids of the food or of the body. In other words,
it is possible, within quite wide limits, for the animal organism to
draw its supply of energy, according to circumstances, either from
food or body proteids or from its stored-up fat.

By Fats and Carbohydrates of Food. — When, in addition to its
reserve of fat, a supply of non-nitrogenous nutrients is afforded in
its food, this range of choice by the organism is still further widened.
In considering the effects of non-nitrogenous nutrients upon the
proteid metabolism, and particularly in the discussion of the mini-
mum of proteids, it became evident incidentally that fat or car-
bohydrates may to a large extent be substituted for proteids in
the food. A certain minimum of proteids was shown to be essential
to the maintenance of the proteid tissues of the body, but proteids
supplied in excess of this amount undergo nitrogen cleavage and
serve substantially as a source of energy. This excess of proteids,
as we have seen, can be replaced in the food by non-nitrogenous
nutrients, particularly the carbohydrates, at least without damage
to the proteid nutrition, as is shown by Voit's results there
cited (p. 134). The later respiration experiments of Pettenkofer
& Yoit show that this is true also as regards the total metab-
olism. As appears from the table on p. 109. a ration of 1500
grams of lean meat sufficed to maintain the dog experimented
upon approximately in equilibrium as regards the income and
outgo of both nitrogen and carbon. When, however, a con-

15°

PRINCIPLES OF ANIMAL NUTRITION.

siderable proportion of this meat was replaced by fat, starch, or
sugar, not only was the nitrogen equilibrium maintained but the
same was true of the carbon, as appears from the following averages
computed from Atwater & Langworthy's " Digest of Metabolism
Experiments." * The results of Pettenkofer & Voit's first series
with 1500 grams of lean meat as given by them are also included
in the table:

Food per Day.

Gain or Loss
by B. ciy.

Meat,
Grms.

Fat,

Gnus.

Starch,
Gnus.

Grape-
sugar.
Gnus.

Nitrogen,
Gnus.

Carbon,
Grms.

Proteids only :

Series I . .

1500
1500

500
500

500
500

100
200

5.3

200

200

0
+ 0.6

+ 0.3
-0.6

-1.8
-1.3

+ 3.3

Average of all (22 experiments)

Proteids and fat:

100 grms. fat (1 experiment) . .
200 " " (5 experiments) .

Proteids and carbohydrates :

Starch (S experiments)

Grape-sugar (3 experiments) . .

+ 8.7

+ 27.1
+ 67.3

+ 9.0

+ 7.2

While it is true, as was stated on page 109, that there is reason
to suppose the carbon balance as computed by Pettenkofer &
Voit to be somewhat in error, this in no way affects the general
showing of the above averages. The introduction into the diet of
100-200 grams of fat or carbohydrates made it possible to dispense
with two thirds of the proteids previously required to maintain the
animal, the remaining 500 grams of meat being nearly or quite suffi-
cient to maintain nitrogen equilibrium. The fat or carbohydrates
added were obviously used by the organism as sources of energy in
place of the proteids (or their non-nitrogenous residue) oxidized
for this purpose on a purely proteid diet, since the stored fat of the
body was not only conserved but even shows a gain.

Rubner's investigations upon the source of animal heat t afford

* U. S. Dept. Agr., Office of Expt. Stations, Bull. 45. Compare Zeit. f.
Biol., 7, 450-480; 9, 6-13 and 450-467.
t Zeit. f. Biol., 30, 125-132.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 151

a similar illustration of this effect of non-nitrogenous nutrients.
Assuming average figures for the nitrogen and carbon content of the
food materials used, he obtained the following results:

Food per Day.

Gain or Loss by Body.

Meat,
Grins.

Fat,
Grms.

Nitrogen,
Grms.

Carbon,
Grms.

Proteids alone (1 experiment)

" and fat (2 experiments).. . .

350

80

30

+ 1.66
-0.08

-2.69
+ 4.46

The possibility of such a substitution of non-nitrogenous nutri-
ents for the food proteids as is illustrated in the foregoing experi-
ments seems, indeed, almost a necessary corollary of the facts con-
cerning proteid metabolism considered on previous pages. We
have seen that, beginning with the fasting metabolism, the effect
of successive additions of proteids to the food is to stimulate the
proteid metabolism. Only a relatively small proportion of the
added proteids is employed by the organism for constructive pur-
poses, the larger part of it undergoing very promptly nitrogen
cleavage and thus constituting, to all intents and purposes, an ad-
dition to the supply of non-nitrogenous material available for
metabolism. It appears quite natural, then, that the portion of
the proteid supply which thus serves substantially as fuel to the
organism should be replaceable in the food by non-nitrogenous
materials which are capable of serving the same purpose.

Fats and Carbohydrates. — The apparent identity of the func-
tions of the fats and carbohydrates as sources of energy which has
been shown in the preceding paragraphs necessarily implies the
possibility of their mutual replacement in the food. Rubner* has
completed the chain of evidence by showing experimentally that fat
and dextrose may thus replace each other. A dog received fur
twelve days a ration of 300 grains of lean meat and 42 or 50 grams
of fat, with the exception of three days, on which varying amounts
of dextrose were substituted for the fat. On six days the respi-
ratory products were determined. Averaging the results for all
the days on which the food was the same, and assuming the lean

* Zeit. f. Biol., 19, 370.

152

PRINCIPLES OF ANIMAL NUTRITION.

meat used to have contained 3.4 per cent, of nitrogen and 12.51
per cent, of carbon, and the fat 76.5 per cent, of carbon, we have:

Food per Day.

Gain or Loss by Animal,

Meat,
Grms.

Fat,

Grms.

Dextrose,

Grms.

Nitrogen,
Grms.

Carbon,
Grms.

Meat and fat

S 300
/ 300
(300
^300
(300

42
50

63.7

79.7

115.5

+ 1.81
+ 0.10
+ 1.78
+ 2.28
+ 1.98

+ 1.27

Meat and dextrose ....

+ 9.31
-7.44
-8.15
+ 6.21

The averages of Pettenkofer & Voit/s results as tabulated on
p. 150 may likewise be regarded in this light.

Relative Values. — The close similarity in the functions of the
several non-nitrogenous nutrients is too obvious to have escaped
early notice, and the investigations of the Munich school of physi-
ologists served both to emphasize the similarity and to follow it
into details. To Rubner, a pupil of Voit, is generally ascribed
the credit of having first placed in a clear light the quantitative
relations of the subject, although v. Hosslin* and Danilewskyf
enunciated similar ideas at about the same time, which, however,
were not based on their own experiments.

As the result of his investigations upon the replacement values
of the nutrients,! Rubner announced the law of "isodynamic re-
placement." This law is, in brief, that the several nutrients can
replace each other in amounts inversely proportional to their physi-
ological heat values, that is, to the amounts of heat which they
would liberate if oxidized to the same final products which
result from their metabolism in the body. In other words, aside
from the minimum of proteids the nutrients are of value to the
organism in proportion to the amount of energy which their meta-
bolism liberates— they are " the fuel of the body." One gram of
fat, for example, when oxidized to carbon dioxide and water, liber-
ates about 9.5 Cals. of energy, while one gram of starch similarly

* Arch. path. Anat. u. Physiol., 89, 333.
t Die Kraftvorrate der Nahrungsstoffe.
% Zeit. f. Biol., 19, 313-

THE RELATIONS OF METABOLISM TO FOOD SUPPLY. 153

oxidized liberates but about 4.2 Cals. The relative values of fat
and stareh, then, are as 9.5:4.2 or as 2.26:1. Similarly, one
gram of proteids oxidized to carbon dioxide, water, and the nitrog-
enous metabolic products of feces and urine liberates (in the dog)
about 4.4 Cals. of energy. So far, therefore, as they are used as
a source of energy simply and not for constructive purposes, their
value, compared with starch, would be as 4.4 : 4.2 or as 1.05 : 1.

A rival theory of "isoglyeosic values," the basis of which has
already been indicated in Chapter II, has been advanced by Chau-
veau * and his school in Paris. According to this school, dextrose
(or glycogen) constitutes the material which is consumed in the
vital activities of the organism. The various nutrients, then, will
be of value to the organism in proportion to the amount of gly-
cogen or dextrose which they can supply, and the chemical equa-
tions already given on pp. 38 and 51 are claimed to show sub-
stantially what that amount is. The carbohydrates, according to
this theory, yield practically their entire store of energy to the
organism, while if the equations mentioned are interpreted liter-
ally the sugar produced from one gram of proteids would, accord-
ing to Chauveau's equation, contain but about 1.^3 Cals. of poten-
tial energy in place of the 4.1 Cals. available from the proteids
according to Rubner. If the proteids are assumed to be split
up in accordance with Gautier's equation the resulting dextrose
would contain about 80 per cent, of their potential energy, and
this figure is used in computing their isoglyeosic value. Similarly,
the sugar derived from one gram of fat would contain about 6.07
Cals. otit of the 9.5 Cals. contained in the original fat. In other
words, while Chauveau does not question that the actual food of
the living cells is of value in proportion as it supplies energy, he
holds that in the complex organism of the higher animals a con-
siderable share of the original potential energy of fats and proteids
is Lost during their conversion into material (carbohydrates) which
the cells can use.

The conception of the mutual replacement of the nutrients on
the basis of the amounts of energy they are capable of liberating
for the use of the organism has proved a fruitful one and been the
basis of much subsequent research. A full discussion of it and

* La Vie et l'Energie chez 1' Animale.

154 PRINCIPLES OF ANIMAL NUTRITION.

of the modifications which later investigation has made necessary
in Rubner's original conclusions, is possible only in connection
with a general study of the energy relations of the food, the animal,
and the environment such as forms the subject of Part II. For
the present we may content ourselves with accepting the general
idea that the relative values of the nutrients depend in very large
measure upon their ability to furnish energy for the vital activi-
ties, deferring until later the consideration of quantitative rela-
tions.

The Non-nitrogenous Ingredients of Feeding-stuffs. —
The discussions of the foregoing paragraphs have had reference to
the effects produced by pure or approximately pure nutrients upon
the metabolism of carnivora. By reason of the simplicity of con-
ditions which is possible in such experiments they are indispensa-
ble in a study of the fundamental laws of nutrition. We must
presume also that the general principles established by such
experiments are applicable to all warm-blooded animals, since
we know of no radical differences in their vital processes.

In making such an application to the nutrition of our domestic
herbivorous animals, however, much caution is necessary to avoid
unwarranted assumptions and conclusions. Two points need espe-
cially to be borne in mind :

First, the food of these animals is, from a chemical point of view,
very heterogeneous. In addition to true proteids, there are present,
especially in coarse fodders, various non-proteid nitrogenous sub-
stances, while the non-nitrogenous nutrients, besides hexose carbo-
hydrates and true fats, include, on the one hand, pentosans and
pentoses, lignin, and all the variety of unknown substances com-
prised under the conventional terms "nitrogen-free extract" and
" crude fiber, " and on the other the waxes, resins, coloring matters,
etc., contained in the "crude fat."

Second, the process of digestion in herbivora, and especially in
the ruminante, as was pointed out in Chapter I. differs materially
from that in carnivora as regards the part played by fermentative
processes, particularly in the solution of the carbohydrates and
related bodies which are so abundant in vegetable materials.

It has been more or less customary to regard the digested por-
tions of the crude fiber and nitrogen-free extract of feeding-stuffs

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 155

as consisting essentially of carbohydrates. The basis for this
assumption is the demonstration by Henneberg that the ultimate
composition of that portion of these two groups of substances
which is not recovered in the feces is substantially that of starch
or cellulose, while Kellner * has more recently demonstrated their
equality in energy value. This fact of itself, however, does not
justify the inference of equal nutritive value, as may be readily
seen in the case of starch. It is obviously not a matter of indiffer-
ence whet her a given amount of this substance is resorbed from the
digestive canal of a steer in the form of sugar or whether, as in some
of Ki'ihn's experiments. 60 percent, of it is converted into methane,
carl »on dioxide, and organic acids, yet the elementary composition
of the "digested" portion would be the same in either case.

The fact is that while the resorbed food of herbivora contains
proteids, carbohydrates, and fats, whose functions in nutrition must
be assumed to be the same as in the carnivora, it is very far from
consisting entirely of them, but contains also a variety of other
substances of whose exact nature and proportions we are compara-
tively ignorant. We know, of course, that the digested non-nitrog-
enous ingredients of feeding-stuffs, taken as a whole, do serve as
sources of energy. When an ox or a sheep is fed exclusively on
ordinary coarse fodders such as hay, straw, or corn stover, the small
supply of proteids that he receives is likely to be little if any in ex-
cess of the minimum demand, and the requirements of the body for
energy must be satisfied very largely by the non-nitrogenous mate-
rials. Moreover, the supply of such substances as starch, sugars,
and true fats in such a case is so small relatively that it appears
difficult to suppose that these alone are sufficient for the needs of
the organism, and one is forced to the conclusion that the ill-known
ingredients of the "crude fiber" and "nitrogen-free extract" are
also utilized.

The separation and identification of these various substances
and the study of their physiological effects presents a problem at
once attractive and laborious and one whose complete solution we
cannot, hope soon to reach. Some few data as to certain classes,
however, are available.

* Compare Part II, Chapter X.

156 PRINCIPLES OF ANIMAL NUTRITION.

Pentose Carbohydrates. — It has already been shown in Chapter
II (p. 24) that such of the pentose carbohydrates as have been
experimented upon are at least partially oxidized in the body, and
that this appears to be especially the case with herbivora, the urine
of these animals seldom containing pentoses.

It is of course conceivable that a substance may be oxidized
in the body without producing any useful effect except in so
far as the resulting heat may be of value to the organism, but it
seems more consonant with our general conceptions of the nature
of metabolism to suppose that the potential energy of any substance
which is capable of entering into the metabolism of the cells may be
utilized as a source of energy for their functions. In the case of the
pentoses, moreover, we have the additional fact, seemingly well
established, that pentoses may give rise, directly or indirectly, to a
production of glycogen. (Compare p. 26.) If we suppose the
latter body to be formed directly from the pentoses, then their nutri-
tive value is established, since that of glycogen is unquestionable.
If, on the other hand, we suppose that the pentoses enable glycogen
to be produced by protecting other materials from oxidation, then
their nutritive value is likewise established, since they serve as a
source of energy to the organism.

Recent respiration experiments by Cremer * seem to fully con-
firm this conclusion. In addition to an only partially successful
trial with a dog, four experiments were made in which the urinary
nitrogen and the respiratory carbon of rabbits were determined on
a diet of varying quantities of rhamnose as compared with a preced-
ing and succeeding day of fasting. No examination of the feces was
made, except to determine the amount of rhamnose contained in
them. Small amounts of this substance were also found in the
urine. Neglecting the carbon and nitrogen of the feces and esti-
mating the urinary carbon from the nitrogen by the use of Rubner's
factor,t 0.7462, the following results have been computed, the two
or three fasting days in each experiment being averaged. The
amount of rhamnose stated is exclusive of that found in feces and
urine.

* Zeit f. Biol., 42, 451.
t Ibid., 19, 318.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 157

Lost from Body.

Nitrogen.
Grms.

Carbon.
Grms.

-p. • , T f Fasting

2.401
2.050
1.612
1.629
1.061
0.855
2.028
2.133

13 831

Experiment I : < , t co P ,

■u. • . TT ( Fasting

12.020
16 542

Experiment II: ^,7 m8 ,

c ( 17 .09 grms. rhamnose

■& • j. ttt S Fasting . . .

10.835
10 742

Experiment III : < -. 0 n,, & ,

r ( 18 . 96 grms. rhamnose

5 338

tj. • , tv 1 Fasting

13 383

Experiment IV : „, „,.6 ,

( 24 . 30 grms. rhamnose

4.482

The conditions in the first experiment were not regarded as
satisfactory. In the other three the loss of fat from the body was
notably diminished by the administration of rhamnose, precisely as
in the experiments of Fettenkofer A: Voit and of Rubner (pp. 147
and 148) with the hexose carbohydrates. The quantitative results
vary considerably in the individual experiments, but in the second
and fourth correspond quite closely to the law of isodynamic
replacement.

Kellner * computes from the results of respiration experiments
in which extracted rye straw was added to a basal ration that the
furfuroids (presumably pentosans) of this material must have con-
tributed to the production of fat to as great an extent as starch or
cellulose. (Compare p. 183.) A fortiori, therefore, they must be
capable of protecting the body fat from oxidation.

Organic Acids. — Mention was made in Chapter II of the fact
that the organic acids, which are found to some extent in the food
and which are produced in large amounts by the fermentation of
the carbohydrates in the digestive apparatus of herbivora, are oxi-
dized in the body. From this latter fact we should anticipate that
they might serve as sources of energy to the organism, and this
anticipation apparently has been confirmed by several investi-
gators.

Zuntz & v. Mering f determined the amount of oxygen con-
sumed by fasting rabbits before, during, and after the injection

* Landw. Vers. Stat., 53, 457.
t Arch, ges Physiol., 32, 173.

i58

PRINCIPLES OF ANIMAL NUTRITION.

into the circulation of sodium lactate. The results per kilogram
and quarter hour were as follows :

Before Injection.

After Injection.

Quarter hours.

Injec-
tion.

Quarter hours.

Fourth.

Third.

Second.

First.

First.

Second.

Third, j Fourth.

c.c.

c.c.

c.c.

c.c.

c.c.

c.c.

c.c. 1 c.c.

c.c.

Apr. 19...

184.7

184.5

190.1

183.3

203.4

185.4

Ii99.0j 188.6

182.2

" 20...

155.3

142.2

164.1

155.6

168.3

156.6

158.5 164.4

160.0

" 22a

142.1

132.6

143.5

138.5

147.2

153.6 153. 31 155.4

157.4

" 226

155.4

157.4

157.1

164.8

155.3 160.7

147.1

154.1

" 28...

159.2

150.7

158.5

155.0

178.1

163.2 158.8

172.9

153.9

May 2...

176.6

185.0

158.2

173.6

171.8

161. 81 173.4

163.3

180.2

" 4...

156.1

167.6

159.9

152.4

166.2

156.0 164.2

159.0

160.1

Totals. ...

1129.6

1120.0

974.31115.5

1199.8

1131. 91167 . 9 1150.7

1147.9

Averages .

161.4 160.0 162.4

159.4

171.4

161.7 166.8 164.4 164.0

160.8

164.2

It being well established that lactic acid is readily oxidized in
the body (compare p. 27), it is evident that in these experiments it
must have protected the body fat from being metabolized, since
otherwise the consumption of oxygen would have increased. .Simi-
lar, although not decisive, results were obtained with sodium buty-
rate. On the other hand, sodium lactate administered by the
mouth caused more or less increase in the oxygen consumption.
Wolfers * has reported confirmatory results with sodium lactate.
Munk f injected sodium butyrate into the veins of fasting rabbits
curarized to eliminate the effects of muscular activity and secure
uniform metabolism, and determined the respiratory exchange by
the Zuntz method (p. 72). The oxidation of sodium butyrate
according to the equation.

C4H7Na02 + 502 = 3C02 + 3H20 + NaHC03

corresponds to a respiratory quotient of 0.6, which is less than that

* Arch. ges. Physiol., 32, 222.
f Ibid., 46, 322.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 159

of the fasting animal. The material lowering of the quotient which
was observed was therefore interpreted as showing that the sodium
1 nil y rate was oxidized, and this conclusion was confirmed by the
strongly alkaline character of the urine and the absence from it
of butyric acid. The amount of sodium butyrate injected during
\\ to 1^ hours was sufficient in the several experiments to supply
from 60 to 100 per cent, of the respiratory demand of the fasting
animal. If this had been oxidized uselessly — that is, if the energy
liberated had not been of use to the organism — then the consump-
tion of oxygen and elimination of carbon dioxide should have in-
creased correspondingly. This, however, was far from being the
cast', as the following fifteen-minute averages for the periods
before, during, and after the injection show:

Acid I Oxygen gP*PP
Injected, Consumed, Excreted|
Grms. c.c.

Respira-
tory
Quotient.

Animal I, weight 1.92 kgs.:

Before injection

During "

After " ,

A nimal II, weight 1 .9 kgs. :

Before injection

During "

After "

Animal III, weight 1.82 kgs

Before injection

During "

After "

Animal IV, weight 1.47 kgs,

Before injection

During "

After "

0.133

0.199

0.206

0.186

260.9
280.0
253.3

290.9
325.2
299.4

305.3
330.9
306.6

278.9
297.6
278.1

196.1
190.5
181.2

228.3
214.6
230.9

243.4
238.0
235". 3

201.0
197.9
205.2

0.75
0.68
0.71

0.78
0.66
0.78

0.79
0.72
0.77

0.72
0.68
0.73

In place of an increase of 60 to 100 per cent, in the respiratory
exchange under the influence of the sodium butyrate, there was an
increase of only 7 to 8 per cent, in the oxygen and none at all in the
carbon dioxide. It is evident, therefore, that the loss of fat from
the body must have been largely diminished, the butyric acid serv-
ing as a source of energy in its place. A stimulating effect upon

i6o

PRINCIPLES OF ANIMAL NUTRITION.

the heart's action was noticed, and Bokai is quoted as having shown
a similar action on the peristaltic movements of the intestines, and
these facts perhaps account for some of the increase of the oxygen,
but Munk shows another reason for most of it. To produce 1 Cal.
of energy by the oxidation of sodium butyrate he computes to re-
quire 0.324 gram of oxygen, while to produce 1 Cal. by the oxida-
tion of fat requires, according to Zuntz & Hagemann (Chapter VIII),
0.302 gram or 6.2 per cent, more in the first case. It would thus
appear that the replacement of fat by sodium butyrate was sub-
stantially isodynamic.

Mallevre * experimented with sodium acetate, whose respira-
tory quotient is 0.5, by the same method as Munk, the amount
injected equaling 85-100 per cent, of the respiratory demand. The
results per quarter hour were:

Weight and Condition.

Sodium
Acetate

Oxygen
Con-

Carbon
Dioxide

Respi-

per Kg.
Weight.

sumed.
c.c.

Excreted.
c.c.

ratory
Quotient*

Grms.

f

I.

Before injection. . . .

176.1

1S3.6

1.04

Weight, 1.44 kgs. !
Just after eating.. |

During " ....

6^201

193.8

167.1

0.86

Residual effect

197.7

152.6

0.76

I

After injection

178.0

171.2

0.96

Weight, 1.5 kgs.. i

II.

Before injection. . . .

195.3

149.6

0.77

After two days' -:

During " ....

6!23i

231.8

168.2

0.71

After

III.

211.2

163.3

0.77

Weight, 1.82 kgs i

Before injection . . .

214.7

165.9

0.77

After two days' -]

During " ....

6'.127(?)

244.8

169.5

0.69

fasting (

After

IV.

217.1

166.6

0.77

Weight, 1.7 kgs. . J
After one day's I

Before injection . . .
During " ....
Residual effect

6!i52

183.4
208.1

209.5

160.5
167.0
164.7

0.87
0.80
0.79

After injection

194.7

164.7

0.85

The decrease in the respiratory quotient, as well as the results
of the examination of the urine, showed that the sodium acetate

* Arch. ges. Physiol., 49, 46Q.

THE RELATIONS OF METABOLISM TO FOOD-SUPFLY. 161

was oxidized in the body. The increase in the amount of oxygen
consumed is much more marked than in Munk's experiments, rang-
ing from 10.4 to 14 per cent. Moreover, as Mallevre points out, in
the oxidation of sodium acetate about the same volume of oxygen
is required to produce a unit of heat as in the case of fat. Appar-
ently, then, while the sodium acetate, like the sodium butyrate in
Munk's experiments, must have largely diminished the metabolism
of the body fat, it also stimulated the total metabolism and was
substituted for the fat in less than the isodynamic ratio. As in
Munk's experiments, a stimulation of the heart action and also an
increased peristalsis of the intestines was observed.

It would seem, then, that lactic and butyric acids, when
introduced into the circulation of the fasting animal, protect
the body fat from oxidation, and replace other nutrients in
isodynamic proportions. Acetic acid, on the contrary, seems in-
ferior to the other two in this respect, and it is of interest to recall
that according to Weiske & Flechsig (p. 123) it apparently has
also less effect in diminishing the proteid metabolism.

Crude Fiber. — As was stated on p. 117, the early experiments
by v. Knieriem * upon the nutritive value of cellulose comprised
respiration experiments as well as determinations of the proteid
metabolism. Combining the results for nitrogen already given with
those for carbon, we have the following:

Number

of

Days.

Food per Day.
(Two Animals.)

Gain or Loss of

Period.

Nitrogen,
Grms.

Carbon,
Grms.

I

9
10
5
4
3

Milk and horn dust

-0.599
+ 0.104
-0.330
-0.318
-0.023

-4.521

II
III

Same + 18.63 grms. crude fiber * . . .
Milk and horn dust

-0.434
-4.868

IV

Vf....

Same + 11 grms. cane-sugar

" + 33 " "

- 1 . 673
+ 5.653

* Water-free.

f Results regarded by the author as of doubtful value.

In addition to its effect in diminishing the proteid metabolism,
the crude fiber in these experiments seems to have been fully as
efficient as the cane-sugar as a substitute for body fat.
* Zeit. f. Biol., 21, 67.

1 62 PRINCIPLES OF ANIMAL NUTRITION.

As we have seen, there has been considerable study of the effects
of crude fiber on the proteid metabolism, but no other comparative
experiments appear to have been made regarding the replacement
values of cellulose and other carbohydrates in a maintenance ration.

The somewhat lower value which seems to be indicated for the
organic acids by the experiments cited in the previous paragraph
has been made the basis of conclusions as to the inferior nutritive
value of cellulose, and Zuntz,* in some comments on Mallevre's
experiments, remarks that the apparent equality between cellulose
and starch observed in experiments on ruminants is to be explained
by the fact that in these animals the starch also undergoes
fermentation, a fact winch the researches of G. Kuhn at Mockern
have since established. In other words, he would say that in case
of ruminants the starch has as low a value as the cellulose rather
than that the cellulose has as high a value as the starch.

Kellner has recently obtained results, to be discussed a little
later, which seem to prove a participation by the digested cellulose
in actual fat production to as great an extent as by starch, and
which therefore seem to put the nutritive value of the form of cellu-
lose used by him beyond dispute.

Utilization of Excess of Non-nitrogenous Nutrients. — No
elaborate scientific investigation is needed to teach us that food
supplied in exceess of the immediate demands of the organism re-
sults in a greater or less storage of material in the body, this material
consisting, in the mature animal, largely of fat. But while the fact
of fat formation is obvious, the exact source of the fat lias been the
subject of as much controversy as almost any physiological question.
As we have seen in the previous section, opinions arc still far from
being unanimous as to the production of fat from proteids, while
until quite recently the same might have been said regarding
the carbohydrates as a source of fat. A very complete critical re-
view of the literature of the subject of fat formation in the animal
body was published by Soskin f in 1894, and to this the writer is in-
debted for a considerable number of the statements and references
on the succeeding pages.

As was stated on p. 29, the older physiologists looked upon the

* Arch ges. Physiol., 49, 447.
f Jour, i Landw., 42, 157.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 163

fat of the food as the sole source of $ie body fat. The contrary
view was first propounded by Liebig * in 1S43. After drawing the
distinction between "plastic materials" (proteids), which serve to
build up the tissue, and "respiratory materials" (non-nitrogenous
substances), which serve as sources of heat, he asserts that any
excess of the latter over the immediate needs of the organism is con-
verted into fat. This proposition, which was based upon observa-
tion and general knowledge rather than upon specific experiments,
led to an active controversy with the adherents of the older view
and to much direct experimental work.

Liebig, while not denying that the food fat was a source of body
fat, maintained that the amount contributed by it was insignificant
and regarded the carbohydrates as the chief source of animal fat.
The controversy turned upon the question of the possibility of
accounting for the body fat by the food fat, both parties tacitly
agreeing that any excess was to be credited to the carbohydrates.
The principal champions of the older view were Boussingault, Dumas,
and Payen.| Boussingault, in particular, brought forward the
results of experiments on milch cows, according to which the fat
of the food fully sufficed to account for that in the milk. They
all. however, ultimately came to acknowledge the substantial accu-
racy of Liebig's view. Thus Dumas & Milne-Edwards % confirmed
the results of Huber & Gundlach,§ cited by Liebig, according to
which bees can produce wax from honey or sugar. Boussingault ||
published the results of new experiments on milch cows as sus-
taining his previous view of the question, but later 1[ convinced
himself by careful and laborious experiments on the fattening of
swine and geese of its untenability and of the correctness of Liebig's
position. Thus in one of his experiments nine pigs gained 103.2 kgs.
of fat in ninety-eight days, while the food contained but 67.6 kgs.,
of which about 8 kgs. was excreted undigested in the feces.
Persoz ** likewise, in experiments with geese, obtained similar

* Aim. Chem. Pharm., 45, 112; 48, 126; 54, 376.

t Annal. de (-'him. et de Physique., 3d ser. 8, 63.

% Ibid., 14, 401).

§ Naturgeschichte der Bicnon, Kassel, 1S42.

|| Annal. de Chim. et de Physique., 3d ser., 12, 153

"IT hoc. cit., 14, 419.

** Annal. de Chim. et de Physique., 14, 408.

164 PRINCIPLES OF ANIMAL NUTRITION.

results and also observed a production of fat by these animals when
fed on food from which all fat had been removed.

Fat. — That the fat of the food may serve directly as a source of
body fat has been shown by Hofmann,* who fasted a dog for thirty
days, thus rendering the body almost fat-free, and then fed for five
days large amounts of fat bacon containing as little lean meat as
possible, and from which there were digested daily 370.8 grams of
fat and 49.4 grams of protein. At the end of the five days the
body of the animal contained 1352.7 grams of fat. Estimating its
fat content at the close of the fasting period at 150 grams, there was
produced daily about 240 grams of body fat. According to the
highest recorded estimates not over 26 grams of this could possibly
have been formed from the protein of the food. Hofmann also
show's from the result of one of Pettenkofer & Voit's respiration
experiments, in which meat and fat were fed, that part of the ob-
served gain of fat must have had its source in the fat of the
food.

The latter investigators also showed in the last of the experi-
ments cited on p. 144 that a large ration of fat alone may result in a
considerable storage of fat. Most of the experiments by the same
investigators in which lean meat and fat were fed show not merely a
diminution of the loss of body fat but an actual increase in its
amount. (Compare the averages on page 150.) The fact is most
strikingly shown, however, in a series in which increasing amounts
of fat were added to a uniform ration of meat which was itself
sufficient to maintain both nitrogen and carbon equilibrium. The
results as given by Pettenkofer & Voit t are contained in the table
at the top of p. 165, those on the basal ration of meat being the
same as those given also on p. 109 for the first series.

It is of course possible to interpret these results as showing that
the fat of the food was oxidized and protected an equivalent amount
of ihe non-nitrogenous residue of the proteids from oxidation and
that the latter were the real source of the fat gained. No necessity
for such an interpretation is apparent, however, and the direct
explanation appears the simpler and more natural.

The results of experiments upon the deposition of foreign fats

* Zeit. f. Biol., 8, 153.
t Ibid., 9, 30.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY.

165

Number

Food.

Nitrogen

of
Excreta.

Total Carbon

of

Excreta.

Gain or

Toss of

Trials.

Moat.

Fat.

Flesh.
'\ -0.034.)

Fat.

3

1500

0

51.0

1S4.5

0

+ 4.3

2

1500

30

49.6

180.6

-42.8

+ 32.4

1

1500

60

51.0

203.6

- 0.6

+ 39.4

2

1 51 II 1

100

47.7

182.4

+ 97.8

+ 91.1

1

1500

100

49.3

174.4

+49.4

+ 109.5

2

1500

150

49.5

193.1

+ 44.8

+ 135.7

in the body which were considered in Chapter II, p. 30, also testify
to the direct formation of body fat from food fat.

Carbohydrates. — Among the experiments of Pettenkofer &
Voit which have been cited in the foregoing pages are several which
show a production of fat upon a ration of lean meat with the addi-
tion of starch or dextrose or of starch alone. A more complete
summary of these experiments * is given below:

Number

of
Experi-
ments.

Food per Day. Gain or Loss of

Meat

Grms.

Fat.

Grms.

»£££«. PG0rmsS'
Grms. Grms-

Fat.
Grms.

Starch . . . -j

Proteids and dextrose

f

1

1
3
3

1
8
1
2
1

' 500

400

500

800

1500

1800

16.9

21.2
20.2

5.6

5.3
13.7

4.5
10.1

450
597
700
200
400
200
450
200
450

-45.0 + 56.2
-58.8+ 3.4
-38.8 +106.4

- 8.1 + 15.0

- 3.1+109.9
-11.3 + 19.5
+40.6i+ 71.5
+ 6.3 + 18.1
+ 70.6 +126.5

Pettenkofer & Voifs Conclusions. — In discussing these results
Pettenkofer & Voit assumed that, as computed by Ilenneberg,f 100
grams of proteids can give rise to a maximum of 51.4 part- of
fat. On this basis they found that, with two apparent exceptions,
the fat of the food, together with that which could be derived from

* Zeit f. Biol., 9, 435.

t Landw. Vers. Stat , 10, 455, foot-note.

1 66 PRINCIPLES OF ANIMAL NUTRITION.

the amount of proteids metabolized, was sufficient to account for
the gain of fat. The}* therefore concluded that the carbohydrates
simply protected these materials from oxidation and regarded the
formation of fat from the former as improbable, being confirmed
in this belief by the observation that the amount of fat produced
was proportional to the proteids rather than to the carbohydrates
of the food. The apparent exceptions they regarded as due to a
retention of undigested starch in the alimentary canal. In brief,
Pettenkofer & Voit, while not denying that carbohydrates aid in
the production of fat, regarded their action as an indirect one.

It should be added that, contrary to the general impression, Voit
did not absolutely deny the formation of fat from carbohydrates,
but regarded it as improbable and unproved. Moreover, he came
later to admit the truth of the opposite view, and even furnished
from his laboratory experimental evidence in its support.

At an earlier date Voit * had likewise made experiments on a
milch cow, the result of which was that not only all the fat of the
milk, but most of the milk-sugar as well, could be accounted for by
the proteids and fat of the food. Voit also examined the numerous
experiments of Dumas, Persoz, Boussingault, and others (p. 163)
upon the origin of animal fat and satisfied himself that, although
they undoubtedly showed, as their authors claimed, a formation
of fat from other ingredients of the food, the amount produced
could at least in the great majority of cases be accounted for by
the proteids of the latter.

It is important to observe that the evidence supporting Voit's
view was negative evidence. The results could be explained on the
hypothesis that the carbohydrates did not contribute to fat pro-
duction, but while a large number of such results might render the
hypothesis very probable, they could not demonstrate its truth. On
the other hand, even a single well-authenticated case in which the
fat and proteids of the food did not suffice to account for the amount
of fat formed in the body would suffice to establish the possibility
of its formation from other materials. A few apparent cases of this
sort among earlier experiments Voit was able to explain plausibly,
but there was one important exception, viz., the experiments of

* Zeit. f. Biol., 5, 79-169.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 167

Lawes £ Gilbert * at Rothamstcd, in 1850, on the fattening of
swine.

Lowes & Gilbert's Investigations. — These experiments consti-
tuted part of a series of feeding trials with fattening sheep and pigs,
undertaken to test the then current view of Boussingault, according
to which the feeding value of stock foods was proportional to their
content of nitrogen. From the results of their extensive experi-
ments, Lawes <t Gilbert concluded that in fattening animals both
the amount of food consumed by a given weight of animal within a
given time and the increase in weight obtained are measured rather
by the supply of non-nitrogenous than of nitrogenous constituents
in the food. This fact of itself strongly suggests a production of
fat from carbohydrates.

In connection with these feeding trials investigations were also
made into the composition of the increase in live wreight during
fattening.! By a comparison of the weight and composition of
one of the fattened pigs with those of an animal supposed to be
precisely similar at the beginning of the fattening the percentage
composition of the increase was found to be approximately :

Water 28.61

Ash 0.53

Proteids 7.76

Fat 63.10

100.00

During the ten weeks of the fattening the animal gained 88
pounds, containing according to the above figures 55.5 pounds of
fat, while the total food consumed contained but 13.7 pounds. In
other words, over three fourths of the fat was formed from other
ingredients of the food.

After the publication of Voit's first paper, Lawes & Gilbert t
presented the results of this and eight other experiments in their

♦Report British Association Adv. Sci., 1852; Jour. Roy. Agr. Soc, 14,
459; Rep. British Asso. Adv. Sci., 1854; Rothamstcd Memoirs, Vol. II.

t Jour. Roy. Agr. Society, 21, 465; Phil. Trans., Part II, 1859, p. 493.

% Rep. British Asso. Adv. Sci., 1866; Phil. Mag., Dec, 1866; Rotham-
sted Memoirs, Vol. IV.

1 68

PRINCIPLES OF ANIMAL NUTRITION.

bearing on the origin of the fat. Nos. 2 to 5 were selections from
the first two series of the experiments of 1850 (designated as I and
II in the table below) and Nos. 6 to 9 were experiments upon the
equivalency of starch and sugar in food reported in 1854 * (desig-
nated below by S). The following table shows the original numbers
of the several experiments and the character of the food consumed :

No

Original Designation.

Food.

Series.

Number.

1
2
3
4
5
6
7
8
9

i

i

i

ii

s
s
s
s

12
1
5
5
1
2
3
4

Bean meal, lentil meal, bran, and barley meal ad lib.

Bean meal, lentil meal, bran, and corn meal ad lib.

Bean meal and lentil meal ad lib.

Corn meal ad lib.

Barley meal ad lib.

Lentil meal and bran, with sugar ad lib.

Lentil meal and bran, with starch ad lib.

Lentil meal and bran, with sugar and starch.

Lentil meal, bran, sugar, and starch ad lib.

From the results of the first experiment, the amount of fat con-
tained in the observed increase in live weight in each case was com-
puted, the animals being assumed to have had at the beginning of
the fattening the composition of the lean pig analyzed and at its
close that of the fat pig. These amounts were then compared with
the amounts which could have been produced from the fat and pro-
teids of the food. In order to make the case as unfavorable as
possible for the carbohydrates the authors assumed:

First, that all the fat of the food was digested and laid up in the
body.

Second, that all the nitrogenous matter of the food was digested,
and that it all consisted of true proteids.

Third, that, after deducting the amount of proteids gained by
the body, the total carbon of the remainder, minus that required to
form urea, was available for fat formation.

The results of the comparison were as follows, calculated per
100 pounds gain in live weight-

*Rep. Brit. Asso. Adv. Sci., 1854.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 169

as co

OS CO

©CN
<N 00

O <N

c o
-tfl 06

4- 00

"* co'

i-i OS

ON O iO

CO HN

r-j ,-t rH

<-H ^ O

O CO

o t-

00 co

-< o

OS <N

o

.O

-

2 «-

45 a

O 3

« g

*

—

O

o

"o
o

<r a

3

>

Oh

O

170 PRINCIPLES OF ANIMAL NUTRITION.

Even on the most extreme assumptions it is only possible to
regard the fat produced as derived wholly from the proteids of the
food in three cases in which an excessive proportion of the latter was
fed. If the probable digestibility of the foods used be considered,
and Henneberg's factor (51-4 per cent.) for the possible production
of fat from proteids be used, the results show even more decidedly
a formation of fat from carbohydrates. In a later paper,* in reply
to criticisms, the authors state that they have reviewed and recal-
culated many of their experiments with the result that, while the
experiments with ruminants (sheep and oxen) failed to furnish con-
clusive evidence of the formation of fat from carbohydrates, a
large number of those with pigs unquestionably showed such for-
mation.

In view of their historical interest it has seemed desirable to
give the results of Lawes & Gilbert's experiments in some detail,
although at the time they hardly secured the recognition which
was due them and Voit's views became the generally accepted
theory for the next twenty -five years. Notwithstanding the latter
fact, however, results of experiments on herbivorous animals speed-
ily began to accumulate which were difficult to reconcile with Voit's
Itypothesis.

Experiments on Ruminants. — Experiments on milch cows were
made by Voit himself, as already noted. G. Kiihn & Fleischer f
a little later discussed the results of two of their extensive feeding
experiments on milch cows in their bearing on this point, and M.
Fleischer \ did the same with the results of similar experiments
made by Wolff and himself. § Their results are tabulated on the
opposite page.

Neither Voit's nor Fleischer's results are such as to require the
assumption of a formation of fat .from carbohydrates. Those, of
Kiihn & Fleischer show a small excess of fat in the milk over that
producible from the fat and proteids of the food, but the authors

* Jour. Anat. and Physiol., 9, 577; Rothamsted Memoirs, Vol. IV.

f Landw. Vers. Stat., 10, 418; 12, 451.

% Virehow's Archiv, 51, 30.

§ Jour. f. Landw., 19, 371, and 20, 395.

THE RELATIONS OF METABOLISM TO FOOD SUPPLY. 17*

Fat of
Fodfler,

Grms.

y . . . I Experiment a

Tr.. , o n. . v \ Experiment I . ..
Kuhn & Heischer: ■>. l tt

Fleischer: | Experiment I.. ... .

318.8
276.0
183.5
183.5
170.5
166.5

Fat from
Protein,

Grms.

401.8
30S . 5
79.5
69.5
158.5
170.0

Total,
Grms.

720.6
5S4 . 5
263.0
253.0
329.0
336.5

Fat of

the Milk,

Grms.

577.5
337.3
277.5
292.0
303.5
290.5

regard the differences as within the limits of error in such experi-
ments.

Studies of the results of fattening experiments with ruminants
give similar results. On the basis of Lawes & Gilbert's determi-
nations of the composition of the increase of live weight in fattening,
the amount of fat produced in such an experiment may be approxi-
mately computed and compared with the amounts of proteids and
fat in the food. Such a comparison by the writer * in seventy-seven
experiments on sheep showed that, with one or two possible excep-
tions, the fat and proteids of the food were sufficient to account for
the amount of fat formed, although in some of the experiments
little margin was left.

Experiments on Sicine. — Experiments with swine, on the other
hand, as Wolff f has shown, have almost without exception given
results which can scarcely be explained except upon the hypothesis
of a formation of fat from carbohydrates. These animals, as Lawes
& Gilbert pointed out in their early papers, are especially adapted
to experiments of this sort, since they consume a relatively large
amount of easily digestible food, have a small proportion of offal to
carcass, and are by nature inclined to lay on fat readily. It was
therefore to be expected that experiments upon swine would show
a production of fat from carbohydrates, if such took place, more
decisively than those upon ruminants.

Experiments on pigs by Weiske & Wildt,J it is true, on the
same plan as those by Lawes & Gilbert, yielded results consistent
with Yoit's theory, showing a formation of 5565 grams of fat in the

* Manual of Cattle Feeding, p. 177.

f Emit lining Lanchv. Nutzth., pp. 354-356

X Zeitschrift f. Biol., 10, 1.

I72 PRINCIPLES OF ANIMAL NUTRITION.

body as compared with a possible 6724 grams from the fat and
proteids of the food. The feeds used, however, were not well suited
to young animals and the gain was abnormally small in proportion
to the food consumed, so that the results could not be expected to
be decisive. Moreover, the presence of non-proteid nitrogen in the
food is not considered in the computation. (See the next paragraph.)

Sources of Uncertainty. — Up to this point the results of experi-
ments on herbivorous and omnivorous animals had been somewhat
conflicting. Before taking up the later investigations it is desir-
able to point out some of the uncertainties attaching to experiments
such as those above enumerated. These relate, first, to the amount
of fat actually produced, and second, to the possible sources of
supply in the food.

The basis for estimating the amount of fat actually produced by
a fattening animal was in two cases a comparison with the amount in
a supposedly similar animal at the beginning of the fattening, the
fattened animal being actually analyzed. In the remainder the
increase in live weight was assumed to have the composition found
by Lawes & Gilbert. It need scarcely be pointed out that the
results of such comparisons can be only approximate and are sub-
ject to a considerable range of error. Only the most decided
results one way or the other can be accepted as at all conclusive.
In experiments on milch cows the production of milk fat can of
course be determined, but the variations in the weight of such an
animal often render any conclusions as to gain or loss of body fat
so difficult that the results as a whole are less satisfactory than
those on fattening.

The possible sources of fat in the food, aside from the carbohy-
drates, are the ether extract and the proteids. As regards the first,
it is certain that not all the digestible ether extract of stock foods
is true fat. With the proteids the case is still worse. In particu-
lar we now know that a portion, and in some cases a considerable
portion, of the total nitrogenous matter of feeding-stuffs consists of
non-proteid material, which so far as we know contributes little if
anything directly to fat production. This is a very important source
of error. Thus the writer * has shown, as has also 8oxhlet,t that if

* Manual of Cattle Feeding, p. 182.

t Compare Soskin, Jour. f. Landw., 42, 203.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 173

account be taken of this fact the teachings of Weiske & Wildt's
experiment cited above are exactly reversed and show a formation
of fat from carbohydrates. A consideration of the same fact, of
course, tends to make the results of all similar experiments, includ-
ing those on milch cows, more favorable to the carbohydrates.

Still further, it is doubtful whether 100 parts of proteids can
actually yield 51.4 parts of fat. The latter number was computed
by Henneberg from the elementary composition of proteids and of
urea to be the maximum amount obtainable. Zuntz,* however,
has called attention to the fact that if the proteids actually split up
in the manner which Henneberg's calculation supposes, the products
must contain all the potential energy of the original material, so
that none can be given off during their cleavage. This is a process
wholly without analogy in the animal body, and, to say the least,
very improbable. It would seem then, that even if we still hold to
a formation of fat from proteids, we must considerably reduce our
estimate of its amount.

Later Fattening Experiments. — All these considerations tend to
strengthen the belief that fat is formed from carbohydrates, and
more recent experiments have demonstrated that such is the fact.
Henneberg, Kern, & Wattenberg,f in experiments undertaken to
determine the rate of gain and the composition of the increase of
fattening sheep, and conducted substantially like those of Lawes
& Gilbert on swine, were the first to furnish proof of the formation
of fat from carbohydrates by ruminants. Wolff| having pointed
out that their results demonstrated that fact, Henneberg discussed
this feature of the experiments in a later publication. § Regarding all
the digested ether extract of the food as pure fat, and assuming that
all the digested nitrogenous matters were true proteids capable of
yielding 51 .4 per cent, of fat, he obtained the results given on p. 174.
Forty-two per cent, more fat was produced than could be accounted
for by the fat and proteids of the food, even on the extreme
assumptions made. Furthermore, not only did some of the
nitrogenous substances of the food undoubtedly consist of non-pro-

* Landw Jahrb., 8, 96.

t Jour. f. Landw., 26, 549.

% Landw. Jahrb., 8, I Supp , 269.

§ Zeit. f. Biol , 17, 345.

174

PRINCIPLES OF ANIMAL NUTRITION.

Digested

Proteids stored up

Proteids available for fat production
Equivalent fat (51 .4 per cent.)

Total from fat and proteids

Actually produced by animal

Proteids,
Grms.

Fat,
Grms.

10220
936

2100

9284

4772

6872
9730

teids, but a high figure was assumed for their digestibility, and in
computing the gain of fat by the animal no account was taken of
the fat of the wool and of the offal. Henneberg's final conclusion
is that no possible errors arising from differences in the animals
compared or from irregularities in the consumption of food can
explain away the above result.

Soxhlet * made similar experiments with swine fattened on rice,
that is, on a feeding-stuff poor in proteids and fat and rich in carbo-
hydrates, with the result that only 17 to 18 per cent, of the fat pro-
duced could be accounted for by the digestible protein and fat of
the food. In two experiments with the same species of animal by
Tschirwinsky f but 43 per cent, and 28 per cent, respectively of the
fat production could be thus accounted for. Of six experiments
on geese by B. Schulze, \ four, in which a comparatively wide nutri-
tive ratio was used, showed that at least from 5 to 20 per cent, of the
fat must have been produced from carbohydrates. Chaniewski §
likewise experimented on geese and obtatined much more decisive
results, from 72 to 87 per cent, of the observed fat production being
necessarily ascribed to the carbohydrates.

Recent experiments by Jordan || have shown that the dairy cow
may likewise produce fat from carbohydrates. In the first experi-
ment a cow weighing 867 pounds was fed for fifty-nine days with
food from which most of the fat had been extracted, the digestible

* Bied. Centr. Bl. Ag. Chem., 10, 674.

t Landw. Vers. Stat., 29, 317.

% Landw. Jahrb., 11, 57.

§ Zeit. f. Biol., 20, 179.

|| N. Y. State Experiment Station, Bulls. 132 and 197.

THE RELATIONS OF METABOLISM TO FOOD SUPPLY.

175

protein of the ration being varied from 1S4 grams to S 11 grams per
day. During this time she gained 8.'> pounds in weight, and her
whole appearance was such as to negative the assumption of any
considerable loss of body fat. In the second experiment one cow
was fed a ration poor in fat, one a normal ration, and one a ration
unusually rich in fat, the protein supply being again varied through
a considerable range. As in the previous case the gain in weight
and the general condition of the cows forbade the assumption that
body fat was drawn upon to any material extent. In all instances
except the last a considerable formation of fat from carbohydrates
was shown.

The following table gives the more important data of the above
experiments :

Experimenter.

Henneberg, Kern, &
Wattenberg

Soxhlet

Tschirwinsky

Schulze

Chaniewski

( 59 davs )

Jordan: -'74 " -

4 "

Total

Equiva-

Fat of

Total

Fat

Animal.

lent Fat,

Food ,

Actually

holism,

Grms.

Grms.

produced.

Grms.

Grms.

Grms.

Sheep

9,284

4,772

2,100

6,872

9,730

Swine -j

3,463

1,779

300

2,079

10,082

7,169

3,685

340

4,025

22,180

Swine \

5,934

3,050

656

3,706

8,577

2,361f

1,213

203

1,416

5,429

f

1,054

3831

222

605

387

1,049

381J

221

602

539

Geese <

785

286J

205

491

515

785

286t

205

491

612

555

194J

203

397

492

555

194$

203

397

471

(

110

55

20

75

269

Geese <

203

105

32

137

640

100

51

9

60

445

(

15,109

7,766

1,490

9,256

17,585

Cows 1

34,661

17,816

2,211

20,027

37,637

i

2,209

1,131

1,504

2,635

3,289

In view of the extreme assumptions made in these computations
as to the possible contribution by the proteids and fat of the food

* Digested protein of food less gain of protein by the animal,
f In original 2572 grms.

% Computed on a different basis from the other experiments
loc. cit , p 84.

Compare

176

PRINCIPLES OF ANIMAL NUTRITION.

to fat production, and of the very large differences between this
amount and the fat computed to have been actually formed, the
possible errors of the method are relatively insignificant, and these
investigations, together with the earlier ones, must be regarded as
establishing the fact of a formation of fat from carbohydrates.

The earliest experiment to be published in full demonstrating
the production of fat from carbohydrates in the bod}'' of the dog,
was by Munk.* The animal was deprived of food long enough to
render it certain that but traces of fat remained in the body. It
was then fed for twenty-four days on a diet consisting of small
amounts of meat, with some gelatine, and large quantities of
starch and sugar. In the body of the animal at the close of the
experiment 1070 grams of fat were found, of which Munk estimates
that at least 960 grams must have been produced during the experi-
ment, while the proteids fed could have produced as a maximum
only 415.3 grams and the meat itself contained but 75 grams of fat.
Even if a formation of fat from gelatine be admitted, a considerable
excess of fat remains unaccounted for except by the carbohydrates
of the food.

Respiration Experiments. — There are not wanting, however, for
final demonstration, experiments with the respiration apparatus, in
which the total income and outgo of nitrogen and carbon has been
determined.

Meissl, Strohmer, & Lorenz,f in very carefully conducted respi-
ration experiments upon swine, using a wide, a medium, and a
narrow nutritive ratio, obtained the following results:

Food,
Grms.

Proteid

Metabolism,

Grms.

Equivalent
Fat,
Grms.

Fat of
Food,
Grms.

Total

from Fat

and Proteids,

Grms.

Fat

Actually

Produced,

Grms.

65.4
64.1
88.0

381.6

33.6
33.0
45.2

196.1

7.9
16.4
15.2

48.6

41.5
49.4
60.4

214.7

353.9

a

413.2

Barlev

208.7

Flesh meal, rice, and
whey

256.3

Almost simultaneously C. Voit % gave a preliminary account of

* Virchow's Archiv, 101, 91.

t Zeit. f. Biol., 22, 63.

% Sitzungsber bayr. Acad d. Wiss. ; Math. Phys. Classe, 1885, p. 288.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 17 7

respiration experiments made in his laboratory by Lehmann &
E. Yoit with geese and by Rubner with a dog which demonstrated
a production of fat from carbohydrates. Rubner's experiment was
shortly afterward published in full.* It was a respiration experi-
ment covering four days immediately following a fortnight's heavy
feeding with meat. On the first two days of the experiment the
animal fasted and on the second two received only starch and cane-
sugar. The results for the last two days were:

Proteid metabolism 15.94 grams.

Equivalent fat, according to Rubner.. 7.65 "

Fat of food 9.40 "

Maximum from fat and proteids 17 . 05 "

Fat actually produced 1 17 . 25 "

Even after making all possible deductions for the fact that some
carbon may have been retained in the body in the form of glycogen
instead of fat, and also for a possible residue of undigested starch in
the alimentary canal at the close of the experiments, Rubner still
computes that at least 40.7 grams of fat must have had its origin
in carbohydrates.

Lehmann & E. Voit's experiments have only recently ap-
peared.! In their introduction they report also the results of ex-
periments on fattening geese made by C. Voit several years previous
to 1SS3. which likewise show a production of fat from carbohydrates.

G. Kuhn and his associates, J at the Mockern Experiment Station,
have demonstrated, by means of respiration experiments in which
starch was added to rations but slightly exceeding the maintenance
requirement, a formation of fat from carbohydrates by ruminants
(oxen). In view of the possibility (see p. 27) that part of the car-
bon of the urine may be derived from the non-nitrogenous matter
of the food, and in order to be on the safe side, the authors assume
as possible that all the carbon of the proteids metabolized may
have been stored up in the body in the form of fat. On this extreme
and improbable assumption their results were as shown on the
following page :

* Zeit. f. Biol., 22, 272.

t Ibid., 42, 619.

% Reported by Kellner; Landw. Vers. Stat., 44, 257.

1 78

PRINCIPLES OF ANIMAL NUTRITION.

Animal.

Period.

Proteid

Metabolism,

Grms.

Equivalent

Fat,

Grms.

Fat of
Food,

Grins.

Maximum

from Fat

and Proteids,

Grms.

Fat

Actually

Produced,

Grms.

I

2a

373.6

259

86

345

423

I

26

382.0

265

81

346

332

II

2

297.4

206

77

283

434

III

2

104.4

72

60

132

281

IV

2

126.9

88

60

148

160

III

3

506.9

351

69

420

375

IV

3.

548.8

380

74

454

388

III

4

980

679

84

763

526

V

2a

232

161

42

203

396

V

2b

268

186

42

228

•407

V

3

149

103

39

142

703

VI

2a

218

151

40

191

304

VI

2b

232

161

35

196

381

VI

3

186

129

43

172

507

In most of these experiments the rations were purposely made
poor in proteids and fat, and in all such cases, with one exception, a
formation of fat from carbohydrates is clearly demonstrated. In
three cases in which large amounts of proteids were fed, as well as in
some similar experiments not included in the above table, it was
possible to account for the fat production otherwise, but such nega-
tive results in no degree weaken the positive teaching of the remain-
ing trials.

The more recent investigations of Kellner et al* at the same
Station, in which starch was added to a basal ration, although under-
taken primarily for other purposes, likewise show the formation of
an amount of fat inconsistent with the hypothesis of its production
from the fat and proteids of the ration only.

The failure of Pettenkofer & Voit to obtain affirmative results
in their earlier experiments appears to be largely explicable, in the
light of more recent knowledge, from the conditions of the experi-
ments themselves. Pfluger f has recalculated their experiments on
the same basis as those upon the formation of fat from proteids
(see p. 109), and has pointed out that in the majority of cases the
total food was, according to his computations, scarcely more than
sufficient for the maintenance of the organism, thus leaving no
excess of any kind for fat production. Moreover, out of those ex-

* Landw. Vers. Stat , 53, 1.
t Arch. ges. Physiol , 52, 239.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY.

179

periments in which the conditions were favorable for a production
of fat from carbohydrates, some actually do show thai result, al-
though the}- were classed by Voit as "exceptional cases," while its
failure to appear in others is explained, according to Pfliiger, by the
increased metabolism due to maltreatment of the animal and the
overloading of its digestive organs with starch.

Whether we admit all of Pfliiger's criticism or not, it is now uni-
versally conceded that the carbohydrates are an important source
of fat. If we are to go further and deny with Pfliiger the production
of fat from proteids, we are brought back, by a curious reversal of
views, substantially to Liebig's classification of the nutrients into
" plastic " and " respiratory, " but, as already pointed out, it ap-
pears altogether probable that the proteids also contribute to fat
production. However this may be, it is clear that in the case
of herbivorous animals, which ordinarily consume relatively little
proteids and fat and large amounts of carbohydrates, the latter are
the most important factors in fattening, and the results of Lawes
<fc Gilbert (p. 167), according to which the gain of fattening ani-
mals is largely determined by the supply of non-nitrogenous matters
in the food, are seen to be in full accord with the most careful physi-
ological investigation.

Evidence from Respiratory Quotient. — The formation of fat from
carbohydrates is a process of reduction. If we suppose all the car-
bon of 100 parts of dextrose, together with the necessary hydrogen
and oxygen, to be united to form fat of the average composition
stated on p. 61, we have the following:

Dextrose.

Equivalent,
Fat.

Residue.

Equivalent,
Water.

Excess of
Oxygen.

Carbon

40.00

6.67

53.33

40.00
6.28
6.01

Hvdrogen

Oxveren

0.39

47.32

0.39
3.12

44 20

100.00

52.29

47.71

3.51

44.20

The excess of oxygen we may further suppose to unite with the
carbon of 41.44 additional parts of dextrose, producing 60.78 parts
of carbon dioxide and 24.86 parts of water. The process would be
an intra-molecular combustion analogous to a fermentation, pro-

180 PRINCIPLES OF ANIMAL NUTRITION.

during carbon dioxide without the intervention of oxygen from out^
side. The latter fact, of course, is equally true whatever substance
combines with the excess of oxygen of the carbohydrate. The
tendency, therefore, will be to increase the respiratory quotient and,
if large amounts of carbohydrates are thus transformed, to even
raise it above unity.

Numerous such instances are on record. Thus Regnault &
Reiset * report a quotient of 1.024 in case of a hen, and Reiset f ob-
tained quotients of 1.004 and 1.054 with a ewe and a boar. Han-
riot & Richet,J in studies on the respiration of man, found that
the ingestion of carbohydrates caused the respiratory quotient to
rise markedly and sometimes to exceed unity. Later Hanriot §
studied the transformations of glucose in the organism of man and
obtained similar but more marked results, the quotient reaching as
high a value as 1.28.

Magnus-Levy | has likewise observed quotients greater than
unity in the case of a dog fed large quantities of carbohydrates, and
Bleibtreu,^[ in experiments on fattening geese in a form of Regnault
respiration apparatus, also verified this fact, as have Kaufmann **
and Laulanie ft in experiments upon dogs with sugar. The exten-
sive respiration experiments of Zuntz & Hagemann^ on the horse
also afford numerous instances of respiratory quotients greater
than unity.

The evidence of the respiratory quotient, then, is entirely in
accord with the conclusions reached by other methods as to the
formation of fat from carbohydrates.

Non-nitrogenous Nutrients of Feeding-stuffs. — It has
become customary to regard the digestible non-nitrogenous ingre-
dients of feeding-stuffs, aside from the ether extract, as consisting
essentially of carbohydrates. As has several times been urged on

* Ann de Chim. et de Phys. [3], 26, 45.

t Ibid. [3], 69, 145.

t Comptes rend., 106, 419 and 496.

§ Archives de Physiol., 1893, p. 248.

|] Arch. ges. Physiol., 55, 1.

IT Ibid., 56,464; 85, 366.
** Archives de Physiol., 1896, 341.
ft Ibid., 1896, 791.
%% Landw. Jahrb., 27, Supp III.

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 181

preceding pages, however, this is far from being the case as regards
the materials actually resorbed from the digestive tract of our
common domestic animals, particularly the ruminants. A demon-
stration of the production of fat from carbohydrates, therefore,
does not necessarily show that the chemically diverse materials
resorbed from coarse fodders, e.g., are available for fat produc-
tion.

As a matter of fact, however, what a large proportion of the
experiments just cited actually show, under a strict interpretation,
is that fat was produced from the non-nitrogenous nutrients of the
rations other than fat. In many of the experiments, it is true, nota-
bly those with swine and with geese, the ration consisted of concen-
trated feeding-stuffs whose " nitrogen-free extract " consisted to a
large extent of hexose carbohydrates. Similarly, in G. Kiihn's ex-
periments the fat production was secured by the addition of starch
to rations slightly above the maintenance requirement. In these
cases, therefore, at least the larger part of the fat production in
excess of that possible from proteids and food fat must be ascribed
to the hexose carbohydrates. In experiments like those of Henne-
berg, Kern & Wattenberg, and of Jordan, on the other hand, a
not inconsiderable proportion of the non-nitrogenous nutrients
was necessarily derived from coarse fodders and was, therefore,
largely of undetermined nature. In such cases it is obviously im-
possible to say whether the fat production was at the expense of
the hexose carbohydrates only or whether the other non-nitrog-
enous ingredients participated in it.

Other considerations, however, seem to render a participation
of these substances in fat production, directly or indirectly, at least
highly probable if not certain.

Crude Fiber. — The experiments of v. Knieriem (p. 161), as we
have seen, seem to show that digested cellulose may be as efficient as
other carbohydrates in protecting the body fat, — that is, as part
of a maintenance ration. The numerous experiments cited on
pp. 117-123 likewise indicate that it has an effect similar to that of
other carbohydrates in diminishing the proteid metabolism. Kell-
ner * has also investigated its value in a fattening ration, using for
this purpose the material resulting from the treatment of rye straw
* Lanclw. Vers. Stat., 53, 278.

1 82 PRINCIPLES OF ANIMAL NUTRITION.

with an alkaline solution under pressure and containing 76.78 pei
cent, of "crude fiber." This material was added to a basal ration
somewhat more than sufficient for maintenance. The results as
regards the proteid metabolism have already been considered
(p. 121); the following table shows the effects also upon the fat
production :

Apparently Digested.

Gain.

Crude | Crude

Fat, | Fiber,

tirms. i Grins.

N. free

Extract,

Cirins.

Protein,
Grins.

Protein,
Grms.

Fat.
Grms.

OxH:
Period 5
" 4

Extracted straw. .
Basal ration

Difference

116
101

3129
1083

3351
2912

654

719

157
43

735

191

Period 3

15
92

2047

1057
1083

439

4773
2912

-95

629

749

114

78
43

544
565

" 4

Basal rat ion

101

191

Ox J:
Period 5
« 4

Extracted straw...

Difference

Starch

-9

110
107

-26

3101
1114

1861

3344
2895

-120

747
836

35

98
33

374

693
223

Period 3

3

85
107

1987

1105
1114

449

4396
2895

-89

764
836

65

91
33

470
472

« 4

Basal ration

Difference

223

-22

-9

1501

-72

58

249

The varying quantities of nutrients digested stand in the way
of a direct comparison of the results. If, however, we reckon 1
gram of digested fat equivalent to 2.25 grams of digested crude
fiber or nitrogen-free extract or protein (isodynamic quantities
according to the usual method of computation), and if we further
convert the gain of proteids into its equivalent amount of fat, on
the same principle, by multiplication by 5.7 and division by 9.4, we
have the results shown in the table on the opposite page.

While no great quantitative accuracy attaches to such a com-
putation, it is sufficient to show that the effect produced in this case

THE RELATIONS OF METABOLISM TO FOOD-SUPPLY

i«J

Carbohydrate £otal *■*

Equivalent Equivalent

of Nutrients, "Ifi?™"
(inns. Grms-

Gain per

Kilogram

Nutrients,

Grms.

Ox H:

Extracted straw, period 5-4
Starch, " 3-4

Ox J:

Extracted straw, period 5-4
Starch, " 3-4

252.8
233.0

218.1
207.3

by the addition to the basal ration of digestible matter five sixths
of which was derived from crude fiber, was not inferior to that
produced by the addition of an equal amount of pure starch.

It would seem that these results may fairly be taken as showing
that the products of the digestion of cellulose by ruminants are
substantially of equal value with those of the digestion of starch.
This, however, by no means warrants the conclusion that starch and
cellulose are of equal value in ordinary feeding-stuffs. The mate-
rial used in these experiments had been so altered mechanically
and freed from incrusting materials by the treatment to which it
had been subjected that 88. 3 per cent, of its organic matter and
per cent, of its crude fiber was digested. The same animals
digested but 52.5 per cent, of the crude fiber of wheat straw, and
the digestible organic matter of the latter proved far less efficient
than that of either starch or extracted straw. A full discussion of
these facts may be more profitably undertaken in connection with a
consideration of the energy relations of feeding-stuffs in Part II;
for the present it may suffice to point out that the difference

I appears to depend on physical rather than chemical causes.

Pentose Carbohydrates.— We have already (p. 156) seen reason
to believe that the pentose carbohydrates may serve as a source of
_;.- to the organism and protect other materials from oxidation.
This, of course, is equivalent to an indirect production of fat. In
the same connection, however, the experiments of Kellner, just
nentioned, were referred to as indicating a direct participation by
these bodies in fat production. About one third of the digested
matter of the extracted rye straw was found to consist of bodies

184

PRINCIPLES OF ANIMAL NUTRITION.

yielding furfural, presumably pentosans, as appears from the follow-
ing modified form of the last table :

Total Carbohydrate Equivalent
of Nutrients.

Total Fat
Equivalent

Pentosans,
Grms.

Other Substances,
Grms.

of Gain,
Grms.

OxH:

Starch, " 3-4

Ox J:

Extracted straw, period 5-4

Starch, " 3^

809
-34

834
-89

1616
1729

1500
1459

613

395

509
284

If we regard the furfuroids as not contributing to the fat pro-
duction, then we must assign to the other nutrients of the extracted
straw a value from 66 to 74 per cent, greater than that of the
digested matter of the starch, a result which is hardly conceivable.
Apparently we must admit that the furfuroids in this case pro-
duced approximately the same effect as the other non-nitrogenous
nutrients and were at least indirectly if not directly a source of fat.

CHAPTER VI.

THE INFLUENCE OF MUSCULAR EXERTION UPON
METABOLISM.

It is a matter of common experience that muscular exertion
results in a very marked increase in the vital activities of the body.
The rate of circulation and respiration is greatly quickened and the
increased metabolism in the organism is shown by the loss of weight
anil by the increased demand for food to make good the destruction
of tissue. Indeed, no other factor even approaches muscular exer-
tion in the extent to which it increases the metabolic activities of
the body.

We have now to. consider in some detail the nature of muscular
exertion and the precise character of its effects upon metabolism.

§ i. General Features of Muscular Activity.

M uacular Contraction.

The work of the muscles is accomplished by contracting, and a
brief consideration of some of the more prominent general features
of muscular contraction will conduce to an intelligent study of the
main subject of the chapter. It will be possible here to consider
this phase of the subject only in its most general outline, and the
reader is referred to works on physiology for details.

When a suitable stimulus, winch in the living animal is usually
a nerve stimulus, is applied to a muscle it contracts; that is, it
tends to grow shorter and thicker. This change is brought about
by a shortening and thickening of the individual fibers of which
the muscle is built up. A single stimulus, such, for example,
as that caused by the making or breaking of an electric circuit,
gives rise to what is known as a simple muscular contraction. If
such a stimulus is repeated with sufficient frequency it produces a

185

1 86 PRINCIPLES OF ANIMAL NUTRITION.

series of simple contractions which fuse together, resulting in a state
of contraction which continues, subject to the effects of fatigue, as
long as the stimulus acts. This form of muscular contraction
has received the name of "tetanus." In the living, animal the
ordinary contractions of the muscles brought about through the
nervous system, even those that seem but momentary, are essen-
tially tetanic in their character.

Chemical Changes during Contraction.— Under the influence
of a stimulus sufficient to produce a muscular contraction there
occurs a sudden and large increase in the chemical changes which
are continually going on even in the quiescent muscle. More mate-
rial is metabolized in the muscle during contraction and energy is
thus liberated for the performance of work.

Our knowledge of the nature of these chemical changes in the
contracting muscle is comparatively meager, but three main features
appear well established:

First, during contraction the neutral or slightly alkaline reac-
tion of the quiescent muscle c1 anges to an acid reaction, probably
through the formation of sarcolactic acid.

Second, there is a large increase in the amount of oxygen taken
up by the muscle from the blood and a still greater increase in the
amount of carbon dioxide given off by it.*

Third, under normal circumstances, judging from the amount
of the urinary nitrogen, there appears to be no considerable increase
in the nitrogenous products of metabolism.

From the increase in oxygen consumed and carbon dioxide given
off we might be led at first thought to suppose that the increased
activity in the muscle during contraction was of the nature of a
simple oxidation. Certain other facts, however, seem to show that
this view of the matter is inadequate.

Oxidations Incomplete. — That the increased metabolism in
the con'racting muscle is not a simple oxidation of some material
t carbon dioxide and water is indicated by the fact of the produc-
tion of lactic or other acid in the muscle. Plainly, if the energy for
muscular contraction is produced by oxidation the oxidation is at
least incomplete.

* Some good authorities doubt whether the carbon dioxide resulting
from muscular exertion actually leaves the muscle in that form. Compare
Schaffer, Text-book of Physiology, 1898, Vol. I, p. 911.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 187

Respiratory Quotient. — By analogy with investigations upon
respiration we may designate the ratio between the oxygen con-
sumed and the carbon dioxide given off by the muscle as the respi-
ratory quotient of the muscle. Numerous investigations upon this
point have shown that during contraction much more carbon diox-
ide is given off than corresponds to the oxygen consumed, or, in
other words, the respiratory quotient of the active muscle is con-
siderably greater than unity.

As early as 1862 Sczelkow * determined the gaseous exchange
between the blood and the muscles of the posterior extremities of
a dog, tetanus being produced by an electric current. He found
that during rest more oxygen disappeared from the blood than
corresponded to the carbon dioxide taken up by it, while during
tetanus, on the contrary, the carbon dioxide considerably exceeded
the oxygen. His results, calculated for the posterior extremities
alone, were as follows:

Experiment.

j Rest . . .
\ Tetanus

( Rest . . .
I Tetanus

j Rest . . .
( Tetanus

j Rest . . .
( Tetanus

j Rest . . .
/ Tetanus

Per Minute.

Carbon

Dioxide

c.c.

Oxygen

c.c.

Respiratory
Quotient.

1.60
10.37

4.10
3.92

0.41
2.65

2.62
12.38

4.25
10.52

0.62
1.18

1.73
10.62

3.21
7.55

0.54
1.41

3.53
12.19

4.71
9.38

0.75
1.30

2.33
12.95

5.82
18.71

0.40
0.80

In the above experiments, with a single exception, the quantity
of oxygen consumed by the active muscles was more than that
taken up in a state of rest, but the increase in the amount of carbon
dioxide given off was still greater, so that the respiratory quotient
was largely increased, exceeding unity in every instance but one.

* Sitzungsber. Wiener Akad. d. Wiss., Math-Naturwiss. Klasse, 45, II, 171.

PRINCIPLES OF ANIMAL NUTRITION.

Chauveau & Kaufmann * have more recently obtained simi-
lar results. Their experiments were made upon the Levator labii
superioris of the horse, both in a state of rest and in a state of activ-
ity consequent upon the consumption of food. From the amount
and composition of blood entering and leaving this muscle the
following results were obtained for the oxygen consumed and carbon
dioxide given off per kilogram of muscle in one minute. On the
average of the three experiments, in round numbers, twenty-one
times as much oxygen was consumed during work as during rest and
twenty-nine times as much carbon dioxide was given off.

Oxygen Consumed.

Carbon Dioxide Given Off.

Experiment.

Rest,
Grms.

Work.
Grms.

Work -*-
Rest.

Rest,
Grms.

Work,
Grms.

Work h-
Rest.

2

.00479
.01167
.00419

.07148
.20190
. 14899

14.9
17.3
35.6

.00365
.01168
.00518

. 12534
. 35488
.25709

34.3

3

30.4

4

49.6

. 00688

.14079

20.5

.00864

.24577

28.5

These facts show plainly that the increased metabolism of the
active muscle cannot consist wholly of a direct oxidation, since the
carbon dioxide given off from the muscle contains more oxygen
than direct experiment shows to have been taken up by the muscle
during the same time.

Oxygen not Essential. — A further and still more striking
proof of the above assertion is found in the fact that the living
muscle can execute a considerable number of contractions in the
entire absence of oxygen.

Setschenow is quoted by Ludwig & Schmidt f as having found
that muscles would contract freely when supplied with oxygen-free
blood, while L. Hermann % has shown that an excised muscle may
continue to contract in a vacuum. The well-known investigations
of Pfliiger § show that frogs may continue to live and execute more
or less extensive motions in an atmosphere of pure nitrogen for

* Comptes rend., 104, 1126, 1352, 1409.

f Verhandl. Sachs Akad. d. Wiss., Math-Phys. Klasse, 20, 12.

% Unters. u. Stoffw. der Muskeln.

§ Arch gcs. Physiol., 10, 313.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 189

several hours, giving out considerable amounts of carbon dioxide,
and Bunge * has made similar observations upon the movements
of certain intestinal worms (Ascarisj in one per cent, salt solution
made as nearly oxygen-free as possible.

Weinland t has shown that in the latter case the energy is
derived chiefly from the cleavage of glycogen with the production
of carbon dioxide and lower fatty acids.

Summary. — The three classes of facts just adduced justify the
conclusion that the chemical changes by which energy is liberated
in a muscular contraction are not simply oxidations, but are of the
nature of a cleavage of some complex substance or substances with
evolution of carbon dioxide. There is, in other words, a sudden
"explosive" decomposition of substances elaborated in the muscle
durins; rest. Of the nature of the material thus broken down we
have little definite knowledge. We can say, however, that if it is
nitrogenous matter its nitrogen is ordinarily retained in the muscle
in some form and that in effect the metabolized material is non-
nitrogenous. The increase in the consumption of oxygen during
work appears to be to a certain extent a secondary process, accom-
plishing the further oxidation of the primary products of metabolism.
At the same time, the fact that the amount of oxygen consumed
responds very promptly to work and also to its cessation shows that
primary products, whatever they may be, are very speedily
oxidized, either in the muscle or elsewhere in the organism.

Thermal Changes during Contraction. — A considerable por-
tion of the energy set free during muscular exertion always takes
ultimately the form of heat. When the muscle acts without shorten-
ing, as when supporting a weight (isometric contraction) — that is,
when no external work is don* — all the metabolized energy takes
the form of heat. If. on the other hand, the weight be lifted (iso-
tonic contraction) — if external work is done — a portion of the
energy takes the form of motion. The interesting question of the
relation between the external work performed and the total amount
of energy metabolized will be considered later. For the present it
is sufficient to state that muscular ■ action always produces heat
and that a very considerable share of the metabolized energy
ultimately takes this form

* Zeit physiol. Chem . 8, 48. f Zeit. f. Biol., 42, 55; 45, 1 13.

19° PRINCIPLES OF ANIMAL NUTRITION.

Muscular Tonus. — The chemical and thermal changes just
enumerated as characterizing the muscle during contraction are
taking place in it to a less extent at all times. Even at rest the
muscle respires and produces heat, as is well illustrated by Sczel-
kow's and Chauveau & Kaufmann's experiments quoted above.

The living muscles of the body are elastic and may be said to
be always slightly on the stretch, as is shown by the fact that when
cut they gape open and that they shorten when their attachments to
the bones are severed. This slight degree of contraction of the resting
muscles has been called muscular tonus, and it is at least a plausible
conclusion that the chemical changes taking place in a quiescent
muscle furnish the energy to maintain this tonus. According to
Chauveau * we may regard the essence of muscular contraction as a
sudden increase in the elasticity of the muscle. He holds that all
the energy liberated by muscular metabolism is converted first into
the elastic force of the muscle and only secondarily into heat. Ac-
cording to this view the slight degree of elasticity of the quiescent
muscle is produced by the constant metabolism going on within it.
In active muscular contraction this process is greatly exaggerated
and the katabolic processes exceed the anabolic, thus giving rise to
a great increase in muscular elasticity which in turn may be con-
verted into work. In repose following work, we may assume that
the substances broken down during contraction are built up again,
while in prolonged repose the two processes must substantially
balance each other.

Muscular tonus is most noticeable during the waking hours,
under the influence of external stimuli to the central nervous sys-
tem, and consequently the rate of metabolism and the heat produc-
tion tend to be greater than during sleep. To this is to be added, as
a further cause of greater metabolic activity during the waking hours,
those continual slight movements of the body which usually take
place even in what is commonly spoken of as a state of rest and
which may be designated as incidental movements

That the total amount of metabolism required for the mainte-
nance of muscular tonus is considerable seems to be indicated by
the observations of Rohrig & Zuntz,| and of Colasanti,| who

* Le Travail Musculaire et l'Energie qu'il Represente. Paris, 1891.
t Arch. ges. Physiol., 4, 57; 12, 522. % Ibid., 16, 157.

INFLUENCE OF MUSCULAR. EXERTION UPON METABOLISM- 191

found that when the motor nerves of the rabbit are paralyzed by
curari the rate of metabolism, as measured by the respiratory ex-
change, falls to about one half the amount during rest and does
not react to changes of external temperature. Pfliiger * computes
from his experiments a similar reduction of about 35 per cent.
Under these conditions the heat production of an animal is insuffi-
cient to maintain its normal temperature, and unless the loss of
heat from the body is hindered by coverings or otherwise it soon
perishes. Frank & F. Voit,| on the contrary, found that curarized
dogs excreted no less carbon dioxide than in the normal state, pro-
vided the body temperature was kept normal.

Secondary Effects of Muscular Exertion.

The greater activity of the muscular metabolism during the
performance of work gives rise to important secondary effects, par-
ticularly upon the circulation and respiration. It is a familiar fact
that in active exercise the action of the heart is largely increased
and the breathing becomes deeper and more rapid, and that ordi-
narily the limit of muscular exertion is set, not by the power of the
muscles themselves, but by the ability of the heart and lungs to
keep pace with the demands upon them.

Circulation. — The circulating blood is the medium by which
oxygen is conveyed to the muscles and carbon dioxide and other
products of their metabolism removed. The latter function is of
special importance, since an accumulation in the muscle of the
products of its own metabolism speedily reduces and ultimately
suspends its power to contract. In active muscular exercise, there-
fore, an increase in the rate of circulation is essential to the con-
tinued activity of the muscles. This increase appears to be brought
about by the accumulation in the blood of the products of metab-
olism, which act as a stimulus to the vaso-motor center. The
result is a dilation of the peripheral blood-vessels, which is aided by
the mechanical effects of muscular contraction. To offset this and
prevent a fall of arterial blood pressure, the visceral capillaries are
probably constricted, while the rapidity and strength of the heart-
beats are largely increased. The rapidity of the circulation as a

* Arch. ges. Physiol., 18, 247. f Zeit. f. Biol., 42, 349.

192 PRINCIPLES OF ANIMAL NUTRITION.

whole is thus greatly augmented, while at the same time a larger
percentage of the total blood passes through the muscles. For
example, in the experiments of Chauveau & Kaufmann, cited
above, the ratio between the circulation in the resting as compared
with the active muscle varied from 1 : 3.35 to 1 : 6.60. Zuntz &
Hagemann,* in their investigations upon the work of the heart,
found the average amount of blood passing through the heart of a
horse per minute to be during rest 29.16 liters and during work
53.03 liters. By this increase in the rate of circulation through
the muscles the carbon dioxide and other injurious products of
muscular metabolism are rapidly removed and an abundant supply
of oxygen is ensured. In fact it is usually true that during work
which is not excessive the venous blood contains less carbon diox-
ide and more oxygen than during rest.

Since the heart is a muscular organ, it is obvious that this in-
crease in the circulatory activity must add materially to its metab-
olism. In the performance of work, therefore, there is an expend-
iture of matter and energy, not only for the work of the skeletal
muscles but likewise for the additional work of the heart. Zuntz
& Hagemann in their experiments upon the horse just mentioned
compute that during moderate work the metabolism due to the
work of the heart amounts to 3.8 per cent, of the total metabolism
of the body.

Respiration. — The greater activity of the circulation conse-
quent upon muscular exertion would be futile were not provision
made for more efficient aeration of the blood in the lungs through an
increased activity of the respiration. The latter appears to be
brought about, like the increase in the circulatory activity, by the
effect of the greater amount of metabolic products in the blood,
acting in this case upon the respiratory center. It has been shown
that an accumulation of carbon dioxide in the blood does not have
this effect, but that a lack of oxygen, such as occurs, for example,
in asphyxiation, provokes powerful movements of the respiratory
organs. In ordinary work, however, whatever may be the case in
excessive muscular exertion, the effect is not caused by a lack of
oxygen, for the blood, as already noted, is usually more arterialized

* Landw. Jahrb., 27, Supp. Ill, 405.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM- 193

than during rest. Apparently the stimulation of the respiratory-
center is brought about by the other products of muscular metab-
olism, whatever they may be, which find their way into the blood.
Under the influence of this stimulus the respiratory movements
increase in frequency or depth or both, thus making possible a
more active gaseous exchange between the blood and the air in the
lungs. This action is usually so efficient that the expired air dur-
ing work contains a smaller proportion of carbon dioxide than it
does during rest, notwithstanding the fact that the total quantity
eliminated is much greater.

Since respiration, like circulation, is maintained by muscular
action, it is true in the former case as in the latter that a greater
activity of the function necessitates a greater metabolism for
that purpose. Zuntz & Hagemann * have recently investigated the
work of respiration in the horse, the augmented respiratory activ-
ity being brought about by an admixture of carbon dioxide to the
inspired air, this resulting in a marked increase in the depth of the
respiratory movements. With the animal upon which most of the
experiments were made they found an increment of from 2.02 c.c. to
5.23 c.c. of oxygen consumed for each increment of one liter in the
volume of air respired. In general, although with some exceptions,
the work of respiration as thus measured increased with the in-
creased depth of the respiratory movements. The results upon
other horses were somewhat variable. It was observed, however,
that in the performance of ordinary work by the horse the effect
was chiefly upon the frequency of respiration rather than its depth.
The former effect the authors believe to involve less work than the
latter and moreover an amount largely independent of the total
volume of air respired.

§ 2. Effects upon Metabolism.

It is obvious from the foregoing paragraphs that the production
of external work is a complex phenomenon. As regards its effects
upon the total metabolism, the main features involved seem to be:

1. An explosive decomposition of some unknown "contractile
substance" in the muscles.

* Landw. Jahrb., 27, Supp. Ill, 361.

194 PRINCIPLES OF ANIMAL NUTRITION.

2. The oxidation somewhere in the organism of the immediate
products of this decomposition to the final excretory products.

3. Since the state of contraction appears to be only an exagger-
ation of the muscular condition during rest, we may reasonably
suppose that there is a continual re-formation of the "contractile
substance " going on.

4. As secondary effects there is a marked increase in the activ-
ity of circulation and respiration, thus involving supplementary
muscular exertion.

It is plain that however interesting and important to the physi-
ologist may be studies of the changes in the muscle itself, from
the point of view of the statistics of nutrition the important thing
is the total effect upon the expenditure of matter and energy by the
organism under varying conditions of work. The energy relations
of the subject will be discussed subsequently in Part II. Here we
are concerned more particularly with the nature of the material
expended in the production of work, and as a matter of convenience
we may, as in the two preceding chapters, take up first the effect
upon the proteid metabolism and second that upon the metabolism,
of the non-nitrogenous substances.

Effects upon Proteid Metabolism.

Earlier Investigations. — Since the muscles, which are the
instruments by means of which work is produced, are composed
essentially of proteid material, it was natural to regard the proteids
as the source of muscular power and to assume that the energy
developed during work was supplied by an increased metabolism
of these substances. This view was supported by the authority of
Liebig, who, however, does not appear to have based it upon any
actual experiments, and it was quite generally, although not
universally, accepted.

C. Voit * appears to have been the first to subject this idea to
investigation. His experiments were made upon a dog weighing
about 32 kilograms. The work performed, by running in a tread-
mill, was considerable, being estimated to average 1.7 kgm. per
second for the whole twenty-four hours. Experiments were made
* Untersuchungen iiber den Einfluss des Kochsalzes, des Wassers, und
der Muskelbewegungen auf den Stoffwechsel. 1SG0. Compare the summary
by E. v. Wolff in Die Erhahrung der landw. Xutzthiere, pp. 386-388.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM- 195

both during fasting and with a daily ration of 1500 grams of lean
meat. The results obtained were as follows :

Number of
Experiment.

Meat
Eaten,
Grms.

Water
Drunk
Grms.

Urine

Excreted,

Grms.

Urea

Excreted,

Grms.

I

•1

"1

1500 ]
1500 j

Rest
Work

Rest

Work

Rest

Rest

Work

Rest

Work

Rest

258
872
123
527
125
182
657
140
412
03

186
518

145
186
143
1060
1330
1081
1164
1040

14.3

II

16.6
11.9
12.3

Ill

10.9
109.8
117.2

IV

109.9
114.1
110.6

The average increase of the proteid metabolism, as measured
by the urea excreted, was in the fasting experiments 11.8 per cent,
and in the experiments with food 4.95 per cent. The absolute
difference in grams, however, was materially less in the fasting
experiments, although approximately the same amount of work
was performed m both cases. A similar experiment upon an
older and quite fat dog while fasting showed an increase of only
6 per cent, in the proteid metabolism.

Subsequently Pettenkofer & Voit * made similar experiments
upon a man, the work consisting in turning a heavy wheel provided
with a brake. The work was performed in the respiration appara-
tus. The results showed a large increase in the carbon dioxide
excreted, but scarcely any effect was noted upon the excretion of
nitrogen, as will be seen from the following table:

Nitrogen

of Urine,

Grms.

Carbon

Dioxide

Excreted

Grms.

Water Excreted.

Oxygen

Taken Up.

Grms.

In

Urine,
Grms.

Evapo-
rated,
Grms.

of

Experi-
ments.

Fusting :

Rest

Work

12.4
12.3

17.0
17.3

716
1187

928
1209

1006
740

1218
1155

821
1777

931
1727

762
1072

832
981

2
1

Average diet :
Rest

3

Work

2

* Zeit. f. Biol., 2, 478.

I96 PRINCIPLES OF ANIMAL NUTRITION.

Pettenkofer & Voit regard the slight increase in the proteid
metabolism which they observed in most cases as a secondary effect
of muscular exertion. They have shown, as we have seen, that
when the cells of the body are abundantly supplied with non-nitroge-
nous nutrients, either in the form of food or of body fat, there is a
tendency to diminish the proteid metabolism. In work, on the con-
trary, large amounts of non-nitrogenous material are oxidized, as
their respiration experiments show. The supply of these nutrients
to the cells is thus diminished, and it is to this that they attribute
the increase in proteid metabolism.

Results like those just given can hardly be interpreted other-
wise than as showing that the non-nitrogenous constituents of the
body or of the food, rather than the proteids, are the source of the
energy expended in muscular work, but the first attempt to com-
pare the amount of work performed with the energy available from
the proteids metabolized was the famous experiment of Fick &
Wislicenus * in 1S66. These observers made an ascent of the
Faulhorn and found that the amount of proteids metabolized
during and after the ascent, as measured by the urea excreted, was
insufficient, according to their computations, to account for more
than one third of the energy required to raise their bodies to the
height of the mountain, making no allowance for the work of the
internal organs, nor for those muscular exertions which did not
contribute directly to the work done.

Fick & Wislicenus found no considerable increase in the uri-
nary nitrogen in their experiment. Subsequent investigators,
among whom may be mentioned Parkes,f Noyes,J Haughton,§
MeissnerJ Schenk,^[ and Engelmann,** have reported appar-
ently conflicting results regarding the influence of work on the
proteid metabolism. In some cases an increase was observed,
while in other cases no material effect was apparent. The increase
when observed was never large except in the experiments of Engel-

* Vrtljschr. Naturf. Gesell. Zurich, 10, 317.

t Phil. Mag. , 4th ser., 32, 1S2.

% Amer. Jour. Med. Sci., Oct., 1807.

§ Brit, Med. Jour., 15, 22.

|| Virchow's Jahresber., 1868, p. 72.

f Centralb. Med. Wiss., 1874. p. 377.

** Archiv f. (Anat. u.) Physiol., 1871, p. 14.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 197

mann, and was entirely insufficient to account for the energy ex-
pended. Oppenheim made the interesting observation that work
pushed to the point of producing dyspnoea caused a marked increase
in the proteid metabolism.

Influence of Total Amount of Food — Kellner's Inves-
tigations.— Doubtless the conflicting results of earlier experiments
are due in part to defective technique, but they arise in part also,
as it would seem, from another cause to which attention was first
called by Kellner in 1879-80. Kellner's experiments were made
upon the horse. They differed from most earlier experiments, first,
in that the comparison was made between different amounts of work
instead of between work and rest, and second, that the individual
periods instead of covering only a few days were extended over two
or three weeks.

Series I. — Kellner's first series * was made primarily for the
purpose of testing the influence of work upon the digestibility of the
food, but the total nitrogen of the urine was also determined. The
methods employed for this purpose were somewhat imperfect, there
being some mechanical loss and probably also a loss of ammonia
from the urine, but the author believes the results of the several
periods to be fairly comparable. The amount of work performed
was measured by a dynamometer. The numerical results of the
measurement have since been shown to be too high, but the relative
amount in the several periods is not thought to be materially
affected by this error. The results of the several periods are briefly
summarized in the following table :

Period.

Work,
Kgm.

Nitrogen.

Digested,
Grms.

In Urine,

Grms.

Live Weight

at Close

of Period,

Kgs.

I.

II.

III.

IV.

V.

625,000
1,250,000
1,875,000
1,100,000

625,000

134.41
128.32
132.72
126.40
129.41

99.0
109.3
116.8
110.2

98.3

534 . 1
529.5
522 . 5
508.8
518.0

While the above figures show a considerable nitrogen deficit,
the urinary nitrogen increased and decreased with the amount of
* Landw. Jahrb., 8, 701.

198 PRINCIPLES OF ANIMAL NUTRITION.

work performed in a manner which can scarcely be explained other-
wise than as a result of the changes in the latter. The ration con-
sumed was amply sufficient for the light work of the first and fifth
periods. When, however, more work was demanded from the
animal, the live weight promptly fell off, showing that the total
ration was insufficient. This insufficiency of the total ration Kellner
believes to be the cause of the increase in the proteid metabolism.

A consideration of the daily results confirms this view. In
passing from periods of lighter to those of heavier work the increase
followed promptly upon the change. In Period III, with the most
severe work, the proteid metabolism continued to increase through-
out the period and apparently had not reached its limit at the
close. Conversely, when the work was diminished in Periods IV
and V it decreased as promptly as it had increased. Finally, it
should be noted that the additional amount of proteids metab-
olized was entirely insufficient to furnish an amount of energy
equivalent to the increase in the work.

In four succeeding series of experiments Kellner * has investi-
gated this phenomenon more fully, some of the sources of error noted
above having been avoided in the later researches. The results, as
will appear, still show a deficit of nitrogen. Kellner estimates that
about 6 grams of nitrogen per day were required for the growth of
hoofs, hair, epidermis, etc., and believes that there was some loss of
urinary nitrogen mechanically and chemically.

Series II. — In this series of experiments the ration, consisting
of 7.5 kilograms of hay and 4 kilograms of beans, was purposely
made rich in protein. In spite of this liberal supply of protein,
however, the same result as in the first experiment was noted to an
even more marked extent. As in the first series, too, the increase
in the excretion of nitrogen promptly disappeared when the amount
of work was diminished.

Series HI. — In this series the animal was brought as nearly as
possible into equilibrium with his food upon rather light work. The
work was then trebled, while at the same time an addition was
made to the non-nitrogenous ingredients of the ration by substitut-
ing for a portion of the beans an amount of oats containing the same
absolute quantity of protein. In this second period there was a
slight increase in the digestibility of the protein and, therefore, a
* Landw. Jahrb., 9, 651.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 199

corresponding increase in the urinary nitrogen (compare Chapter
Y), but this was small compared with the much greater amount of
work performed. Moreover, it did not, as in the first series of
experiments, augment from day to day during the period of severe
work. The following table shows the principal results of this
series, the figures for urinary nitrogen and for live weight being
given for the first and second halves of each period :

Work,
Kgm.

Nitrogen.

Live

Period.

Digested,
Grms.

In Urine,
Grms.

Weight,
Kg.

I

810,000

2,430,000
810,000

173.6

178.8

178.8

j 158.9
1 164.1
j 174.0
I 174.8
j 166.4
j 171.4

560.3

II

556.8
541.3

Ill

539.7
542.5

543 . 3

Series IV. — Upon the basis of the foregoing facts Kellner deter-
mined the maximum amount of work which his horse could perform
on a fixed medium ration without causing an increase in the proteid
metabolism. One kilogram of starch was then added to the ration
and the maximum amount of work that could be performed upon
this new ration without causing such an increase was determined.
In the nature of the case this determination could not be of the
highest accuracy, but it is amply sufficient for our present purpose.
The principal results are given in the following table, the amount
of work being expressed by the number of revolutions of the dyna-
mometer, since relative results are all that are required:

Work,
Rev.

Nitrogen.

Live

Period.

Digested,
Grms.

In Urine,
Grms.

Weight,
Kg.

I

Ua

116

Ill

IV

I

II

r

Without
starch

1 With I
f starch (

300
600
600
500
400

800
600

1
1

y 121.1
1

J l

j- 120.1 j

107.2 540.0
11C. 2 538.3
115.6 533.1

109.4 532.5
109.6 530.7

115.5 517.1

109.6 515.4

PRINCIPLES OF ANIMAL NUTRITION.

Kellner estimates that the maximum amount of work which
could be performed on the ration containing starch was 700 rev-
olutions as compared with a maximum of 500 revolutions without
starch. Even if this estimate of Kellner's be regarded as high, it is
evident from the figures given that the addition of the starch enabled
materially more work to be performed without an increase in the
proteid metabolism. The results obtained in this and the subse-
quent series have been made the basis of interesting computations
regarding the utilization of the potential energy of the food which
will be considered in Part II.

Series V. — This series was precisely similar to the preceding one,
except that the addition of non-nitrogenous matter to the ration was
made in the form of oil by substituting flaxseed for linseed meal.
The protein of the ration remained unchanged, while the fat was
increased by 203 grams. The results were entirely similar to those
with starch, as the following table shows :

Work,
Rev.

Nitrogen.

Live

Period.

Digested, In Urine,
Grms. Grms.

Weight,
Kg.

In

Without [
}■ addition <
of fat

With
)■ addition -i
j of fat 1

500
500
550
550

700
700
650
650

1 f

[ 159.0 -j

] 1
1 (

[ 153.9 -j

J I

148.9
149.2
147.5
153.0

148.1
153.9
145.6
145.0

496.5

16

493.2

Ha

485.8

116

la

16

479.4

476.0
469.0

Ua

466.4

116... .

400.8

While Kellner's method of investigation may be regarded as
somewhat imperfect and necessarily giving but approximate results,
yet it suffices to bring out in a very striking manner the intimate
relation existing between the supply of non-nitrogenous nutrients
in the food of a working animal and the effect of the work upon the
proteid metabolism. In conclusion, it should be noted that in
all Kellner's experiments there was a fairly abundant supply of
protein. Whether the same result would be obtained on a ration
containing the minimum amount of proteids required by the organ-
ism is not shown. In no case was the increase in the proteid metab-
olism, when observed, sufficient to supply energy equivalent to the
additional work done.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 201

Later Investigations. — In 1882 North * made experiments
upon himself in which a considerable amount of work, mainly walk-
ing from 30 to 47 miles while carrying a load of about 27 pounds,
was performed on one day of each experiment. The account of the
experiments does not give sufficient data for computing the total
amount of work performed, but it was evidently very considerable
and resulted in a marked increase in the excretion of nitrogen. It
is not possible, however, to determine whether the total food was
adequate for the work days, but it was no greater then than during
the periods of rest.

Argutinsky,f in experiments upon himself, observed as a result
of rather severe work a very marked increase in urinary nitrogen
which continued at least three days after the cessation of the work.
Munk X subsequently criticised Argutinsky's results on the ground
that the supply of non-nitrogenous nutrients in his diet was insuffi-
cient. Krummacher § obtained results quite similar to those of
Argutinsky, but his experiments are open to the same criticism as
those of his predecessor, namely, an insufficient supply of non-
nitrogenous nutrients, as he himself points out in a later paper.
Hirschfeldt || failed to observe any material increase in the nitrogen
excretion as the result of work upon a diet containing a considera-
ble excess of food over the amount required for maintenance. This
was true both upon a diet containing little protein and one abun-
dantly supplied with this nutrient.

Pfliiger, like Liebig, regards protein as the sole source of mus-
cular energy. As yet only a preliminary sketch of his investiga-
tions has been published. "f He fed a lean dog upon prepared lean
meat, that is. upon a nearly pure proteid diet, for seven months.
The animal remained apparently in perfect health and was able to
perform a large amount of work. Under the influence of the work
the excretion of nitrogen was observed to increase somewhat, but
not sufficiently to account for the energy expended in the work.
This phenomenon Pfliiger explains by supposing that during work

* Proc. Roy. Soc, 36, 14.

t Arch. ges. Physiol., 46, 552.

% Arch. f. (Anat. u.) Physiol., 1890, p. 552.

§ Arch. ges. Physiol., 47, 454.

U Virchow's Archiv., 121, 501.

H Arch. ges. Physiol., 50, 98.

202

PRINCIPLES OF ANIMAL NUTRITION.

the organism economizes in its demands for proteids elsewhere than
in the muscles. The further interesting observation was made that
with continuous work the proteid metabolism, which at first showed
an increase, diminished again and even reached its original value.
With a ration containing but little protein and much non-nitrogenous
material, a small increase of the proteid metabolism was observed
as the result of work. The preliminary account of the experi-
ments affords no adequate data for computing the sufficiency of
the total food.

Krummacher,* in his second investigation, made three separate
experiments. In the first of these the total food was estimated to
be approximately sufficient for maintenance (38 Cals. per kilogram),
while in the other two it was much in excess of this. The following
table shows the total amount of food per kilogram, expressed as
Calories of metabolizable energy, t the amount of work performed,
and the percentage increase of the proteid metabolism:

Energy of Food.

Work •

Measured,

Kgm.

Increase

Total.
Cals.

Per Kg.
Weight Cals.

Metabolism,
Per Cent.

Experiment I

II

Ill

2459
5034
5701

38
04
72

153,070
321.540
401,965

21

22

7

The work done consisted in turning an ergostat. It has been
shown by subsequent investigators that not over 30 per cent, of
the energy of the body material metabolized in the performance of
work in this way can be recovered in the work actually done.
Assuming this high figure, and further that Krummacher's esti-
mate of the maintenance requirements is accurate, it appears that
the food in these experiments was insufficient to supply the energy
required for the amount of work actually done.

It was observed, as in other experiments of this nature, that the
increased excretion of nitrogen continued for a day or two after the
cessation of the work. Only in the first experiment, however, was
even the total proteid metabolism during the periods of work, to-
gether with the excess above the rest value observed on succeeding
* Zeit, f. Biol., 33, 108. t See Chapter X.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 203

days, sufficient to supply an amount of energy equal to that
actually measured on the ergostat, so that at least the larger share
of the energy must have been derived from non - nitrogenous
materials.

Zuntz & Sehumburg.* in investigations upon soldiers, observed
an increase of the proteid metabolism as the result of marching,
carrying a considerable weight . The increase, however, seemed to
bear no direct relation to the amount of work performed, but rather
to the conditions under which it was done. Thus excessive heat or
sultriness of the atmosphere, resulting in unusual fatigue, was ac-
companied by an increased excretion of nitrogen. The increase
continued during the two days following the work.

Frentzel f experimented upon dogs. In the first series the ani-
mals were fed pure fat, while in the second series no food was given.
The work, which was done upon a tread power, was considerable.
In the first series there was an increase of 9.25 per cent, in the nitro-
gen excretion in the- work experiments, while in the second series a
maximum increase of 44.26 per cent, was computed, which, how-
ever, is believed by the author to be too high. A method of com-
putation which he considers more nearly correct makes the increase
in the second period 13.31 per cent. In the first scries of experi-
ments the food consisted of 150 grams of fat per day except upon
one of the work days, when only SO grams were consumed. No data
are given regarding the sufficiency of this ration, but according to
E. Yoit's compilation J it would appear hardly adequate for the
maintenance of a dog of the weight used (36 kilograms). The work,
therefore, even in the first series, was probably done upon insuf-
ficient food. In neither case was the increase in the amount of
protein metabolized equivalent in energy content to the actual
amount of external work done, and in the first series .even the total
proteid metabolism was not. while if we allow for the consumption
of energy in internal work, heat production, etc., it was not suf-
ficient in either series.

Atwater & Sherman £ have reported observations upon the

* Arch. f. (Anat. u.) Physiol., 1895, p. 378.

t Arch. ges. Physiol., 68, 212.

tZeit. f. Biol., 41, 115

§ U. S. Dept. A<rr., ( Iffice of Experiment Stations, Bull. 98.

204 PRINCIPLES OF ANIMAL NUTRITION.

food consumption, digestion, and metabolism of three bicyclers
during a six-day contest. They find that, in spite of an apparently
liberal diet containing large amounts of protein, all three riders
lost considerable proteid tissue during the race. The conditions of
the investigation were not such as to permit of a determination
of the sufficiency of the food consumed, but the computations by
Carpenter of the actual amount of work done seem to render it very
probable that the body fat must have been drawn upon to a con-
siderable extent.

Recapitulation. — The investigations above cited seem to show
beyond a doubt that when work is performed upon food less than
sufficient to maintain the body and supply the amount of energy
required for the work the proteid metabolism is somewhat increased.

Whether the converse of this is true, namely, that when the
food is sufficient such an increase in the proteid metabolism does
not occur, is not so clear, for the reason that in most, if not all, of
the cases we have no adequate data as to the sufficiency of the
food. It is plain, however, that the question is not so easily inves-
tigated as might appear at first sight, and that the final solution of
the relations of work to proteid metabolism can only be reached by
means of investigations in which the total metabolism both of matter
and energy is determined.

Gain of Proteids during Work. — Caspari and Bornstein have
recently made further investigations into the possibility of a gain
of* protein as a result of work which was mentioned above in con-
nection with Pfliiger's experiments.

Caspari * experimented upon a dog which received an amount
of food computed to have been fully sufficient for its maintenance
and to supply energy for the work done. Furthermore, a consider-
able portion of the non-nitrogenous nutrients of the ration, consist-
ing largely of carbohydrates, was given shortly before the work was
done, while in some cases additional sugar or fat was given at that
time. In the first experiment, work was performed upon three
successive days. Upon the second of these there was a consider-
able increase of the urinary nitrogen, but upon the third its amount
fell below that of the rest period. The average for the three days
of work was almost exactly equal to the value found for the last

* Arch. ges. Physiol., 83, 509.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 205

day of the rest period and less than the average for the four pre-
vious rest days.

In the second experiment the work was continued for four days,
then a rest day intervened, and then the work was continued for
five more days. At the outset there was a slight increase of the
proteid metabolism, but in the second period of five days it showed
a marked decrease resulting in a progressive gain of nitrogen by
the body, as is shown in the following tabular statement of the daily
averasre results:

Day.

External
Work
Done,
Cals.

Nitrogen

of Food,

Grms.

Average

Nitrogen

of Feces,

Grms.

Nitrogen

of Urine,

Grms.

Gain or

Loss of

Nitrogen,

Grms.

1-5

5- 7

0

0

0

597

467

597

596

0

595

590

593

588

586

25.11
25.11
25.11
25.11
25.11
25.11
25.11
25.11
25.11
25 . 1 1
25.11
25.11
25.11

1.89
1.89
1.89
1.78
1.78
1.78
1.78
1.78
1.78
1.78
1.78
1.78
1.78

23 . 68

22.00

21.98

24.72 (?)

23.32

23.23

21.83

22.06

20.82

19.64

20.39

19.87

19.79

' -0.46
+ 1 22

7-8

8-9

+ 1.24
— 1 39

9-10

10-11

11-12

12-13

13-14

14-15.

+ 0.01
+ 0.10
+ 1.50
+ 1.27
+ 2.51
+ 3.69
+ 2.94
+ 3 46

15-16

16-17

17-18

+ 3.54

This gain Caspari ascribes to an actual growth of the muscles as
the effect of exercise, this growth according to him taking the form
of a hypertrophy of the fibers. No determinations of the gain or
loss of carbon were made.

Bornstein,* who had previously investigated the possibility of
increasing the store of proteids in the body by the addition of pro-
teids to the food, has also contributed to the investigation of this
phase of the question. His experiments were made upon himself.
For seven days lie consumed a uniform ration containing a moder-
ate amount of protein and sufficient non-nitrogenous nutrients,
according to previous experience, to maintain his body. The latter
was in equilibrium with the food as regards nitrogen from the first
day. Then the proteid supply was increased by approximately
50 per cent, by the ingestion of pure proteids and light work (17,000

* Arch. ges. Physiol., 83, 540.

206 PRINCIPLES OF ANIMAL NUTRITION.

kgm. per day) done by turning an ergostat. As a result of the in-
creased supply of proteids in the food the proteid metabolism in-
creased promptly, reaching its maximum upon the fifth day, when
it very slightly exceeded the supply. From that time, however,
it decreased gradually during the remaining thirteen days of the
experiment, so that a gain of proteids by the body resulted, which
was still in active progress when the experiment was discontinued.
Counting from the time when the proteid metabolism reached its
maximum the average gain of nitrogen per day was

First five days 1 . 28 grams

Last five days 2.06 "

Average of all 1 . 475 "

The author computes that 22 per cent, of the proteids added to
the food was stored up in the body. In a previous similar experi-
ment without work it was found that only 16 per cent, was thus
stored.

Two respiration experiments with the Zuntz apparatus were
made during the work. The difference between their results and
those of similar experiments during rest was used as the basis for
computing the actual amount of energy metabolized in the body for
the performance of work. This was found to be equal to 0.0100875
Cal. per kgm. external work, which is equivalent to 171.5 Cals. for
the whole daily work of 17,000 kgm. Assuming the original ration
to have been a maintenance ration, Bornstein computes that the
portion of the added proteids which was actually metabolized was
insufficient to supply the energy necessary for the work done and
that some of the fat of the body was drawn upon. The loss in
live weight was found to agree with this assumption.

The above investigations seem to show, not only that work may
be done without increasing the proteid metabolism but that it may
actually result in diminishing it, a fact which appears in harmony
with the common observation that the tendency of exercise is to
build up the muscular tissue.

Summary. — While the results which have been cited are not in
all respects conclusive, and while further investigation is required
to fully elucidate the relations of muscular exertion to proteid metab-
olism, the following general conclusions seem to be justified by the
evidence now available:

INFLUENCE OF MUSCULAR EXERTION UION METABOLISM. 207

1. The non-nitrogenous ingredients of the food or of the tissues
are the chief source of muscular energy. In by far the gr< ;ater
number, if not all, of the experiments upon this subject the amount
of protein metabolized, as measured by the nitrogen excretion, was
insufficient to furnish energy equivalent to the work done, the de-
ficiency being in many cases very great . This statement, it will be
observed, does not assert that the proteids are not concerned in the
production of this energy. We may regard it as very probable that
the non-nitrogenous matter metabolized has first entered into the
structure of the muscular protoplasm, which, as we know, consists
largely of proteids. but in a contraction it is largely, if not wholly,
the non-nitrogenous groups contained in the protoplasm which are
metabolized rather than the nitrogenous groups.

2. With insufficient food there may be a considerable increase
in the proteid metabolism as a result of muscular exertion, espe-
cially when pushed to exhaustion.

3. This increase is far from sufficient to supply energy for the
work actually done, is not usually proportional to it, and seems

ident to a considerable degree upon accompanying conditions.

4. With sufficient food the increase of the total proteid metab-
olism consequent upon muscular exertion is at the most slight and
possibly equal to zero.

5. In some cases a storage of proteids has been observed to
result from the performance of work.

Functions of Proteids. — If the above conclusions are admitted,
it is possible to suppose that in a muscular contraction under favor-
able conditions — that is, when there is an abundant supply of non-
nitrogenous material— there is no increased metabolism of the
ids. This view of the subject would regard the question as
being simply one of the relative supply of nutrients, the energy
being evolved from non-nitrogenous nutrients when these are in
abundance, while in default of them the proteids are drawn upon.

Another view of the subject, however, is possible, and perhaps
more probable. It would appear that muscular exertion tends to
produce two opposite effects upon the proteid metabolism: first,
to break down additional protein, as is shown when work is done
upon insufficient food; and second, to build up proteid tissue when

208 PRINCIPLES OF ANIMAL NUTRITION.

the food is sufficient, as is illustrated in the experiments of Caspari
and Bornstein.

As a basis for a tentative hypothesis, it seems allowable to sup-
pose that both these processes — that of anabolism and katabolism
of proteids — are continually taking place in the muscle and that
both are exaggerated by exercise. In other words, we may imagine
that the performance of work by a normally developed muscle
requires an increased proteid katabolism, which is balanced, at least
in the course of the twenty-four hours, by a corresponding increase
in the proteid anabolism. With a liberal supply of food proteids,
then, a part of the latter would, during rest, simply undergo nitro-
gen cleavage and be used virtually as " fuel," but when work was
done they (or part of them) would be used to replace the proteids
katabolized in the muscles. Upon this hypothesis, the proteids
might play a not unimportant part in the production of muscular
work without any evidence of it appearing in an increased nitrogen
excretion. It is to be remarked, however, that even on this suppo-
sition the proteids could not be regarded as furnishing all, or even,
in many cases, a large share, of the energy liberated. On insuffi-
cient food, the hypothesis would assume that the energy supply is
deficient and that proteids which would otherwise be used for
muscular anabolism are diverted to use as "fuel," probably under-
going a preliminary nitrogen cleavage and furnishing their non-
nitrogenous residue to the muscles as a source of energy.

The above tentative hypothesis implies that if work were per-
formed upon a ration containing only the minimum amount of
proteids required during rest, it would cause an increase of the
proteid metabolism, no matter how much non-nitrogenous mate-
rial was supplied, because there would be no proteids available
which could be diverted to repair the waste assumed to be occa-
sioned by muscular activity. Up to the present time, however,
we possess no experimental investigation of this phase of the ques-
tion.

However this may be, we know that the performance of work
requires a well-developed muscular system. To produce and de-
velop such a system, a liberal supply of protein is essential, while
we may reasonably suppose that to maintain it involves a larger
proteid supply in the food than is required to maintain the proteid

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 209

tissue on a lower level. This fact alone would indicate the need of
a reasonably liberal supply of protein in the food of working animals.
If the hypothesis above outlined be approximately correct, it is
necessary that the food also contain protein which during rest may
be simply a source of heat, but which during work may be diverted
to repair the increased waste of nitrogenous tissues caused by ex-
ertion. This accords with the well-established fact that the dieta-
ries selected by athletes and others who undertake severe physical
exertion are almost invariably rich in protein.* It is of course
difficult to say how far the large amount of proteids in the dietaries
of athletes represents a real physiological demand and how far it is
a matter of tradition or of taste, but it hardly seems likely that so
universal an opinion should be lacking in some considerable basis
of fact.f

Effects upon the Carbon Metabolism.

In the foregoing paragraphs we have seen that as a rule the
total proteid metabolism is not much affected by muscular exertion.
While proteids undoubtedly have important functions in connection
with the production of work, it is nevertheless true that normally
the energy liberated in muscular contraction is derived chiefly or
wholly from the breaking down of non-nitrogenous material.
Moreover, even in those cases in which a considerable increase of
the proteid metabolism has been observed, its amount has been
entirely insufficient to account for the extra evolution of energy.
It therefore becomes of especial importance to consider the effects
of work upon the carbon balance.

The Gaseous Exchange. — Since the influence of muscular ex-
ertion upon the proteid metabolism is at most small, it is possible to
compare the carbon metabolism during work and rest without
material error upon the basis of the gaseous exchange simply, and
a- a matter of fact a large share of our knowledge of the subject

3 upon determinations of the respiratory exchange.

Is Largely Increased. — The fact that muscular work largely
increases the evolution of carbon dioxide and water and the con-
sumption of oxygen by the organism is too familiar from ordinary

* For a summary of American experiments bearing upon this point see
Atwater & Benedict, Ho-ton Medical and Surgical Journal, 144, G01 and 629
1 ompare, however, Chittenden, Physiological Economy in Nutrition,
New York, Stokes Co., Idol.

2IO

PRINCIPLES OF ANIMAL NUTRITION.

experience and too well established scientifically to require more
than illustration. The fact of such an increase was shown in the
researches of Lavoisier. Scharling,* who as early as 1843 con-
structed an apparatus somewhat like the Pettenkofer respiration
apparatus (see p. 70), states in his account of his experiments
that moderate work increases the excretion of carbon dioxide and
that it is also greater shortly after a meal. Of other early researches
upon this point may be mentioned those of Hirn f in 1857, and
especially those of Smith J in 1859. The investigations of Petten-
kofer & Voit § in 1S66 appear to have been the first to be executed
in accordance with modern methods. Their results have already
been cited in their bearing upon the influence of work on proteid
metabolism, but may be repeated here:

Nitrogen

of Urine,

Grms.

Carbon

Dioxide

Excreted,

Grms.

Water Excreted.

Oxygen

Taken

Up.

Grms.

Number
of Experi-
ments.

Tn
Urine.
Grms.

Evapo-
rated.
Grms.

Fasting :

Rest

12.4
12.3

17.0
17.3

716
1187

928
1209

1006
746

1218
1155

821
1777

931
1727

762
1072

832
981

2

AYork

1

Average diet:
Rest

3

Work

2

Subsequent investigatory such as Speck, || Hanriot & Richet,*|T
Katzenstein,** Loewy,-j-f p-nd many others have fully confirmed the
results of the early experimenters. The increase in the oxygen
taken up was not actually demonstrated in all of these experiments,
but it was in some and may be reasonably inferred in the remainder-

* Ann Chem Pharm . 15, 214

f Comptes rend rfo" de Physique de Colmar, 1857; Revue Scientifique,
ler Semestre. 1887.

J Phil. Trans. 18*&, p 681.

§ Zeit f Biol., 9, 478

|| Schriften de*" Gesell. der ges Naturwiss zu Marburg, 1871; Arch, klin,
Med , 45, 461.

1 Comptes '•end., 104, 435 and 1865; 105, 76; Ann. de Chim et de Phys.,
(6), 22, 485.

** Arch £es Physiol., 49, 330.

tt Ibid , 49, 405.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 211

Effects are Immediate. — Experiments like those of Petten-
Icofcr A: Yoit, extending over twenty-four hours, give simply the
total effect of the performance of work upon the carbon balance.
By the use of the Zuntz type of apparatus, however, it is possible
to follow the gaseous exchange in its details through successive
short periods as well as to determine the amount of oxygen con-
sumed. The data thus obtained give a clear picture of the imme-
diate effects of work upon metabolism and have led to the extensive
use of this type of apparatus in experiments of this nature. The
results of these experiments agree with common experience in
showing that these effects appear very promptly and soon reach
their maximum, disappearing as promptly after the work ceases.
In other words, the increase in the carbon metabolism is very
closely confined to the time during which the work is actually
performed.

The Respiratory Quotient. — The ratio between carbon dioxide
produced and oxygen consumed, commonly known as the respira-
tor}- quotient, as has been pointed out in Chapter III, enables us
to form a fairly clear idea as to the general nature of the total mate-
rial metabolized, and hence much study has been bestowed upon
the relation between these two quantities.

Is Variable.- — We have already seen that the respiratory quo-
tient may vary considerably during repose, being largely deter-
mined by the nature of the food. The same thing is true of the
respiratory quotient during work.

Zuntz,* in experiments on a fasting dog, obtained the follow-
ing values for this quotient:

Rest: Standing

" Lying

Horizontal locomotion
Locomotion up hill. . .
Horizontal draft

Number of
Experi-
ments.

Average

Respiratory

Quotient.

2

0.69

6

0.71

8

0.73

5

0.77

10

0.77

In Zuntz & Hagemann's f experiments upon the horse the respi-
ratory quotient in the single work periods ranged from 0.729 upon a

♦Arch. ges. Physiol., 68, 191.

t Landw. Jahrb., 27, Supp. Ill, 296-331.

212 PRINCIPLES OF ANIMAL NUTRITION.

ration of green alfalfa to 0.996 upon hay, straw, and oats. The
averages obtained for different forms of work were as follows :

Walking, nearly horizontal 0 . 865

" up a slight incline 0.847

" " " steeper incline 0.900

Draft, nearly horizontal 0.890

Walking with load, horizontal 0.840

" up incline 0.893

Trot, nearly horizontal 0 . 882

" with load, nearly horizontal 0 . 873 •

" horizontal draft 0.927

The total range of the respiratory quotient in these experiments
was 0.84 to 0.93. It is thus seen to be higher with herbivorous
animals, subsisting largely upon carbohydrates, than with the dog.

Change Caused by Work. — Chauveau states as the result of his
investigations upon the origin of muscular power that the per-
formance of work always increases the respiratory quotient.

His first experiments * were made upon a man who had fasted
for sixteen hours. The work consisted in the alternate ascent and
descent of a staircase, the work of ascending being equal to about
29,000 kgms. in the seventy minutes of the experiment. Samples
of the expired air were taken by the Tissot apparatus f for five
minutes at a time at intervals during the work and the respiratory
quotient determined by a comparison of its composition with that
of the normal atmosphere. The following were the results for the
respiratory quotient:

Immediately before work 0 . 75

First to fifth minute 0 . 84

Tenth to fifteenth minute 0 . 87

Fortieth to forty-fifth minute 0 . 95

Sixty-fifth to seventieth minute 0. 74

♦Comptes rend., 122, 11G3.

f Archives de Physiol., 1896, p. 563. The apparatus is of the Zuntz type.

INFLUENCE CF MUSCULAR EXERTION UPON METABOLISM- 213

A second experiment,* begun after fifteen hours' fasting, was
divided into two periods. The first was similar to the previous
experiment, but lasted for thirty minutes only, the work of ascent
equaling in that time about 30,000 kgms. The subject then rested
for a time during which he consumed 105 grams of butter. Two
hours after the ingestion of the butter the experiment was repeated,
samples of the expired air being taken for three minutes at a time
The results as regards the respiratory quotient were as follows:

Fasting.

Three minutes before beginning work 0 . 706

Twelfth to fifteenth minute 0 . 804

Twenty-seventh to thirtieth minute 0.812

Rest 0.812

Two Hours after Ingestion of Butter.

Three minutes before beginning work 0 . 666

Twelfth to fifteenth minute 0 . 783

Twenty-seventh to thirtieth minute 0.809

In conjunction with Laulanie t he has also experimented on
dogs and rabbit 3, the muscular contractions being caused by electric
shocks. The method of determining the respiratory exchange, as
described by Laulanie, consisted in using a Pettenkofer type of
apparatus with a small but constant known rate of ventilation.
The outgoing air passed through a small gasometer, but the current
could be shunted and the sample of air contained in the gasometer
analyzed. No details of the experiments or of the methods of cal-
culation are given. The first table on the following page contains
Laulanie's summary of the results. §

An even greater increase in the respiratory quotient has been
observed by other investigators. Thus Hanriot & Richet || found

*Comptes rend., 122, 1169

t Ibid., 122, 1244, 1303: Archives de Physiol . 1896, p. 572.

X Archives de physiol . 1896, pp. 619 and 636.

§ Energetique Musculaire, p. 70

|| Comptes rend., 104, 435 and 1865: 105, 76.

214

PRINCIPLES OF ANIMAL NUTRITION.

Food.

No. of
Expta.

Respiratory Quotient.

Before
Work.

During
Work.

After
Work.

Rabbit . .
Dog ... .
Dog ... .

Ad libitum

7
5
2

0.880

0.776
1.016

0.970
0.849
1.027

0.799
0 733

Fasting from 1 to 7 davs..

Abundantly fed with milk porridge.

1.033

in the increments of carbon dioxide and oxygen over the rest values
quotients much greater than unity and reaching in one case 3.5 (?).
Speck * likewise found an increase in the respiratory quotient as
the result of work. Although he observed numerous exceptions,
he regards it as the rule that it increases with the severity of the
work.

On the other hand, Katzenstein,f in experiments on men, found
in some cases no considerable increase in the respiratory quotient
during work. He gives the following average results, of which
those in the first table do not relate to exactly the same subjects
in the three cases:

Turning Ergostat.

Repose 0. 754

Light work 0.824

Heavy work 0.823

Walking.

Subject
No. 1.

Subject
No. 2.

Subject
No. 3.t

Subject
No. 4.

Repose

0.801
0.805

0.799

0.73
0.77
0.79

0.77
0.82
0.865

0.75

Horizontal locomotion

0.895

Locomotion up hill

0.86

In all cases, the determinations of the respiratory exchange
covered only a few minutes soon after the work began, and

* Arch. klin. Med., 45, 461.
t Arch. ges. Physiol., 49, 330.
X A very corpulent individual.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 215

no mention is made of the nature of the diet except in one case
(fasting). The individual results were rather variable, but most
weight is given to those on Subject No. 1, with whom most of the
experiments were made. Katzenstein believes Speck's results to
be due in part to a change in the rate of respiration, causing the
excretion of carbon dioxide to exceed its actual production (p. 73),
and in part to a deficiency of oxygen in the tissue of the contracting
muscles.

Loewy,* like Katzenstein, found that work pushed to the point
of producing a considerable degree of fatigue raised the respiratory
quotient, while moderate work did not. Rapid turning of the
wheel of the ergostat, preventing full breathing, or compression
of the upper arm by means of a rubber band, produced the same
effect, which he attributes to a lack of oxygen. The most marked
results seem to be those for the first few minutes of work, although
in one case work continued for ten to twenty minutes and producing
fatigue raised the respiratory quotient.

Probably the most extensive and carefully conducted investiga-
tions of this nature are those of Zuntz and his associates upon the
dog, and particularly on the horse. Some data from the latter
investigations have already been given on p. 212. The following
table adds to the averages there quoted those for the corre-
sponding rest periods. In these experiments there was a distinct
lowering of the respiratory quotient instead of an increase. In

Kind of Work.

Periods.

Respiratory Quotient.

Repose.

Work.

Walking nearly horizontal

a, b, e, /, i, 0

". b, e, 0

a,b,e, /, i, n

b, e, f, i

e, i, 0

e, i, 0, 0

a, e, /, 0

e, i, 0

9,f,i

0.943
0.940
0.953
0.956
0.915
0.915
0.943
0.915
0.943

0.865

up slight grade

" steeper grade

0.847
0 900

Draft, nearly horizontal

0 890

Walking with load, nearly horizontal.. .

" up a grade

Trot, nearly horizontal

0.840
0.893

0 882

" " " with load

" horizontal, with draft

0.873
0 927

* Arch. ges. Physiol., 49, 405.

2l6

PRINCIPLES OF ANIMAL NUTRITION.

all cases the animal was liberally fed, usually with oats, hay, and
cut straw.

Variation during Work. — In their experiments cited above,
Chauveau & Laulanie find that the rise of the respiratory quotient
which they regard as the invariable result of muscular exertion
occurs promptly upon the beginning of the work, and the same thing
is shown by the earlier results of Chauveau. As the work is con-
tinued, however, the quotient shows a tendency to fall again, some-
time even going below its original rest value, while in a period of
rest following work a still further decrease is observed. The
results of their experiments * are contained in the table on the
opposite page.

Zuntz & Hagemann f also report a number of experiments on
the horse in which the respiratory exchange was determined in suc-
cessive periods of work. The following are their results for the
respiratory quotient :

No. of

Successive Values of Respiratory Quotient.

Aggregate Length
of Work Periods,

Experiment.

1

2 3

Min.

37

38

.917
.913
.929
.925
.920
.865
.928
.910
.974
.863
.911
.949
.936
.931

.865
.806
.889
.948
.931
.868
.921
.926
.905
.820
.922
.934
.909
.904

897

875
911

837

871

878
883

80
121J

41

42

102
100

43

45

92£
54

46

34

47

48

58

65

63

66

73

71

67

78

124

78

96

75

The results cited in the foregoing paragraphs would appear to
justify the general conclusion that in the case of fasting animals or

* Comptes rend., 12?, 1244.
t hoc. cit., pp. 290-292.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 217

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218 PRINCIPLES OF ANIMAL NUTRITION.

of those insufficiently fed the respiratory quotient is increased by
the performance of work, while with well-fed animals, especially
those receiving an abundance of carbohydrates, this effect is not
apparent. As the work is continued, there appears in many cases
to be a tendency toward a diminution of the quotient, while in rest
following work a still further decrease may occur.

Nature of Non-nitrogenous Material Metabolized. — As already
pointed out, a comparative study of the final products of metab-
olism during rest and work does not itself afford direct evidence
as to the nature of the material actually metabolized in a muscu-
lar contraction, but simply shows the total effect of the contraction
itself and of the secondary activities resulting from it upon the
make-up of the schematic body. When we attempt to go further
than this, other methods of investigation are requisite, although
experiments like those already cited may afford important con-
firmatory evidence.

Conclusions from Respiratory Quotient. — The significance
of the respiratory quotient in experiments upon work has already
been illustrated in Chapter III (p. 76). Neglecting any slight error
due to small changes in the proteid metabolism, the variations in
the respiratory quotient as outlined in the foregoing paragraphs
enable us to trace the corresponding changes in the nature of the
carbon metabolism.

The metabolism of a fasting animal at rest is, as was shown in
Chapter IV, largely a metabolism of fat. Corresponding to this,
the respiratory quotient of such an animal approaches the value
0.7 for pure fat, although never quite reaching it, since some pro-
tein is always metabolized. Numerous instances of this fact are
seen in the experiments already cited. When such an animal per-
forms work, the respiratory quotient has been found to increase
materially, thus showing that, in addition to the fat, carbohydrate
material is being metabolized. This is entirely in accord with the
well-established fact that muscular exertion causes the glycogen,
both of the muscles and of the liver, to decrease and even disappear
entirely. With an animal at rest and liberally supplied with car-
bohydrate food, on the other hand, the respiratory quotient ap-
proaches or even reaches unity, showing that the metabolism is
essentially carbohydrate in character. When work is required of

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 219

such a subject, little change is noted at first in the respiratory quo-
tient. The cells of the body being richly supplied with carbohy-
drates apparently utilize these as the most readily available source
of energy. In either case, however, continued work makes large
demands upon the non-nitrogenous materials available, the store
of carbohydrates in the body is rapidly depleted, and the fat of the
body is drawn upon to an increasing extent as a source of energy,
the necessary result being a diminution in the respiratory quotient.
In the experiments of Chauveau & Laulanie only the respira-
tory quotients corresponding to the total metabolism are given, and
consequently the changes in the character of the metabolism indi-
cated above can only be traced qualitatively. In Zuntz & Hage-
mann's investigations the increments of the carbon dioxide and
oxygen over the rest values are given, and from them the propor-
tion of oxygen applied respectively to the oxidation of fat and of
carbohydrates is computed. The following average results for the
various forms of work show clearly that the ratio of fat to carbo-
hydrates metabolized may vary through a very wide range.

Kind of Work.

Periods.

Oxygen per Minute
applied to the Oxida-
tion of

Fat,
c.c.

Carbohy-
drates, c.c.

Walking; nearly horizontal

a, b, e, f, i, 0

a, b, e, 0

a, b, e, f, i, n

b. e, /, i
e, 1, 0

e, i, 0, 0
a, e, }, 0

e, i, 0

g, f, i

4.3638

10.433

8.665

8.882

5.962

8.525

7.852

12.718

14.007

2 9962

" up a slight grade

7 465

" " " steeper grade

15 215

Draft, nearlv horizontal

12 992

Walking with load, nearly horizontal . .
" up a grade

3.317

14 892

14 201

" with load, nearly horizontal

" " draft, horizontal

16.023
45 050

The Intermediary Metabolism. — As stated, the conclusions
drawn from the respiratory quotient relate, strictly speaking, to
the total effect of muscular exertion upon the store of matter in the
body. The results of such experiments show that, as a consequence
of a given amount of work, a certain quantity of fat and of carbo-
hydrates has been oxidized somewhere in the organism.

220 PRINCIPLES OF ANIMAL NUTRITION.

Many eminent physiologists, however, notably Zuntz and his
pupils, go further and regard both the fat and the carbohydrates of
food or body tissue as immediate sources of muscular energy and as
of value for this purpose in proportion to their content of potential
energy — that is, to their heats of combustion. In other words, t hey
hold that either fat or carbohydrates may be in effect directly
metabolized by the muscular tissue and that each under like condi-
tions yields substantially the same proportion of its potential energy
in the form of mechanical work.

On the other hand, Chauveau * and Seegen f and their followers,
as has already been indicated, regard the carbohydrates as the im-
mediate source of energy for all the vital activities and hold that fat
(or proteids) must first be converted into dextrose by the liver before
it can be utilized. It is particularly with regard to muscular exer-
tion that this theory has been elaborated, the conclusions as to other
forms of vital activity being to a considerable extent based upon
analogy with the former.

Functions of the Liver. — According to this theory the material
which is actually metabolized in a muscular contraction is a carbo-
hydrate, viz., either the dextrose carried to the muscle by the blood
or the glycogen which is stored up in it. Muscular activity is thus
brought into intimate relations with the sugar-forming function of
the liver, and a chief office of that organ is considered to be the
preparation of the necessary carbohydrate material from the various
ingredients of the food. The main facts which have been estab-
lished may be summarized as follows (compare Chapter II, §§ 1
and 2) :

1. Dextrose is being constantly formed by the liver, which not
only modifies the carbohydrates of the food but likewise appears to
produce dextrose from proteids and particularly, according to this
school of physiologists, from fat.

2. Dextrose is as constantly being abstracted from the blood by
the tissues, particularly the muscular tissues, as is shown by the
constancy of the proportion of dextrose in the blood.

3. The dextrose content of the blood is, according to Chauveau,

* La Vie et l'Energie chez l'Animale.
t Die Zuckerbildung im Thierkorper.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 221

maintained during fasting until the very last stages of inanition.
When it finally disappears there is a rapid fall in the body temper-
ature and death speedily follows.

4. Both the production of dextrose by the liver and its con-
sumption in the tissues appear to be augmented by muscular exer-
tion.

The latter fact is shown by the well-known experiments of
Chauveau & Kaufmann * upon the masseter muscle of the horse.
Comparing the amount of blood passing through the muscle and
the decrease in its percentage of dextrose in rest and in work they
found that the consumption of dextrose in the two cases was in the
proportion of 1 : 3.372. Subsequent experiments f upon the Leva-
tor labii superioris of the horse, the results of which as to the gaseous
exchange have already been cited (p. 188), gave the following
figures for the dextrose abstracted from the blood per kilogram of
muscle in one minute :

Rest, Grms.

Work, Grms.

Work -5- Rest.

Experiment 2

! 0.00598 (?)

0.07026 (?)
0.22303

0.12852

11.75

3

4

0 . 06358

0.03976 (?)

3.51
3.23

Average

0.03644

0.14027

3.85

The authors also call attention to the fact that in these two
series of experiments the arterial blood supplied to the active muscle
contained a higher percentage of dextrose than that supplying the
same muscle in a state of repose, notwithstanding the consumption
of this substance by the muscle, and conclude that muscular activ-
ity stimulates the production of dextrose by the liver. The observa-
tion of Kiilz,]: that prolonged muscular exertion may cause the dis-
appearance of glycogen from the liver, may perhaps be interpreted
as sustaining this conclusion.

* Comptes rend., 103, 974, 1057, 1153.
t Ibid., 104, 1126, 1352, 1409.
j Arch. ges. Physiol., 24, 41.

222 PRINCIPLES OF ANIMAL NUTRITION.

Muscular Glycogen. — Especial interest attaches in this connec-
tion to the behavior of the glycogen of the muscles. Nasse *
appears to have been the first to show that the muscular glycogen
is consumed during contraction. This result has been abundantly
confirmed by other investigators, notably by Weiss, f while, as just
stated, Kiilz has shown that the same thing is true of the glycogen
of the liver.

It has also been shown that glycogen accumulates in muscles
whose activity has been suspended by section of their nerves or other-
wise. An early statement to this effect, unaccompanied by experi-
mental proof, is by MacDonnel.J Chandelon § investigated the
influence upon the glycogen content of the hind leg of a rabbit of,
first, ligature of the arteries, and second, section of the motor nerves.
The first treatment caused a large loss and the second a large gain
of glycogen. Morat & Dufourt | confirmed these results and also
found that the formation of muscular glycogen was more rapid in a
fatigued quiescent muscle than in a normal one, while Aldehoff *["
has shown that in a fasting animal glycogen persists longer in the
muscles than in the liver and reappears first in the former when food
is given.

In view of these facts it can hardly be doubted that the muscu-
lar glycogen is in some way a source of energy to the muscles, being
destroyed during contraction and stored up again during rest.

Chauveau's Interpretation. — By a comparison of their results for
dextrose just cited on p. 221 with those for the gaseous exchange
of the muscle as given on p. 188, Chauveau & Kaufmann show that
during rest there was a storage of dextrose and of oxygen in the
muscle. During work, on the contrary, more carbon dioxide was
produced by the muscle than corresponded to the amount of dex-
trose which was abstracted from the blood, and this carbon dioxide
contained more oxygen than was supplied to the muscle by the

* Arch. ges. Physiol., 2, 97; 14, 482.

t Sitzungsber. Wiener Akad der Wiss., Math-Nat. Klasse, 64, II, 284.

% Proc. Roy. Irish Acad., Ser. I, 7, 271.

| Arch. ges. Physiol _, 13, 626.

|j Archives do Physiol , 1892, 327 and 457.

«[ Zeit. f. Biol., 25, 137.

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 223

blood during the same time. The average results, computed in
milligrams per minute, were:

During Rest, During Work,
Mgrms. Mgrins.

Oxygen from blood

in C02 produced

" required to oxidize dextrose taken up from

blood

Carbon of COa produced

" " dextrose taken up

0.11803
0.08424

0.58305
0.03160
0.21862

2.48490
3.15052

2.35055
1.18128
0.88118

During rest the muscle was storing up both carbohydrate (gly-
cogen) and oxygen, thus supplying itself with a reserve of potential
energy. During activity this reserve, as well as the supply brought
by the blood, was drawn upon for the performance of work.

The fluctuations of the respiratory quotient resulting from mus-
cular exertion are explained by Chauveau in outline as follows:

At first there is a rapid oxidation of the stored glycogen of the
muscles and of the dextrose of the blood, resulting in a respiratory
quotient approaching unity. As the work progresses the store of
carbohydrate material in the organism becomes relatively exhausted,
unless there is a large supply of it in the food, and to meet the
demands of the muscles an increased production of dextrose from
the fat of the food or of the body takes place in the liver. This
change, however, according to the equation proposed on p. 38, con-
sumes 67 molecules of oxygen for each 18 molecules of carbon diox-
ide produced. This process, superadded to the combustion of
carbohydrates in the muscles, results in the observed lowering of
the respiratory quotient. The further lowering of the quotient
during a succeeding rest period results from the great diminution
in the amount of carbohydrates oxidized in the muscles, the for-
mation of carbohydrates from fat in the liver still continuing
for a time in order to replenish the exhausted store of muscular
glycogen.

Fat as a Source of Muscular Energy. — According to the above
theory, fat is only indirectly a source of muscular energy, in
that it serves for the production of dextrose in the liver, and the

224 PRINCIPLES OF ANIMAL NUTRITION.

same thing is held to be true of protein so far as it contributes
energy for muscular exertion.

As we have seen in Chapter II, however, the formation of dex-
trose from fat in the liver is by no means universally admitted, and
Chauveau's ingenious theory as to the immediate source of muscu-
lar energy has not lacked opponents. If it is true, fat has a much
lower value for that purpose than corresponds to its potential
energy as measured by its heat of combustion. If it be assumed
to be converted into dextrose in accordance with the equation on
p. 38, it is easy to compute that about 36 per cent, of its potential
energy will be liberated as heat in the process and that consequently
only the 64 per cent, remaining in the resulting dextrose will
be available to the muscles. Consequently the relative values of
fat and dextrose for the production of work will be as 162 to 100
and not as 253 to 100.

While the evidence of the respiratory quotient is not incon-
sistent with Chauveau's theory, it is also not inconsistent with the
view which supposes fat to be directly metabolized for the produc-
tion of mechanical work. The difference lies, not in the amounts
of carbon dioxide and oxygen evolved but in the place where and
the form in which the energy is liberated, and the question can
therefore be satisfactorily discussed only on the side of its energy
relations.

Postponing that discussion for the present, it may be remarked
here that while it appears to be true, as already stated, that the
muscular glycogen and the dextrose of the blood are a source of
muscular energy, and perhaps the most readily available one, it
by no means follows that they are the only source. The muscle
contains other non-nitrogenous reserve materials besides glycogen,
and notably a not inconsiderable amount of fat and of lecithin;
Moreover, recent investigations (see pp. 63 to 05) have shown that
the amount of the muscular fat is greater than was formerly sup-
posed, and that some of it cannot be extracted with ether and
behaves almost as if in chemical combination. Indeed, it appears
not improbable that both fat and carbohydrate molecular groupings,
as well as proteids, enter into the structure of living protoplasm.
Finally, not only the muscle but the blood which nourishes it
contains fat as well as carbohvhrates, the former indeed being more

INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM- 225

abundant than the latter. There would seem to be no inherent
difficulty, then, in supposing that the fat of the muscle and of the
blood serves directly as a source of energy, although the writer is not
aware of any investigations upon the influence of the contraction
of a muscle upon its fat-content.

PART II.

THE INCOME AND EXPENDITURE OF ENERGY.

CHAPTER VII.
FORCE AND ENERGY.

Force is defined as whatever is capable of changing the rate of
motion of a mass of matter. When a force acts upon a mass, im-
parting to it a certain velocity, it does work, the amount of work
being measured by the product of the force into the distance through
which it acts. Energy may be defined as the capacity to do work.
Any mass of matter which can act upon another mass in such a
way as to change its rate of motion is said to possess energy.

Kinetic and Potential Energy. — In studying energy we
distinguish between kinetic energy, or the energy due to motion,
and potential energy, or the energy due to position. The falling
weight of a pile-driver at the instant it strikes the pile possesses a
certain amount of kinetic energy and does a corresponding amount
of work on the pile. When it is raised again a certain amount of
work is done on it, and when it comes to rest at the top of the ma-
chine a corresponding amount of energy is stored up in it as poten-
tial energy. As long as the weight is supported at this point it
does no work, but simply possesses the possibility of doing work.
When it is allowed to fall again, this potential energy due to its
position is converted into the actual or kinetic energy of motion,
and when it reaches the point from which it was raised and strikes
the pile it does work upon the latter exactly equal to that formerly

226

FORCE AND ENERGY. 227

stored up in the weight as potential energy, which again was equal
to the energy expended in raising it.

An even simpler example of the conversion of potential into
kinetic energy and vice versa is a swinging pendulum. When at
rest for an instant at the end of a vibration it possesses a certain
amount of potential energy, corresponding to its vertical height
above the lowest point of its arc. When it reaches this lowest,
point, so far as the mechanism of which it forms part is concerned
it has no more potential energy because it cannot fall any farther.
In place of this, however, neglecting mechanical resistances, it con-
tains an exactly equivalent amount of kinetic energy, due to its
motion. During the second half of the swing this kinetic energy
is expended in again raising the pendulum, and when it has all been
expended the pendulum will (in the absence of external resistance)
have been raised to exactly the same height as before above its
lowest point. In other words, its kinetic energy will have been re-
converted into an equivalent amount of potential energy and so
the alternate conversion and re-conversion goes on as long as the
pendulum continues to swing.

The same facts which have been illustrated above in the case of
the motion of visible masses of matter are likewise true of molecular
and atomic motions. When molecules of carbon dioxide and water
are converted into starch in the green leaves of the plant, work is
done upon them by the energy of the sun's rays. Their constituent
atoms are forced apart and compelled to assume new groupings. In
this process a certain amount of kinetic energy has disappeared
and the resulting system of starch molecules and oxygen molecules
contains a corresponding amount of potential energy. Under
suitable conditions the reverse process may also take place. The
atoms may. so to speak, fall together and resume their old positions,
producing the original amounts of carbon dioxide and water and
giving off in the process the exact amount of kinetic energy which
was originally absorbed. This energy may appear in the form of
heat, as in ordinary combustion, or in any other of the various
forms of energy, according to circumstances.

The last example is but an illustration of the general fact that
in every chemical reaction there occurs a transformation of energy
which most commonly takes the form of an evolution or absorption

228 PRINCIPLES OF ANIMAL NUTRITION.

of heat. That branch of science which deals with the connection
between chemical and thermal processes is known as thermo-chem-
istry. Since kinetic energy in the animal is derived from chemical
processes, and since it largely takes the form of heat, we may regard
the study of the transformations of energy in the organism as con-
stituting a branch of thermo-chemistry and proceed to a consider-
ation of the fundamental laws upon which the latter subject is
based.

The Conservation of Energy. — In any system of bodies not
acted on by external forces the sum of the potential and kinetic
energy is constant. In other words, while the ratio of potential
to kinetic energy may vary, and while each may take various forms,
as mass-motion, heat, electric stress, etc., there is no loss of energy
in these conversions. Energy, like matter, is indestructible. This
great law of the conservation of energy was first clearly enunciated
by Mayer, and forms the foundation of all modern conceptions of
physical processes. In the case of the swinging pendulum used
above as an illustration the total energy of the system composed
of the earth and the pendulum is constant, a portion of it simply
alternating between the potential and kinetic states. So, too, in
the system of atoms of carbon, hydrogen, and oxygen, the potential
energy contained in the system before the starch is burned is simply
converted into the kinetic energy of heat, while the total energy
of the system remains the same.

Initial and Final States. — An important consequence of the
law of the conservation of energy, which was first deduced and
demonstrated experimentally by Hess in 1840, is known as the law
of initial and final states. This law is that in any independent
system the amount of energy transformed from the potential to
the kinetic form, or vice versa, during any change in the system,
depends solely upon the initial and final states of the system and
not at all upon the rapidity of the transformation or upon the kind
or number of the intermediate stages through which it passes.
Although this law is true in the general form here stated, it was
originally propounded as related to chemical reactions and forms
the basis of the science of thermo-chemistry. If we start with
starch and oxygen and end with the corresponding quantities of
carbon dioxide and water, the amount of kinetic energy evolved is

FORCE AND ENERGY. 229

the same, no matter whether the starch be burned almost instanta-
neously in pure oxygen or whether it be subjected to slow oxidation
in the tissues of a plant buried in the soil; whether carbon dioxide
and water are the immediate products of the action or whether the
starch be previously transformed into maltose, glycogen, dextrose,
lactic acid, etc., etc., as in the body of the animal. We have simply
to determine the potential energy of the system in its initial and in
its final state, and the difference is equal to the amount of kinetic
energy evolved during the change. The truth of this law, as ap-
plied to chemical processes, has been fully demonstrated by the
researches of Berthelot and Thomsen. That the same law applies
to the processes taking place in the body of the animal is exceed-
ingly probable, a 'priori, and has been demonstrated experimentally
by the researches of Rubner and of Atwater and his associates.

Heats of Combustion. — We have no means of determining
the total amount of potential energy contained in a system, but can
only measure that portion which is manifested by the change to
the kinetic or the potential form during some change in the system.
In other words, we may assume the potential energy of the system
in some particular state as zero and obtain a numerical expression
for its potential energy in some other state as compared with this
standard state. For the latter we shall naturally select that one in
which no further conversion of potential into kinetic energy can,
according to our experience, take place.

In the case of organic substances, such as those entering into
the metabolism of the animal, the system consists of the substance
itself and oxygen, and the state of complete oxidation is the one in
which experience shows that no further evolution of kinetic energy
is possible by chemical means. Thus, to recur to the example of
starch, if one gram be oxidized in accordance with the equation

CyE, 0O5 + 602 = 6C02 + 5H20,

the amount of heat evolved will be 4183 cals.,* this being the amount
of energy converted from the potential to the kinetic form. From
the system represented by the second member of the above equa-
tion we can get no further evolution of heat. We therefore repre-

* For the units of measurement see the following paragraph.

230 PRINCIPLES OF ANIMAL NUTRITION.

sent its potential energy by 0 and accordingly that of the system
starch + oxygen by 4183 cals. for each gram of starch. This value
is called the heat of combustion of starch, and shows how much
energy can be liberated from this substance by its conversion into
C02 and H20. It is common to speak of this as the potential energy
of the starch, and the expression has the advantage of brevity.
but it should not be forgotten that it is really the potential energy
of the system C6H10O5 + 602 as compared with the system
6C02 + 5H20.

In like manner the heat of combustion of any organic com-
pound, or of any mixture of compounds such as a feeding-stuff,
represents the amount of energy which a given weight of it evolves
in the form of heat when completely oxidized. In the case of
nitrogenous bodies the final products are C02, H20, N2, and in
case of proteids S03.

Heats of combustion may be determined at constant pressure
or at constant volume. When the substance is burned under ordi-
nary atmospheric pressure the amount of heat evolved may include,
besides that due to the difference in the chemical energy of the
substance before and after burning, a mechanical component due
to the fact that the volume of the products is not the same as that
of the original substances. If it is greater, work is done in
overcoming atmospheric pressure and the heat production is
diminished by a corresponding amount. In the contrary case, work
is done by the atmosphere upon the products of combustion and
heat is evolved. When the substance is burned in a confined
volume of oxygen, as in the bomb-calorimeter, the possibility of
such mechanical action is eliminated and we obtain a quantity of
heat representing solely the difference in chemical energy. The heats
of combustion at constant volume are therefore, from a theoretical
point of view, the more correct. On the other hand, however, all
ordinary processes of combustion, including those occurring in the
animal organism, take place under atmospheric pressure, which is
practically constant, and therefore the actual heat value of a sub-
stance oxidized in the body is measured by its heat of combustion
at constant pressure. If there is no change in volume during the
combustion, then the two heats of combustion are, of course, iden-
tical. This is the case, for example, with the carbohydrates, wrhich

FORCE AND ENERGY. 231

form so large a part of the food of herbivorous animals. Further-
more, the difference in the case of the other common nutrients is so
slight that the heats of combustion as determined with the bomb-
calorimeter may be used without appreciable error in computing
the metabolism of energy in the body. The only substance involved
in such computations for which the correction needs to be made is
methane, CH4, the heat of combustion of which is at constant
volume 13.246 cals. per gram and at constant pressure 13,344 cals.

Units of Measurement. — The unit of force is the dyne, which
is defined as the amount of force required to produce in a mass of
one gram, in one second, an acceleration of one centimeter per
second.

When a .orce acts upon a mass, the amount of work clone is
measured by the product of the force into the distance (measured
along the direction of the force) through which it acts. The unit
of work is the erg, which is defined as the work done by a force of
one dyne acting through one centimeter.

Energy has been defined as the power of doing work, and is
measured by the amount of work done, that is, in ergs. Since,
however, the erg is a very small quantity, it is often more con-
venient in practice to use a multiple of it. For this purpose the
quantity 1010 erg=l Kilojoule (J) is a convenient unit. Energy is
also frequently expressed in units based on weight instead of mass,
the most common being the gram-meter, the kilogram-meter, and
the foot-pound. The gram-meter is the work done against gravity
in raising a weight of 1 gram through 1 meter. Since, however, the
force of gravity, and consequently the weight of a given mass, varies
at different points on the earth's surface, it is necessary to state
also where the weight is taken. At the level of the sea, in temperate
latitudes, the force of gravity equals 980.5 dynes. Under these
conditions, then, doing 1 gram-meter of work would be equivalent
to exerting a force of 980.5 dynes through 100 cm., which equals
98,050 ergs. The kilogram-meter (kgm.) is the work done againsl
gravity in raising 1 kilogram through 1 meter, and is accordinuly
1000 times the gram-meter or 98,050,000 ergs. The foot-pound
is the work done against gravity in raising 1 pound through 1 foot
and accordingly equals 13,550,000 ergs.

In addition to mechanical energy the animal produces heat.

232 PRINCIPLES OF ANIMAL NUTRITION.

For the measurement of heat various units are in use, but the ones
most commonly employed in physiology are the small and the large
calorie. The small calorie (cal.) is defined as the amount of heat
required to raise the temperature of 1 gram of water through 1° C.
Since, however, the specific heat of water varies somewhat with the
temperature, it is necessary to specify the average temperature
of the water. The temperature of 18° C. has been quite commonly
used for this purpose, the resulting unit being indicated by the
abbreviation cal18. Atwater & Rosa,* however, in their work
writh the respiration-calorimeter, have employed the temperature
of 20° C, designating their unit by cal20. The difference between
the two is very slight, 1 cal20 equaling 1.0002 cal18. The large
calorie (Cal.) is the amount of heat required to raise the tempera-
ture of one kilogram of water through 1° C, or is equal to 1000 small
calories. The temperature at which the large calorie is measured
may be indicated as in ease of the small calorie.

The calorie, however, while commonly used, and while in some
respects a convenient unit, is in a sense not a rational one. Since
heat is one form of energy, and since, in accordance with the law of
the conservation of energy, there is a fixed relation between it and
other forms of energy, a rational unit would be one bearing a simple
numerical relation to the units employed to measure other forms
of energy, or in other words, the erg or some simple multiple of it.
As already noted, the Kilojoule (J) is a convenient unit for this
purpose. It has two advantages over the Calorie : first, it permits
of a direct comparison of heat with other forms of energy (expressed,
of course, in units of the same system) ; and second, it is an " abso-
lute" unit, that is, it is based on the fundamental units of space,
mass, and time, and has a perfectly definite magnitude, while the
Calorie has not unless the temperature at which it is measured is
stated. To this may be added that in discussing physiological
relations it avoids the sometimes confusing implication that the
quantities of energy dealt with actually exist in all cases as heat.

The relation between the Calorie and the Kilojoule is as follows:

1 Cal18 = 4 . 183 J = 41,830,000,000 ergs ;
U - 0 . 2391 Cal18 = 10,000,000,000 ergs.

* U. S. Dept. Agr., office of Expt. Stats., Bull. 03, p. 55.

FORCE AND ENERGY.

233

Since, however, most of the results of investigations upon the
physiological relations of energy are expressed in calorics (often
without any statement of temperature) it will be more convenient
in the following pages to employ this unit rather than the more
rational Kilo joule.

Finally, since measurements of mechanical energy (as in experi-
ments with working animals) have been commonly made in weight
units, it is necessary to know the relation of these to the calorie.
These relations are included in the following table, the force of
gravity being taken as 980.5 dynes:

EQUIVALENCE OF UNITS OF ENERGY.

Ergs.*

Kilojoules.

Gram-
meters.

Kilogram-
meters.

1 Kiloioulc =

1010
980.5X102
9S0.5X105
135.5X105
4.183X107
4.183X10'°

101989

1000

138.2

426.6

426600

1 kilogram-meter = . . . .

1 foot-pound =

1 eah s

980.5--i(r

980.5-r-lO5
135.5-hIO5

0.004183

4.183

0.001

0.1382
0.4266

1 Cal, 8

426 6

Foot- „„i
1 t-til im-
pounds.

Cali8.

1 Kilojoule =

73S.1
0.007236
7.236

239 . 1
0.002344
2.344
0.3239

0 2391

1 gram-meter =

0 2344 -=-10*

1 kilogram-meter =

0 . 002344

1 foot-pound =

0 . 000324

1 cal-

3.087
30S7.

0.001

1 Cats

1000

* From Ostwald, Grundriss der allgemeinen Chemie.

CHAPTER VITI.
METHODS OF INVESTIGATION.

The food is the sole known source of energy as well as of matter
to the body of the warm-blooded animal, and the total income of
potential energy, according to the principles laid down in the pre-
ceding chapter, is represented by the heat of combustion of the
food.

A portion of this food, as we have seen in Part I, is metabolized
in the body, while part of it escapes complete oxidation and is re-
jected as undigested matter in the feces, as metabolic products in
feces, urine, and perspiration, and as combustible intestinal gases.
All these substances still contain more or less of their original store
of potential energy and collectively constitute one main division of
the outgo of energy. We may call it, for brevity, the outgo of
potential energy. A portion of the food may also be applied to the
production and storage of tissue (protein and fat) in the body, and
this, from our present point of view, is to be classed with the
outgo of potential energy.

The potential energy of the remaining portion of the food, viz.,
that which is completely oxidized, may take various transitory
forms in the organism, but ultimately it leaves it in one of tw.i
forms of kinetic energy, viz., as mechanical work or as heat. Here
we have the second main division of the outgo of energy, viz., the
outgo of kinetic energy. These relations may be briefly expressed
in tabular form, as shown at the head of the opposite page.

As in the corresponding chapter of Part I, it is proposed to con-
sider here simply the general principles of the more important
methods available for determining the income and outgo of poten-
tial and kinetic energy, without entering into technical details.

234

> Potential energy.

METHODS OF INVESTIGATION. 235

J neon n :

Food
Outgo:

Feces

% Urine

Perspiration
Combustible gases
Storage of tissue

Work I „. .

Heat \ Kinetic energy.

Determination of Potential Energy.

The Energy of the Food. — The potential energy of the food is
conveniently measured by converting it into the kinetic form of heat;
that is, by determining its heat of combustion. This determina-
tion is effected by means of an instrument known as a calorimeter,
in which the heat produced by the complete combustion of a known
weight of the substance under examination is absorbed by some
calorimetric substance and its amount measured by the change of
temperature or of physical state of the latter. The calorimetric
substance ordinarily employed is water, the increase in tempera-
ture of a known weight of this substance giving directly the amount
of heat in calories. It is, of course, essential either that all the heat
produced shall be transferred to the calorimetric substance or that
it shall be possible to correct the observed results for any heat that
may escape absorption.

Another essential is that the oxidation shall be complete, a
condition whose fulfillment it is by no means easy to secure Two
general methods have been employed for this purpose. The first
was that of Thompson,* as used by Frankland and subsequently
modified by Stohmann,f in which the oxidation is effected by
means of pure potassium chlorate corrections being made for the
heat evolved in the decomposition of the latter substance. The
second method, which has almost entirely replaced the first, con-

* Described by Frankland, Proc. Roy. Inst, of Great Britain, June 8,
1866, and Phil. Mag (4), 32, 182.

t Jour. pr. Chem , 127, 115; Landw. Jahrb., 13, .513.

236 PRINCIPLES OF ANIMAL NUTRITION.

sists in burning the substance without any admixture in highly
compressed oxygen contained in a lined steel bomb as first devised
by Berthelot * and subsequently modified by Mahler, Hempel, and
Atwater. With this type of calorimeter very accurate and com-
paratively rapid work may be done.f

Frankland was the first to undertake determinations of the
heats of combustion of foods and food ingredients, using the origi-
nal form of the Thompson calorimeter. Subsequent investigators,
of whom may be especially mentioned Stohmann, v. Rechenberg
and Danilewski, Berthelot and his associates, Rubner, and Atwater,
Gibson & Woods, have continued these investigations with im-
proved apparatus and more refined methods, J and we now possess
a considerable mass of data as to the heats of combustion of the
more important ingredients of animals and plants and of the prod-
ucts of metabolism. Atwater § gives the following summary of
the results on record up to July, 1894 (see pp. 237-9).

In the course of recent investigations into the energy relations
of the food of man and of domestic animals a considerable amount
of data has also been secured regarding the heats of combus-
tion of foods and feeding-stuffs. A summary of the results of
such determinations on 276 samples of human foods of various
kinds has been published by Atwater & Bryant. || No similar
compilation of heats of combustion of feeding-stuffs is as yet avail-
able.

It need hardly be pointed out that, taken by themselves,
such results furnish no measure of the relative values of the
various feeding-stuffs. Like a chemical analysis, they supply
but a single factor, albeit an important one, for such a com-

* Ann. de Chim. et de Phys., (5), 23, 160.

t For the technical details of the method reference may be had to the
published descriptions of the apparatus or to Wiley's Principles and Prac-
tice of Agricultural Chemical Analysis, Vol. Ill, p. 569.

% For a historical sketch of the development of calorimetry, as applied
to food substances, compare Atwater, " Chemistry and Economy of Food,"
U. S. Department of Agriculture, Office of Experiment Stations, Bull. 21, pp.
116-126.

§ Ibid., pp. 127 and 128. Compare also Rep. Storra Expt. Station, 1899,
p. 73.

11 Rep. Conn. Storrs Expt. Station, 1899, p. 97.

METHODS OF INVESTIGATION.

237

HEATS OF COMBUSTION OF ORGANIC SUBSTANCES.

Bert helot
Method.

Rert he-
lot and

Asso-
ciates.

Albuminoids, etc.

Gluten

Elastin

Plant fibrin . . . .
Serum albumin.

Syntonin

Hemoglobin . . .
Milk casein

Yolk of egg

Legumin

Vitellin

Egg albumin

Muscle, extractives and fat re-
moved

Crystallized albumin

Muscle, fat removed

5990 . 3

5S32.3

5910
5626 . 4

8112.4

Stoh-

niann

and

Lang-

bein.

5780 . 6
5687.4

5728.4

5504.2

Blood fibrin 5529 . 1

Harnack's albumen

Wool

Congluten

Fibrin of skin

Peptone

Fish glue

Chondrin

Ossein

Fibroin

* "hit in

Tunicin

Paraglobulin

Am ids, E'

I'p ,

Glycocoll

Alanin

Leucin

Sarkosin

Hippurif acid

Aspartic acid 12911.1

Tvrosin 5915.9

5240 . 1
5342 . 4
5410.4
5095 . 7
4655
4146.8

5961.3

5941.6

5917.8

5907.8

5885 . 1

5867

5S49 . 6

5840.9

5793 . 1

5745 . 1

5735.2

5720 . 5

5672

5662.6

5640.9

5637 . 1

5553

5510.2

5479

5355 . 1

5298.8

2530 . 1
3133.6
1370.7
6536 ■"'

5130.6
5039 . 9
4979.6
4650.3

5659 :;

2541.9
3129.1
1355 . 5
6525 . 1
4505.9
5668.2
2899

Asparagin

Kreat in (crysl .)

" (water-free)

Trie acid

Guanin

Caffein

3396.8 3514

3714.1

. . vi-:, 1
2749 . 9
3891 7
5231 1

27o 1

Thompson-Stohmann Method.

Stoh-
mann
and
Asso-
ciates.

5717

5579

5598
5324

5511
5362

5637

2465
3053

5642
3428

B.

Dani-
lewski.

6141
6231

5785
5573

5709

5069
5493
4909

2537

! (3206)

2021

Rub-
ner.

5950

5778*
5656*

2523

Gibson.

* Calculated a-h-free.

238 PRINCIPLES OF ANIMAL NUTRITION.

HEATS OF COMBUSTION OF ORGANIC SUBSTANCES {Continued).

Fats.

1 Animal:

Fat of swine

Fat of oxen

Fat of sheep

Fat of horse

Fat of dog

Fat of goose

Fat of duck

Fat of man

Butter fat

Sperm oil

2. Vegetable:

Olive oil (expressed)

Berthelot
Method.

Berthe- Stoh"

oeiiue mann

lot and anj

Asso- Lang-

ciates. bein.

Thompson-Stohmann Method.

Stoh-
mann
and
Asso-
ciates.

9476.9
9485 . 7
9493 . 6

Poppy-seed oil (expressed) ,
Rape-seed "

Ether extract of various seeds .

Carbohydrates, etc.

1. Pentoses:

Arabinose

Xylose

Fucose

Rhamnose (water-free)

(cryst.)

2. Hexoses:

Sorbinose ,

Galactose

Dextrose

Fructose

3. Heptoses:
Glucoheptose

4. Disaccharids :

Cane sugar

Milk "

" " (cryst.)

Maltose

" (cryst.)

Trehalose

" (cryst.)

5. Trisaccharids :
Meletriose

" (cryst.)

Melezitose

3714
3739.9

9215.8

3762

3732 . 8
3961.7
3777 . i

3722
3746
4340.9
4379 . 3
3909 . 2

3714.5
3721.5
3742.6
3755

3955.2

3951 . 5

3736.8

3949.3

3721.8

3947

3550.3

9380
9357
9406
9410
9330
9345
9324
9398
9192

9328
9471
9442
9489
9619
9130
9467

3695

3659
3692

3866
3877
3663

4020 . 8
3400.2
3913.7

B.
Dani-
lewski.

9686

Rub-
ner.

9423

4001

Gibson.

9515

9427
9530

9185
10001

9471

3754

3921
3710

METHODS OF INVESTIGATION.

239

HEATS OF COMBUSTION OF ORGANIC

SUBSTANCE

3 {Continued).

tmann Method.

|

Berthelot
Method.

Thompson-Sto!

Berthe-
lot and
Asso-
ciates.

Stoh-
niann
and
Lang-
bein.

Stoh-

mar.n
and
Asso-
ciates.

B.

Dani-
lewski.

Rub-

ner.

Gibson.

6. Polysaccharids:

Glycogen

4190.6
4185.4
4182.5
4112.3
4133.5

4112.4
3997.8
3679.6

9352.9

4146
4123

4070

4317
3908

9226
9429

1960
3019
1745
2397

( fellulose

4200
4228
4180.4
4187.1

7068

4164

Dextran

Alcohols.
Ethyl alcohol

Glycerin

4001.2
3676.8

3490 . 4

3959

Inosite

Acids.

Palmitic

Stearic

Oleic

9494.9

Malonic

Succinic

Tartaric

1998.2
3006.2

Citric

2477.9

parison. Just as the chemical analysis shows the total amounts
of various substances or classes of substances present, so the heat
of combustion shows the total amount of potential energy which
has been stored up in the feeding-stuff. In both cases the knowl-
edge thus acquired must be combined with data, secured in an
entirely different way, as to the availability of these ingredients or
this energy before we can form a judgment as to the relative values
to the animal.

Computation of Heats of Combustion. — The heat of com-
bustion of a mixture of various organic substances, such as are
contained in ordinary foods and feeding-stuffs, is equal to the sum of
the heats of combustion of the single ingredients. If the latter are
known we may obtain the heat of combustion of the material in
question either by a direct calorimetric determination or by deter-
mining chemically the proportions of the several ingredients and
multiplying the amount of each into its known heat of combustion.

240 PRINCIPLES OF ANIMAL NUTRITION.

The first method, when available, is obviously to be preferred,
and is to be regarded as indispensable in all exact investigations
into the energy relations of the food of man or of animals. With
materials whose proximate composition is fairly well known, how-
ever, the agreement between the computed and the actual heat of
combustion is very close, as has been shown by Wiley & Bigelow *
and Slosson | for hulled cereals and cereal products. Atwater &
Bryant, in their publication just referred to, have discussed this
question very fully in relation to human foods and have proposed
a series of factors for the ingredients of the various classes of foods
by whose use they obtain a most satisfactory agreement with the
calorimetric results.

On the other hand, in case of vegetable products containing
much woody and fibrous material the actual heat of combustion
is higher than that computed under the ordinary interpretation of
the results of chemical analysis. Thus the actual heat of combus-
tion of unhulled oats was found by Wiley & Bigelow to be about
4.5 per cent, higher than the computed value, and Merrill J has
obt ained similar results for wheat middlings and bran and for hay
and silage. This obviously arises from the presence among the
ill-known bodies constituting the so-called lignin and incrusting
substances of compounds having higher heats of combustion than
the common carbohydrates. It is not impossible that a series of
factors similar to those of Atwater & Bryant might be worked out
for different classes of stock foods, so that their heats of combustion
might be computed from their chemical composition. In view,
however, of the comparative ease and rapidity with which direct
calorimetric results can be accumulated it may be doubted whether
such an undertaking would repay the labor involved.

Methods have also been proposed and somewhat extensively
used for computing the heat of combustion of the digested portion
of the food. This phase of the subject, however, can be more
profitably considered later.

The Energy of the Excreta. — For the visible excreta (feces
and urine) substantially the same method is available as for the food,

* Jour. Am. Chem. Soc, 20, 304.
t Wyoming Expt. Station, Bull. 33.
t Maine Expt. Station, Bull. 67, p. 169.

METHODS OF INVESTIGATION. 241

viz., a determination of the heat of combustion. An element of
uncertainty, however, which is ordinarily not met with in the case
of the food, arises from the ready decomposability of the excretory
products, which is liabie to result in a loss of energy during the
drying necessary to prepare them for combustion. The urea of
the urine, in particular, is very readily converted into the volatile
ammonium carbonate. Comparative determinations of nitrogen
in the fresh and in the dried urine will show the amount of nitrogen
lost in drying, and on the assumption that only urea is decomposed
the loss of energy can be readily computed from the known heat of
combustion of that substance. Atwater & Benedict * have found
this assumption to be substantially correct for human urine, and
the same may be presumed to be the case with the urine of carniv-
ora. It has usually been assumed to be applicable also to the
more complex urine of herbivora, although without, so far as the
writer is aware, any experimental proof.

A greater or less loss of nitrogen has also been observed in the
drying of the feces of domestic animals, particularly of horses and
sheep, but the nature of the material decomposed has not }ret been
investigated, and the same is true of the possible decomposition of
non-nitrogenous materials in both urine and feces. Atwater &
Benedict (loc. cit.) found the loss of nitrogen from human feces to
be insignificant.

Computation of Energy. — The computation of the energy of
the visible excreta is much less satisfactory than in the case of the
food on account of our inferior knowledge of the proportions and
chemical nature of their ingredients.

The Urine. — Formerly the urine was assumed to be substan-
tially an aqueous solution of urea, and numerous computations of
its energy content were made on this basis, particularly in connec-
tion with estimates of the metabolizable energy of the proteids,
while the same method has been applied also in estimating the
metabolizable energy of feeding-stuffs. Rubner f was the first to
demonstrate the serious nature of the error involved in this assump-
tion and to show that the energy of the urine is materially greater
than the amount thus computed. In the urine of the dog he found

* U. S. Dept. Agr., Office of Experiment Stations, Bull. 69, p. 22.

t Zeit. f. Biol., 20, 265; 21, 250 and 337; 42, 302. Compare Chapter X.

242

PRINCIPLES OF ANIMAL NUTRITION.

the energy content to be from 6.7 to 8.5 Cals. per gram of nitrogen
in place of 5.4 Cals. as computed on the assumption that only urea
was present, while for human urine he has obtained values ranging
from 6.42 Cals. to 8.87 Cals. per gram of nitrogen, and Tangl *
has reported even higher figures.

Kellner f has shown that the difference is still greater in the
urine of an ox receiving only coarse fodder, the actual energy being
about six times that computed on the above assumption and nearly
175 per cent, of that computed after allowing for the hippuric acid
present. In subsequent investigations \ he finds that the energy
content of the urine of cattle is much more nearly proportional to
its carbon than to its nitrogen, being approximately 10 Cals. per
gram of carbon.

In six cases reported by Atwater & Benedict § in the course of
their investigations with the respiration-calorimeter, the amount of
energy found in human urine from a mixed diet as compared with
that computed from the nitrogen reckoned as urea was:

Energy.

Total

Nitrogen,

Grms.

Actual,
Cals.

Computed,
Cals.

Experiment No.

5

72.43

511

392

6

64.29

504

348

7

70.60

569

382

8

77.90

658

421

9

71.72

597

388

10

77.76

589

421

Here, too, it is evident that a computation on the basis of the
urea yields results much below the truth, and later experiments by
the same authors have fully confirmed this result.

The Feces. — Our knowledge of the proximate principles con-
tained in the feces is so small that no satisfactory computation of
their energy content is possible, except perhaps in the case of car-
nivora on a purely meat diet, where the total amount of feces is

* Arch. f. (Anat. u.) Physiol., 1899, p. 261.

t Landw. Vers. Stat., 47, 275.

I Ibid., 53, 437.

§ U. S. Dept. Agr., Office of Experiment Stations, Bull. 63.

METHODS OF INVESTIGATION. 243

small. On a mixed diet containing any considerable proportion of
vegetable matter, and particularly in the case of herbivorous ani-
mals consuming Large amounts of coarse fodders, only an actual
determination of the heat of combustion can be depended upon.
Since the feces of these animals contain a larger proportion of the
indigestible lignin, etc., than does their food, the heat of combustion
of the feces is correspondingly higher, but its actual value must
obviously depend to a considerable degree on the character of the
food.

Combustible Gases. — Since it is impracticable to collect sepa-
rately the combustible intestinal gases, we must of necessity com-
pute the amount of potential energy carried off in these substances.
This computation has been based on the amount of carbon con-
tained in these gases, determined in the manner indicated on p. 72,
upon the assumption that only methane (CHJ was present. It
has been shown that this gas exists in considerable amounts in
the digestive tract of herbivora, and it is probable that the above
assumption is substantially accurate, although a small amount of
hydrogen has been found by some observers. In experiments by
Fries.* at the Pennsylvania Experiment Station, in which both
the carbon and hydrogen of the combustible gases excreted by a
steer consuming chiefly timothy hay were determined, the follow-
ing ratios of hydrogen to carbon were obtained:

Period A.

First day,

1:2.900

Second day,

1:2.916

Period B.

First day,

1:2.978

Second day,

1:2.947

Period C.

First day,

1:2.899

Second day,

1:2.951

Period D,

First day,

1:3.051

Second day,

1:3.096

Average,

1:2.967

Computed for CH<,

1:2.976

* Proc. Soc. Prom. Ag. Sci., 1902.

244 PRINCIPLES OF ANIMAL NUTRITION.

These results tend strongly to substantiate the belief that the
combustible gases practically consist of methane only.

Perspiratiori. — In view of the relatively minute amounts of
organic matter contained in the perspiration it has generally been
regarded as a negligible quantity. The data given on p. 48 for
the nitrogen of the sensible perspiration would afford some approxi-
mate data for computing the amount of energy contained in it.

The Energy of Tissue Gained. — The amount of potential energy
stored up in a gain of tissue, or the amount liberated in the kinetic
form in case the gain is negative, cannot, of course, be made the
subject of a direct determination. The amounts of protein and of
fat gained or lost can, however, be determined by the methods
described in Chapter III, and their energy content computed from
average figures. The errors involved are those incident to the
method of computation from the carbon and nitrogen balance,
which have already been considered in the chapter cited, and those
arising from uncertainty as to the exact energy content of the
material gained by the body.

Protein. — Just as computations of the gain or loss of protein
by the body have been based upon the average composition of the
proteids, so computations of its energy content have been based
upon the average heat of combustion of these substances. The
compilation by Atwater on pp. 237-9 contains the available data
up to 1894.

For approximate computations the value 5.7 Cals. per gram has
been commonly used, while in more exact computations it has
been assumed that the gain of protein by the animal has substan-
tially the heat value as well as the chemical composition of fat-free
muscular tissue (see p. 63), and the average of Stohmann's two
determinations, viz., 5.652 Cals. per gram, has been employed.
Kohler's investigation * of the composition of fat and ash-free
muscular tissue (p. 64) included determinations of the heats of
combustion which are reproduced on the opposite page.

Fat. — Rubner, in his computations, employs the round number
9.4 Cals. per gram for fat, while Kellner uses the value 9.5 Cals.
Benedict & Osterberg,| whose determinations of the composition of

* Zeit. physiol. Chem., 31, 479.
f Amer. Jour. Physiol., 4, 69.

METHODS OF INVESTIGATION.

245

No. of
Samples.

Heat of Combus-
tion per Gram,
Cals.

Cattle

4
2
2
3
2
2

5. 677H

5.6387

5.6758

Horse

5.5990*

Rabbit

5.6166

Hen

5.6173

human fat are given on p. 61, found for the heat of combustion of
the same twelve samples values ranging from 9.474 Cals. to 9.561
Cals. per gram, the average being 9.523 Cals. Other results are
noted in the table on pp. 237-9.

Determination of Kinetic Energy.

Mechanical Work. — The energy of the mechanical work done
by the animal upon its surroundings is derived, as was seen in Part
I, immediately from body materials and mediately from the food,
and is one of the two forms in which kinetic energy leaves the body.

The energy of the mechanical work done by the animal may be
measured in various ways, the consideration of which belongs
to the domain of mechanics and lies outside the scope of the
present work. In general two classes of appliances have been used :

First, dynamometers proper, in which the work is expended in
overcoming a known resistance, produced either by friction or by
a magnetic field, the work done being measured by the tension of a
spring or by the amount of electric energy produced.

Second, the tread power, in which the work, aside from that of
locomotion, consists in lifting the body vertically and is propor-
tional to the product of the mass of the body into the distance
through which it is raised.

Heat. — The second form in which kinetic energy leaves the
body is heat. In an animal doing no work all the energy arising
from the metabolism in the body ultimately takes this form, and
even when mechanical work is done the larger share of the outgo
of kinetic energy consists of heat. Part of this heat is imparted to
the surroundings of the animal by conduction and radiation and a

* Contained an average of 3.65 per cent, glycogen.

246 PRINCIPLES OF ANIMAL NUTRITION.

part is expended in the evaporation of water from skin and lungs
and takes the form of the latent heat of water vapor.

Animal Calorimeters. — The direct determination of the heat
produced by an animal, especially a large animal, is not an easy
task. It requires in the first place a calorimeter large enough to
contain the animal and in the second place, for experiments of any
length, the maintenance of a sufficient ventilating current of air
under such conditions as shall not affect the accuracy of the calori-
metric determination, while the latent heat of the water vapor
carried out in the air-current must also be taken account of. In
other words, such an apparatus must be at once a respiration appa-
ratus and a calorimeter, and hence the name respiration-calorimeter
has come to be applied to it. Various forms of animal calorimeters
have been devised, some of which may be briefly mentioned.

Lavoisier & Laplace,* in their investigations upon the origin of
animal heat, employed an ice-calorimeter, in which the heat is
measured by the amount of ice melted. Crawford f investigated
the same subject using a water-calorimeter, as did, later, Dulong J
and Dcspretz,§ while more recently Wood,|| and still later
Reichert,^" have also employed the water-calorimeter.

The ice-calorimeter, however, necessarily subjects the animal
to an abnormally low temperature, while with the water-calorim-
eter it has been found very difficult to secure a uniform heating of
the different strata of water. These facts led to the employment
of air as the calorimetric substance, the heat being measured either
by the increase in the volume of a confined body of air at a constant
pressure or the increase in the pressure at constant volume, and
untjl quite recently the most exact methods have been based on
this principle.

Scharling,** Vogel,tt and Him, ft between 1849 and 1864, used

* Hist. Acad. Roy. d. Sc, 1780, 355.

t Experiments and Observations on Animal Heat. London, 1788.

% Ann. de Chim. et de Phys. (3), 1, 440.

§ 76m?., (2), 26,337.

|| Smithsonian Contributions, 1880.

t Univ. Med. Mag., Phila., 2, 173.

** Jour. pr. Chem., 48, 435.

ft Arch. d. Ver. f. Wiss. Heilk., 1864, p. 442.

XX Recherches sur l'equivalent m£chanique de la chaleur. Paris, 1858,

METHODS OF INVESTIGATION. 247

crude forms of the air-calorimeter. In 1S85 Richet * described an
air-calorimeter for small animals, the heat being measured by the
increase in the volume of a confined portion of air at constant press-
ure. His experiments were of short duration (1 to H hours) and
no specific statement is made regarding ventilation and no mention
of any determinations of the latent heat of the water vapor.

In 1886 d'Arsonval f described a differential air-calorimeter,
and in 1890 \ two other forms of animal calorimeter, the first being
a water-calorimeter of constant temperature with automatic regu-
lation of the flow of water, for which a high degree of accuracy is
claimed, and the second an air-calorimeter, but he reports no ex-
periments with either form. In the same year Laulanie § (see p. 70)
described briefly a Regnault respiration apparatus which was also
used as a calorimeter, and has subsequently reported some results
obtained by its use.

One of the best known forms of animal calorimeter is that of
Rubner.|| This is essentially a Pettenkofer respiration apparatus,
the walls of the chamber being double and the whole surrounded
by an air space which in its turn is surrounded by a jacket con-
taining water kept at a constant temperature. The amount of
heat given off to the calorimeter is measured by the expansion
under constant pressure of the confined volume of air between the
two walls of the respiration chamber, while from comparative de-
terminations of moisture in the ingoing and outcoming air the heat
removed in the latent form is computed.

Rosenthal •[ has constructed a somewhat similar instrument in
which the respiratory portion is a Regnault apparatus, while the
heat is measured by the increase in pressure of the air at constant
volume, instead of by the increase in its volume as in Rubners
apparatus. Both instruments are therefore air-calorimeters, and the
numerical values of their readings must be determine! experimen-
tally for each instrument. These two forms of apparatus are of a size
sufficient for experiments with small animals (rabbits or small dogs).

* Archives de Physiol , 1885, II, 237.

t Jour, de l'An.it. el Physiol., 1886.

X Archives de Physiol., 1890, pp. 610 and 781.

§Ibid., p. 571.

|| Calormetrische Methodik, Marburg, 1891; Zeit. f. Biol., 80, 91.

Tf Arch. f. (Anat. u.) Physiol., 1894, p. 223.

248 PRINCIPLES OF ANIMAL NUTRITION.

In 1894 Haldane; White & Washbourne * described a form of air-
calorimeter in which the expansion caused by the heat produced by
the animal in one chamber is balanced by that produced by a flame
of hydrogen burning in a second similar chamber. The calorimeter
is essentially one of constant volume, but the heat is computed
from the amount of hydrogen burned.

Laulanie \ in 1895 described a Pettenkofer apparatus with small
ventilation (see p. 71) which served also as an air-calorimeter, and
still later \ has described a differential water-calorimeter. Kauf-
mann,§ as mentioned on p. 69, has determined the respiratory
exchange of animals during short periods in a confined volume of
air. The apparatus consisted simply of a zinc receptacle which
served also as a radiation calorimeter. The internal temperature
and that of the surrounding air were measured by recording ther-
mometers and the loss of heat calculated according to Newton's
law. The atmosphere in the apparatus was saturated with water-
vapor at the start, so that the moisture excreted by the animal was
condensed and no correction for the heat of vaporization was neces-
sary.

By far the most important form of respiration-calorimeter yet
devised, however, not only as regards accuracy but particularly
in view of the range of work of which it is capable, is that of Atwater
& Rosa,|| the respiratory part of which has already been mentioned
(pp. 72 and 79). In this apparatus water is used as the calori-
metric substance, but in the form of a constant current instead of a
large stationary mass. As described by the authors the appara-
tus consists of a Pettenkofer respiration apparatus provided with
special devices for the accurate measurement, sampling, and analy-
sis of the air-current. A current of cold water is led through copper-
absorbing pipes near the top of the respiration chamber and takes
up the heat given off by the subject. The volume of the water used
being measured, and its temperature when entering and leaving
being taken at frequent intervals, the amount of heat brought out

* Jour. Physiol., 16, 123.

t Archives de Physiol., 1895, p. 619.

t Ibid., 1898, pp. 538 and 613.

§ Ibid., 1896, p. 329.

1 U. S. Dept. Agr., office of Experiment Stations, Bulletins 63 and 69.

METHODS OF INVESTIGATION. 249

in the water-current is readily calculated. To this is added the
latent heat of the water-vapor brought out in the ventilating air-
current. By means of ingenious electrical devices, a description
of which would occupy too much space here, the temperature of the
interior of the apparatus is kept constant, and any loss of heat by
radiation through the walls or in the air-current is prevented. In
'■xperiments the apparatus has given extremely accurate re-
sults.

An especial advantage of this apparatus is that it is practicable
to make it of large size, and also to continue the experiments for an
indefinite length of time. The original apparatus was of a size
sufficient for experiments on man, while all previous forms were
restricted to experiments on small animals. Recently a modified
At water-Rosa apparatus has been completed under the writer's
direction at the Pennsylvania Experiment Station, with the co-
operation of the Bureau of Animal Industry of the United States
Department of Agriculture, of a size sufficient for investigations
with cattle, and still larger ones are in process of construction.

Computation of Heat Production. — The respiration-calorim-
eter, in its more perfected forms, is a complicated and costly appara-
tus both in construction and use, and, moreover, is a rather recent
development. It was natural, therefore, that attempts should be
made to determine the heat production indirectly by computations
based on the kind and amount of matter oxidized in the body.

We may conveniently distinguish three distinct although closely
related methods of attacking the problem, all of which assume as a
fundamental postulate that the oxidation of a given substance in
the body liberates the same amount of energy as does its oxidation
outside the organism. In the next chapter we shall examine into
the correctness of this postulate; for the present we are con-
cerned simply with the methods of computation based on it.

Computation from Gaseous Exchange. — From a knowledge of the
ultimate composition and heat of combustion of a substance it is
easy to compute the amount of heat which will be produced by the
oxidation of an amount of it sufficient to yield a unit of carbon
dioxide or to consume a unit of oxygen. Conversely, then, we can
compute from the carbon dioxide evolved or the oxygen consumed
in a given time the corresponding amount of energy liberated.

25°

PRINCIPLES OF ANIMAL NUTRITION.

Such computations have been made by different authors for the
three principal classes of nutrients, viz., the proteids, carbohydrates,
and fats, the results of a few of which are as follows:

Magnus-
Levy.*

Zuntz.t

Kaufmann.J Laulani^.§

Per

Liter

co2

Cals.

Per

Liter

o,

Cals.

Per

Liter
COa
Cals.

Per

Liter

o3

Cals.

Per
Liter

co3

Cals.

Per Per
Liter Liter

0, co„

Cals. Cals.

Per

Liter

Oa

Cals.

Proteids |[

5.464 4.289

5.644
6.628

4.476 5.569
4.686 6.648

4.647 . . .
4.650 6.571
5.056

4.95

4.6

Fat

6.586
4.915
4.976

4.676
4.915
4.976

4.6

5.056

Starch

5.047

5.047

Cane-sugar

5.09015.090

4.95

1

Kaufmann also computes from his theoretical equations already
given in Part I (pp. 38 and 51) the evolution of heat per liter of
oxjrgen in the various processes of partial oxidation which he be-
lieves to take place in the body, with the following results :

Albumen to fat and urea 4 . 646 Cals.

" " dextrose and urea 4.460 "

Fat (stearin) to dextrose 4.067 "

Disregarding the minor differences in the figures of different
authorities, it is evident that the amount of heat produced bears a
much more constant relation to the oxygen consumed than to the
carbon dioxide produced. For the fats and proteids,. especially, the
difference is comparatively small. In the case of an animal metab-

* Arch. ges. Physiol., 55, 9.

\Ibid., 68, 191."

% Archives de Physiol., 1896, pp. 329, 342, 757.

§ Ibid., 1898, p. 748.

|| As pointed out on pp. 7 1-75 the determination of the respiratory exchange
corresponding to a unit of proteids is not a simple matter In the table
Kaufmann's and Laulanie's figures are based upon the theoretical equation
(p. 75) for the conversion of albumin into carbon dioxide, water, and urea,
while those of Magnus-Levy and Zuntz are derived largely from determina-
tions and estimates by Rubner (Zeit. f. Biol , 21, 363) and others of the
proximate composition of the urine of meat-fed animals. As will appear
later, these figures are not applicable to the urine of herbivora.

METHODS OF INVESTIGATION. 251

olizing substantially proteids and fat, then, such as a fasting animal
or one consuming only those two nutrients, a determination by any
of the methods indicated in Chapter III of the amount of oxygen
consumed will afford the basis for at least an approximately correct
computation of the energy liberated during the same time, par-
ticularly when, as is often the case, the proteid metabolism consti-
tutes but a small proportion of the total metabolism. For the
carbohydrates the figures are somewhat higher, and where these
bodies constitute a considerable portion of the food the error will
be more serious, but even then the results will be of value and
especially will afford relatively correct figures for the heat produc-
tion on the same diet at different times.

The computation from the gaseous exchange of the amount of
energy liberate d assumes a more exact form in case it is desired
to determine the increment arising from some change in the
conditions of the experiment, notably from an increase in the
muscular work done. In the latter case, as we have seen (Chap-
ter VI), the increased metabolism is largely or wholly that of non-
nitrogenous matter. Such being the case, we can compute in the
manner indicated on p. 76 from the increments of carbon dioxide
and oxygen caused by the work the proportion of each gas corre-
sponding respectively to the oxidation of fat and of carbohydrates,
and from this it is easy to compute the corresponding amounts of
energy. Thus, to take the example from Zuntz's investigations
there given, the increments of oxygen and of carbon dioxide pro-
duced by the performance of 1 kgm. of work in the case of a dog
were computed to be divided as follows:

Oxygen

ConMimed,
c.c.

Carbon Dioxide

Produced,

c.c.

By fat

" carbohydrates

0.6939
0.9765

0.4905
0.9765

Total

1.6704

1.4670

From this, using Zuntz's factors and assuming that there was no
change in the proteid metabolism, the total excess of energy liber-
ated in the body during work over that metabolized during rest is
computed as follows:

252 PRINCIPLES OF ANIMAL NUTRITION.

Energy from fat 4.686 cals. X 0.6939 = 3.252 cals.

" carbohydrates ... 5 . 047 cals. X 0 . 9765 = 4 . 927 "

Total 8.179 "

It is obvious that this method of computation affords the means
of comparing the total energy metabolized during the performance
of a measured amount of work with the quantity recovered in the
work itself. It has been extensively used for this purpose by Zuntz
and bis associates, especially in his investigations in conjunction
with Lehmann and Hagemann* upon work production in the horse,
which will be considered in a subsequent chapter. The same
authors f show that the error introduced by the assumption of
unchanged proteid metabolism is too small to be of any significance.

Computation from Total Excreta. — The method just described
naturally leads up to a computation based on the gaseous exchange
combined with a determination of the urinary products, particu-
larly nitrogen. The latter shows the total amount of proteicls
metabolized. If we also know, or can compute with sufficient
accuracy, the carbon, hydrogen, and oxygen of the urinary solids
we have the data from which to compute the portion of the respira-
tory exchange due to the protein (see p. 75) and the corresponding
amount of energy liberated. The residues of carbon dioxide and
oxygen can then be distributed between the fats and carbohy-
drates in the manner already described. This method has been
extensively employed by Kaufmann.J As already stated, he com-
putes the gaseous exchange of the proteids on the assumption of an
oxidation to carbon dioxide, water, and urea only, an assumption
which, as we have seen, is in some cases considerably wide of the
truth.

It is, of course, essential that experiments by this method shall
cover a sufficient length of time to ensure that the nitrogen excretion
corresponds with the actual proteid metabolism. It is therefore
inapplicable to periods of from a few minutes to an hour or so, such
as have been generally employed in experiments based on the gas-
eous exchange only. Kaufmann's experiments extended over five

* Landw. Jahrb., 18. 1; 23, 125; 27, Supp. III.

■[Ibid., 27, Supp. III., p. 251.

% Archives de Physiol., 1896, pp. 329, 342, 757.

METHODS OF INVESTIGATION. 253

hours, but it is open to serious question whether such a period is
sufficiently long.

Rubner * has made extensive use of a method substantially the
same as that just outlined, but differing in details. The computa-
tion is based upon the total nitrogen and carbon (determined or
estimated) of urine, feces, and respiration for twenty-four (or
twenty-two) hours, the feces being regarded as substantially a
metabolic product. The oxygen consumption is not determined.
From the results for nitrogen and carbon the proteid and fat meta-
bolism is computed in the manner explained in Chapter III (p. 78).
For each gram of carbon in the fat metabolized Rubner reckons
12.31 Cals. of energy, equivalent to 9.4 Cals. per gram of fat, while
for each gram of excretory nitrogen (urine and feces) he uses an
energy value based on previous experiments f hi which the actual
heats of combustion of proteids and the products of their meta-
bolism were determined. These results will be considered in another
connection (Chapter X). The resulting values for the evolution
of energy corresponding to each gram of excretory nitrogen are :

Fasting (mammals) 24 . 94 Cals.

(birds) 24.35 "

Lean meat fed 25. 98 "

Extracted lean meat fed 26.66 "

These factors were obtained in experiments on dogs and in
strictness apply only to carnivorous animals. By their use. espe-
cially if average figures are assumed for some of the minor quanti-
ties, such as the carbon of the feces and urine, the determination
of the heat production of a quiescent animal in this indirect way
becomes a relatively simple matter, while comparisons with direct
calorimetric results have shown it to be quite accurate. As was
pointed out on p. 78, however, when carbohydrates enter largely
into the diet the results are ambiguous, and this fact as well as
the marked differences in the character of the excreta forbid its
application to herbivorous animals.

Cleavages. Hydrations, etc. — Both the above methods of comput-
ing the heat production of an animal assume that the gaseous ex-

* Zeit. i. Biol., 19, 313; 22, 40; 30, 73.
f Ibid., 21, 250 and 337.

2 54 PRINCIPLES OF ANIMAL NUTRITION.

change is brought about by what is, in effect, a process of oxidation
simply. That many other chemical processes take place in the
body is, however, well known, and Berthelot * in particular lays
special stress upon the possibility of numerous cleavages, syntheses,
hydrations, and dehydrations in which the respiratory quotient
may vary between wide limits and in which the heat production is
not necessarily proportional to either the oxygen consumed or the
carbon dioxide generated. An example of such a process is the
formation of fat from carbohydrates, which, as we have seen, may
be regarded in the light of an intra-molecular combustion in which
no oxygen from outside is consumed, but in which there is an evolu-
tion of heat. As an illustration of the opposite possibility — an
evolution of heat without production of carbon dioxide — Berthelot
instances f the oxidation of a molecule of ethyl alcohol by suc-
cessive atoms of oxygen to ethyl aldehyde, acetic acid, glycollic
acid, oxyglycollic acid, oxalic acid, and finally carbon dioxide and
water. Only in the last of these stages is there an evolution of
carbon dioxide, yet in each stage there is an evolution of heat vary-
ing from 39.9 Cals. to 73.3 Cals. per atom of oxygen.

But while the possibility and even probability of similar reac-
tions in the body of the animal cannot be denied, it certainly
seems very questionable, in the light of the results to be considered
in the next chapter, whether they have any material bearing upon
the determination of the general balance of energy. We know at
least approximately the final products of metabolism, and accord-
ing to the law of initial and final states (p. 228) the intermediate
reactions can only affect the total amount of energy liberated in
case some of the intermediate products are retained in the organism.
The only material which we know to be stored up in any consider-
able quantity in the normal body, however, is fat, and the amount
of this we can at least approximately determine. It is of course
possible that in an experiment covering a few minutes only, these
intermediate reactions may seriously affect the result, but in an
experiment covering several hours or a whole day we can hardly
conceive such to be the case. Indeed we may probably go still
further. It seems to be a general physiological law that the func-
tions of the organism are adjusted to a certain average composition
* Chaleur Animale, Part I. t Loc, cit., p. 44.

METHODS OF INVESTIGATION. 255

of its tissues and fluids, and that even a comparatively small varia-
tion in the latter calls into action compensatory processes. A
striking illustration of this is seen in the promptness with which the
respiratory and vascular mechanism reacts to the changes produced
in the blood by muscular activity (compare Chapter VI). It seems
improbable, therefore, that any sufficient accumulation of the in-
termediate products of metabolism can take place to seriously in-
fluence the results of any but very short experiments. That the
methods employed involve other sources of error has already ap-
peared, but with due allowance for these it would appear that the
results are worthy of a large degree of confidence.

Computation from Carbon and Nitrogen Balance. — The method
of computing the heat production from the total excreta, as cm-
ployed by Rubner and others for carnivorous animals, wo have seen
to be inapplicable to herbivora. It, however, shades naturally into
a third method, of general applicability, which consists in combining
with a determination of the carbon and nitrogen balance by means
of the respiration apparatus direct determinations of the potential
energy of the food and of the visible excreta by the methods already
indicated. Kellner has made extensive use of this method, and the
following example, taken from his earliest investigations,* will
serve to show clearly the nature of the method. The ox experi-
mented upon was fed daily 8.5 kgs of meadow hay. Respiration
experiments showed that on this ration there was a daily gain by
the animal of 6.2 grams of nitrogen and 127.2 grams of carbon,
equivalent to 37.2 grams of protein and 140 8 grams of fat. the
potential energy of which can be computed from the data on p. 244.

From determinations of the heats of combustion of food, feces,
and urine, assuming the combustible gases excreted to consist only
of methane, the balance of energy is computed as in the table on
p. 256.t

Having included under the head of outgo all the known forms
in which potential energy as such may be disposed of, the balance
of 14,819.5 Cals. is regarded as having been liberated as kinetic
energy, and, since no external work was performed, to have taken
finally the form of heat. Short of an actual calorimetric experi-

* Landw. Vers. Stat., 47, 275.

t The figures are the corrected ones given in Landw. Vers. Stat., 53, 9.

256

PRINCIPLES OF ANIMAL NUTRITION.

Income,
Cals.

Outgo,
Cals.

Food

32,177.3

Feces

11,750.3

Urine

1,945.0

Methane .

2,113.7

211.2

Fat "

1,337.6

14,819.5

32,177.3

32,177.3

ment, this is the most accurate method available for determining
the heat production of an animal during a considerable period of
time. To short periods it is inapplicable for obvious reasons.

Heat Production and Heat Emission. — In conclusion, it is
important to remember that what is determined more or less accu-
rately by all these indirect methods is the amount of energy which
takes the kinetic form, and in the absence of mechanical work
finally appears as heat. In other words, what is determined is the
heat production by the animal. On the other hand, the results ob-
tained with an animal calorimeter show the amount of heat given
off by the animal during the experiment, that is, the heat emission.
But these two, heat production and heat emission, are by no means
necessarily equal. On the one hand, heat produced may be tem-
porarily stored in the body, or, on the other hand, heat retained in
the body from a previous period may be given off along with that
actually produced during the experiment.

This is sufficiently obvious in case of changes in the body tem-
perature, but even when the latter remains constant the possibility
of a temporary storage of the materials of the food, and especially
of water, in the body, must be considered. If, for example, the con-
sumption of water in an experiment exceeds the total amount giver.
off in the visible and gaseous excreta, the quantity of heat required
to warm the excess of water to the temperature of the body remains
in the animal as sensible heat. The heat is produced but not
emitted. If, on the other hand, the excretion of water exceeds the
consumption, sensible heat is removed from the body in this excess
and the emission of heat exceeds the production by a corresponding
amount. What is true of water is of course true also, ceteris paribus,
of the total income and outgo of matter, although the water, on

METHODS OF INVESTIGATION. 257

account of its large amount and high specific heat, constitutes the
most important factor. The skillful investigator will, of course,
seek to plan his experiments so as to avoid these fluctuations so far
as possible, but they can rarely be completely eliminated and
therefore we cannot expect that the emission of heat will correspond
exactly to the production.

CHAPTER IX.
THE CONSERVATION OF ENERGY IN THE ANIMAL BODY.

Throughout the preceding chapter, particularly in considering
the indirect methods of animal calorimetry, it has been assumed
that the law of the conservation of energy applies to the animal
body. This is the fundamental postulate upon which all study of
nutrition from the standpoint of energy is based, and it is of prime
importance, therefore, to examine into the experimental evidence
upon which it is based.

The processes of metabolism are essentially chemical processes,
and, like other chemical reactions, are accompanied by thermal
changes, resulting as a whole in a liberation of kinetic energy.
From this point of view, then, the subject may be regarded as a
branch of thermo-chemistry.

The applicability of the law of the conservation of energy, and
in particular of the law of initial and final states, to the most diverse
chemical reactions has been amply demonstrated by the investiga-
tions of Hess, Berthelot, Th< imsen, and others. It might seem, then.
in view of the chemical nature of metabolism, that we were justified
in assuming the same law to apply also to the reactions taking place
in the body, especially since investigations in other fields of science
have led us to regard it as one of the fundamental laws of the uni-
verse. On the other hand, however, the reactions occurring in the
body are vast in number, are of the most varied character — oxida-
tions, reductions, syntheses, cleavages, hydrations, etc. — are infi-
nitely more complex than those which the chemist can produce in his
laboratory, and finally, our knowledge of them is as yet but very
partial and fragmentary. Moreover, the matter composing the
body is living matter, and whatever view we may take as to the
nature of life the properties of living matter differ from those of

258

THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 259

dead matter, and we have no scientific right to assume in advance
of the evidence that no special forces arc operative in the former.
In brief, whatever may be the probabilities in the case the applica-
bility of the law to living beings as logically requires experimental
demonstration as did its applicability in physics or chemistry, and
no little labor has been within the past few years devoted to this
problem.

Nature of Evidence. — Before proceeding to a consideration
of the experiments bearing upon this question it will be well to
make clear the nature of the evidence required.

If the law of the conservation of energy applies to the animal,
the following are necessary consequences of it :

1. In an animal doing no work on its surroundings and neither
gaining nor losing body substance, the potential energy (heat of
combustion) of the food will be equal to the potential energy of the
excreta plus the kinetic energy given off in the form of heat plus
the energy expended in producing physical and chemical changes in
the body.*

2. In an animal doing work on its surroundings, but neither
gaining nor losing body substance, the potential energy of the food
will be equal to the potential energy of the excreta plus the energy
of the heat given off plus the energy of the work done plus the
energy expended in producing physical and chemical changes in
the body.

3. In an animal doing no work on its surroundings, but gaining
or losing body substance, the potential energy of the food will equal
the potential energy of the excreta plus the energy of the heat given
off plus the potential energy of the gain by the body (a loss by
the body being regarded as a negative gain) plus the energy ex-
pended in producing physical and chemical changes in the body.

4. In an animal doing work on its surroundings and gaining or
losing body substance the potential energy of the food will equal
the potential energy of the excreta plus the energy of the heat given
off plus the energy of the work done plus the potential energy of the
gain by the body (a loss by the body being regarded as a negative

♦Such as changes of temperature or aggregation, cleavages, syntheses,
etc. In case these resulted in an evolution of energy, this term of the equa-
tion would, of course, have a negative sign.

260 PRINCIPLES OF ANIMAL NUTRITION.

gain) plus the energy expended in producing chemical and physical
changes in the body.

In actual experimentation it is practically impossible to so
adjust the food that there shall be absolutely no gain or loss of body
substance, although its amount can be made relatively small.
Experiments on this subject, then, necessarily fall under Cases 3
or 4, and as a matter of fact, in all the experiments hitherto
made, the subject has either done no mechanical work or this
work has been converted into heat inside the calorimeter and
measured along with that directly given off by the body, so that
all these experiments fall under Class 3.

The quantities to be determined, then, are

1. Potential energy of food.

2. Potential energy of excreta (feces, urine, hydrocarbons, etc.).

3. The heat produced (including that into which any mechani-
cal work is converted).

4. The potential energy of the gain or loss of body substance.

5. The energy expended (or evolved) in producing changes in
the body.

If we can determine accurately these five factors, and having
done so find the equality stated under 3 to exist in a large number
of cases, we shall be justified in the conclusion that the law of the
conservation of energy applies to the animal organism.

The methods by which the first four of the above factors may be
determined formed the subject of the preceding chapter. As re-
gards the fifth, it has commonly been assumed that in an experi-
ment begun and ended at the same hour of the day and under com-
parable conditions, which has been preceded by a considerable
period of uniform feeding and other conditions, and in which the
subject was in apparent good health, the initial and final states of
the body are substantially the same. While it seems highly prob-
able that this is true, an actual demonstration of its truth is not an
easy matter. With respect to the body temperature in particular
it is worthy of note that even a slight variation would materially
affect the results. Thus in a 1000-pound ox, assuming an average
specific heat of O.S, a variation of one fifth of a degree Celsius would
correspond to 160 Cals. The rectal temperature affords the best
available means of control on this point, and a very ingenious

THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 261

method for its determination at frequent intervals has been de-
scribed by Benedict cv. Snell.* While it is true that the rectal
temperature is not necessarily the average of that of the whole body,
we may probably assume with safety that the variations of the two
will substantially correspond and therefore that the error introduced
by the use of the former will be insignificant.

The question of possible chemical and physical changes in the
make-up of the tissues has already been considered in the preceding
chapter, where it was pointed out that their effect is in all proba-
bility negligible in experiments of any considerable duration.

Early Experiments. f — From a slightly different point of view
the question under consideration may be stated as that of the source
of animal heat. Is the energy given off by the animal in this form
(in the absence of external muscular work) equivalent to the heat
produced by the oxidation of the same materials outside the body?
In this form the question could scarcely fail to attract attention as
soon as man began to observe and reflect upon the phenomena of
nature.

The ancients regarded the "animal heat" or "vital heat" as
"innate" and having its source in the heart. In more recent times'
it was attributed in a vague way to chemical action, and later was
also explained as resulting from mechanical action and in particular
from the pulsation of the blood in the blood-vessels. Our real
knowledge of the subject, however, dates from the discovery of
oxygen and from those researches by Lavoisier and others which
established the true nature of combustion and laid the foundations
of modern chemistry.

Black I discovered that carbon dioxide was produced in animals
by a process of combustion, and Lavoisier. § along with his more
purely chemical researches, studied the question of animal heat and
advanced the hypothesis that respiration consists essentially of a
slow oxidation of the carbon and hydrogen of the food by the oxygen
of the air, and that this slow combustion is the source of the animal
heat.

* Arch ges Physiol., 90, 33.

t This paragraph follows substantially the historical introduction to
Rubner's paper, "Die Quelle der thierischen Warme.'' cited below.
X Lectures on Chemistry, edited by Robison, Edinburgh, 1803
§ Hist. Acad. Roy. d. Sci., Paris, 1780, 355.

262 PRINCIPLES OF ANIMAL NUTRITION.

The first part of this hypothesis was readily susceptible of verifi-
cation by a quantitative determination of the oxygen taken up and
the carbon dioxide given off, but the second portion was too bold to
secure general acceptance. Lavoisier, therefore, with the aid of
Laplace, subsequently attempted to secure experimental evidence
as to its truth. To this end they determined the amount of heat
given off by a guinea pig in an ice-calorimeter, while in a second
experiment the animal was placed under a bell-jar and the produc-
tion of carbon dioxide determined. Having previously determined
by means of the ice-calorimeter the heat of combustion of carbon,
the results of these two trials gave them data for comparing this
amount with that produced by the animal. The computed amount
of heat was 25.41 Cals. ; that produced by the animal 31.82 Cals.

Several sources of error were inherent in the experimental
methods adopted, of some of which Lavoisier was aware, which
tended to make the computed amount of heat too small. Taking
these into consideration, Lavoisier considered that the experiment
substantially confirmed his hypothesis.

At about the same time Crawford * was investigating the same
subject, and while his methods were rather primitive and his results
less accurate than those of Lavoisier and Laplace, his general con-
clusions were the same. Of later experiments may be mentioned
especially those of Despretz f and of Dulong.J Both investigators
employed very similar apparatus, viz., a water-calorimeter through
which a current of air was passed, the respiratory products and the
heat being determined in the same experiment. The proportion of
the oxygen consumed which united with hydrogen was also deter-
mined. Both investigators found more heat than they could ac-
count for by the oxidation of tissue and concluded that chemical
action is the chief but not the only source of animal heat.§

With the advance of physiological knowledge and the recogni-
tion of the multiplicity and complexity of the processes taking place
in the body, the combustion theory of the origin of animal heat
lost rather than gained ground. A few clear-sighted physiologists

* Experiments and Observations on Animal Heat, 1788.

t Ann. de Chim. et de Phys. (2), 26, 337.

tlbid. (3), 1, 440.

§ Compare Liebig's discussion of their experiments, Thierchemie, p. 28.

THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 263

still adhered to the unity and simplicity of the combustion theory,
but in general various subsidiary hypotheses were brought in to
account for the observed surplus, such as the motion of the blood,
friction, imbibition, etc.

Rubxer's Experiments. — The demonstration of the law of the
correlation and conservation of energy in the inorganic world sup-
plied the clue to a rational explanation of the energy manifestations
in the living organism, while the subsequent developments of thermo-
chemistry served also to demonstrate a material source of error in
the older experiments on animals. In those experiments the com-
puted heat production was based upon the amounts of carbon and
hydrogen oxidized and the heats of combustion of those elements,
the nitrogenous compounds not being considered. The body, how-
ever, does not oxidize free carbon and hydrogen, but various organic
compounds, while among its excreta are likewise incompletely
oxidized bodies. The computed heat production, therefore, in the
early experiments could not fail to be seriously erroneous. From
the new point of view, therefore, there appeared no reason to seri-
ously doubt that the animal heat has its sole source in the metab-
olism of food and tissue, or, in other words, that the law of the con-
servation of energy applies to the animal body. The first to under-
take an experimental demonstration of this fact by modern methods
was Rubner.*

His object being primarily to investigate the source of animal
heat, his experimental method could be somewhat abbreviated from
the general method outlined on p. 260. No external mechanical
work having been done by the animals, we have Case 3 of the
four possible ones there mentioned. If we let

F= potential energy of food,
E= " " " excreta,

G= " " " gain by body,

if = heat produced,

then, assuming the initial and final states of the body to be the same,
we have

F = E + G + H,

* Zeit. f. Biol., 30, 73.

264

PRINCIPLES OF ANIMAL NUTRITION.

which may also be given the form

H=(F-G)-E.

Rubner determined summarily the value of the quantity F — G
in the second member of the last equation by the method described
in Chapter VIII, p. 253, while the actual heat production was deter-
mined by means of his respiration-calorimeter.

The quantities actually determined in these experiments were
the weight and nitrogen content of feces and urine, the carbon
dioxide of respiration, and the heat produced. The carbon of feces
and urine was estimated from their nitrogen and the absence of
combustible gases in the respiratory products was assumed. From
the total excretion of nitrogen and carbon the amounts of protein
and fat metabolized are computed, it being assumed that all the
excretory carbon is derived from these two substances. The corre-
sponding amount of potential energy, equivalent to the expression
F — G in the equation above, can readily be computed from the heats
of combustion of fat and protein. From this the potential energy
of the excreta must be subtracted, and this Rubner virtually com-
putes from their total nitrogen on the basis of results obtained in
previous experiments with similar food.

A comparison of the heat production as thus computed with that
actually measured by means of the calorimeter gave the following
results :

Food.

Length
of Experi-
ment.
Days.

Total Heat.

Computed,

Cals.

Measured,
Cals.

Percentage
Difference.

Fasting ■}

Fat

Meat and fat ■]

Meat j

Total

5
2
5

8

12

6

7

1296.3
1091.2
1510.1
2492.4
3985 . 4
2249 . 8
4780.8

1305.1
1056.6
1495.3
2488 . 0
3958.4
2276.9
4769 . 3

45

17406.0

17349.7

+ 0.69
-3.15
-0.97
-0.17
-0.68
+ 1.20
-0.24

-0.32

While some of the individual experiments show not inconsider-
able discrepancies, the averages of computed and measured heat

THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 265

agree very closely and, granting the entire validity of the numerous
assumptions involved in this method, would seem to approach a
demonstration of the applicability of the law of the conservation of
energy to the metabolism of the animal. Aside from errors in the
estimation of the carbon of the excreta from their nitrogen, which
are probably small, the chief elements of uncertainty are the
assumptions as to the nature of the material metabolized in the
body and as to the heat of combustion of the excreta. As regards
the former point, Rubner himself points out (loc. cit., pp. 118-121)
that a portion of the carbon of the respiration may be derived from
glycogen, and even bases upon the calorimetric results in one case
a computation of the extent to which this may have occurred. The
latter, however, is obviously begging the question, and in his main
computations Rubner assumes that only protein and fat were meta-
bolized.

Laulanie's Experiments. — By means of his differential water-
calorimeter, Laulanie * has determined the respiratory exchange
and the heat production of animals, both fasting "and fed. The
nitrogen excretion does not appear to have been determined.
From the respiratory exchange the heat production is computed,
using the data given on p. 250, and compared with that obtained
calorimetrically. In the fasting experiments an evolution of 4.6
Cals. of heat is computed per liter of oxygen consumed. In the
experiments in which food was given the author computes from the
respiratory quotient the distribution of the oxygen between fat and
carbohydrates, neglecting the protein because it yields the same
amount of heat per unit of oxygen as does fat, and thence calculates
the heat production. Preliminary tests of the calorimeter, by
allowing water to cool in it, gave respectively 101.3 per cent., 100.9
per cent., and 99.7 per cent, of the theoretical results. The experi-
ments show a close agreement between the observed and computed
amounts of heat, as appears from the table at the top of page 266.

Atwater & Benedict's Investigations. — By far the most
extensive and complete data regarding the conservation of energy
in the animal body are those afforded by the investigations of
Atwater & Benedict f upon man. The experiments were made

* Archives de Physiol , 1898, p. 748.

t U. S. Dept. Agr., Office of Experiment Stations, Bull. 109 and 136;
Memoirs Nat. Acad. Sci., 8, 235.

266

PRINCIPLES OF ANIMAL NUTRITION.

Subject.

Two guinea-pigs. . .

Rabbit

Duck

Dog (2 expts.)

Total of all fast-
ing txpts

Two clogs

Guinea-pigs

Rabbit

Duck

Dogs

Food.

Third day of fasting

Second day of fasting. . .

Fasting for 2 days

" " 2 and 4 days.

300 grms. of meat.

Mixed diet rich in
carbohydrates

Legth
of

Experi-
ment,

Hours.

5*

4

12*

Oxygen
Con-
sumed,
Liters.

8.112

7.650

8.800

30.205

82 812
51.683
31.787
46.445
21.603
180.398

Resp.
Quo-
tient.

0.791
0.752
0.750
0.758

0.766
0.816
0.917
0.893
0.885
0.973

Heat Production.

Ob-

serv'd,
Cals.

Com-
puted
Cals.

Comp,
+ Obs.

315
190
480
943

935

741
23(1
050
.040
19'

100.6
99.8

100.3
98.2

98.8
99.2
99.0
98 6
98.2
100.5

with the aid of the respiration-calorimeter of Atwater & Rosa (p..
248), and in addition to the great pains bestowed to obtain accurate
results are especially distinguished by the fact that all the quantities
involved were, so far as possible, subjected to direct measurement,
estimates being avoided with the necessary exception of the poten-
tial energy of the gain or loss by the body. The sublingual or
axillary temperature of the subject was also measured in every case.
The following results of one of the earliest experiments (No. 5) may
serve to illustrate the general features of them all :

Income,
Cals.

Outgo,

Cals.

2655
2655

143

128

Loss by body :

-24

Fat

-73

Heat production

2379

102
2655

Aside from the loss of 97 Cals. by the body as computed from the
carbon and nitrogen balance, all the quantities in the above state-
ment represent actual determinations of energy and the account
balances within 102 Cals., which is 3.8 per cent, of the total energy
of the food or 4.1 per cent, of the computed heat production. To
put the matter in a slightly different way, the heat production as
computed by Kellner's method (p. 255) from the carbon and nitrogen
balance and the energy of food and excreta exceeds by 102 Cals,

THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 267

the heat production actually measured by the calorimeter. This
experiment was one of the two showing the greatest percentage
difference between the computed and the observed heat production.
In the following statement are tabulated the results of all the ex-
periments reported up to 1902, arranged without regard to the subject
of the experiment or the nature of the diet, but divided into two
groups according as active muscular work was or was not performed.

Gain

by

Body,

Cals.

Heat
Production.

Com-
puted,
Cals.

Ob-
served ,
Cals.

Difference.

Cal-
ories.

Per
Cent.

Work
Done,
Cals.

Rest Experiments :

No. 5

" 7

" 8

" 9

" 10

" 13

" 14

" 15

" 16

" 17

" 18

" 19

" 20

" 21

" 22

" 23

" 24

" 25

" 26

" 27

" 28

Totals

Work Experiments :

No. 6

" 11

" 12

" 29

" 30

" 31

" 32

" 33

" 34

Totals

Totals, rest and work

- 97

-204
4-266
+ 150
+ 159
+ 186
+ 158
+ 70
+ 88
+ 13S
+ 166
+ 330
-211
-266
+ 597
+ 75
+ 571
+ 396
+ 213
+ 137
+ 182

+ 3525

-415
-391
-308
-255
-234
-164
-347
-451
-388

2481
2434
2361
2277
2268
2112
2131
2357
2336
2289
2367
2220
2339
2304
2 ISO
2216
2238
2242
2043
2125
2067

47387

3829
3901
3922
3515
3479
3439
3573
3669
3629

2379
2394
2287
2309
2283
2151
2193
2362
2332
2276
24S8
2279
2303
2279
2258
2176
2272
2244
2085
2123
2079

-102

- 40

- 74 1
+ 32!
+ 15;
+ 39
+ 62
+ 5

- 4

- 13
+ 121
+ 59

- 36

- 25
+ 78

- 40
+ 34

+

-4.1
-1.6
-3.2

+ 1.4
+ 0.7
+ 1.8
+ 2.9
+ 0.2
-0.2
-0.6
+ 5.1
+ 2.7
-1.5
-1.1
+ 3.6
-1.8
+ 1.5
+ 0.1
+ 2.0
-0.1
+ 0.6

-2953 32956
• r,72 80343

47552

3726
3932
3927
3589
3470
3420
3565
3632
3487

+ 165 +0.35

103 -2.7

+ 31
+ 5
+ 74

- 9

- 19

- 8

- 37
-142

+ 0.8
+ 0.1
+ 2.1
-0.3
-0.6
-0.2
-1.0
-1.2

32748
80300

250
186
200
255
249
249
196
197
250

-208.-0.03 2032
- 43-0.05

268

PRINCIPLES OF ANIMAL NUTRITION.

In the former case the observed heat production includes the heat
into which the work was converted.

The total of all the experiments shows an almost absolute agree-
ment between the computed and the observed results. To a trifling
extent, however, this arises from a compensation between the rest
and work experiments, the computed heat tending to be slightly
too small in the former and slightly too great in the latter, but the
agreement in each series is so close as to amount to a demonstra-
tion of the applicability of the law of the conservation of energy to
the metabolism of the animal organism.

Benedict and Milner* in eleven later experiments on men
with a respiration calorimeter of the Regnault-Reiset type like-
wise obtained a good agreement between the observed and com-
puted heat production, as did also Benedict t in eighteen subse-
quent trials, the aggregate results of the two series being:

Computed
Heat Produc-
tion, Cals.

Observed
Heat Produc-
tion, Cals.

Difference.

Calories.

Per Cent.

Benedict & Milner . . .

95 075
102 078

95 689
101 336

+614
-742

+0.7
-0.7

The writer,! in conjunction with Fries, has shown that the
same thing is true of cattle, the aggregates of fifty -nine experi-
ments being:

Computed heat production . . 976 204 Cals.
Observed " " .. 980 234 "

Difference + 4030 Cals. = +0.4 per cent.

* U. S. Dept. Agr., Office of Experiment Stations, Bull. 175, 262.
t " Influence of Inanition on Metabolism," Carnegie Institution of Wash-
ington, Publication 77, 512.

% Jour. Am. Chem. Soc, 35, 1794.

CHAPTER X.

THE FOOD AS A SOURCE OF ENERGY— METABOLIZABLE

ENERGY.

With the establishment of the law of the conservation of
energy in its application to the animal body, and with the
development of the methods of calorimetric research briefly out-
lined in Chapter VIII, it has become possible to study success-
fully the problems of animal nutrition from a new standpoint, re-
garding the food as primarily a source of energy to the body and
tracing, to some extent at least, the transformations which that
energy undergoes in the organism and particularly the extent to
which the latter utilizes it for various purposes.

Some data regarding the total energy of foods and their constitu-
ents have already been given in Chapter VIII. It was there pointed
out, however, that the total energy, taken by itself, does not fur-
nish a measure of the nutritive value of a substance. It is now
necessary to enter upon the question of the availability of this
energy to the organism.

Total and Metabolizable Energy. — The heat of combustion
of the food represents to us its total store of potential energy. By
no means all of this potential energy, however, is accessible to the
organism. A part of what the animal eats is not food at all in a
physiological sense, but is simply waste matter which passes through
the digestive tract unacted upon. Furthermore, that part of it
which is digested and resorbed is not completely oxidized in the
body, but gives rise to the formation of excretory products which are
still capable of liberating energy by oxidation. We have, there-
fore, at the outset, to distinguish between the total, or gross,
energy of the food eaten, represented by its heat of combustion,
and the portion of that energy which can be liberated and utilized in

269

2 7° PRINCIPLES OF ANIMAL NUTRITION.

the organism. It is only this latter portion, of course, of which the
body can avail itself, and the term available energy has, therefore,
very naturally been proposed for it.

As will appear later, however, the terms available and availa-
bility may also be employed, and have actually been used, in a more
restricted sense to designate that part of the energy of the food
which can be applied directly by the organism to purposes other
than simple heat production. In order to avoid the confusion of
terms thus arising it has been proposed to modify the term available
by the words gross and net. The gross available energy, according
to this terminology, signifies all of the total energy of the food
which can be utilized by the body for any purpose whatever;
that is, it is available energy in the first of the two senses defined
above. Similarly, the net available energy signifies the available
energy in the second sense, or energy available for other purposes
than simple heat production. The term " fuel value " has also been
employed by some writers, notably by At water, to designate the
gross available energy.

It appears to the writer desirable, however, to avoid the double
use of the word available, even with the somewhat awkward modi-
fying terms proposed. Strictly speaking, what is meant by gross
available energy in the above sense is that portion of the potential
energy of the food which the digestive and metabolic processes of
the organism can convert into the kinetic form, and its measure,
according to the principles enunciated in Chapter VII, is the differ-
ence between the potential energy of the food and the potential
energy of the various forms of unoxidized matter rejected by the
organism. In other words, it is that fraction of the energy of the
food which can enter into the metabolism of energy in the body.
The writer, therefore, tentatively proposes for it the term metabo-
lizable energy, as expressing the facts without any implication as to
the uses made by the body of the energy thus metabolized.

Metabolizable energy, then, may be briefly defined as potential
energy of food minus potential energy of excreta, including under
excreta, of course, all the wastes of the body, visible and invisible.
The method is analogous to that of the determination of digestibility.
In both cases it is a calculation by difference, and the result shows
simply the maximum amount of matter or of energy put at the dis-

THE FOOD AS A SOURCE OF ENERGY. 271

posal of the organism without affording any clue to the use made
of it by the latter, that is, to its availability in the more restricted

In actual investigation, of course, the metabolizable energy of
the food is most accurately found by means of direct determinations
of the heats of combustion of the food and the waste products.
Except in the case of the intestinal gases no serious difficulties
stand in the way of these determinations, and with the present im-
proved and simplified methods of calorimetry it may fairly be
expected that, in exact experiments, at least the energy of the food,
feces, and urine will be directly determined, while it is not impossi-
ble that more extended investigations than are now available may
enable us to make, for different classes of materials, a fairly accurate
estimate of the intestinal gases. As results accumulate from such
investigations we shall gradually acquire a fund of information
regarding; the amount of metabolizable energy contained in foods
and feeding-stuffs which it is perhaps not chimerical to suppose may
one day largely take the place of our present tables of composition
and digestibility.

Up to the present time, however, but a comparatively small
number of experiments upon domestic animals are on record in which
the metabolizable energy of the food has been actually determined.
In a somewhat larger number of cases the loss of energy in feces
and urine has been determined, and in others that in the feces only.
As regards human food the data are somewhat more abundant,
but nevertheless by far the greater part of our scientific knowledge
of foods and feeding-stuffs is expressed in terms of (conventional)
chemical composition and apparent digestibility. If, therefore, we
would not forego the advantages which may be anticipated from a
study, from the new point of view, of the accumulated results of
the last half-century of experimental work in this domain, it is im-
portant that we be able to estimate as accurately as may be the
metabolizable energy of the food from its known or estimated com-
position and digestibility. Not a little labor has been expended
upon both aspects of the subject, particularly by Rubner in relation
to the carnivora and man, by Atwater and his associates with rela-
tion to human nutrition, by Kellner as regards ruminants, and by
Zuntz and his associates in the case of the horse.

272 PRINCIPLES OF ANIMAL NUTRITION.

§ i. Experiments on Carnivora.

The comparative simplicity and completeness of the digestive
processes of carnivora, together with the great variations which can
be made in their diet, have made them favorite subjects for physio-
logical experiments. It is possible to feed a dog or cat on what are
close approximations to simple nutrients for a sufficient length of
time to permit an accurate determination of the waste products,
while with herbivora this is impracticable for obvious reasons.

While earlier experimenters, among whom may be mentioned
Frankland,* Traube.f and Zuntz,{ have concerned themselves with
the question of the energy values of foods and nutrients, it is to the
fundamental researches of Rubner that we owe not merely more
accurate determinations of metabolizable energy, but in particular
a clearer conception of its actual significance in nutrition. Rubner's
experiments § were made chiefly with dogs and were directed
toward the determination of what he designates as the physiological
heat value of the more important proteid foods, corresponding
substantially to what is here called the metabolizable energy.

Proteids. — As regards the non-nitrogenous ingredients of the
food, Rubner assumes that, so far as they are digested, their metab-
olizable energy is the same as their gross energy, or, in other words,
that there are no waste products. For example, if a dog is given a
certain amount of starch and none appears in the feces it is assumed
that the starch has simply undergone hydration and solution in the
digestive tract without material loss of energy and that conse-
quently the full amount of energy contained in the starch is avail-
able in the resorbed sugar for the metabolism of the body. In
herbivora we know that there is a considerable production of gas-
eous hydrocarbons by fermentation in the digestive tract. The
respiration experiments of Pettenkofer & Voit on dogs, however
(compare p. 72), showed but a small excretion of such gases, while
Tappeiner || denies the presence of methane in any part of the dog's
alimentary canal. In the case of carnivora, then, the above

* Phil. Mag. (4), 32, 182.

f Virchow's Archiv., 29, 414.

% Landw Jahrb., 8, 65.

§ Zeit. f. Biol., 21, 250 and 337.

|| Quoted by Rubner, ibid., 19, 318.

THE FOOD AS A SOURCE OF ENERGY. 273

assumption is at least in harmony with current opinion. Rubner's
experiments were therefore directed to the determination of the
metabolizable energy of the proteids.

The earlier computations of the metabolizable energy of the
proteids by Frankland, Traube, Danilewski, and others * were af-
fected by two sources of error. First, the heats of combustion as
determined by the imperfect calorimetric methods then available
were seriously in error. Second, the manner of computing the
metabolizable energy from these data has been shown by Rubner
to be incorrect. Previous to his investigations the metabolizable
energy of the proteids had been very generally computed by deduct-
ing from their gross energy the energy of the corresponding amount
of urea. In other words, it was assumed that all the nitrogen of the
proteids was split off in the form of urea and excreted in the urine,
which was accordingly regarded as being practically an aqueous
solution of urea, and that the non-nitrogenous residue of the proteids
was completely oxidized to carbon dioxide and water. Rubner's
results show that this assumption is seriously erroneous and gives
too high results for the metabolizable energy.

In the first place, it neglects entirely one of the waste products,
viz.. the feces. The latter are to be regarded in the carnivora,
especially on a proteid diet, as a true excretory product, comparable
to the organic matter of the urine and containing at most but traces
of undigested food. This was earl}'' pointed out by Bischoff &
Voit f and is now generally admitted by physiologists. (Compare
p. 47.) In Rubner's experiments somewhat over 3 per cent, of
the energy of the proteid food was found in the feces.

In the second place. Rubner shows that the urine is far from
being a simple solution of urea.^ His previous investigations § had
shown that the extractives of lean meat the form of proteid most
commonly used in such experiments, pass through the system un-
changed and are excreted in the urine, thus increasing its content oi
energy. By feeding meat previously treated with water to remove

* Cf. Rubner, loc. cit., p. 341.

t Erniihrung des Fleischfressers, p. 291; compare also Miiller, Zeit f
Biol., 20, 327; Rieder, ibid., 20, 378; Tsuboi, ibid., 35, 68.
X Compare Chapter VIII, p. 241.
§ Zeit. f. Biol., 20, 265.

274 PRINCIPLES OF ANIMAL NUTRITION.

these extractives, he demonstrates that in this case also the urine
is far from being a simple solution of urea. With a daily excretion
of 13.22 grams of total urinary nitrogen, there was found in the urine
0.105 gram of kreatinin, 0.656 gram of cyanuric acid, and an un-
determined amount of phenol. The proportion of carbon to nitro-
gen in the urine was also notably higher than in urea, viz., 0.523: 1
in place of 0.428: 1, or an excess of about 20 per cent. Rubner
concludes that the only sure method of ascertaining the amount of
potential energy carried off in the urine is the direct determination
of its heat of combustion. Accordingly, in the experiments under
consideration, the urine was dried on pumice-stone and burned in
the calorimeter, a correction being made for the urea decomposed
during the drying. Danilewski,* about the same time, also re-
ported determinations of the heat of combustion of the dry matter
of human urine which, like Rubner's, showr an excess over that
c imputed from the urea present.

The materials experimented on by Rubner were prepared lean
meat, such as has been commonly used in feeding experiments,
and meat with the extractives removed by treatment with water,
the gross energy of each being determined by burning the dried
material in the calorimeter after having removed the fat by extrac-
tion with alcohol and ether.j The prepared material (in the moist
state) was fed to clogs for from five to eight days, during all or a
portion of which time the feces and urine were collected and their
content of nitrogen and energy determined. The amounts fed are
not stated, but the percentage of the total nitrogen fed which
reappeared in the feces is given. A third experiment on a fasting
dog was added in which the urine of the second, third, and fourth
days was collected and examined.

So far as the proteids are metabolized in the body all their nitro-
gen which does not reappear in the feces will be found in the urine.
On this basis the nitrogen per gram of dry proteids metabolized in
these f xperiments wras divided as shown in the following table. In
the case of the fasting animal, Rubner believes himself justified, on the
basis of other experiments, in assuming that the nitrogenous tissue

* Arch. ges. Physiol., 36, 230.

t Subsequent investigations have shown that the material thus prepared
still contains traces of fat.

THE FOOD AS A SOURCE OF ENERGY.

275

metabolized had substantially the same composition and heat-value
a^ the lean meat of the first experiment, and the feces are also
assumed to be similar.

Food.

Nitrogen of
Food,
Grms.

Nitrogen of
Feces,
Grms.

Nitrogen of
Trine,
Grms.

Lean meat

Extracted lean meat

Nothing (body tissue)

0.1540
0.1659
0.1659

0.0024
0 . 0023
0.0023

0.1516
0.1636
0.1636

The energy of the excretory products, calculated per gram of
nitrogen, was as follows:

Food.

Urine,
Cats.

Feces,
Cats.

Lean meat

Extracted lean meat

Nothing

7.450
6 . 695
8.495

70.290
81.515

A comparison of the above results for the urine with the energy
of urea (5.41 Cals. per gram of nitrogen) fully confirms the conclu-
sions already drawn from its chemical composition.

From the figures of the last two tables, together with the heats
of combustion found for the food consumed, viz.,

Lean meat, fat removed 5 . 345 Cals, per gram

" " extractives and fat removed... 5. 754 " " "

we can readily compute the energy of the excreta and by difference
the metabolizable energy of the food per gram, as follows:

Energy of food

" feces

" urine

Metabolizable energy

Lean Meat.

Cals.

0.1683
1.1294
4 . 0473

5.3450

Ext racted
Lean Meat.

Cals. Cals.

5.3450

0 1854

1 . 0945
4.4741

Nitrogenous
Body Tissue.

Cals. Cals. I Cals.

5.7540 5.3450

0.1683

1.2878

3 . 8889

5.345O5.7540 5.7540

5.3450 5.3450

276

PRINCIPLES OF ANIMAL NUTRITION.

Rubner makes a slight correction in the above figures for the
energy of hydration and solution. The energy of the proteids was
determined in the dry state. They were fed, however, moist, and
it is known that an evolution of heat takes place when dry proteids
are brought in contact with water. Consequently the potential
energy of the moist proteids is less than that computed from the
calorimetric results. Rubner estimates this loss (Joe. cit., p. 307) at
0.5 per cent. The urea leaves the body in solution. Its solution
in water, however, causes an absorption of heat equal to 2.4 per
cent, of the total energy of the urea, and accordingly (neglecting
other organic matter) the heat value of the urine is higher than that
calculated from the calorimetric results upon the dried urine. Both
these errors tend to make the metabohzable energy appear too
great. Rubner's corrections are as follows:

Lean Meat
Ca)s.

Extracted

Lean Meat.

Cals.

Nitrogenous

Body Tissue,

Cals.

Metabolizable energy as above

Energy of hydration

" " solution

Corrected metabolizable energy

4.0473
0 . 02G9
0.0199
4 . 0005

4.4741
0.0288
0.0215
4.4238

3.

0 . 0269
0.0199
3.8421

The energy lost in hydration is of course, practically a diminu-
tion of the gross energy of the food. The energy absorbed in the
solution of the urea can be regarded either as a part of the energy of
the excreta or as being a part of the general expenditure of energy
by the body in internal work. (See the next chapter.)

Rubner * 1ms also computed the metabolizable energy of a num-
ber of proteids for which direct determinations are wanting. For
this purpose he uses the results of Stohmann f for the gross energy
and assumes, first, that the nitrogen will be divided between feces
and urine in the same ratio as in the experiment on extracted lean
meat, and second, that the energy of these excretory products per
gram of nitrogen will be the same as in that experiment. He thus
obtains the following results:

* hoc tit., p. 351.

t Landw. Jahrb., 13, 513.

THE FOOD AS A SOURCE OF ENERGY.

277

Substance.

Per Cent.
Nitrogen.

Gross

Energy

Per Grm.,

Cals.

Loss in

Excreta,

etc.,

Cals.

Motaboliz-
able Energy

I Per Grin.,
Cals.

Paraglobulin

Egg albumin

( lasein

Syntonin

Fibrin

Lean meat

Conglutin

Crystallized albumin . . .
Nitrogenous bodv tissue

15.6
15.7
15.2
16.6
16.6
15.4
17.5
19.2
15.4

5.634
5.577
5.715
5 . 754
5.508
5.345
5.359
5.595
5.345

1.263
1.270
1.311
1.329
1.329
1.345
1.390
1.555
1.503

4.371
4 . 307

404
424
179
000
969
090

3.842

§ 2. Experiments on Man.

Protein. — Rubner * has also reported a single experiment on a
man upon a diet of meat with a slight addition of fat. The results,
expressed in the same manner as those given in the preceding sec-
tion, that is, per gram of dry matter of the meat, were —

Energy of food 5 . 599 Cals.

" feces 0.434 Cals.

" urine 1.027 "

Metabolizable energy 4. 138 "

5.599 " 5.599 "

Quite a number of determinations are on record of the ratio be-
tween the nitrogen and the energy content of human urine. Rub-
ner | reports the following results upon various diets, including the
experiment on meat just quoted :

Diet Energy Per Grm.,

Nitrogen.

Mother's milk 12. 10 Cals.

Cow's milk— infant 6 . 93 "

" —adult 7.71 "

.Mixed diet, poor in fat 8.57 "

(I a a it tl o qo u

" " rich in fat 8.87 "

<t it a u u o a a a

« » « a— boy................. 6.42 "

Mixed diet — boy 7 . 50 "

Meat 7.69 "

Potatoes 7.85 "

* Zeit. f. Biol., 42, 272. t Ibid., P- 302.

278 PRINCIPLES OF ANIMAL NUTRITION.

With the exception of the mother's milk, the results show but a
slightly greater range than those on the dog. The results of Atwater
& Benedict,* cited on p. 242, when computed per gram of nitrogen,
give the following results:

Experiment No. 5 7.055 Cals.

" 6 7.839 "

" 7 8.060 "

" 8 8.447 "

" 9 8.326 "

"10 7.575 "

The same authors report t the average of 46 determinations as
7.9 Cals. per gram of nitrogen. Tangl % has reported materially
higher figures, especially for diets containing large amounts of
carbohydrates and fat.

In the case of a mixed diet more or less of the potential energy
of the feces may be derived from the non-nitrogenous nutrients
of the food, and we should hardly be justified in making for these
experiments a computation like that made for the meat diet. The
rather small range of the figures in most cases, however, would
seem to show that the metabolizable energy of the proteids of ordi-
nary mixed dietaries is substantially the same as that found by
Rubner for carnivora. Tangl's results perhaps suggest the possi-
bility of the occasional presence in human urine of non-nitrogenous
m alters similar to those found so abundantly in that of ruminants.

Rubner's Computations. — Rubner's earlier researches did not
include experiments upon man, but from the results given in the
foregoing section he endeavored to compute approximate factors
for the metabolizable energy of the mixed diet of man.§ For this
purpose he estimates that, on the average, 60 per cent, of the pro-
tein of the diet is derived from animal sources and 40 per cent, from
vegetable. For the animal protein he uses the value found above
for lean meat , and for vegetable protein the average of the values
for syntonin and fibrin (since these have an ultimate composition

* U. S. Dept. Apr., Office of Experiment Stations, Bull. 69.
I Eteporl Stuns Expt. Station, 1899, p. 100.
X Arch. f. (Anat. u.) Physiol., 1899, 261.
§ Loc. tit., p. 370.

THE FOOD AS A SOURCE OF ENERGY. 279

similar to that of the proteids of the grains). Correcting these
values for the error involved in the usual computation of protein
from nitrogen, he obtains as the average metabolizable energy of
the protein (N X 6.25) of a mixed diet 4.1 Cals. per gram.

For the fat and carbohydrates it is assumed that all their poten-
tial energy is metabolizable, but an allowance is made in the latter
case for the error due to the ordinary computation of the carbo-
hydrates by difference and for some minor sources of uncertainty.
Rubner's final averages are —

Protein (N X6.25) 4.1 Cals. per gram.

Fat 9.3 " "

Carbohydrates 4.1 " " "

The value for protein, by the method of computation, includes
an allowance for the metabolic products contained in the feces, but
neither it nor the values for the other nutrients include any estimate
for the loss through imperfect digestion. In other words, they
refer to the digested nutrients.

These figures were designed expressly for computing the metab-
olizable energy of human dietaries, and even for that purpose are
confessedly only approximations. In the absence of more exac
figures, however, they have been somewhat extensively used for
computing the metabolizable energy of the digested portion of the
food of domestic animals. For purposes of approximate estimates
such a use of them was perhaps justifiable, but in too many cases
their origin seems to have been forgotten and a degree of accuracy
ascribed to them which they do not possess. As will be shown
presently, later investigations have yielded materially different
results for the metabolizable energy of the several classes of nutri-
ents in the food of herbivorous animals.

Later Experiments. — Quite recently Rubner * has published the
results of some experimental investigations into the validity of the
averages or "standard figures " given above. In these experiments
the weights and heats of combustion of food, feces, and urine were
determined calorimetrically and the metabolizable energy as ob-
tained from these data was compared with that computed by the use
of the above factors. In making the latter calculation an allowance

*Zeit. f. Biol., 42, 261.

280

PRINCIPLES OF ANIMAL NUTRITION.

was made for the percentage loss in the feces equal to that observed
in the actual experiment. The results for the metabolizable energy
per day were —

Diet.

From

Calorimetric

Data,

Cals.

Computed,
Cals.

Potatoes only

Rye bread, bolted flour

" " unbolted flour

1911.4
2060.4
1773.1
2400.5
2698.8
2574.1
2549 . 6
1746.8
1765.5

1911.5
2079.3
1758.6

Mixed diet, poor in fat

2376.0

" rich " "

2600.0

" " " " " and carbohyd's | jx
Mixed diet — growing boys -j jx

2608.0
2610.0
1724.3
1737.3

As above noted, the computed results include a deduction for
the energy of the undigested matter in the feces. Rubner finds that
the heat of combustion of the organic matter of the latter varies
but little even on extremes of diet, so that the loss through this
channel is approximately proportional to the amount of the ex-
cretion. In the experiments on mixed diet the percentage loss of
energy in the feces varied from 4.3 per cent, to 7.9 per cent, of
the energy of the food.

Atwater's Investigations. — By far the most extensive data
as to the metabolizable energy of human foods and dietaries are
those derived from the investigations upon human nutrition made
under Atwater's direction by the United States Department of
Agriculture with the cooperation of Wesleyan University, the
Storrs Experiment Station, and various other experiment sta-
tions. Atwater & Bryant * have summarized these results in a
preliminary report of which the essential features are given in
the following paragraphs.

From the best data available, the heats of combustion of the
protein, carbohydrates, and fats of various classes of foods are esti-
mated. In these estimates account is taken as fully as possible of
the proportion of nitrogen in proteid and non-proteid forms, and
of the varying percentage of nitrogen in different proteids, the nitro-
gen factors used being those quoted on p. 6. The accuracy of

* Report Storrs Agr. Expt Station, 1899, p. 73.

THE FOOD AS A SOURCE OF ENERGY. 281

these estimates is checked by a comparison of the computed with
the actual heats of combustion of 276 different samples of food, the
average results showing a close agreement. Assuming the potential
energy of the urine to be all derived from the proteids, the average
of 7.9 Cals. per gram nitrogen given above (p. 278) corresponds to
1.25 Cals. per gram of protein (NX6.25) metabolized. The loss of
energy in the feces is estimated from a number of digestion experi-
ments upon single foods, the results being checked by a comparison
of the actual and computed apparent digestibility in 93 digestion
experiments on mixed diet. Finally, the proportions of the several
nutrients which are derived from different classes of foods in
average mixed diets are computed from the results of 185 dietary
studies. The final results thus obtained for the metabolizable
energy or ''fuel value'! of the nutrients are shown in the table on
page 282.

The average results for the ordinary mixed diet of man were —

Protein 4.0 Cals. per gram

Carbohydrates 4.0 " " "

Fat 8.9 " "

These factors are smaller than those proposed by Rubner, largely
because they relate to the total and not to the digested nutrients.
Comparisons of the computed with the actual metabolizable energy
of mixed dietaries, using the factors of the above table, gave con-
cordant result .

§ 3. Experiments on Herbivora.

The Mockern Investigations. — The larger share of our present
knowledge regarding the metabolizable energy of the food of her-
bivora is due to the investigations upon mature cattle which have
made by Kellner * since 1894 at the Mockern Experiment
Station. In the earlier series of experiments (including those by
G. Kuhn, reported by Kellner f) additions of commercial wheat
gluten and of starch were made to a basal ration consisting exclu-
sively of coarse fodder (hay or straw). In the later series of ex-
periments additions of the same substances and of oil and beet
molasses on the one hand, and of coarse fodders on the other hand,
were made to a mixed basal ration.

* Lanchv. Vers. Stat., 47, 275: 50, 245; 53, 1. f Ibid., 44, 257.

282

PRINCIPLES OF ANIMAL NUTRITION.

Nutrients

Propor-

Total

Fuel Value.

Kind of Food
Material.

Furnished rieats ot .energy

by Each Cpmbus- Njffito per Grm.

Group per tion per Aptuallv ln A\?""

100 GrmsJ Grm. Af": l> able Nu-

Total. ^ole trients.

Per Grm.

Available
Nutrients.

Per Grm.
Total
Nutri-
ents.

Protein :

Meats, fish, etc ...
Eges

Grms.

43.0

6.0

12.0

Cals.
5.65
5.75
5.65

Per Cent.
97
97
97

Cals.
5.50
5.60
5.50

Cals.
4.40
4.50
4.40

Cals.

4.25

4.35

Dairy products . . .

4.25

Animal food ....
Cereals

61.0

31.0
2.0
5.5
0.5

5.65

5.80
5.70
5.00
5.20

97

85
78
83
85

5.50

4.95
4.45
4.15
4.40

4.40

4.55
4.45
3.75
3.95

4.25
3.70

Legumes

Vegetables

Fruits

3.20

.2.90

3.15

Vegetable food .
Total food

Fat:

Meat and eggs ....
Dairy products . . .

39.0
100.0

60.0
32.0

5.65
5.65

9.50
9.25

85
92

95
95

4.80
5.20

9.00
8.80

4.40
4.40

9.50
9.25

3.55
4.00

9.00
8.80

Animal food ....
Vegetable food .

92.0

8.0

9.40
9.30

95

90

8.95
8.35

9.40
9.30

8.95
8.35

Total food

Carbohydrates :

Animal food. . . .
Cereals

100.0

5.0
55.0

1.0
13.0

5.0
21.0

9.40

3.90
4.20
4.20
4.20
4.00

95

98
9S
97
95
90

8.90

3.80
4.10
4.05
4.00
3.60
3.85

9.40

3.90
4.20
4.20
4.20
4.00
3.95

8.90

3.80
4.10

Legumes

Vegetables

Fruits

4.05
4.00
3.60

Sugars

3.95

3.85

Vegetable food .
Total food

95.0

100.0

4.15
4.15

97
97

4.00
4.00

4.15
4.15

4.00
4.00

In each experiment the digestibility of the ration was deter-
mined in the usual manner, and also the carbon of food, feces, urine,
and respiration (including methane, etc.), and the nitrogen and
heats of combustion of food, feces, and urine. The experiments
were made with every precaution and extended over a sufficient
length of time to ensure normal results. In each experiment the
respiratory products were determined in four or five separate periods
of twenty-four hours each. No such complete experiments with

THE FOOD AS A SOURCE OF ENERGY. 283

other classes of herbivorous animals have been reported, although
partial data are available from experiments on horses and swine.

Method of Stating Results. — The determination of the
metabolizable energy of a given ration by experiments like the
above is, in principle, very simple, although requiring many appli-
ances and much technical skill. When, however, we attempt to
generalize the results much greater difficulties are encountered
than in the cases previously considered.

In investigations upon carnivora and upon man the metaboliz-
able energy, as we have just seen, is usually computed upon the
total nutrients of the food — that is, upon the total amounts of
protein, carbohydrates, and fat — the deduction for the loss of
energy in the feces being included in the factors employed. This
is possible because the amount of potential energy thus removed
is small in itself and subject to relatively small variations on ordi-
nary diet and also because the crude nutrients composing the food
are largely chemical compounds which are at least fairly well
known.

The food of herbivora, on the contrary, is both more complex
and less well known chemically and contains a much larger and very
varying proportion of indigestible matter. As a consequence the
feces, instead of being chiefly an excretory product, consist mainly
of undigested food residues with but a small proportion of meta-
bolic products, and contain a large and variable part of the total
potential energy of the food. For all these reasons it seems likely
that any attempt to compute general factors for the metab-
olizable energy of the crude nutrients of feeding-stuffs similar to
those of Rubner or Atwater for the nutrients of human foods would
be confronted by almost insuperable difficulties.

It was natural, then, to attempt to eliminate these difficulties
by computing the results upon the digestible nutrients of the feed-
stuffs, but even here considerable difficulties arise. The di-
g( sted nutrients, particularly in the case of coarse fodders, are far
from being the pure protein, carbohydrates, and fats which our
ordinary statements of composition and digestibility assume them
to be. Furthermore, a considerable and a variable proportion of
the waste of proteid metabolism in the herbivora takes the form of
hippuric acid, a body less completely oxidized than urea, and ac-

284 PRINCIPLES OF ANIMAL NUTRITION.

cordingly containing more potential energy, while the urine of
sheep and cattle also contains not a little non-nitrogenous matter
of some sort. Finally, the slow and complicated process of diges-
tion in the herbivora is accompanied by fermentations and the
evolution of gaseous hydrocarbons (methane), and perhaps of
hydrogen, both of which carry off a more or less variable propor-
tion of the potential energy of the food. By means of experiments
with approximately pure nutrients it is possible to secure factors
for the metabolizable energy of the digested nutrients of con-
centrated feeding-stuffs, but in the case of coarse fodders about
all that is practicable in this direction is to compute the results
of experiments upon the total digestible matter.

There is possible, however, a third method, viz., to compute the
metabolizable energy upon the total organic matter of the feeding-
stuff, expressing it either as Calories per gram or pound of organic
matter or as a percentage of the gross energy. In the latter form
the result would be analogous to a digestion coefficient and would
show what proportion of the total energy of the material, as- deter-
mined by combustion in the calorimeter, was capable of being met-
abolized in the body. This method of expressing the results has
certain advantages in directness and simplicity, and especially in
putting the whole matter on the basis of energy values. In the
succeeding paragraphs the available data will be considered from
both the standpoints last named.

METABOLIZABLE ENERGY OF ORGANIC MATTER.

For a discussion of the matter from this standpoint we have to
rely almost entirely upon the Mockern investigations already men-
tioned.* In the case of those earlier experiments in which the ration
consisted exclusively of a single coarse fodder the computation of
the metabolizable energy of the latter is, of course, readily made.
In the experiments in which the food under investigation was added
to a basal ration the computation is somewhat less simple. The
details of both methods will be best explained by illustration.

* For later results on timothy hay, clover hay, maize meal, broom corn,
and oats, see Armsby & Fries: U. S. Dept. of Agriculture, Bureau of Animal
Industry, Bulletins 51 and 74; and Tangl; Landw. Jahrb., 34, 1.

THE FOOD AS A SOURCE OF ENERGY. 285

Total Organic Matter.

Coarse Fodders. Fed Alone. — For Ox Ff, fed exclusively on
meadow hay, Kellner obtained the following results * per day and
head :

Ingest a.

7,263f grams meadow hay 32,177 . 3 Cals.

Excreta.

2,547 f grams feces 11,750.3 Cals.

13,675 " urine 1,945.0 "

158.4 " methane 2,113.7 "

Total excreta 15,809.0 "

Difference 16,368.3 "

Had the ration exactly sufficed for the maintenance of the ani-
mal, the difference of 16,368.3 Cals. would represent exactly its
metabolizable energy. In reality, however, the nitrogen and car-
bon balance indicated a gain by the animal of 37.2 grams of protein
(Xx6.00t) and 140.8 grams of fat, equivalent to 1548.8 Cals., so
that the amount of energy actually converted into the kinetic form
was 16,368.3-1548.8 = 14,819.5 Cals. The potential energy of the
140.8 grams of fat, however, while it was not actually rendered
kinetic, might hare been had the needs of the organism required it.
Its retention in the potential form was, in a sense, temporary and
accidental, and its energy should properly be considered as a part
of the metabolizable energy of the food.

With the gain of protein, however, the case is different. Its
total potential energy equals 211.2 Cals., but not all of this is
capable of conversion into kinetic energy. According to Rubner's
results (p. 275) each gram of urinary nitrogen derived from the met-
abolism of the protein of lean meat corresponds to 7.45 Cals. If
this result is applicable to the forms of protein consumed by her-
bivora (and we shall see later that there is good reason to believe
that such is approximately the case), then the metabolism of the
37.2 grams of protein gained would have added 46.2 Cals. to the
observed potential energy of the urine, while the remaining 165
Cals. would have taken the kinetic form and should, therefore, be
regarded as part of the metabolizable energy of the food.

* hoc. cit., 53, 9. f Dry matter. X Compare pp. 67, 68.

286

PRINCIPLES OF ANIMAL NUTRITION.

In other words, to get at the actual metabolizable energy of the
ration in this experiment we must add to the observed potential
energy of the urine the amount of 46.2 Cals. by which it would have
been increased had all the protein of the food been metabolized, or,
what is the same thing, must subtract this amount from the ob-
served difference between food and excreta. This leaves 16,322.1
Cals. as the metabolizable energy of 7263 grams of dry matter or
6750 grams of organic matter in meadow hay, and the metabolizable
energy per gram of organic matter is therefore 2.418 Cals.

Computed in the above manner, the several experiments of this
category gave per day and head the following results :

Ani-
mal.

A

II

V

VI

XX

I

B
III
IV

Ration.

Meadow hay I . .

A. .

B. .

M. '.
II.

Average .

Meadow hay and oat straw .
Clover

o '3

Energy of

Food
Cals.

6750 32177.3
7816 36975.1

7199

7125

809

6815

7107

7328
7074

34211.5
33855 . 4
37167.3
32252 . 2

33794.4
34603 . 2
33405 . 1

Feces.
Cals.

Urine
(Cor-
rected),
Cals.

11750.3 1991.2
15521.111925.7*
153)2.2 1559.3*
13765.2 1737.9*
13880.7 3224.6
14669.0 1686.9

Meth-
ane.
Cals.

Metabolizable
Energy.

Total,
Cals.

Per
Grm.
Or-
ganic
Mat-
ter,
Cals.

2113.7 16322.1 2.418

3137.2 16388.1 2.097
2268.5 15071.52.093
2480.6|15871.7 2.228
2646.1 17415.9 2.230

2092.3 13804.0 2.026

14576.1
15505.1
15250.6

1440.3 '2331.2
1549.6* 2070. 1

15446.8
14878.4

1481.5* 2491 .3,14181 .7

2.182

2.173
2.031
2.004

* Energy of urine computed from its carbon content.

It should be noted that the figures for the energy of the feces in
these and in all the succeeding experiments include that of the met-
abolic products contained in them. While the latter are not derived
directly from the food they are a part of the expenditure made by
the body in the digestion of the food, and there is. therefore, the same
reason for including their energy as for including that of the organic
matter of the urine.

Both contain a certain amount of potential energy, derived
ultimately from the food, which has escaped being metabolized in

THE FOOD AS A SOURCE OF ENERGY.

287

the body and so is to be deducted from the total energy of the
food to obtain its metabolizable energy.

Experiments on timothy hay made by the writer,* in which the
amount of methane excreted was estimated from the amount of non-
nitrogenous nutrients digested, gave the following results, the cor-
rection for the gain or loss of nitrogen being computed in a slightly
different way from that explained above :

ENERGY PER GRAM ORGANIC MATTER.

Experiment I.
Cals

Experiment II.
Cals.

Experiment VI.
Cals.

Steer 1

2.104
2.007
1.904

1.838
2.164
1.824

2.139

" 2

2.175

" 3

2.176

Average

2.005

1.942

2.163

Average of all

2.037

It should be noted that the above figures are, as already stated,
approximate only. The energy of the methane was estimated, while
the determinations of the energy of the urine were not, in all cases,
satisfactory. We are probably justified, however, in regarding
the results as a close approximation to the truth.

Coarse Fodders Added to Basal Ration. — As an example of
this class of experiments we may take Periods 4 and 7 with Ox H.f
The rations in the two periods wrere as follows :

Total Weight.

Containing Organic Matter.

Period 4,
Kgs.

Period 7,
Kgs.

Period 4, | Period 7,
Grms. 1 Grms.

Difference.
Grms.

Meadow hay

4
3

1

8
3
1

3198 6495

23S6 2413

818 835

3297

Molasses-beet pulp

Peanut meal

27

17

8

12

6402

9713

3341

* Penna State Experiment Station, Bull 42, p. 153.
t hoc cil., 53, 278-335.

288

PRINCIPLES OF ANIMAL NUTRITION.

The potential energy of food and excreta (that of the urine cor-
rected to nitrogen equilibrium) and by difference the amounts of
metabolizable energy were:

Food,
Cals.

Feces,
Cals.

Urine

(Corrected),

Cals.

Methane,

Cals.

Metaboliz-
able Energy,
Cals.

Period 7

" 4

46,275.0
30,338.1

14,104.8
8,574.9

2,593.0
1,795.0

3,564.2

2,579.4

26,013.0
17,388.8

Difference ....

15,936.9

5,529.9

798.0

984.8

8,624 . 2

The metabolizable energy of the additional 3341 grams of or-
ganic matter eaten in Feriod 7 was therefore 8624.2 Cals. This
added food was intended to consist of hay, but the unavoidable
variations in the moisture content of the feeding-stuffs resulted in a
slightly greater consumption of the other ingredients of the ration
also. Of the 3341 grams of additional organic matter, 3297 grams,
as the previous table shows, were from the hay and 44 grams from
the basal ration. If, then, we would ascertain the metabolizable
energy of the added hay only, we must subtract from the difference
of 8624.2 Cals. between the two rations the metabolizable energy of
this 44 grams of organic matter from the other feeding-stuffs.

But while the gross energy of the latter is known, its metabo-
lizable energy cannot be computed exactly, since it is impossible to
determine what part of the energy of the excreta was derived from
this particular portion of the ration. By assuming, however, that
the same percentage of its gross energy was metabolizable as was
the case with the basal ration, and that its non-metabolizable energy
was similarly distributed between the various excreta, we may
compute a correction which, although not strictly accurate, will not,
in view of the small quantities involved, introduce any serious error.
In this case the gross energy of the 3297 grams of organic matter in
the added hay was 15,728.6 Cals., and the table takes the form
shown on the opposite page.

As thus computed, the metabolizable energy of the 3297 grams
of organic matter added to the basal ration in the form of hay was
8504.8 Cals., equal to 2.580 Cals. per gram. The total correction
amounts to 119.4 Cals., and even a considerable relative error in it
would not materially change the final results.

THE FOOD AS A SOURCE OF ENERGY.

289

Food,
Cals.

Feces,
Cals.

Urine

(Corrected),

Cals.

Methane,
Cals.

Metaboliz-

able Energy,

Cals.

Period 7

" 4

46.275.0 14,104.8

30.338.1 1 8,574.9

2,593.0
1,795.0

3,564 . 2
2,579 . 4

26,013.0
17,388.8

Difference ....
Correction ....

15,936.9
-208.3

5,529.9
-58.9

798.0
-12.3

984.8
-17.7

8,624.2
-119.4

Percentages.. .

15,728.6
100.0

5,471.0
34.78

785.7
5.00

967.1
6.15

8,504 . 8
54.07

In these computations it is assumed that the increased metabo-
lizable energy of the ration is derived entirely from the added feed-
ing-stuff, or, in other words, that the latter exerted no influence
either upon the digestibility of the basal ration or upon the propor-
tion of its energy lost in urine and in hydrocarbons. That such is
the case we have no means of proving, and it is, indeed, unlikely
that it is exactly true. The metabolizable energy of the added
feeding-stuff as above computed includes any such effects — that is, it
represents the net result to the organism of the added coarse fodder.

Table I of the Appendix contains the results of all the experi-
ments of this sort, computed in the manner illustrated above. It
will be noted that in all but two cases the correction is less than
in the above example. In each case the table shows also the per-
centage of the gross energy of the feeding-stuff which was found to
be metabolizable and the percentage carried off in each of the
excreta.

Summary. — The results of the foregoing determinations of the
metabolizable energy of the organic matter of coarse fodders ar<>
summarized in the table on page 290, which shows the gross and
metabolizable energy per gram of organic matter and also the
percentage of gross energy found to be metabolizable.

Concentrated Feeding-stuffs. — The metabolizable energy of
the organic matter of a concentrated feeding-stuff when added to
a basal ration can, of course, be computed by the same metln « 1 as in
the case of added coarse fodders, but, as we shall see, some special
difficulties arise in its application.

The only commercial concentrated feeding-stuff upon which
such experiments have been reported is beet molasses, although

290

PRINCIPLES OF ANIMAL NUTRITION.

Per Gram Organic
Matter.

Gross

Energy,

Cals.

Metaboliz-

able

Energy,

Cals.

Per Cent.

Metaboliz-

able.

Meadow Hay .

Sample I...

" A .

B, Ox V.
B. "VI.

B, average.

M

II

V, OxF...
V, " G...

V, average

VI, OxH, Period 2.
VI, " H, " 7.
VI. "J

" VI, average

Average of seven samples
Timothy Hay (approximate) . .

Oat Straw
OxF ..
" G...

Average.

Wheat Straw .

OxH

" J

Average.

Extracted Rye Straw :

OxH

" J

Average.

4.767
4.731

4.752

4.760
4.734

U.743 j

4.771

4.751
4.670

\ 4.816 {

U.743 j

4.251

2.418
2.097
2.093
2.228

2.161

2.230
2.026
1.933

2.087

2.010

2.520
2.580
2.540

2.547

2.213
2.037

1.760
1.688

1.724

1.411
1.540

1.475

3.261
3.164

3.213

50.72
44.32
44.06
46.88

45.47

46.86
42.80
40.75
44.00

42.38

52.82
54.07
53.24

53.38

46.56
43.62

36.54
35.05

35.80

29.75
32.47

31.11

76.71
74.45

75.58

experiments were also made by Kellner with wheat gluten, starch,
oil, and extracted straw, the aim of which was to determine the
metabolizable energy of the various digestible nutrients.

As an illustration of this class of experiments we may take one
upon molasses with Ox F,* comparing Period 3, on the basal ration,
* hoc. cit., 53, 172-227.

THE FOOD AS A SOURCE OF ENERGY.

291

with Period 6, on the same ration with the addition of molasses.
Comparing, first, the organic matter of the two rations we have
the following:

Total Organic

Matter Fed,

Grms.

Organic Matter in

Molasses,

Grms.

Period 6

S262
6630

1632

1702

" 3

0

1702

In the period with molasses 70 grams less of the basal ration
was consumed than in the period without, and a correction must
accordingly be made for this in the way explained on page 288.
The energy of food and excreta in the two experiments (that
of the urine being corrected to nitrogen equilibrium), together with
the correction for the 70 grams of organic matter, is shown in the
following table :

Food,
Cals.

Feces.
Cals.

Urine,
Gals.

Methane,
Cals.

Metaboliz-

able Energy,

Cals.

Period 6

" 3

37,946.2
31,327.8

11,365.8
9,599.2

1,786.1
1,530.0

2,397.9
2,560.7

22,396.4
17,637.9

Correction ....

6,618.4
+ 330.8

1,766.6
+ 101.3

256.1
+ 16.2

-162.8
+ 27.0

4,758 . 5
+ 186.3

6,949 . 2

1,867.9

272.3

-135.8

4,944 . 8

Dividing the metabolizable energy of the molasses, 4944.8 Cals.,
by the number of grams consumed, 1702, gives the metabolizable
energy of 1 gram of organic matter as 2.905 Cals.

Real and Apparent Metabolizable Energy. — The above
figures, however, demand more critical discussion. While the addi-
tion of molasses to the basal ration increased the amount of poten-
tial energy carried off in the feces and urine, it diminished that in
the methane; that is, it acted in some way to check the fermen-
tation in the digestive tract to which this gas owes its origin. In
other words, under the influence of the molasses the loss of energy
by fermentation of the basal ration was diminished by 135.8 Cals.,
and this amount, by the method of computation, is added to the
metabolizable energy of the molasses.

292

PRINCIPLES OF ANIMAL NUTRITION.

Moreover, the loss of energy in the feces is a complex of sev-
eral factors. The amounts of organic matter and of the several
nutrients excreted in the feces in the two periods (not corrected for
the 70 grams difference in organic matter consumed) were as
follows :

Organic
Matter,
Grms.

Protein,

Grms.

Crude
Fiber,
Grms.

Nitrogen-
free
Extract,
Grms.

Crude

Fat,

Grms.

" 3

2132
1797

403
284

595
527

1068
924

66

62

Difference

335 119

68

144

4

In addition to protein and nitrogen-free extract, which may
possibly represent indigestible material in the molasses, the feces
contained 68 grams more crude fiber and 4 grams more fat in Period
6 than in Period 3. These cannot have been derived from the
molasses, since the latter does not contain these ingredients. This
feeding-stuff, in other words, diminished the apparent digestibility
of the fiber and fat of the basal ration. As a matter of fact, the
ingredients of molasses being practically all soluble in water, it is
probable that nearly all the difference in the amount digested is
due to the diminished apparent digestibility of the basal ration
under the influence of the molasses.

The figure above given for the metabolizable energy includes all
these effects; that is, it shows the net result as regards energy ob-
tained from molasses fed under the conditions of these experiments,
the nutritive ratio of the basal ration being 1 : 5.S and that of the
molasses ration 1 : 6.4. To get at the actual amount of energy set
free from the molasses itself we should need to subtract from the
metabolizable energy as calculated above the energy corresponding
to the decreased excretion of methane and to add to it the metabo-
lizable energy corresponding to the decrease in the amounts of crude
fiber and ether extract digested, assuming that all the excess of
protein and nitrogen-free extract in the feces was derived from the
molasses. Computed in this way * the real metabolizable energy

* One gram of crude fiber = 3.3 Cals., and one gram of ether extract =
8.3 Cals. See p. 332.

THE FOOD AS A SOURCE OF ENERGY. 293

of the organic matter is 2.977 Cals. per gram. This would be a mini-
mum figure, while if we assume, as suggested above, that the mo-
lasses is entirely digestible, this figure is still too low and should be
increased to equal the gross energy of the organic matter.

If, however, either one of these latter values were used in com-
puting the metabolizable energy of rations, the results would obvi-
ously be too high unless corrections were made for the effect upon
the apparent digestibility of the other feeding-stuffs in the ration.
The figure first computed, while including several different effects,
nevertheless seems better adapted for use in actual computations
under average conditions, while the second gives the more accurate
idea of the store of metabolizable energy contained in the feeding-
stuff regarded by itself. The distinction is analogous to that
between apparent and real digestibility, and we may accordingly
speak of the apparent and the real metabolizable energy of feeding-
stuffs.

The whole of our present discussion of the metabolizable
energy of the organic matter (total or digestible) of food materials
relates to the apparent metabolizable energy. This is obvious as
regards the concentrated feeds from the above example, and logic-
ally applies also to those cases in which coarse fodders were added
to the basal ration, while in the case of the coarse fodders used alone
the distinction vanishes or is reduced to one between apparent and
real digestibility. The experiment with beet molasses well illus-
trates the difficulties in the way of determining the actual metabo-
lizable energy of feeding-stuffs which cannot be used alone.

Beet Molasses. — In two later experiments the addition of
molasses increased instead of diminishing the excretion of methane.
The results of the three experiments upon molasses, computed in
the same manner as the experiments upon coarse fodders, are con-
tained in Table II of the Appendix.

In the last two experiments 10 to 12 per cent, of the energy of
the molasses was lost in the products of intestinal fermentation,
but this was more than counterbalanced by its less effect upon the
digestibility of the rations, so that the final result is a higher figure
for the apparently metabolizable energy than in the first experi-
ment. Summarizing the results per gram as in the case of the
coarse fodders we have:

294

PRINCIPLES OF ANIMAL NUTRITION.

Gross

Energy,

Cals.

Apparently

Metabolizable

Energy,

Cals.

Per Cent
Metabolizable.

Sample I

4.084
| 4.188

1

2.905
3 . 308
3.044

71.16

II, OxH

" "J

79.00
72.70

Average, Sample II

3.176

75.85

Starch. — In a considerable number of the trials commercial
starch was added to the basal ration. The earlier experiments by
Kiihn were intended primarily to throw light on the possible for-
mation of fat from carbohydrates (compare p. 177). In them,
starch was added to a ration of coarse fodder only and the nutritive
ratio was purposely made very wide, the result being that more or
less of the starch escaped digestion. In the later experiments by
Kellner the starch was added to a mixed ration. Except in the
first two experiments the nutritive ratio was a medium one and
but traces of starch escaped digestion. It will be convenient,
therefore, to tabulate these two classes of experiments separately,
as has been done in Tables III and IV of the Appendix, the com-
putations being made as in the previous cases.

The same remarks which were made on p. 291 concerning the
distinction between real and apparent metabolizable energy apply
to these results. As computed they represent the net gain to the
organism from the consumption of starch and are the algebraic sum
of several factors. In particular, there was a considerable loss of
energy in the feces, even in the later experiments in which but
traces of the starch itself escaped digestion. In other words, the
starch either lowered the digestibility of the basal ration or in-
creased the formation of fecal metabolic products or both. The
method of computation adopted virtually looks upon this as part
of the necessary expenditure in the digestion of the starch. On
the other hand, there are several cases in which there was a de-
crease in the outgo of potential energy in the urine, even after the
results are corrected to nitrogen equilibrium. This, from our pres-
ent point of view, is credited to the starch and increases its
apparent metabolizable energy.

THE FOOD AS A SOURCE OF ENERGY.

295

The results on starch, expressed in Calories per gram of organic
matter, may be summarized as follows:

Gross

Energy,

Cals.

Apparent

Metaboiiz-

able

Energy,

Cals.

Per Cent.

Metaboliz-

able.

Kiihn's Experiments :

Sample I, Ox III

" " " IV

Average

Sample II, Ox V, Period 2a
" " " " 26
u u VI> it 26

<« ll ll ll II q

Average

Average of I and II

Kellner's Experiments :

Samples I and II Ox B

H ' II II II It /™i

Average

Sample III, Ox D

Average

Sample IV, Ox H

" J

Average

Average of III and IV . . .

4.249
4.249

3.029
2.705

4.249

4.236
4.236
4.236
4.236

2.867

3.347
3.161
3.018
2.964

4.236
4.243

4.165
4.165

4.165

4.156
4.156
4.156

4.151

4.180
4.180

4.180
4.168

3.123
2.995

2.027
2.028

2.028

2.792
2.969
3.214

2.992

3.313
3.017

3.165
3.079

71.21
63.71

67.46

78.95
74.68
71.26
69.98

73.72
70.59

48.62
48.68

48.65

67.20
71.44
77.32

71.99

79.22
72.16

75.69
73.84

"Wheat Gluten. — Seven experiments upon commercial wheat
gluten are reported, three by Kiihn and four by Kellner. The
chemical composition of the dry matter of the three samples of
gluten employed is shown in the first table on the next page.

In Kiihn's experiments the gluten caused a marked increase in
the apparent digestibility of the basal ration, which by our method
of computation augments the apparent metabolizable energy of
the gluten, so that in one case it amounts to over 101 per cent, of
the gross energy. The correction for organic matter is also rela-

296

PRINCIPLES OF ANIMAL NUTRITION.

Kiihn's

Experiments,

Per Cent.

Kellner's Experiments.

Oxen B and C,
Per Cent.

OxD,
Per Cent.

Ash

1.36

87.88
0.47
8.07
2.22

2.86
83.45

0.08
13.35

0.26

2.80

Crude protein

82.67

Crude fiber. .

0.43

Nitrogen-free extract

13.38

Ether extract

0.72

100.00

100 . 00

100.00

lively large. In Kellner's experiments the variations are not so
great. Computed as before, the results are as shown in Table V of
the Appendix. Summarizing Kellner's figures, as probably the
more accurate, we have per gram of organic matter —

Gross Energy.
Cais.

Apparent

Metabolizable

Energy,

Cals.

Per Cent.
Metabolizable.

Sample I, Ox B, Period 1

" " " " 3

" " " c

5.675
5 . 675
5.675

3.019
3.719
4.062

53.18
65.55
71.61

Average

5.675
5. SOS
5.742

3.600
4.061
3.831

63.45

Sample II, Ox D

69.90

Average of I and II

66.68

The wheat gluten was by no means pure protein and the above
figures of course apply to the feeding-stuff as a whole, including its
fat and carbohydrates as well as its protein. The question of the
metabolizable energy of the latteT will be considered subsequently.

Peanut Oil. — Three experiments with this substance are re-
ported by Kellner. In the first the oil was given in the form of an
emulsion, prepared by saponifying a small portion of the oil with
sodium hydrate, and was completely digested In the second and
third experiments it was emulsified with lime-water. In this form
it was less well digested, and in one case (Ox F) affected the digesti-
bility of the basal ration unfavorably. The results per gram of
organic matter, computed as before, constitute Table VI of the
Appendix and are summarized in the following table:

THE FOOD AS A SOURCE OF ENERGY.

297

Gross Energy,
Cals.

Metabolizable
Energy,

Cals.

Per Cent.
Metabolizable.

Sample I, Ox D

" 11, " F

" " " G

9.493
J- 9.464 j

7.382
4.973
5.623

77.76
52.52
59.39

Average, II

5.298

55.96

Summary. — The foregoing results may be conveniently sum-
marized in the table below, which shows the average gross energy
per gram of organic matter, the percentage of this gross energy
carried off unmetabolized in the various excreta, and the apparent
metabolizable energy expressed both per gram of total organic
matter and as a percentage of the gross energy:

Apparent

Percentage Loss in

Metabolizable

*

En'gy

per
Grm.

Energy.

Or-

Per

ganic

Grm.

Per

Mat-

Or-

Cent.

ter,

Feces.

Urine.

Methane.

ganic

of

Cals.

Mat-
ter.
Cals.

Gross
En'gy.

4.751
4 670

40.96
47 27

5.71
2.61

6.77
6 50*

2.213
2 037

46 56

Timothy hav

43 62

Oat straw

4.816
L743

4.251
4.188

56.80

58 . 22

12.75

9.93

2.08

2.37

-0.79

2.91

5.32

8.30

12.46

11.31

1.724
1.475
3.213
3.174

35 80

Wheat straw

31 11

Extracted rve straw

75 58

75.85

Starch, Kiihn's experiments

4.243

19.59

-0.92

10.74

2.995

70.59

" Kellner's experiments:

Heavv rations

4.165
4.168

55.91
17.61

-2.07
-0.66

-2.49
9.21

2.028
3.079

48 65

Medium rations

73 84

Wheat gluten, Kellner's experi-

5.742

20.16

13. OS

0.08

3.831

66.68

9.493

2 1. 34

-1.08

-1.02

7.382

77.76

(1 <. « y

9.464
9.464

64.77
41.00

-1.19
1.37

-16.10
-1.76

4 . 973
5.623

52 52

" " G

59 . 39

* Estimated.

Digestible Organic Matter.
As appears especially from the figures of the last table, the loss
of energy in the feces is the one which is subject to the greatest vari-
ation. In other words, the digestibility of a feeding-stuff is the

298

PRINCIPLES OF ANIMAL NUTRITION.

most important single factor in determining its content of metabo-
lizable energy. We may eliminate this factor by computing, on
the basis of the determinations of digestibility, the energy of the
digested organic matter and the proportion of this energy which
was lost in urine and methane or was metabolizable. In this way
we may secure figures which will be useful as a basis for estimat-
ing the energy values of rations in experiments in which it has
not been determined, and which will also afford, from some points
of view, a better idea of the relative extent of the losses other than
those in the feces.

Coarse Fodders Alone. — In the cases in which coarse fodder
constituted the exclusive ration the computation from the data
given on p. 286 and the amounts of organic matter apparently
digested in the several experiments is very simple and yields the
following results per gram digested organic matter:

Feed.

Gross
Energy.

Loss in

Metabolizable
Energy.

mal.

Urine,
Per
Cent.

Methane.

Per

Cent.

Per

Cent.

Per
Grm.
Cals.

A

II

V

VI

XX

T

Meadow hay I

" A

" li

" B

" M

" 11

4.509
4 . 408
4.317
4.398
4.452
4.371

9.75
8.98
8.25
8.65
13.85
9.59

10.35
14.62
12.00
12 . 35
11.36
11.90

79.90
76.40
79 . 75
79.00
74.79
78.51

3.603
3.368
3.443
3.474
3.330
3.432

4.409
4.377

9.85
4.95

12.09
12.33

78.06
82.72

3.442

Average lor timothy hay .

3.620

Coarse Fodders Added to Basal Ration. — From the re-
sults contained in Table I of the Appendix we may compute in sub-
stantially the same manner the total and metabolizable energy of
the digestible organic matter of the coarse fodders which were
added to the basal rations. In the table referred to, a correction
was introduced for the small differences in the amount of the basal
rations consumed in the periods compared. In the present com-
putations it has been assumed that the organic matter of these
small differences possessed the same digestibility as the total organic
matter of the basal ration. For example, in the case of Ox H,

THE FOOD AS A SOURCE OF ENERGY. 299

Periods 4 and 7, the amounts of digestible organic matter in the

two rations were:

Period 7 7106 grams

Period 4 4845 "

Difference 2261 "

The table shows, however, that in Period 7 the animal received 44
grams more of total organic matter in the basal ration than in
Period 4. In the latter period the digestibility of the organic
matter was found to be 75.7 per cent. Consequently, of the
excess of 2261 grams of digestible organic matter in Period 7
44X0.757 = 33 grams maybe regarded as derived from the basal
ration and 2261 — 33 = 2228 grams from the meadow hay added.
The corresponding corrected amounts of energy as given in the
same table are —

Total Cals.

Per Grm. Digested

Organic Matter

Cals.

Energy of added hav (corrected)

" " corresponding feces

15728.6
5471 0

" " digested matter

10257. G
8504.8

4 604

Metabolizable energy

3 817

The table on the next page contains the results of these com-
putations expressed per gram of digested organic matter. Kell-
ner* has made the same comparison in a slightly different man-
ner. His results for the gross energy of the digested matter are
given subsequently (p. 310). Those for metabolizable energy do
not differ materially from those here given.

Concentrated Feeding-stuffs. — The results of experiment
upon concentrated feeding-stuffs may of course be computed in the
same manner as those upon coarse fodders just considered. In the
case of materials like starch, oil. and gluten, however, which differ
widely from ordinary feeding-stuffs and which produce material
and readily traceable effects upon the digestibility of the basal
ration, relatively little value attaches to computations of the appar-
ent metabolizable energy, and only the average results with these
materials have been included in the summary on page 301 for the
* Loc. at , 53, 414 and 447.

3°°

PRINCIPLES OF ANIMAL NUTRITION.

"3

o

P4

Total

Energy,

Cals.

i
Loss in

Apparent

Metabolizable

Energy.

fa
'a
<

Urine.
Per Cent.

Methane,
Per Cent.

Per Cent.

Per Grm.,

Cals.

F
G

1

2

2

7
2

2

1

I
I

5
5

Meadow Hay :

V

" VI

" VI

Oat Straw :

" II

Wheat Straw :

" I

Average

Extracted Straw :

Sample I

" I

Average

4.356
4.496

8.61
7.72

10.20
12.58

81.19
79.70

3.537
3.583

H
H
J

4.426

4.531
4.604
4.506

8.17

8.32
7.66
9.64

11.39

7.74
9.43
9.33

80.44

83.94
82.91
81.03

3.560

3.803
3.817
.3.651

F
G

4.547

4.441
4.586

8.54

5.30
4.32

8.83

10.17
14.42

82.63

84.53
81.26

3.757

3.754

3.726

H
J

4.514

4.488
4.397

4.81

4.75
6.49

12.30

20.11
19.67

82.89

75.14
73.84

3.740

3.373
3.247

H
J

4.443

4.240
4.164

5.62

-0.52
-1.29

19.89

13.99
14.58

74.49

86.53
86.71

3.310

3.668
3.611

4.202

-0.91

14.29

86.62

3.640

sake of completeness. Those upon peanut oil have been omitted,
since the varying effect upon digestibility and upon the methane
fermentation makes the results as computed in this way appear
of questionable significance.

Summary. — The average results upon the various materials
experimented with are summarized on the opposite page.

As appears from the figures of the table, the apparent metabo-
lizable energy of the digestible organic matter of the different coarse
fodders is quite uniform. At first sight it appears somewhat sur-
prising that oat straw should show more favorable results than hay,
but the reason is readily seen in the smaller loss which takes place
in the urine; in wheat straw this loss is somewhat larger, while that

THE FOOD AS A SOURCE OF ENERGY.
ENERGY OF DIGESTED ORGANIC MATTER.

301

Total
Energy.

Cats.

Loss in

Apparent

Metabolizable

Energy.

Urine,
Per
Cent.

Methane,

Per

Cent.

Per
Cent.

Per

Grm.,

Cals.

Meadow hav (seven samples)

4.439
4.377
4.514
4.443

9.62
4.95
4.81
5.62

11.52
12.33
12.30
19.89
14.29
12.52
13.42
11.12
0.02

78.86
82.72
82.89
74.49
86.62
84.24
87.77
89.80
83.39

3.501
3.620

Oat straw

Wheat straw

3.740
3.310

Extracted straw

4.202 -0.91

3.640

Beet molasses, Sample II

Starch, Kiihn's experiments

" Kellner's experiments *

Wheat gluten, Kellner's experiments..

4.124
4.192
4.012
5.749

3.24

-1.19

-0.92

16.59

3.473
3.679
3.603
4.792

* Average of Samples III and IV.
in the methane is considerably larger, resulting in a materially
lower figure for metabolizable energy.

The results summarized in the two preceding tables, it should
be remembered, include, as already pointed out, all the effects pro-
duced by the addition of the material under experiment to the
basal ration; that is, they give the apparent metabolizable energy.
In the case of the coarse fodders no other method of computation
is practicable, and the same would be true in most instances of
ordinary concentrated commercial feeding-stuffs. In such cases it
is rarely possible to distinguish with accuracy between the energy
derived from the material experimented with and the subsidiary
effects of the latter upon the digestibility of the several in-
gredierits of the ration or upon the losses of energy in urine and
methane. We may anticipate, therefore, that the results of future
determinations of the metabolizable energy of ordinary feeding-
stuffs will of necessity be expressed substantially in the summary
manner here employed.

With the nearly pure nutrients used in many of Kellner's ex-
periments the case is different. Here it is possible to take account,
to a large degree, of the secondary effects, such as those, for exam-
ple, which in the case of wheal gluten result in figures exceeding
1 i( (per cent, for the apparent metabolizable energy, and to compute
results which represent more nearly the actual metabolizable energy
contained in the substances themselves. In these cases, therefore,

302

PRINCIPLES OF ANIMAL NUTRITION.

the averages of the tables are of less significance than the resu^s
given in the following pages, where the digestible nutrients are
made the basis of the computation.

ENERGY OF DIGESTIBLE NUTRIENTS.

The foregoing paragraphs have dealt with the apparent
metabolizable energy of feeding-stuffs, and the results have
been expressed in terms of total or of digestible organic matter,
or as percentages of gross energy. We now turn to a con-
sideration of such data as are available regarding the several con-
ventional groups of nutrients into which the food of herbivorous
animals is ordinarily divided and inquire whether it is possible to
compute average factors for their metabolizable energy which
shall be useful in themselves and be of value particularly for pur-
poses of comparison with earlier experiments. This was the special
purpose of Kellner's investigations, and his experiments simply
valuable data on these points as regards cattle and presumably
other ruminants, which may be supplemented to a certain extent
from experiments by other investigators upon horses and swine.
In considering the experiments from this standpoint, Kellner's
discussion and methods of computation have been closely followed,
the attempt being made to compute as accurately as possible the
real metabolizable energy of the several nutrients.

Gross Energy.
If it were possible to add pure nutrients to a basal ration and
be sure that they would have no effect upon the utilization of the
latter, it would be a comparatively simple matter to determine their
real metabolizable energy. As a matter of fact, however, as has
been seen, this is not possible. Not only is it impracticable to secure
large quantities of pure nutrients, but each such addition to the basal
ration is liable to affect especially the digestibility of the latter.
Consequently the difference in metabolizable energy between the
two rations fails to represent correctly the real metabolizable energy
of the nutrient added. In order to compute the latter we must
have a basis for correcting the results foT the small variations in the
amounts of other nutrients digested, and for this purpose we need
to know the total or gross energy of the digested matters.

THE FOOD AS A SOURCE OF ENERGY.

3°3

Crude Fiber. — In four of his experiments on hay fed alone,
Kellner * determined the heats of combustion of t ho crude fiber of

the food and of the feces with the following results per gram:

Crude Fiber of
Hay, Cals.

Crude Fiber of
Feces, Cals. .

I....

11....

III....

IV....

4 . 4350
4.3907
4.4548
4 . 4230

4 . 7378
4.7423
4.9037
4 . 7426

It appears from these figures that the crude fiber of meadow
hay has a higher heat value than pure cellulose (4.1854 Cals. accord-
ing to Stohmann), obviously due to the admixture of compounds
richer in carbon, while the indigestible crude fiber of the feces has
a still higher heat value. Merrill f has also reported similar results
for the crude fiber of oat hay, clover silage, and oat and pea silage,
as follows:

Crude Fiber of Fodder
Cals. per Grin.

Crude Fiber of Feces.
Cals. l or Orm.

< >.-it hay

4 . 405
4.610
4.667

4 . 662
5.215
4.820

< lover silage

Oat and pea silage ....

It follows that the digested portions of the crude fiber must
contain less potential energy than the crude fiber of the feed, and
from the known digestibility of the latter it is easy to calculate
what the heat of combustion of the digested portion must be.
Kellner 's results, after deducting 5.711 Cals. per gram for the slight
amounts of nitrogenous matter still contained in the crude fiber,
were as shown on the next page.

The average result shows that not only the chemical com-
position but likewise the heat of combustion of the digested crude
fiber varies but little from that of pure cellulose. Merrill's figures,
computed in the same manner from the data of the digestion
experiments reported by Bartlett,J but without the correction for

* Loc. rit., 47,299.

fMaineExpt. Station, Bull. 67, p. 170.

+ Ibid., pp. 140 and 150, and Report, 1898, p. 87.

3°4

PRINCIPLES OF ANIMAL NUTRITION.

Crude
Fiber,
Grins.

Equivalent

Energy,

Cal3.

i\

In hay..
" feces .

2832
1034

12532.8
4869.2

Digested fiber

Heat of combustion per gram

II -I

III

IV

In hay .
" feces.

Digested fiber

Heat of combustion per gram .

In hay .
" feces

Digested fiber

Heat of combustion per gram.

In hay. .
" feces.

Digested fiber

Heat of combustion per gram.

Average heat of combustion per gram.

1798

2394

822

7663.
4,

10503

3878,

6
2623

0
1

1572

2329
769

6624
4

10367
3754

9
2143

1
0

1560

1978
716

6613

4

8732
3479

7
2396

0
2

1262

5252
4

1623
2196

nitrogenous matter, give the following results per gram for the
digested crude fiber:

Oat hay 4.161 Cals.

Clover silage 4. 123 "

Oat and pea silage 4. 584 "

Ether Extract. — Similar determinations by Kellner * on the
ether extract of hay and feces yielded the following results per gram :

Ether Extract
of Hay. Cals.

Ether Extract
of Feces. Cals.

I

II
III

IV
V

Average. .

9.1604

| 9.3240 |

9.0554
9.1062

9 . 7690
9.8923
9.8646
9.8314
9 . 7640

9.1940

9.8243

* Loc. cit., 47, 301.

THE FOOD AS A SOURCE OF ENERGY. 305

A calculation similar to that made for the crude fiber yielded the
following figures for the heat of combustion of the digested portion :

I 8.239 Cals.

II 7.802 "

III 8.185 "

IV 8.267 "

V 8.685 "

That these results are more or less discordant is not surprising
in view of the uncertain elements involved in the determinations.
Applying the average figures for the energy per gram of the ether ex-
tracts to the total amounts eaten and excreted in the five experiments
taken together, we have for the average energy of the apparently
digested ether extract 8.322 Cals. per gram, a figure considerably
below the results recorded on p. 238 for either animal or vegetable
fats. It must be remembered, however, that the ether extract of
the feces contains more or less metabolic products, so that the
above result does not represent the actual energy of the digested
ether extract. It does, however, represent the energy correspond-
ing to the difference between food and feces with which we reckon
in computing rations, and from this point of view it is of value.

Nitrogen-free Extract. — The nitrogen-free extract cannot
be separated and examined like the crude fiber and the ether ex-
tract, but it is possible to arrive at an estimate of its heat of com-
bustion indirectly. For this purpose Kellner assumes the average
heat of combustion of the proteids (proteid nitrogen X 6.25) as
5.711 Cals. per gram and that of the non-proteids as equal to that
of asparagin, viz., 3.511 Cals. per gram. By subtracting from the
gross energy of food or feces as directly determined the energy of
the amounts of proteids, non-proteids, crude fiber, and ether ex-
tract shown by analysis to be present, he computes the heat of
combustion of the nitrogen-free extract. Furthermore, by compar-
ing the results on food and feces as in the case of the crude fiber the
heat of combustion of the digested portion may be computed.
The results per gram of such a computation for the same four ex-
periments were : *

* Loc. cit., 47, 303-306.

3°6

PRINCIPLES OF ANIMAL NUTRITION.

N.-fr. Extract

of Hay,
Cals. per Gram

N -fr. Extract

of Feces.
Cals. per Gram.

Digested N -fr.

Extract.
Cals. per Gram.

I

4.5713

4.6547
4.5029
4 . 6081

5.2834
5.4212
5.1058
5.2484

4.203

II

4.146

Ill

4.246

IV

4.335

4.584

5.2G5

4.232

In view of the indirect nature of the computation the results
agree as well as could be expected and show that, as might be
anticipated from its chemical composition, the heat of combustion
of the digested portion of the nitrogen-free extract did not vary
widely from that of starch.

Digested Matter of Mixed Rations. — The Mockern experi-
ments afford accurate data as to the energy of the total digested
matter of a large number of mixed rations. Kellner * has com-
pared this with the computed energy of the same material. For
this computation the factors used were : for fat , S.322 Cals. per gram ;
for crude fiber and nitrogen-free extract, the average of Stohmann's
figures for starch and cellulose, 4.184 Cals. per gram; for protein
provisionally, 5.711 Cals. per gram. Of the fifty-nine experiments,
twelve, in which large amounts of wheat gluten or oil were fed,
showed sufficient differences to indicate that the figures assumed
for protein and fat were too low as applied to these two materials.
In the other forty-seven cases the differences were nearly all less
than 2 per cent, of the total amount and were in both directions.

The special interest of these results lies in the fact that they
show that we may safely use the above figures as indicated on p.
302 to correct the results reached from a comparison of two rations.

Xitrogex-free Extract of Starch. — As an example of Refi-
ner's method of computation we may compare the results fcr Ox H
in Period 3. with starch, and in Period 4, on the basal ration. The
total energy of the apparently digested matter (compare Table
IV of the Appendix) was —

Period 3, with starch 28,718 Cals.

Period 4, without starch 21 ,763 "

Difference 6,955 "

* hoc. til., 53, 407.

THE FOOD AS A SOURCE OF ENERGY.

3° 7

A slightly less amount of the basal ration was eaten in Period 3
than in Period 4. The difference in crude nutrients and in esti-
mated digestible nutrients was as follows:

Total.
Grms.

Estimated Digestible.

Grms.

Equivalent
Energy, Cals.

4
13)
23 J

2
24

11.4

100.4

Crude fiber

Nitrogen-free extract. . . .

111.8

This amount of 112 Cals. should be added to the energy of
the digested matter of Period 3 or subtracted from that of Period 4
in order to render them comparable, thus making the real difference
due to the starch 7067 Cals. Still further, the starch diminished
the digestibility of the other nutrients of the ration by the following
amounts :

Grms.

Equivalent
Energy, Cals.

Protein

118 '
17
9

673.8
71.1
74.9

S19.8

Had these amounts been digested in Period 3 as in Period 4, the
energy of the digested matter of the ration would have been 820
Cals. greater, and the difference between the two periods would
have been 7887 Cals. The digestible nitrogen-free extract was
1876 grams more in Period 3 than in Period 4. Assuming all of
this to be derived from the starch, we have for the energy of each
gram of digested nitrogen-free extract 7887-=- 1876 = 4.204 Cals.

The following table* contains the results of all the starch
experiments computed in the manner just outlined:

* hoc. cit., 53, 412.

308 PRINCIPLES OF ANIMAL NUTRITION.

ENERGY OF DIGESTED NITROGEN-FREE EXTRACT OF STARCH.

Ox III 4.283 Cals.

Ox IV 4.202 "

Ox V (Period 2a) 4.380 "

Ox V (Period 26) 4.324 "

Ox VI (Period 26) 4. 159 "

Ox B 4.050 "

Ox C 4.000 "

OxD 4.099 "

Ox F 4.219 "

OxG 4.213 "

OxH 4.204 "

Ox J 4.095 "

Average 4 . 185 "

Carbohydrates of Extracted Straw. — Computed in the
same manner as the experiments upon starch, the two experiments
upon this substance gave the following results : *

OxH 4.278 Cals.

OxJ 4.216 "

Average 4.247 "

This average is slightly higher than would be computed on the
assumption that the digested crude fiber and nitrogen-free extract
had the heat values respectively of the digested crude fiber of hay
and the digested nitrogen-free extract of starch.

Peanut Oil. — Four experiments upon this substance similarly
computed give the following results; *

Ox D 8 508 Cals.

OxE 8 845 "

OxF 8 820 "

OxG 9.112 "

Average 8.821 "

*Loc c%t ,53 413 and 414

THE FOOD AS A SOURCE OF ENERGY. 309

As in the case of the ether extract of hay, the energy of the
digested fat is less than that of the original material, which was
9.47S Cals. per gram.

Protein of Wheat Gluten. — Comparing the experiments with
and without this material exactly as in the case of the starch, we
have the following results* for the energy of the digested protein:

Ox B (Period 1) 5. 728 Cals.

Ox B (Period 3) 5.817 "

Ox C (Period 3 5.712 "

Ox D (Period 4) 6.040 "

Ox E (Period 4) 6.009 "

Ox III (Period 3) 6.166 "

Ox III (Period 4) 6.277 "

Ox IV (Period 3) 6.061 "

Average 5 . 976 "

In these trials three different kinds of gluten were used which
were prepared by somewhat different processes. The averages for
the three sorts separately were as follows.:

No. 1 5.732 Cals.

"2 6.025 "

" 3 6.168 "

5.975 "

The above figures refer to the so-called crude protein, that is, to
nitrogen X 6.25. The proteins of wheat, however, contain con-
siderably over 16 per cent, of nitrogen. Using Ritthausen 's factor,
namely. 5.7. for the computation of protein from nitrogen reduces
the amount of protein in the gluten and increases that of the
nitrogen-free extract by the same amount. The energy of the
digested protein when computed on this basis equals 6.148 Cals.
per gram.

Organic Matter of Coarse Fodders. — For the total digested
organic matter of hay and straw the following heat values per gram
were computed : *

* hoc. cit., 53, 412 and 414.

3IQ PRINCIPLES OF ANIMAL NUTRITION.

Meadow hay I, Ox A 4509 Cals.

"A, "II 4408 "

" B, " V 4317 Cals.

4QP57
B, "VI 4398

" M, " XX 4452 "

" II, " 1 4371 "

" " V, " F 4355 Cals. | „

"V, " G 4495 " )

" VI. " H 4534 " j

" VI, " H 4601 " V 4535 "

" VI, " J 4502 " )

Average of 7 kinds 4437 "

Oat straw, Ox F 4443 Cals.

OxG 4584 "

Average 4513 "

Wheat straw, Ox H 4553 Cals.'

Ox J 4387 "

Average 4470 "

The digestible matter of the straw has apparently about the same
heat value as that of hay.

Metabolizablc Energy.

Protein. — A portion of the gross energy of the digested protein
is removed in the urea and other nitrogenous products of metabo-
lism, and in addition to this there is to be considered the possibility
of a loss of energy by fermentation in the digestive tract.

Losses in Methane. — In nine of the Mockern experiments in
which wheat gluten or flesh-meal was added to the basal ration, the
amount of carbon excreted in the form of hydrocarbons per day
and head was as tabulated on the opposite page.

The differences between the excretion writh and without gluten
are small in amount and are sometimes positive and sometimes
negative, the averages being probably within the limit of experi-
mental error. The percentage losses of energy in methane as

THE FOOD AS A SOURCE OF ENERGY.

Period.

Gluten
Added,
Grms.

Carbon in Form of Hydrocarbons.

From Basal
Ration,
Grms.

With

Addition of

Gluten,

Grms.

Differences,
Grms.

Kiihn :

Ox III

3

4
3

2a
26

1

3
3

4

680
1360

680
1000*
1000*

1700
1700
1700
1600

186.4
186.4
187.7
148.7
148.7

205.7
207.6
187.6
162.9
157.4

+ 19 3

" III

+ 21 2

" IV

- 0 1

" XX

+ 14.2

" XX

+ 8.7

Kellner :

Ox B

171.6

208.9
208.9
183.0
166.1

184.2

211.0
200 9
167.1
170.7

+ 12.6

+ 2.1

" B

- 8.0

" C

-15.9

" D

+ 4.6

Average

191.7

187.4

- 4.3

* Flesh-meal.

computed in Table V of the Appendix, like the figures just given
for the carbon of the methane, lead to the conclusion that the pro-
tein of the food does not participate in the methane fermentation.
Those figures were :

Ox III, Period 3 10.81 per cent.

III,

IV,

B,

B,

C,

D,

4 5.08

3 - 1 . 26

1 0.08

3 - 1 . 62

3 -3.69

4 1.91

Average -0.83 "

Kellner * reaches the same conclusion by comparing the ratio
of the methane carbon to the amount of digested carbohydrates
(nitrogen-free extract + crude fiber) in the several periods. The
former amounted to the following per cent, of the latter in his
experiments :

*Loc. cit., 53, 420.

312

PRINCIPLES OF ANIMAL NUTRITION.

d_„„i t>„*: Basal Ration

.basal Kation, . r"i. ■*»»..
ppl.rPnt + Oluten,
i'er Cent. Per Cent

Ox B

2.94
2.94
2.71
2.75

2.87

2.84

2.96
2.82
2.41
2.71
3.19

2.82

" B

" C

" D

" E. .
Average

Had the large quantities of digestible protein added to the basal
rations produced any material amount of methane, that fact must
have been reflected in the above percentages. This method of
comparison takes into account the probable effect of the carbo-
hydrates of the wheat gluten in increasing the production of
methane, and the substantial agreement of the results with and
without protein leads to the same conclusion as the preceding
data. It seems fair to presume that this conclusion applies to
protein in general, although a strict demonstration of it, especially
for coarse fodders, would have its difficulties.

Losses in Urine. — While the assumption that the urine is
essentially an aqueous solution of urea leads to grave errors in the
case of the carnivora, this is still more emphatically true of the urine
of herbivora, particularly of ruminants. The presence in the urine
of herbivora of hippuric acid and other nitrogenous compounds less
highly oxidized than urea has of course long been known, while,
as stated on p. 27, the presence of eonsiderable amounts of non-
nitrogenous organic matter was subsequently demonstrated by
Henneberg and by G. Kiihn in the urine of ruminants.

It follows from these facts that the energy content of the urine
of these animals must be higher in proportion to its nitrogen than
is the case with carnivora or with man, but the experimental dem-
onstration of this fact and the realization of the extent and im-
portance of the difference are of comparatively recent date.

Cattle. — It is to Kellner * that we owe the first direct determi-
nations of the potential energy of the urine of cattle. The two
animals used in the experiment were fed, the one (A) on meadow
hay, and the other (B) on meadow hay and oat straw. The results
as regards the urine were as follows, per day and head:

* Loc. cit., 47, 275.

THE FOOD AS A SOURCE OF ENERGY.

3*3

Ox A.

Ox B

Total nitrosen 61 .28 grams.

" carbon 203.20

Hippunc acid 145.00

Total energy 1945 . 00 Cals

46.63 grams.
161.30 ."
126.40 "
1549.40 Cals

Assuming all the nitrogen not contained in the hippuric aeid to
have been in the form of urea, we have the following as the distri-
bution of the carbon and of the energy of the urine:

Ox

A.

Ox B

Amount.

Per Cent.

Amount Per Cent

Carbon :

In hippuric acid

" urea

" other compounds. .. .

Grms.

87.48
21.40
94.32

43.05
10.53
46.42

Grms

76.26 47.28
15.75 9.76
69.29 42.96

Total

Energt/

In hippuric acid

" urea

" other compounds. . . .

203.20

Cals.
821.30
271 .40
852 . 30

100.00

42 . 23
13.95
43.82

161.30

Cals.
715.90
199.60
633 . 90

100.00

46.20
12.88
40.92

Total

1915.00

100 00

ir> in 40 100 00

While the assumption that all the nitrogen was present either
as hippuric acid or urea is not strictly correct, still the figures suffice
to show, first, that a considerable proportion of the energy of the
proteids of the food may be removed in the hippuric acid, and
second, that the urine contains relatively considerable amounts of
non-nitrogenous organic matter. Had the energy of the urine
been computed from its nitrogen reckoned simply as urea, the
' s would have been as follows :

Ox A.

OxB.

Calculated from N as urea

Actually present

331.6 Cals.
1945.0 "

252.3 Cals
1549.4 "

In experiments by the writer on the maintenance ration of
cattle,* determinations of the total energy of the urine of steers
* Penna. Experiment Station, Bull 42, p. 150.

3*4

PRINCIPLES OF ANIMAL NUTRITION.

were likewise made. Calculated per gram of nitrogen the results
were as follows:

Feed.

Steer No. 1.

Steer No. 2.

Steer No. 3.

Timothv hay and corn meal

37 . 79 Cals.
40.64 "
19.29 "
25.02 "
11.24 "

28.35 Cals.
34.25 "
18.01 "

10.77 "

28.82 Cals.

12.47 "

" " and starch

Wheat straw, corn meal, and linseed meal

10.95 "

The methods employed to prepare the urine for combustion
were not altogether satisfactory, and the range of possible error
is rather large. In but two cases, however, was the energy of the
urine less than twice that corresponding to its nitrogen calculated
as urea (5.434 Cals.), while in one case it reached over seven times
that amount. Neither carbon nor hippuric acid having been deter-
mined, no computations can be made as to the amount of non-
nitrogenous matter present.

Jordan * has reached similar results on the urine of cows, the
average energy content per gram of nitrogen being as follows:

Total Nitrogen,
Grms.

Potential Energy,
Cals.

Energy per Grm.
Nitrogen, Cals.

Cow No. 12:

87.0
78.8
42.8
65.5

1658.3
1547.2
1323.5
1452.5

19.06

" 2

19.63

" 3

30.93

Cow No. 10

22.18

As in the writer's experiments, the energy per gram of nitrogen
varies within wide limits, being greatest when the total nitrogen
of the urine is least. In other words, it would appear that the
non-nitrogenous ingredients of the urine of cattle are subject to
less fluctuation than the nitrogenous ingredients.

Kellner's later experiments have fully confirmed his earlier
results, as will appear in greater detail in subsequent paragraphs.
He finds that the carbon rather than the nitrogen of the urine is
the measure of its potential energy, and that an estimate of 10
Cals. per gram of carbon gave for his experiments results closely
approximating the truth. t

* New York State Experiment Station, Bull. 197, p. 28.
t Loc. cit., 53, 437.

THE FOOD AS A SOURCE OF ENERGY. 315

Other Species. — We may probably assume without serious error
that the results obtained with cattle apply in general to sheep and
other ruminants. No direct determinations of the energy of the
urine of the horse or the hog have yet been reported, but Zuntz &
Hagemann * have made some estimates of it in the case of the
horse on a mixed ration of hay, oats, and straw. They determined
the total carbon and total nitrogen of the urine and, on the assump-
tion that only urea and hippuric acid are present, compute the
proportion of each of these, and thence the energy of the urine.
They thus find the potential energy of the latter, per gram of nitro-
gen, equal to 15.521 Cals. Neither hippuric acid nor energy having
been determined directly, it is impossible to check the above com-
putation or to ascertain whether any non-nitrogenous organic
matter was present. It is to be noted, however, that the ratio of
carbon to nitrogen in the urine was much lower than in Kellner's
experiments on cattle, viz.:

Zuntz & Hagemann 1.526 : 1

Kellner, Ox A 3.315 : 1

Ox B 3.458 : 1

This fact clearly indicates that at least there was very much less
non-nitrogenous matter present in the former case.

Meissl, Strohmer & Lorenz f in their respiration experiments
on swine likewise determined carbon and nitrogen in the urine.
Computed by the method of Zuntz & Hagemann the energy of the
urine averaged 9.55 Cals. per gram of nitrogen, while the average
ratio of carbon to nitrogen was 0.745 : 1. These results would
seem to indicate that the loss of energy in the urine of the hog
is not very much greater than in that of the carnivora.

Mktabolizable Energy of Protein of Concentrated Feeds.
— Accepting it as demonstrated that there is no material loss of
potential energy in the form of fermentation products of protein,
the data regarding the energy of the urine just considered afford
the basis for an approximate estimate of the metabolizable energy
of the digested protein.

Cattle. — Kellner's experiments upon cattle afford data for com-
puting the metabolizable energy of the digested protein of wheat

* Landw. Jahrb., 27, Supp. Ill, 239. f Zeit. f. Biol., 22, 63.

316

PRINCIPLES OF ANIMAL NUTRITION.

gluten and of beet molasses. The method of computation is pre-
cisely similar to that already employed for calculating the metabo-
lizable energy of the total organic matter; that is, the results upon
the basal ration are subtracted from those upon the ration con-
taining the material under experiment.

Taking as an example the results upon wheat gluten with Ox C
in Periods 1 and 3 we have the following comparison :

Digested.

Energy

of Urine,

Cals.

Gain of
Nitrogen

by

Animal,

Grms.

Protein,
Grms.

Crude
Fiber,
Grms.

Nitrogen-
Free
Extract.
Grms.

Ether

Extract.

Grms.

Period 3

1694
59S

1279
1289

5648
5464

34

40

2592 . 8
1666.4

20 31

" 1

16.01

Difference

1096

-10

184

-6

920.4

4.30

The difference of 4.3 grams in the amount of nitrogen gained
by the animal is equivalent to 32 Cals. which would otherwise have
appeared in the urine. This added to the 926.4 Cals. actually
found makes a total of 958.4 Cals. for the increase in the potential
energy of the urine due to the 1096 grams of protein digested.
There are also differences in the amount of non-nitrogenous matters
digested, particularly of the nitrogen-free extract. As Tables I, III
and IV of the Appendix show, both starch and crude fiber, as repre-
sented by the extracted straw, tend to diminish t lie amount of energy
carried off in the urine. These differences were observed when from
2 to 2.5 kilograms of these substances were added to the basal
ration. If the differences are proportional to the amount fed, the
energy corresponding to the small difference observed in this ex-
periment would not exceed 15 or 20 Cals., and may be neglected,
while the maximum difference in any experiment of the series
would probably not exceed 70 to 75 Cals. Assuming that all the
additional protein digested came from the wheat gluten, we have
for the corresponding energy of the urine

958.4 -f- 1096 = 0.874 Cals. per gram protein digested.

Subtracting this from the total energy of the digested protein as
found on p. 309, viz., 5.975 Cals., we have 5.101 Cals. as the metabo-

THE FOOD AS A SOURCE OF ENERGY.

3J7

lizable energy of one gram of digested protein of wheat gluten in
this experiment.

For the four experiments upon this substance, computed as in
the above example, the results were as follows:

Protein
digested

from
Ciluten,
Grms.

Difference in
Energy of trine.*

Total,
Cals.

Per Grm. of

Protein,

Cals.

Ox B, Periods 1 and 3 2185

" C, Period 3 1096

" D, " 4 10.56

" E, " 4 1148

Average 1371

2547.3

958.4
1061.1
1362.1

1.166
0.874
1.005
1.186

1482.2

1.081

* Corrected to nitrogen equilibrium.

Subtracting from the total energy of the digested protein the
potential energy carried off in the urine we have for the metab-
olizable energy of one gram of protein

5.975 Cals. -1.0S1 Cals. =4.894 Cals.

If we use Ritthausen's factor, 5.7, for proteids, the average
digested protein becomes 1250 grams and the loss of energy in the
urine 1.190 Cals. per gram of protein. Subtracting this from 6.148
Cals., the gross energy of one gram of NX5.7 (p. 309), wye have for
the metabolizable energy of the latter 4.958 Cals. per gram.

The average increase in the energy of the urine for each addi-
tional gram of nitrogen excreted in these experiments (6.756 Cals.)
was almost exactly the same as Rubner found in his experiment
on extracted lean meat (6.695 Cals.). This may be taken as indi-
cating that the process of proteid metabolism is substantially the
same in both classes of animals, while the fact that the result is
notably greater than the energy of urea shows that in the herbivora
as in the carnivora other waste products than urea result from the
proteid metabolism.

In three other experiments beet molasses was added to the
basal ration, resulting in the digestion of an inn-eased amount of
nitrogenous matter. Computing the results as in the case of the

3x<

PRINCIPLES OF ANIMAL NUTRITION.

wheat gluten, and assuming that the large amounts of soluble
carbohydrates digested had no effect on the potential energy of the
urine, the results were as follows :

Protein Digested

from Molasses,

Grins.

Difference in Energy of Urine.*

Total, Cals.

Per Grm. Protein,
Cals.

Ox F

117
160
122

256.1
240.3
192.6

2 189

" H

" J

Average

1.502
1.579

133

229.7

1.727

* Corrected to nitrogen equilibrium.

It will be seen that the loss of energy in the urine is much
greater than in the case of the gluten or than in Rubner's experi-
ments with carnivora. Since it is improbable that the soluble
carbohydrates of the molasses escape oxidation, it would appear
that some of the nitrogenous material of the latter must have
passed through the system unmetabolized. Kellner suspects that
it is made up in part at least of xanthin bases.

If we consider the nitrogen of the molasses to represent crude
protein (NX6.25) with a heat value of 5.711 Cals. per gram, the
metabolizable energy per gram would be 3.984 Cals. In view,
however, of the fact that only a very small proportion of the nitro-
gen of the molasses is in the proteid form, such a calculation seems
of doubtful value.

Swine. — In the investigations of Meissl, Strohmer and Lorenz *
upon the production of fat from carbohydrates (p. 176) the carbon
and nitrogen of the urine were determined in six experiments. .

Applying to the results Zuntz & Hagemann's method of
computation (p. 315) we obtain the following estimates for the
energy per gram of nitrogen in the urine of the hog in these
experiments and for the corresponding metabolizable energy of the
digested protein:

* Zeit. f. Biol., 22, 63.

THE FOOD AS A SOURCE OF ENERGY.

3X9

Experi-
ment
No.

Feed.

Nitrogen

as L'rea,

Grnis.

Nitrogen

as Hi] i

puric

Acid.

Grms.

Total
Energy
of L rine,

Cals.

Energy
per Grm.

of

Nitrogen

Cals.

Metabo-
lizable
Energy

per

Grm.

Protein,

Cals.

Rice

Barley

Whey, rice, and flesh' meal
Nothing

9.58
9.22
13.04
59.89
9.35
6.48

0.88
1.04
1.04
1.17
0.45
0.29

115.7
125.6
146.5
410.0
83.7
56.4

11.06

12.24

10.40

6.72

8.54

8.33

3.941
3.753
4.048
4.636
4.344
4.379

Komauth & Arche * report the following results on the urine of
swine fed chiefly upon cockle:

Experiment
No.

Nitrogen,
Grms.

Carbon ,
Grms.

Ratio,
C :N.

1

10.56
10.30
10.41

10.30
9.53
9.96

0.975 : 1
0.926 : 1
0.957 • 1

2

3

Average

10.42

9.93

0.953 : 1

The results, computed as in the previous case, make the average
energy content of the urine 10.27 Cals. per gram of nitrogen,
equivalent to a metabolizable energy of 4.067 Cals. per gram of
protein.

In the two fasting experiments of Meissl, Strohmer & Lorenz
the ratios of carbon to nitrogen and of computed energy to nitro-
gen are similar to those obtained with fasting carnivora. The
abundant supply of proteids in the diet in the fourth experiment
seems to have had the effect of reducing these ratios to values
comparable with those obtained by Rubner for extracted meat
and by Kellner for the digested protein of wheat gluten. These
facts seem to indicate clearly that the nature of the protcid meta-
bolism in all these animals is substantially the same. In the ex-
periments in which ordinary grains were used, the computed energy
content of the urine is notably greater relatively to its nitrogen.
How far the excess of carbon found in these cases was due to an

* Landw. Vers. Stat., 40, 177.

320 PRINCIPLES OF ANIMAL NUTRITION.

increased formation of hippuric acid and what part of it, if any, is
to be ascribed to the presence of non-nitrogenous matter in the
urine, the experiments afford no means of estimating.

The Horse. — Zuntz & Hagemann's results on the horse, p. 315,
although the result of feeding mixed rations, may be conveniently
considered here. The computed energy of the urine was 15.521
Cals. per gram of nitrogen, equivalent to 2.483 Cals. per gram of
protein. Assuming for the latter, as before, a value of 5.711 Cals.,
there remains for the metabolizable energy 3.228 Cals. per gram.

Protein of Coarse Fodders. — Almost the only data on this
point are those afforded by Kellner's experiments upon cattle. In
those in which coarse fodders were used alone we can of course
compute the metabolizable energy of the protein directly from the
amount digested and from the energy of the urine. In those
experiments in which coarse fodders were added to a basal ration
we can compare the two experiments in the same manner as those
upon gluten, neglecting, as in that case, the differences in the non-
nitrogenous nutrients digested.

Passing over the details of the computation, the final results,
including the metabolizable energy of the digested protein com-
puted upon the assumption that its gross energy equals 5.711
Cals. per gram, are as given in the table on the opposite page.*

The writer's experiments on timothy hay, the results of which
as regards the energy of the urine have already been given on p. 314,
when computed in the same manner as the above experiments give
the following results for the metabolizable energy of the digested
protein :

Steer 1 2.625 Cals.

" 2 2.830 "

" 3 3.716 "

Average 3 . 057 "

Influence of Non-nitrogenous Matter of Urine. — In the previous
paragraphs there appeared reasons for supposing that the processes
of proteid metabolism are essentially the same in all domestic

* The figures given in this table for digested protein, energy, etc., refer
solely to that derived from the coarse fodder and not to that of the total
ration.

THE FOOD AS A SOURCE OF ENERGY.

321

Protein

(NX6.25)

Digested,

Grms.

Difference in
Energy of I'rine.*

Total,
Cals.

Per Grm. of

Protein

Digested,

Cals.

Metaboliz-

able Energy

per Grm.

Digestible

Protein,

Cals.

Meadow Hay :
No. I, Ox A
" II,
" V,
" V,
" VI,
" VI,
" VI,
" A,
" B,
" B,
" M.

" I.

" F.

" G.

" H,

" H,

" J.

" II.

" V.

" VI.
" XX.

Average

Oat Straw :
No. II, Ox F.
" II, " G.

Average . . .

Wheat Straxv :
No. I. Ox H.
" I. • J.

Average . .

Period 1 .

440
342
137
146
193
220
213
413
451
458
540

323

35

48

42

11

14

1991.3

16S6.9

583.2

556.5

781.4

798.0

930 . 5

1925.7

1559.3

1737.9

3224 . 6

526

.933

.257

.812

.049

.632

.36S

4.662

3.456

3 . 794

5.973

1434 . 1

354.2

274.0

4.439

10.120
5.710

314.1

289.7
413.2

7.47!

(?)
29 . 520

351.5

(?)

1.185

0.778
1 . 454
1 . 899
1.662
2.079
1.343
1.049
2.255
1.917
-0.262

1.272

-4.409
■0.001

■1.767

(?)
-23.809

(?)

* Corrected to nitrogen equilibrium.

animals and consequently that the metabolizable energy of the
proteids cannot be widely different. In these results upon coarse
fodders we meet an apparent contradiction of this conclusion, the
metabolizable energy of the digestible protein as above computed
being quite variable and much lower than the values found for pure
proteids, while in the straw we get largo negative values.

These latter results, however, while appearing at first sight para-
doxical, furnish the clue to the apparent contradiction. In the
case of the straws it is evident that a very considerable part of the
potential energy of the urine must have been contained in non-
nitrogenous substances, and that the latter must have been derived
largely from the non-nitrogenous matter of the food. We have
already seen, however, that these non-nitrogenous excretory prod-

322 PRINCIPLES OF 4N1MAL NUTRITION.

ucts are a normal constituent of the urine of cattle both on hay and
on mixed rations. Their effect on the computation becomes more
obvious in the case of the straws, simply because of the relatively
small amount of protein in the latter feeding-stuffs. In these cases
we get impossible results when we assume that all the potential
energy of the urine is derived from the proteids metabolized, but it
is clear that the results on the hays must be affected by the same
error, and there is little question that the low and variable results
noted in the table are to be explained in part in this way. We
know no essential difference between the real proteids of the differ-
ent coarse fodders, nor between those of coarse fodders and grain,
nor any reason why they should not be metabolized in substantially
the same way in the body and possess approximately the same
metabolizable energy. It would seem more reasonable, then, to
assume that the proteids of coarse fodders are metabolized sub-
stantially like those of concentrated fodders, and to take provision-
ally the results obtained for the protein of wheat gluten as repre-
senting approximately the metabolizable energy of the digested
protein of the total ration, while we regard the remaining energy
of the urine as derived largely from the non-nitrogenous nutrients
of the food.

Hippuric Acid. — The statement last made, however, requires
some modification. Not a little of the potential energy of the urine
of cattle is contained in the hippuric acid which these animals
excrete so abundantly. This being a nitrogenous product, it is
natural to look upon it as derived from the proteids of the food,
but it must not be forgotten that thh is only partially true. Its
glycocol portion originates in the proteids, but its phenyl radicle
appears to be derived in these animals largely, if not wholly from
the non-nitrogenous ingredients of the food (compare p. 45). If
the metabolism of one gram of protein is arrested at the glycocol
stage by the presence in the organism of benzoic acid, there has
already been liberated from it about 3 Cals. of energy, while about 2.7
Cals. remain in the glycocol. The resulting hippuric acid, however,
contains about 11.6 Cals. of potential energy, or more than the
original protein. In this case, then, the larger share of the energy
of the excretory product (8.9 Cals. out of 11.6 Cals), although con-
tained in a nitrogenous substance, is derived ultimately from the

THE FOOD AS A SOURCE OF ENERGY. 323

non-nitrogenous mutter of the food. It is clear, then, that the
non-nitrogenous radicle of the hippuric acid and the non-nitrogen-
ous organic matter of the urine together represent a large share
of the potential energy of the latter, and that it is (mitt" as in-
correct to compute the metabolizable energy of the protein on the
assumption that all the energy of the urine is derived from it as it
is, on the other hand, to simply deduct from its gross energy the
energy of the equivalent amount of urea.

Ether Extract. — Our only data upon this ingredient are fur-
nished by the four experiments upon steers by Kellner in which
peanut oil was added to the ration. In the first two experiments
this oil was emulsified by means of a small quantity of soap made
from the same oil. The result was a milky fluid which was readily
digestible and which caused no considerable decrease in the digesti-
bility of the basal ration. In the second two experiments the oil
was emulsified with lime-water, giving a thickish mass which was
not very well digested and which, in the case of Ox F particularly,
caused a considerable decrease in the digestibility of the crude fiber
and nitrogen-free extract of the basal ration. It should be noted
that in the experiment with Ox E the oil was not added to a basal
ration, but was substituted for a part of the bran. From Table VI
of the Appendix we obtain the summary tabulated on the next
page, showing the effects of the oil upon the ioss of energy in the
gaseous hydrocarbons and in the urine, the results of the experi-
ment on Ox E being included.

Upon the evidence of these four experiments, bearing in mind
that the one with Ox E was upon the substitution of oil for bran,
we should not be inclined to ascribe to the fat of the food any con-
siderable effect either upon the formation of hydrocarbons or upon
the amount of potential energy carried off in the urine. As regards
the hydrocarbons, the differences in the cases of Oxen D and G are
insignificant. In the case of Ox F, on the contrary, the production
of hydrocarbons was reduced nearly one half; this it may be noted
was the case in which there was a considerable effect upon the
digestibility of the basal ration. As regards the energy of the urine,
the differ* nces, except in the case of Ox E, are relatively small and
are in both directions.

Provisionally, therefore, we are probably justified in assuming

324

PRINCIPLES OF ANIMAL NUTRITION.

Ani-
mal.

Period.

Energy of Urine
(Corrected). Cals.

Energy of
j Methane, Cals.

D

3

1

3

1

5
3

5
3

With oil

2851.2
2407 . 0

2909 0

D

Basal ration

2957 0

Differences

-55.8

2026.2
2312.9

-48 0

F

With oil

2640 8

F

Basal ration

2950 4

Differences

-286.7

1455.0
1530.0

-309.6

F

With oil

1369 1

F

Basal ration

2560.7

Differences

With oil

Basal ration

Differences

G
G

-75.0

1452.1
1359.6

-1191.6

2371.2

2524 . 7

92.5

-153.5

as Kellner does that none of the energy of the fat was lost either in
the hydrocarbons or in the urine, and that consequently the metab-
olizable energy of the digested fat was the same as its gross energy,
namely, 8.821 Cals. per gram, as given on p. 308. If we assume that
the ether extract of hay behaves like the peanut oil, taking no part
either in the production of methane or in the loss of energy through
the urine, its metabolizable energy would likewise be the same as
its gross energy, namely, 8.322 Cals. per gram, as computed on p. 305.
No results upon the metabolizable energy of the ether extract are
available in the case of other species of herbivorous animals.

Carbohydrates. — Those of Kellner's experiments in which
starch, as a representative of the readily digestible carbohydrates,
and extracted straw, consisting largely of "crude fiber," were added
to the basal ration afford data for an approximate computation of
the metabolizable energy of this group of nutrients in the ox, and
experiments by Lehmann, Hagemann & Zuntz afford partial data
for the horse.

Starch. — The results of the Mockern experiments, as recorded
in Tables 111 and IV of the Appendix, show that the starch had
but a slight effect upon the amount of potential energy carried off
in the urine of the ox, although the general tendency was to

THE FOOD AS A SOURCE OF ENERGY.

325

diminish it slightly. On the other hand, the formation of hydro-
carbons was markedly increased except in two cases. It has al-
ready been shown that the proteids of the food do not take part in
the production of these gases, and that the same is probably true
of the fat under normal conditions. Neglecting the small effects
upon the urine, therefore, we may compare directly the increase in
the digested carbohydrates with the increase in the gaseous hydro-
carbons, using for this purpose the differences between the two
rations uncorrected for the slight variations in the consumption of
dry matter.

Taking first the last five of Kellner's experiments.* which seem
to represent the most normal conditions, we have the following:

Difference in Carbohydrates
Digested.

Difference in

Crude Fiber,
Grais.

Nitrogen-free

Extract,

Grms.

Methane,
Cals.

Ox D, Period 2

" F " 4

-64
-64
-50
-26
- 9

+ 1388
+ 1609
+ 1598
+ 1861
+ 1501

+ 424.4
+ 822.0

" G " 4

" H, " 3

+ 645.8
+ 604.5

"J " 3

+ 769.9

Totals

-213

+ 7957

3266.6

Assuming that the same proportion of hydrocarbons is pro-
duced in the fermentation of crude fiber as in that of starch, we
may compare the algebraic sum of the two with the energy of the
methane as follows :

3266 . 6 Cals. - (7957 - 213) = 0 . 422 Cals. per gram.
Subtracting the latter result from the gross energy of the digested
nitrogen-free extract of starch, we have for the metabolizable
energy of the latter

4 . 185 Cals. - 0 . 422 Cals. = 3 . 763 Cals. per gram.

In the experiments on Oxen B and C the basal ration w-as a
heavy one, with a rather wide nutritive ratio, and already con-
tained large amounts of digestible carbohydrates. Under these cir-
cumstances the added starch was very imperfectly digested, while
*Loc. cit , 53. 422.

326

PRINCIPLES OF ANIMAL NUTRITION.

the production of hydrocarbons was diminished. Kellner suggests
that the latter effect may have been due to a partial suppression
of the organisms causing the methane fermentation by other species,
and suspects that the presence of large amounts of carbohydrates
along with little protein favors this result. At any rate, the con-
ditions are evidently unusual if not abnormal.

In Kiihn's experiments the starch was added to a ration of
coarse fodder. The nutritive ratio was wide, but the absolute
amount of carbohydrates was much less than in the two experiments
by Kellner just mentioned, less starch appeared to escape diges-
tion, and the production of hydrocarbons was increased in every
case. The following are Kiihn's* results:

Difference in Carbohydrates
Digested.

Difference in
Energy of

Crude Fiber,
Grms.

Nitrogen-free
Extract, Grms.

Methane,
Cals.

Ox III, Period 2

-220

-180
-195
-130
-176
-146
- 88
-156

1529
1408
1537
1539
2619
1468
1554
2587

706.2

" IV, " 2

856.7

" V, " 2o

" V, " 2b

752.6
665.5

" V, " 3

1181.0

" VI, " 2a

729.5

" VI, " 2b

649.9

" VI, " 3

1407.0

Totals

-1291

14241

6948.4

Assuming as before the equivalence of crude fiber and nitrogen-
free extract as regards the production of hydrocarbons we have

6948. 4 Cals. -(14241 -1291) = 0.537 Cals. per gram,
4 . 185 Cals. - 0 . 537 Cals. = 3 . 648 Cals. per gram.

Determinations by Lehmann, Hagemann & Zuntz f of the
amount of methane produced by the horse will be considered in
connection with the metabolizable energy of crude fiber. Zuntz J
has pointed out that the fermentation of the food in the horse takes
place largely in the coecum and after the more digestible carbo-
hydrates have been resorbed. Accordingly he regards the metabo-

* hoc. tit., 44, 570.

f Landw. Jahrb., 23, 125.

% Arch. ges. Physiol., 49, 477.

THE FOOD AS A SOURCE OF ENERGY.

327

lizable energy of starch and similar bodies in this animal as equal
to their gross energy, viz., 4.185 Cals. per gram in the case of
starch.

Kxtracted Straw. — The two experiments in which extracted
straw was added to the basal ration, when computed as in the case
of the starch experiments, give the following results:

Difference in Carbohydrates
Digested.

Difference in

Crude Fiber,
Grms.

Nitrogen-free

Extract,

Grms.

Methane,
Cals.

Ox H, Period 5

2046
1987

439
449

1425.1

" J " 5

1425.2

4033

888

2850.3

The loss of energy in the hydrocarbons equals 0.579 Cals. per
gram of total digestible carbohydrates (of which 82 per cent, was
crude fiber), and the corresponding metabolizable energy of the
carbohydrates is 3.668 Cals. per gram. This is a materially lower
figure than Kellner found for starch and indicates that the loss of
energy in the gaseous products of fermentation is greater in the
case of crude fiber than in that of the more soluble carbohydrates,
an indication which, as we shall see, is confirmed by the results of
other experiments.

Carbohydrates of Coarse Fodders. — Upon the same two
assumptions, viz., that the carbohydrates are the sole source of the
gaseous hydrocarbons, and that the latter represent the entire loss
of energy from the digested carbohydrates, we may compute the
metabolizable energy of the total digestible carbohydrates of the
various coarse fodders exactly as in the case of the extracted straw,
the results being tabulated on the next page.

If we average the results for each feeding-stuff and compute
them as in the foregoing cases, our findings are as given on p. 329,
where the rations are arranged in the order of their crude fiber
content. In computing the metabolizable energy, the gross energy
of the digested carbohydrates has been assumed to be the average

328

PRINCIPLES OF ANIMAL NUTRITION.
COARSE FODDERS ALONE.

Digested Carbohydrates.

Energy of

Animal.

Crude Fiber,
Grms.

Nitrogen-
free
Extract,
Grms.

Methane,
Grms.

A

Meadow hav I

1262

2713

2113.7

II

" A

1765

2610

3137.2

V

" B

1572

2315

2268.5

VI

" B

1642

2420

2480.6

XX

" M

1560

2999

2646 . 1

I

" II

1266

2348

2092.3

B

" and oat straw. .

1702

2357

2331.2

III

Clover " " " " ....

1676

2226

2670.1

IV

<« it a tt it

1565

2145

2491.3

COARSE FODDER ADDED TO BASAL RATION.

Period.

Difference in
Carbohydrates Digested.

Energy of

Methane,

Cals.

Animal.

Crude Fiber,
Grms.

Nitrogen-
free
Extract,
Grms.

F

G
H
H
J
F

1
2

2

7
2
2
1
1
1

Meadow hav V

" V

"• " VI

" VI

" VI

Oat straw II

546
538
703
739
683
694
595
821
829

836

886

1129

1236

1213

721

684

524

616

689.9
907.4
727.2
898.0
984.8
679.2

G
H
J

" II

Wheat straw I

" I

923.4
1213.0
1281.0

of the results given on pp. 304 and 306 for the digested crude fiber
and nitrogen-free extract of coarse fodders, viz., 4.226 Cals. per
gram.

As a whole, the figures given on p. 329 show a tendency
toward an increased production of methane with an increase in
the proportion of crude fiber, but considerable variations are
found in individual cases. It is evident, therefore, from these
results, as well as from those already cited in connection with the
experiments upon starch and upon molasses, that a variety of
factors influence the extent of this fermentation.

THE FOOD AS A SOURCE OF ENERGY.

329

100 Parts Digested

Carbohydrates

Contain

Energy of
Methane
per Grm.
Digested

Carbo-
hydrates,

' Cals.

Metaboliz-
able Energy
of Total
Digested
Carbo-
hydrates
per Grm.,
Cals.

Crude
Fiber.

Nitrogen-
free
Extract.

31.7
34.2
35.0
37.3
38.6
40.4
41.9
42.6
47.8
59.1

68.3
65.8
65.0
62.7
61.4
59.6
58.1
57.4
52.2
40.9

0.532
0.580
0.579
0.458
0.569
0.597
0.574
0.678
0.595
0.894

3.694

"" M

3.646

" II

3.647

" VI

3.768

" V

3.629

" B

3.657

" " and oat straw

3.652

Clover " " " "

Oat straw II

3.548
3.631

Wheat straw V

3.332

A comparison of the methane production with the digestibility
of the feeding-stuffs shows in general that the former is greatest
when the latter is least, that is, with the feeding-stuffs which tend
to remain longest in the digestive tract. Here too, however, excep-
tions occur, and it would appear that the physical condition of the
feeding-stuff is not without its influence. The exceedingly com-
plicated nature of digestion in ruminants, and the fact that it is a
chemical rather than a physiological process, and is therefore sub-
ject to considerable variations according to the nature and amount
of the food, render it difficult, if not impossible, with our present
knowledge to compute very trustworthy averages for the amount
of energy carried off in this way.

Crude Fiber. Ruminants. — Both the ultimate composition and
the heat of combustion of the digested nitrogen-free extract have
been shown to agree quite closely with those of starch, and the
nutritive value of the former has commonly been assumed to be
the same as that of the latter. If we are justified in somewhat
extending this, and assuming that the nitrogen-free extract of
coarse fodders suffers the same loss by the methane ferment ai inn
as docs starch, the figures of the preceding paragraphs supply
data for computing the corresponding loss suffered by the
crude fiber.

33°

PRINCIPLES OF ANIMAL NUTRITION.

In the case of the extracted straw, for example, there was
digested in the total of the two experiments :

Crude fiber 4033 grams

Nitrogen-free extract 888 "

Assuming the loss of energy in the methane to have been 0.422
Cal. per gram of nitrogen-free extract digested (the same as that
found by Kellner for starch, p. 325), the 888 grams of these sub-
stances correspond to a loss of 374.7 Cals. Subtracting this from
the total loss of 2850.2 Cals., we have 2475.5 Cals. as the energy of
the methane produced from 4033 grams of crude fiber, which is equal
to 0.614 Cal. per gram. The total energy of the digested crude fiber
was shown on p. 304 to be approximately 4.220 Cals. per gram.
Subtracting the loss in the methane, 0.614 Cal., leaves 3.606 Cals.
as the metabolizable energy of one gram of digested crude fiber.
A similar computation of the average results upon the other coarse
fodders affords the figures of the following table for the metabo-
lizable energy of one gram of digestible crude fiber :

Digestible Crude Fiber of

Loss in Methane,
Cals.

Metabolizable

Energy,

Cals.

Extracted straw

Hay fed alone

" added to basal ration

Oat straw added to basal ration . .
Wheat straw added to basal ration

0.614
0.909
0.614
0.783
1.219

3.606
3.311
3.606
3.437
3.001

The loss of energy in methane, as thus computed, is in all
instances greater than in the case of starch. Owing, however, to
the slightly higher value obtained for the gross energy of the
digested crude fiber, the difference in metabolizable energy between
starch and crude fiber is somewhat less marked, and is hardly
sufficient of itself to justify assigning a materially lower nutritive
value to the latter.

It is worthy of note also that the loss in the methane appears to
be a very variable one, justifying the conclusion already reached
that other factors than the proximate composition of the food ma-
terially affect the extent of the methane fermentation.

The Horse. — The production of methane by the horse appears
to be much less copious than that by ruminants. Lehmann, Hage-

THE FOOD AS A SOURCE OF ENERGY. 331

mann tfc Zuntz * in eight respiration experiments obtained the
following results, the hydrocarbons being computed as methane:

Crude Fiber Digested.

Methane Excreted.

698.

5

grams

26 . 8 grams

538.

9

<<

33.4

tt

451.

.7

a

13.0

it

<<

a

20.0

tt

tt

tt

16.4

tt

11

tt

31.0

tt

a

a

22.1

a

it

tt

23.0

it

As already noted on p. 326, Zuntz f has pointed out that the
fermentation of the food in the horse takes place largely in the
coecum and after the more digestible carbohydrates have been
resorbed. The authors consequently compute the excretion of
methane entirely upon the crude fiber of the food. On the average
of the eight somewhat discordant experiments, in which the food
consisted of oats. hay. and cut straw, 100 grams of digested crude
fiber yielded 4.7 grams of methane, which corresponds exactly with
the results reported by Tappeiner \ for the artificial fermenta-
tion of cellulose. In the same experiments an excretion of approxi-
mately 0.203 gram of hydrogen per 100 grams digested crude fiber
was observed. Deducting the corresponding amounts of energy
from the energy of the apparently digested cellulose we have —

Total energy of I gram 4 . 220 Cals.

Energy of CH4 (0.047 gram). 0.627 Cal.

Energy of H (0.002 gram)... 0.070 " 0.697 Cal.

Metabolizable energy of 1 gram 3 . 523 Cals.§

While less methane is apparently produced by the horse than
by the ox, the assumption that it all arises from the fermentation
of the crude fiber gives the latter a metabolizable energy not greatly
different from that found in the case of the ox. It is of course

* Landw. Jahrb., 23, 125.
t Arch. ges. Physiol., 49, 477.
% Zeit. f. Biol., 20, 88.

§ As computed by the authors, 3.487 Cals. on the basis of. 4. 185 Cals.
total energy per gram of crude fiber.

332

PRINCIPLES OF ANIMAL NUTRITION.

implied in this that the metabolizable energy of the digested nitro-
gen-free extract is the same as its gross energy.

Summary. — The results recorded in the preceding paragraphs
regarding the metabolizable energy of the several classes of digesti-
ble nutrients are summarized in the following table :

METABOLIZABLE ENERGY OF DIGESTIBLE NUTRIENTS.

Cattle,

Cals. per

Grm.

Horse,

Cals. per

Grm.

Swine,

Cals. per

Grm.

Protein (NX6.25):

From wheat gluten

" (NX5.7)

" beet molasses

" mixed grain

" " ration of oats, hay, and straw

" meadow hay

" timothy hay

" straw

4.89-4
4.958
3.984

4.083

3.228

Fat:

From peanut oil

" hay (ether extract) ,

1.272
3.057(?)

(?)

8.821
8.322

Carbohydrates :

Starch, Kellner's experiments . .

" Kilhn's
Nitrogen-free extract (assumed)
Crude fiber, of extracted straw .
" hay fed alone.

763
648

added to basal ration

oat straw

wheat straw

mixed ration

3.606
3.311
3.606
3.437
3.001

4.185

3.523

Perhaps the most striking thing about these figures is the wide
range of the results upon the same class of nutrients. For reasons
already stated, this is most noticeable with the protein, but it is
sufficiently marked also with the other two groups. Moreover,
such meager data as we possess regarding other animals than the ox
indicate that the results vary with the species of animal, a fact
which should not surprise us, but which, nevertheless, adds mate-
rially to the complexity of the subject and greatly widens the range
of necessary investigation. It is obvious, therefore, that at present
our knowledge is too imperfect to allow of the assignment of average
values for the metabolizable energy of the different classes of

THE FOOD AS A SOURCE OF ENERGY. 333

nutrients (as ordinarily determined) even for a single species of
animal.

The results tabulated above, however, are amply sufficient to
justify the statement on p. 279 that Rubner's averages are not appli-
cable to herbivorous animals, and that the metabolizable energy
as computed with their aid is likely to vary widely from the truth.
Indeed, since Rubner's factor for fat (9.3 Cals. per gram) is 2.27
times that for carbohydrates and protein (4.1 Cals. per gram) a
computation of the metabolizable energy of feeding-stuffs or rations
as it has not uncommonly been made simply gives a series of figures
about 4.1 times as great as that obtained for total digestible matter
when the digestible fat is reduced to its starch equivalent by multi-
plication by 2^. So far, then, as a comparison of one feeding-stuff
or ration with another is concerned, this process adds no whit to our
knowledege. It does, it is true, give some idea, albeit an inade-
quate one, of the total amount of metabolizable energy present. As
yet, however, our accurate knowledge of the energy requirements of
domestic animals for various purposes is comparatively meager.
If we base our computations on the feeding standards now current,
wo simply repeat with them the useless multiplication performed
on the feeding-stuffs. On the other hand, if we take the results of
such exact experiments on the metabolism of energy as are available,
then, as the above results show, we shall be computing the energy
requirements upon one basis and the energy supply upon a mate-
rially different one.

Significance of the Results. — A much more fundamental prob-
lem than that raised in the foregoing paragraph confronts us when
we come to reflect upon the general method by which it has been
attempted to compute the metabolizable energy of nutrients, and
to consider the real significance of the results. In so doing we may
properly confine ourselves to the results upon cattle, those for horses
and for swine being more or less fragmentary and uncertain. By
far the larger proportion of the results above tabulated, as well
as the most important of them, are based on experiments in
which additions were made to a basal ration, the computation being
by difference. As wa- pointed out in discussing the apparent
metabolizable energy of the organic matter on previous pages,
and as was specifically illustrated in the case of one experiment on

334 PRINCIPLES OF ANIMAL NUTRITION.

molasses (p. 201), the difference in the metabolizable energy of the
excreta is the algebraic sum of the differences in the energy of
methane, urine, and the several proximate ingredients of the feces,
and some of these differences may be positive and others negative.
The computations of the metabolizable energy of the organic matter
as made in the earlier paragraphs give the net result to the animal
under the condition of the experiment and include all the secondary
effects upon digestion, etc.

In the computations here considered Kellner's methods have
been followed. In the first place the influence of the added feed
upon the digestibility of the basal ration has been eliminated by
basing the computation upon the digested matter. Still further,
such effects as the decrease of the methane excretion in certain of
the experiments with molasses, oil, and starch, and the diminished
export of energy in the urine under the influence of starch and ex-
tracted straw, have not entered into the computation. In other
words, the endeavor has been to determine the actual amount of
energy liberated by the breaking down of the molecules of the di-
gested starch or protein or fat in the organism without regard to
these various incidental effects; that is, to determine the real and
not the apparent metabolizable energy.

Either method of computation would seem to be entirely defensi-
ble, and our choice between them will be largely determined by
the point of view. For the purposes of the physiologist, desirous
of tracing the details of the chemistry and physics of metabolism,
the results obtained by the latter method will be of more interest.
On the other hand, the student of nutrition who is especially in-
terested in the problems of feeding will not fail to note that the
results thus reached represent, from his standpoint, only a part of
the truth. They show (barring errors of detail) how much energy
is liberated in the body from the several nutrients, but the loss or
saving of energ)^ in the incidental processes constitutes just as real
a part of the balance of energy which he wishes to determine as the
energy liberated from the nutrients themselves, and must be taken
account of in his calculations. Whether this can best be done by
using some such factors as those just tabulated and then making
a correction for these incidental gains and losses, or whether the
method followed in the earlier paragraphs is to be preferred, it

THE FOOD AS A SOURCE OF ENERGY. 335

would probably be premature to attempt to decide at present.
Pending further investigation and experience however, it should
be remembered that the figures on p. 332 will give, in most cases,
too high results for the metabolizable energy of mixed rations, while
the same thing is still more emphatically true of Rubner's factors.

One additional point requires mention. In discussing the
metabolizable energy of protein it was pointed out (p. 320) that
it is at least a plausible hypothesis that the proteids are metabo-
lized in the herbivora substantially as in carnivora, and that the
excess of energy in the urine is derived from the non-nitrogenous
ingredients of the food. If we accept this hypothesis, however,
and assume the metabolizable energy of protein (N X 6.25) to
he in the neighborhood of 4.9 Cals. per gram, then the figures
for the non-nitrogenous nutrients are subject to a still further
deduction, especially in the case of coarse fodders. If we were to
assign to the fat its full value as given, it would not be difficult to
compute the metabolizable energy of the carbohydrates on this
basis, and probably a set of factors could be worked out which
would correspond to the actual results obtained with mixed rations.
These, however, if successfully obtained, would be substantially
identical with the results given on previous pages for the apparent
metabolizable energy of total or of digestible organic matter, and
it does not appear that the former would offer sufficient advantages
over the latter to justify the labor involved in their computation.

CHAPTER XI.

INTERNAL WORK.

§ i. The Expenditure of Energy by the Body.

Having considered the food in the light of a supply of energy
to the animal, it now becomes desirable to take a more general
view of the subject and inquire into the uses to which the energy
of the food is applied in the organism.

We have already distinguished between that portion of the
potential energy of the food which is convertible into kinetic energy
in the body, and which we have here called metabolizable energy,
and that portion of it which is rejected for one reason or another
in the potential form in the various excreta. This latter portion
we may dismiss from consideration for the present. The former
portion — the metabolizable energy — as common experience informs
us, is applied to two main purposes. First, it supplies the energy
for carrying on the various activities of the body. Second, if the
supply is in excess of the requirements of the body a portion of it
may temporarily escape conversion into the kinetic form and be
stored up as gain of tissue, notably of fat. We may say briefly,
then, that the metabolizable energy of the food is used, first, for
the production of " physiological work " and second, for the storage
of energy.

Physiological Work. — The term " physiological work " in the
previous sentence is employed in a somewhat loose and general
sense to designate all those activities in the body which are sus-
tained by the metabolizable energy of the food and whose ultimate
result is the production of heat or motion. A more definite idea
of what the term includes may be gained by a consideration of the
chief factors which go to make up the physiological work of the

body.

336

INTERNAL WORK. 337

Work of the Voluntary Muscles. — The most obvious form
of physiological work is that performed by the contraction of the
voluntary muscles, either in the performance of useful work or in
the various incidental movements made during the waking hours.
In a sense the production of muscular work may be said to be the
chief end of the metabolizable energy of the food, inasmuch as
all the other activities of the body (apart from the reproductive
functions and, of course, from mental activities) are accessory to
this. In amount, however, the energy of muscular work is much
less than the energy expended in other forms of physiological work
and consumes a comparatively small percentage of the metaboliz-
able energy of the food.

Internal Work. — The body of an animal in what we commonly
speak of as a state of rest is still performing a large amount of work.
The most evident forms of this are the work of circulation and res-
piration. In addition to these, however, there are less obvious kinds
of work whose total is probably very considerable. The body is an
aggregate of living cells. The living cell, however, is constantly
carrying on activities of various sorts, and these activities require
a supply of energy, although how much of the energy of the food
is consumed in the various processes of secretion, osmosis, karyoki-
nesis, etc., it is difficult to say.

In the numerous varieties of internal work the energy involved
passes through various forms. Ultimately, however, since it
accomplishes no work upon the surroundings of the animal, it
is converted into heat and leaves the body either by radiation and
conduction, as the latent heat of water vapor or as the sensible
heat of the excreta.

Work of Digestion and Assimilation. — Logically the work
of digestion and assimilation would be classed as part of the internal
work of the body, but motives of convenience make it desirable to
consider it separately.

In a fasting animal, with the digestive tract empty, the various
forms of internal work indicated above go on with a considerable
degree of constancy, and the resulting heat production is quite
uniform under like conditions. If food be given to such an animal
there results very promptly an increase in the excretion of carbon

338 PRINCIPLES OF ANIMAL NUTRITION.

dioxide and the absorption of oxygen and in the amount of heat
produced. In general terms this is brought about in four ways :

First, the muscular work required for masticating and swallow-
ing the food and moving it through the digestive apparatus involves
an expenditure of energy which finally gives rise to the evolution
of heat.

Second, the activity of the various secreting glands of the diges-
tive tract is stimulated, again making a demand for energy and
giving rise to an increased heat production.

Third, the work of the resorbing cells likewise makes demand
for energy.

Fourth, the various fermentations, cleavages, hydrations, and
syntheses which the food ingredients undergo in the course of diges-
tion, resorption, and assimilation may occasion in individual cases
either an evolution or an absorption of energy, but taken as a whole
result in the production of a greater or less amount of heat and con-
sume a corresponding amount of the metabolizable energy of the food.

Production of Heat. — The body temperature of the healthy
warm-blooded animal is practically constant, any considerable
variation from the average indicating some serious disturbance of
the animal economy. Since this temperature is ordinarily higher
than that of the environment, a continual production of heat is
necessary to maintain it.

As stated above, the various forms of internal work, including
the work of digestion and assimilation, give rise in the aggregate
to the evolution of a large amount of heat, and this heat is of course
available for the maintenance of the body temperature.

"Whether its amount is sufficient for this purpose, or whether
under any or all circumstances there is a production of heat for its
own sake, simply to keep the animal warm, is still a debatable
question. Many eminent physiologists, notably Chauvcau and
his associates, hold that the primary function of metabolism is to
furnish energy for the physiological processes going on in the body.
They hold that the potential energy of the food is converted imme-
diately into some form of physiological energy, which in its turn,
in fulfilling its functions, is converted into heat which serves inci-
dentally to maintain the body temperature. In other words, they
regard heat as substantially an excretion and would consider that

INTERNAL WORK.

339

in the course of organic evolution those forms have survived in
which the incidental heat production was sufficient to meet the
demand of the environment.

Other physiologists no less eminent hold that at least an ex-
ceptional demand for heat (low external temperature) may be met
by a direct combustion of food or body material for that purpose
We shall have occasion later to give further consideration to these
divergent views.

Summary. — The following scheme may serve to summarize
what has been said above regarding the uses to which the energy of
the food is put in the body, the possible direct heat production being
considered, for convenience, as part of the physiological work of
the body in order to include it among the other forms of the
expenditure of energy :

• Energy of excreta.

Gross energy

Metabolizable
energy

Physiological
work

Work of voluntary
muscles.

Internal work.

Work of digestion and
assimilation.

Heat production.
Storage of energy.

For the sake of directness of statement, language has been used
above which seems to imply that the food is directly oxidized some-
what like the fuel in a locomotive. While statistically the effect is
the same as if this were the case, it must not be forgotten that the
bod}' itself constitutes a reservoir of potential energy and that
the energy liberated in its various activities comes primarily from
the potential energy stored up in its various tissues, while the func-
tion of the food is to make good the loss this occasioned.

The metabolism of matter and energy in the body might be
compared to the exchange of water in a mill-pond. The water in
the pond may represent the materials of the bod}' itself, while the
water running in at the upper end represents the supply of matter
and energy in the food, and that going down the flume to the mill-
wheel the metabolism required for the production of physiological
work as above defined. The water flowing into the pond does not
immediately turn the wheel, but becomes part of the pond and
loses its identity. Part of it may be drawn into the main current

340 PRINCIPLES OF ANIMAL NUTRITION.

and enter the flume comparatively soon, while another part, may
remain in the pond for a long time. Pursuing the comparison still
further, as but a small proportion of the energy liberated in the de-
scent of the water in the flume takes the form of mechanical energy,
most of it being converted into heat, so in the body but a small
proportion of the energy expended in physiological work takes
ultimately the form of mechanical energy. Finally, if we compare
the flow of water in the stream below the dam to the heat produc-
tion of the body, that flow may be increased, in case of need, in two
ways, viz., by opening the gate wider and letting more water pass
through the flume (increase of physiological work) or by lowering
the dam and allowing more water to flow over, corresponding to a
heat production for its own sake, if such takes place.

The succeeding sections of this chapter will be devoted to a con-
sideration of the expenditure of energy in the various forms of in-
ternal work, including that of digestion and assimilation, while the
subjects of the production of external work and of] the storage of
energy may be more appropriately considered in subsequent chap-
ters.

§ 2. The Fasting Metabolism.

If an animal be deprived of food for a sufficient length of time
to empty the digestive tract, and kept in a state of rest as regards
muscular exertion, the expenditure of energy in external work and
in the work of digestion and assimilation are both eliminated, while
there can be, of course, no storage of energy. Under these condi-
tions the metabolism of energy in the organism is confined to the
maintenance of those essential vital activities which were grouped
above under the term " internal work " in the narrower sense, to-
gether with any direct production of heat for its own sake. The
fasting animal, then, affords the most favorable opportunity to
study the laws governing the expenditure of energy for the internal
work of the body. The fasting metabolism has already been con-
sidered in Part I from the side of the matter involved; here we are
concerned with its energy relations.

Nature of Demands for Energy.
Without attempting to enter into details, it may be said that
the internal work of the fasting organism may be roughly classified

INTERNAL WORK. 341

as muscular, glandular, and cellular. To the demand for energy
for these purposes we have probably to add, at least in some cases,
a direct demand for heat production.

Muscular Work. — The more obvious forms of muscular work
in the quiescent animal are circulation and respiration. To these
are to be added as minor factors any movements of other in-
ternal organs, and especially the general tonus of the muscular
system, while finally, the various incidental movements made by
such an animal, although not logically belonging in the category
of internal work, practically have to be classed there in actual
experimentation. It would be aside from the purpose of this
volume to enter into any detailed consideration of these forms of
internal work, but a few general statements regarding their amount
may be of interest.

Circulation. — The work performed by the heart is determined
by two factors, viz., the weight of the blood moved and the mean
arterial pressure overcome. Quite divergent results have been ob-
tained by various investigators for the former factor, while the
latter is more readily determinable. Zuntz & Hagemann* estimate
the output of blood by the heart of the horse from a comparison
between the blood gases and the respiratory exchange, and compute
the expenditure of energy in circulation to be 5.01 per cent, of the
total metabolism of the horse in a state of rest and 3.77 per cent,
during moderate work. Hill f estimates the average work of the
heart in man at about 24,000 kilogram-meters in twenty-four hours.
As the velocity of the circulation increases, the friction in the pe-
ripheral blood-vessels, and consequently the arterial pressure, rap-
idly augments, so that in case of severe muscular exertion, for ex-
ample, the work of the heart may readily become excessive.

Respiration. — The work of respiration consists essentially of
an expansion of the thorax against the resistance caused by the
atmospheric pressure and the elasticity of the lungs and the rib
cartilages. Zuntz & Hagemann £ estimate its amount in the
horse at about 4.7 per cent, of the total metabolism.

Muscular Tonus. — As was pointed out in Chapter VI, the living

* Landw. Jahrb., 27, Supp. Ill, 371.

t Schiiffer's Text-book of Physiology, II, 43.

X Loc. cit.

34^ PRINCIPLES OF ANIMAL NUTRITION.

muscle is in a constant state of slight tension or tonus, and is con-
stantly the seat of metabolic activities which we may presume
serve, in part at least, to maintain that tonus. This is, of course,
equivalent to saying that there is a continual liberation of kinetic
energy in the resting muscle, which temporarily takes the form of
muscular elasticity but ultimately appears as heat. As to the
amount of energy thus liberated exact information seems to be lack-
ing, but in view of the relatively large mass of the muscles as com-
pared with that of the other active tissues we may assume that it
is not inconsiderable. The same thing would seem to be indicated
also, as noted in Chapter VI (p. 191), by the great decrease in the
metabolism and heat production ordinarily observed as the result
of paralysis of the motor nerves by curari.

Incidental Muscular ]York. — It is rare that an animal, even
when at rest in the ordinary sense, does not execute more or less
motions of various parts of the body, all of which involve a conver-
sion of potential energy into the kinetic form. Even apparently
insignificant movements may materially increase the amount of
metabolism. Zuntz & Hagemann,* for example, report a respira-
tion experiment upon a horse in which the uneasiness caused by the
presence of a few flies in the chamber of the apparatus caused an
increase of over 10 per cent, in the metabolism. Johanson, Lan-
dergren, Sonden & Tigerstedt,f in two-hour periods, found the fol-
lowing average and minimum values per day and kilogram weight
for the excretion of carbon dioxide by a fasting man during sleep,
the results plainly showing the increased metabolism due to rest-
lessness :

Third day (first day of fasting)

Fourth "

Fifth " (very restless)

Six tli "

Seventh "

Average,

Minimum,

Grms.

Grms.

7.296

6.744

7.704

6.768

8.136

7.524

7.488

6.684

7.212

6.564

Subsequently Johanson J compared the excretion of carbon
dioxide by a fasting man when simply lying in bed (awake) with

♦Landw. Jahrb., 23, 161.

t Skand. Arch. f. Physiol., 7, 29.

| Ibid.. 8, 85.

INTERNAL WORK.

343

that obtained when all the muscles were as perfectly relaxed as
possible. The results per hour were :

Lying in bed 24 . 94 grams

Complete muscular relaxation 20.72 "

Furthermore, there is more or less muscular exertion involved
during the waking hours in maintaining the relative position of the
different members of the body. This is notably true of the effort
of standing. In experiments with the respiration-calorimeter
under the writer's direction* the heat production of a steer per
minute while standing and lying was found to be approximately
as follows:

Lying,
Cals.

Standing
Cals.

Ratio, Lying to

Standing.

Period A
" B
" C
" D

1 : 1.321

1 : 1.332

1 : 1 . 296

1 : 1.286

Zuntz f found an even greater difference in the case of the dog,
the average oxygen consumption per minute being —

Lying 174.3 c.c.

Standing 245 . 6 "

In experiments of any considerable duration on normal animals
it is impossible to avoid more or less expenditure of energy in this
incidental muscular work, while it is often a matter of difficulty
to make the different periods of an experiment comparable in this
respect.

Glandular Work. — The activity of the various secretory, ab-
sorptive, and excretory organs may be conveniently summarized
under this head. While the purpose of the glandular metabolism
i~. in the majority of cases, primarily a chemical one. the accom-
plishment of this purpose involves an expenditure of energy which,

* Proc Soc Prom. Agr Sci , 1902.
t Arch. ges. Physiol , 68, 191.

344 PRINCIPLES OF ANIMAL NUTRITION.

so far as it is not removed from the body in the potential form in
the secretions or excretions, ultimately takes the form of heat.

Moreover, the fundamental features of glandular metabolism
appear to be indentical with those of muscular metabolism. Thus
Henderson * has shown that the active submaxillary gland of the
dog does not lose nitrogen as compared with the inactive gland,
but does lose weight, evidently from the metabolism of non-
nitrogenous matter. Similarly, Barcroft f found the respiratory
exchange of the same gland during activity to be three or four times
that during rest. If we may accept these results as typical, we
must conclude that glandular, like muscular metabolism is largely
at the expense of non-nitrogenous matter, and shall not hesitate to
summarize the two together as parts of the internal work of the
body.

Cellular Work. — While both muscular and glandular work
are forms of cell activity, a passing mention may be made for the
sake of completeness of such processes as imbibition, filtration,
osmosis, protoplasmic motion, karyokinesis, etc., which, while
taking place in the various organs, are so general in their nature and
form so essential a part of our conception of cell life that it seems
proper to speak of them collectively as cellular work. As to the
quantitative importance of these activities, so far as they can be
differentiated from the special functions of the various organs, we
lack the data for forming any definite conception, although it
would appear that it must be small.

Heat Production and Regulation.

As we have just seen, the forms of internal work are numerous
and some of them are not readily accessible to measurement. All
of them, however, have this in common, that the energy used in
their performance ultimately assumes the form of heat.

This being the case, while the single factors making up the
internal work are not readily determined, a determination of the
total heat produced by a fasting animal in a state of rest (either
directly or by computation from the amount and kind of matter
metabolized) will show the total amount of energy consumed in the

* Am. Jour. Physiol., 3, 19. f Journal of Physiol , 27, 31.

INTERNAL WORK.

345

performance of the internal work and how it varies under varying
conditions. Carnivorous animals, with their short and relatively
simply digestive canal, lend themselves most readily to experiments
of this sort although rabbits and guinea-pigs have been employed
to some extent, as well as, for short periods, men.

Constancy Under Uniform Conditions. — Attention has al-
ready been called in Chapter IV to the relative constancy of the
total metabolism of the fasting animal, particularly as compared
with the total mass of active tissue in the body. This constancy
has been especially emphasized by Rubner,* and forms the basis
of his determinations of the replacement values of the several
nutrients which will be considered in the following chapter.
With a rabbit the following daily averages, computed per 100
parts of nitrogen in the body, were obtained:

Day of Experiment.

Nitrogen in
Urine.

Fat
Metabolized.

Third to eighth

Ninth to fifteenth

2.16
2.19

16.2
13.8

Since the ratio of proteids to fat metabolized did not vary
greatly in these trials, the total amount of carbon dioxide ex-
creted may be taken as an approximately accurate measure of
the total metabolism. For the several days of the experiment,
this was as follows:

Average Live

Weight,

Grms.

Carbon Dioxide Excreted.

Day.

Per Head,

Grms.

Per Kg.

Live Weight,
Grms.

Fifth

2091
2002
1907
1864
1764
1731
1716
1697

36.1
31.8
30.3
29.2
30.2
27.4
27.4
25.5

17.26

Seventh

15.90

Ninth

15.90

Tenth

15.65

Twelfth

17.18

'1 hirteenth

Fourteenth

Fifteenth

15.81
15.95
15.90

*Zeit. f. Biol., 17, 214; 19, 312.

34^ PRINCIPLES OF ANIMAL NUTRITION.

With a dog the following results were obtained :

Day.

Live

Nitrogen

Weight,

in Urine,

Grms.

Grms.

9190

4.23

8920

2.89

8620

3.65

8190

2.59

8030

2.41

7890

2.53

7970

2.98

7830

3.02

Fat
Metab-
olized,
Grms.

Carbon Dioxide
Excreted.

Per Head,
Grms.

Per Kg.

Live
Weight,
Grms.

First

Second

Fourth

Tenth

Eleventh ..
Twelfth . . .
Thirteenth
Fourteenth

51.74
45.94
42.90
45.55
41.83
36.48
37.45
33.80

1S7.4
157.5
146.9
151.7
140.4
127.9
134.8
125.0

20.70
17.83
17.99
18.70
17.86
16.13
17.06
18.12

Rubner also quotes the following results by Kuckein on a cock:

^ Carbon Dioxide per

u&y- Kg. Live Weight.

Third 21 . 73 grams.

Fifth 21.47 "

Seventh 21.43 "

Rubner 's experiments on a guinea-pig * show a similar constancy,
the heat production being computed from the total metabolism:

n Heat Production

y* per Kilogram.

First 149.9 Cals.

Second 162.6 "

Third 156.5 "

Fourth 140.5 "

Fifth 137.3 "

Sixth 150.6 "

Seventh 157.4 "

Eighth 155.6 "

Ninth 162.6 "

Concerning this point Rubner says : f " The uniformity of the
fasting metabolism proves that, in spite of the undoubted limita-
tation of all the voluntary functions which can cause a consump-
tion of matter, no further reduction of the metabolism is possible,
* Biologische Gesetze, p. 15. t Loc. cit., 19, 326.

INTERNAL WORK. 347

and we recognize from this that we have to do here with a constant
metabolism which is indissolubly connected with life itself. The
animal in the fasting state adjusts itself to the minimum metabolism.".

In other words, the metabolism and consequent heat production
of the fasting, quiescent animal speedily reaches a minimum which
represents the aggregate demands of the vital activities of the
organism for energy; that is, which represents the internal work of
the body in the sense in which the words are here used, plus the
metabolism required for any direct production of heat which may
be necessary to maintain the normal temperature of the animal.

The relative importance of the internal work in the narrower
sense and of the direct heat production as regards their demands for
a supply of energy will appear more clearly when we consider, in
the following paragraphs, the effects of varying conditions, and
particularly of the thermal environment, upon the heat produc-
tion of the fasting animal.

Influence of Thermal Environment on Heat Production.* — An
animal, particularly in the temperate zones, is subject to consider-
able variations of external conditions, particularly of temperature,
which, in the first place, tend to affect the rate at which it emits
heat, and secondarily, within certain limits to modify the amount
of heat produced in the body.

Body Temperature. — As regards their body temperature,
animals have been divided into two great classes : the cold-blooded
(poikilothermic), whose temperature as a rule differs but slightly
from that of their surroundings, and the warm-blooded (homoio-
thermic), whose temperature remains approximately constant dur-
ing health whatever be that of their surroundings. Since all our
domestic animals, as well as man himself, belong to the second
group, it alone will be considered in the following paragraphs.

Since the animal is constantly producing heat in the various
ways already indicated, it is obvious that in order to maintain a
constant body temperature it must be able to give off this heat at
the same average rate at which it is produced. Ranke illustrates
this necessity in a striking manner by computing that if the

* The discussion of this subject follows to a considerable extent that of
Ranke in the introduction to his " Einwirkung des Tropenklimas auf die
Ernahrung dea Menschen," Berlin, 1900.

34§ PRINCIPLES OF ANIMAL NUTRITION.

body of a man were unable to give off the heat which it pro-
duces, a single day would suffice to raise it to a pasteurizing
temperature, while in the course of a year, at the same rate, a
temperature of over 17,000° C. would be reached.

Furthermore, since the external conditions of temperature are
subject to frequent and sudden changes, it is obvious that the
balance between heat production and emission must be capable of
prompt adjustment to varying circumstances.

Thermic Range. — The ability of the animal body to adapt
itself to changes of temperature has, however, often been ex-
aggerated. As a matter of fact this adaptation is possible only
within a comparatively narrow range, and unless we hold fast to
this fundamental idea we are in danger of reaching fallacious
and absurd conclusions. Man has considerably extended the range
of climate within which he can exist by means of clothing, shelter,
artificial heat, and even to a slight extent artificial refrigeration,
and this fact often leads unconsciously to an overestimate of the
possible thermic range. These means of artificial protection re-
sult essentially in modifying the temperature to which the body is
actually exposed, and the same is true in a less degree of the differ-
ences in the summer and winter coats of animals. The fact still
remains that the actual thermic range of any species is and must be
strictly limited. All life implies a certain amount of metabolism,
and consequently of heat production. With rising temperature a
point must sooner or later be reached at which the animal is unable
to impart this heat to its surroundings as fast as it is produced, and
in which the rise in temperature necessarily resulting will prove
fatal. With falling temperature a point will be reached at which
the greatest possible amount of metabolism in the body will be
unable to equal the rate at which heat is lost to the surroundings
and the animal will perish from cold. Both the maximum and
minimum points and the extent of the thermic range will vary for
different species and varieties of animals, but at best the range is
relatively small.

Means of Regulation. — Within the thermic range of a given
animal the adjustment -to its thermal environment may be effected
in one or both of two ways, viz., by a regulation of the rate of emis-
sion of heat or by a variation in the heat production.

INTERNAL WORK. 349

Regulation of Rale of Emission. — Heat is giveD off by the body
in four principal ways: (1) by conduction; (2) by radiation; (3)
by evaporation of water: (4) as the sensible heat of the excreta.

By conduction, heat is transferred directly from the body to
its surroundings, including such solid objects as it may be in con-
tact with and particularly the air. The rate of loss in this way
will depend upon the relative temperature and conductivity of
the surface of the body and of the substances with which it is in
contact, and in case of the air will be also influenced by the rate
of motion of the latter relatively to that of the body.

By radiation, a constant exchange of heat goes on between the
body and objects not in immediate contact with it. Since the body
is usually warmer than its surroundings, the net result of this ex-
change is a loss of heat by the body, the amount of which depends
upon the specific radiating power of the surface of the body and
upon the difference in temperature between the latter and sur-
rounding objects.

By evaporation of water from the skin, and to a less degree
from the mucous membrane of the air-passages, a lar«;e amount
of heat may be removed as latent heat of vaporization. The
amount of water evaporated from the skin, and consequently the
rate at which heat is carried off, will depend in part on the
amount transpired by the skin, but when this is abundant,
chiefly upon the relative humidity of the air and upon its rate of
movement.

Finally, the heat removed in the excreta is relatively small, and
in the case of the fasting animal in particular is insignificant as
compared with the losses through the other three channels.

In general we may say that the rate of emission of heat in all
of the first three ways named is determined by two sets of condi-
tion-, viz., those relating to the environment of the animal (tem-
perature, relative humidity, movement of air) and those relating
to the animal itself and particularly to its surface.

The conditions of the first set, of course, are beyond the control
of the organism. Their tendency is to produce- the same effect upon
the rate of emission of heat that they would upon that of a lifeless
body, viz., to increase it as the temperature of the surroundings is
lowered and their conducting power increased. In the case of the

350 PRINCIPLES OF ANIMAL NUTRITION.

living animal this tendency is offset by the regulative mechanism
acting upon the second set of conditions, so that, e.g., a fall in
the temperature of its surroundings within certain limits instead
of increasing the rate of emission, as in the case of a lifeless body,
has no effect upon it. This regulation of the rate of emission is
effected chiefly by means of changes in the temperature and state
of moisture of the skin, brought about on the one hand through the
vaso-motor mechanism and on the other through the special nerves
of perspiration.

Variations of external temperature acting upon the peripheral
nerves influence by reflex action the activity of the vaso-motor
nerves which regulate the caliber of the minute blood-vessels.
Exposure to cold causes a contraction of the capillaries of the
skin and a relaxation of those of the viscera. As a result more
blood passes through the latter, while the flow through the skin
is diminished, the latter becomes paler, and since the heat given
off is not fully replaced by the blood current, its temperature falls.
Exposure to heat has the contrary effect. The capillaries of the
skin relax, more blood flows through them, the skin becomes flushed
and its temperature rises, while the flow of blood to the viscera is
checked. A fall in the temperature of the skin, however, tends to
diminish the rate of emission of heat both by conduction and radia-
tion, while a rise in its temperature has the opposite effect, thus
counteracting the tendency of changes of external temperature.
In other words, the "emission constant" of the skin changes to
meet changes in external conditions. So exactly are these mech-
anisms adjusted in health that within certain rather narrow limits
they maintain the rate of emission of heat, and consequently the
average temperature of the body, very nearly constant.

Obviously, however, there must be a limit above which the
temperature and radiating power of the skin cannot be increased
to compensate for a rise in external temperature. The second
method of regulation then comes more markedly into play through
the familiar act of perspiration, or sweating. At high temperatures
the activity of the sweat-glands is greatly stimulated, in part
doubtless by the more abundant supply of blood to the skin, but
chiefly by reflex stimulation of the special nerves which control the
secretion of sweat. The evaporation of the relatively large amount

INTERNAL WORK.

351

of water thus supplied to the surface of the skin is a powerful means
of refrigeration, as we know no less from common experience than
from scientific determinations, the evaporation of a single gram of
water requiring approximately 0.592 Cal. of heat. With very-
high temperatures, especially in a humid atmosphere, however,
even this method of disposing of the heat becomes insufficient and
the extreme upper limit of the thermal range is passed.

These two methods of regulation of the body temperature are
often spoken of collectively as "physical" regulation.

Variations in Amount of Heat Produced. — Just as there is a
superior limit beyond which the regulation of the body tempera-
ture by the means above described cannot be carried, so it is obvious
that there must be a lower limit of regulation. However much the
cutaneous circulation maybe reduced, the skin will always lose heat
to a sufficiently cold environment faster than it is being generated
by the internal work of the body. Under these circumstances the
only method by which the temperature of the animal can be main-
tained is an increase in the rate of generation of heat.

That changes of external temperature affect the amount of heat
generated was shown by the experiments of Lavoisier and the
observations of Liebig, but Liebermeister * appears to have been
the first to clearly enunciate the theory of regulation by variations
in the rate of production. The fact of such regulation has been
fully demonstrated by numerous subsequent investigators. As a
typical example we may take the well-known experiments of Theo-
dor | on a cat, some of the results of which are as follows:

Temperature,
Deg.
Cent.

CarbonDioxide

Excreted,

Grms.

Oxygen

Taken Up,

Grms.

Temperature.
Deg.
Cent.

CarbonDioxide

Excreted,

Grms.

Oxygen

Taken Up.

Grms.

-5.5

-3.0

0.2

5.0

19.83
18.42
18.24
17.90

17.48
18.26
19.95

14.82

12.3
16.3
20.1
29.6

17.63
15.73
14.34
13.12

17.71

14.74
12.78
10.87

Numerous other investigators have obtained similar results,
but the effect of low temperature in stimulating the heat produc-
tion of warm-blooded animals is too well established to require an

* Arch f. (Anat. u.) Physiol., 1860, pp. 520 and 589; 1861, p. 661.
t Zeit. f. Biol., 14, 51.

352 PRINCIPLES OF ANIMAL NUTRITION.

extended citation of authorities here. Some of Rubner's * more
recent results, however, are of interest as showing the delicacy of
the reaction. The experiments were made on fasting dogs in a
state of complete rest, the heat production being computed from
the total metabolism of carbon and nitrogen:

Tempera- Heat Production per

ture, Deg. C. Kg. in 24 Hours.

13. 8 78.68 Cals.

14.9 74.74 "

17.3 69.78 "

18.0 67.06 "

I

rll.8 40.60 "

J 12.9 39.13 "

11 I 15.9 35.99 "

1 17. 5 35.22 "

/ 13.4 39.65 "

III] 19.5 35.10 "

(27.4 30.82 "

This method of regulation of the body temperature is often briefly
designated as "chemical" regulation.

Just how the additional generation of heat is effected is not so
clear. From the fact that the muscles are the seat of a very large
part of the heat production of the body we should naturally be
inclined to look to them as the source of the increase. In quite a
number of experiments on man, of which those of A. Loewy f and
of Johansson \ may be especially mentioned, a stimulation of the
heat production with falling temperature was only observed when
there was visible muscular action, such as shivering, while in the
other cases only the " physical " regulation occurred. Any contrac-
tion of the muscles would of course be a source of heat, but the in-
crease with falling external temperature has been repeatedly observed
with animals in the absence of this obvious cause. Whether in
such cases there is an increase in the tonus of the muscles, involv-
ing an increase in their metabolism, or whether, through some
form of reflex stimulation, the rate of oxidation is accelerated

* Biologische Gesetze, p. 10.

f Arch. ges. Physiol., 46, 189. ♦

X Skand. Arch. f. Physiol., 7, 123.

INTERNAL WORK.

353

simply for the sake of the heat produced is still an unsettled ques-
tion and one which, for our present purpose, we need not pause
to consider. As to the fact of the increase there is no question.

Critical Temperature. — In early writings upon this subject
the influence of external temperature in increasing or diminishing
the heat production of the body was frequently spoken of as if it
were of unlimited application, and the same idea has passed more
or less fully into the popular literature of the subject. But little
reflection is necessary, however, to show that this cannot be the
case. Common observation teaches us that neither our own metab-
olism nor that of our domestic animals, as roughly measured by
the consumption of food, is affected, for example, by the difference
between winter and summer to any such extent as would correspond
to the difference in average temperature. Moreover, if every rise
in external temperature diminished the heat production, there
would be a temperature at which no heat production at all would
occur and at which, therefore, life could exist without metabolism,
which is a contradiction in terms. This extreme case renders clear
the fundamental error of this view, viz., that of regarding the heat
production as an end in itself and not as, substantially, an incident
of the general metabolism.

Carl Voit* was the first to demonstrate by exact scientific
experiments the limits within which the influence of temperature
upon metabolism (the so-called chemical regulation) is confined.
His experiments were a continuation of those of Theodor (p. 351),
and were made upon a man weighing about 70 kgs. and wearing
ordinary clothing. After exposure for some time to the tempera-
ture to be tested he passed six hours in the chamber of the respira-
tion apparatus, fasting and in complete rest. During the six hours
the excretion of carbon dioxide and nitrogen was as follows:

Temperature.
DeK 1

Carbon

I lioxide.

i inns.

Urinary

Nitrogen
( inn-.

Temperature.
Deg. C.

Carbon
Dioxide.

Grm.s.

Urinary

Nit rogen.

( Irms.

4.4

6.5

9.0
14.3
16.2

210.7
206 . 0
192.0
155.1
158.3

4.23

4.05
4.20
3.81
4.00

23.7

24.2
26.7
30.0

164.8
166.5
160.0
170.6

3.40
3.34
3.97

* Zeit. f. Biol., 14, 57.

354

PRINCIPLES OF ANIMAL NUTRITION.

Later and more comprehensive experiments with animals by
Rubner have given corresponding results. Thus with two guinea-
pigs the following figures were obtained in 24-hour experi-
ments : *

Mature Animal.

Young Animal.

Temperature

Temperature

COa per Kg.

Temperature

Temperature

COa per Kg.

of Air.

of Animal,

and Hour,

of Air,

of Animal,

and Hour,

Deg. C.

Deg. C.

Grms.

Deg. C.

Deg. C.

Grms.

0

37.0

2.905

0

38.7

4.500

11.1

37.2

2.151

10

38.6

3.433

20.8

37.4

1.766

20

38.6

2.283

25.7

37.0

1.540

30

38.7

1.778

30.3

37.7

1.317

35

39.2

2.266

34.9

38.2

1.273

40.0

39.5

1.454

A later experiment by Rubner f upon a dog, in which the heat
production was measured by a calorimeter, gave the following
results :

Temperature of Air. Heat Production per Kg.

7.6° 0 83.5 Cals.

15.0° " . 63.0 "

20.0° " 53.5 "

25.0° " 54.2 "

30.0° " 56.2 "

The uniform testimony of these various experiments is that for
each species there is a certain external temperature at which the
metabolism and consequent heat production reach a minimum.
With man in ordinary clothing it would appear to lie at about
15° C.,l with the dog at about 20° C, and with the guinea-pig at
about 30°-35° C. Below this point the heat production rises or
falls with changes of external temperature; or, in other words, the
constancy of the body temperature is secured, in part at least, by

* Biologische Gesetze, p. 13.

t Arehiv f. Hygiene, 11, 285.

% Rubner (Biol. Gesetze, p. 30) says that for naked man it is about 37° C.

INTERNAL WORK. 355

means of the so-called " chemical " regulation, that is, by variations
in the production of heat.

Above this point the heat production, instead of a further de-
crease, shows an increase, which, however, is slight as compared
with the differences observed as a result of the "chemical" regu-
lation: Here we are obviously in the domain of the "physical"
regulation — the regulation by changes in the emission constant of
the skin. This temperature at which the chemical regulation
o ases, and which presumably varies for different species of animals,
Ranke calls the critical temperature. Below it the regulation is
chiefly "chemical," above it chiefly "physical." The slight in-
crease in the metabolism above the critical point is plausibly ex-
plained as due to the greater activity of the organs of circulation,
respiration, and perspiration required for the "physical" regula-
tion.

Rubner's experiments also show that the portion of the thermic
range lying above the critical temperature falls into two distinct
subdivisions. For a certain distance above that point, the factors
chiefly concerned in the regulation of the body temperature are
conduction and radiation, which keep pace with the rising tem-
perature in the manner already explained. At the same time,
there is a small increase in the rate of evaporation of water, approxi-
mately equivalent to the slight increase in the metabolism above
the critical temperature to which attention has just been called.
Matters go on in this way through a certain range of temperature
until the regulative capacity of the vaso-motor mechanism is
utilized to its maximum. If the external temperature still rises,
the emission of heat by conduction and radiation begins to decrease
as it would in a lifeless object, and the deficit thus occasioned is
made up by a sudden increase in the exhalation of water vapor,
coinciding, in man, with the production of visible perspiration.
This sudden increase in the activity of the sweat-glands is accom-
panied, as we should expect, by an increase in the total metabolism
and consequent heat production.

These phenomena are well illustrated by Rubner's experiments
with a fasting dog, already partially cited on the opposite page.
The following table shows the amount of heat carried off by con-
duction and radiation and as latent heat of water-vapor at the

356

PRINCIPLES OF ANIMAL NUTRITION.

several temperatures, and the same facts are also shown graphically
in the accompanying diagram.

1

Temperature
of Air.
Deg. C.

Total Heat

Production,

Cals.

Disposed of by

Conduction

and
Radiation,

Cals.

As Latent

Heat of Water

Vapor,

Cals.

7.6
15.0
20.0
25.0
30.0

83.5
63.0
53.5
54.2
56.2

71.7
49.0

37.3
37.3
30.0

11.8
14.0
16.2
16.9
26.2

7.6 C

15 C

20 C

25 C

3(TC

It appears, then, that a certain minimum heat production,
corresponding to the metabolism at the critical temperature, is
inseparably connected with the life of the animal. The very fact
that the heat production at this temperature is a minimum shows
that its amount is not determined by the needs of the organism for
heat. If the latter were the controlling condition, a rise of exter-
nal temperature should still further reduce the generation of heat,
while as a matter of fact it is accompanied by a slight increase up
to the point where the amount of heat produced overpasses the
ability of the organism to dispose of it and death results. The
natural conclusion is that the metabolism at the critical tem-
perature is that which is necessary for the performance of the
various functions of the organism, and that the heat production
at this temperature, therefore, represents the amount of energy
necessarily consumed in the internal work of the body. This is,
of course, Rubner's conclusion (p. 346) in a slightly altered form.

The case is not unlike that of a room in which a fire must be
kept burning for some purpose — a kitchen, for example. In winter,
changes in external temperature may be met by burning more or

INTERNAL WORK. 357

less fuel. As spring advances, the fire is reduced until it is just
sufficient for the necessary work. If the weather still continues
to grow warmer, since the fire cannot be further reduced the excess
of heat is gotten rid of by opening the windows more or less, while,
to carry out the analogy, in very hot weather we may sprinkle the
floor or wet the walls to secure relief from heat through the evapora-
tion of water.

Modification of Conception of Critical Temperature. —
In our discussion thus far we have considered chiefly the influence
of external temperature on metabolism and heat production. This
is, however, by no means the only condition affecting the heat
balance of the body. Of the other meteorological factors, three
call for special mention, viz., wind, insolation, and in particular
relative humidity.

Wind. — In a perfectly still atmosphere, the layer of air next to
the skin becomes warmed and loaded with water vapor and con-
stitutes to a certain degree a protective envelope which is removed
with comparative slowness by gaseous diffusion. A current of air,
by removing this protecting layer and bringing fresh portions of air
in contact with the body, increases the emission of heat both by
conduction and by evaporation of water. This is in accord with the
common experience that a degree of cold which can readily be
borne when the air is still becomes intolerable in a brisk wind, while,
on the other hand, the oppressiveness of a very hot clay is sensibly
relieved by even a slight breeze. The effect of wind, then, is to
transpose the thermic range of the animal to a higher place in the
thermometric scale, and to correspondingly raise the critical tem-
perature.

Insolation. — The direct rays of the sun impart a considerable
amount of heat to the body. The effect of insolation, therefore, is
the reverse of that of wind, viz., to transpose the thermal range
and the critical temperature downward. A similar effect is pro-
duced, of course, by the sun's heat when reflected from surrounding
objects, or by the radiant heat from hot objects, the earth, for ex-
ample. On the other hand, the radiation from the body into space
during the night, especially at high altitudes and through a dry,
clear atmosphere, may have a very considerable effect in the con-
trary direction.

35 8 PRINCIPLES OF ANIMAL NUTRITION.

Relative Humidity. — The relative humidity of the air affects the
emission of heat in two principal ways. At low temperatures,
where the evaporation of water plays a subordinate role, it increases
the rate of emission by increasing the conductivity and specific
heat of the air, and also the conductivity of the skin and the body
covering (hair, fleece, clothing), these effects outweighing its in-
fluence in diminishing the relatively small amount of evaporation.
Moist cold is, therefore, more trying than dry cold.

At high temperatures, on the other hand, where a large pro-
portion of the heat is removed by evaporation, a high relative
humidity f by checking this evaporation, hinders the emission of
heat, this effect overbalancing any slight increase in conductivity.
Moist heat is accordingly more oppressive than dry heat.

An increase in the relative humidity, then, abbreviates the
thermal range at both ends, while at moderate temperatures it
appears to have but little effect, a diminution of the loss by evap-
oration being compensated for by an increase in radiation and
conduction.

Critical Thermal Environment. — From the above it is
obvious that the so-called critical temperature is not a constant,
even for the same species or the same individual, but that other
factors than the temperature of the air materially affect it.

AY hat is constant (relatively at least) is the rate at which heat
is produced in the body by the metabolism necessary to sustain its
various physiological activities, that is, by its internal work In
order to maintain the normal body temperature, the total outflow
of heat through its various channels must, at its minimum, be equal
to the amount thus liberated in the organism. The outflow of
heat, as we have seen, is affected directly or indirectly by the
external conditions, and largely by the three just mentioned. In-
numerable combinations of these conditions are possible, and any
one of them whose combined effect upon the animal is to make
the outflow of heat equal to the rate of evolution due to the
internal work will constitute a critical point in the above sense.
Any change in such a set of conditions which tends to increase the
outflow of heat will, like a fall in temperature, be met chiefly by an
increased heat production. Any change tending in the opposite
direction will be compensated for by the effects upon the organ-

INTERNAL WORK. 359

ism whch have already been described and which result in maintain-
ing the rate of emission of heat at a point enough higher than before
to provide for carrying off the extra heat arising from the physio-
logical work of the regulative mechanism itself. In other words,
instead of a critical temperature, we get the conception of a critical
thermal environment, which may be reached under a variety of
conditions, and below which we have the domain of "chemical"
regulation, while above it is the region of "physical" regulation.

Influence of Size of Animal on Heat Production. — The total
metabolism of a large animal is necessarily greater than that of a
small one of the same species, but it is not proportional to the
weight, being relatively greater in the smaller animal under com-
parable conditions.

Relation- of Heat Production to Surface. — Bergmann *
appears to have been the first to connect the fact just stated with
the relatively greater surface of the smaller animal, but we are in-
debted to Rubner f for the first quantitative investigation of this
phase of the subject. His experiments were made on six dogs
whose weights varied from 3 to 24 kilograms each. The total
metabolism (proteids and fat) of each of these animals in the fasting
state was determined in from two to thirteen experiments, and
from their results the average heat production of each animal was
computed. The table on page 360 J shows the air temperature and
the computed heat production per kilogram live weight in each
experiment, and also the same corrected to the uniform tempera-
ture of 15° C. This correction is made on the basis of Theodor's
experiments (see p. 351), according to which a difference of 1°
Centigrade caused the amount of oxygen taken up by the cat to
vary 1.11 per cent. The first series consists of a selection from
Pettenkofer & Voit's experiments.

Whether we consider the observed or the corrected heat pro-
duction we find that with the single exception of the corrected
result for No. VI the amount per unit of live weight increases as the
weight itself decreases.

* Cited by Rubner.

t Zeit. f. Biol , 19, 535.

X The figures of the table are computed from those given by Rubner in
loc. cit., p 540, and differ in some cases from the summary given in loc. cit.,
p 542.

360

PRINCIPLES OF ANIMAL NUTRITION.

No. of

Date.

Live

Weight,

Kgs.

Air Tem-
perature,
Deg. C.

Heat Production
per Kg.

Ani-
mal.

Observed,
Cals.

Corrected
to 15°,

Cals.

I «

Pettenkofer & Voit's
experiments

Average

30.96

29.87
31.44
30.38

17.1
17.7
16.2
13.9

38.99

31.82
37.39
36.54

39.90
32.77
37.89
36.09

30.66

24.11
23.75
23.27

16.2

15.0
15.0
15.0

36.18

41.40
40.22
41.10

36.66

June 19, 1883

41.40

" 21, "

40.22

IT J

" 23, "

41.10

1

Average

1

23.71

19.80
19.01
18.79

15.0

16.9
14.5
16.0

40.91

47.95
45.71
42.79

40.91

Feb. 24, 1882

48.91

" 28, "

45.48

TTT J

Mch. 1, "

43.22

1

Average

1

19.20

18.20
17.20

15.8

13.9
16.6

45.48

50.72
41.54

45.87

f

Jan. 12, 1880

50.11

" 14, "

42.29

IV J.

Average

1

17.70

9.05

8.83

8.68

8.53

11.11

10.87

15.3

19.2
20.9
20.2
21.0
18.4
20.0

46.13

66.32
60.28
64.88
60.66
61.16
57.86

46.20

Dec. 21, 1881

69.10

" 22, "

64.19

" 23, "

68.58

" 24, "

64 . 66

V -I

May 2, 1882

63.42

" 3, "

61.04

Average . .

9.51

6.84
6.36
6.14
6.83
6.69
6.56
6.40
6.66
6.50
6.36
6.21
6.15
5.98

19.95

15.8
23.6
20.7
18.2
18.0
15.0
16.5
14.6
16.4
16.3
15.9
18.4
19.2

61.86

65.01
63.65
58.13
71.07
76.85
71.60
75.03
61.55
54.91
53.64
52.57
61.06
54.24

65.16

Dec. 5, 1881

65.77

" 6, "

69.70

" 9, "

61.79

Feb. 1, 1882

73.56

2, "

79.39

" 3. "

71.60

4, "

76.23

VI J

Jan. 27, 1883

61.11

" 28. "

55.73

" 29, "

54.39

Feb. 2, "

53.09

" 10, "

63.22

" 11, "

56.73

Average

6.44

3.34
3.05
2.91

17.6

15.0
12.7
20.6

63.02

84.45
97.86
80.00

64.79

f

Jan. 30, 1880

84.45

Feb. 1, "

95.41

VII \

3, "

84.88

1

Average

I

3.10

16.1

87.44

88.25

■

INTERNAL WORK.
SUMMARY.

361

Average Live

Weight,

Kgs.

Heat Production per Kg.

Relative Heat

No. of
Animal.

Observed,
Cals.

Corrected

to 15°,

Cals.

Production

(Corrected),

Cals.

I

30.66

23.71

19.20

17.70

9.51

6.44

3.10

36.18
40.91
45.48
46.13
61.86
63.02
87.44

36.66
40.91

45.87
46.20
65.16
64.79

88.25

100

II

112

Ill

125

IV

126

V

178

VI

177

VII.. . .

241

Rubner also determined approximately the surface exposed by
his animals, in part by direct measurement and in part by calcu-
lation, and computed the heat production per square meter of sur-
face, with the following results :

No. of Animal.

Surface,
Sq. Cm.

Heat
Production per
Square Meter,

Cals.

I

10750
ssi ).-,
7500
7662
5286
3724
2423

1046
1112
1207
1097
1183
1120
1214

II

Ill

IV

V

vr

VII

He also cites * the results of experiments by Senator on the heat
production of fasting dogs, and a respiration experiment by Reg-
nault & Reiset, as follows:

Live Weight,
Kgs.

Calculated
Surface,
Sq. Cm.

/

Heat Production.

No.

Per Kg.

Live Weight,

Cals.

Per Square
Meter of Sur-
face, Cals.

VIII

10.80
7.52
6.09
5.68
5.40
4.24
5.59

5423
4285
3722
3534

3462
2924
3508

52.31
r»3.76
63.04
68.40
74.16
69.12
72.82

1035

IX

X

XI

XII

XIII

XIV

'.Ml
1031
1101
1 1 ;>7
1003
1154

* Loc. cit., p. 551.

362

PRINCIPLES OF ANIMAL NUTRITION.

With one exception, the results per square meter agree very well
with those of Rubner, both absolutely and relatively.

Rubner has also shown in later experiments * that the same
thing is substantially true of guinea-pigs, both at zero and at the
temperature of about 30 degrees, at which the heat production is at
its mimimum (critical temperature). He likewise points outf
that the well-known rapid metabolism of children as compared
with adults is, so far as the available data show, quite closely pro-
portional to their relative surface, and observations on the diet of
a dwarf | gave a like result.

Richet,§ working with an air-calorimeter of constant pressure,
in which the heat production was measured by the amount of
water displaced by the expansion of the air, obtained the following
results on rabbits, and similar results upon guinea-pigs are also
reported :

Number of
Experiments.

Live Weight,
Kgs.

Heat
per Kg.,

Cals.

Total Heat r™ o
Expressed in ! The Same
c.c. of Water 1 P% Uj:,t of

Displaced. Surface.

5

10

12

4

6

7

2.0-2.2
2.2-2.4
2.4-2.6
2.6-2.8
2.8-3.0
3.0-3.2

4.730
3.985
3.820
3.650
3.570
3.320

119
110
115
119

125
130

130
129
127
128
127

It would appear from the description of the experiments that
only the heat given off by radiation and conduction was measured,
no specific statements being made as to ventilation or as to the loss
of heat as latent heat of water-vapor. The experiments were also of
short duration, ranging from sixty to ninety minutes.

The same author in later experiments || determined the respi-
ratory exchange*! of rabbits of different weights. Computing the

* Biologische Gesetze, pp. 17-18.

t Zeit. f. Biol., 21, 390.

% Biologi.sche Gesetze, p. 9.

§ Archives de Physiol , 1885, II, 237.

|| Ibid., 1890, pp. 17 and 483; 1891, p. 74; Comptes rend , 109, 190.

% By means of an apparatus described briefly in Comptes rend., 104, 435

INTERNAL WORK.

)63

results per square centimeter of surface by the use of Meeh's for-
mula (p. 364) he obtained the following figures, while similar
results are also reported on guinea-pigs, rats, and birds.

Number of
Experiments.

Average Live
Weight. Kgs.

Carbon Dioxide

per Square Cm.

of Surface,

Mgr*.

4

24.0

2.65

5

13.5

2.60

7

11.5

2.81

4

9.0

2.81

3

6.5

2.69

3

5.0

2.57

6

3.1

2.71

4

2.35

2.70

E. Voit * has recently published an extended compilation of
results bearing upon this point, including experiments on man, dogs,
rabbits, swine, geese, and hens, the heat production being in most
cases computed from the metabolism of carbon and nitrogen. The
results when computed per square meter of surface, while they
show not inconsiderable variations in some individual cases, never-
theless as a whole substantially confirm the conclusion that the
fasting metabolism is in general proportional to the surface. Still
more recently Oppenheimer f has shown that the law also holds
good for infants.

Causes of Variations. — In comparing experiments made upon
different animals by different observers at different times some
variation in the results would naturally be expected. The experi-
ments compiled by Voit were not all made at the same temperature,
but the range in most cases is relatively small and can hardly have
exerted any considerable influence. Differences between the differ-
ent animals as t<> their normal rate of emission of heat (thickness of
coat, quality of skin) may perhaps have also had an effect, although
probably a small one.

A more important source of error seems to lie, as Voit points
out, in the computation of the results to unit surface, what is
actually measured, of course, being the total heat production of
the animal. In solids which are of the same shape, that is, which

*Zeit. f. Biol., 41, IIP,.

ilbid., 42, 147.

364 PRINCIPLES OF ANIMAL NUTRITION.

are geometrically similar figures, the surface is proportional to
the two-thirds power of the volume. If we let S = surface and
V= volume, then S = kV*, in which k is a constant for any given
form. Putting W= weight, if the bodies have the same specific
gravity we may substitute W for V in the above equation, and we
then have

S = kW*, k = J-.

On the assumption that the bodies of animals of the same species
constitute similar figures and have the same specific gravity, the
value of k has been determined for several species, as follows (the
weight being expressed in grams and the surface in square centi-
meters) :

Man..". 12.9 Meeh (Zeit. f. Biol., 15, 425).

Dog 11.2 Rubner (Ibid., 19, 548).

Rabbit 12.9 Rubner (Ibid., 19, 553).

Horse 9.02 Hecker (Zeit, f. Veterinark., 1894).

Hen 10.45 Rubner (Zeit. f. Biol., 19, 553).

Guinea-pig .... 8.89 Rubner (Biol. Gesetze, p. 17).

Rat 9.13 ) „ , _. ,

V 4 P2 f Ru'jner (Zeit. f. Biol., 19, 553).

The heat production per unit of surface in most of the foregoing
experiments is computed by the use of these factors. The results
of such computations, however, are necessarily approximations
only. While animals of the same species are of the same general
shape, we can by no means regard them as being exactly similar
figures in the geometrical sense, nor can we safely assume them to
be of exactly the same specific gravity, since changes in the
amount of contents of stomach and intestines, and particularly in
the quantity of fat in the body, would cause greater or less variations.
Moreover, the state of fatness has, as Voit points out, still another
effect. As an animal grows fat, the increase in size is mainly
transverse and not longitudinal, the effect being like that of in-
creasing the diameter of a cylinder of fixed length.* In such a
case, however, the increase in the surface is not proportional to the
two-thirds power of the volume, nor to the square root of the vol-

* In the case of an animal, of course, we have the additional fact that the
deposit of fat is not of uniform thickness over the whole surface of the body.

INTERNAL WORK. 365

lime, as Voit states. The curved surface of the cylinder will be
proportional to the square root of its volume, while the surface of
the two ends will be proportional to the volume, and the ratio of
total surface to volume will depend upon the ratio of length to
diameter, being greater as the latter becomes less.

Obviously, the calculation of the surface of an animal from its
weight is a more or less uncertain one, and it is not surprising that
the results should be somewhat fluctuating. It seems very doubt-
ful, however, whether the larger differences found in Voit's com-
pilation can be explained in this way, and Voit shows that there
is another factor to be considered, viz., the mass of active cells in
the body, which lias a material bearing on the results. Before
proceeding to a discussion of this point, however, it is desirable to
consider briefly the significance of the general fact of the close
relation between heat production and surface.

Significance of Results. — Let us imagine an animal exposed to
its " critical thermal environment " (p. 358) to gradually shrink in
size while the external conditions remain the same. Under such
circumstances the loss of heat to its surroundings will tend to in-
crease relatively to its mass — that is, the body, like an inanimate
object, will tend to cool more rapidly. This tendency can be met
and the body temperature maintained in only two ways, viz., either
by some modification of its surface — e.g., thicker hair — which will
lower what we may call its emission constant, or by a relative in-
crease in its rate of heat production.

The results which we have been considering show that in
general the emission constant, i.e. the rate of heat emission per
unit of surface, is substantially the same in small and large animals,
and that the greater loss of heat in the former case is met by an
increased production. In this aspect the effect is simply an ex-
tension of the influence of falling temperature, the increased de-
mand for heat being met by an increased supply, so that the extent
of surface appears as the determining factor of the amount of met-
abolism.

In the case of an animal exposed to a temperature below the
critical point, howevor, the increased demand for heat appears to be
met largely by a stimulation of those processes of metabolism which
do not result in any visible form of work, while the internal work,

366

PRINCIPLES OF ANIMAL NUTRITION.

in the more restricted sense of the ordinary functions of the internal
organs, does not seem to be materially affected. Are we justified
in assuming the same thing to be true in our imagined shrinkage
of an animal? In other words, is the work of the internal organs
proportional to the mass of the body and is the increased heat
production in the smaller animal due to the same cause as that
observed when an animal is exposed to a falling temperature?

It appears quite clear that this question must be answered in
the negative. It is a well-known fact that the circulation, respira-
tion, and other functions are as a rule more active in small
than in large animals, and this greater activity must necessarily
result in the evolution of relatively more heat. If we raise the
temperature of the surroundings to a point corresponding to the
critical thermal environment, we may, as we have seen, regard the
heat production as representing the internal work in the narrower
sense. Rubner * reports experiments of this sort upon four guinea-
pigs at 0° C. and at 30° C, which gave the following results for the
production of carbon dioxide :

COo per Hour at 0° C.

COz per Hour at 30° C.

Animal,
Grms.

Per Kg.

Weight.
Grms.

Per Square

Meter Surface,

Grms.

Per Kg.

Weight,
Grms.

Per Square

Meter Surface,

Grms.

617
568
223
206

2.905
3.249
4 162
4.738

27.85
30.30
30.47
31.56

1.289
1 J 29
1.778
1.961

12.35
10.53
12.14
13.16

With the first and third of these animals direct experiment
showed that the minimum production of carbon dioxide (critical
point) was reached at about 30°-35°, and we may fairly assume
this to be true of the other two. At 30° C, then, we may assume
that the " chemical " regulation was practically eliminated and that
the observed metabolism was that due to the work of the internal
organs. Under these conditions, as the figures show, the metab-
olism was still approximately proportional to the surface of the
animal, and consequently greater per unit of weight in the smaller
than in the larger animals.

* Biologische Gesetze, pp. 12-18.

INTERN.1L WORK. 367

Strong confirmation of this conclusion is afforded by the exper-
iments previously cited. In many of them, notably in Rubner's,
the range of size is so great that to regard the differences in heat
production as arising from a direct stimulation of the metab-
olism, as in the case of a fall in the external temperature, leads
to improbable consequences. Thus a comparison of the largest
with the smallest dog in Rubner's experiments (p. 361) shows
that if we regard the heat production of the former as represent-
ing simply the work of the internal organs, over 56 per cent, of the
heat production of the smaller animal must, on the supposition
that the internal work is proportional to the mass of the body, have
arisen from some other source. Such an enormous increase in the
metabolism of the body simply for the sake of heat production
can hardly be regarded as probable. Still further, if we assume
(compare p. 354) a temperature of about 20° C. to represent the
critical point for the dog, then, on the hypothesis that the necessary
internal work per unit of weight is the same, we find that a fall of one
degree in temperature must have produced about six times the
effect upon the metabolism of the smallest dog that it did on that
of the largest one, wrhile if we take the other alternative and seek to
explain the results on the assumption of a higher critical tempera-
ture for the smallest dog, we find for the latter about 36^° C.

Taking these considerations along with the results of Rubner's
trial- with the four guinea-pigs, it seems most reasonable to assume,
in default of more extensive investigations directed to this specific
point, that the critical temperature is substantially the same for
large and small animals of the same species and that the work of
the internal organs is approximately proportional to the surface
of the animal.

Substantially the same conclusion has been reached by v. Hoss-
lin * from a quite different point of view. He points out that the
increased production of heat below the critical temperature is not
proportional to the difference in temperature between the body and
urroundings, as it should be, according to Newton's law, if the
emission constant of the surface remained the same. Taking as an
example Theodor's experiments (p. 351) he makes the following
comparisons :

* Arch. f. (Anat. u.) Physiol., 1888, p. 323.

368

PRINCIPLES OF ANIMAL NUTRITION.

External

Difference Between Body and
External Temperature.

Carbon Dioxide in 12 Hours.

Degrees.

Total,
Degrees.

Relative.

Total,
Grms.

Relative.

30.8

20.1

12.3

0.2

-5.5

7.2
17.9
25.7
37.8
43.5

1.0

2.5
3.6
5.25
6.0

12.03
14.34
17.76
18.24
19.83

1.00
1.19
1.48
1.52
1.65

It would appear from these figures that even below the critical
temperature the "physical" regulation plays a large part in the
regulation of the body temperature, being simply supplemented
by the "chemical" regulation, and that therefore the demand for
heat has not the determining influence upon the heat production
which Rubner supposes. According to v. Hosslin the apparent
dependence of the total metabolism upon the surface is only a par-
ticular case of a general morphological law and he points out :

First, that since, according to him, the velocity of the circula-
tion does not vary greatly in large and small animals, the average
amount of blood passing through the organs, and consequently
their supply of oxygen, will be proportional to the total cross-
section of the blood-vessels, which again, similar form being
assumed, will be proportional to the two-thirds power of the
volume (or weight) of the body.

Second, that the capacity of the alimentary canal to digest and
resorb food and thus to supply material for metabolism is limited
in the same proportion.

Third, that the work of locomotion — substantially the only
form of external work in the wild state — at a given speed is pro-
portional to the two-thirds power of the weight.

In short, v. Hosslin claims that all the important physiological
activities of the body, including, of course, its internal work and the
consequent heat production, are substantially proportional to the
two-thirds power of its volume, and that since the external surface
bears the same ratio to the volume, a proportionality necessarily
exists between heat production and surface. According to this
view, then, the heat production of the fasting animal at the criti-
cal temperature represents the internal work, which is proportional

INTERNAL WORK.

369

to the two-thirds power of the volume of the body, while below
this point there is superadded a stimulating effect upon the heat
production, which, since it acts through the surface, we may
assume to be proportional to the latter.

Comparison of Species. — In the foregoing discussion compari-
sons have been made between large and small animals of the same
species, with the result that both their internal work and their
total fasting metabolism appear to be closely proportional to their
surface. Going a step further and comparing the average results
of the several species with each other, E. Yoit * reaches the inter-
esting and striking result that the same relation of total fasting
metabolism to surface is substantially true as between different
species. The following table contains the averages, with the addi-
tion of the fasting metabolism of the horse as computed by Zuntz
& Hagemann, which Voit believes with good reason to be too
low :

Average Tem-
perature,
Deg. C.

Average
Weight, Kgs.

Fasting Metabolism.

Per Kg.,
Cals.

Per Square
Meter, Cals.

Horse

Swine

9.1 (?)
20.1
14.3
18.0
18.2
15.0
18.5

441

128

64.3

15.2

2.3

3.5

2.0

11.3
19.1
32.1
51.5
75.1
66.7
71.0

>948
1078

Man

1042

Dog

Rabbit

Goose

1039
776
967

Hen

943

"With the exception of the rabbit, the average heat production
of these various animals per unit of surface does not show any
greater variations than have been observed between different
animals of the same species, more or less of which, as we have seen,
can probably be accounted for by errors in the estimate of the
surface of the body.

Accepting the fact of the general proportionality of heat pro-
duction to surface, and passing over for the moment the excep-
tional case of the rabbit, it is plain that the considerations which
have been adduced in discussing the results upon the same

* Loc. cit., p. 120.

37° PRINCIPLES OF ANIMAL NUTRITION.

species will in the main apply to a comparison of different species.
It is true that what data we have indicate that there may be more
or less difference between the critical temperatures for different
species, but in view of the enormous range in the size of the animals
experimented on this cannot largely modify the results. Any
reasonable assumptions as to critical temperatures and as to rates
of variation per degree in heat production would still leave the
corrected results substantially proportional to the surface. Appar-
ently we must conclude that in all these different species, as well
as in larger and smaller animals of the same species, the internal
work, as measured by the total metabolism at the critical tem-
perature, is substantially proportional to the surface.

How generally this may be true we have at present no means
of judging. It is clear, however, that in the process of organic
evolution one of the very important factors has been the demand
for heat exerted by the environment upon the animal. This has
been met to some extent by modifications in the coat of the animal,
but to a very large degree by changes in the rate of heat produc-
tion, with the result that, other things being equal, those forms have
survived whose normal heat production, resulting from internal
work alone, was sufficient to maintain their temperature under the
average conditions surrounding them without, on the one hand,
calling largely into play the processes of " chemical " regulation,
nor, on the other hand, producing so much heat as to render it
difficult for the body to get rid of it.

Relation of Heat Production to Mass of Tissue. — As
already indicated, E. Voit, in his article cited above, has shown
that while the heat production is in general proportional to the sur-
face, there is also another determining factor, viz., the mass of the
active cells in the organism, a rough measure of which is the total
nitrogen of the body exclusive of that of the bones and the skin.
This conclusion is based chiefly on experiments with fasting animals.
As the weight of such an animal decreases, its relative surface must
increase, and, as was shown on p. 364, probably more rapidly than
in proportion to the two-thirds power of the weight. Under these
circumstances we should naturally expect that the relative heat
production would increase, but as a matter of fact it rather shows
a tendency to decrease. E. Voit, in discussing the results of Rubner

INTERNAL WORK.

371

and others, computes the heat production per unit of surface, and
also compares it with the amount of nitrogen computed to be
present in the organs (if the animal on the several days of the ex-
periment. The following results of one of Rubner's experiments
with rabbits are typical of those obtained in this way:

Average

Live
Weight ,
Grms.

Heat Production per Day.

Day of Fasting.

Total,
Cals.

Per Kg..
Cals.

Per

Square

Meter of

Surface,

Cals.

Per 100

Nitrogen,
Cals.

Third

Fifth

2185
2093

155
117
102
97
95
88
81
72

71.0

55.9
50.8
50.5
51.6
50.7
49.2
47.8

730
556
499
488
494
463
452
428

310

243

Seventh

2007
1923
1841
1735
1646
1507

220

Ninth

221

Tenth and twelfth .. .

227

Thirteenth and fourteenth ..
Fifteenth and sixteenth ....
Seventeenth and eighteenth

222
218
219

The heat production per unit of surface is seen to decrease at
first rapidly and later more slowly, while the heat production per
unit of weight shows but a slight decrease and that per unit of
nitrogen scarcely any. From these and other similar results, Voit
concludes that the law of the proportionality of heat production to
surface as enunciated by Rubner and as extended by himself must
be limited in its application to animals in like bodily condition,
and that an animal with a low stock of nitrogenous tissue will,
under the same conditions, show a lower heat production per unit
of surface than a well-nourished animal. The exceptionally low
average for the rabbit noted on p. 369 he explains on this hypoth-
esis as resulting from the frequent use for such experiments of
animals in a poorly nourished and "degenerate" condition re-
sulting from long confinement.

The result has an interesting bearing in another direction.
Most of the experiments cited by Voit were probably made at tem-
peratures below the critical points for the several animals. In
our previous discussion we have assumed that under these circum-
stances the heat regulation is accomplished largely by "chemical"
means — by variations in the rate of production. In these cxperi-

372

PRINCIPLES OP ANIMAL NUTRITION.

ments, on the contrary, since the heat production decreased along
with the decrease of nitrogenous tissue, we see the regulation of
body temperature effected by a diminution in the rate of emission
of heat, which, however, was in most cases less marked than in the
instance just cited. Either we must conclude that the abnormal
condition arising from fasting enables the animal to diminish the
rate of emission of heat to an extent not possible to the well-
nourished one, or we may suppose that in the latter case the stimu-
lation of the metabolism by the abstraction of heat begins before
the possibilities of "physical" regulation have been exhausted;
that, in other words, the domains of "chemical" and "physical"
regulation overlap. Obviously the latter conclusion is entirely in
harmony with v. Hosslin's views as stated on pp. 367-8.

§ 3. The Expenditure of Energy in Digestion and Assimilation.

General Conception.

Food Increases Metabolism. — That the consumption of food
increases the metabolism and consequent heat production in the
body has been known since the time of Lavoisier, who observed
the oxygen consumption of man to increase materially (about 37
per cent.) after a meal. Regnault & Reiset * also, among their
respiration experiments on animals, report the following results
for the oxygen consumption of two rabbits while fasting and after
eating :

Animal.

Fasting,
Grras.

After Eating,
Grms.

A

2.518
2.731

3.124
3.590

B

Subsequent investigations by Vierodt, Smith, Speck, Fredericq,
v. Mering & Zuntz, Wolfers, Potthast, Hanriot & Richet,f Magnus-
Levy, Zuntz & Hagemann, Laulanie, and others, some of which will
be considered more specifically in subsequent paragraphs, have fully
confirmed these earlier results, so that the fact of an increased met-
abolism consequent upon the ingestion of food is undisputed.

* Ann. de Chim. et de Phys. (3), 26, 414.

t Ibid. (6), 22, 520.

INTERNAL WORK. 373

Cause of the Increase. — Two possible explanations of the
above fact naturally suggest themselves, viz., that, on the one hand,
the more abundant supply of food material to the cells of the
body may act as a direct stimulus to the metabolic processes, or,
on the other hand, that the increased metabolism may arise from
the greater activity of the organs of digestion, or finally, that both
causes may act simultaneously.

The results obtained by Speck,* who found that the increase
began very promptly (within thirty minutes) after a meal, would
indicate that it can hardly be due to a stimulating action of the
resorbed food upon the general metabolism, but must arise, in
large part at least, from the activity of the digestive organs.
Specific investigations upon this point were undertaken by Zuntz &
v. Mering.f They found that glycerin, sugar, egg-albumin, puri-
fied peptones, and the sodium salts of lactic and butyric acids \
when injected into the circulation caused no material increase in
the amount of oxygen consumed as determined in successive short
periods by the Zuntz form of respiration apparatus. It is well estab-
lished that some of these substances do increase the metabolism
when given by the mouth, and the authors verified this fact for sugar
and for sodium lactate and likewise showed that substances like
sodium sulphate, which are not metabolized in the body, caused a
similar rise in the metabolism when introduced into the digestive
tract. They therefore conclude that the effect of the ingestion of
food upon the metabolism is due chiefly to the expenditure of energy
required in its digestion. Wolfers § and Potthast,|| in experiments sup-
plementary to those just mentioned, obtained confirmatory results.

On the other hand, Laulanie,!" in the experiments mentioned
on p. 180 in their bearings upon the formation of fat from carbo-
hydrates, obtained almost as marked an increase in the oxygen
consumption subsequent to the injection of sugar into the circula-
tion as after its administration by the mouth.

* Arch, exper. Pathol, and Pharm., II, 1S74, p. 405.
t Arch. pes. Physiol., 15, 634; 32, 173.

\ The results of their experiments upon organic acids have already been
cited in Chapter V, p. 157, in another connection.
§ Arch. ges. Physiol., 32, 222.
|| Ibid., 32, 280.
T[ Archives de Physiol., 1S9G, p. 791.

374 PRINCIPLES OF ANIMAL NUTRITION.

On the whole, however, and in view of the patent fact that the-
activity of the digestive apparatus consequent upon the consump-
tion of food must lead to an expenditure of energy, the results of
Zuntz & v. Mering appear to have been generally accepted as proof
that it is this influence rather than any direct effect of the resorbed
food upon the metabolism to which the increase of the latter after
a meal is to be ascribed. This increased expenditure is often,
although rather loosely, spoken of as the "work of digestion."

Factors of Work of Digestion. — In the process of digestion
we are probably safe in assuming that the muscular work of pre-
hension, mastication, deglutition, rumination, peristalsis, etc., con-
stitutes an important source of heat production. A secondary
source of heat production, which we may designate as glandular
metabolism, is the activity of the various secretory glands which
provide the digestive juices, to which may be added also the work
of the resorptive mechanisms. Furthermore, the various processes
of solution, hydration, cleavage, etc., which the nutrients undergo
during digestion contribute their share to the general thermic effect.

Fermentations. — To the above general sources of heat produc-
tion during the digestive process, there is to be added as a very
important one in the case of ruminating animals the extensive fer-
mentation which the carbohydrates of the food undergo. We have
already seen that a considerable fraction of the gross energy of these
bodies is carried off in the potential form in the combustible gases
produced. A further portion is liberated as heat of fermentation.
This latter portion forms a part of the metabolizable energy of the
food as defined in the preceding chapter, since it assumes the kinetic
form in the body. Since, however, it appears immediately as heat,
it can be of use to the body only indirectly, as an aid in maintaining
its temperature. While, therefore, it does not constitute work in
the strict sense of the term, the heat produced by fermentation
constitutes a part of the expenditure of metabolizable energy in
digestion, and therefore is included under the term " work of diges-
tion " in the general sense in which the term is frequently used.

Warming Ingesta. — The food, and particularly the water, con-
sumed by an animal have to be warmed to the temperature of the
body. To the extent that this warming of the ingesta is accom-
plished at the expense of the heat generated by the muscular, gland-

INTERNAL WORK. 375

ular, and fermentative actions indicated above, it does not call for
any additional expenditure of energy and so does not, from the
statistical point of view, constitute part of the "work of digestion."
If, however, at any time the warming of the ingesta requires more
heat than is produced by these processes — if, for example, a large
amount of very cold water is consumed — it is evident that the
surplus energy required will be withdrawn from the stock otherwise
available for other purposes, and to this extent will increase the
expenditure of energy consequent upon digestion.

The Expenditure of Energy in Assimilation. — While our
knowledge of the changes which the nutrients undergo after re-
sorption is very meager, we may regard it as highly probable that
they undergo important transformations before they are fitted to
serve directly as sources of energy for those general vital activities
of the body represented in gross by the fasting metabolism.

Thus the various cleavage products formed in the course of
digestive proteolysis are synthesized again to proteids, while the
proteids, when the supply is large, undergo, as was shown in Chap-
ter V, rapid nitrogen cleavage, leaving a non-nitrogenous residue
as a source of energy. According to some authorities, as we have
seen, the resorbed fat undergoes conversion into dextrose in the
liver before entering into the general metabolism of the body.
Even the carbohydrates, at least so far as they are not directly
resorbed as dextrose, seem to undergo more or less transformation
before entering into the general circulation.

In brief, there seems good reason to believe that the crude mate-
rials resulting from the digestion of the food undergo more or less
extensive chemical transformations before they are ready to serve
as what Chauveau calls the "potential" of the body — that is, as
the immediate source of energy for the vital functions. Of the
nature and extent of these transformations we are largely ignorant.
So far as they are katabolic in their nature, a liberation of energy is
necessarily involved. Any anabolic processes of course would
absorb energy, but the energy so absorbed must come ultimately
from the katabolism of other matter, and in all probability there
would be more or less escape of kinetic energy in the process.

Moreover, as was pointed out in the opening paragraphs of
Chapter II in discussing the general nature of metabolism, as well

376 PRINCIPLES OF ANIMAL NUTRITION.

as in the Introduction, the vital activities are intimately connected
with the katabolic processes going on in the protoplasm of the
cells. As was there stated, it is highly probable that the molecules
of the protoplasm are much more complex than those of the pro-
teids, fat and carbohydrates of the food (compare pp. 17 and 224).
To what extent it is necessary that the resorbed nutrients shall be
synthesized to these more complex compounds before they can
serve the purposes of the organism we are hardly in position to
say, but so far as it is required it can be accomplished only by an
expenditure of energy derived ultimately from the food and con-
stituting a part, and not impossibly a large part, of the work of
assimilation.

Summary. — The considerations of the foregoing paragraphs
make it plain that the. exercise of the function of nutrition, as is the
case with the other functions of the body, involves the expenditure
of energy. In general, we may say that this energy is expended for
the two purposes indicated in the title of this section, viz., for diges-
tion, or the transformation of the crude materials of the food and
their transference to the fluids of the body, and for assimilation, or
the conversion of these resorbed materials into the "potential" of
the organism. Each of these two general purposes is served by a va-
riety of processes, and the attempt to assign to each its exact share
in the increased metabolism brought about by the ingestion of food
is a physiological problem at once interesting and complicated.

For our present purpose, however, viz., a consideration from the
statistical point of view of the income and expenditure of energy
by the organism, we are concerned primarily with the total ex-
penditure caused by the ingestion of food rather than with the
single factors composing it. As a matter of convenience it may be
permissible to retain the designation above given, viz., the work of
digestion and assimilation, but it should not be forgotten that other
processes may conceivably be concerned in the matter. In par-
ticular, any increased heat production resulting from a direct stimu-
lation of the metabolic processes or of the incidental muscular
activity of the animal by the resorbed food, such for example, as
Zuntz & Hagemann * have observed with the horse as a result of
abundant feeding, particularly with Indian corn, would be included
under the term as here used.

* Landvv. Jahrb., 27, Supp. Ill, 234 and 259.

INTERNAL WORK. 377

Experirm ntal Results.

General Methods. — It follows from what has been said above
that two general methods, or more properly two modifications of
one general method, may be employed to determine the total ex-
penditure of energy due to the ingestion of food.

First, since the energy expended in the various processes out-
lined above is ultimately converted into heat, we may determine the
heat production of the animal while fasting and compare with it the
heat production during the digestion and assimilation of a known
amount of food. The excess of heat produced in the latter case as
compared with the former will represent the increased expendi-
ture of energy in the work of digestion and assimilation.

Second, we may determine the total income and outgo of energy
in the fasting and in the fed animal by one of the methods indicated
in Chapter VIII. In this case the extent to which the net loss of
energy by the body has been diminished by means of the food will
show how much of the mctabolizable energy of the latter has been
utilized by the organism in place of that previously drawn from the
metabolism of tissue. The part of the mctabolizable energy not
thus utilized has obviously been expended in 'some of the various
operations of digestion, assimilation, etc. The two methods are com-
plementary, in the one case the expenditure for digestion, assimila-
tion, etc, being determined directly and in the other by difference.

A point of some importance, at least logically, is that the deter-
minations should be made below the point of maintenance. The
term assimilation as above defined includes all those processes by
which the resorbed nutrients are prepared for their final metabo-
lism in the performance of the vital functions. When we give food
in excess of the maintenance requirement, however, there is added
to this the set of processes by which the excess food is converted
into suitable forms for more or less temporary storage in the
body. These may be presumed to consume energy, and as it would
seem, to a more or less variable extent. At any rate, we have
no right to assume in advance that the relative expenditure of en-
ergy above the maintenance point in the storage of excess material
is the same as that below the maintenance point for the processes
of assimilation as above defined. In other words, it is not necessa-
rily nor even, it would seem, probably the case that the resorbed

378 PRINCIPLES OF ANIMAL NUTRITION.

portion of a maintenance ration is first converted into the same
materials (particularly fat) that are deposited in the body when
excess food is given, and that these materials are then metabolized
in the performance of the bodily functions. It is at least conceiv-
able, if not likely, that a much less profound transformation, and
one involving a smaller loss of energy, suffices to prepare the re-
sorbed nutrients for their functions as " potential " than is required
for their storage as gain of tissue.

Finally, the comparison need not necessarily be made, and in-
deed in case of most agricultural animals cannot well be made, with
the fasting state. While this method is the simpler when practi-
cable, a comparison of the total heat production or of the balance
of energy on two different rations (both being less than the mainte-
nance requirement) will afford the data for a computation by differ-
ence (exactly similar to that employed in the determination of
metabolizable energy in Chapter X) of the expenditure of energy in
the digestion and assimilation of the food added to the basal ration.

The most important quantitative investigations upon the work
of digestion are those of Magnus-Levy * on the dog and on man,
and those of Zuntz & Hagemann f upon the horse.

Experiments on the Dog. — In Magnus-Levy's experiments
the respiratory exchange of the animal was determined by means
of the Zuntz apparatus at intervals of one or two hours during
fasting and after feeding. The single periods were twenty-five to
thirty minutes long, and the external conditions were maintained
as uniform as possible.

Fat. — Fat (in the form of bacon free from visible lean meat),
when given in quantities not materially exceeding in heat value
the fasting metabolism, resulted in a slight increase of the latter,
beginning about one to three hours after eating, reaching its maxi-
mum between the fifth and ninth hours, and disappearing about the
twelfth hour. The maximum increase observed was 12 per cent.,
seven hours after eating. In amounts largely exceeding the equiv-
alent of the fasting metabolism the effect of fat was somewhat
more marked and longer continued, a maximum increase of 19.5
per cent, being observed in one case seven hours after eating, while

* Arch. ges. Physiol., 55, 1.
f Landw. Jahrb., 27, Supp. III.

INTERNAL WORK.

379

the metabolism was still slightly above its fasting value after eight-
een hours. The respiratory quotient in every case sank to a value
closely corresponding to that for the oxidation of pure fat.

The experiments do not permit an exact estimate of the total
increase of the metabolism during the twenty-four hours, since
the observations were not always made at hourly intervals and
but few of the trials extended over a full day. By selecting,
however, the two in which the data are most complete and com-
puting as accurately as may be the average rate of consumption
of oxygen per minute, it is possible to obtain an approximate
expression for the total heat production. For this purpose the
average oxygen per minute is multiplied by 1440 and this product
by the calorific equivalent of the oxygen, viz.. 3.27 Cals. per gram
in this case, and the following results obtained, the heat production
during fasting being in each instance- that found in the particular
experiment under consideration :

Energy

(if Food,

Cals.

Heat Production in 24 Hours.

No. of /? *
Experiment. Grms'

1 asl ing,
Cals.

With
Food,
Cals;

Increase.

Cals.

Per Cent.
of Food.

100 131.6

64 and 68 305 5

1250
2902

972
1055

991
1142

19

87

1.53

2.99

Carbohydrates.— Carbohydrates produced a more marked
effect upon the metabolism than did fat, and one which showed
itself more promptly. In the experiments on the clog the food
consisted of rice, cither alone or with the addition of small amounts
of fat, sugar, or meat; in other words, the animal was on a mixed
diet in which carbohydrates predominate! 1.

< »n the average of a series of six experiments in which the food
(•(Hoisted of 500 grams of rice, 200 grams of meat, and 25 grams of
fat, the metabolism increased by fully 30 per cent, within the first
hour and continued to increase more slowly until the maximum of
39 per cent, was reached at the sixth to eighth hour. From thai
time it decreased to 25 percent, in the twelfth hour and then rather
suddenly dropped nearly to the fasting value. The respiratory
quotient rose from 0.78 during fasting to 0.90 in the first hour, and

38o

PRINCIPLES OF ANIMAL NUTRITION.

reached very nearly 1.00 by the third hour, remaining at substan-
tially this value for sixteen to eighteen hours and not falling to the
fasting value in twenty-four hours. Two parallel experiments in
which 400 grams of meat were fed showed that a part, but by no
means all, of the above increase was to be ascribed to the 200 grams
of meat. The small amount of fat given can hardly have affected
the result. The author estimates that of the total calculated in-
crease of 22 per cent, over the fasting metabolism about 5 per
cent, may have been due to the proteids of the food and the
remainder to the carbohydrates. This conclusion is confirmed by
the results of two experiments in which rice, sugar, and fat were
given. The increase in the metabolism was of precisely the same
character as in the other experiments, but less in amount.

In all these experiments the food was in excess of the fasting
metabolism. In another series in which the food, consisting of rice,
either alone or with a small amount of sugar, was about equivalent
to the fasting metabolism, the increase in the metabolism was
slightly less, although otherwise the results were similar to those
of the other trials.

Computing the results per twenty-four hours, as in the case of the
fat, we have the following approximate figures for the three series :

Food*
Grms.

Metab-

oliza-

ble
Energy

of

Food.t

Cals.

Heat Production in 24 Hours.

No. of
Experiment.

Fast-
ing,
Cals.

With
Food,

Cals.

Increase.

Cals.

Per
Cent, of
Food.

68, 70, (

71, 73, -

74, and 75 (

84 and 87 !
107 i

Proteids 71.3 )

Carbohydrates.. 375.0 [•
Fat 31.0 )

Proteids 28.1 )

Carbohydrates.. 457.5 r
Fat 25 . 0 )

Proteids 18.75)

Carbohydrates. . 225 . 00 f
Fat )

2121

2226

999

1040

1132

991

1271
1292
1080

231

160

89

10.89
7.19
8.91

* Rice estimated to contain 75 per cent, carbohydrates and 1 per cent,
nitrogen.

t Computed by the writer, vising Rubner's factors.

INTERNAL WORK.

38i

Proteids. — Proteids in the form of meat or a mixture of meat
ami flesh-meal, with in some eases small amounts of fat, caused a
very marked and prompt increase in the metabolism of the dog.
The maximum effect was usually reached about the third or fourth
hour and continued with but slight diminution up to the seventh
or eighth hour with small rations and as long as to the twelfth or
fiftt enth hour with large rations. As in the case of fat and carbo-
hydrates, the increase was greater with large rations, but its amount
largely exceeded that caused by either of the two former groups of
nutrients, reaching in some cases 90 or more per cent, of the fasting
value.

'The result- were more irregular than in the preceding experi-
ments, and were apparently influenced by a peculiar effect of the
food upon the type of respiration. The author, however,* com-
putes from three selected series of experiments the following
approximate averages for the twenty-four hours:

Proteids
Eaten,

Grm>.

'Heat Production in 24 Hours.
Metnho-

No. of

I xperiment.

lizable
Energy

of Food, Fasting,
Calfi- Cals.

With
Food,
Cals.

Increase.

Cals.

Per Cent,
of Food.

83 and SO 82.5

102 ■ 106 230 (l

95 ■• 96 37n 6

338 1030
943 963

1.520 1059

1086
1079
1303

56
116
244

16.57
12.30
16.05

The amount of the proteid metabolism was not determined in

experiments, but the author points out that they were made

on the first day of the feeding, and that it is probable that the

prot< id metabolism, and consequently the heat production, would

have increased more or less had the feeding, particularly with

3S of food, been continued longer.

Bone, when fed in large quantities to the dog, was found to
cause a greater increase in the metabolism than corresponded to the
nitrogenous matter estimated to have been resorbed from it, and
the difference is ascribed to the mechanical effect upon the digestive
tract.

* Loc. cit., p. 78.

382

PRINCIPLES OF ANIMAL NUTRITION.

Experiments on Man. — Magnus-Levy's experiments upon man
were made substantially like those upon the dog, the subject lying
upon a sofa, as completely at rest as possible, and breathing through
a mouth-piece.

Fat. — Two experiments with fat, computed in the same way as
those upon the dog, gave the following results :

Fat
Eaten,
Grms.

Energy

of Food,

Cals.

Heat Production in 24 Hours.

No. of Experiment.

Fasting,
Cals.

With
Food,
Cals.

Increase.

Cals.

Per Cent,
of Food.

81

94.0
195.6

893
1855

1537
1524

1547
1582

10

58

1.12

21

3.13

Carbohydrates. — Numerous experiments on a man were made
in which the diet consisted chiefly of bread, and a smaller number
in which the effect of sugar was studied. With bread the increase
in the metabolism was more prompt than in the experiments on the
dog, but smaller in amount, varying from about 12 to as high as 33
per cent., according to the amount eaten. By the end of the third
hour the effect had nearly disappeared, but it was then followed
by a second increase, less in amount but continuing longer, which
the author suggests may have been due to the commencement of
intestinal digestion. With sugar (both cane and grape) the increase
was equally prompt, although rather less in amount, but dis-
appeared entirely after two or three hours. None of the experi-
ments extended over more than ten hours and usually over less, and
the data given are insufficient for a satisfactory computation of the
total increase for the twenty-four hours. The respiratory quotient
was considerably raised, but did not reach 1.00 in any case.

Proteids. — Experiments upon the effect of proteids on the
respiratory exchange yielded results similar to those obtained with
the dog, but do not permit of a satisfactory computation of averages
for the twenty-four hours.

Mixed Diet. — Results with a mixed diet the ingredients of
which are not specified have been reported by Johansson, Lander-

INTERNAL WORK.

383

gren, Sonden & Tigerstedt.* The experiments were made in a
large Pettenkofer respiration apparatus and extended over twenty-
two hours, the results being computed to twenty-four hours. The
total heat production, as computed from the carbon and nitrogen
balance, and the computed metabolizable energy of the food were:

Energy of Food,
Cals.

Heat Production,
Cals.

First day

4141.4
4277.9

0

0

0

0

0
4355.9
3946.4

(?)
2705.3
• 222(1 4
2102.4
2024 . 1
1992.3
1970.8
2436.9
2410.1

Second "

Third "

Fourth "

Fifth "

Sixth "

Seventh "

Eighth "

Ninth "

The above figures furnish a striking example of the constancy of
the fastinu' metabolism, and of the marked increase brought about
by the consumption of food. Omitting the results for the first day
of fasting and for the first day of the experiment we obtain the
following averages:

Average energy of food 4193.4 Cals.

M( tabolism :

With food 2517.4 "

Fasting 2022.4 "

Increase.

Total 495.0 "

Per cent, of food 11 . 76 Per cent.

It is to be noted, however, that the food in this experiment was
considerably in excess of the fasting requirements, so that there
was a notable storage of material and energy in the body.

Summary. — The results of the foregoing approximate computa-
tions of the increased expenditure of energy for twenty-four hours
are summarized in the following table, which also includes a com-
parison of the metabolizable energy of the food with the fasmig
metabolism:f

* Skand. Arch. Physiol., 7, 29.

t Rubner (Gesetze des Energieverbrauchs bei <lcr Ernahrung, Leipsic and
Vienna, 1902) has subsequently obtained much higher figures.

3«4

PRINCIPLES OF ANIMAL NUTRITION.

Food.

Metabolizable

Energy of
Food, Cals.

Excess Above

Fasting

Metabolism,

Cals.

Digestive Work
in Per Cent.
of Metaboliz-
able Energy.

Fat:

Experiments on man \

" dog S

893
1855
1250
2902

-644
+ 331

+ 278
+ 1847

1.12
3.13
1.53
2.99

2.19

Chiefly Carbohydrates :

2121

2226

999

+ 1081
+ 1094

+ 8

30.89
7.19
8.91

8.99

Proteids :

Experiments on dog -J

338

943

1520

-692

-20

+ 461

16.57
12.30
16.05

.

14.97

Mixed Diet :

Experiments on man

4193

+ 2171

11.76

It is clear that proteids caused the greatest increase in the
metabolism and fat the least, while the carbohydrates occupied an
intermediate position. In the case of fat the increase in the heat
production seems to show a slight tendency to become greater
with amounts of food largely in excess of the fasting metabolism,
but with the carbohydrates and proteids no distinct effect of this
sort is apparent.

These results, particularly those on proteids, afford a good illus-
tration of the fact that the increase in the heat production caused
by the ingestion of food is not due solely to the increased muscular
work involved, since if we were to suppose the latter to be the case
it is not apparent why the proteids, which are digested pretty
promptly and with comparative ease, should cause seven times as
much work as the fats. The results certainly suggest strongly that
a large part of the heat production in the former case arises from
the considerable chemical cleavage which the proteids undergo in
digestion and still more from the stimulative effect of food proteids

INTERNAL WORK. 385

on the nitrogen cleavage; in other words, that what was called
on p. 375 the work of assimilation is an important factor.

Results on Fat. — The relatively small increase in the metabo-
lism resulting from the ingestion of fat is worthy of notice as bear-
ing upon the hypothesis, already several times referred to, that it
undergoes a cleavage, into dextrose, carbon dioxide, and water in
the liver, and that the resulting dextrose is the material which
serves as the source of potential energy for the general metabolism.
As was pointed out in Chapter V (p. 153), however, the dextrose
derived from one gram of fat according to the commonly accepted
equation would contain about 6.1 Cals. of potential energy out of
the 9.5 Cals. contained in the original fat. In other words, over
one third of the energy of the fat would be liberated as heat in the
intermediary metabolism supposed to take place in the liver.
While the heat production was not directly measured in Magnus-
Levy's experiments, and while the method of computation em-
ployed may be open to criticism in details, his results certainly fail
to indicate any such large increase hi the metabolism as this hypoth-
esis would require.

It should be noted, in conclusion, that the above experiments
did not include a determination of the work of mastication and in-
gestion of the food, and also that, according to the author, there
was little if any production of fat in the experiments in which carbo-
hydrates were fed.

Experiments on the Horse. — Zuntz, Lehmann & Hagemann*
have investigated the effect of digestive work and also of the masti-
cation of the food on the metabolism of the horse, the respiratory
exchange being determined by the Zuntz method and a correction
made for the cutaneous and intestinal respiration. In addition to
this, however, other data were secured which serve the authors as
the basis for computations of the energy metabolism of the animal
and of the available energy of the digested food. Since their most
important conclusions as to digestive work are based in large part
on the results of these computations it is necessary to consider
their method in some detail.

Method of Computation. — At six different times between the

* Landw. Jahrb., 27, Supp., Ill, pp. 271-285.

386 PRINCIPLES OF ANIMAL NUTRITION.

years 1888 and 1891 digestion experiments were made * in which
the total nitrogen metabolism f and the carbon of the food and of
the visible excreta were determined. The ration in every case
consisted of hay and a mixture of six parts of oats with one of cut
straw; the chemical composition of these feeds was quite similar
in the several experiments, the greatest variation being in the last
experiment (October 16-22, 1S91).

From the results of these experiments the metabolism of
energy in the respiration experiments is computed in the following
manner :

First, the results of the several digestion experiments are com-
1 lined in such a way as to give an average corresponding to the ration
during the respiration experiment. E.g., in Period 1 {he. tit., p. 256)
the ration consisted of 6 kgs. of oats, 1 kg. of straw, and 6 kgs. of
hay. As no single digestion experiment was made on just this
ration, the results of the first one are taken four times, those of the
second three times, and those of the third once, and the sums divided
by eight. These averages are taken as representing the digestibility
and the urinary carbon and nitrogen during the respiration experi-
ment.

Second, from the average carbon and nitrogen of the urine as
thus obtained its content of urea and hippuric acid is computed,
and from these data, on the assumption of average composition for
the metabolized proteids, the portion of the elements of the latter
completely oxidized in the body, from which again the amount of
oxygen required and of carbon dioxide produced is computed.

Third, from the computed amount of crude fiber digested,
assuming it to have the composition C6H10O5 and that 100 grams
yield 4.7 grams of methane, is computed the oxygen required for
its oxidation and the carbon dioxide resulting.

Fourth, after subtracting the amounts of oxygen and carbon
dioxide, as above computed, corresponding to the proteids and
crude fiber oxidized, from the totals found in the respiration experi-
ment, the remainders are divided between fat and carbohydrates

* Loc. cit., pp. 211-236.

t The nitrogen of the feces was determined in the air-dried material.
Subsequent experience has shown that there is some loss of nitrogen in air-
drying.

INTERNAL WORK.

337

in the manner described on page 76 on the assumption that the fat
has the composition C 76.54 per cent., II 12.01' per cent., 0 11.45
per cent., and the carbohydrates that of starch.

Fifth, <ni the basis of the chemical processes thus computed the
amount of energy set free is estimated from the known (average)
licai s of combustion of the materials oxidized.

While the calculation involves numerous assumptions, and
while, therefore, the result is of the nature of an approximation
most of the assumptions are so nearly correct as not to contain the
possibility of serious error. The two which seem most questionable
are the peculiar method of computing the digestibility of the food
and the proteid metabolism, and the computation of the proximate
composition of the urine.

Influence of Food Consumption on Metabolism. — The
influence of the ingestion of food in increasing the oxygen con-
sumption and the energy metabolism of the animal is illustrated by
the following tabulation of the results obtained in Period b(loc. cit.,
p. 282). (The animal was standing quietly, but otherwise was' in
a >tate of rest.)

In the Morning,
Fasting.

Immediately After Feeding.

Later Stage After
First Feeding.

No. of
Experiment.

Per Kg. Live

Weight per
Minute.

a> si

° a
■ ~s
/. 1

x ,°

5""*

3

X

Feed Eaten.

Per Kg. Live

Weight per

Minute.

Hi

0

e es

■££

to1*5

3
O

H

Per Kg. Live

Weight per

Minute.

a

h)

_-'

- 0

£:- ■

t S-g

2 0 u

Hi

Oats
and

Straw.
Grms.

Hay,
Grms.

X i °

og

£ S3 «»

w-s °

Hi

Og

O

% "'a
- ~ ;.

Hi

u

a
0

49

T2300
L2300

2100
2280
3180
3150

[2300

2330

i)
[0

1500"!
1500J

1000
1420
0
0
1000]
] 130
1650
2500]

3.602
3.613

18.365
18.798

19.159
19.220
19.304
16.134

18.318

20.450
19 . 333

3 5

50

2 0

51

.52

3.226
3.304
3.510
3 . 246
3.130
3.499
3.310

16.380
16.784
17.613
16.359
16.928
17.748
ir, 219*

10.5
10.5
1(1.5
11.0
10.5
11 .0
17.5

3.418
4.039
3.745
3.584

17.431
20.889
18.913
17.647

0.6

(IS

0.5
0.6

3.823
3.737
3.739
3.169
3.564

4.174
3.914

53

54

4.5

56

3 5

57

4 0

58

2 5

en. .

3. m;
3.242

17.516
17.474
16.272

11.0
11 .0

11.0

3.71 6
3.385

18.931
17.247

0.5
0.5

3 5

61

3.5

Averages . .

3 . 339

16.929

11.5

2173

917

3.648

18.510

0.6

3.704

18.787

3.5

* Animal was uneasy.

338

PRINCIPLES OF ANIMAL NUTRITION.

The average energy metabolism thirty-six minutes after eating,
computed as previously described, is somewhat more than 9 per cent,
greater than that shortly before eating, and a still further increase
was observed at the end of three hours. The effect is precisely
similar to that observed in Magnus-Levy's experiments. It was
not, however, followed through the twenty-four hours, as in some
of those experiments.

Comparison of Hay and Grain. — It was found further that
coarse fodder (hay) produced a much more marked effect than did
grain. The following comparison of the average of the experi-
ments of Period c on an exclusive hay diet with that of Period / on
a mixed ration illustrates this fact :

Peril id c.

Perioil /.

Time since last fed

2 . 6 hrs.
About 10.5 kgs.*

2 . 8 hrs.

Ration :

Hay

4.75 kgs.
6.00 "

Oats

Straw

1.00 "

Total digested nutrients (fatx

2.5)

Per kilogram and minute:

Oxygen consumed

Carbon dioxide given off

Energy set free (computed)

4125 grms. f

3.9837 c.c.

3.65S6 "
19.552 cals.

5697. grms.f

3.6986 c.c.

3.6695 "

18.339 cals

Notwithstanding the greater total weight of food consumed in
Period /, and the much larger amount of digestible matter contained
in it, the oxygen consumption and the computed amount of energy
liberated are notably greater in Period c, on the hay ration. The
average time which had elapsed since the last feeding, as well as the
external conditions, having been substantially the same in both
periods, X and the animal having been in a state of rest, the effect
is ascribed to an increase in the expenditure of energy in diges-
tion due to the difference in the physical properties of the two
rations. This difference is chemically characterized by the greater

* The exact amount of hay eaten is not stated. The digestible matter
is computed from the composition of the hay by the use of Wolff's coeffi-
cients.

| Computed in the manner described above, p. 386.

X It varied considerably in the individual experiments composing Period/.

INTERNAL WORK. 389

proportion of crude fiber in the hay ration. Ascribing the differ-
ence in digestive work entirely to the crude liber, the authors en-
deavor to estimate the expenditure of energy on this ingredient
as follows:

Digestive Work for Crude Fiber.— The hay ration con-
tained 1572 grams less of (estimated) digestible matter and 648
grams more of total crude fiber than the mixed ration. The com-
puted evolution of energy per head for the twenty-four hours was
greater by 772 Cals. in the hay period. On the basis of Magnus-
Levy's results the authors assume that the expenditure of energy
in the digestion of the nutrients exclusive of crude fiber equals 9
per cent, of the total energy of the digested matter. For 1572
grams (fat being reduced to its starch equivalent) this. amounts to
4.1X1572X0.09 = 580 Cals. Accordingly, the energy metabo-
lism should have been 580 Cals. less in Period. c than in Period /.
It was actually 772 Cals. greater, a difference of 1352 Cals. This
difference is ascribed to the presence of the 648 grams more of total
crude fiber, and corresponds to 2.086 Cals. per gram. With an
average digestibility of 55 per cent, this would equal 3.793 Cals.
per gram of digested crude fiber, an amount slightly exceeding its
metabolizable energy as computed on p. 331. In other words, it
would appear that all the metabolizable energy of the crude fiber
(or even more, should the digestibility fall below the percentage
aed) is consumed in the work of digestion and converted into
heat . leaving none available for external work, and this result seems
to coincide strikingly with the results obtained by Wolff* by an
entirely different method. (Compare Chapter XIII, §2.)

It is to be observed, however, that the basis of Zuntz <Sc Ilage-
mann's computation is the difference between the energy required
for the digestion of the 648 grams of crude fiber and that required
for the digestion of an equal amount of fiber-free nutrients. To
get at the total expenditure upon the digestion of the crude fiber
we should make the following computation:

The nutrients other than crude fiber digested were in Period /
5124 grams and in Period c 260S grams, a difference of 2516 grams.
The corresponding difference in the work of digestion would, on the

* Grundlagen, etc*, Neue IVitriige, 1887, p. 94

39° PRINCIPLES OF ANIMAL NUTRITION.

above assumptions, bo 4.1X2516X0.09 = 928 Cals. Adding this,
as before, to the observed difference of 772 Cals. gives a total of
1700 Cals. as the effect of the 648 grams of crude fiber, which equals
2.623 Cals. per gram. With a digestibility of 55 per cent., this
corresponds to 4.768 Cals. per gram of digested crude fiber, or
materially more than its metabolizable energy.

Uncertainties of the Computation. — The whole method of
computation, however, is open to serious criticism on at least two
points, aside from the rather indefinite statements as to the amount
of hay consumed in Period c and as to the distribution of the ration
between the three feedings in Period /.

First, the estimate for the work of digestion of the nutrients
other than crude fiber which forms the basis of the computation is
derived chiefly from the experiments of Magnus-Levy on dogs and
man. Those experiments were not only made with highly digesti-
ble food, but the digestive work is computed as a percentage of the
total (gross) energy of the food. The food of the horse contained
in the dry matter 40.94 per cent, of indigestible substances in
Period / and 54.37 per cent, in Period c, or if we leave out of account
the crude fiber the corresponding figures are 31.99 per cent, and
58.45 per cent. A considerable part of the work of digestion un-
doubtedly consists of muscular work, which must be performed
on the indigestible as well as the digestible matter of the food.
Moreover these indigestible matters, by their mechanical stimulus
and by acting in a certain sense as diluents, may perhaps cause a
more abundant secretion of the digestive juices. These facts are
entirely ignored when the figures for digestive work derived from
experiments on dogs and man are applied simply to the digested
food of the horse.

Second, the method of computation assumes that the difference
between the metabolism on the two rations which was observed 2.7
hours after eating would have retained the same absolute (not rela-
tive) value during the twenty-four hours. The justification for this
assumption is found in a comparison * of the results of a single res-
piration experiment, made one half hour after feeding, with the
average of two experiments in which the excretion of carbon

* hoc. cit., p. 218.

INTERNAL WORK. 39 x

dioxide was determined for twenty-four hours in a Pettenkofer
respiration apparatus. After allowing for the work of mastica-
tion in the latter experiment the results were found to agree
within 8.8 per cent. The authors, therefore, conclude that with
regular feeding the respiratory exchange during the forenoon
hours, when their experiments were made, corresponds substan-
tially to the average metabolism for the twenty-four hours, exclu-
sive of the work of mastication. It is to be remarked, however,
that this conclusion is not fully in harmony with the results
quoted on p. 3S7, which plainly show a marked decrease in the
metabolism during the night. Moreover, numerous other deter-
minations of the respiratory exchange at the same hours and on
similar food show quite wide variations. In view of this discrep-
ancy, as well as of the somewhat narrow basis of comparison, it
certainly appears questionable whether a computation of Periods
c and / for twenty-four hours can be safely made.

Zuntz & Hagemann's results unquestionably show that the
work of digestion is greater with coarse fodder than with grain.
That this difference is due, at least in large part, to the greater
amount of crude fiber in the former is extremely probable. In
view, however, of the two sources of uncertainty just pointed out,
as well as of the numerous minor assumptions involved in the calcu-
lations, we must conclude that the data available are insufficient
for an accurate quantitative estimate of the digestive work re-
quired by crude fiber.

Work of .Mastication . — The foregoing computations relate
to the expenditure of energy in the digestion of the food after it has
entered the stomach. The same authors have also determined the
increase in the gaseous exchange caused by mastication, degluti-
tion, etc. For this purpose they compare * the excretion of carbon
dioxide and the consumption of oxygen during the time actually
occupied in eating with the corresponding amounts during rest as
found from the average of a number of experiments made under
identical conditions. On the assumption that the proteid metabo-
lism is unaltered, the proportion of carbohydrates and fat metabo-
lized and the corresponding amounts of energy are computed by

* I. in-, r ,'/., p. 271 .

392

PRINCIPLES OF ANIMAL NUTRITION.

the method described on pp. 76 and 252. The following is a sum-
mary of the results computed per kilogram of feed :

Fodder.

No. of
Experi-
ments.

Oxygen

Consumed,

Liters.

COj

Excreted,

Liters.

Equivalent

Energy,

Cals.

Oats and cut straw (6 : 1) . . . .
Hay

8
8
8
2

7

12.964

33.840

20 . 072

7.133

6.171

10.679

27.813

17.677

6.205

4.980

64.17
167 44

Hay, oats, and cut straw ....
Maize and cut straw (6 : 1) . . .
Green alfalfa

100.79
35 . 72
30 42

Computed for oats alone

47 00

" " maize alone . . .

13.80

•

As was to have been expected, the work of mastication proves
to be much greater in the case of hay than in that of grain. Maize
gave a remarkably low result, while the lowest was obtained with
green fodder. Even when the results on the latter are computed
per kilogram of dry matter, they are still about 40 per cent, lower
than those on hay. A few experiments on old horses with defect-
ive teeth gave somewhat higher results for the mixture of oats
and cut straw.

The absolute amount of energy expended in mastication, etc., is
very considerable. On the average of three periods, on a ration
consisting of 5.6 kgs. of oats, 0.93 kgs. of cut straw, and 5.18 kgs.
of hay, it is computed at 1287.1 Cals., an amount equal to 11.2 per
cent, of the total metabolism during rest.

Conclusions. — The researches of Zuntz & Hagemann are of
great value in that they demonstrate the large proportion of the
energy of the food which is consumed in its prehension, mastication,
digestion, and assimilation in the case of herbivorous animals, and
that this proportion is largely influenced by the physical character
of the food. Thus the hard but brittle maize required much less
energy for its mastication than the softer but tougher and more
woody oats, and the dry matter of the green alfalfa decidedly less
than that of the hay. These results indicate quite clearly that no
accurate estimates of the work of mastication can (at least in the
present state of our knowledge) be based on the chemical compo-
sition of feeding-stuffs. As noted above, Zuntz & Hagemann
attempt to compute the work of digestion upon that basis. It

INTERNAL WORK. 393

certainly seems open to question, however, whether in this case also
other properties than those expressed by the percentage of crude
fiber may not materially affect the result,* and it will be wise, until
the subject receives further investigation, to accept their compu-
tations as tentative and approximate.!

* Compare Kellner's results on cattle, Chapter XIII, § 1.

f A somewhat extended critique by Pfeiffer of these researches, together
with replies by Zuntz & Hagemann, will be found in Landw. Vers. Stat., 54,
101; 55, 117; 56, 283 and 289.

CHAPTER XII.
NET AVAILABLE ENERGY— MAINTENANCE.

The organic matter contained in the body of an animal we have
learned to regard in the light of a certain capital of stored-up energy,
at the expense of which the vital activities of the organism are
carried on. The function of the food is to make good the losses
thus occasioned. The food is frequently spoken of as "the fuel of
the body." In a certain limited sense the comparison is admissible,
but it may easih' be pushed too far, and a closer analogy is that with
a stream of water supplying a reservoir and serving to replenish the
drafts made upon it for water.

The food in the form in which it is consumed, however, is by no
means ready to enter directly into the composition of the tissues of
the body and add to its store of potential energy, but on the con-
trary, as we have seen, a very considerable amount of energy must
be expended in the separation of the indigestible matters from the
digestible and in the conversion of the latter into such forms as are
suitable for the uses of the living cells of the body.

When, therefore, we give food to a quiescent fasting animal we
do two things: we supply it with metabolizable energy, depending
in amount upon the quantity and nature of the food, to take the
place of the energy expended in its internal work, but we at the
same time increase its expenditure of energy by the amount neces-
sary to separate the metabolizable from the non-metabolizable
energy of the food.

The case is analogous to that of a steam-boiler which is fired
by means of a mechanical stoker driven by steam from the same
boiler. Each pound of coal fed into the fire-box is capable of
evolving a certain amount of heat, representing its metabolizable
energy in the above sense, and that heat is capable of producing a

394

NET AVAILABLE ENERGY— MAINTENANCE. 395

certain quantity of steam. A definite fraction of the latter, how-
ever, is required to introduce the next pound of coal into the furnace
and therefore is not available for driving the main engine. To
recur to the illustration of the reservoir, it is as if the water, instead
of simply flowing into the reservoir, actuated a pump or a hydraulic
ram which lifted part of it to the required level.

Gross and Net Availability. — As stated in Chapter X, the
difference between the potential energy of the food and that of the
excreta represents the maximum amount of energy which is avail-
able to the organism for all purposes. This quantity has some-
times been designated as gross available energy, but has here been
called metabolizable energy.

A portion of this metabolizable energy, however, as just pointed
out, has to be expended in the various processes which have been
grouped together under the term work of digestion and assimilation.
This portion ultimately takes the form of heat, thus tending to
increase the heat production of the animal by a corresponding
amount, and becomes unavailable for other purposes in the body,
since, so far as we know, the organism has no power to convert heat
into other forms of energy. The remainder of the metabolizable
energy of the food represents the amount which that food con-
tributes directly towards the maintenance of the capital of potential
energy in the body. It is the measure of the net advantage derived
by the body from the introduction into it of the food.* From this
point of view the energy remaining after deducting the expenditure
caused by the ingestion of the food from its metabolizable or gross
available energy has been called the net available energy. There
are obvious objections to the use of the words available and avail-
ability in two senses, but no better term for net available has
yet txen suggested, while the use of available energy in the sense
of metabolizable energy has become quite general. It appears
necessary, therefore, to retain for the present the modifying words
gross and net to avoid ambiguity.

Distinction between Availability axd Utilization. — The
net available energy of the food in the above sense represents the

* As will appear later, this somewhat broad statement appears to be sub-
ject to modification in certain cases in which there is an indirect utilization
of the heat resulting from the work of digestion and assimilation.

396 PRINCIPLES OF ANIMAL NUTRITION.

net contribution which it makes to the demands of the vital func-
tions for energy or, in other words, its value as part of a mainte-
nance ration. This must be clearly distinguished from its value
for the storage of additional energy in the body — that is, its value
for productive purposes. In the latter case it is quite possible that
the conversion of the digested nutrients into suitable forms for
storage (fat of adipose tissue, ingredients of milk solids, proteids of
new growth, etc.) involves a greater expenditure of energy than is
required to convert them into forms fitted to serve as sources of
energy to the body cells (work of assimilation). The consideration
of this question belongs in the succeeding chapter, but meanwhile
it is important to bear in mind that the net available energy, in
the sense in which the term is here employed, is a distinct con-
ception from that of the utilization of energy in fattening, milk
production, etc., and has reference to the availability of the energy
of the food for maintenance.

It is evident from the above paragraphs that the value of a
feeding-stuff to the animal is not measured solely by its metaboliz-
able energy, since materials containing the same proportion of the
latter may require the expenditure of very unequal amounts of
energy for their digestion and assimilation and, therefore, may
contain very unequal amounts of net available energy. Plainly,
then, it is a matter of much importance to know the net avail-
ability of the metabolizable energy of the various nutrients and
feeding-stuffs, and thus to learn the proportions in which they may
replace each other.

§ 1. Replacement Values.

We have already seen (Chapter V, p. 148) that, aside from a
certain minimum of proteids, the several nutrients can mutually
replace each other to a very large if not to an unlimited extent,
either one or all serving, according to circumstances, to supply the
demand for energy.

In 1882 v. Hosslin * published an extended discussion of Petten-
kofer & Voit's respiration experiments from this point of view,
using such data regarding the potential energy of the nutrients as
were then available. He calls attention to the wide range of re-

* Virchow's Archiv, 89, 333.

NET AVAILABLE ENERGY— MAINTENANCE.

397

placement possible, quoting also Lawes & Gilbert's conclusions *
on the same point drawn from their experiments on fattening swine,
and asserts that the nutrients replace each other according to their
content of available energy. Danilewsky f also advanced similar
views, but Rubner % appears to have been the first to investigate
the subject experimentally.

Isodynamic Values. — We have already seen that the total
metabolism of a fasting animal is approximately constant, repre-
senting the rate at which the store of matter and energy in the body
is drawn upon to support the necessary internal work. If we deter-
mine the total metabolism of such an animal and then give it a
known quantity of some nutrient, as fat, e.g., the loss of tissue will
be diminished by a certain amount, which will represent the net
available energy of the nutrient and which may be compared with
the amount fed. Similarly, a second and third nutrient may be fed
and thus their relative values for the prevention of loss of tissue be
determined. For example, a dog after fasting for six days was
given on the seventh and eighth days 720 and 760 grams respect-
ively of fresh lean meat. The average nitrogen and fat metab-
olism for the fifth and sixth days (fasting) and the seventh and
eighth days was as follows : §

! Total Nitrogen
Food. Excretion
Grms.

Fat

Metabolized

Grms

Temperature
Deg C.

Nothing (fifth and sixth days) ....
Meat (seventh and eighth days) . .

3.16
20.63

75.92
30.72

18.0
19.2

Difference

+ 17.47

-45.20

+ 1.2

The result of the feeding with meat was, of course, a great in-
crease in the proteid metabolism. The increase of 17.47 grams in
the nitrogen excreted was equivalent to 113.38 grams of dry matter
of the meat. The metabolism of this amount of proteid matter,
therefore, enabled the organism to diminish the metabolism of fat

* Phil Trans . 160, 541

t Die Kraftvorrate dor Xahrungsstoffe ; Arch. ges. Physiol., 1885, p. 230.
% Zeit. f. Biol , 19, 313

§The original account of the experiments is contained in Zeit. f. Biol.,
19, 313; these figures are the corrected values given in ibid., 22, 45.

39«

PRINCIPLES OF ANIMAL NUTRITION.

by 45.20 grams. For the prevention of loss of tissue in this experi-
ment, then, 250 parts of the dry matter of the meat were apparently
equivalent to 100 parts of fat. The food, however, was given at the
temperature of the room. To warm it and the 100 c.c. of water
consumed to the temperature of the body would require an amount
of heat equal to that produced by the oxidation of 1.4 grams of fat.
Adding this to the 45.2 grams above gives 46.6 grams of fat as the
equivalent of 113.38 grains of dry matter of the meat, or a ratio of
100:243.

Another similar experiment gave as a final result a ratio of
100: 253, or after correction for the warming of the food 100: 243,
and a third longer experiment with extracted lean meat (syntonin)
yielded the ratio 100:227, or corrected as before 100:225.

If now, from the results of Rubner's determinations of the met-
abolizable energy of the proteids (p. 276), we compute the amount of
each which contains the same quantity of metabolizable energy as
100 grams of fat and compare it with the above ratios we have the
following as the amounts equivalent to 100 grams of fat:

Dry Matter of —

Computed
from Met-
abolizable
Energy,
Grms.

Found in

Experiments

on Animals,

Grms.

Lean meat:

First experiment

Second "

Extracted meat

235
235
213

• 243
243
225

The computed and observed equivalents differ by only 4.3 per
cent, and 5.6 per cent, respectively, and hence Rubner concludes
that protein replaces fat in metabolism substantially in inverse pro-
portion to its " physiological heat value," or, in other words,, to its
metabolizable energy.

Rubner has also made similar experiments with cane-sugar and
starch, comparing them in each case with the body fat, as in the
above experiments, and has also made trials in which grape-sugar
was substituted for the fat of the food. In computing the results
of these experiments any change in the proteid metabolism was
reduced to its equivalent in fat as computed from its metabolizable

NET AVAILABLE ENERGY-MAINTENANCE.

399

energy. The following table contains the final results, including
those on proteids just given :

EQUIVALENT TO 100 GRMS. OF FAT.

Dry Matter of —

Computed

from

Metabolizable

Energy,

Grms.

Found in

Experiments

on Animals,

Grms.

Lean meat

235
213

235

229

255

243

243

225

( 234

\ 235

( 234

232

( 258

\ 254

( 255

Extracted meat

Cane-sugar

Starch

Grape-sugar

The equivalents found by experiment correspond quite closely
with those computed from the metabolizable energy, and on these
facts Rubner bases the law of isodynamic replacement, which may
be briefly stated as follows: In amounts less than a maintenance
ration the nutrients replace each other in inverse proportion to their
metabolizable energy. The quantities which thus replace each other
are accordingly said to be isodynamic. It need scarcely be pointed
out that the minimum of proteids required for the maintenance of
the nitrogenous tissues is not included under this law.

Rubner is careful to limit this law to small amounts of food.
In his earlier publications he states that it holds only below the main-
tenance ration ; later * he asserts that it obtains up to an excess of
about 50 per cent, over the maintenance ration.

Isoglycosic Values. — Mention has already been made of the
theory of isoglycosic values maintained by Chauveau and his school,
according to which the net available energy of the digested nutrients
is measured by the amount of sugar they are considered to be capa-
ble of producing in the organism according to the equations given
in Chapter II. Chauveau f computes that the metabolism of 100
parts of proteids according to Gautier's scheme (p. 51), together
with the partial oxidation of the resulting fat (p. 38), would yield
* Biologische Gesetze, p. 20. . f Comptes rend., 126, 1073.

400 PRINCIPLES OF ANIMAL NUTRITION.

81.5 parts of dextrose. Laulanie * computes that 100 parts of

fat, carbohydrates, and albumin would produce the following

amounts of dextrose:

100 parts of fat produce 161 parts of dextrose

100 " " starch produce 110 " " "

100 " " sucrose produce 105 " " "

100 " " albumin produce 80 " " "

The corresponding isoglycosic values would be as follows,

Rubner's isodynamic values being added for comparison:

Isodynamic
Weights.

Isoglycosic
Weights.

Fat

100
229
235
255
235
213

100
146
153
161

201

Starch

Extracted meat

Albumin

It is evident that the chief point of difference is the relative
value of fat and carbohydrates.

Experiments on Maintenance. — As regards the relative values of
the several nutrients in a maintenance ration the above conclusions
are in part based on theoretical considerations and in part are de-
ductions from the experiments upon the influence of work on the
respiratory quotient and upon the nature of the non-nitrogenous
material metabolized which were considered in Chapter VI, pp.
211 to 225. Contejean,t however, has made direct experiments
upon the replacement values of fat and carbohydrates.

His experiments were made with dogs. In the first series the
animal, weighing about 20 kgs., received a basal ration of 500 grams
of meat (1000 in the first period), estimated to be ample to main-
tain nitrogen equilibrium. To this were added in the several
periods varying amounts of lard, sugar, and gelatin. The live
weight of the animal was taken daily at the same hour and under
uniform conditions, and the urinary nitrogen was determined.
No mention is made of the fecal nitrogen. The total heat produc-

* Energetique musculaire, p. 101.
t Archives de Physiol., 1896, p. 803.

NET AVAILABLE ENERGY— MAINTENANCE.

401

tion for the four days of each experiment (excluding preliminary
feeding) is computed from the proteid metabolism as measured
by the urinary nitrogen, on the assumption that all the fat con-
tained in the meat and all the non-nitrogenous nutrients added
were metabolized. An exception is made in the fourth period,
however, in which the author computes from a comparison of the
gains of nitrogen and of live weight that there was a gain of about
50 grams (?) of fat by the animal. The resuUs are contained in
the first six columns of the following table :

_o

'V.
0

Food.

Gain or

Loss of

Weight,

Grms.

Gain or

Nitrogen,
Grms.

Loss of

Equivalent

Flesh,
Grms.

Esti-
mated
Heat
Pro-
duction,
Cals.

Cor-
rected
Heat
Pro-
duction,
Cals.

I

1000 grms. meat

-395
-170

+ 50

+ 335 (?)

+ 152

-105

+ 19.39

- 1.86

+ 1.81
+ 6.36
+ 6.04

- 1.10

+ 570

- 55

+ 53

+ 187 (?)
+ 170

- 32

4548
3903

5326

5486

3811

4088

6190

500 " "

:}
:}
:}
:}
:}

II 1

III]

ivj

vj

VI j

40 " lard
500 " meat. . .

80 " lard
500 " meat...
100 " lard
500 " meat.. .
100 " sugar . .
500 " meat...
100 " gelatin .

4981
5354
4566
3980
4773

Making the comparison of fat and carbohydrates, as the essen-
tial point, it would appear from Contejean's results that 100 grams
of sugar was fully as efficient as 80 grams of fat,, while according
to Rubncr's figures about 180 grams of sugar would be required.
Corresponding to this is the lower computed heat production in the
sugar period, the excess in the fat periods being ascribed to the
cleavage of fat believed to occur in the liver.

If, however, there is justification for computing the gain of fat
by the body in the fourth period by subtracting the gain of flesh
from the total gain in weight, the same method is equally applicable
to the other periods. By its use the figures of the last column of
the table have been computed by the writer. While the heat
production in the sugar period as thus estimated is still below that
of the fat periods, the rather wide range in the results of the latter
serves to illustrate the uncertainties of such computation.

402

PRINCIPLES OF ANIMAL NUTRITION.

In a second series of experiments a ration of 150 grams of meat
and 100 grams of lard appeared to be equivalent to one of 300 grams
of meat and 50 of lard. In a third series, fat, sugar, and gelatin
were each given for two days to a fasting dog, the live weight * and
urinary nitrogen being determined daily. The results were as
follows :

Date.

Live Weight, Kgs.

Food.

Urinary

Nitrogen,

Grms.

Dec. 24

25 . 780

25 . 125

24 . 765

24. 780 +.095 feces

24. 016 +.064 "

24.215

23.920

23. 870 +.038 "

23 . 500

23.200

Nothing

u

200 grms. sugar

200 " "

Nothing

200 grms. fat

200 " "

Nothing

200 grms. gelatin

" 25

5.56

" 26

6.05

" 27

5.59

" 28

4.13

" 29.

4.59

" 30

6.56

" 31

6.85

Jan. 1

4.97

2

28.77

Neglecting the variations in the urinary nitrogen, Contejean
makes the following comparison of the daily loss of live weight,
from which he draws the conclusion that 200 grams of sugar,
equivalent to 792 Cals., was more efficient in maintaining the ani-
mal than 200 grams of fat, equivalent to 1876 Cals.

Gain or Loss of Live Weight per Day.

Average for fasting —377 grams

Sugar :

First day +110 grams

Second day -100 "

Average +5 "

Fat:

First day —295 grams

Second day - 12 "

Average — 154 "

* In taking the live weight any feces voided during the previous twenty-
four hours were added to the weight of the animal, so that the computed gain
or loss of weight does not include the feces.

NET AVAILABLE ENERGY— MAINTENANCE. 4°3

Experiments in which Work was Done. — Somewhat earlier in
point of time than the above experiments by Contejean were similar
ones by Chauveau * in which the animal performed a uniform
(unmeasured) amount of work per day. No attempt was made to
determine the equivalence between food or body metabolism and
the work performed, but the latter was simply used as a means of
increasing the metabolism, while the relative value of the several
nutrients in maintaining the store of energy in the body was esti-
mated from the effect upon the live weight. The experiments,
therefore, are not, properly speaking, work experiments, but belong-
in the same category as those of Contejean — that is, they aim to
show in what proportions the nutrients may replace each other in
a maintenance ration.

In the first series the basal ration consisted of 400 grams of
lean meat, to which was added in alternate six- or five-day periods
either 51 grams of lard or an isodynamic quantity (121 grams) of
cane-sugar. In one period 128.5 grams of dextrose was used in-
stead of the cane-sugar. The animal (bitch) averaged about 16.8
kgs. in weight. The gain or loss of weight in each period (differ-
ence between first and last weighings) was as follows :

Period 1 Lard 0 grams

" 2. . Cane-sugar +170 "

" 3 Lard - 10 "

" 4 Cane-sugar +290 "

" 5 Lard -265 "

" 6 Dextrose 0 "

" 7 Lard -295 "

In the first four periods the cane-sugar seems to have caused a
gain in weight as compared with practical maintenance on the lard.
During the last three periods the animal was in heat and a loss of
weight upon the lard ration resulted, which was arrested on the
dextrose ration. Water was given ad libitum for several hours after
the work, but withdrawn at least twelve hours before weighing.
No record is given of the amount of it consumed or of the water
content of the materials fed.

In another experiment, in which twice as much work was done,

* Comptes rend., 125, 1070; 126, 795, 930, 1072.

4°4 PRINCIPLES OF ANIMAL NUTRITION.

fat and cane-sugar replaced each other in isoglycosic proportions,
viz., 110 grams of fat and 168 of cane-sugar. In this case the
amount of water consumed was uniform, viz., 400 grams. The gain
or loss of live weight in five-day periods was :

Period 1 Sugar + 35 grams •

" 2 Fat -160 "

" 2 " —Omitting first day . . - 20 "

A third experiment, in which amounts of sugar intermediate
between the isoglycosic and isodynamic equivalents of the fat
were fed, showed a gain on the former as compared with practi-
cally no change on the fat.

In a second series of experiments isoglycosic amounts of lard
(110 grams) and cane-sugar (168 grams) were alternated every five
or three days for eighty-five days, the basal ration consisting of 500
grams of lean meat, and 400 grams of water being consumed per
day. The estimated heat values of these rations were respectively
1513 Cals. and 1145 Cals., but notwithstanding this difference they
appeared to be equally efficient in maintaining the live weight.

Whatever weight may attach to the deductions from the exper-
iments upon work production, it is hardly necessary to urge that
such a method of investigation as that employed in the above
trials, while it may afford useful indications, is altogether too
crude to disprove the theory of isodynamic values based upon
Rubner's more elaborate experiments.

Respiration Experiments. — Kaufmann * has also reported respi-
ration experiments in support of the views regarding the interme-
diary metabolism promulgated by Chauveau. In his experiments
the nitrogen excretion, respiratory exchange, and heat production
of dogs variously fed were determined, in five-hour periods, by
means of a radiation calorimeter in which the products of respira-
tion were allowed to accumulate. (See pp. 69 and 248.) From the
theoretical equations given in Chapter II he computes the figures
given on the opposite page for the consumption of oxygen, produc-
tion of carbon dioxide, and heat evolution in the various reactions.

Besides determinations of the fasting metabolism the experi-
ments included feeding exclusively with meat and also with rations
rich in carbohydrates and in fat. For each diet, on the basis of the
* Archives de Physiol., 189G, pp. 329, 342, and 757.

NET AVAILABLE ENERGY— MAINTENANCE.

4°5

Per Grm. of Substance.

Oxygen

Con-
sumed,
loiters.

Carbon

1 (ioxide

Produced,

Liters. 1

Heat
Evolved,

Cals.

Heat
per Liter
of ( >xvgen

Con-
sumed,

Cals.

Albumin to fat and urea

" dextrose and urea

" CO,, H,0 " "

Stearin " " " " dextrose . .

" " andHX>

Dextrose " " " "

0.4S1
0.713
1.045
0.840
2.043
0.744

0.4777
0.5480

0.8720
0.2257
1.4290
0.7440

234
180

857
417
500
762

646
460
647
067
650
056

determination of the respiratory products, the author assumes a
scheme of metabolism in accordance with the theory, and finds that
the heat production as computed on this assumption agrees quite
closely with that actually determined.

Aside from questions of method, particularly whether a five-
hour period is sufficiently long, it is to be remarked that the results
of Kaufmann's experiments are ambiguous. They show that it is
possible to interpret the facts in accordance with his theory, but
they do not exclude the possibility of other explanations. For this
reason it seems unnecessary to cite the experiments in detail, and
for the same reason they are at best but confirmatory evidence in
favor of the theory of isoglycosic values.

§ 2. Modified Conception of Replacement Values.

The theory of isodynamic replacement as announced by Rubner
constituted the first systematic application of the general laws of
energy to the problems of animal nutrition. As such it has exerted
a profound influence upon subsequent study of the subject in that
it has been chiefly instrumental in leading to a practical application
of the long-known fact that the food is primarily a supply of energy.
It was based, of course, upon the conception that the law of the
c< >nservation of energy obtains in the animal body, and in subsequent
experiments, which have been described in Chapter IX, Rubner
gave at least a partial demonstration of the truth of this concep-
tion.

Rubner's general ideas still form the basis of our views regard-

4°6 PRINCIPLES OF ANIMAL NUTRITION.

ing the metabolism of energy in the body, but, as was natural, his
first conclusions have undergone more or less modification, in part
at his own hands.

Digestive Work. — The law of isodynamic replacement as
stated above is equivalent to saying either that all the metabolizable
energy of the food below a maintenance ration is net available
energy or that the percentage availability of all the nutrients
experimented with is the same. The latter supposition, however,
appears to be negatived by the results of Magnus-Levy and others
on digestive work.

If, however, a fraction of the metabolizable energy of the food
is applied to the work of digestion and assimilation, it is plain that
this fraction cannot serve directly for tissue building. In his first
paper, Rubner, while not denying the fact of the consumption of
energy in digestive work, appears to regard its amount as insignifi-
cant, although what he specifically claims is that the total metabo-
lism below the maintenance ration is not increased by the inges-
tion of food. In support of this view he gives the results of three
experiments in which fat was fed; that is, the nutrient which, ac-
cording to Magnus-Levy's later results, causes the least digestive
work. Of these, one on a dog, in which approximately a mainte-
nance ration was given, showed no increase of the metabolism over
the fasting state. In the other two experiments, one on a dog and
one on a rabbit, more fat was consumed than corresponded to the
fas! ing metabolism, and an increase of the latter was observed
amounting to approximately 3 per cent, and 12 per cent, respec-
tively. Feeding with bone also caused an increase of about 12
per cent.

In later pul 1; rations,* however, he recognizes the apparent
inconsistency between the effects of small and large amounts of
food, and propounds a hypothesis to explain it which, in its general
features at least, seems in harmony with the observed facts. This
hypothesis is outlined in the following paragraphs, although in a
slightly different manner than by Rubner.

Indirect Utilization of Heat Resulting from Digestive
Work. — In Chapter XI we acquired the conception of the critical
thermal environment. According to the ideas there advanced,

* Biologische Gesetze, Marburg, 1887, p. 20; Gesetze des Energiever-
brauchs bei der Ernahrung, Leipsic and Vienna, 1902.

NET AVAILABLE ENERGY— MAINTENANCE. 4° 7

the heat production of a quiescent, fasting animal below the critical
point is made up of —

1. The heat produced by the internal work.

2. The heat produced by the processes of "chemical" regula-
tion.

The first of these we may regard as substantially constant, while
the latter varies to meet varying conditions and thus maintain the
constancy of body temperature. When we give food to such an
animal we introduce a third source of heat, viz., the work of diges-
tion and assimilation. Other conditions remaining the same, the
tendency would be to raise the temperature of the body, and this
tendency can be overcome either by means of " chemical " or " physi-
cal" regulation. Recurring to the illlustration of the room on
p. 356, it is as if a second fire were kindled in it. To maintain con-
stant temperature, either the first fire must be lowered or the win-
dows must be opened.

The fact, however, that below the critical point the heat regula-
tion of the body appears to be largely " chemical " renders it prob-
able that the regulation is effected by the former method; that is,
that the heat produced by the work of digestion is utilized to warm
the body and that correspondingly less energy is withdrawn from
that stored in the tissues of the body.* Under these circum-
stances the total heat production of the animal would not be in-
creased by the ingestion of food, and all the metabolizable energy
of the food would be apparently available ; that is, we should have
the phenomenon of isodynamic replacement.

Digestive Work Above Critical Point. — The statements cf
the last paragraph refer to conditions below the critical point.
Above this point no such indirect utilization of the heat resulting
from digestive work is possible, since the heat production has
already been reduced to the minimum due, as was concluded on
p. 356, to internal work. The excess of heat arising from the work
of digestion is then disposed of by " physical " means.

Thus Ilubner t obtained the following results for the carbon

* Loewy (Arch. ges. Physiol., 46, 189; quoted by Magnus-Levy, ibid., p.
116) claims to have shown that such a substitution or compensation does
not take place in man.

t Biologische Gesetze, pp. 17-25.

408

PRINCIPLES OF ANIMAL NUTRITION.

dioxide produced per square meter by guinea-pigs at 0° C. and at
30° C. (critical temperature), when fasting and after the consump-
tion of food ad libitum.

PER SQUARE METER OF SURFACE.

Fasting.*

Fed.

Live Weight,
Grms.

At 0° C.
COa, Grms.

At 30° C.
CO.,, Grms.

Live Weight,
Grms.

At 0° C.
CO,, Grms.

At 30° C.
COa, Grms.

617
568
223
206

27.85
30.30
30.47
31.56

12.35
10.53
12.14
13.16

670
520
240
220

Average . . .

29.49
29.08
34.07
30.59

14.10
16.19
17.69
18.94

Average.. .

30.05

12.05

30.81

16.73

* Already cited on p. 366.

Comparing the averages we see that at 0° C, considerably below
the critical point, the consumption of food did not materially in-
crease the total metabolism per unit of surface. On the other hand,
at a temperature close to the critical point the average heat pro-
duction was increased nearly 39 per cent, by the consumption of
food.

It appears also that at this higher temperature the heat produc-
tion of the fed animals was no longer proportional to their surface,
but was relatively greater in the smaller animals. Rubner explains
this by the supposition that (the animals being fed ad libitum) the
consumption of food by the animals was in proportion to their fast-
ing metabolism ; that is, to their surface. Under these circumstances
the factor of surface enters twice, and the heat production is approx-
imately proportional to the square of the surface.

Rubner* has also made calorimetric determinations of the heat
production of a dog at different temperatures with the results
shown on the opposite page. Not only did the feeding increase
the heat production, but it eliminated the effect of rising tempera-
ture in diminishing it; that is, it lowered the critical temperature.

Critical Amount of Food. — The very probable hypothesis of
a substitution of the heat produced by the work of digestion for that

* Sitzungsber. der k. bayer. Akad. d. Wiss., Math.-phys. Classe, 15, 452.

NET AVAILABLE ENERGY-MAINTENANCE.

409

Fasting.

Fed Small Amount of Meat.

Temperature,
Deg. C.

Heat

Production,

Cals.

Temperature,
Deg. C.

Heat

Production,

Cals.

13.2
19.5

27.4

39.65
35.10
30.82

19.5
18.2
23.7

24.8

42.64
41.13
41.83
41.10

arising, below the critical point, from the "chemical"' regulation of
the body temperature affords a very reasonable explanation of the
apparent discrepancy between the law of isodynamic replacement
as propounded by Rubner and the no less certain fact that the work
of digestion and assimilation makes a demand on the body for
energy, which energy finally takes the form of heat and is not
available for other purposes.

A consequence of this hypothesis, however, which is sufficiently
obvious has indeed been pointed out, but hardly seems to have re-
ceived the attention which it deserves in view of its important
bearing on the theoretical aspects of metabolism.

If we give increasing amounts of food to a fasting animal we
progressively increase the evolution of heat due to digestive work,
and this heat, according to the hypothesis, if the thermal environ-
ment is below the critical point, is substituted for the heat pre-
viously produced by the metabolism of tissue. There must be a
limit to the possibility of this substitution, however, just as there
must be to the " chemical " regulation of body temperature (p. 353),
since otherwise there would be a ration on which all the heat of the
body was derived from the work of digestion and the internal work
was performed without evolution of heat. The limit is indeed the
same in both cases and is reached when all the heat previously
evolved by the processes of "chemical" regulation has been re-
placed by the heat arising from digestive work . Beyond that
point the conditions are the same as in the fasting animal above
the critical point, and the excess of heat is gotten rid of by
" physical " regulation. We may call the amount of food whose in-
gestion produces the quantity of heat necessary to just reach this
limit the critical amount of food. Below that amount the apparent

4i°

PRINCIPLES OF ANIMAL NUTRITION.

availability of the metabolizable energy of the food will be 100 per
cent, or we shall have isodynamic replacement. Above that
amount we shall have an availability depending upon the relation
of the work of digestion and assimilation to the total metabolizable
energy.

Graphic Representation. — The critical amount of food will
depend chiefly upon two things, viz., the distance below the critical
thermal environment at which the experiment is made and the
amount of energy that has to be expended in the digestion and
assimilation of the food. The greater the former quantity, the
more of the total metabolism of the animal will be due to the " chemi-
cal " regulation and therefore capable of being replaced, while the
greater the work of digestion the less food must be consumed to
furnish by its digestive work the heat necessary to a complete
substitution.

On the two coordinate axes OX and OY let distances along OX
represent the metabolizable energy of the food consumed and dis-
tances along OY the effect of this food upon the store of potential
energy in the body. In the first instance, let us take the case of a

NET AVAILABLE ENERGY— MAINTENANCE. 41 1

fasting animal and suppose the thermal environment to be at the
critical point. The distance OA may then represent the loss of
potential energy (tissue) from the body caused by the internal work.
If now we supply the animal with food 80 per cent, of whose met-
abolizable energy is available, with any given amount of energy
thus supplied, as OB = AC, SO per cent, of that energy, represented
by CD. will serve to maintain the store of potential energy in the
body, while 20 per cent., or DB' , will be absorbed by the work of
digestion, etc.. and converted into heat. Accordingly if we assume
that the work of digestion is proportional to the amount of food
eaten, the line AD will indicate the availability of the particular
food and may be represented algebraically by the equation

y = ax,

in which a=tan D AC = the percentage availability.

We may also represent the heat production on the same axes.
With no food it will be CE equal to OA. With an amount of food
equal to OB it will be equal to OE + DB' = BF, and the line EF,
expressed algebraically by

y=(l-a)x,

will represent the lawT of heat production.

Let us next suppose that, the animal being again deprived of food,
the external demand for heat is increased, by a fall of temperature,
e.g., and that to meet this demand the metabolism is increased by
an amount AG, and the heat production consequently by the equal
amount EH. If we now give the same food as before, its real availa-
bility will be unchanged and will be represented by the line GJ,
parallel to AD. Up to the critical amount of food, however, the
beat resulting from the digestive work will, as we believe, be sub-
stituted progressively for that represented by EH and resulting
from the metabolism AG. The apparent availability, therefore,
will be represented by the line GK, making an angle of 45° wdth the
axes, and the heat production by the line HL, parallel to OX.
When the food consumed reaches an amount OM at which the line
GK intersects AD, the limit of this substitution is reached, since
the amount of digestive work, KN, equals the amount of additional
metabolism AO caused by the fall in temperature. In other words,

412 PRINCIPLES OF ANIMAL NUTRITION.

OM is the critical amount of food. Beyond this amount the energy
expended in the work of digestion will become waste energy, serving
simply to increase the outflow of heat, and the apparent and real
availability of the food will coincide.

Plainly, the critical amount of food will vary with circumstances.
If the experiment is made at or above the critical thermal environ-
ment for the fasting animal the smallest quantity must cause an
increase in the heat production and the critical amount will be 0
(or, mathematically, a negative quantity). As the external con-
ditions fall below the critical thermal environment, the point K will
be further and further removed from A until finally the point of
intersection might even lie above OX, that is, above the mainte-
nance ration. The relative availability of the food, too, will be a
factor in determining the critical amount. Thus if the true availa-
bility of the food were expressed by the line AP instead of AD, the
point of intersection would lie at R and OR' would be the critical
amount of food.

§ 3. Net Availability.

The modified conception of replacement values discussed in
the preceding section and in the introductory paragraphs of this
chapter renders it evident that both the theory of isodynamic re-
placement, as first announced and later modified by Rubner, and
the rival theory of isoglycosic replacement are but aspects of the
more general question of the availability of the metabolizable
energy of the food. That the several nutrients are of use to the
body and can replace each other in the food in inverse ratio to
their available energy is simply a necessary consequence of the law
of the conservation of energy. The important question is how
much of their energy is really available. Rubner's theory regards
all the metabolizable energy of the food as virtually available,
directly or indirectly, for maintenance, and this view has been quite
generally accepted. Chauveau's theory of isoglycosic replacement
has the merit of distinctly recognizing the fact of a possible expen-
diture of energy in the assimilation of the digested food, but, on the
other hand, it takes no account of the digestive work, and moreover,
so far as maintenance values are concerned, rests, as we have seen,
upon a rather insecure foundation. Plainly, the real question at

NET AVAILABLE ENERGY-MAINTENANCE. 4*3

issue can only be settled by experiments in which the actual availa-
bility of the energy of the food or of its various ingredients is deter-
mined.

Determinations of Net Availability.

Since the net available energy of the food is equal to its metabo-
lizable energy minus the energy expended in digestion and assimila-
tion, the two general methods for the determination of the latter
quantity which were outlined in the preceding chapter (p. 377) are
also, from the converse point of view, methods for the determination
of net availability. In our study of digestive work we considered
chiefly the results of direct determinations of the increase in the
heat production due to the ingestion of food; for our present pur-
pose the results of any accurate determinations of the metabolism
upon varying known amounts of the same food may be used.

The experimental evidence available is far from being as full as
could be wished, but in the following paragraphs the attempt has
been made to summarize such data as are accessible. In consider-
ing these results it should be remembered that, as explained on
p. 396. the net available energy means the energy available for
maintenance. In a considerable number of the experiments to be
considered, more or less gain was made by the animals, but it seems
better to give the results of each series of experiments in full, re-
serving a discussion of the results with productive rations for a
subsequent chapter.

Experiments on Carnivora. — The most extensive data regarding
the metabolism of the carnivora in its relations to the food supply
are those afforded by the investigations of Pettenkofer & Voit and
of Rubner. These have already been considered in Chapter A'
from the standpoint of matter and chiefly in a qualitative way;
we have now to study them quantitatively in their bearing upon
the income and expenditure of energy by the body.

In Pettenkofer & Voit's experiments, and in the earlier ones by
Rubner, the quantities of energy involved must be computed from
the chemical data. In Rubner's experiments upon the source of
animal heat, cited in Chapter IX, the actual heat production of the
animals was determined, but in no case was there a direct determi-
nation of the total income and expenditure of energy, and in par-
ticular the data as to the energy of the food are incomplete. For

414

PRINCIPLES OF ANIMAL NUTRITION.

the study of replacement values by Rubner's method the latter
factor was not necessary, but for a determination of the percentage
availability of the energy of the food it is indispensable. In the
following paragraphs the necessary computations of energy have
been made by the writer, using Rubner's factors as far as possible.*

In the case of Pettenkofer & Voit's experiments the average
results given in Chapter V have been made the basis of the compu-
tation.

Proteids. — From the average results obtained by Pettenkofer
& Voit f with different amounts of lean meat (see p. 104), the met-
abolizable energy of the food and of the resulting gain (or loss) by
the body may be computed as follows :

Metabolizable

Energy of

Food,

Cals.

Computed Heat Production.

Food,
Grms.

From F Fat
Igg*. Cals.

Total,

Cals.

Body,
Cals.

0

500

1000

1500

0

442

883

1325

146

530

954

1325

S95
443
179

-38

1041

973

1133

1287

-1041
-531
-250

+ 38

* The following factors were used in computing these experiments:
Metabolizable Energy of Food :

Bacon (Speck), 92.2 per cent, fat (Zeit. f. Biol., 30, 138).

1 grm. pork fat, 9.423 Cals. (ibid., 21, 333).

1 grm. butter fat, 9.216 Cals. (U. S. Dept. Agr., Office of Expt. Stations
Bull. 21, p. 127).

1 grm. cane-sugar, 4.001 Cals. (Zeit. f. Biol., 21, 266).

1 grm. grape-sugar, 3.692 Cals. (Stohmann, Zeit. f. Biol., 22, 40).

1 grm. starch, 4.123 Cals. (Stohmann, ibid., 19, 376).

Fresh lean meat, 3.4 per cent, nitrogen.

1 grm. nitrogen in meat, 25.98 Cals. (Zeit. f. Biol., 21, 321).

1 grm. nitrogen in syntonin, 26.66 Cals. (ibid., 21, 309).
Energy of Metabolism :

1 grm. excretory nitrogen (urine and feces).

(a) No proteids fed :

Birds, 24.35 Cals. (Zeit. f. Biol., 19, 367).
Mammals, 24.94 Cals. (ibid., 22, 43).

(b) Meat fed, 25.98 Cals. (Ibid.).

(c) Syntonin fed, 26.66 Cals. (ibid.).

1 grm. carbon in fat, 12.31 Cals. (ibid.).
f Zeit. f. Biol., 7, 489.

NET AVAILABLE ENERGY— MAINTENANCE.

415

As compared with the fasting state, the 883 Cals. of metaboliz-
able energy supplied, for example, in 1000 grams of meat diminished
the loss of energy by the body by 1041 - 250 = 791 Cals. The latter
quantity, then, represents the extent to which the 883 Cals. supplied
in the food aided in maintaining the stock of potential energy in the
body, while the remaining 92 Cals. was consumed in the work of
digestion and assimilation as defined on previous pages; that is, it
increased by this amount the heat production of the animal. Ac-
cordingly we compute that in this case 89.6 per cent, of the metabo-
lizable energy of the meat was available, while the digestive work
consumed 10.4 per cent. Computing the other experiments in the
same way we have —

Metabolizable
Energy of
Food, Cals.

Gain Over

Fasting

Metabolism,

Cals.

Net Avail-
ability,
Per Cent.

442

883

1325

510 115.4

791 89.6

1079 81.5

From Rubner's experiments * with proteids (see p. 106) the
following figures are computed in the same manner as those above:

Meat.

Extracted meat -J
Meat

I:

Food,
Grms.

0
415

0
740

0
740

0
390
350

0
580

Metab-
olizable
Energy
of Food,
Cals.

0
367

0
654

0
939

0
347
309

0
512

Heat
Produc-
tion,
Cals.

573*
596*
793*

825*
931*
959*
261f
334 f
379f
528f
681f

Gain.

Total,
Cals.

-573
-229
-793
-171
-931

- 20
-261
+ 13

- 70
-528
-169

Over
Fasting
Metab-
olism,
Cals.

344

622
9li

274
191

359

Net
Avail-
ability,

Per
Cent.

93.74

95.15

!»7.o:>

7s.1 is
61.80

70.12

Tem-
perature,
Deg. C.

19.2
19.6
18.0
19.2
14.9
15.6

* Computed.

t Calorimetric determinaaon.

* Zeit. f. Biol., 22, 43-48; 30, 117-135.

416

PRINCIPLES OF ANIMAL NUTRITION,

To the above results we may add those of Magnus-Levy's deter,
minations (p. 381) of the work of digestion and assimilation in the
dog on a meat diet as follows :

Metabolizable

Energy of

Food,

Cals.

Expended in

Digestion and

Assimilation,

Cals.

Net Available.

Grms.

Total,
Cals.

Per Cent.

82.5
230.0
370.6*

33S

943

1520

56
116
244

282

827

1276

83.43

87.70
83.95

* In excess of maintenance requirements.

The wide range of the results obtained by Rubner would seem to
indicate either that the net availability of the energy of the pro-
teids may vary with different animals and under different conditions
or that the experimental methods were not sufficiently sharp for the
purpose now in view. The value of an average drawn from such
results is questionable, but for the sake of comparison it is included
below along with those derived from Voit's and Magnus-Levy's
experiments, Yoit's first result being omitted because impossible.
The figures express the average net availability as a percentage
of the metabolizable energy.

A'oit's experiments 85.60 per cent.

Rubner's experiments S2.80 " "

Magnus-Levy's experiments 85 . 03 '

Fat. — Computing the results obtained by Pettenkofer & Voit *
and by Rubner f upon the effects of fat on the total metabolism
(see pp. 144—140) in the same manner as those upon the proteids,
and adding Magnus-Levy's results (p. 379), we have the table
opposite.

Rubner's and Magnus-Levy's results do not differ widely, and
their average, 96.4 per cent., indicates a relatively small expendi-
ture of energy in the digestion and assimilation of fat, which does
not appear to materially increase above the maintenance require-
ment. Most of Pettenkofer & Voit's experiments give materially
lower results above that point, and the one case in which the food

* Zeit. f. Biol, 5, 370; 7, 440-443; 9, 3-13.
f Ibid., 19, 328-334; 30, 123.

NET AVAILABLE ENERGY-MAINTENANCE.

417

Food.

Pettenkofer & Voit's Experiments

Nothing

100 grms. fat

350 " fat

500 " meat

500 " meat; 100 grms. fat
500 " " 200 " "

Rubner's Experiment* :

Nothing

200 grms. bacon

Nothing

39.75 grms. butter fat

Nothing

100 grms. fat

Nothing

40 grms. bacon

Magnus-Levy's Experiments :

Fasting

131 . 6 grms. fat

Fasting

305 . 5 grms. fat

Metab-
olizable
Energy
of Food,
Cals.

0

942

3298

442

1384

2326

0
1738

0
356

0
942

0
348

0
1250

0
2902

Gain.

Total,
Cals.

-10S6
-275
+ 87S
-554
+ 329
+ S37

-658

+ 1016

-373

-17

-466

+ 428

-261

+ 49

-972

+ 259
-1055
+ 1760

Over
Fasting
Metab-
olism or

Basal
Ration,

Cals.

811
1964

883
1391

1674
356
894
310

1231
2815

Net
Avail-
ability,
Per Cent.

86.1

59.6

93.7

73.8

98.6

100.0

94.9

89.1

98.5
97.0

supply was below the amount required for maintenance also gives
a rather low availability as compared wth that obtained by the
other experimenters.

Carbohydrates. — Tabulating as in the previous cases the re-
stilts of Pettenkofer & Voit * and of Rubner f (see pp. 146-152),
and adding those of Magnus-Levy (p. 380), we have the figures
shown on the next page.

As was the case with fat, most of Pettenkofer & Voit's experi-
ments give figures notably lower than those obtained by the other
two investigators. The averages of the latter, omitting the figures
which exceed 100 per cent., are:

Rubner's experiments 88.9 per cent.

Magnus-Levy's experiments 91.0 "

* Zeit. f. Biol., 9, 485.

t Ibid., 19, 357-379; 22, 273.

4i8

PRINCIPLES OF ANIMAL NUTRITION.

Food.

Pettenkofer & Voit's Experiments :

Nothing*

450 grms. starch; 16.9 grms. fat
597 " " 21.2 "

700 " " 20.2 "

500 " meat

500 " meat; 200 grms. starch
500 grms. meat; 200 grms. dextrose

Rubner's Experiments :

Nothing

76.12 grms. cane-sim.ir . .
104.97 "
Nothing

Metab-
olizable
Energy
of Food,
Cals.

97.3 grms. cane-sugar

17.0 " " "

143.0 " " "

Nothing

42 . 96 grms. starch (digested)

Nothing

57.38 grms. starch (digested)

Nothing

94.36 grms. cane-sugar; 67.96 grms.

starch; 4 . 7 grms. fat

300 grms. meat ; 63 . 7 grms. dex t ros< i
300 " " 79.7 "

300 " " 115.5 "

Magnus-Levy's Experiments :

Chiefly rice

0
2015
2661
3076
442
1316
1180

0
305
420

0

389

68

572

0
177

0
244

0

702
500
559
691

2121

2226

999

Gain.

Total,
Cals.

-1098
+ 353
-198
+ 853
-554
+ 137
+ 108

-436

-116

-22

-451

-87
-374
+ 190f
-302
-138
-354
-140
-302

+ 365f

-126

-84

+ 34

+ 850

+ 934

-81

Over
Fasting
Metab-
olism or

Basal
Ration,

Cals.

Net
Avail-
ability,
Per Cent.

1451

900

1951

691
662

320
414

364

77
641

164

214

667

42
160

1890

2066

910

72.0
33.8
63.4

79.1

89.7

104.9
98.6

93.6
113.2
112.0

92.6

87.8

95.0

71.1J

83. 7X

S9.1
92.8
91.1

* Fasting metabolism estimated from previous experiments.
f Gain of carbon assumed to be all in the form of fat.
X Of dextrose added.

Experiments on Herbivora. — Comparatively few experiments
have been reported from which the net availability of the food of
herbivorous animals can be computed, and as regards the common
farm animals in particular there is an almost entire lack of data,
although numerous experiments upon the relative value of various

#£7" AVAILABLE ENERGY- MAINTENANCE.

419

materials for productive feeding have been reported and will be
considered in the following chapter.

Fat. — Rubner's experiments include one * in which fat was
fed to a rabbit with the following results:

Metabolizable energy of food .

Total gain

Gain over fasting metabolism
Net availability

Fasting.

0 Cals.
-101 "

Fed 26.1 Grms.
Bacon.

227 Cals.
+ 122 "
223 "
98. 2%

In connection with his investigations upon cellulose, v. Knie-
riemf also experimented upon the influence of fat on the metabo-
lism of the rabbit. The basal ration consisted of milk, to which
was added in the second period 3,94 grams of dry butter fat per day.
Computing the amounts of energy by the use of Rubner's factors
the results were :

•

Metabolizable <-•-.:_ r-„i
Energy. Cals. Gain, Cals.

Net Availability,
Per Cent.

Milk and butter fat

207.3
169.8

— 19.5
-55.2

Milk

Difference

37.5

35.7

95.2

Carbohydrates. — Paibner % reports three experiments with
cane-sugar on a cock from which the following results are com-
puted :

Food.

Metaboliz-
able Energy
of Food,
Cals.

Gain.

Total,
Cals.

Over Fast-
ing Metab-
olism, Cals.

Net

Availability,

Per Cent.

Nothing

34 grms. cane-sugar

Nothing

45 grms. cane-sugar
50 " " "

0
136

0
180
200

-239
-121
-258
-101
- 53

118

157

205

86.8

87.2
102.5

* Zeit. f. Biol., 19, 333. f Ibid., 21, 119. J Ibid., 19, 366.

420

PRINCIPLES OF ANIMAL NUTRITION.

From the comparisons of cellulose and cane-sugar made by v.
Knieriem (loc. cit.) and cited on p. 161, the following figures for the
net availability of the energy of the latter substance may be com-
puted :

c

03

>>
c3

P

"3
d

Food per Day.

Metab-
olizable
Energy
of Food,
Cals.

Gain.

Net

a

X

Total,
Cals.

Over

Basal

Ration,

Cals.

Avail-
ability,
Per Cent.

TTT

5
4
3

Milk..

350.1
393.7

480 . 7

-37.9
-15.9
+ 69.9

IV

V

" +11 gnns. cane-siigar. .

" +33 " " " ..

22.0
107.8

50.5
.82.5

A series of experiments by May * upon the effect of fever on
metabolism affords incidentally a few data bearing on the availa-
bility of the energy of dextrose. In his experiment No. 5 (loc. cit.,
p. 23) the ingestion of 30 grams of grape-sugar, an amount approxi-
mately equivalent to the fasting metabolism, caused no increase in
the computed heat production as compared with that during fasting.
In this experiment there was no fever. In Experiment No. 6 (p. 25),
with fever, the ingestion of the same amount of grape-sugar pro-
duced a computed gain of 2.88 grams carbon as fat, but caused no
increase in the computed heat production. Experiment No. 7
(p. 26) was similar to No. 6, but showed a decrease in the computed
heat production, which, however, coincided with a decrease in the
fever. On the whole, .May's results appear in accord with Rubner's
hypothesis of a substitution of the heat resulting from digestive
work for that arising from the metabolism of tissue.

Pentoses. — Cremer's experiments f with rhamnose upon rabbits,
cited in Part I, p. 157, afford data for computing the net availa-
bility of this representative of the pentoses. For this purpose
Cremer computes from the excretion of nitrogen and carbon (neg-
lecting the feces), in the manner described in Chapter VIII, p. 253,
the amount of energy liberated by the metabolism of protein and
fat in the body, assuming that the rhamnose, after deducting the
small amounts in feces and urine, was completely oxidized. The
following are the results for each day of the four experiments:

* Zeit. f. Biol., 30, 1.

t Ibid., 42, 451.

NET AVAILABLE ENERGY— MAINTENANCE.

421

Food, Grms.

Metabolizable

Energy of Food,

Cals.

Loss bv Body,
Cals.

Experiment I :

Nothing

0

45.3

0

0

66.8

0

0

74.1

0

0

0

0
72.9
22.0

147.4
114.0
113.3

180.7
111.6

184.8

129.1

54.3

113.1

113.4

146.0

141.4

53.3

98.1

Rhamnose, 11.584 grms.. . .
.Nothing

Experiment II :

Nothing

Rhamnose, 17.09 grms

Nothing

Experiment III :

Nothing

Rhamnose, 18 . 96 grms

Nothing

" (av'ge of two days)

Experiment IV :

Nothing

tt

Rhamnose, 18 . 66 grms

5.64 * "

* The total amount of rhamnose (24.3 grms.) was given on the first day,
but it is estimated from the results for the carbon excretion that this amount
of it was not metabolized until the second day.

The results as to net availability obtained by comparison with
the several fasting days vary considerably, as the following state-
ment shows, several of them exceeding 100 per cent. :

Experiment I.

Compared with first day 74 per cent.

" " third day Negative

Experiment II.

Compared with first day 103 per cent.

" third day 110 " "

Experiment III.

Compared with first day 101 per cent.

" third day 79 " "

Second and third with fourth and fifth days. 80 " "

Experiment IV.

Third compared with second day 121 per cent.

" " " fourth day 88 " "

422

PRINCIPLES OF ANIMAL NUTRITION.

J>'

The great variations in the results, as well as the large propor-
tion of cases in which the availability appears to exceed 100 per
cent., show that little value attaches to them as quantitative deter-
minations, although they undoubtedly show that rhamnose pos-
sesses a comparatively high nutritive value.

Crude Fiber.— The experiments of v. Knieriem have already
been cited in Chapter V in their general bearings upon the metabo-
lism of matter. As was there noted, certain corrections were neces-
sary on account of the residue of undigested cellulose remaining
in the digestive canal at the close of the experiment. The results
given below are based on those computed by the author, as sum-
marized on p. 161, on the assumption that the resorption of the
remaining digestible crude fiber was complete after two days.

W .

ifr'

0

6

Food per Day.

Metab-
olizable
Energy
of Food,
Cals.

Gain.

Net

u

Total,
Cals.

Over

Basal

Ration,

Cals.

Avail-
ability,
Per Cent.

I

9
10

5

Milk

341.7
374.6
350.1

-46.8
- 6.9 -j

-37.9

39.9
31.0

II
III

" + 22 grms. crude fiber |

for eight days j

Milk

121.3*
126. 5f

* Compared with Period I.

t Compared with Period III.

It is evident from the above figures that while the experiments
show qualitatively a nutritive value for the cellulose, they are in-
sufficient for a quantitative determination of its amount.

In striking contrast with these results are the conclusions drawn
by Zuntz & Hagemann from their experiments upon the horse
which have already been considered in the previous chapter (pp.
389-391). As was there explained in detail, these investigator
have estimated the expenditure of energy in the digestion of crude
fiber from a comparison of the computed heat production in two
sets of experiments in which the proportion of coarse fodder eaten
differed considerably, it being assumed that 9 per cent, of the metab-
olizable energy of the nutrients other than crude fiber was consumed
in their digestion. On this basis the digestive work caused by the
crude fiber is computed at 2.086 Cals. per gram, or rather more than

NET AVAILABLE ENERGY— MAINTENANCE. 423

its average metabolizable energy. In other words, it is computed
that under the conditions of these experiments, with a ration more
than sufficient for maintenance, the net availability of the energy
of the crude fiber was practically zero.' The authors report no
experiments upon rations below the maintenance requirement, but
appear to regard the metabolizable energy of the crude fiber as being
indirectly available, under such conditions, substantially in the
manner assumed by Rubner and already explained.

As has been noted, Zuntz & Hagemann's conclusions as to the
value of crude fiber for work production are in apparent harmony
with those of Wolff, which will be discussed in the next chapter, but
on the other hand they contrast sharply with the results of Kellner
(see Chapter XIII, § 1), who observed a high percentage utilization
of the energy of one form of crude fiber in the ration of fatten-
ing cattle. On previous pages some reasons were presented for
questioning the quantitative accuracy of Zuntz & Hagemann's
computations, but even aside from these their conclusions as re-
gards the value of crude fiber are difficult to reconcile with obvious
facts. Thus they compute (loc. cit., p. 280) that the expenditure
of energy in the mastication and digestion of average straw is
greater than its metabolizable energy, so that for the horse this
material has a negative value. When forming part of a mainte-
nance ration we mt.y probably assume that below the critical
amount of food (p. 408) the heat generated during the digestion of
the straw would be of use to maintain the body temperature, but
this could not possibly suspend the expenditure of energy in the
various forms of internal work, such as respiration and circulation.
Since, however, by hypothesis, the straw can contribute no energy
directly for these purposes, it follows that the consumption of this
material alone cannot reduce the loss of tissue below the amount
requisite to supply energy for the internal work, while on an
exclusive straw ration above the critical amount of food the more
straw the animal consumed the sooner it would starve.

Organic Acids. — The results of a considerable number of ex-
periments in which salts of organic acids were mjected into the
blood have already been presented in Chapter V (p. 157). The
general result was that lactic and butyric acids caused little or
no increase in the heat production of the animal — in other words

424

PRINCIPLES OF ANIMAL NUTRITION.

that practically all their potential energy was available to prevent
loss of tissue. In such experiments, of course, there is no digestive
work in the proper sense. What they indicate is that what we
have called rather loosely the work of assimilation for these sub-
stances is practically zero. Acetic acid, on the other hand, was
found by Mallevre to increase the consumption of oxygen by from
10 to 17 per cent., indicating a considerable waste of energy directly

or indirectly.

Timothy Hay. — The experiments described in the foregoing
paragraphs relate to pure or nearly pure nutrients. Experiments
upon a steer have been made by the writer in conjunction with
Fries in which the availability of the apparent metabolizable .energy
of timothy hay has been determined. To a basal ration consider-
ably below the maintenance requirement, consisting of 3250 grams
of hay and 400 grams of linseed meal, three different additions of
timothy hay were made, the digestibility of the ration in each
period being determined, and likewise the total balance of nitrogen,
carbon, and energy by means of the respiration-calorimeter. The
results as to energy, uncorrected for the very small differences in
the organic matter of the basal ration consumed and for the
changes in the live weight of the animal, were as follows : *

Period A.

Period B.

Period C.

Period D.

Outgo,
Cals.

Income,
Cals.

Outgo,
Cals.

Income,

Cals.

Outgo,

Cals.

Income,
Cals.

Outgo,
Cals.

Income,

Cals.

14,923

8,590

974

1,251

9,482

20,297

11,477
1,125
1,374

11,222

25,198

29,647

Excreta:

Urine

6,446
863
996

6,618

14,276

1,220

1,896

12,255

Methane

Metabolizable . . .

14,923

14,923
6,618
2,449

20,297
10,206

20,297
9,482

724

25,198

10,606
616

25,198
11,222

29,647

29,647
12,255

Heat produced . .

9,067

11,183
1,072

9,067

9,067

10,206

10,206

11,222

11,222

12,255

12,255

* Proc. Soc. Prom. Agr. Sci„ 1902.

For a full discussion of the revised figures and for later results on clover
hay and maize meal, see U. S. Dept. Agr., Bureau of Animal Industry, Bul-
letins 61 and 74.

NET Ay AIL ABLE ENERGY— MAINTENANCE.

425

Subtracting the results on the basal ration of Period A from those
of the other periods, as in previous cases, we have the following :

Metabolizable

Energy,

Cals.

Gain of Tissue,
Cals.

NetAvailability,
Per Cent.

Period B

9,482
6,618

-724
-2,449

A

Difference

2,864

11,222
6,618

1,725

616
-2,449

60 24

Period C

A

Difference

4,604

12,255
6,618

3,065

1,072
-2,249

66 57

Period D

" A .

Difference

5,637

3,521

62 46

Average

63 09

Strictly speaking, only the first of the above percentages repre-
sents the net availability for maintenance, since the other two
include some gain. From the difference observed between the
metabolism of the animal standing and lying, however, it was
computed approximately what the gains would have been had the
same position been maintained for the whole twenty-four hours,
with the following results:

Metabolizable
Energy, Cals.

Gain, Standing,
Cals.

Net Availability,
Per Cent.

Period B

9,482
6,618

-1,606
-3,507

" A

Difference

2,864

11,222
6,618

1,901

-550
-3,507

66 37

Period C

" A

Difference

4,604

12,255

6,618

2,957

23
-3,507

64 2-3

Period D

" A

Difference

5,637

3,530

62.62

Average

64.41

426

PRINCIPLES OF ANIMAL NUTRITION.

Metabolizable
Energy, Cals.

Gain, Lying,
Cals.

Net Availability,
Per Cent.

Period B

9,482
6,618

1,157
-1,046

" A

2,864

11,222
6,618

2,203

2,136
-1,046

76.92

Period C

" A

4,604

12,255
6,618

3,182

2,743
-1,046

69.12

Period D

" A

Difference

5,637

3,789

67.22

71.08

The results are likewise shown graphically on the accompanying
diagram, in which the full line represents the average availability

3000

X

' 0

2000

0 /

y'

1000

/'

O .

<

/'

0

<
0

/''

y^

s

^1000

0'

oy
y

—2000

y

y'

-3000

0'''

y

-4000

7

aoo a

W0 9

WO ic

<W0 11

000 12

000

METABOLIZABLE ENERGY, CALS.

NET AVAILABLE ENERGY— MAINTENANCE.

.427

observed and the broken lines that computed respectively for
standing and lying, while the points indicate the results for each
period. As computed standing, the results are all practically at or
below the maintenance point, and their fairly close agreement with
each other and with those actually observed indicates that the net
availability of the metabolizable energy of this sample of timothy
hay was between 63 and 65 per cent.

Summary. — The foregoing data as to availability are sum-
marized in the following table, those experiments in which the total
ration was less than the maintenance requirement being separated
from those in which more or less gain by the body took place:

Experiments on Carnivora.
Below Maintenance.

Froteids :

Per Cent

Pettenkofer & Voit -

115.4
89.6

f 93.7

95.2

Rubner -j

97.0

61.8

. 70.1

Magnus-Lew 1

83.4

87.7

Fat:

Pettenkofer & Voit

86.1

Rubner

100.0

Experiments on Herbivora.
Below Maintenance.

Fat : per Cent,
v. Knieriem 95 . 2

Starch :

Pettenkofer & Voit 33 . 8

v> u ( 92-6

Kubner - „

( 87.8

Magnus-Lew (rice) 91 . 1

Dextrose :

Rubner 71.3

Cane-sugar :

Rubner

Starch and Cane-sugar :

Rubner 95.0

428

PRINCIPLES OF ANIMAL NUTRITION.

Pentoses -

Cremer

Crude Fiber:
v. Knieriem.

Timothy Hay:
Armsby & Fries

Above Maintenance. Above Maintenance.

Proteids: Per Cent.

Pettenkofer & Voit 81 . 5

Rubner 79.0

Magnus-Levy 84 . 0

Fat: Fat-

i 59.6 Rubner

Pettenkofer & Voit ] 93 . 7

( 73.8

t 98.6

Rubner { 94.9

( 89.1

Magnus-Levy j 97 Q

Starch:

r 72.0

Pettenkofer & Voit \ 63.4

( 79.1

/ • s (89.1

Magnus-Levy (nee) | ^

Dextrose :

Pettenkofer & Voit 89 . 7

Rubner 83.7

Cane-sugar : Cane-sugar :

Rubner 112.0 v. Knieriem

Cane-sugar and Starch :

Rubner 95.0

74.0

(?)
103.0
110.0
101.0

79.0

80.0
121.0

88.0

121.3
126.5

63-65

Per Cent.
. 98.2

82.5

NET AVAILABLE ENERGY— MAINTENANCE. 4*9

It scarcely seems possible to draw any well-founded conclusions
regarding the net availability of the several nutrients from such
widely divergent results as those tabulated above, even if the ex-
treme and obviously incorrect figures be discarded. Two things,
however, seem worth}' of remark.

First, in but few cases does the net availability of the food reach
100 per cent., and most of those results relate to cane-sugar or rham-
nose; that is, to cases in which some of the gain of carbon which is
computed as fat may have been in the form of a carbohydrate. It
would seem fairly safe to conclude, therefore, that no such complete
substitution of the heat resulting from digestive work for that re-
sulting from the general metabolism took place as Rubner's hypoth-
esis supposes. Apparently, under the conditions of these experi-
ments, there was, in most cases at least, a material loss of energy
in digestive work.

Second, there is no clear indication of a smaller loss of energy
below than above the maintenance ration, although the wide range
of the results renders a definite conclusion upon this point hazardous.
This question, however, may be more properly considered in con-
nection with a study of the utilization of the net available energy of
the food.

finally, it is to be said that if the validity of the conception of
a critical amount of food, as developed on p. 409, be admitted —
that is, of an amount of food below which the heat resulting from
the work of digestion and assimilation is substituted for that pro-
duced by the general metabolism, while above it no such substtu-
tion takes place — a very important element is lacking for the
interpretation of the above experiments, except, perhaps, those on
timothy hay, in which the uniformity of the results with varying
amounts seems to show clearly that all the rations supplied more
than the critical amount of food. If that conception is correct,
to determine the real availability of the energy of a food it is
necessary to compare the effects of two quantities both of which
are greater than the critical amount. On the other hand, the
complete substitution of energy supposed by Rubner could only
be demonstrated by comparing quantities less than the critical
amount, while a comparison of quantities below the latter
amount (including, of course, fasting) with those exceeding it

43° PRINCIPLES OF ANIMAL NUTRITION.

can give only mixed results varying with the quantities com-
pared.*

It seems tolerably clear, then, that the whole subject of the net
availability of foods and nutrients needs reinvestigation by more
rigorous methods and with due regard to the amounts of the food
materials compared and to the thermal environment of the animals
experimented upon.

Discussion of Results.

For the reasons just stated, any strict quantitative discussion
of the above results seems impossible. At the same time, certain
general conclusions may be at least tentatively deduced from, them
which, even though to a considerable extent speculative, may at
least serve provisionally as a connecting thread between the known
facts.

Influence of Amount of Food on Availability. — In the fore-
going paragraphs it has been tacitly assumed that the amount of
food eaten has no influence on its availability, or, to state it in
another way, that the expenditure of energy in digestion and assimi-
lation is proportional to the quantity of food. To express the same
thing in mathematical terms, we have assumed, in constructing the
diagram on p. 410, that the net available energy is a linear function
of the metabolizable energy.

While it seems highly probable that such is the case the only ex-
periments bearing specifically upon this question of which the writer
is aware are those upon timothy hay just cited. An examination
of the graphic representation of the results strongly supports the
hypothesis that the net availability of the food is independent of
its amount, but the evidence of so few experiments must naturally
be accepted with some reserve. The other recorded results, as
computed above, apart from the possible source of uncertainty
pointed out on p. 429, show such considerable variations in indi-
vidual cases that it scarcely seems possible to reach any definite
conclusions from them regarding the influence of quantity of food.
As will appear in the next chapter, the extensive respiration exper-
iments made in recent years at the Mockern Experiment Station by
G. Kiihn and O. Kellner upon fattening cattle indicate that the

* Rubner, in his latest publication (Gesetze des Energieverbrauchs bei < ler
Ernithrung, Leipsic and Vienna, 1902) has also adopted this view.

NET AVAILABLE ENERGY- MAINTENANCE. 431

actual gain obtained (expressed in terms of energy) , at least within
certain limits, is proportional to the amount of metabolizable energy-
supplied in excess of maintenance. This would mean that above
the maintenance ration the energy required for digestion and
assimilation plus that consumed in the chemical changes incident
to the formation of new tissue (compare p. 396) is proportional
to the amount of food. If this be true it seems more reasonable
to conclude that each of these forms of work separately is propor-
tional to the amount of food than to assume a compensation between
the two, and granting this, we should have every reason to suppose
that the same proportionality would hold good for the work of
digestion and assimilation below the maintenance requirement.

Character of Food.— The investigations of Zuntz & Hagemann
(pp. 385-393) have shown that, in the case of the horse at least
and doubtless with other animals also, the work of digestion and
assimilation varies with the kind of food, a result which is entirely
in accordance with what we should expect. For reasons stated in
describing their experiments, their results are to be regarded as
qualitative rather than quantitative, but they suffice to demon-
strate the very marked difference as regards availability which
exists between the relatively pure nutrients employed in the exper-
iments of Pettenkofer & Voit, Magnus-Levy, Rubner, and others
and the feeding-stuffs consumed by our herbivorous domestic
animals, and to show the fallacy involved in applying the results
of the former experiments directly to the latter case. The same
conclusion is also indicated by the few results upon timothy hay
on p. 424.

Unfortunately no other direct determinations of the availability
of the food of herbivorous animals in amounts belowT the mainte-
nance ration are on record, so that we are unable to compare either
different feeding-stuffs or different species of animals in this respect.
The extensive investigations of the Mockern Experiment Station
mentioned in the previous paragraph show how large a proportion
of the metabolizable energy of the food of fattening animals becomes
economically waste energy, thus fully confirming the conclusions
drawn from Zuntz & Hagemann's experiments upon the horse, but
they afford no means of distinguishing between the work of diges-

43 2 PRINCIPLES OF ANIMAL NUTRITION.

tion and assimilation and the energy expended in converting the
resorbed material into permanent tissue.

The Maintenance Ration. — As already defined, net available
energy is that portion of the metabolizable energy of the food
which serves to make good the losses of potential energy arising
from the internal work plus the work of digestion and assimilation,
or, in other words, which contributes towards the maintenance of
the stored-up capital of energy. We may, therefore, appropriately
consider the bearings of the known facts regarding availability
upon the amount of food required for maintenance.

Relations to Availability.— Not a little effort has been
expended in determining the maintenance requirements of • farm
animals on the more or less tacit assumption that this quantity is
a constant for the same animal, and the same assumption has even
more largely controlled in computations based on the experimental
data obtained.

By the maintenance ration, of course, we understand a ration
just sufficient to prevent any loss of tissue — that is, of potential
energy — by the animal. To accomplish this we must give a ration
containing net available energy equal in amount to the potential
energy lost when no food is given. Expressed thus in terms of net
available energy, the maintenance requirement under given condi-
tions is a constant and is equal to the energy of the fasting metabo-
lism.

The maintenance requirement, however, particularly in the case
of farm animals, has not usually been expressed thus, since the
necessary data are lacking, but in terms of total digestible matter
or of real or supposed metabolizable energy. When thus expressed,
however, it is apparent in the light of the foregoing discussion that
the maintenance requirement must be a variable, depending upon
the availability of the metabolizable energy of the food. Referring
again to the graphic representation on p. 410, it is evident that,
under the conditions there represented, with an availability ex-
pressed by tan DAC, the amount of metabolizable energy required
for maintenance will be equal to OS. Furthermore, it is equally
evident that as the availability decreases and the angle DAC con-
sequently becomes more acute OS will increase. Only when the
critical amount of food, OM, is greater than the fasting metabolism

NET AVAILABLE ENERGY— MAINTENANCE. 433

and the point K falls above the axis OX will there be an apparent
exception to this law. In that case, since the energy expended in
digestion and assimilation seems to be indirectly utilized, the ap-
parent availability will be 100 per cent, and the metabolizable
energy required for maintenance will be constant and equal to the
energy of the fasting metabolism.

This case might and perhaps does occur with animals whose food
consumes little energy in digestion, such as the carnivora. As was
pointed out on p. 412, however, an increase in the work of digestion
tends to reduce the critical amount of food and there would appear
to be good reason for believing that, in ruminants at least if not in
the horse, it lies considerably below the point of maintenance.

Relative Value of Grain and Coarse Fodder. — We know
from the investigations of Zuntz <fc Hagemann (pp. 3S5-393) that
the work expended in the digestion of coarse fodders is, in the horse
and presumably therefore in other animals, materially greater than
that caused by grain. It follows, then, that a unit of digestible
matter or of metabolizable energy should have more value for
maintenance in the latter than in the former.

That such is the case WTth cattle is rendered probable by experi-
ments by the writer.* In the absence of a respiration apparatus
the nutritive effect of the rations was judged of from the live weight
and the proteid metabolism during relatively long periods and the
methane production was computed from the carbohydrates digested.
A ration in which only about 24 per cent, of the digested organic
matter was derived from coarse fodder, as compared with rations
consisting exclusively of coarse fodder, gave the following results
for the metabolizable energy of the maintenance ration per day
and 500 kgs. live weight:

Exclusive coarse fodder, 12 experiments. . . . 12,771 Cals.
Largely grain, 3 experiments 11,023 "

Such determinations of the maintenance requirements of the
horse as have been made tend to confirm the results obtained with
ruminants. Wolff, in his investigations upon work production
described in the following chapter, has computed the maintenance
requirements of the horse in the manner there explained both from

* Penna. Expt. Station, Bull. 42, p. 159.

434 PRINCIPLES OF ANIMAL NUTRITION.

his own experiments and from those of Grandeau and LeClerc,
with the following results per 500 kgs. live weight :

Total Digestible Nutrients,*
Grms.

On hay alone 4586

About equal parts hay and grain 4190

About f grain and I hay (Grandeau) 3626

Zuntz & Hagemannj from the results of a respiration experi-
ment with the horse, make a still lower estimate of the maintenance
requirement, viz.. 3265 grams total nutrients per 500 kgs. live
weight on a ration of which about four sevenths was grain, but
after allowing for the differences in crude fiber content compute a
satisfactory agreement between their results and "Wolff's. Since
their estimate for the work of digestion of crude fiber is really
based on the difference in digestive work required by coarse fodder
and by grain this is equivalent to showing that the latter is more
valuable for maintenance than the former.

On the other hand, Grandeau and LeClerc X in later experiments
on exclusive hay feeding found that the live weight was almost
exactly maintained for a month on 8 kgs. of hay per day, the total
digested nutrients being as follows:

Animal.

Live Weight ,
Kgs.

Total Digestible Nutrients.

Per Head,
Grms.

Per 500 Kgs.,
Grms.

No. 1

395

419
413

2892
3036
3058

3660
3622
3701

" 2

" 3

These figures do not materially exceed the average computed by
Wolff from their previous experiments on heavy grain rations. The
horses had a half-hour's walking exercise daily, so that the ration
seems to have been amply sufficient for maintenance, and no reason
for the divergent result is obvious.

While none of these comparisons have the conclusiveness of

* Including fat X 2.4.

f hoc. cit., pp. 422-4.

X L'aliinentation du Cheval du Trait, 3d memoir, pp. 23-31.

NET AVAILABLE ENERGY— MAINTENANCE. 435

complete metabolism experiments, their results as a whole indicate
clearly that the metabolizable energy of the grains is more valu-
able for maintenance than that of the coarse fodders, a fact un-
doubtedly due to the greater expenditure of energy in the digestion
and assimilation of the latter.

The maintenance ration of horses, cattle, and sheep, then, as
ordinarily expressed (i.e., in units of digestible matter or of metabo-
lizable energy) is not a constant but a variable, depending on the
availability of the metabolizable energy, and such a statement of it,
to be definite, must be accompanied by a statement of the kind of
feed used.

Xo similar experiments upon swine appear to have been made.
The ordinar\' feed of this animal, however, probably varies less in
availability than that of ruminants, and it may be presumed that
no such striking differences would be found.

Value of Crude Fiber. — As a result of Wolff's conclusions con-
cerning the apparent worthlessness of crude fiber for work production,
as discussed in the succeeding chapter, and of Zuntz & Hagemann's
estimates regarding its digestive work (p. 389), there has been a
tendency to ascribe the difference between grain and coarse fodders
to the greater amount of crude fiber in the latter, forgetting that
what these investigators have actually shown is simply the lower
value of the digestible matter from coarse fodders, and that their
conclusions regarding crude fiber are deductions from the observed
facts. Kellner's more recent experiments (see p. 182 and Chapter
XIII. § 1) have demonstrated that at least one form of crude fiber
is nearly as efficient in producing a gain of fat by cattle as is
starch. A fortiori, therefore, it should be equally valuable for
maintenance. We have as yet no sufficient evidence to justify us
in ascribing the difference between grain and coarse fodder to the
crude fiber as such aside from its influence on the mechanical
structure of the material.

Influence of Thermal Environment. — It has been not
uncommonly assumed that the maintenance requirement of an
animal is affected by changes in the temperature and other external
factors which combine to determine the refrigerating effect of the
environment; in other words, the heat production of the animal
has been looked upon more or less distinctly as an end in itself.

436 PRINCIPLES OF ANIMAL NUTRITION.

We have already seen reason to believe that this is the case to a
very limited extent only, even in the fasting animal, and to a still less
degree in one consuming food. If we are justified in thinking that
the critical amount of food for herbivorous animals is ordinarily
less than the maintenance requirement, it follows that the heat
production on a maintenance ration is in excess of the actual needs
of the organism for heat by an amount depending upon the avail-
ability of the metabolizable energy of the food, and that this excess
of heat is disposed of by "physical" regulation. That such is the
case appears to be clearly indicated by the writer's experiments
upon timothy hay (p. 424), since there was obviously no such in-
direct utilization of the heat resulting from the work of digestion
and assimilation as takes place, according to Rubner's theory,
below the critical amount of food. If, now, the temperature to
which such an animal is exposed falls, it is in accord with all that
we know regarding the regulative processes in the body to suppose
that the additional draft on it for heat will be compensated for by
a fall in the emission constant rather than by an increased produc-
tion of heat, or, to put it in another way, that some of the heat
resulting from digestive work will be utilized to maintain the tem-
perature of the animal instead of being at once dissipated.

No exact experiments upon the influence of external tempera-
ture on the maintenance requirement appear to have been made,
but Kern, Wattenberg & Pfeiffer * have investigated the influence
of the greater exposure to cold caused by shearing upon the metabo-
lism of sheep consuming a maintenance ration. A slight decrease
in the proteid metabolism was found to result, due, as Pfeiffer con-
jectures, to a more rapid growth of wool after shearing, but the
corresponding difference in the metabolism of energy is insignificant.
The removal of a nine-months fleece appears to have caused at first
an increased excretion of carbon dioxide, but this practically dis-
appeared within four or five days and is probably to be attributed
to greater muscular activity on the part of the shorn animals.
Comparing the results before shearing with those obtained from
five to sixteen days after, we have the following averages, the
amount of water-vapor given off being only an approximate esti-
mate:

* Jour. f. Landw., 39, 1.

NET AVAILABLE ENERGY— MAINTENANCE. 437

Carbon dioxide

per Day,

Grms.

Estimated
Water-vapor

per Day,
Grms.

Before shearing (4 experiments) . . .
After (4 " )...

719.6

725.1

1939
434

The total metabolism, as indicated by the excretion of carbon
dioxide, shows scarcely any increase as a result of the shearing, and
if we accept Pfeiffer's suggestion that the result for the first of the
four days (736 grams) may have been slightly affected by the stimu-
lation of movement above noted, the difference becomes still less.
On the other hand, the difference in the amount of water-vapor
given off is very striking and apparently admits of but one con-
clusion, viz., that drawn by Pfeiffer, that the unshorn animals upon
a maintenance ration produced an excess of heat which was gotten
rid of by evaporation of water, while the shorn animals, instead of
meeting the greater refrigerating effect of their surroundings by an
increased metabolism, simply evaporated less water and thus com-
pensated for the increased loss of heat by radiation and conduction.

Even in the case of man. where the digestive work is much
less than in the herbivora, the heat production on a mainte-
nance ration may be in excess, and even large \y in excess, of the
minimum requirement, it being simply a question of clothing,
temperature, etc. This has been most strikingly demonstrated
by Ranke,* who shows that with relatively high temperature and
humidity the heat production on a maintenance ration may be so
great as to even produce pathological effects and that under such
circumstances the consumption of food is instinctively reduced
below the maintenance requirement.

Sanborn,! in experiments upon the maintenance ration of swine,
found the amount of middlings required, per hundred pounds of live
weight, to be as tabulated on the next page. The second summer
experiment is not comparable with the others, since the smaller
animal would require a relatively greater maintenance ration.
The remaining experiments seem to show a lower requirement for
maintenance in winter than in summer.

* Einfluss des Tropenklimas auf die Ernuhrung des Menschen, and Zeit.
f. Biol., 40, 288.

t Mo. State Agr. Coll., Bull. 28, pp. 5 and 6.

43s

PRINCIPLES OF ANIMAL NUTRITION.

Live Weight,
Lbs.

Maintenance

Requirement,

per 100 Pounds,

Lbs.

Winter (temp, about 40° F.) ... . j
Summer ( " " 80° F.) .... -j

173.5

171.6

173.6

48.3

1.65
1.89
2.02
2.07

On the other hand, Cooke,* in a series of experiments on swine
at the Colorado Station, found the following amounts of computed
digestible matter required for maintenance per hundred pounds live
weight of animals weighing from 85 to 182 pounds per head:

In hot weather 0.93 lbs.

In moderate weather 1 . 25 "

In cold weather 1 .41 "

Consumption of Water. — A not inconsiderable amount of energy
is usually required to raise the ingesta to the temperature of the
body. This is particularly true of the water consumed, especially
in case of the herbivora, both by reason of its relatively large amount
as compared with the dry matter of the food and on account of its
high specific heat. At first thought it might seem that the warming
of the ingesta is part of the work of digestion, since it is an expendi-
ture of energy in preparing the food for assimilation. This same
matter or its equivalent, however, finally leaves the body, in the
form of various excreta, at body temperature, thus carrying off as
sensible heat substantially the same amount of energy which was
imparted to it when its temperature was raised, and this heat it
imparts in cooling to the environment of the animal. It would
seem, then, that the warming of the ingesta may be more logically
regarded as a part of the general draft for heat which the surround-
ings make upon the-, animal, the process being simply a little fes's
direct than the loss of heat by radiation and conduction through
the skin.

From this point of view the influence of the consumption of cold

food and particularly of cold water will be subject to the same

general laws as the other forms of the demand for heat. On a

ration supplying less than the critical amount of metabolizable

♦Private communication.

NET AVAILABLE ENERGY-MAINTENANCE. 439

energy any increase in the consumption of water (taking this as the
typical case) will increase the metabolism by an amount sufficient
to warm the water to the body temperature. Above the critical
amount of food the excess of heat arising from the digestive work
will, we may reasonably suppose, be applied to the warming of the
additional water consumed, and only when this is insufficient will
an increased metabolism be required to make up the deficit. In
case of farm animals, however, it would appear that the waste heat
even on a maintenance ration is ordinarily sufficient, and more
than sufficient, to supply all the energy needed for warming the
ingesta.

The Time Element. — One important factor in modifying the
results of the demand for heat, particularly with relation to the
water consumption, is what we may call the time element. Hitherto
it has been tacitly assumed that all the factors making up the
demand for heat act at a uniform rate. As a matter of fact this is at
best only partially true. Ordinarily a farm animal is watered but
once or twice per day and then consumes a relatively large amount
in a few minutes. A sudden demand for heat is thus set up, since
this water must be raised to body temperature within a compara-
tively short time. It is quite conceival >le, therefore, that the demand
for heat may temporarily exceed the supply, requiring the deficit
to be made up by an increased metabolism, while if the same water
consumption were distributed uniformly over the twelve or tw< nty-
four hours no such effect would be produced. Such a temporary
increase in the heat production, however, cannot be made up for
later when the heat production is in excess, but is a permanent loss.
Once converted into heat, the energy of food or tissue has, so to
speak, escaped from the grasp of the organism, which appeal- to
have no power to reconvert it into any other form of energy. We
may plausibly suppose that these considerations constitute a partial
explanation of the advantages observed in practice from the warm-
ing of drinking-water and the installation of self -watering devices
in the stable.

What is true in regard to the consumption of water is of course
equally true of other forms of the demand for heat. The time ele-
ment is an important factor. Thus an exposure of an hour or two
in a cold yard or to a cold rain may cause an increased metabolism

44" PRINCIPLES OF ANIMAL NUTRITION.

and heat production although the average conditions for the twenty-
four hours may be such that the necessary production of heat by
the internal work and the work of digestion and assimilation would
be more than sufficient for the needs of the animal.

Influence of Size of Animal. — The discussion of the heat
production of the fasting animal in Chapter XI led us to the con-
clusion that under comparable conditions, at least for the same
species of animal, the internal work is probably approximately
proportional to the surface of the body. This, however, is equiva-
lent to saying that the quantity of net available energy required
for maintenance is proportional to the body surface. Furthermore,
if we are right in supposing that the available energy is a linear
function of the metabolizable energy, the amount of the latter
required for maintenance will also be proportional to the surface of
the body. Referring once more to the diagram on p. 410, if OA is
proportional to the body surface, then OS, which for a given food
bears a fixed ratio to OA, must also be proportional to the surface.
If the critical point, K, lies above the maintenance requirement,
then the metabolizable energy required for maintenance will equal
the fasting metabolism, and this, as shown on pp. 359-363, is pro-
portional to the surface.

Apparently, then, we are justified in concluding that the mainte-
nance requirements of different normal animals of the same species
are proportional to their body surface, or, for approximate computa-
tions, to the two-thirds power of their live weights. It must not be
overlooked, however, that the results upon which this conclusion is
based were obtained largely with the dog, an animal which when at
rest lies down, and which, therefore, in these experiments was in a
state of almost complete muscular relaxation. Our common farm
animals, on the contrary, pass a considerable portion of their time
standing, which involves an expenditure of energy in muscular
work. This expenditure we should naturally assume to be pro-
portional to the mass to be sustained rather than to its surface,
and if this be true we have here a second determining factor in the
maintenance requirement. How important this factor is it is diffi-
cult to say, although the writer's results with a steer (p. 343) in-
dicate that it is a large one. Its tendency would be to make the
maintenance requirement increase more rapidly than the surface.

NET AVAILABLE ENERGY— MAINTENANCE. 44*

Moreover, so far as we can judge from the accounts of Faibner's
experiments, it would seem likely that what were designated on
p. 342 as incidental muscular movements are a more important
factor in determining the maintenance requirements of farm ani-
mals than they are in fixing that of the dog.

While, therefore, we are probably justified in retaining pro-
visionally the computation of the maintenance requirement in
proportion to the real or estimated surface, it should be with a clear
understanding that it is at present a deduction from experiments
on other species and under more or less different conditions.

Effect of Fattening on Maintenance Requirement. — An interesting
question, and one of practical importance, is what effect the pro-
gressive change in weight of the same animal as it is fattened has
upon its maintenance requirement. We can hardly suppose that
the internal work of the body will be materially increased by such a
gain. The increased mass of tissue must involve, of course, some
increase in metabolism, but all that we know of metabolism of adi-
pose tissue indicates that it is very sluggish. The most important
effect might be anticipated to be an increase in the muscular ex-
ertion required in standing, perhaps counterbalanced to a greater or
stent by the tendency of the fat animal to pass more of its
time in a recumbent position.

Zuntz & Hagemann* have investigated the effect of a load
carried on the back upon the metabolism of the horse, and have
found the latter to be proportional to the total mass (horse plus
load), but the applicability of this result to another species of ani-
mal and to an increase of weight caused by fattening may perhaps
be questioned. The only experiments upon cattle bearing on this
point are those of Kellner,f who has compared the maintenance
requirements of fattened and unfattened cattle. It being impossible
to hit upon exactly the maintenance ration, it is computed from the
actual results. In case there was a loss of tissue the maintenance
requirement of the animal is computed by subtracting the poten-
tial energy of the excreta from the potential energy of food plus
tissue lost; in other words, the replacement of energy claimed by
Ilubner is assumed to occur. When there was a gain of tissue, on

* Landw. Jahrb., 27, Supp. Ill, 269.
t Landw. Vers. Stat., 50, 24.5; 53, 14.

442

PRINCIPLES OF ANIMAL NUTRITION.

the other hand, the amount of metabolizable energy required to
produce it is computed on the basis of the results upon utilization
obtained in other experiments, this larger amount being added to
the energy of the excreta and the sum of the two subtracted from
the potential energy of the food; that is the energy of digestion
and assimilation above the maintenance ration is assumed to be
waste energy.

Computed in this way, and assuming further that the mainte-
nance requirements of different animals are substantially propor-
tional to the two-thirds powers of their live weights, the results are
as follows:

No. of
Animals.

Live

Weight,

Kgs.

Stable
Tem-
perature,
Deg. C.

Main-
tenance
Require-
ment,
Cals.

Observed :

Unfattened

Fattened

Computed to same live weight

Unfattened

Fattened

632

7S5

800
800

15.2
15.7

15.2
15.7

13,470
19,671

15,760
19,920

KcUner concludes from these figures that the maintenance re-
quirements of fattened animals are greater per unit of surface than
those of unfattened ones.

These experiments, it is true, were on different animals and the
individuality of the animal is an important factor in determining
the maintenance requirement. The results on the seven unfattened
animals, when computed to 600 kgs. live weight, show a range of
1760 Cals., or 13.54 per cent, of the average, while the three results
on fattened animals, computed to 800 kgs. live weight, show a
range of 2420 Cals., or 12.16 per cent, of the average. Moreover,
in making up the average of the unfattened animals, one animal
was excluded on the ground that the results were probably abnor-
mally high, but the same animal is subsequently included among
the three fattened animals the results on which are averaged.

Even after making all allowances for these facts, however, the
results for the fattened animals are decidedly higher relatively

NET AVAILABLE ENERGY— MAINTENANCE.

443

than for the unfattened, but how much higher can hardly be deter-
mined from such averages.

Comparing the results on the one animal common to the two
series of experiments we have —

Live Weight,
Kgs.

Maintenance,
Cals.

Observed:

Unfattened

611.5
750

800
800

16,835.6
18,959.6

20,140
19,800

Computed to 800 kgs.:

Unfattened

Fattened

According to the above figures the maintenance ration of this
animal was practically proportional to the two-thirds power of its
live weight. On the other hand, however, its maintenance require-
ment in the unfattened state was much higher than the average
for the seven unfattened animals, while after fattening it did not
difler materially from the average for the three fattened animals.
If, then, we are to regard the above result as correct we must
assume that by chance all three of the fattened animals had a
higher normal rate of metabolism than the seven unfattened ones,
which is not exactly probable. Although this leaves the question
in a rather unsatisfactory state, it would seem that we must be
content to let it rest there pending further comparative experi-
ments on identical animals in different stages of fattening.

CHAPTER XIII.
THE UTILIZATION OF ENERGY.

According to the conceptions discussed in the preceding chap-
ter a certain portion of the metabolizable energy of the food is
consumed in what has been called in a broad sense the work of
digestion and assimilation, while the remainder constitutes net
available energy and contributes to the maintenance of the store of
potential energy in the body. If the food is sufficient to supply
net available energy equal to that dispensed by the internal work
of the body, the balance between income and expenditure of energy
is just maintained. If we increase the food beyond this maintenance
requirement we supply the body with an excess of net available
energy. In general terms we can say that this excess may be
disposed of in two ways : it may be utilized for the peformance of
external work, or it may give rise to a storage of potential energy
in the body in the form of new tissue,* particularly of fat tissue.
It appears probable, however, that neither of these processes takes
place without more or less loss of energy in the form of heat.
This is certainly true of the performance of muscular work, as has
already been mentioned (p. 189) and as will be shown in detail
on subsequent pages. Out of the total potential energy of the
material metabolized rather more than one third, in the most fav li-
able case, is actually recovered in the form of external work, the
remainder taking the form of heat. In this case, then, we might
speak of the coefficient of utilization of the energy as being about
one third.

In the utilization of surplus energy by storage of tissue it
appears likely that there must be also a loss of energy, although,

* From this point of view the production of milk is to be regarded as
the formation of new tissue.

444

THE UTILIZATION OF ENERGY. 445

as will appear later, we are not yet in a position to make any such
definite statements regarding its amount as in the case of muscular
work, and although the writer's few results on timothy hay cited
on p. 424 afford no indication of such a loss, the utilization of the
metabolizable energy for the production of gain seeming to have
been practically equal to its net availability. It is obvious, how-
ever, that the conversion of the resorbed nutrients of the food into
the ingredients of tissue involves profound chemical changes, and
we can hardly suppose that these take place without some evolution
of heat. As a good illustration we may take the case of a carbo-
hydrate. As resorbed into the blood it appears to be in the form
of a sugar, and it would seem that this sugar can serve, without any
very extensive chemical changes, to sustain the metabolism incident
to the internal work of the body; that is, that it is oxidized more
or less directly in the various tissues to supply energy for their
physiological work. When, however, a surplus of a carbohydrate
is to be utilized for the storage of energy in the form of fat, the case
is different. The formation of fat from a carbohydrate is chemi-
cally a process of reduction, and the oxygen which is removed
from the carbohydrate must unite with the carbon and hydrogen
either of other molecules of the carbohydrate or cf other in-
gredients of food or tissue, in either case giving rise to an evolu-
tion of heat. If we suppose the transformation to take place
according to the equation given in Chapter II (p. 24), the re-
sulting fat would contain about 87 per cent, of the energy of
the dextrose. Whether this percentage expresses the actual facts
of the case or not, it is very improbable that this or any similar
synthetic process takes place in the body without the evolution
of some heat.

Provisionally, then, we seem justified in assuming that only a
part of the net available energy supplied to the organism above
the maintenance requirement can be utilized to increase the store
of potential energy in the body, and we may speak in this case, as
in that of muscular work, of the coefficient of utilization. Repro-
ducing here the essential parts of the graphic representation
on p. 410, we may now complete it so as to represent in a general
and qualitative way the relations indicated above, assuming pro-
visionally that the effects are linear functions of the food. As

446

PRINCIPLES OF ANIMAL NUTRITION.

before, OG represents the fasting metabolism at a temperature
below the critical point and OM the critical amount of food at this
temperature. Then the line GKS represents the availability of
the food, HLS' the heat production, and OS the maintenance
requirement. Beyond the point S we may assume that the net
availability of the food remains the same, represented by the line
ST. But a fraction of this net available energy, however, can be

recovered as mechanical work, and its utilization will therefore be
represented by some such line as SV, while the heat production will
be correspondingly increased as represented by S'V. Similarly
the proportion of the net available energy which in the quiescent
animal is stored up in the form of new tissue may be expressed by
a line SU and the corresponding heat production by S'V . What
the relation between the proportions utilized in the two cases is we
do not know, and the diagram is intended to be simply schematic;

'IHE UTILIZATION OF ENERGY. 447

Dut we do know that the proportion is materially greater in the
latter case, since the heat production of a fattening animal is ob-
viously much less than that of a working animal utilizing the same
amount of food.

In the following pages the attempt has been made to bring
together the more important experimental evidence bearing upon
the utilization of food energy for the production of tissue and of
work. Before, however, proceeding to a consideration of our present
knowledge upon the subject, attention should be called once more
to the fact that we are here dealing with it from the statistical point
of view of the balance between income and expenditure of energy
of the body.

In an animal performing work, each muscular contraction
metabolizes a certain quantity of energy, part of which finally
appears as heat and part as mechanical work. Besides this, how-
ever, a secondary result is an increase in the activity of the organs
of circulation and respiration which requires the expenditure of a
certain amount of energy, this energy ultimately taking the form
of heat and being added to that resulting directly from the activity
of the skeletal muscles. When we compare the actual external
work done with the total energy metabolized for its performance,
and so compute the coefficient of utilization, we group all these
sources of heat production and regard them as, from the economic
standpoint, a waste of energy, just as in a heat engine the energy
which escapes conversion into work is regarded as waste energy not-
withstanding the fact that the loss is inevitable. So, too, in the pro-
duction of new tissue we look upon total gain of potential energy
by the body as constituting the net useful result of the feeding,
and the coefficient of utilization in this case, as in that of muscular
work, would express the relation which this bears to the net avail-
able energy supplied in the food. That the effect of abundant,
food may be in some cases to stimulate the metabolism of tissue or
the " incidental " muscular work (p. 342) is rendered probable by
Zuntz & Hagemann's results with the horse (see p. 376). All these
effects are part of the necessary expenditure of energy by the body,
and however interesting physiologically are statistically sources of
loss.

448 PRINCIPLES OF ANIMAL NUTRITION.

§ i. Utilization for Tissue Building.

Under this head we have to consider almost exclusively ex-
periments upon the fattening of mature animals. While the growth
of young animals and the production of milk are both forms of
tissue building, the experimental data available seem too scanty
to justify including them in the scope of the present work. For
convenience we may first bring together the recorded results and
later discuss them in their more general bearings.

One difficulty, however, is encountered at the outset in our
inadequate knowledge of the net availability of nutrients and
feeding-stuffs, as pointed out in the foregoing chapter. Until this
gap is filled it is of course impossible to compare the gain of energy
by the body with the supply of net available energy. Accordingly
the results of the experiments upon productive feeding can at present
be utilized only to determine what proportion of the metabolizable
energy of the food is recovered in the gain of tissue, and the experi-
ments cited in the following paragraphs will be considered from
this point of view.

Experimental Results.

Experiments on Carnivora. — In connection with the dis-
cussion of net availability in the preceding chapter a number of
experiments were cited (p. 428) in which more or less gain was
made by the animals. In addition to these Rubner * has made a
preliminary report of investigations upon the effect of abundant
feeding on the heat production A dog weighing 25 kgs received
successively isodynamic amounts of lean meat, fat, and carbo-
hydrates (kind not stated) equivalent to 155 per cent, of its fasting
metabolism, a two-days' fast intervening in each case between the
different rations. Few details are given, but presumably the
methods were those of Rubner's other experiments already de-
scribed (compare p. 253). In a second experiment the effects of
two different amounts of meat were also compared In the follow-
ing table the results of these experiments have been put into the
same form as those on net availability in the preceding chapter, the
data given being per day and head:

* Sitzungsber. k. bayer. Akad. der Wiss., Math -ph vs. Classe, 15, 452

THE UTILIZATION OF ENERGY.

449

Metabnlizable

Energy of

Food.

Cals.

Total Gain.
Cals.

Gain Over Fasting
Metabolism.

Total,
Cals.

Per Cent, of

Energy of

Food.

Nothing

Fat

Carbohydrates

Meat •]

0
1549
1549
1549
1463
2181

-944
+ 540
+ 509
+ 418
+ 332
+ 805

1484
1453
1362
1276
1749

95.8
93.8

87.9
87.2
80.2

Experiments by Gruber * upon the formation of fat from pro-
teids (see p. 112) afforded the following results, computed f by
the use of the factors given on p. 414:

Nothing

1500 grms meat

1st series. .

2d series . . .

Average . .

Metabolizable

Energy of
Food

Cals.

Total Gain
Cals.

1325
1325

1325

-743

250
296

273

Gain Over Fasting
Metabolism.

Total

Cals

993
1039

1016

Per Cent, of

Energy of

Food.

74.9
78.4

70.7

The difficulty in interpreting these results as well as those tabu-
lated on p. 428. as already stated, lies in our imperfect data regard-
ing the net availability of the materials below the point of mainte-
nance. Rubner, in discussing his results, assumes an availability
of 100 per cent., or in other words that the fasting metabolism is
the measure of the amount of metabolizable energy required for
maintenance. He accordingly subtracts this amount from the
total metabolizable energy of the food and regards the remainder
as excess food, which may be utilized for the storage of energy.
The percentage utilization of this excess was as shown in the follow-
ing table, to which Gruber's results, computed by the writer in the
same way, have been added:

* Zeit f Biol , 42, 409.

f From the last two complete days of each series

45°

PRINCIPLES OF ANIMAL NUTRITION.

Mainte-

Metab ■
olizable

nance Re-
quirement

Excess

.

Percent
age

Energy of

(Fasting

Food

Food
Cais.

Metab-
olism),
Cals.

Cals.

tion.

Fat

1549
1549

944
944

G05
605

540
509

89.3

Carbohydrates

84.1

Meat :

(

1549

944

605

418

69.1

1463

944

519

332

63.9

|

2181

944

1237

805

65.1

1325

743

582

250

43.0

1325

743

582

296

50.9

As was shown in the preceding chapter, however, while the
recorded determinations of net availability are far from satisfactory
they show with a considerable degree of probability that there is
some loss of energy below the maintenance point and that 100 per
cent, of net availability is at least not ordinarily reached. A lower
net availabilty, however, means a larger maintenance requirement,
and this in turn results in a larger computed percentage utilization
of the excess food.

In the following table the latter percentage has been computed
by the writer for most of the experiments tabulated on p. 428, as
well as for those of Rubner and Gruber just cited, on the assump-
tion that the net availability below the maintenance requirement
was :

Meat 85 per cent.

Fat 98 " "

Starch 90 " "

Cane sugar 96 "

The factor for meat is the average of all the results on p. 427;
that for fat is based on Magnus-Levy's results upon digestive work;
those for starch and cane-sugar are the averages of Rubner's re-
sults, omitting those which exceed 100 per cent. By dividing the
fasting metabolism by the above percentages we may compute the
amount of metabolizable energy required for maintenance on the
above assumption, while subtracting this from the metabolizable
energy of the food leaves the amount of excess food, which can be
compared with the observed gain.

THE UTILIZATION OF ENERGY.

45 1

Fasting

Metab-
olism.
Cals.

Metab- I Com.
olizable putec]
Energy Main-
,,"' , tenance
Food, Cals.
Cals.

Excess
Food ,
Cals.

Gain,
Cals.

Per-
centage
Utiliza-

• tion.

Proteids (meat) :

Pettenkofer & Voit

[

Rubner ■{

I
Gruber j

Fat:

Pettenhofer & Voit -1

Rubner I

Starch :

Pettenkofer & Voit

Rubner ("carbohydrates", as-
sumed to be starch)

i 'ane-sugar :

Rubner

Cane-sugar and Starch (93 per
cent, availability) :
Rubner

1041
261
944
944
944
743
743

1325

347
1549
1463
2181
1325
1325

1086 3298
554*j 942
554* 1884

658
466
261
944

1098

1098

554"

944

451

302

1738
942
348

1549

2015
3076

874

1549

572

702

1225

307

1169

1169

1169

920

920

1108
565
565
671
476
266
963

1220

1220

616

1049

470

325

100

38

40

13

380

418

294

332

1012

805

405

250

405

296

2190

878

377

329

1319

837

1067

1016

466

428

82

49

586

540

795

353

1S56

853

258

137

500

509

102

190

377

365

38.0
32.5
110.0
112.9
79.6
61.7
73.1

40.1
87.3
63.5
95.2
91.8
59.8
92.1

44.4
46.0
53.1

101.8

186.3

96.8

While as a whole the results of the computation would seem to
indicate that the percentage utilization for tissue building is less
than the percentage availability, the remarkable range of the
figures and the uncertain basis upon which they are computed do
not encourage any attempt at a critical discussion.

Experiments on Man. — The only respiration experiments upon
man which the writer has been able to find in which any large
amount of excess food was given are those of Johansson, Lander-
gren, Sonden & Tigerstedt f already cited on p. 383 in their bearing
on the subject of digestive work. If we assume, on the basis of

* Loss on basal ration.

t Skand Arch. f. Physiol., 7, 29.

452

PRINCIPLES OF ANIMAL NUTRITION.

Magnus-Levy's results, that the work of digestion in man equals
about 9 per cent, of the metabolizable energy of the food, the
average results of the experiments are as follows:

Fasting metabolism 2022 . 4 Cals.

Metabolizable energy of food 4193.4 "

Computed maintenance requirement 2222.5 "

Excess food 1970.9 "

Gain 1676.0 "

Percentage utilization 85 . 0 per cent.

The computation gives a somewhat lower percentage for the
utilization of the excess food than that assumed for the availability
of the maintenance food.

Experiments on Swine. — Meissl, Strohmer & Lorenz * in their
investigation upon the sources of animal fat made six respiration
experiments with swine, the results of which afford some data as to
the utilization of their food by these animals. In Experiments V
and VI, made on two different animals, no food was given, and the
following results were obtained, the energy equivalent to the loss
of tissue being computed as in Rubner's experiments in the pre-
vious chapter:

Experi-
ment.

Tempera-
ture.
Deg. C.

Hours

Since

Last

Feeding.

Live

Weight,
Kgs.

Loss of

Nitrogen.

Grms.

Loss of
Carbon,
Grms.

Total
Metab-
olism,
Cals.

Metab-
olism per
100 Kgs.

Live

Weight.t

Cals.

V

VI.... \

20
20
20.4

24
12

72

140
120
120

9. SO
9.55
6.77

224.51
375.78
194.93

2607
2291

2083
2029

The experiment begun only twelve hours after the last feeding
obviously gave too high results, owing to the presence of food in
the digestive canal. That this source of error was substantially
eliminated after twenty -four hours appears probable from the close
agreement of the results with those obtained after seventy-two
hours. The average fasting metabolism per 100 kgs. live weight
is 2056 Cals. and this average has been made the basis of the com-
putations which follow, except in Experiment I. This experiment

* Zeit. f. Biol., 22, 63.

t Assumed to be proportional to the two-thirds power of the weight.

THE UTILIZATION OF ENERGY. 453

having been made on the same individual as Experiment V, the
result of the latter is used directly.

In Experiments I and II the ration consisted of rice, in Experi-
ment III of barley, and in Experiment IV of rice, flesh-meal, and
whey. In all cases large amounts of food were consumed and a
rapid production of fat was observed. The digestibility of the food
was determined. Its metabolizable energy has been computed by
the writer from the results of the digestion experiments by the use
of the following factors:*

1 gram digestible protein 4.1 Cals.

1 " nitrogen-free extract. . . 4.2 "

1 " " crude fiber 3.5 "

1 " " ether extract 8.8 "

No attempt was made in these experiments to determine the
methane, if any existed, in the respiratory products. The results
per day and head were as follows:

Experiment.

Tem-
perature,
Deg. C.

Live

Weight ,

Kgs.

Computed

Fasting

Metabolism,

Cals.

Metab-
olizable
Energy.
Cals.

Energy of
Gain, Cals.

Nutritive
Ratio.

I

II

Ill

IV

18.0
18.5
19.3
16.7

140

70

125

104

2607
1621
2386
2111

7157
7167
5125 f
6129

3464
4048
1774
2556

1 :15.4
1 :14.1
1 : 9.3

1 : 2.4

No determinations were made of the actual requirements for
maintenance as distinguished from the fasting metabolism, and
hence the data are lacking for a computation of the net availability
of the metabolizable energy of the food on the one hand and the
percentage utilization of the excess food on the other. Cooke's re-
sults mentioned on p. 438, however, seem to give some indication
that the maintenance demand of swine may not be greatly in excess
of the fasting metabolism. If in these experiments we assume the
same net availability as that just assumed in the case of man, viz.,
91 per cent., we obtain the following figures:

* Compare p. 332.

t In this experiment the ether extract of the feces exceeded that of the
food by 23.95 grms. This excess has been assumed to have a heat value
of 4.2 Cals. per grm. and a corresponding amount deducted from the com-
puted energy of the other digested nutrients.

454

PRINCIPLES OF ANIMAL NUTRITION.

Food.

Com-

Com-

Metab-

puted

Nutri-

puted

olizable

Main-

tive

Fasting

Energy

tenance

Ratio.

Metab-

of

Re-

1-

olism,

Food,

quire-

Cals.

Cals.

ment,
Cals.

15.4

2607

7157

2865

14.1

1621

7167

1781

9.3

2386

5125

2622

2.4

2111

6129

2320

Excess
Food,
Cals.

Gain,
Cals.

Per-
cent-
age
Utiliza.
tion.

I

II

III

IV

Rice

Barley

Rice, flesh-meal,
and whey

4292
5386
2503

3809

3464
4048
1774

2556

80.7
75.2
70.9

67.1

It is interesting to note that the utilization as thus computed
diminishes as the proportion of protein in the ration increases, a
result which the low average figures obtained on pp. 427 and 450
for the availability and the percentage utilization of the proteids
would lead us to expect. Obviously, however, too much value
should not be attached to such computations as the above.

Kornauth & Arche * in an investigation on the feeding value of
cockle have also made respiration experiments with a swine. The
food consisted in Period II of cockle, barley, and maize, and in«
Period III of rape-cake, barley, and maize, the amounts of the
several nutrients actually digested being nearly the same in the
two periods. In each period two respiration experiments were
made which gave concordant results. The following table contains
the average results for each period computed on the same basis as in
the experiments of Meissl, Strohmer & Lorenz. No fasting experi-
ments having been made, the average results of the experiments by
the last-named authors have been used, the average live weight of
50 kgs. being taken as the basis.

J

s

&

Food.

Nutri-
tive
Ratio.
1.

Esti-
mated
Fasting
Metab-
olism

Cals.

Metab-

olizable

Energy

of

Food,

Cals.

Com-
puted
Main-
tenance

Re-
quire-
ment,
Cals.

Excess
Food,
Cals.

Gain,
Cals.

Per-
centage
Utiliza-
tion.

II

Cockle, barley, and
maize

6.7
6.2

1296
1296

3057
3101

1424

1424

1633
1677

1170
1095

71.7

III

Rape-cake, barley,
and maize

65.3

* Landw. Vers. Stat., 40, 177.

THE UTILIZATION OF ENERGY. 455

The percentages as thus computed are seen to agree fairly well
with the ones computed for those of Meissl, Strohmer & Lorenz's
experiments in which the proportion of protein in the food was
similar.

Experiments on Ruminants.— Experiments upon ruminants
necessarily differ in one important respect from those hitherto con-
sidered. With carnivora and with swine it is possible to determine
the fasting metabolism, or, in other words, to trace the line repre-
senting the net availability or the utilization throughout its entire
extent. With herbivora, and particularly with ruminants, this is
practically impossible, for obvious reasons, and the course of the
lower portion of the line is imaginary. This, however, is no obsta-
cle to a determination of the net availability or percentage utiliza-
tion of the food within the limits as to amount prescribed by the
nature of the animals. As is clear from the graphic discussion of
the problem on pp. 410 and 446, all that is necessary is to determine
the gain or loss of energy by the body corresponding to two different
amounts of food above or below maintenance. A simple com-
parison of differences then gives in the one case the percentage
utilization and in the other the net availability of the energy of the
food added.

The Mockern Experiments. — The very extensive and elabo-
rate investigations upon cattle at the Mockern Experiment Station
by G. Kiihn and Kellner,* which have already been discussed in
relation to the metabolizable energy of the food, are also our chief
source of knowledge regarding the utilization of this energy by
ruminants and will necessarily constitute the principal basis of the
present discussion .f

These experiments were chiefly upon the fattening of mature
cattle, various additions being made to basal rations which were
themselves in almost every case more than sufficient for maintenance.
The actual gain of carbon and nitrogen by the animals, both on the
basal and the augmented rations, was accurately determined, and
from the data thus obtained the gain of proteids and fat and of
energy was computed in the usual way. By a comparison of the

* Landw. Vers. Stat, 44, 257; 47, 275; 50. 245; 53, 1.

t For a summary of important later results and a full discussion of the
subject, compare Kellner, Die Ernahrung der landwirtschaftlichen Nutztiere,
Berlin, 1905.

45 6 PRINCIPLES OF ANIMAL NUTRITION.

gains on the basal and on the augmented ration, then, we may deter-
mine what proportion of the metabolizable energy of the added
food was stored in the gain of tissue. In other words, we may
determine two points on the line SU in the figure on p. 446,
thereby determining the line if it is a straight line.

If the added metabolizable energy of the larger ration were de-
rived solely from the material added, the result would show the utili-
zation of the energy of that material. As we have seen, however, in
connection with the discussion of the metabolizable energy of the food
in Chapter X, this is rarely if ever the case with herbivorous animals.
The difference in metabolizable energy between two rations usually
includes, in addition to the real metabolizable energy of the added
food, differences in the digestibility of the original ration and in
the losses in urine and methane. Accordingly, we are here con-
fronted with the same alternative as before, viz., whether to attempt
to eliminate these secondary effects and base our computations
on the real metabolizable energy of the feeding-stuff under experi-
ment or to take the apparent metabolizable energy as representing
the actual amount of energy contributed to the metabolism of the
body. In the one case, if successful, we shall obtain a result which
will be physiologically correct but which when applied in practice
will require modification for the secondary effects just mentioned.
In the other case we shall have a summary expression including
all these results, but with the disadvantage of being more or less
empirical in its nature. Either method has its advantages and
disadvantages. In the present case we shall use the apparent
metabolizable energy of feeding-stuffs as computed on pp. 285-297
and in Tables I-VI of the Appendix as the basis of computation.
This does not, of course, affect the absolute amount of energy
utilized from a unit weight of the material, but only the percentage
calculated upon the metabolizable energy.

Sources of Uncertainty in Computation. — While the computation
of the energy utilized from feeding-stuffs in the manner just indi-
cated is in principle very simple, certain complications arise in its
execution from the impossibility of securing exactly comparable
conditions of experiment. Two of these in particular require
consideration here.

Differences in Organic Matter Consumed. — As was noted in the

THE UTILIZATION OF ENERGY. 457

discussion of the metabolizable energy of feeding-stuffs, the un-
avoidable slight variations in the moisture-content of the latter
in the Mockern experiments resulted in slight differences in the
amounts of organic matter of the basal ration consumed in the
several periods. A comparison, then, between two periods, as re-
gards metabolizable energy and resulting gain, shows the effect of
the added feeding-stuff plus the effect of this small difference.
For the metabolizable energy an approximate correction was com-
puted. In order to make a similar correction in the resulting gain
of tissue, however, it is necessary to know to what extent this
difference in metabolizable energy contributed to the observed
gain; that is, to know the percentage utilization of the basal ration.
No direct determinations of this factor, however — that is, no com-
parisons of the results of feeding different amounts of the basal
ration — were made. In his discussion of the results Kellner virtually
assumes a percentage of utilization by subtracting from the total
metabolizable energy of the food the average amount required for
maintenance as determined by his own experiments and then com-
paring the energy in excess of the maintenance requirement with
the resulting gain.

Differences in Live Weight. — The live weights of the animals in
the Mockern experiments differed considerably in the different
periods. This would probably result in differences in the require-
ments for maintenance, although the data at hand seem insufficient
to satisfactorily determine the relation between live weight and
maintenance (see p. 441). Kellner assumes that the maintenance
ration is in proportion to the two-third power of the live weight, a
result which has already been shown to correspond fairly well with
the results upon Ox B, although in apparent conflict with the aver-
age results obtained on other animals.

Utilization of Basal Ration. — In order to be able to correct the
results for differences in organic matter consumed and differences
in live weight, it is necessary, as has just been pointed out, to know
the percentage utilization of the basal ration. This Kellner assumes
in assuming a maintenance ration. There are, however, serious ob-
jections to this method of procedure. First, the maintenance ration
used by Kellner is an average, based on results which were obtained
with a number of animals, not including all those used in the fatten-

458 PRINCIPLES OF ANIMAL NUTRITION.

ing experiments, and which show a range of 13.5 per cent, of the
average. Second, the computed maintenance ration is based upon
experiments with coarse fodder only. We have seen reason to be-
lieve, however (pp. 388-391) that the net availability of the metabo-
lizable energy in coarse fodders is decidedly less than in case of con-
centrated feeds, and that consequently more metabolizable energy
would be required for maintenance on a ration composed of coarse
fodder than on one containing concentrated feeds, as did Kellner's
basal rations. In other words, Kellner's assumed maintenance
ration is probably somewhat too large and his computed utiliza-
tion of the basal ration, therefore, also somewhat too high. Third,
it is by no means demonstrated that the maintenance ration of
fattened as compared with unfattened animals is, as Kellner as-
sumes, in proportion to the two-third power of the live weight.

In the absence of any direct determinations of the utilization
of the basal rations, however, there seems to be no course open but
to follow substantially Kellner's method of computation and assume
a maintenance ration for each of the animals in proportion to the
two-thirds power of its live weight during the period under con-
sideration.

Computation of Results. — The method of computing the correc-
tions for the differences in live weight and in the amount of the
basal ration consumed may be illustrated by the same two periods
which were used on pages 288-9 to exemplify the computation of
metabolizable energy, viz., Periods 4 and 7 with Ox H, on meadow
hay. In Period 4, on the basal ration, the live weight was 668.9
kgs., the computed maintenance requirement 13,989.1 Cals., and
the gain by the animal 2003.2 Cals. The percentage utilization
therefore was as follows :

Metabolizable energy of ration 17,388.8 Cals.

Computed maintenance requirement . . 13,989.1 "

Excess food 3399.7 Cals.

Gain 2003.2 "

Percentage utilization 58.9 %

In Period 7 the total metabolizable energy of the ration was
26,013.0 Cals. and the gain 5643.2 Cals. Of the excess of 8624.2

THE UTILIZATION OF ENERGY.

459

Cals. over Period 4, however, it was computed that 119.4 Cals. were
due to an increased consumption of the ingredients of the basal
ration, leaving 8504.S Cals. as the metabolizable energy of the
added hay. This 119.4 Cals., however, contributed to the increased
gain of 3640.0 Cals. made by the animal. If we assume the per-
centage 5S.9 just computed to apply to it, the corresponding gain
would be 119.4 X 0.5S9 or 70.3 Cals., leaving 3569.7 Cals. as the
gain produced by the 8504.8 Cals. of metabolizable energy derived
from the meadow hay.

In Period 7, however, the animal weighed 736.0 kgs., and his
computed maintenance requirement was therefore 14,909.6 Cals. of
metabolizable energy, or 920.5 Cals. more than in Period 4. In
other words, if he had weighed no more in Period 7 than in Period
4, there would have been 920.5 Cals. more metabolizable energy
which could have served to produce a gain of tissue. Assuming, as
before, that 58.9$ of this would be stored in the body, the result-
ing gain would have been 920.5 X 0.589 or 542.2 Cals. Adding this
to the gain of 3569.7 Cals. just computed makes a total of 4111.9
Cals. as the computed gain to be credited to 8504.8 Cals. of metab-
olizable energy in the hay added, which is equivalent to a per-
centage utilization of 48.4 per cent. Expressed in tabular form,
the results of these comparisons are as follows: —

3

a

<

11

11

— '

c

5
-

7
4

Average

Live W
Kgs.

Metabo-
lizable

Energy,

1 ,[

Com-

] m ted
Mainte-
nance,
Cals.

Excess
over
Mainte-
nance,
Cals.

Energy
0 i Gain

(Cor-
rected 1,

( lals.

8, c ^

M O <"

$■■8 £

Meadow Ha y . VI:

Basal ration + hay

Correction for organic matter

736.0
668. 9

26.013.0
-119.4

14,909.6
13,989.1

11,103.4

-119.4

10,984.0
4-920.5

5,643.2

-70.3

5.572.9
I 542.2

Correction for live weight . . .

25,893.6

17,388.8
8,504.8

1 1 .904 . 5
3,399.7

6.115.1
2.003.2

58.9

8,504.8

4,111.9

48.4

Table VII of the Appendix contains the details of the computa-
tions of percentage utilization according to the above method.
The results differ somewhat from those reported by Kellner *
* hoc. cit., pp. G3, 133, 226, and 334.

460 PRINCIPLES OF ANIMAL NUTRITION.

first, because they include a correction for the differences in or-
ganic matter consumed, and second, because the energy of the gain
has been corrected for the amount of nitrogen retained in the body
in the same manner as the energy of the urine (compare p. 285),
viz., by deducting 7.45 Cals. per gram of nitrogen. In most cases
the metabolizable energy is that already computed in Tables I to
VI of the Appendix, being based on actual calorimetric determina-
tions in food and feces. In two instances (distinguished by being
bracketed) the metabolizable energy has been computed by the
writer from such data as are available.*

In Table VII the final results are expressed as percentages of
the metabolizable energy utilized. By combining them with the
results contained in the six preceding tables of the Appendix they
may likewise be expressed as percentages of the gross energy of
the several materials and also as energy utilized per gram of total
organic matter. The summary on pp. 461-2 contains the results
expressed in all of these ways.

Earlier Experiments. — The earlier respiration experiments
of Henneberg & Stohmann f on oxen, in 1S65, while made in accord-
ance with the experience then available, are now known to be de-
fective in several respects. The respiratory products were deter-
mined for twelve hours only, while the same authors subsequently
showed that twenty-four hours was the minimum time necessary.
The food consumed on the respiration day was less than the average
for the whole experiment, but how much less does not appear, and
finally the methods used for the determination of the hydrocarbon
gases excreted have subsequently been shown to give too low re-
sults. It seems useless therefore to enter into an elaborate com-
putation of the results. In the later experiments of the same
authors % with sheep, these sources of inaccuracy were largely re-

* The data used in these computations are as follows .

For Ox IV the average results for Periods la and \b have been com-
puted on the assumption that the heat values of food and excreta per gram
in Period 1/; wore the same as those determined in Period la

For Ox V the metabolizable energy in Period 3 has been computed by-
adding to that in Period 2a 3 345 Cals. for each gram of organic matter in
the starch added, this being the metabolizable energy computed for the
starch in Period 2a.

t Neue Beitriige, p 287. J hoc. cit , p 68.

THE UTILIZATION OF ENERGY.

461

ENERGY UTILIZED.

Per Cent, of

Metabo-

lizable

Energy.

Per Cent, of

Gross

Energy.

Per Grm.

Total

Organic

Matter, t'als.

Meadow Hoy :

Sample V, Ox F

40.4
36.2

16.5
15.9

0 780

V, " G

0.756

Average

38.3

50.4
48.4
34.8

16.2

26.5
26.1
18.5

0.768

Sample VI, Ox H, Period 2

1.266

VI. " H, " 7...

1.247

VI, " J

0.883

44.5
41.4

38.8
33.4

23.7
20.0

14.2
11.7

1.132

Average V and VI

0.950

Oat Straw :
Ox F

0.682

" G

0.564

Average

36.1

10.8
24.0

12.9

3.2

7.8

0.623

Wheat Straw :

Ox H

0.153

" J

0.373

17.4

67.3
58.6

5.5

51.6
43.6

0.263

Extracted Rye Straw :

OxH

2.194

" J

1.854

Average

63.0

58.5
83.4
50.2

47.6

41.6
65.9
36.5

2.024

eel Molasses :
Sample I, Ox F

1.700

II, " H

2.760

II, " J

1.529

Average

66.8

50.0
49.2

51.2

35.6
31.3

2.145

Stirrh — Kiihn's Experiments :

Sample I. Ox III

1.514

I, " IV

1.331

Average

49.6

53.2
53.7
59.7

48.1
40.0

33.5

42.0
40.1

34.3
32.6

1.423

Sample II. Ox A". Period 2a

II. " V, " 26

II. " V, '• 3

" II. " VI, " 2b

II. " VI, " 3

1.779
1.699

1 . 452
1.380

Average

50.4
50.0

37.3
35.4

1.578

Average I and II

1.501

462

PRINCIPLES OF ANIMAL NUTRITION.

ENERGY UTILIZED {Continued).

Per Cent, of
Metabo-
lizable
Energy.

Per Cent, of

Gross

Energy.

Per Grm

Total
Organic

.Matter

Starch — Kellner's Experiments .

Sample I and II, Ox B. . . .

" I " II, " C....

Average.

Sample III, Ox D.

Ill, " F.

" III, " G.

Average.

Sample IV, Ox H .
" IV. " J.

Average

Average III and IV

Wheat Gluten — Kiihn's Experiments :

Ox III, Period 3

" III, " 4

Average.
Ox IV ... .

Wheat Gluten — Kellner's Experiments :

Sample I, Ox B, Period 1

I, " B, " 3

" I, " C

Average

Sample II, Ox D

Average of I and II.

Peanut Oil .

Sample I. Ox D

II. " F

" II, " G

Average

65.4
57.6

61.5

53.7

64.8
65.8

61.4

56.0

54. S

55.4
58.4

45.3

48.0

46.7

58.2

36.9
49.7
43.2

43.3
37.3
40.3

51.6
65.1
69.4

67.3

31.8
28.0

29.9

36.1
46.2
50.9

44.4

44.4
39.5

42.0
43.2

37.0
35.8

36.4
58.9

19.6
32.6
30.9

27.7
26.1
26.9

40.1
34.2
41.2

37.7

1.325
1.168

1.247

1.500
1.922
2.116

1.846

1.855
1.652

1 . 754
1.800

2.2S9
2.213

2.251
3.645

1.115

1.849
1.850

1.605
1.516
1.561

3.811
3.238
3.903

3.571

THE UTILIZATION OF ENERGY. 463

moved, but the experiments were upon maintenance feeding only
and afford no data for a computation of utilization.

A series of respiration experiments on sheep was made by
Kern A: Wattenberg at the Gottingen-Weende Experiment Station
in L879, the results of which were reported after Kern's death by
Henneberg & Pfeiffer.* Varying quantities of nearly pure proteids
(conglutin or flesh-meal) were added to a basal ration of hay and
barley meal, the amount of proteids in the ration being regularly
increased by about 50 grams in each of four successive periods and
then similarly diminished through three more periods.

The experiments suffered from some defects in technique which
later experience has remedied, the results most strikingly affected
being those for the amount of methane excreted. For the first
two periods no results are reported; for the remaining periods they
are quite variable, and those on different days of the same period
differ widely. The authors consider that their figures represent
the minimum amount present, and in their final computations use
the average of all the five periods as the basis for estimating the
quantity of carbon excreted in this form. The amounts as actually
determined showed a considerable diminution in the periods in
which most proteids were fed, contrary to Ktihn's results, but it is
worthy of note that the average proportion of carbon dioxide to
methane was not much different from that found by the latter.
The determinations of carbon dioxide in the respiratory products
likewise showed considerable fluctuations from day to day, but as
the results are mostly the average of three or four trials of twenty-
four hours each it may be assumed that these variations are more
or less compensated for. The respiratory products were determined
for both animals together, although ail the other data were secured
for each individual. The results given on the following pages,
therefore, are the totals for both animals.

It is stated that addition of proteids to the ration resulted in
the diminution, and final disappearance in the middle period, of the
hippuric acid of the urine, but the actual amounts present are re-
ported only for the first and last periods. It is not possible, there-
fore, to make any satisfactory computation of the energy of the
urine or of the proper factor for the nietabolizable energy of the
digested proteids of the total ration. By another method of com-
* Jour. f. Landw., 38, 215.

464

PRINCIPLES OF ANIMAL NUTRITION.

putation, however, it seems possible to secure an approximate idea
of the relation of added food to gain.

By subtracting from the food digested in Periods 1 1- VI the
average amount digested in Periods I and VII, on the basal
ration, we find the amounts of added food, consisting chiefly of
proteids. Reckoning the metabolizable energy of the added pro-
teids at 4.958 Cals. per gram (compare p. 317), that of the crude
fiber and nitrogen-free extract at 3.674 Cals., and that of the ether
extract at 8.322 Cals., we get the approximate metabolizable energy
of the added food, and can compare it with the energy of the corre-
sponding gain. Thus for Period II we have the following:

DIGESTED.

Protein.*

Grms.

Crude
Fiber,
Grms.

Nitrogen-
free
Extract,
Grms.

Ether

Extract

Grms.

Period II

211.33
101.05

280 . 77
277.91

643.22
633.12

20.88

Periods I and VII

21.60

Difference

110.28

Cals.
546.8

2.86

10 10

-0.72

Equivalent metabolizable
energy

12

Cals.

47.6

.96

Cals.

-6

* Protein of basal ration and of feces equals N X 6.25; that of conglutin
or flesh-meal equals its total organic matter.

GAIN.

Protein Grms.

Fat. Grms.

Period 11

15.00
6.85

69.27
19.66

Periods I and VII. . .

Difference

Equivalent energy

8.15

Cals.

46.3

49.61
Cals.

471.5

The figures for the gain are those given by the authors, based
on the assumption of a uniform excretion of methane throughout
the experiments; the gain of protein includes that contained in the
wool produced. The animals gained slightly in weight, in addition
to the growth of wool. Computed on this basis, the percentage of
the energy of the added food which was utilized was as follows:

THE UTILIZATION OF ENERGY.

465

Period.

Metabolizable

Energy of

Added Food,

Cals.

Energy of

Resulting Gain.

Cals.

Per Cent.
Utilized.

( II

Conglutin: - III

( IV

588.4
1100.3
1639.2
1131.7

454.9

517.8
741.8
1106.8
672.5
315.7

88.00
67.42
67 51

Flesh- meal: j y.

59.41
69.39

A computation based on the observed amounts of methane
would affect the above figures in two ways. First, if the added
proteids diminished the production of methane, this was equivalent
to an increase in the apparent metabolizable energy of the food,
and the figures for the latter must be correspondingly increased.
Second, the gain of fat will also appear relatively greater in the
intermediate periods, II-VI, and the figures for the energy of the
gain must also be increased. Computed on this basis the results
are:

Period.

Energy of

Added Food,

Cals.

Energy of

Resulting Gain,

Cals.

Per Cent.
Utilized.

( II

Conglutin: 1 III

! iv

Flesh-meal: -J yrr'

715.4
1245.8
1902.3
1288.2

582.1

605.7

842.4

1288.8

780.7

403.6

84.68
67.63
67.76
60.59
69.33

Xo obvious explanation of the exceptionally high results ob-
tained in Period II presents itself. Those of the remaining
periods do not seem to indicate any considerable differences in the
utilization of different quantities. The figures are notably higher
than those computed from the Mockern experiments, but in view
of the uncertainties attaching to them too much stress should not
be laid on this fact.

Discussion of Results.

As was pointed out at the beginning of this section, and as was
further apparent in considering the results of experiments upon
carnivora, our knowledge of the net availability of the energy of
feeding- stuffs and nutrients is too imperfect to permit the experi-

466 PRINCIPLES OF ANIMAL NUTRITION.

mental results above detailed to be discussed from the standpoint
of the percentage utilization of the net available energy.

Furthermore, even confining ourselves to a consideration of the
utilization of the metabolizable energy of the food, we have already
seen that the recorded results upon carnivorous animals show such
wide divergencies as to render it difficult if not impossible to draw
any quantitative conclusions from them.

For the present, accordingly, our discussion of the utilization
of energy must be confined chiefly to the results which have been
reached with herbivora. and in the main to the Mockern experi-
ments, and we must content ourselves with an attempt to trace the
relations between metabolizable energy and energy utilized, or, to
look at the subject from the other point of view, with determining
the proportion of the metabolizable energy of the food which is
expended in the combined work of digestion, assimilation, and
tissue building. From the practical standpoint this is of course
the important thing, since either form of expenditure of energy
constitutes, in the economic aspect of the matter, a waste but it
is nevertheless to be regretted that it is at present impossible to
further analyze this waste.

Influence of Amount of Food. — As in the discussion of net
availability in Chapter XII, we have thus far assumed the energy
utilized to be a linear function of the net available or of the metabo-
lizable energy of the food. Before proceeding further it becomes
important to consider how far this assumption is justified by the
facts on record.

Carxivora. — Of the experiments upon camivora recorded on
preceding pages, those of Rubner with different amounts of meat,
when computed by his method (that is, assuming an availability of
100 per cent, below the maintenance point, as on p. 450). appear to
indicate that the utilization above that point is constant. If, how-
ever, a lower percentage of availability is assumed, as on p. 451,
this constancy disappears. None of the other results there sum-
maiized seem suitable for discussion from this point of view.

Swine. — If in the experiments of Meissl, Strohmer & Lorenz, as
computed on p. 454, we express the estimated metabolizable energy
of the excess food as a percentage of the fasting metabolism, Ave
have the following comparison of the percentage utilization with

THE UTILIZATION OF ENERGY.

467

the relative amount of excess food, to which may be added Ivor-
nauth & Arche's results similarly computed:

1 Ixcess Over

l'..-ting

Metabolism,

Per Cent.

Percentage
Utilization.

Meissl :

Experiment I

133
250

74
ISO

126
129

80.7
75.2
70.9
07. 1

71.7
65.3

II

Ill

IV

Korncmth it- A rche :

Experiment III

IV

While there is some variation in the percentage utilization, as
would naturally be expected in experiments with different animals,
the range in the relative amount of excess food is much greater
and there is no indication of a connection between the two.

Ruminants . — The earlier Mockern experiments by G. Kiihn
include one upon wheat gluten and two upon starch in which two
different quantities were added to the basal ration of the same
animal. The final results were as follow- :

Animal.

Period

Vide 1 to
asal Ration

Kgs

Percentage
Utilization

of Metaboliz-
able Energy.

Wheat gluten . . . . \

f

Starch.

Ill

111

V

V

V

VI

VI

3

4

2a

26

3

26

3

0.68

1.36

2.0

2.0

3.5

2.0

3.5

45.3
48.0
53.2
53.7
59.7
48.1
46.6

These results do not indicate that any material effect is exerted
upon the utilization of the metabolizable energy of the food by
the amount consumed, since the differences are small in themselves
and in both directions.

The results, reported by Pfeiffer, of experiments upon the addi-
tion of varying amounts of proteids to a basal rati'ui as computed
by the writer (p. 465) , likewise show a fairly constant percentage

468

PRINCIPLES OF ANIMAL NUTRITION.

utilization of the energy of the proteids used, with the exception
of the strikingly higher result of the first period.

A similar conclusion may be drawn from a study of the Mockern
results as a whole, as recorded in Table VII of the Appendix.
While the computed percentages in each series vary more or less
in the different experiments, the differences are in most cases not
large and appear to bear no relation either to the total quantity
of food given or to the amount of the particular food under experi-
ment which was added to the basal ration, but to be due rather
to individual differences in the animals. This is strikingly shown
in the following table, in which the results upon hay, wheat gluten,
and starch are arranged in the order of the percentage utilization :

Metaboliz-

Total Excess

Percentage

able Energy

Over Com-

Utilization

Feeding-stuff.

Animal.

Period.

of Added

puted Main-

of Metabo-

Foo( 1 ,

tenance,

lizable

Cals.

Cals.

Energy.

[

J

2

7875

12,192

34.8

G

2

5726

9,780

36.2

F

1

5506

10,184

40.4

H

7

8505

11,905

48.4

I

H

2

7875

11.275

50.4

f

B

1

4483

15,129

36.9

1

D

4

5713

17,373

37.3

|

C

3

6033

19,635

43.2

Wheat gluten «[

III

3

2913

8,982
11,401

45.3

III

4

5332

48.0

B

3

5507

16,153

49.7

I

IV

3

3645

7,132

58.2

'

VI

3

8264

12,364

46.6

VI

26

5038

9,138

48.1

Starch — Kiihn'sexpts. . -

IV

III

2
2

4350
4998

3411
6 592

49.2
50.0

l

V

2a

5425

8,821

53.2

V

3

9658

13,054

59.7

■

D

2

4420

16,080

53.7

J

3

4826

9,142

54.8

H

3

6668

10,068

56.0

Starch — Kellner's expts -l

C

2

3027

16,829

57.6

F

4

5009

9,686

64.8

1

B

2

3291

13,937

65.4

G

4

5387

9,441

65.8

But while this is true of each series by itself, a comparison of
the two series upon starch leads to a different conclusion. In
Kuhn's experiments the basal rations consisted largely or exclu-
sively of coarse fodder. In Kellner's experiments the starch was

THE UTILIZATION OF ENERGY. 469

added to a materially heavier basal ration containing considerable
grain and therefore already tolerably rich in starch and other carbo-
hydrates. In spite of the smaller average amounts of starch added,
then. Kellner's results in a sense represent the percentage utiliza-
tion of larger quantities of starch than do Kuhn's; that is, they
represent the utilization of starch at a greater distance above the
maintenance ration. The average utilization (pp. 461-2) was —

Kuhn's experiments 50.0 per cent.

Kellner's experiments, moderate rations ... 58.4 " "
heavy rations 61.5 "

It would appear, then, from these figures that the metaboliz-
able energy of starch was more fully utilized in rations containing a
relatively large quantity of it. At least a partial explanation of
this seems to be afforded by the variations in the production of
hydrocarbons (methane). As was mentioned in discussing the
metabolizable energy of starch, the conditions in Kuhn's experi-
ments were such as to permit a considerable proportion of the
starch to undergo the methane fermentation, while the more abun-
dant supply of it in Kellner's experiments resulted in reducing, or
in some cases wholly suppressing, this fermentation of the starch.
The effect of this, as there pointed out. was to make the metaboliz-
able energy per gram greater in Kellner's than in Kuhn's experi-
ments, but it has also another result. As we have seen, the methane
fermentation constitutes part of the work of digestion, in the general
sense in which that term is here employed, the amount of the latter
being measured by the heat evolved. This amount being less in
Kellner's than in Kuhn's experiments, the net availability of the
metabolizable energy of the starch should be greater, and, other
things being equal, the storage of energy (gain of tissue) should also
be greater.

Kellner * computes that for each 100 grams of starch digested
there was produced, on the average, methane corresponding to the
following amounts of carbon:

In Kuhn's experiments 3.0 grams

In Kellner's experiments 2.3 "

* Loc tit., p. 423.

4JO PRINCIPLES OP ANIMAL NUTRITION.

An approximate computation of the probable differences in the
heat evolved by the fermentation, based on such data as are avail-
able, gives as a result 0.159 Cal. per gram of starch, or somewhat
more than one-half the difference in average utilized energy, viz.,
0.265 Cal. per gram. The data on which the computation is based,
however, are too uncertain to allow us to attach very much value
to the results, except perhaps as an indication that the supposed
cause of the difference in the utilization of the energy is insuffi-
cient to fully account for the effect.

Conclusions. — It cannot be claimed that the above results are
sufficiently extensive or exact to permit final conclusions to hi
drawn, but their general tendency seems to be in favor of the hy-
pothesis that the proportion of energy utilized is substantially inde-
pendent of the- quantity of food, provided that the changes in the
latter are not so great as to modify the course of the fermentations
in the digestive tract. The results upon starch just considered
seem to indicate that if the variations in quantity or make-up of
the ration are pushed beyond that point, a difference in the pro-
portion of the energy utilized may be caused by a difference in the
digestive work; in other words, that it is the availability that is
modified rather than the proportion of the available energy which is
recovered as gain. While not denying that the latter function may
be also modified, either directly as the effect of varying amounts
of food, or indirectly by changes in the chemical nature of the sub-
stances resorbed from the digestive tract under varying conditions
of fermentation, it seems probable that the main effect is that upon
availability.

It is to be observed that the rations used in these experiments,
while not heavy fattening rations, still produced very fair gains.
The experimental periods were comparatively short and hence
the testimony of the live weight itself is liable to be misleading.
Taking the actual gains of fat and proteids as shown by the respi-
ration experiments, however, and comparing them with the compo-
sition of the increase of live weight in fattening as determined by
Lawes iV- Gilbert, it appears that the total gain per day was equiva-
lent to from 0.9 to 2.5 pounds gain in live weight per day in the ex-
periments on coarse fodder, while in those upon concentrated feeds
the corresponding range is from 1 to 3 pounds.

THE UTILIZATION OF ENERGY. 47 J

It may be remarked further that the rations in Kiihn's experi-
ments differed materially from those ordinarily used in practice,
both as to their make-up and their very wide nutritive ratio, so
that the conditions may fairly be regarded as in a sense abnormal.
Kellner's rations represent more nearly normal conditions, and
they fail, as we have seen, to give any clear indications of an in-
fluence of amount of food upon the proportion of energy utilized.
Whether other feeding materials show a behavior analogous to that
of starch, future investigations must decide. In the meantime
we are apparently justified in discussing such results as are now
on record upon the provisional hypothesis that, within reasonable
limits, the utilization of energy is independent of the amount of
food, or, in other words, is a linear function.

Influence of Thermal Environment. — The influence of the
thermal environment of the animal upon its heat production and
upon the net availability of the energy of the food has already been
fully discussed in previous pages and needs only a brief consider-
ation here.

Ruminants. — We have already found reason to think that in
ruminants the heat production on the ordinary maintenance ration
is in excess of the needs of the body. Kiihn's and Kellner's results
show us that from 25 to 72 per cent, of the metabolizable energy
of the food supplied in excess of the maintenance requirement was
converted into heat, so that the heat production was frequently
increased 40 or 50 per cent, above that which was observed on the
maintenance ration. Under these circumstances we can hardly
suppose that any moderate changes in the thermal environment
would sensibly affect either the availability of the food energy or its
percentage utilization.

The writer is not aware of any exact determinations of the
influence of the thermal environment upon the heat production of
fattening ruminants, but the above conclusion is in harmony with
the practical experience of many feeders that moderate exposure
to cold is no disadvantage, btit rather an advantage in maintaining
the health and appetite of the animals, and it appears also to have
the support of not a few practical feeding trials.*

* Compare Henry, " Feeds and Feeding," second edition, p. 364, and
Waters, Bulletin Mo. Bd. Agr., September, 1901, p. 23.

472 PRINCIPLES OF ANIMAL NUTRITION.

Naturally this can be true only within limits, and exposure to
very low temperatures, especially in a damp climate, and particu-
larly to cold rains, causing a large expenditure of heat in the evapo-
ration of water from the surface of the body, may very well pass
the limit and cause an increase in the metabolism simply to main-
tain the temperature of the body. Finally, the time element, as
pointed out on p. 439, is one to be taken into consideration.

Swine. — As was remarked on p. 435, the work of digestion is
doubtless less with the swine than in ruminants, on account of the
more concentrated nature of his food, and as was shown on p. 438,
the maintenance requirement appears to be affected by the thermal
environment. The same reason would tend to make fattening
swine more susceptible to this influence than fattening ruminants.
This conclusion is borne out by the experiments of Shelton * at the
Kansas Agricultural College, who found that swine kept in an open
yard during rather severe weather required 25 per cent, more corn
to make a given gain than those sheltered from extreme cold.

Influence of Character of Food. — Attention was called in the
previous chapter to the fact that the expenditure of energy in the
digestion and assimilation of the food is largely dependent upon the
nature of that food, but as was there pointed out, we have few
quantitative determinations of the differences. Experiments of
the class now under consideration show marked variations in the
proportion of the metabolizable energy of different foods which
is utilized, and we should naturally be inclined to ascribe these
variations to differences in the work of digestion and assimilation
rather than to differences in the physiological processes involved
in tissue production.

The data recorded in the foregoing pages constitute only a
beginning of the study of the utilization of the energy of feeding-
stuffs, but a brief consideration of the main results will prove at
least suggestive.

Concentrated Feeding-stuffs. — As we saw in connection
with the discussion of the metabolizable energy of feeding-stuffs in
Chapter X, the Mockern experiments, to which we owe the larger
share of our present knowledge regarding the metabolism of energy
in farm animals, were made for the purpose of comparing the
* Rep. Prof, of Agriculture, 1883.

THE UTILIZATION OF ENERGY. 473

principal classes of nutrients rather than commercial feeding-stuffs.
Accordingly such representative materials as starch, oil, and glu-
ten were largely used, and we have as ye1 but few determinations
either of the metabolizable energy of ordinary concentrated feeding-
stuffs or of its percentage utilization. We have already considered
to some extent the advantages and disadvantages resulting from
making the pure nutrients, on the oue hand, or actual feeding-stuffs,
on the other, the starting-point for investigations. Passing over
this question for the present, we may conveniently group together
here such results as are on record for materials other than coarse
fodders.

Starch. — Starch, as a representative of the readily digested car-
bohydrates, has, as we have seen, received a large share of atten-
tion. The results obtained are tabulated in the Appendix, and
have already been partially considered in their bearings upon the
influence of amount of food. It was there noted that the earlier
series of experiments by Kiihn, in which the starch was added to
a ration of coarse fodder only, gave results differing decidedly from
those obtained later by Kellner from the addition of starch to a
mixed fattening ration. Among the latter experiments, more-
over, were two (animals B and C) which were exceptional in that
very large total amounts of starch were contained in the ration,
relatively large amounts escaping digestion, while none of the added
starch underwent the methane fermentation.

A clear.. image of the fate of the total potential energy supplied
to the organism in the starch is best obtained by a study of its per-
centage distribution among the several excreta, the work of digestion,
assimilation, and tissue building, and the gain secured, as in the
table on page 474. in which each of the three sets of experiments
indicated above is given separately. The figures for the work of
digestion, etc., are, of course, obtained by difference.

As pointed out in the discussion of metabolizable energy, the
percentage of the muss energy carried off in the feces includes, as
here computed, not only the energy of the undigested portion of the
starch itself, but also that of the portion of the basal ration which
escaped digestion under the influence of the starch. This is espe-
cially true of Kellner's experiments with moderate rations, in which
little or no starch could be detected in the feces. Similarly, the

474

PRINCIPLES OF ANIMAL NUTRITION.

PERCENTAGE DISTRIBUTION OF GROSS ENERGY OF STARCH.

In
Feces.

In

Urine.

Work of
Diges-
tion,
Assimi-
lation,
and
Tissue
Build-
ing.

In
Gain.

Kiihn's experiments.

Kellner's experiments:
Moderate rations. . . .

Kellner's experiments:
Heavy rations

Averages :

Kiihn's experiments .
Kellner's experiments:

Moderate rations. . .

Heavy rations

Ill

IV

V

V

VI

VI

D

F

G

H

J

B
C

2

2

2a

26

26

3

2

4

4

3

3

2
2

59.60
52.22

19.59

17.61
55.91

-1.29
-1.01

1.03
-0.27
-2.01
-0.88
-3.27

0.73

0.35
-2.32

1.14

-3.25
-0.89

-0.92

-0.66
-2.07

10.06

12.01

11.20

9.86

8.86

11.87

6. OS

11.41

8.98

7.38

11.85

-4.96
-0.01

10.74

9.21
-2.49

35.61
32.41
36.95
34 . 58
36 . 96
37.38
31 . 10
25 . 24
26.42
34.82
32.66

16.82
26.68

35.19

30.64

18.75

35 . 60
31.30
42.00
40.10
34.30
32.60
36.10
46.20
50.90
44.40
39.50

31.80
28.00

35.40

43.20
29.90

negative losses in the urine and, in two cases, in the methane
mean, of course, that under the influence of starch the metabolic
or other processes were so modified that less of the potential energy
of the basal ration was lost through these channels. The starch,
so to speak, borrowed energy from the basal ration. In brief, the
figures of the table give us a picture of the aggregate net results of
supplying 100 units of additional potential energy in the form of
starch, or in other words, of the "apparent" utilization.

As between Kiihn's results and those of Kellner upon moderate
rations, the chief difference, as already noted, is the less evolution
of methane in the latter and, apparently as, in part, a consequence
of this, the smaller expenditure of energy in the work of digestion,
etc. Combined with the slightly smaller loss in the feces, this
results in making the energy utilized a much larger percentage of
the gross energy. Apparently Kellner's figures correspond most
nearly to normal conditions of feeding and may be taken to repre-
sent the average utilization of starch under these circumstances.

THE UTILIZATION OF ENERGY.

475

In Kellner's two experiments on heavy rations the enormous losses
in the feces cut down the percentage utilization to a very low
figure and thus render difficult a direct comparison with the
other averages.

While the above form of stating the results appears the simplest
and most direct, it is of interest also to eliminate the influence of
varying digestibility by computing the percentage distribution
of the gross energy of the apparently digested portion of the starch.
This is particularly the case since Kellner's computations of his
experiments are made in a somewhat similar way. Combining
the data given on p. 461, regarding the percentages of metaboliz-
able energy utilized, with those on p. 301 for the energy of the
apparently digested matter, we have the following:

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED STARCH.

In Urine.
Per Cent.

In Methane.
Per Cent .

Wrork of
Digestion,
Assimilation,
and Tissue
Building.
Per Cent.

In Gain.
Per Cent.

Ki'ihn's experiments

Kellner's experiments:

Moderate rations

Heavy rations

-1.19

-0.92
-4.95

13.42

11.12

-6.15

43.89

37.36
42.77

43.88

52.44
68.33

Kellner's computations are made in a different manner.* Omit-
ting in the computation of metabolizable energy the correction for
nitrogen gained or lost, he compares the period in which starch was
fed with that on the basal ration substantially as has been done
above. He then, however, introduces a correction for the influence
of the starch upon the digestibility of the basal ration. For ex-
ample, comparing Periods 3 and 4 on Ox H, he finds in the manner
shown on p. 307, Chapter X, that the equivalent of 820 Cals. less
of the basal ration was digested in the period in which starch was
added to it. while there is a further correction of 112 Cals. to be
made for the Less amount of organic matter of the basal ration con-
sumed in Period '■'>. making a total difference of 932 Cals. Of the
energy of the digestible matter of the basal ration, 79.9 per cent.

* Compare Landw. Vers. Stat., 53, 450.

476

PRINCIPLES OF ANIMAL NUTRITION.

was found to be metabolizable, so that the above difference in
energy would correspond to 745 Cals. of metabolizable energy.
Of the metabolizable energy of the basal ration in excess of main-
tenance, 59.6 per cent, was recovered in the gain. If, then, the
differences in organic matter consumed and in the digestibility of
the basal ration had not offset some of the effect of the starch in
Period 3, there would have been 745 Cals. more of metabolizable
energy disposable from the basal ration, and presumably the gain
resulting from this would have been 59.6 per cent, of 745 Cals., or
444 Cals. We have, then, by this method the following:

Energy of
Gain,

Cals.

Period 3 minus Period 4.. .
Correction for live weight.

Correction for organic matter and for decreased
digestibility

Percentage utilization

3752
40

3712

444

4156

56. 6#

Kellner's results, then, assuming that the corrections are accu-
rate, represent respectively the metabolizable and the utilizable
energy of the digested matter of the starch itself, while the results
as computed on the preceding pages represent, as was there pointed
out, a balance between the various negative and positive effects of
the addition of starch. In other words, Kellner attempts to com-
pute the real as distinguished from the apparent utilization of the
energy of the starch. The comparison on the opposite page of the
percentages obtained in this way with those computed on p. 461
will therefore be of interest.

Kellner also computes by his method the distribution of the
gross energy of the digested starch in Kiihn's experiments and in his
own experiments on moderate rations. As calculated in Chapter X,
pp. 325-6, the average loss of potential energy in methane was 12.7
per cent, in Kiihn's experiments, and 10.11 per cent, in Kellner's,
while none of the potential energy of the digested starch passed

THE UTILIZATION OF ENERGY.

477

UTILIZATION OF METABOLIZABLE ENERGY OF STARCH.

Animal.

Period.

Real Utiliza-
tion as

Computed by
Kellner.
-;rCent.

Apparent
Utilization as
Computed on

p. 461.
Per Cent.

f

Ill

2

46.2

50.0

IV

2

49.0

49.2

J

V
V

2a
2b

51.3

52.6

53.2
53.7

1

VI

26

48.0

48.1

VI

3

46.8

46.6

Kellner's experiments:

■I

D

2

54.2

53.7

J

F

4

63.2

64.8

Moderate rations

G

4

65.2

65.8

H

3

56.6

56.0

1

J

3

55 . 2

54.8

....]

B
C

4
2

61.4
56.4

65.4
57.6

Averages .

49.0

58.9
58.9

50.0

Kellner's experiments:

Moderate rations

58.4

61.5

into the urine. In the two cases, then, S7.30 per cent, and 89.89
per cent, respectively of the potential energy of the digested starch
was metabolizable. Of this metabolizable energy 49.0 per cent,
and 58.9 per cent, respectively was recovered in the gain. Com-
bining; these figures we have —

DISTRIBUTION OF ENERGY OF DIGESTED STARCH.

Tn Urine
Per Cent.

In Methane
Per Cent

Work of

Digestion.

Assimilation,

and Tissue
Building
Per Cent

In Gain
Per Cent.

Kiihn's experiments. .

Kellner's experiments;

Moderate rations

12.70
10.11

44.52
36.95

42.78
52.94

The final results for the energy recovered in the gain of tissue,
whether expressed as a percentage of metabolizable energy or of
energy of digested matter, are substantially the same numer-
ically as those reached by the former method of computation, but
this agreement is purely accidental, and the significance of the

478 PRINCIPLES OF ANIMAL NUTRITION.

figures is essentially different, as already explained. From the re-
sults last given, assuming the gain of energy to have been entirely
in the form of fat, Kellner * computes that the conversion of starch
into fat in cattle takes place according to the following scheme :

Starch 100.00 grams

+ Oxygen 38.69

Yield:

Methane 3.17 grams

Water 23.40 "

Carbon dioxide 88.78 "

Fat 23.34 "

138.69 grams 138.69 "

Oil. — Applying to Kellner's three experiments upon the addition
of oil to a basal ration the same method of computation which was
used for the starch — that is. computing the apparent utilization —
we have the results shown in the two following tables:

DISTRIBUTION OF GROSS ENERGY OF OIL.

Sample I

" II |

Average of Sample II

In Feces.
Per Cent.

24.34
64.77
41.00
52.89

In Urine.
Per Cent.

-1.08

-1.19
1.37

0.09

In
Methane.
Per Cent.

■1.02

■16.10

1 .76

8.93

Work of
Digestion
Assimila-
tion and

Tissue
Building.
Per Cent.

37.66
18.32
18.19
18.25

In Gain.
Per Cent.

40.10
34.20
41.20
37.70

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED OIL.

Sample I

II -j

Average for Sample II

Animal,

Period.

In Urine.
Per Cent.

-1.42

-3.38

2.32
-0.53

In
Methane
Per Cent.

- 1.34
-45.09

- 3.01
-24.35

Work of
Digestion
Assimila-
tion and

Tissue
Building
Per Cent.

49.76
52.01
30.83
41.42

In Gain.
Per Cent.

53.00
97.06
69.86
83.46

* Loc. cit., 53, 452.

31

THE UTILIZATION OF ENERGY.

479

As was noted in the discussion of metabolizable energy in
Chapter X, the results on Ox F appear to be exceptional, but those
upon the other two show considerable differences, and it is evident
that further investigation will be necessary to obtain satisfactory
data upon the effect of oil fed in this way.

Kellner's method of computation, based upon the provisional
conclusion on p. 323, Chapter X, that oil has substantially no effect
upon the loss of energy in urine and methane under normal condi-
tions, gives the following results:

PERCENTAGE OF METABOLIZABLE ENERGY UTILIZED.

As Computed
by Kellner.

As Computed
on p. 462.

Ox D

" F

52.2

51.6
65.1
69.4

" G

59.4

DISTRIBUTION OF ENERGY OF DIGESTED OIL.

Ani-
mal.

Period.

In LTrine,
PerCent.

In Methane,
Per Cent.

Work of
Digestion,
Assimilation
and Tissue
Building,
Per Cent.

In Gain,
Per Cent.

Sample I

" II

D

G

3
5

0
0

0

0.5

0
0

47.8
40 6

52.2
59 4

0

-2 2

44.2
40.3

55 8

Average computed

61 4

The numerical results of these experiments show more clearly
than was the case with the starch the difference in the two methods of
computation. Both methods agree, however in showing that the
combined expenditure of energy in the digestion and assimilation of
the oil and in tissue building is very considerable. We have already
seen that the expenditure of energy in the digestion of fat by cai -
nivora and by man is comparatively small. If we are justified in
assuming that the same thing is true of ruminants, the result just
reached signifies that the digested fat undergoes extensive trans-
formations before being finally deposited in the adipose tissue.

480

PRINCIPLES OF ANIMAL NUTRITION.

Until, however, we have satisfactory determinations of the per-
centage utilization of fat by carnivora, or of its net availability
in ruminants, or both, no final conclusion on this point is possible.
Wheat Gluten. — The three samples of this feeding-stuff experi-
mented upon contained respectively 87.88, 83.45, and 82.67 per
cent, of crude protein in the dry matter, the remainder being
chiefly starch, with the exception of 2.22 per cent, of ether
extract in the first lot. A reference to the results obtained for
the metabolizable energy will show that they were variable and
also that, especially in the earlier experiments, the incidental
effects were large. Tabulating the results as in case of starch
and oil we have the results contained in the tables on this and
the opposite pages.

DISTRIBUTION OF GROSS ENERGY OF AVHEAT GLUTEN.

g

"a
<

III
111

Av.

IV

B
B
C

Av.
U

5
"C
10

Ph

3

4

3

1
3
3

4

In Feces.
Per Cent.

In

Urine.

Per Cent.

In
Methane.
Per Cent .

Work of
Diges-
tion.
Assimila-
tion, and
Tissue
Building.
Per Cent.

In Gain.
Per Cent.

f

1

Kukn's experiment-. . j

1

I
Kellner's experiments

f
Sample I <J

-10.38
- 1.28

17.85
21.71

10. SI
5.08

44.72
38.69

37.00
35.80

- 5.83
-16.17

30.16
22.55
20.89

19 . 78
16.18

16.58
13.52
11.19

7.95
-1.26

0.08
-1.62
-3.69

41.70
42.35

33.58
32.95
40.71

36.40
58.90

19.00
32.60
30.90

I

24.53
15.80
20.16

13.76
12.39
13.08

-1.74
1.91
0.08

35.75

43. SO
39.78

27.70
26.10

26.90

The exceptionally small loss of energy in the urine in the case
of Ox IV, Period 3, and the total suppression of the methane fer-
mentation, as well as the fact that the metabolizable energy was
apparently greater than the gross energy, seem to justify exclud-
ing this experiment from the average, although there was appar-
ently nothing abnormal in the ration fed. In the experiment
with Ox D, Period 4, the nutritive ratio was very narrow
(1 J3.3), and Kellner considers this a probable explanation of the

THE UTILIZATION OF ENERGY.

4S1

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED MATTER.

"3
E

1

In Urine.
Per Cent.

In
Methane.
Per Cent.

Work of
Digestion,
Assimila-
tion, and

Tissue
Building.
Per Cent.

In Gain.
Per < cin.

Kellner's experiments:
Sample I

r

j

1
1

r

■-!
I
I

III
III

Av.
IV

B
B
C

Av
D

3
4-

3

1
3
3

4

16.17
21.44

9.79
5.02

40.50
38.24

33.54
35.30

18.81
13.92

23.74
17.46

14.15

7.39

-1.07

0.11
-2.10

-4.67

39.38
36.44

48.06
42.57
51.46

34.42
50.71

28.09
42.07
39.06

Sample II

18.45
14.72

16.59

-2.22
2.27
0.02

47.35
52.01
49.68

36.42
31 00

Average of I and II

33 71

relatively small utilization of the protein as computed by his
method. (See below.) An unexpected result is that while the
earlier sample of gluten seems to have increased the methane
fermentation, the later samples, although containing more starch,
caused a decrease in the methane production except in case of
OxD.

Digestible Protein. — Kellner does not attempt to compute the
energy utilized from the wheat gluten as a whole by his method, but
uses the results as a basis for computing the utilization of the energy
of the digested protein. He finds that of the metabolizable energy
of the latter, computed in the manner described in Chapter X
(p. 316); the following percentages were recovered in the gain.

Ox B 45.0 per cent.

OxC 42.7 " "

Ox III 45.1 " "

Ox IV 48.8 " "

Average 45 . 2 " "

Ox I) 32.9 " "

The average loss of energy in the urine was found (p. 317) to be
19.3 per cent, of the gross energy of the digested protein. Applying

482

PRINCIPLES OF ANIMAL NUTRITION.

this average to the above figures, and assuming with Kellner that
the protein does not take part in the methane fermentation, we
have the following:

DISTRIBUTION OF ENERGY OF DIGESTED PROTEIN.

Animal.

In Urine.
Per Cent.

In Methane.
Per Cent.

Woik of
Digestion.
Assimilation
and Tissue
Building.
Per Cent

In Gain.
Per Cent.

B

- 19.30

0 -

44.38

46.24
44.30
41.32

36.32

C

34.46

Ill

36.40

IV

39.28

Average ....

r>

44.07
54.15

36 63
26.55 '

There is a wide discrepancy between these results and those
computed on p. 465 from the experiments of Kern & Wattcnberg
upon sheep with conglutin and flesh-meal Omitting the apparently
exceptional result of Period II, we have the following as the per-
centages of the (computed) metabolizable energy of the digested
proteids which was utilized in those experiments :

Conglutin

Aver age ....
Flesh-meal . . .

Average ....

Period.

Ill

V
VI

Ter Cent

67.63
67.76

67 70

60.59
69.33

64.96

While the gain in these cases includes a considerable growth
of wool, it seems difficult to suppose that this alone can have made
the conditions so much more favorable for the storing up of the
added protein as to account for the great difference between these
results and Kellner's, and it must apparently be left to further
investigation to clear up the matter.

THE UTILIZATION OF ENERGY.

483

It need hardly be added that none of these results are directly
comparable with those computed above, after another method, for
the wheat gluten as a whole.

Beet Molasses. — The results of the three experiments upon beet
molasses show such great differences, as was noted in Chapter X
and as is further apparent from the following table, that any dis-
cussion of them would evidently be premature:

DISTRIBUTION OF GROSS ENERGY OF BEET MOLASSES.

"e3

E
'S
•<

—
0

'u,
(B

In Feces.
Per Cent.

In Urine.
Per Cent.

In

Methane.
Per Cent.

Work of
Digestion.
Assimilation.
and Tissue
Building.
Per Cent.

In Gain.
Per Cent.

" II j

F
H
J

6

6
6

26.87

5.40

14.45

3.92
3.16

2.67

-1.95
12.44

10.18

29.56
13.10
36.20

41.60
65.90
36.50

Average

9.92

2.92

11.31

24.65

51 20

Rice. — The two experiments upon swine by Meissl, Strohmer &
Lorenz, when computed as on p. 454, show that of the (estimated)
metabolizable energy of the food approximately the following per-
centages were recovered in the gain :

Period 1 80 . 7 per cent.

II

75.2

Average 78.0 " "

These results are notably higher than any obtained in experi-
ments on ruminants. Like the results on barley and cockle below
they are the expression in another form of the well-known supe-
riority of the swine as an economical producer of meat.

Barley. — For the utilization of the energy of this grain the
single experiment by Meissl, Strohmer & Lorenz gives 70.9 per cent,
of the (estimated) metabolizable energy.

Mixed Grains. — For mixed grains Kornauth & Arche's results
on swine give figures which do not differ materially from the result
just computed for barley, viz.:

Experiment II 71.7 per cent.

Ill 65.3 " "

484

PRINCIPLES OF ANIMAL NUTRITION.

Coarse Fodders. — Kellner's results upon hay, straw, and ex-
tracted straw are the only data regarding the utilization of the
energy of this class of feeding-stuffs which we as yet possess. Only
those experiments in which coarse fodder was added to a mixed
basal ration are available for a computation of this sort.

Meadow Hay. — The two kinds of meadow hay (V and VI) used
in Kellner's experiments gave the following results for the distri-
bution of their energy, computed as in previous instances:

DISTRIBUTION OF GROSS ENERGY OF MEADOW HAY.

In

Feces.

Per Cent

In

Urine.

Per Cent.

In

Methane.
Per Cent.

Work of
Digestion,
Assimila-
tion, and

Tissue
Building.
Per Cent.

In Gain.
Per Cent.

Sample V .

Sample YI .

Average of V and VI .

F
G

Av,

H
H
J

{ Av,

49.81
44.80

4.32

4.26

5.12
0.94

24.25.
28.10

47.30

37.07

34 . 78
34.30

4.29

5.24
5.00
6.33

6.03 I 26.18

4.87
6.15
6.13

35.38
41.34

5.52
4.91

5.72

5.87

26.32
27.97
34.74

29.68
27.93

16.50
15.90

16.20

26.50
26.10
18.50

23.70
19.95

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED MATTER.

"3

a

<

■a

O

a.

In Urine.
Per Cent.

In
Methane.
Per Cent.

Work of
Digestion.
Assimila-
tion, and

Tissue
Building.
Per Cent.

In Gain.
Per Cent.

r

Sample V J

F

G

Av.

H
H

J

Av.

1
2

2

7
2

8.61

7.72

10.20
12.58

48.39
50.85

32.80

28.85

i
I

I

Average of V and VI . .

8.17

8.32
7.66
9.64

11.39

7.74
9.43
9.33

49.62

41.63
42.77
52 . 83

30.82

42.31
40.14
28.20

8.54
8.34

8.83
10.78

45.75
49.08

36.88
31.80

THE UTILIZATION OF ENERGY.

485

Computed by Kellner's method, the percentage of the metabo-
lizable energy of the hay which was recovered as gain of tissue was
as follows, as compared with the results obtained by the writer's
method:

PERCENTAGE OF METABOLIZABLE ENERGY RECOVERED.

Computed by
Kellner's
Method.

Computed by

the Writer's
Method.

Hay V !

Ox F

42.8

40 4

" G

Average

37.7

36.2

40.2

} 49.9 -j
35.8

38.3

i

Hay VI i

Ox H, Period 2....
u H, " 7 ...
" J

50.4

48.4
34.8

I

Average

42.8
41.5

44.5
41 4

Computing the results upon the gross energy of the digested
matter of the hay, Kellner obtains the following:

DISTRIBUTION OF ENERGY OF DIGESTED MATTER.

In Urine.
Per Cent.

In Methane.
Per Cent

Work of

Digest ii in.

Assimilation,

and Tissue
Building.
Per Cent.

In Gain.
Per Cent.

Hav V

8.2
8.8

11.5

9.0

48.00
48.10

32.3

"VI

34.1

Average

8.5

10.3

48.00

33.2

As in some previous cases, the numerical results of the two
methods of computation do not vary greatly, but their essentially
different significance should not be forgotten.

Oat Straw. — For the single sample of this feeding-stuff experi-
mented on, the results, arranged in the same order as before, were
as follows:

486

PRINCIPLES OF ANIMAL NUTRITION.

DISTIBUTION OF GROSS ENERGY OF OAT STRAW.

Animal.

Period.

In Feces.
Per Cent.

In Urine.
Per Cent.

In
Methane.
Per Cent.

Work of
Digestion,
Assimila-
tion, and

Tissue
Buili!i:ig.
Per Cent.

22.34
23.35

In Gain.
Per Cent.

F

2

1

56.77

56 . 86

2.29
1.86

4.40
6.23

14.20

G

1 1 . 70

Average

56.81

2.08

5.31

22.85

12.95

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED MATTER.

Animal.

Period.

In Urine.
Per Cent.

In Methane.
Per Cent.

Work of
Digestion,
Assimilation,
and Tissue
Building.
Per Cent.

In Gain.
Per Cent.

F

2

1

5.30
4.32

10.17

14.42

51.73

54.12

32.80

G

27.14

Average

4.81

12.30

52.92

29.97

PERCENTAGE OF METABOLIZABLE ENERGY RECOVERED.

Computed by
Kellner's Method.

Computed by the
Writer's Method.

Ox F

39.9
35.3

38.8
33.4

" G

37.6

36.1

DISTRIBUTION OF ENERGY OF DIGESTED MATTER (KELLNER).

Average F and G.

In Urine.
Per Cent.

4.7

In Methane.
Per Cent .

12.2

Work of
Digestion,
Assimilation,
and Tissue
Building.
Per Cent.

51.9

In Gain.
Per Cent.

31.2

THE UTILIZATION OF ENERGY.

487

Wheat Straw. — Tabulating the results upon wheat straw in the
same manner as those for oat straw we have —

DISTRIBUTION OF GROSS ENERGY OF WHEAT STRAWr.

Animal.

Period.

In Feces.
Per Cent.

In Urine.
Per Cent.

In
Methane.
Per Cent.

Work of
Digestion.
Assimila-
tion, and

Tissue
Building.
Per Cent.

In Gain.
Per Cent.

H

1
1

60.41
56.03

1.88
2.85

7.96
8.65

26.55

24.67

3.20

J

7.80

Average . . .

58.21

2.37

8.31

25.61

5.50

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED MATTER.

Animal.

Period.

In Urine.
Per Cent.

In Methane.
Per Cent.

Work of
Digestion,
Assimilation,
and Tissue
Building.
Per Cent.

In Gain.
Per Cent.

H

1

1

4.75
6.49

20.11
19.67

67.03

56.12

8.11

J

17.72

Average

5.62

19.89

61.57

12.92

PERCENTAGE OF METABOLIZABLE ENERGY RECOVERED.

Computed by
Kellner's Method.

Computed by the
Writer's Method.

Ox H

11.2
24.3

10.8
24.0

" J

17.8

17.4

DISTRIBUTION OF ENERGY OF DIGESTED MATTER (KELLNER).

Average of
H and J.

In urine 5.6

In methane 20 . 0

Work of digestion, assimilation, and tissue building. 61 .2

In gain 13.2

100.0

488

PRINCIPLES OP ANIMAL NUTRITION.

Extracted Straw. — As previously noted in another connection,
this material consisted of rye straw which had been treated with an
alkaline liquid under pressure, substantially as in the manufacture
of straw paper. It contained in the water-free state 76.78 per cent.
of crude fiber and 19.96 per cent, of nitrogen-free extract. Con-
siderable interest attaches to the results obtained upon this sub-
stance as representing to a degree the crude fiber of the food of
herbivorous animals. Computed as before, these results were:

DISTRIBUTION OF GROSS ENERGY OF EXTRACTED STRAW.

Animal.

Period.

In Feces.
Per Cent.

In Urine.
Per Cent.

In
Methane.
Per Cent.

Work of
Digestion,
Assimila-
tion, and

Tissue
Building.
Per Cent.

In Gain.
Per Cent.

H

5
5

11.35
14.14

-0.46
-1.11

12.40

12.52

25.11
30.85

51.60

J

43.60

Average ....

12.75

-0.79

12.46

27.98

47.60

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED MATTER.

Animal.

Period.

In Urine.
Per Cent.

In Methane.
Per Cent.

Work of
Digestion,
Assimilation,
and Tissue
Building.
Per Cent.

In Gain.
Per Cent.

H

5
5

-0.52
-1.29

13.99
14.58

28.29
35.89

58.24

J

50.82

Average

-0.91

14.29

32.09

54.53

PERCENTAGE OF METABOLIZABLE ENERGY RECOVERED.

Computed by
Kellner's Method.

Computed by the
Writer's Method.

Ox H

67.5

58.7

67.3
58.6

" J

63.1

63.0

THE UTILIZATION OF ENERGY.

489

DISTRIBUTION OF ENERGY OF DIGESTED MATTER (KELLNER).

Average of
H and J.

In urine 0.0

In methane 14.0

Work of digestion, assimilation, and tissue building. 31 .7

In gain 54.3

100.0

As was noted in discussing the results upon metabolizable
energy, the treatment to which the straw was submitted left
it in a condition in which its digestibility, and consequently its
percentage of metabolizable energy, compared favorably with that
of starch. As we now see, this analogy extends also to its effect
in producing gain, the figures showing in this respect a slight
superiority on the part of the extracted straw, as appears from
the following summary:

RECOVERED IN GAIN.

Starch (Kellner's

Experiments on

Moderate

Rations).

Extracted Straw.

Per cent, of gross energy

" " " apparently digested energy.
" " " metabolizable energy..

43.4
53.1
59.0

47.6
54.5
63.0

The reason for this strikingly high value of the extracted straw
as compared with the low value indicated for crude fiber by the
results of Zuntz and Wolff will be considered in a subsequent para-
graph.

Summary. — For convenience of reference the foregoing results
may be summarized in the tables on pages 490 and 491,
showing respectively the percentage distribution of the gross
energy of the feeding-stuffs, that of the energy of the appar-
ently digested organic matter, and the percentage utilization of

49°

PRINCIPLES OF ANIMAL NUTRITION.

the metabolizable energy according to the two methods of com-
putation adopted:

DISTRIBUTION OF GROSS ENERGY.

Work of

Diges-

In

In

In

tion, As-

In

Feces.

Urine.

Methane.

simila-

Gain.

Per Cent.

Per Cent.

Per Cent, tion, and

Per Cent.

T issue

Building.

Per Cent.

Concentrated Feeding-stuffs :

Starch, Kiihn's experiments. . . .

19.59

-0.92

10.74

35.40

" Kellner's experiments,

moderate rations

17.61

-0.66

9.21

30.64

43.20

heavy rations. . .

55.91

-2.07

-2.49

18.75

29.90

Oil, average, Sample II

52.89

0.09

-8.93

18.25

37.70

Wheat gluten, Kellner's expts. .

20.17

13.08

0.08

39.78

26.90

Beet molasses, Sample II

9.92

2.92

11.31

24.65

51.20

Coarse Fodders :

Meadow hav

41.34

4.91

5.87

27.93

19.95

Oat straw

56.81
58.21

2.08
2.37

5.31
8.31

22.85
25.61

12.95

5.50

Extracted straw

12.75

-0.79

12.46

27.98

47.60

DISTRIBUTION OF ENERGY OF APPARENTLY DIGESTED MATTER.

In

Urine.
Per Cent.

In
Methane.
Per Cent .

Work of
Digestion,
Assimila-
tion and

Tissue
Building.
Per Cent.

In
Gain.

Per
Cent.

Concentrated Feeding-stuffs :

Starch, Klihn's experiments

" Kellner's experiments, moder-
ate rations

" Kellner's experiments, heavy

rations

Oil, Sample II

Wheat gluten, Kellner's experiments.. . .

Coarse Fodders :

Meadow hay

( ).tt straw

Wheal si raw

Extracted straw

-1.19

-0.92

-4.95

-0.53

16.59

8.34

4.81

5.62

-0.91

13.42

11.12

- 6.15

-24.35

0.02

10.78

12.30
19.89
14.29

43.99

37.36

42.77
41.42
49.62

49 . 08
52.92
61.57
32.09

43.88

52 11

68.33
83.46
33.71

31.80
29.97
12.92
54.53

THE UTILIZATION OF ENERGY. 491

PERCENTAGE UTILIZATION OF METABOLIZABLE ENERGY.

Real

Utilization

as Computed

by Keilner.

Apparent
Utilization.

By Ruminants.

Concentrated Feeding-stuffs :

Starch, Kiihn's experiments

" Kellner's expts., moderate rations
" heavy rations... .

Oil, Sample II, Ox G

Wheat gluten, Kellner's experiments

Conglutin, Kern

Flesh-meal, Kern

Coarse Fodders :

Meadow hay

Oat straw

Wheat straw

Extracted straw

By Swine.

Rice

Barley

Mixed grain

49.0

58.9

58.9

59.4

45.2*

67.7*

65.0*

41.5
37.6
17.8
63.1

78.0
70.9
68.5

50.0
58.4
61.5
69.4
40.3

41.4
36.1
17.4
63.0

* Of protein.

The Expenditure of Energy in Digestion, Assimilation, and
Tissue Building. — As was shown in the introductory paragraphs on
p. 4G6_ the recorded data do not permit us to distinguish between the
energy expended in the digestion, resorption, and assimilation of
the various feeding-stuffs experimented upon and the energy which
we have reason to believe is required for the conversion of the assim-
ilated material into tissue. Accordingly these two factors have
been grouped together in the foregoing summaries of results. Some
interesting facts are revealed, however, by a comparison of the
total expenditure of energy for these two purposes in the several
cases. Kellner's results, a - the latest and apparently most accurate
and representative, have been made the chief basis of the compari-
son, the figures being those computed by the writer and therefore
showing the aggregate net effect upon the balance of energy, 1 hat is,
the "apparent" utilization.

Coarse Fodders. — A comparison of the coarse fodders with
each other brings out the interesting fact that while the percentage
of the gross energy recovered in the gain varied from 5.5 to 47.6.

492

PRINCIPLES OF ANIMAL NUTRITION.

the percentage expended in digestion, assimilation, and tissue build-
ing varied only from 22.85 to 27.98. Expressing the same thing in
absolute figures, we have the following:

ENERGY PER GRAM OF ORGANIC MATTER.

Gross,
Cals.

Expended in Diges-
tion, Assimilation,
and Tissue Building,
Cals.

Oat straw

4.751
4.816
4.743
4.251

1.327
1.100
1.214
1.190

Average

4.640

1.208

In other words, the combined energy required to separate the
digestible from the indigestible portion of one gram of organic
matter, resorb it, and convert the resorbed portion into tissue was
not greatly different for these four materials. They differed widely
in their nutritive effect, not because of a greater or less expendi-
ture of energy for these purposes, but chiefly because the same
expenditure of energy resulted in making a much larger amount of
material digestible in some cases than in others.

Concentrated Feeding-stuffs. — A still more striking result is
reached when we compare the results on coarse fodders with those
on concentrated feeding-stuffs. Taking the figures of Kellner's
experiments for the latter, and omitting his results on heavy
rations of starch, we have the following data for starch, oil, and
wheat gluten:

ENERGY PER GRAM OF ORGANIC MATTER.

Gross,
Cals.

Expended in

Digestion,

Assimilation,

and Tissue

Building,

Cals.

Oil

4.168
9.464

5.742

1.277

1.728

2.284

Gluten (Kellner) ....

We thus reach the seemingly paradoxical result that the total
expenditure of energy in the production of new tissue is decidedly

THE UTILIZATION OF ENERGY.

493

greater in the case of these three materials, and notably the last
two, than in the four coarse fodders previously tabulated.

The paradox largely disappears, however, when we remember
that while the larger share of the work of digestion has to do with
the total dry matter of the food, the work of assimilation and tissue-
building has to be performed only upon the digested matter, and
that the proportion of the latter is much larger in the starch, oil,
and gluten than in the coarse fodders. We have already (pp. 375
and 445) seen reason to suppose that the processes of assimilation
and tissue building consume a considerable share of the metaboliz-
able energy of the food, although we are still ignorant as to how
much and as to how the proportion differs with different materials,
and the above results serve to confirm this conclusion.

If, simply as an illustration, we assume that the uniform pro-
portion of 30 per cent, of the metabolizable energy of the several
feeding-stuffs is thus consumed, then if we deduct this amount from
the totals above computed we shall have the work of digestion alone
as follows:

ENERGY PER GRAM OF ORGANIC MATTER.

Metaboliz-
able
Energy
(p. 297).
Cals.

Assumed
Work of
Assimilation
and Tissue
Building
(30 Per Cent.
of Metaboliz-
able), Cals.

Total Ex-
penditure
as Above,
Cals.

Work of

Digestion Alone,

Cals.

Meadow hay

Oat straw

Wheat straw

Extracted straw

Starch (Kellner)

Oil

Wheat gluten (Kellner) .

2.213
1.724
1.475
3.213
3.079
5.298
3.831

0.664
0.517
0.443
0.964
0.923
1.589
1.149

1.327
1.100
1.214
1.190
1.277
1.728
2.284

0.663]

0.192J
0.354
0.139
1.1.5

This arbitrary assumption reduces the work of digestion of the
starch to about one half that expended upon a like amount of mate-
rial in the form of coarse fodders which yield chiefly carbohydrates
tn the organism. Moreover, we must remember that in the case of
starch there is a considerably greater loss of energy in the methane
fermentation than with the same amount of total organic matter
in coarse fodders, and that this loss is included in the work of diges-
tion. The high figure found for the wheat gluten we might be

494 PRINCIPLES OF ANIMAL NUTRITION.

inclined to explain by its well-known effect in stimulating the met-
abolism in the body — that is, by supposing that for this substance
our assumption of 30 per cent, for the work of assimilation and
tissue building is too low.

The computed work of digestion is small in the case of the oil,
as the results obtained in other experiments would lead us to expect.
At the same time it should be remembered that the figures given
are derived from two experiments only, while a third gave quite
different results, showing in particular a decidedly higher figure for
the combined work of digestion, assimilation, and tissue building.
It is obvious, therefore, that further investigation is necessary to
fix the value of oil in this respect.

Crude Fiber. — Finally, it will be observed that our arbitrary
assumption results in making the work of digestion of the extracted
straw less than two thirds that of starch. We should naturally
suppose that the mechanical work involved in digestion would be
fully as great' in the case of the former as in that of the latter, while,
as the figures for methane show, the extracted straw underwent a
more extensive fermentation than the starch. Obviously, the
mechanical and chemical treatment to which the straw was sub-
jected so modified the cellulose and removed incrusting matters as
to produce a material which behaved substantially like starch in the
alimentary canal, both as regards its digestibility and its relation to
ferments.* Correspondingly, the total work of digestion, assimila-
tion, and tissue building is not widely different in the two cases. It
is only when we arbitrarily assume a high percentage for the work of
assimilation and tissue building, as was done above for the sake of
illustrating the general cfliestion, that this difference and that in the
amount of metabolizable energy combine to give the relatively low
figure for digestive work noted above.

§ 2. Utilization for Muscular Work.

When a muscle is subjected to a suitable stimulus (normally a
nerve stimulus) there occurs, as we have seen, a sudden and rapid
increase in its metabolism. This increased metabolism appears to

* Lehmann (Landw. Jahrb., 24, Supp. I, 118) had previously shown that
the apparent digestibility of the crude fiber and nitrogen-free extract of straw
and chaff thus treated was increased by from 79 to 133 per cent.

THE UTILIZATION OF ENERGY. 495

consist largely in a breaking down or cleavage of some substance or
substances contained in the muscle, resulting in a rapid increase in
the excretion of carbon dioxide and the consumption of oxygen by
the animal. In this process of breaking down or cleavage there is a
corresponding transformation of energy, a portion of the potential
energy of the metabolized material appearing finally as heat, while
a part may take the form of mechanical energy. The inquiry
naturally arises what proportion of the total energy liberated during
the increased metabolism is recovered as mechanical work and what
proportion takes the form of the (for this purpose) waste energy of
heat. The question is not only one of great theoretical interest to
the physiologist, but the efficiency of the working animal regarded
as a machine for the conversion of the potential energy of feeding-
stuffs into mechanical work is also of the highest practical im-
portance.

Efficiency of Single Muscle. — A large amount of experi-
mental work has been devoted to the study of the single muscle as a
machine. The subject is a complicated one, and unanimity of views
upon it has by no means been attained, especially as to the mechan-
tf muscular contraction. As regards the efficiency of the muscle
as a converter of energy, however, one fact is perfectly well estab-
lished, viz.. that it varies within quite wide limits.

If the two ends of a muscle be attached to fixed points, so that
it cannot shorten, a suitable stimulus will still cause it to contract
in the technical sense of the word; that is, a state of tension will
be set up in the muscle tending to pull the two supports nearer
together (isometric contraction). In such a contraction there is
an expenditure of potential energy and a corresponding increase
of muscular metabolism, but no external work is done. In other
words, all the potential energy finally takes the form of heat and
the mechanical efficiency is zero. This is the case, for example, in
the standing animal. A not inconsiderable muscular effort is
required to maintain the members of the body in certain fixed
positions, and a corresponding generation of heat takes place, but
no mechanical work is done.

But even when the muscle is free to shorten and thus do mechan-
ical work, its efficiency is found to be variable, the chief determin-
ing factors being the load and the degree of contraction. The

496 PRINCIPLES OF ANIMAL NUTRITION.

maximum efficiency of the muscle is reached when the load is such
that the muscle can just raise it, while this maximum load dimin-
ishes as the muscle contracts until when the latter reaches the limit
of shortening it of course becomes zero. Conversely, if the muscle
be stretched beyond what may be called its normal length, as is
the case in the living body, the weight which it can lift, and conse-
quently its efficiency, is increased. In these respects the muscle
behaves like an elastic cord, and some authorities, notably Chau-
veau,* regard the essence of muscular contraction as consisting of
a direct conversion of the potential energy of the "contractile
material" of the muscle into muscular elasticity.

Efficiency of the Living Animal. — According to the above
principles the greatest efficiency of a muscle would be obtained
when it was loaded to its maximum at each point in the contraction ;
that is, when the load diminished uniformly from the maximum
corresponding to the initial length of the muscle to zero at the point
of greatest contraction. Such conditions, however, rarely if ever
obtain in the animal. Of its many muscles some serve largely
or wholly to maintain the relative positions of the different parts of
the body, and consequently have an efficiency approaching zero.
Others contract to a varying extent and under loads less than the
maximum. Some muscles, owing to their anatomical relations,
work at a less mechanical advantage than others, while the extent
to which a given group of muscles is called into action will vary
with the nature of the work.

If, then, the efficiency of the single muscle is variable, that of
the body as a whole would seem likely to be even more so, thus
rendering it difficult to draw any trustworthy direct conclusions
as to the efficiency of the bodily machine from studies of the effi-
ciency of the single muscle. Moreover, the performance of labor
by an animal sets up various secondary activities, notably of the
circulatory and respiratory organs, which consume their share of
potential energy and yet do not contribute directly to the per-
formance of the work, and the extent of these secondary activities
varies with the nature and the severity of the work. When, there-
fore, as is here the case, we consider the whole animal in the light
of a machine for converting the potential energy of the food into
* Le Travail Musculaire. Paris, 1891.

THE UTILIZATION OF ENERGY. 497

mechanical work, we are perforce, by the very complexity of the
problem, driven to the statistical method of comparing the total
income and outgo of energy in the various forms of work.

THE UTILIZATION OF NET AVAILABLE ENERGY.

Both the activity of the skeletal muscles in the performance of
work and the supplementary activity of the muscles concerned in
circulation, respiration, etc., is carried on at the expense of energy
stored in the muscles themselves or perhaps in the blood which
circulates through them. The body thus suffers a loss of energy
which is replaced from the energy of the food. If, then, we supply
a working animal, in addition to its maintenance ration, with an
amount of food exactly sufficient to make good the loss, the total
energy metabolized in the performance of the work will repre-
sent the net available energy of the excess food, since this by
definition is that portion of the gross energy which contributes
to the maintenance of the store of potential energy in the
body.

It is true that in our discussion of the net available energy of
the food we regarded it as making good the losses that occur below
the maintenance requirement, and the question may arise whether
the availability as thus measured is the same as the availability for
the production of muscular work. In reality, however, the two
cases are not radically different. Even below the point of mainte-
nance the internal work of the body consists very largely of muscu-
lar work, and it is the energy metabolized in the performance of this
work which appears to constitute the chief demand for available
food energy. It would appear highly probable, therefore, that the
net availability of the metabolizable energj' of the food will be found
to be substantially the same whether that energy be employed to
prevent a loss from the body as a consequence of its internal work
below maintenance or on account of the performance of external
work above maintenance.

If, then, we cause an animal to perform a known amount of
external work and measure the increase in the amount of energy
metabolized in the body, we may regard the latter as representing
net available energy derived from previous food, and a comparison

498 PRINCIPLES OF ANIMAL NUTRITION.

of this quantity with the work done will give the coefficient
of utilization for the particular animal and kind of work experi-
mented on.

The Efficiency of the Animal as a Motor.

The relation just indicated between the work performed and the
total energy metabolized in its performance is not infrequently re-
garded as expressing the efficiency of the animal as a motor, but it
should be clearly understood that this is true only in a limited sense.
A coefficient computed in the manner outlined above takes account
only of the loss which occurs in the conversion of the stored energy
of the body into external mechanical work. It neither includes the
expenditure of energy required for the digestion and assimilation of
the food, nor does it take account of the large amount of energy con-
tinually consumed in the internal work of the animal machine.' It
does not, therefore, furnish a direct measure of the economy with
which the animal machine uses the energy supplied to it, but is
comparable rather to the theoretical thermo-dynamic efficiency of
a steam-engine. With this limitation, however, the phrase may be
used as a matter of convenience.

Quite extensive investigations upon this point are already on
record. They have generally taken the form of what may be called
respiration experiments. The respiratory exchange of carbon di-
oxide and oxygen has been determined, first, in a state of rest, and,
second, during the performance of a measured amount of work.
From the difference between these two values the quantity of ma-
terial metabolized and the amount of energy consequently liberated
have been computed and compared with the energy recovered in
the form of mechanical work.

This method of experimentation has been largely developed and
employed by Zuntz and his associates * in experiments upon man,
the dog, and especially the horse. Since the present work relates
especially to the nutrition of domestic animals, the results upon the
latter animal are of peculiar interest, but their study may be ad-
vantageously preceded by a somewhat brief consideration of the
results upon the dog and upon man.

* Compare Chapter VIII, pp. 251-2

THE UTILIZATION OF ENERGY.

499

Experiments on the Dog. — The following experiments by
Zuntz,* while not the earliest upon record, may serve to illustrate
the general methods employed and as introductory to the more
elaborate experiments upon the horse.

The following table shows the average oxygen consumption and
carbon dioxide excretion, determined by the Zuntz apparatus, of
a dog when lying, standing, and performing work upon a tread-
power, and also the amount of work done, all computed per minute:

Weight

No of
Ex-
peri-
ments.

Respiration per

Minute.

Work per Minute.

of Ani-
mal
and
Load
Kgs.

Oxy-
gen
c.c.

0O2.
c.c.

Respir-
atory
Quo-
tient.

Work

of
Ascent
Kgm.

Work

of
Draft.
Kgm.

Dis-
tance
travel-
led,
Meters.

6
2

174.3
172.
245.6
725.3
1285 3
1028.8

124.7
12.3.8
170.2
525.2
990.6
798.9

0.71
0.72
0.69
0.73

0.77

0.77

( Magnus- Levy) ...

26.932
26 . 67-!
27.17?

8

5

10

Ascending slight incline.

steeper "
Draft nearly horizontal .

1 3 . 23

365 . 82

22 . S3

202.91

78 . 566
79.497
70.420

The work per minute as given in the above table does not in-
clude the energy expended in horizontal locomotion. The work of
draft is the product of the distance traversed into the draft ; the
work of ascent equals the same distance multiplied by the sine of
the angle of ascent. A remarkable increase (41 per cent.) in the
metabolism when standing over that when lying was observed
(compare p. 343) but does not enter into the subsequent com-
putations.

The two experiments on ascending a grade afford data for com-
puting the increased metabolism corresponding, on the one hand,
to one gram-meter of work done against gravity, and, on the
other, to the transportation of one kilogram through one meter
horizontally. The latter, of course, is not work in the mechanical
sense, but it requires the consumption of a certain amount of
material, the liberated energy being employed in successive liftings
of the body and in overcoming internal resistances and ultimately
appearing as heat. It includes, of course, the increased metab-
olism required for the maintenance of the erect position.

* Arch. ges. Physiol., 68, 191.

5°°

PRINCIPLES OF ANIMAL NUTRITION.

If from the totals given in the table we subtract the figures
for rest, we have the following as the increments of the respiration
due to the work, including the work of standing:

Oxygen,

C.C.

Carbon Dioxide.

C.C.

Ascending slight incline . .
" steeper "

551.0
1111.0

■00.5
865.9

The weight of the animal and the distance traversed having
differed somewhat, the results may be rendered comparable by com-
puting them per kilogram of weight and per meter of distance trav-
ersed— that is, by dividing in each case by the product of weight
into distance. Expressing the results in gram-meters and cubic
millimeters for convenience we have —

Oxygen
c mm

Carbon Dioxide
c mm.

Work of Ascent,
gr.-m.

Ascending slight incline.
" steeper "

260.40
523.93

189.27
408.35

6.252
172.512

If we let x equal the oxygen consumption required for the trans-
portation of 1 kg. through 1 meter and y that required per gram-
meter of work of ascent we have

x+ 6. 252?/ = 260. 40 c.mm.
z+172.512?/ = 523.93 c.mm.

whence we have

£ = 250.49 c.mm.
y= 1.585 c.mm.

A similar computation for the carbon dioxide gives

Locomotion, per kg. and meter 181 .033 c.mm.

Per gram-meter of work of ascent .... 1.317 c.mm.

and the corresponding respiratory quotient is 0.723.

With these data in hand it is easy to compute the increased
respiratory exchange corresponding to one gram-meter of work of
draft as follows:

THE UTILIZATION OF ENERGY.

501

O.xyc
c.c

m,

Carbon
c

Dioxide,
c.

Total

174

479
36

30

36
19

1028.80
689.85

124.70

346.55
30.07

798 90

Rest

Transportation of 27.175
through 70.42 meters ...

kgs.

Ascent — 22.S3 kgm

Total

501 32

Remains for draft

338.95

297 58

For one gram-meter of work of draft we have, therefore,

Oxygen 1 . 6704 c.mm

Carbon dioxide 1 . 467 c.mm

Respiratory quotient 0.87S

It appears from the above that the work of draft required
somewhat more metabolism than the same amount of work of
ascent. The individual experiments of this and other series like-
wise show that variations in the speed and in the angle of ascent
affect the result. For the present, however, we may confine our-
selves to a consideration of the average figures.

It remains to compute from the results for oxygen and carbon
dioxide the corresponding amounts of energy liberated. The data
are insufficient for an exact computation. It having been shown,
however (compare Chapter VI), that even severe work causes but a
slight increase in the proteid metabolism, the author assumes that
the additional metabolism in these experiments was entirely at the
expense of carbohydrates and fat and computes the proportion of
each from the respiratory quotient. The results are admittedly
not exact. Besides the uncertainty just mentioned, there is the
possibility that irregularities in the excretion of carbon dioxide
may affect the respiratory quotient in short trials and, more-
over, we must bear in mind the possibility of various cleavages
and hydrations as affecting the evolution of energy in such experi-
ments (compare Berthelot's criticism on p. 254). The author does
not, however, regard these possible errors as very serious. Com-
puted on this basis the results are as follows, expressed both in
terms of heat (calories) and in gram-meters (1 cal. equals 425
gram-meters) :

502 PRINCIPLES OF ANIMAL NUTRITION.

For 1 gram-meter, ascent 0.0076681 cal. =3.259 gr.-rm

" 1 " " draft 0.008180 " =3.476 "

" locomotion per kg. and meter . . 1.1787 cals. = 500.95 "

According to the above figures the performance of one gram-
meter of work required the metabolizing of material whose potential
energy was equal to 3.259 gr.-m. in the one case and 3.476 gr.-m. in
the other. In other words, these amounts of net available energy
were liberated in the kinetic form in the body, one gram-meter in
each case being recovered as external work while the remainder
ultimately took the form of heat.

This is equivalent to a utilization of 30.7 per cent, of the net
available energy in ascent and of 28.77 per cent, in draft. It is to
be noted that these figures refer only to that portion of the in-
creased metabolism which is applied to the production of external
work and do not include that necessary for the transportation of the
animal's weight. The corresponding ratio for this portion could
only be obtained on the basis of complicated and uncertain compu-
tations of the mechanical work of locomotion. If, however, instead
of this we assume that this most common form of muscular activity
is performed with the same economy as the work of ascent, we can
conversely compute the mechanical work of locomotion for 1 kg.
through 1 meter as

500 . 95 gr.-m. X . 307 = 153 . 8 gr.-m.

Experiments on Man. — In connection with his experiments on
the dog already described, Zuntz * cites the results of a number of
experiments with man upon the work of locomotion and of ascent,
the average results of which are summarized in the table opposite,
to which have been added the results of later experiments by
Frentzel & Reach. |

Experiments on the Horse. — Very extensive investigations on
the production of work by the horse have been made by Zuntz in
conjunction with Lehmann and Hagemann.J Some of the results
of these investigations have already been discussed in their bearing
on the question of digestive work (pp. 385-393), and the method

* Lor tit., p. 20S.

t Arch, ges Physiol., 83, 494.

% Landw Jahr., 18, 1; 23, 125; 27, Supp III.

THE UTILIZATION OF ENERGY.

5°3

Weight
(with
Appara-
tus),
Kgs.

Energy Expended in

Horizontal
Velocity

per
Minute,
Meters.

Experimenter.

Loco-
motion
per Kg.
and Meter,

Kgm.

Per Kgm.
Work of

Ascent,
Kgm.

Grade.
Per Cent.

Katzenstein

55.5
72.9
67.9
80.0
88.2
72.6
81.1
80.0

86.5
86.5

0.334
0.217
0.211
0.288
0.263
0.284
0.231
0.244

0.219
0.233

0 . 230
0.251

2.857
3.190
3.140
3.563
3.555
2.913
2.921
2.729

[2.746 |
j- 2.846 -j

74.48
71.32 I
71.46 J
51.23 /
43.34 f
62.04 )
60.90 J-
56.54 )

66.94 1
3,. 92 1

y

63.95 !

34 . 58 J

9 6-13 3

j
Sehumburg & Zuntz ■{

I

Frent zel :

Normal gait

Slow "

6.5
30.7-62.0

23.0-30.5

Reach :

Normal gait

Slow "

65.8
55.8

23.3

of computing the total metabolism in the rest experiments has
been explained; it remains to consider the results of the work
experiments. The larger proportion of the experiments were
upon the same horse (No. Ill), and the summaries and averages
on subsequent pages represent chiefly the results with this animal.

The work was done upon a special tread-power located in the
open air, and during the rest experiments the animal likewise stood
in the tread-power. The inclination of the platform of the power
could be varied, and it could also be driven by a steam-engine, so
that by setting it horizontal the work performed by the animal was
reduced to that of locomotion alone. The distance traversed was
measured by a revolution-counter, and in the experiments on draft
the animal pulled against a dynamometer.

The large number of experiments (several hundred) are grouped
by the authors into fourteen periods according to the season (winter
or summer) and the kind and amount of food consumed, each of
these periods including a considerable number of experiments both
on rest and on different forms of work. On each day from two to
eight experiments were usually made, some on rest and some on
work of various sorts. The average of all the rest experiments in
each period is then compared with similar averages for the various

5°4

PRINCIPLES OF ANIMAL NUTRITION.

kinds of work in order to eliminate so far as possible the influence
of variations in external temperature and in the feeding, as well as
to reduce the probable error of experiment.

Work at a Walk. — The experiments may be grouped into
those in which the work was performed respectively at a walk and a
trot. Those of the former category, being the more numerous, may
be considered first.

Work of Locomotion. — The following detailed comparison of the
experiments of Period a upon rest and upon walking without load
or draft will serve to further explain the method :

REST EXPERIMENTS. PERIOD a.
Ration, 6 Kg. Oats, 1 Kg. Straw, 6-7 Kg Hay.

No. of Experiment.

Per Kg Live Weight
and Minute.

Respira-
tory
Quotient.

Air Tem-
perature
Deg C.

Relative
Velocity
of Wind.

Hours
Since Last
Feeding.

Oxygen
c.e.

Carbon

Dioxide

c.c.

37a

3.94
3.92
3 98
4.06
4.11
3.89
3 71

3.81
4 02
3.42
4.04
3 86
3.63
3.44

0.968
1.025
0.861
0.997
0.940
0.933
0.929

-5.0
-0.5
2.0
5.3
4.7
2.0
9.0

0

1
1

3
1
1
3

3.0

386

2.5

38/

5.6

39a

2.0

44a

1.5

45(7

3.5

46a

15

Average

Corrected * ....

3.94
4.04

3.75
3.S6

0.950

2.5

1.4

2.8

In the same period eight experiments were made in which the
tread-power was set as nearly horizontal as possible and driven by
the steam-engine, the animal being simply required to maintain his
place on the power. The results for oxygen were as shown in the
first portion of the following table:

* A comparison of Zuntz's method with the results obtained in the Pet-
tenkofer respiration apparatus showed that the gaseous exchange through
the skin and intestines amounted to about 2\ per cent, of the pulmonary-
respiration in case of the oxygen and 3 per cent in case of the carbon di-
oxide. These additions are accordingly made to the figures of the respira-
tion experiments and the results designated as "corrected."

THE UTILIZATION OF ENERGY.

5°5

WALKING WITHOUT LOAD OR DRAFT.
Per Kg. Live Weight.

PERIOD a.

Live

Weight

Kgs.

Obser\ ed.

Oxygen
to

Equivalent

No. of
Experiment.

Per Minute.

Work of

Ascent ,

Per Meter

Traveled.

Gr.-m.

\\ ork.

Oxygen
c.c. .

Distance

Traveled

Meters.

Work of

Ascent ,

Kgm.

Per

Minute.

c.c.

Per Meter

Traveled.

c.mm.

40d

446

456

429
434
428
428
430
430
434
434

9.0
11.3
12.2
12.7
10.8
11.7
12.3
11.2

57
■ 87
94
95
92
99
98
93

0.57
0.84
0.89
0.87
0.70
0.74
0.79
0.76

10

10

9

9

8
8
8
8

5.1
7.3

8.2
8.7
6.9
7.8
8.4
7.3

89

M
88

■iod

466

46c

476

47c

92
74
79
86

78

Average . . .
Corrected. .

430.9

11.405

89.338

0.764

8.643

7.463

83 . 793

85 . 888

If from the oxygen consumption in each of the above experiments
we subtract the average rest value for the same period (3.94 c.c.)
the remainder will represent the increase due to the work, as shown
in the seventh column, and this divided by the distance traveled
gives the figures of the eighth column.

The average respiratory quotient of that part of the respiration
due to the work in these eight experiments was 0.894. On the
very probable assumption that the work caused no material change
in the metabolism of either proteids * or crude fiber, or in other
words, that the energy for work was derived substantially from solu-
ble carbohydrates and fat, the calorific equivalent of 1 c.c. of oxygen
is computed and the following calculation of energy made for the
average of the eight experiments (compare pp 76 and 251). These
results are not corrected for cutaneous and intestinal respiration.

Per Kg. Live Weight per Minute.

Oxygen combined with fat 3 4415 c.c.

Oxygen combined with starch 4.0215 "

Total 7.4630 "

Equivalent energy 36.420 cals.

* The authors show that even a considerably increased proteid meta-
bolism would not "materially affect the computation of energy.

506

PRINCIPLES OF ANIMAL NUTRITION.

Energy per Meter Traveled (Including Work of Ascent).

Per kg. total mass * 0.3948 cal.

„ , ,. . , ( 0.4077 "

Per kg. live weight j 0 1733 kgm

Work of ascent 8.643 gr.-m.

Determinations of the work of locomotion were made in six
different periods, or thirty-five experiments in all. The average
for each period, computed in terms of energy as in the above
example, is given in Table VIII of the Appendix. It is to be noted
that these results still include the small amount of work expended
in ascending the slight incline. This factor is determined in the
manner shown in the following paragraph.

Work of Ascent. — In four periods experiments were made (thir-
teen in all) upon the work of ascending a moderate grade at a walk.
The average results, computed on the same basis as before, are
contained in Table IX of the Appendix.

By comparing the average results of these two series of experi-
ments in the manner explained on p. 500, letting x equal the oxygen
or energy required per kilogram live weight for locomotion through
1 meter horizontally and y the corresponding quantities for the
performance of 1 gram-meter of work of ascent we have the follow-
ing equations:

For Oxygen.
x+ 4.395?/= 83.480 c. mm.
x + 107.041?/ = 222. 941 c.mm.

For Energy.
x+ 4. 395?/ = 0.4035 cal.
x+ 107. 041y=l. 0795 cals.
Solving these we obtain the following values respectively for
the work of locomotion per meter and for the energy expend,'! in

Oxygen,
c mm.

Energy.

cals.

Kgm.

Locomotion per meter:

Per kg live weight

" " total mass .

77 . 509
75 . 048
1359.00

0.3746
0.3618

6.5858

0.1592
0.1538

Ascent, per kilogram-meter

2 . 7990

* Weight of animal plus weight of apparatus carried.

THE UTILIZATION OF ENERGY.

5°7

doing 1 kgm. of work of ascent, and the utilization of the available
energy in the latter case is 35.73 per cent.

Work of Draft. — For the work of draft at a walk, up a slight
incline, the results tabulated in Table X of the Appendix were
obtained.

Giving x and y the same significance as before, and letting z
represent the oxygen or energy corresponding to one gram-meter of
.vork of draft, we have the following equation, based on the results
per kilogram live weight and meter traveled:

x + o. 115^+153. 1272 = 306.561 c.mm. = 1.5021 cals.

Substituting in this the average values of x and y obtained as in-
dicated in the previous paragraph, but from a larger number of
experiments, we have

2= 1 .4504 c.mm.= .007143 cal. per gram-meter.

The above details of a few of the experiments may serve to illus-
trate the methods of computation employed. Similar determina-
tions were made upon various forms of work under differing condi-
tions, the results of which will be given later.

Correction for Speed. — Before final data could be obtained,
however, it was found necessary to take account of the speed of the
animal, since comparisons of the various periods showed that the
metabolism due to the work of locomotion at a walk increased
materially as the velocity increased.

To compute the necessary correction, the authors divide the
thirty-five experiments of Table VIII into three groups according
to the speed. For each group the oxygen and energy correspond-
ing to the work of ascent are computed, using the values of y given
on the previous page (1359 c.mm.; 6.5858 cals.), and subtracted
from the total, leaving the following as the amounts per kilogram
live weight due to horizontal locomotion:

No. of
Experi-
ments.

Velocity

per Minute,

Meters.

Oxygen
Consumed

per Kg
and Meter,

c.mm.

Respira-
tory
Quotient.

Oxygen Re-
calculated to
Respirator;

Quotient of
O.SG, c.mm

Increase of
Oxygen per

Meter
Velocity,

c.mm.

Heat Value
of Oxygen
per Meter

(Corrected).
cals.

6

20

9

78.00
90.10
98.11

66.69
76 0 1
80 ''7

0 . 896
0.848
0.873

67.32
75.80
81.23

0.697
0.683

0.3363
0.3787
0.4058

5o8

PRINCIPLES OF ANIMAL NUTRITION.

On the average, an increase of 1 meter per minute in the speed
was found to cause an increased metabolism corresponding to —

Oxygen 0 . 692 c.mm.

Energy 0.00345 cal.

A similar computation for the experiments on ascending a con-
siderable grade without load or draft showed a similar difference,
which, however, seemed to be chiefly or entirely due to variations
in the work of locomotion. When the amount of the latter was
computed with the correction for speed just given, the metabolism
due to the actual work of ascent seemed to be independent of the
speed, the only exception being two experiments at a rapid walk in
which over exertion of the animal was suspected.

In the thirteen experiments on the work of ascending a moderate
grade contained in Table IX, the average speed was 81.95 meters
per minute, while in the thirty-five experiments with which they
are compared (Table VIII) the average speed was 90.16 meters.
From the table on p. 506 we compute that the consumption of
oxygen (R.Q. = 0.86) and the corresponding energy values per kilo-
gram and meter at these speeds would be —

Oxygen
c.mm.

Energy,
cats.

At 90.16 M. velocity

" 81.95 M. " "

75 . 80
70.05

0.3746
0.3462

Substituting this corrected value of x in the equations on
p. 506, we have as the corrected value of y per kilogram-meter for
ascending a moderate grade

6 . 851 cals. = 2.912 kgm. = 34 . 3 per cent.

In brief, a correction for the value of x is computed, using the
first value of y, and then this corrected value of x is used to com-
pute the corrected value of y. In other words, the method is one
of approximation, but the errors of the corrected values are pre-
sumably less than the unavoidable errors of experiment.

Effect of Load. — In a number of experiments the horse carried
on the saddle a load, consisting of lead plates, corresponding to that
of a rider. The mere sustaining of such a weight at rest was found

THE UTILIZATION OF ENERGY. 509

to increase the gaseous exchange, the total metabolism being sub-
stantially proportional to the total mass (horse + load), but in com-
puting the work experiments the same rest values are used as for
the preceding experiments; that is, the results include the work
required to simply sustain the weight as well as that required to
move it. Computing the results in the same manner as before the
authors obtain for an average speed of 90.18 meters per minute
the following results:

Locomotion per Meter.

Per kg. live weight 0.5004 cal. = 0.212G kgm.

" " total mass 0.3914 " =0.1663 "

Ascent.

Per kilogram-meter 6.502 cals. = 2.7640 " = 36. 19g

A comparison of these figures with those on p. 506 shows
that for this animal a load of 127 kgs. caused about 8 per cent,
increase in the energy expended, per kg. of total mass, in horizon-
tal locomotion, but no increase in that expended per kilogram-
meter in ascent.

Work of Descent. — In descending a grade the force of gravity
acts with instead of against the animal and tends therefore to
diminish the metabolism. On the other hand, however, as the
steepness of the grade increases the animal is obliged to put forth
muscular exertions to prevent too rapid a descent, and this tends
to increase the metabolism. It was found that an inclination of
2° 52' caused the maximum decrease in the metabolism. At 5° 45'
the metabolism was the same as at 0°, while on steeper grades it
was greater than on a level surface.

\Y< irk at a Trot. — A smaller number of experiments were made
upon work at a trot under varying conditions. In trotting, the up
and down motion of the body is much greater than in walking, while
but a small part of the muscular energy thus expended is available
for propulsion. It was therefore to be expected that the energy
required for horizontal locomotion would be greater at a trot than
at a walk, and the results of the experiments corresponded fully
with this expectation, the computed energy per meter being found
to be

Per kg. live weight 0 . 5G60 cal.

" " mass (horse + load) 0.5478 "

5IQ

PRINCIPLES OF ANIMAL NUTRITION.

at a speed of 195 meters per minute. The fact of such an increased
expenditure of energy in trotting as compared with walking has
also been confirmed by the results of Grandeau, which will be con-
sidered in another connection. It was also found that in trotting,
unlike walking, the work of locomotion was independent of the
speed within the limits experimented upon (up to a speed of 206
meters per minute, or about 7£ miles per hour). A load of 127.2 kgs.
increased the work of locomotion per kg. of mass by about 10 per
cent, as compared with the increase of 8 per cent, at a walk. One
experiment on work of ascent and one on horizontal draft, both
without load, showed a utilization of, respectively, 31.96 percent,
and 31.70 per cent., but two other experiments on horizontal draft,
in which the work was thought to have been excessive, gave an
average of only 23.35 per cent.

Summary. — The final results of the experiments upon the horse
may be summarized as follows:

Work at a Walk.

Available Energy
Expended

cals

Kgm.

Utiliza

tion

Per

Cent

Work at a Slow Trot.

Available Energy
Expended.

cals.

Kgm

Utiliza-
tion.
Per
Cent.

For 1 kgm. work of ascent,
without toad :

10.7 % grade

18.1$ grade

For 1 kgm. work of ascent,
with load :

15.8$ grade

For 1 kgm. work of draft :

0.5 % grade

8.5 % grade

Locomotion per kg mass per
meter without load :
Speed of 78 00 M. per min
" " 90.16 " " "
" " 98.11 " " "
The same with load :

Speed of 90.18 M per min

6.8508
6.9787

6.502

2.911634.3
2.9660 33.7

2.7634

36.2

7.5190 3.1960 31.3
10.3360 4.3930 22.7

0.3256
0 . 3666
0.3929

0.3914

7.3647*

7.4240*
10.0780f

0.5478J
0.6007J

3.1300*

31.96*

3.1550*31.7*
4.2820t23.4f

* Single experiment.

i Two experiments. Work probably excessive.

% Independent of speed.

THE UTILIZATION OF ENERGY. 511

Conditions Determining Efficiency.

From the results recorded in the preceding paragraphs it appears
that, as we were led to expect from a consideration of the efficiency
of the single muscle, the efficiency of the animal as a converter of
potential energy into mechanical work varies with the nature of the
work and the conditions under which it is performed, although the
variations are perhaps hardly as great as might have been expected.
In general, we may say that in the neighborhood of one third of the
potential energy directly consumed in muscular exertion is recov-
ered as mechanical work. This appears to be a high degree of effi-
ciency as compared with that of any artificial transformer of poten-
tial energy yet constructed. The steam-engine, the chief example
of such transformers, even in its most highly perfected forms, rarely
utilizes over 15 per cent, of the potential energy of the fuel, while
in ordinary practice one half of this efficiency is considered a good
result.

The comparison is misleading, however, for three reasons: First,
the figures given in the preceding pages relate to the utilization of
the net available energy of the food. As we have seen, however,
a certain expenditure of energy in digestion and assimilation is
required to render the food energy available, while still another
portion of the latter is lost in the potential energy of the excreta.
In the case of herbivorous animals, these two sources of loss very
materially reduce the percentage utilization when computed upon
the gross energy of the food. Second, the comparison takes no
account of the large amount of energy consumed continually
throughout the twenty-four hours for the internal work of the
body of the animal, and which continues irrespective of whether
the animal is used as a motor or not. Third, the expenditure
of energy in locomotion is not considered in computing the
efficiency of one third. When these three points are allowed for
but little remains of the apparent superiority of the animal as a
prime motor, even omitting from consideration the greater cost of
his fuel (food).

It remains now to consider somewhat more specifically the in-
fluence upon the efficiency of the animal machine of some of the
more important conditions.

5"

PRINCIPLES OF ANIMAL NUTRITION.

Kind of Work. — Of the forms of work investigated, that of
ascent, that is, of raising the weight of the body (with or without
load), appears to be the one which is performed most economically.
The horse in ascending a moderate grade without load showed an
efficiency of 34.3 per cent., while with a load of 127 kgs. a slightly
higher efficiency was obtained, viz., 36.2 per cent. (The latter
figure, however, includes some estimated corrections for speed.)
For the dog (p. 502) the average result was 30.7 per cent. For
man the figures of the table on p. 503 correspond to from 28.1 to
36.6 per cent.

The efficiency, however, was found to decrease with the steep-
ness of the grade. Thus with the horse it fell from 34.3 to 33.7
per cent., with an increase of the grade from 10.7 to 18.1 per cent.
The experiments of Loewy on man, averaged on p. 503, show the
same result in a more striking manner. Taking separately the
experiments on each subject we have the following:

Grade
Per Cent.

Efficiency.

A L.
Per Cent.

J L
Per Cent.

L Z
Per Cent.

23
30 5
36.6

34 3
34 3
29 0

36.1
32 6
32 3

36.6
36 6
32 2

The work of horizontal locomotion consists largely of successive
liftings of the weight of the body. It might therefore be expected
from the above results that this work would be performed even more
economically than that of ascent, since it is obviously the form of
muscular activity for which animals like the horse and dog are
specially adapted. In the case of the walking horse, Kellner * has
proposed a formula based on mechanical considerations, for com-
puting the work of locomotion. Zuntz f has applied this formula
to the animal used in his experiments and computed the mechanical
work of locomotion at the three speeds for which the total metabo-
lism was also determined (p. 507).

Landw. Jahrb. 9 658.
t Ibid , 27. Supp III. p 314

THE UTILIZATION OF ENERGY.

5r3

A comparison of these figures, expressing the total metabolism
in its mechanical equivalent, is as follows:

Speed

Meters per

Minute.

Per Kg. Muss and Meter.

Total
Metabolism
Gram-meters.

Computed

Work

Gram-meters.

Percentage
Efficiency.

78.00
90.16
98.11

138.4 49.14
155.8 54.54
167.0 58.40

35 . 5

35.00

34.97

This computation gives an efficiency slightly greater than that
obtained for the ascent of a grade without load, and in so far tends
to confirm our conjecture, but the basis on which the work of loco-
motion is computed can hardly be regarded as sufficiently accurate
to give this result the force of a demonstration.

The work of draft appears to be performed considerably less
economically than that of ascent or locomotion. Thus, for the
horse, the efficiency for nearly horizontal draft was found to be 31.3
per cent, at a walk, and in one experiment at a trot 31.7 per cent., as
against 34-36 per cent, for ascent. In two other experiments at a
trot, in which the work may have been excessive, a much lower
efficiency was found, viz., 23.4 per cent. For draft up a grade of
8.5 per cent, at a walk the efficiency was greatly reduced, viz., to
22.7 per cent. The above figures refer to the work of draft only,
after deducting the energy required for locomotion and ascent. A
similar difference was likewise observed with the dog (p. 502), the
efficiency in nearly horizontal draft being 28.S per cent, as compared
with 30.7 per cent for work of ascent.

Experiments on man, not cited in the above pages, in which
the work was performed by turning a crank, have shown decidedly
lower figures for the percentage utilization.

Speed and Gait. — The energy expended by the horse in loco-
motion at a walk was found to increase with the speed at the
rate of 0.00334 cal. per meier and kilogram mass for each in-
crease of 1 meter in the speed per minute. Kellner's mechanical
analysis of the work of locomotion mentioned above divides it
into two parts, viz., that expended in lifting the body of the

5 14 PRINCIPLES OF ANIMAL NUTRITION.

animal and that expended in imparting motion to the legs. The
former portion is regarded as constant, while the latter portion
would increase with the speed. The very close proportionality
between the work thus computed and the total metabolism, as
shown -by the table on the preceding page, is a strong confirma-
tion of the correctness of both methods and places the conclusion
as to the influence of speed upon metabolism beyond reasonable
doubt. It is to be remembered, however, that it is the total
metabolism per kilogram and meter which increases with the speed.
The percentage utilization of the energy, so far as the data at our
command enable us to determine, apparently remains constant.
Practically, however, it is the former fact which interests' us, since
the expenditure of energy in locomotion is comparable to that in
internal work and has only an indirect economic value. A similar
effect of speed on the metabolism in horizontal locomotion was
observed by Zuntz * in experiments on man. In those with the dog,
on the other hand, the variations in speed were between 64.2 and
85.9 meters per minute, but no material difference in the metabo-
lism due to locomotion was observed.

In trotting, a horse expends much more energy per unit of hori-
zontal distance than in walking. Thus, trotting at an average
speed of 195 meters per minute (a little over 7 miles per hour), as
compared with walking at an average speed of 90.16 meters per
minute, gave the following results for the metabolism per kilo-
gram mass and meter distance.

Trotting 0.5478 cal.

Walking 0.3666 "'

On the other hand, speed is, so to speak, obtained more econom-
ically at the trot than at the walk. In the averages just given the
speed was increased by 116 per cent., while the metabolism was in-
creased by only 49 per cent. The same result is reached in another
way by computing, by means of the factor given at the beginning of
this paragraph (0.00334 cal.), the theoretical walking speed which
would give a metabolism equal to the average metabolism in trot-
ting. We find this to be 147 meters per second, as compared with 195
meters at a trot. Moreover, it was found that at the trot the metab-
olism did not increase with the speed, within the limits of the ex-
* Arch. ges. Physiol, 68, 198.

THE UTILIZATION OF ENERGY. 515

periments. These, however, did not include speeds above 206
meters per minute (about 7h miles per hour), and the work was done
on a tread-power, so that there was no air resistance. At this
moderate speed it is not probable that the latter factor would be a
large one, but it is one which increases as the square of the
velocity, so that at high speeds it constitutes the larger portion of
the resistance. At high speeds, too, the muscles contract to a
greater degree, thus decreasing their efficiency, and additional auxil-
iary muscles are called into play, both directly and to aid the in-
creased respiration. It is a matter of common experience that while
a horse is able to travel for a number of miles consecutively at 6 to
7 miles per hour, drawing a considerable load, he can maintain his
highest speed for only a short time even without load, and does this
only at the cost of largely increased metabolism. It is evident then
that there is a limit beyond which an increase of trotting speed
must increase the metabolism with comparative rapidity.

Load. — Supporting a load on the back while standing was found
to increase the metabolism of the horse Xo. Ill approximately in
proportion to the load — that is, the metabolism computed per unit
of mass (horse + load) increased but very slightly. In locomotion
with a load the metabolism is, of course, increased, since the load
as well as the body of the animal must be lifted at each step. The
increase over the metabolism at rest and without load, both walking
and trotting, was found in the case of Horse III to be somewhat
greater (8-10 per cent.) than the increase in the mass moved
(horse + load).

After making allowance for this increase in the work of locomo-
tion, the efficiency in ascent with a load was found to be unaffected
by the latter; that is, the energy expended in lifting a unit of mass
(horse + load) through a unit of distance remained substantially
the same. Indeed the figure obtained (36.2 per cent.) is slightly
higher than that without load (34.3 per cent). Interesting indi-
vidual differences in the above particulars were, however, observed
between Horse Xo. Ill and some of the other animals experimented
upon, particularly Nos. II and XIII, which form the subject of a
succeeding paragraph.

Species and Size of Antmal. — In ascending a moderate grade,
the efficiency seems to be about the same in the horse and in

5i6

PRINCIPLES OF ANIMAL NUTRITION.

man, while in the dog it is apparently somewhat less, as is seen
from the following comparison:

Grade,
Per Cent.

Efficiency,
Per Cent.

Man

23

10.7
18.1
17.2

35.7
34.3
33.7
30.7

Horse

<<

Dog

The energy expended in horizontal locomotion, on the other
hand, showed more marked differences, viz.:

Speed. Meters
per Minute.

Energy Expended

per Kg. Mass
per Meter, Kgm.

Dog 78 . 57

Man 42.32-74.48

Horse 78.00

0.501
0.211-0.334

0.138

The relatively high figure for the dog is perhaps due in part to the
considerable muscular effort apparently required (p. 499) to main-
tain the erect posture. It has been shown by v. Hosslin,* however,
by a mechanical analysis of the work of locomotion, that the latter
docs not increase as rapidly as the weight of the animal, but in
proportion to its two-thirds power, or, in other wprds, approximately
in proportion to the surface. If we compare the experiments upon
different species of animals on this basis — that is, if we divide the
total energy expended by the animal for locomotion by the product
of the distance traversed into the two-thirds power of the weight
— we obtain the following figures :

Dog 1 . 501 kgm.

Man 0 . 861-1 . 274 kgm.

Horse 1 . 058 kgm.

Computed in this way, the figures for the horse and those for man at
a comparable speed (74.48 M. per min.) do not differ greatly, and
v. Hosslin's conclusions are to this extent confirmed. The figures
for the dog still remain higher than the others. If, in the case of
* Archiv f. (Anatomie u.) Physiol., 1888, p. 340.

THE UTILIZATION OF ENERGY. 517

this animal, we compare the total metabolism in locomotion with
that during standing instead of lying, as was done in the case of
the horse, the figure is reduced to 1.303 kgm., or not much higher
than in the case of man. It must be remembered, however, that
the figures above given for man include the metabolism due to
standing.

Individuality. — Zuntz & Hagemann's investigations show that
the efficiency of the horse is affected to a considerable degree by
the individual differences in animals. The experiments whose
results are summarized on p. 510 were upon a single animal
(Xo. III). In addition to these a small number of experiments
were made with several other animals, mostly old and more or less
worthless ones, besides the considerable number upon Horse Xo. II
previously reported by Lehmann & Zuntz.* The results are com-
puted by the authors in terms of energy and corrected for speed
upon the basis of the results obtained with Horse Xo. III.

In a single case the work of ascent required slightly less expen-
diture of energy than with Horse Xo. Ill, and in another case the
work of horizontal locomotion, computed to the same live weight
in proportion to the two-thirds power of the latter (see the oppo-
site page) was also less than for Horse Xo. Ill, but as a rule these
old, defective horses gave higher results. For ascent, omitting
one exceptional case, the range was as follows:

Per Kgm. of Work.

Minimum 5 . 906 cals. = 39 . 84 per cent, efficiency

Maximum 9.027 " =26.07 " " "

Horse Xo. Ill .. . 6.851 " =34.30 " "
With one very lame horse (string-halt) the figures reached the
maximum of 12.343 cals., or an efficiency of only 16.6 per cent.

A similar range was observed in the results on horizontal loco-
motion. Reduced to a speed of 78 M. per minute and to the live
weight of Xo. Ill, the range was as follows:

Per Meter and Kilogram Live Weiqht.

Minimum 0 . 284 cal.

Maximum 0 . 441 "

Horse Xo. Ill 0.336 "

* Landw. Jahrb., 18, 1.

5i8

PRINCIPLES OF ANIMAL NUTRITION.

The very lame horse mentioned above gave a still higher figure,
viz., 0.566 cal.

A somewhat larger number of experiments with Horse No. XIII
brought out the interesting fact that the increase in the metabo-
lism caused by carrying a load on the back was markedly less than
in the case of No. Ill, both at rest and in motion.

PER KILOGRAM MASS (HORSE 4- LOAD).

Without I.oar),
cals. per Minute.

With Load,
cals. per Minute.

Standing :

Horse XIII

15.990
18.311

cals. per Meter.
0.389
0.367

0.553

0.548

14.670
18.389

cals. per Meter.
0.388
0.391

0.488
0.601

" III

Walking horizontally :
Horse XIII

" III....

Trotting Horizontally :
Horse XIII

" III

While, without load, Horse No. XIII showed a greater metabo-
lism, both while walking and trotting than did Horse No. Ill, the
additional effort required for carrying a load was relatively less,
so that in every case the metabolism per unit of mass, instead of
increasing, remained unchanged or even diminished. The percent-
age efficiency of the animal in ascending a grade was also not
materially affected by the load, while with Horse No. Ill it ap-
peared to increase slightly.

The experiments with Horse No. II previously reported,* when
recalculated f in the same manner as the later ones, likewise show
interesting individual differences. For horizontal locomotion,
after correcting for varying speeds, we have per kilogram mass
(horse + load) the following:

Horse No. II,
cals. per Meter.

Horse No. Ill,
cals. per Meter.

Walking without load

" with load

0.415
0.385
0.499
0.415

0.367
0.391
0.548
0.601

Trotting without load

" with load

* Landw. Jahrb., 18, 1.

t Ibid., 27, Supp. Ill, 355

THE UTILIZATION OF ENERGY.

519

As these figures show, No. II was decidedly inferior to No. Ill
in walking without load. In trotting, on the other hand, he was
somewhat the superior of No. Ill, or in other words the change
from walking to trotting caused much less increase in his metabo-
lism. Like No. XIII. he carried a load with decidedly less expendi-
ture of energy than did No. 111. For the forms of work in which
the percentage efficiency could be measured the results were as
follows, the grades, however being not exactly the same for No. II
as for No. Ill :

Horse No. II,
Per Cent.

Horse No. Ill,
Per Cent,

Ascending, moderate grade . . .

heavier grade

Draft, nearly horizontal

'33.2
31.7
29.0
22.4

34.3
33.7
31.3

22.7

It seems a fair presumption that such individual differences
as those above instanced are caused, in large part at least,
by differences in the conformation of the animals resulting from
heredity or "spontaneous" variation. A strain of horses which
has been bred and trained especially for the saddle through a
number of generations might very naturally be expected to be
more efficient in carrying a load than a strain which has been bred
for speed in harness or strength in draft, while the latter might as
naturally excel the former in efficiency at the trot or in draft.
Similarly, a race of horses developed in a hilly country might be
expected to be more efficient in ascending a grade than one in-
habiting a flat region. It would seem, too, that these differences
may be not inconsiderable. The results cited suggest an interest-
ing line of thought and investigation for the student of breeding.

Training and Fatigue. — It is a familiar experience that any
unaccustomed form of work is much more fatiguing at first than it
is later. This is due in part to the fact that in making unfamiliar
motions more accessory groups of muscles are called into activity
than are necessary later when more skill has been acquired. The
experience of a learner on the bicycle is an excellent example of
this. In the second place, however, simple exercise of a group of

520

PRINCIPLES OF ANIMAL NUTRITION.

muscles in a particular way seems to increase their average mechan-
ical efficiency.

Graber,* in two series of experiments upon himself, obtained
the following figures for the excretion of carbon dioxide during
rest, horizontal locomotion, and hill climbing, all the trials being
made about the same length of time (four to five hours) after the
last meal:

Work of
Ascent,
Kgm.

Carbon Dioxide
Excreted in 20
Minutes, Grms.

Series I:

Rest

Horizontal locomotion

Hill climbing without practice

" " after 12 days' practice.

Series II (2 months later) :

Rest

Horizontal locomotion

Hill climbing without practice

" " after 14 days' practice.

5892
6076

7376
7539

9 . 706*
19.390*
40.982
32.217

12 . 833
22.418
38.832f
31.001

* Some carbon dioxide may have escaped absorption,
f Some carbon dioxide lost.

Schnyder f has confirmed and extended Gruber's results. In
experiments in a treadmill upon two different subjects he ob-
tained the following figures for the work performed per gram of
carbon dioxide excreted in excess of that given off during rest:

Kgm.

( Without training 218 . 13

No- X \ After 2 months' training 253.18

I Without training 243.93

No. 2 A After 6 days' training 285 . 52

| " 55 " " 319.40

. . I Without training 302 . 76

No. 2 (second series) j M ^ 4? dayg, training 404 39

That the greater efficiency after training is not due solely to a
diminished use of accessory muscles is shown by Schnyder's experi-
ments on convalescents. His results were as follows:

* Zeit, f. Biol., 28, 466
t Ibid., 33, 289.

THE UTILIZATION OF ENERGY.

521

No, 1 — Climbing a hill

No. 2— Treadmill

No. 3— Treadmill

Work per
Gram, Car-
bon Dioxide,
Kgm.

(First trial 215.18

\ 18 days later 306 . 18

First trial 182 . 70

2 days after first trial 248 . 34

10

12

14

15

21

" " 253.74

" " " 238.85

" " 210.87

" " " 227.04

; " " " 227.50

1 2\ months after first trial 441 . 17

First trial 231 .24

2 days after first trial 231 .24

4 " " " " 286.25

In walking the same distance (468 M.) No. 1 excreted the following
excess of carbon dioxide over the rest value :

First trial 4 . 505 grams

A week later 3 . 690 "

A month later 2. 780 "

It appears from these results that the gradual strengthening of the
muscles during convalescence results in a more economical per-
formance of their work, largely independent of amy special training
for a particular kind of work. It seems a justifiable conclusion,
therefore, that a part of the gain due to training arises from its
direct effect in strengthening the muscles, as well as from the in-
creased skill acquired in their use. Conversely, the effect of fatigue
in increasing the relative metabolism, as shown by Loewy,* would
seem to be in part a direct effect. Schnyder summarizes the matter
in the statement that it is not the work itself, but the muscular
effort required, which determines the amount of metabolism.

In the case of domestic animals kept chiefly for work, however,
we may safely assume that they are constantly in a state of training,
and that the results obtained by Zuntz and his associates on the
horse are applicable to work done by normal animals within the
limits of the experimental conditions.

* Arch. ges. Physiol., 49, 405.

522 PRINCIPLES OF ANIMAL NUTRITION.

Relative Values of Nutrients. — In the foregoing discussion
it has been tacitly assumed that the stored-up energy of the pro-
teids, fats, and carbohydrates of the body is all net available energy,
ready to be utilized directly for the production of mechanical work.
As we have seen, however, on previous pages, a school of physiolo-
gists, of which Chauveau may stand as the representative, denies
this, and holds that the fat in particular must be converted into
a carbohydrate before it can become directly available.

In discussing the source of muscular energy in Chapter VI it
was shown that the recorded results as regards the nature of the
material metabolized were insufficient to decide the question, since
the final excretory products are qualitatively and quantitatively
the same whether the fat is directly metabolized in the muscle or
undergoes a preliminary cleavage in the liver or elsewhere in the
body. The results as to energy, however, would be materially
different in the two cases. The dextrose resulting from the cleavage
of fat, according to Chauveau's schematic equation (p. 38), would
contain but about 64 per cent, of the potential energy of the fat, the
remainder being liberated as heat. We cannot, however, suppose
that the energy of this dextrose can be utilized by the muscle any
more completely than that of dextrose derived directly from the
food. It follows, then, that the percentage utilization of the total
energy metabolize^ during muscular work should be materially
greater when the metabolized material consists largely or wholly of
carbohydrates than when it consists chiefly of fat. By supplying
food consisting largely of one or the other of these materials, it is
possible to bring about these conditions, and a determination of
the respiratory exchange and the nitrogen excretion will then
afford a check upon the nature of the material metabolized and the
means of computing the utilization of its potential energy.

Investigations of this sort have been reported from Zuntz's
laboratory. The earliest of these were by Zuntz & Loeb * upon a
dog, the method being substantially the same as that with which
the preceding pages have made us familiar. Their final results for
the energy metabolized per kilogram and meter traveled (including
the work of ascent) were:

* Arch. f. (Anat. u.) Physiol., 1894, p. 541.

THE UTILIZATION OF ENERGY. 523

Diet.

Respiratory
Quotient.

Energy, cals.

Proteids only

Chiefly fat.

" " (body freed from carbohydrate- by
phloridzin)

Much sugar with proteids

" " and little proteids

0.78
0.74

0.71
0.83
0.88

2.58
2.43

2.71
2.58
2.63

The differences are quite small, while, as Zuntz points out, if
2.6 cals. represent the demand for energy per unit of work when
carbohydrates are the source it should, according to Chauveau's
theory, rise to about 3.6S cals. when the energy is derived exclu-
sively from fat.

Altogether similar results have been recently reported from
Zuntz's laboratory by Heineman,* and by Frentzel & Reach, j in
experiments on man.

In Heineman's experiments the work, which was never exces-
sive, consisted in turning an ergostat, the respiratory exchange
being determined by means of the Zuntz apparatus and the total
urinary nitrogen being also determined. From these data, reckon-
ing 1 gram of urinary nitrogen equivalent to 6.064 liters of
Oxygen,J the average amount of energy metabolized on the vari-
ous diets, and the proportion derived respectively from proteids,
fats, and carbohydrates, is computed. By comparison with rest
experiments the increments of oxygen and carbon dioxide clue to
the work were determined, and from these the energy consumed
per kilogram-meter of work was calculated upon three different
assumptions: first, that the proteid metabolism was not increased
by the work; second, that it increased proportionally to the oxy-
gen consumption; third, that as large a proportion of the energy
for the work was furnished by the proteids as is consistent with
the observed respiratory exchange. The results are summarized
in the following table :

* Arch. pes. Physiol., 83, 441.
t Ibid., 83. 477.

* Zuntz, Arch. ges. Physiol., 68, 204.

524

PRINCIPLES OF ANIMAL NUTRITION.

Predominant Nutrient.

Respira-
tory
Quo-
tient.

Total Energy
Supplied by

Fat,
Cala.

Car-
bohy-
drates,
Cals.

Pro-
teids,
Cals.

Energy per Kgm.
of Work.

First
As-
sump-
tion,
cals.

Second

As-
sump-
tion,
cals.

Third

As-
sump-
tion,
cals.

Fat

it::

Carbohydrates. \u"

As much proteids as
possible

0.783
0.724
0.805
0.901

0.796

3829
4422
3414
1543

3381

1379

246

1823

3374

1620

163
163
139
139

377

10.98

9.39

11.15

10.67

11.40

9.35

io!e>3

11.27

10.35

9.27

10.46

10.37

10.64

The subject was not able to consume even approximately
enough proteids to supply the demands for energy, so that the
experiments are virtually a comparison of the utilization of fat and
carbohydrates in different proportions. With the exception of the
third group, the results seem to show that the energy of the fat
metabolized was utilized, if anything, rather more fully than that
of the carbohydrates.

Frentzel & Reach experimented upon themselves, the work
being done by walking in a tread-power; otherwise the methods
were similar to those of Heineman. In computing the results of
the experiments on a carbohydrate and a fat diet they assume that
there was no increase in the proteid metabolism as a consequence
of the work. For the experiments on a proteid diet they com-
pute the results both on this assumption and also on the assump-
tion of a maximum participation of the proteids in work produc-
tion. Calculated in this way the total evolution of energy per kilo-
gram weight and meter traveled was as given in the table on p. 525.
The results show a slight advantage on the side of the carbo-
hydrates, which in the case of Frentzel is regarded by the authors
as exceeding the errors of experiment. They compute, however,
that it is far too small to afford any support to Chauveau's theory.

Zuntz * has recalculated Heineman's results, using slightly
different data but reaching substantially the same result. He
shows, however, that they are affected by the influence of train-
ing already discussed on p. 519. Arranging the experiments in
chronological order, it becomes evident that the work was done
* Arch. ges. Physiol., 83, 557.

THE UTILIZATION OF ENERGY.

525

Frentzel — fat diet:

First week

Second week. . . .

Average

Frentzel — carbohydrate diet:

First week

Second week

Average.

Frentzel — proteid diet :

First assumption . . .

Second assumption. .
Reach — fat diet:

First week

Second week

Average.

Reach — carbohydrate diet :

First week

Second week

Average.

0.900

Respiratory
Quotient.

Energy per Kg.
and Meter, cals.

0.766
0.778

2.088
2.049

0.773

2.066

0.896
0.880

1.932
2.031

0.889

1.980

I 0.799 j

1.933
1.824

0.805
0.766

2.259
2.034

0.781

2.119

0.899
0.901

2.202
2.005

2.086

with increasing efficiency, largely independent of the food, and the
fact that most of the experiments with fat came later in the series
than those with carbohydrates largely, although perhaps not en-
tirely, accounts for the observed difference in efficiency, while the
low figure for proteids is accounted for by the fact that these were
among the earliest experiments. A similar effect appears in the
experiments of Frentzel & Reach, although it is less marked, since
walking is a more accustomed form of work than turning a crank.
On the whole, Zuntz concludes that these experiments warrant the
conclusion that in work production the materials metabolized in
the body replace each other in proportion to their heats of combus-
tion— that is, in isodynamic and not isoglycosic proportions.

THE UTILIZATION' OF METABOLIZABLE ENERGY.

ihe investigations just discussed give us fairly full data as to
the utilization of the stored-up energy of the body in the produc-
tion of external work, and this, as we have seen (p. 497), is sub-
stantially equivalent to a knowledge of the utilization of the net

526 PRINCIPLES OF ANIMAL NUTRITION.

available energy of the food. These determinations by Zuntz and
his co-workers, however, do not bring the energy recovered as
mechanical work into direct relation with the energy of the food;
that is to say (aside from such computations of available energy as
those made by Zuntz & Hagemann * for the food of the horse) ,
they do not tell us how much of the energy contained in a given
feeding-stuff we may expect to recover in the form of mechanical
work, but only what proportion of the stored-up energy resulting
from the use of this feeding-stuff is so recoverable.

It is the former question rather than the latter, however, which
is of direct and immediate interest to the feeder of working animals.
The feeding-stuffs which he employs are comparable to the fuel of
an engine, and the practical question is how much of the energy
which he pays for in this form he can get back as useful work.

Methods of Determination. — Two general methods are open
for the determination of the percentage utilization of the energy
of the food.

It is obvious that if we know the net availability of the energy
(gross or metabolizable) of a given food material we can compute
its percentage utilization in wrork production from the data of the
foregoing paragraphs with a degree of accuracy depending upon
that of the factors used. For example, if we know that the net
available energy of a sample of oats is 60 per cent, of its gross energy,
then if the oats are .fed to a draft horse utilizing, according to Zuntz
& Hagemann, 31.3 per cent, of the net available energy, it is obvious
that the utilization of the gross energy of the oats is 60X0.313 =
18.78 per cent. An entirely similar computation could of course be
made of the percentage utilization of the metabolizable energy of
the oats.

Unfortunately, however, as we have already seen, our present
knowledge of the net availability of the energy of feeding-stuffs and
nutrients for different classes of animals is extremely defective, and
extensive investigations in this direction are an essential first step
in the determination of the percentage utilization of the energy
of feeding-stuffs in work production by this method. Until trust-
worthy data of this sort are supplied, results like those of Zuntz &
Hagemann can be applied to practical conditions only on the basis
* Landw. Jahrb., 27, Supp. Ill, 279 and 429.

THE UTILIZATION OF ENERGY. 527

of more or less uncertain estimates and assumptions regarding the
expenditure of energy in digestion and assimilation such as those
discussed in Chapter XI, § 3.

The second possible general method for the determination of
the percentage utilization of the energy of the food in work pro-
duction is that employed in the determination of the utilization in
tissue production. Having brought the animal into equilibrium as
regards gain or loss of tissue and amount of work done with a suit-
able basal ration, the material to be tested is added and the work
increased until equilibrium is again reached. The increase in the
work performed compared with the energy of the material added
would then give the percentage utilization of the latter.

The accurate execution of this method would require the em-
ployment of a respiration apparatus or a respiration-calorimeter
for the exact determination of the equilibrium between food and
work, while the skill of the experimenter would doubtless be taxed
in the endeavor to so adjust food and work as to secure either no
gain or loss of tissue or equality of gain or loss in the two periods
to be compared. Indeed, it may safely be said that exact equality
would, as a matter of fact, be reached rarely and by accident, and
that as a rule it would be necessary to correct the observed results
for small differences in this respect. To make such corrections
accurately, however, requires, as we have seen in § 1 of this
chapter, a knowledge of the net availability and percentage utili-
zation of the food, and we are thus brought back to the necessity
for more accurate knowledge upon fundamental points.

The extensive investigations of Atwater & Benedict * upon man
appear to be the only ones yet upon record in which the actual
balance of matter and energy during rest has been quantitatively
compared with that during the performance of a measured amount of
work. Unfortunately, however, the gains and losses of energy by
the bodies of the subjects in these experiments were relatively
considerable, while the experiments thus far reported seem to
afford no sufficient data for computing the net availability of the
food for maintenance or its percentage utilization for the production
of gain. Moreover, the authors appear to regard the measurements

* U. S. Depart incut Agr., Office of Experiment Stations, Bull. 109; Mem-
oirs Xat. Acad. Sci., 8, 231.

S28 PRINCIPLES OF ANIMAL NUTRITION.

of the work done as not altogether satisfactory. In a preliminary
paper * Atwater & Rosa compute a utilization of 21 per cent.
Inasmuch as they have not further discussed the question of the
utilization of the food energy for work production it would seem
premature to attempt to do so here. It may be remarked, however,
that the figures given seem to indicate a rather low degree of effi-
ciency for the particular form of work investigated (riding a station-
ary bicycle).

Wolff's Investigations.

The horse, being par excellence the working animal, has natu-
rally been the subject of experiments upon the relation of food to
work. While as yet the respiration apparatus or calorimeter has
not been applied to the study of this phase of the subject, two ex-
tensive and important series of investigations have been made
upon the work horse, viz., by Wolff and his associates in Hohen-
heim and by Grandeau, LeClerc, and others f in Paris, in which the
attempt has been made to judge approximately of the equilibrium
between food and work from the live weight and the urinary nitro-
gen.

Grandeau's experiments were made for the Compagnie generate
des Voitures in Paris, and were directed specifically toward a scientific
investigation of the rations already in use by the company and to
a study of the most suitable rations for the different kinds of ser-
vice required of the horses. They were, therefore, while executed
with the greatest care and exactness, largely "practical" in their
aim.

Wolff's experiments were made at the Experiment Station at
Hohenheim and were broader in their scope, being directed largely
to a determination of the ratio of (digested) food to work. The
following paragraphs are devoted chiefly to an outline of Wolff's
experiments, but with more or less reference also to Grandeau's
results.

Methods. — In discussing the effects of muscular exertion on
metabolism in Chapter VI, mention was made of the interesting

* Phys. Rev., 9, 248; U. S. Dept. Agr., Office of Experiment Station, Bull.
98, p. 17.

t L'alimentation du Cheval de Trait, Vols. I, II, III, and IV, and Annales
de la Science A«ronomique, 1892, I, p. 1; 1893, I, p. 1; and 1896, II, p. 113

THE UTILIZATION OF ENERGY.

529

results obtained by Kellner regarding the influence of excessive
work upon the proteid metabolism of the horse. It was there shown
that when the work was increased beyond a certain amount there
resulted a prompt increase of the urinary nitrogen and at the same
time a steady falling off in the live weight. The method employed
in Wolff's experiments, and which originated with Kellner, is based
upon this fact. It may perhaps be best illustrated by one of
Kellner's earliest experiments,* in which starch was added to a
basal ration, the results of which have already been referred to in
Chapter VI (p. 199).

In the first period the daily ration consisted of 6 kgs. of oats
and 6 kgs. of hay, while in the second period 1 kg. of rice starch was
added. Digestion trials showed that there was digested from these
rations the following:

Period I,
Grms.

Period II,
Grms.

Increase.
Grms.

Crude protein

757.07

636.10

3874.36

279.45

750.53

713.40

4488 . 15

275.43

— 6.54

" fiber

Nitrogen-free extract

+ 77.30
+ 613.79

Ether extract

- 4 02

5546.98

6227.51

+ 680.53

The work was performed in a special sweep-power which was
so constructed as to act as a dynamometer. With a uniform draft
of 76 kgs., the daily work in the four subdivisions of the first
period consisted of 300, 600, 500, and 400 revolutions respectively,
while in the two subdivisions of the second period it was 800 and
600 respectively. From the daily results for live weight and urinary
nitrogen and from a comparison with another period in which 1.5
kgs. of starch was fed, Kellner concludes that the maximum amounts
of work which the animal could perform vithout causing an increase
in its proteid metabolism and a decrease in its live weight were for
the first period 500 revolutions and for the second period 700 revo-
lutions. The difference of 200 revolutions, then, represents the
additional work derived from the added starch. Two hundred
revolutions with a draft of 76 kgs. equaled 438,712 kgm., to which
is to be added the work of locomotion, estimated by Kellner (com-
* Landw. Jahrb., 9, 670.

53° PRINCIPLES OF ANIMAL NUTRITION.

pare p. 539) at 100,000 kgm., making the total additional work
538,712 kgm. Kellner compares this difference with the increased
amount of nitrogen-free extract digested, 613.79 grams, neglecting
the small differences in the other nutrients. As corrected in a later
publication,* the results are as follows:

613.79 grms. starch =2527. 601 Cals. = 1,071,698 kgm.
538,712-1,071,698 = 50.27 per cent.

If we base the calculation upon the difference in total organic
matter digested, the percentage will of course be somewhat smaller.

It was discovered later that the indications of the dynamometer
used in these experiments and many subsequent ones were untrust-
worthy, so that no value attaches to the percentage computed above,
"but it serves just as well to illustrate the method employed, and
which was followed in the whole series of experiments. In brief,
the attempt is to find in the indications of live weight and urinary
nitrogen a partial substitute for the determination of the respira-
tory products. As Kellner and Wolff do not fail' to point out, the
results are but approximations, and in any single experiment may
vary considerably from the truth, but on the average of a large
number of experiments it was hoped that satisfactory results might
be reached. In later experiments rather more importance seems
to be attached to the effects upon live weight than to those upon
urinary nitrogen, but it should be noted that the live weight showed
remarkably small variations from day to day, under the carefully
regulated conditions of the experiments, and was quite sensitive
to changes in the amount of work done.

The experiments may be conveniently divided into three groups.
The first of these f includes the years 1877 to 1886, inclusive, in which
the work done was compared with the total digested food. The
second % covers the experiments of 1886-1891, in which the digested
crude fiber was omitted in computing the work-equivalent of the
food, while the third group § includes the experiments of 1891-1894
with a new and more accurate form of dynamometer.

* Wolff, Grundlagen, etc., p. 89.

t Grundlagen fur die rationelle Futterung desPferdes, 1886, 66-155; Neue
Beitrage, Landw. Jahrb., 16, Supp. Ill, 1-48.

X Landw. Jahrb , 16, Supp. Ill, 49-131, and 24, 125-192.
§ Ibid., 24, 193-271.

THE UTILIZATION OF ENERGY.

531

Experiments of 1877-1886. — During the years named, in addi-
tion to the preliminary investigations necessaiy in working out the
method, a large number of experiments were made on three different
animals. The rations consisted largely of hay and oats in some-
what varied proportions, together with smaller amounts of other
feeding-stuffs. In three experiments on starch and four on oats a
comparison of the increase in digested nutrients * with the in-
creased work which could be done gave the following results : f

Increase in Digested]

Nutrients,

Grms.

Increase in Work Done

at 76 Kg. Draft,

Revolutions.

Nutrients Equivalent

to 100 Revolutions,

Grins.

Starch

677.3
577.0

217
175

312

Oats

318

315

The Maintenance Requirement. — As already stated, it was
discovered later that the dynamometer used was unreliable and
gave too high readings, so that the above result cannot be em-
ployed to compute the utilization of the energy of the added food.
It does, however, in its present form, enable us to compute the
maintenance requirements of the horse by subtracting from the
total digested food the nutrients equivalent to the work performed
(i.e., 3.15 gramsXthe number of revolutions). The results of such
a computation made by Wolff % are given on p. 532.

The actual live weights in these experiments were somewhat
below the normal weights, which were regarded as being about
533 kgs. for No. I, 500 kgs. for No. II, and 475 kgs. for No. III.
Wolff considers the maintenance requirements to be independent
of minor changes in weight, and on the basis of the above "normal"
weights computes the maintenance requirements per 500 kgs. live
weight as follows:

Horse 1 4143 grams

" II 4260 "

" III 4167 "

Average 4190 "

* The algebraic sura of the differences in the single nutrients is used, and
in this and the succeeding comparisons the digested fat is multiplied by 2.44.
f Loc. tit., pp. 125-129.
% Loc tit., pp. 99 and 132.

532

PRINCIPLES OF ANIMAL NUTRITION.

No. of
Experi-
ments.

Total

Nutrients,

Grms.

Nutritive
Ratio.

Live

Weight,

Kgs.

No. of
Revolu-
tions.

Equiva-
lent
Nutrients,
Grms.

For

Mainte-
nance,
Grms.

Horse II:

1881-82

1882-83....
1883-84....

4

7
4
6

6305.6

5831 . 1
6748.3
5920.2

1:5.79

1:6.64
1:6.37
1:7.26

521

477
486
457

600

546
662
567

1890

1720

2085
1786

4416

4111

4663
-.134

Average...

Horse HI:

1881-82

1882-83

1883-84

1885

17

6
6
5
4

6078.4

5313.8
6061.3
5734 . 8
5761.2

1:6.80

1:7.16
1:6.88
1:7.55
1:7.57

473

454
469
473
473

577

404
683
580
575

ISIS

1273
2152

1827
1811

4260

4041
3909
3908
350

Average.. .

21

5717.8

1:7.29

467

5.1

1766

3952

By means of a comparison of the results by groups * Wolff
shows that the maintenance requirement as thus computed is appar-
ently independent of the amount of work done and of the nutritive
ratio, and from this uniformity concludes that the relative efficiency
of the food for work production is unaffected by these factors,
within the range of his experiments.

A series of similar experiments on Horse No. Ill in 1885-86, f
computed in substantially the same way, gave results for the main-
tenance ration agreeing well with those of earlier years, viz.,

Period 1 3934 grams total nutrients

" II 3984 "

" III and V 4001 "

" VII6 4094 "

" VIII 4094 "

Average 4021 "

with an average live weight of 475 kgs., equivalent to 4230 grams
per 500 kgs. In a succeeding period (IX), however, in which hay
alone was fed, a decidedly higher result was obtained, viz., 4357
grams per head, or 4586 grams per 500 kgs.

* hoc. cit., pp. 135 and 137.

t Landw. Jahrb., 16, Supp. Ill, 32.

THE UTILIZATION OF ENERGY.

533

Experiments of 1886-91.— In the experiments thus far de-
scribed, with the exception of the last, the proportions of grain and
coarse fodder in the rations were not widely different, the latter
furnishing on the average fully one half of the dry matter fed.
Consequently the experiments were not calculated to bring out any
difference in the nutritive value of the two such as is indicated by
the results of the one trial with hay alone.

Grain vs. Coarse Fodder for Maintenance. — The results
obtained by Grandeau & LeClerc upon the maintenance ration of
the horse when fed a mixture containing about 75 per cent, of
grain fully confirm the indications of Wolff's trial with hay.
Their experiments have been very fully discussed, and in part
recalculated, by Wolff * in their bearing on this question. The
three horses experimented on were fed two different amounts of
the same mixture in several different thirty-day periods, eighteen
such periods in all being available for comparison. In all of them
the animals were led daily, at a walk, over a distance of about four
kilometers. Wolff estimates the amount of work of locomotion by

means of the formula — ( -
2\g

and by subtracting the equivalent

amount of nutrients from the total digested obtains the amount re-
quired for maintenance. The results are as follows:

No. of
Experi-
ments.

Live
Weight,

Kgs.

Digested

Nutrients,
Grms.

Nutrients
Equiva-
lent to
Work,
Grms.

For Maintenance.

Per

I lead,

Grms.

Per

500 Kgs.,

Grms.

Heavier Ration :

Horse I

" II

" III

3
5
4

416.6
405.9
439.0

3553
3432
3625

110

108
119

3443
3324
3506

4132
4078
3994

Average

420.5

411.0
441.2

3537

3060
3310

112

108
119

3425

2952
3191

4068

Lighter Ration :

Horse II

" III

2
4

3636
3617

Average

426.1

3185

114

3071

3626

The results, and particularly those on the lighter ration, which
appeared ample for maintenance, are much lower than those com-
* Landw. Jahrb., 16, Supp. ill, 73-81.

534

PRINCIPLES OF ANIMAL NUTRITION.

puted in the previous paragraph. The difference is too great to be
ascribed to experimental errors in estimating the small amount of
work done, and can most reasonably be ascribed to the difference
in the character of the ration. Apparently the horse, like cattle
(p. 433), requires less digestible food for maintenance when the
latter consists largely of grain than when it is chiefly or wholly coarse
fodder.

Direct experiments by Wolff * likewise show that the digestible
nutrients of concentrated feed (oats) are more valuable for work
production than those of coarse feed (hay). The experiments
were made in the manner already described, the draft being uni-
formly 60 kgs. Although tne measurements of the work actually
done are probably incorrect, it may be assumed to have been
substantially proportional to the number of revolutions of the
dynamometer. A ration of 3 kgs. of hay and 5.5 kgs. of oats served
as the basal ration, to which was added in one case 4 kgs. of hay
and in another H kes. of oats. The nutrients digested in each

case

and the equivalent

amount of work secured were

Ration.

Digested.

£ >

^o

Protein,
Grms.

Crude
Fiber,
Grms.

Nitrogen-
free
Extract,
Grms.

Ether
Ex-
tract,
Grms.

Total
(Fat X

2.4),
Grms.

■SpS

I-III

v....

7 kgs. hav, 5.5 kgs. oats
3 " " 5.5 "

822 . 58
626 . 46

816.68
422 . 74

3889.64
3068 . 46

186.72

1S4.7S

5973 . 62
4561.13

750
350

196.12

393.94

821.18

1.94

1412.49
353.12

5434.21
).-.f,l .13

400

VI...

v....

3 kz>. hay, 7 kgs. oats...
3 " " 5.o " " ..

1 .5 kg*, oats

754 . 52
626 16

355 . 24
393 . 94

3719.24
3068.46

252.17
184.78

700
350

128.06

-67.50 650.78

67.39

873.08
249.45

350

The relative value of the digested matter of hay and of oats for work
production in these trials was thus approximately as 5 : 7.

In the earlier experiments (p. 531) it was found that when oats
or starch were added to a basal ration, approximately 315 grams
of digested nutrients were required to produce the amount of
work represented by 100 revolutions at 76 kgs. draft. Converting
this result and the one just given for oats into kilogram-meters,
Wolff computes that 100 grams of digested nutrients was equivalent,
* hoc. cit., pp. 84-95.

THE UTILIZATION OF ENERGY.

535

in round numbers, to 85,400 kilogram-meters in the earlier experi-
ments and to 90,480 in the one just cited. While these figures are
not correct absolutely, they are probably comparable, being ob-
tained with the same apparatus. In the later experiment the
work of locomotion is computed by Wolff's formula, which gives
higher results than Kellner's. Taking this into account we may
regard the agreement of the two equivalents as satisfactory.

Value of Crude Fiber. — In all the experiments with con-
centrated feeds the additional nutrients digested from the added
food contained no crude fiber, the apparent difference, indeed,
being in most cases, as in the above experiment, negative. When
hay was added, on the other hand, over one fourth of the addi-
tional nutrients digested consisted of crude fiber. If, now, we
neglect this crude fiber and compare the work and the fiber-free
nutrients we have 1018.55^-4.00-= 254.64 grams of fiber-free nutri-
ents per 100 revolutions, or a figure corresponding almost exactly
with that obtained for the fiber-free nutrients added in oats or
starch. In other words, it would appear from this result that the
digested crude fiber of hay is as valueless for work production as it
appears to be for maintenance.

If, however, the crude fiber is valueless both for maintenance
and work, then by omitting it altogether from our computations we
ought to get results for the maintenance ration and for the ratio of
nutrients to work which are independent of the proportion of grain
to coarse fodder in the ration. Confirmatory evidence of this sort
is abundantly furnished by Wolff's experiments and likewise by
the results of Grandeau on maintenance. Taking first the averages
of the experiments of 1877-1886 (p. 532) we have —

Nutrients Digested.

No. of
Revolu-
tions at
76 Kgs.

Equiva-
lent

Nutrients.
Grms.

Fiber-free Nutrients
fur Maintenance.

Total,
Grms.

Crude
Fiber,
Grms.

Without
Crude
Fiber,
Grms.

Per Head,
Grms.

Per .',1 H 1
K-.
Grui.^.

Horse I

• II ... .
" III...

6306
6078
5718

815 | 5491
978 I 5100
809 ! 4909

600
577
561

1890

ISIS

1766

3601

3282

143

3378
3282
3306

3322

536

PRINCIPLES OF ANIMAL NUTRITION.

The results of the series made in 1885-86 on Horse No. Ill
(p. 532), computed in the same way, give the following as the
amounts of fiber-free nutrients required for maintenance :

Per Head.
Grms.

Per 500 Kgs.

Live Weight,

Grms.

Period I

3270
3186
3242
3342
3316
3170

3442
3353
3413
3549
3490
3335

" II

" III and V

" VII

" VIII

" IX

Average

3254

3430

From Grandeau's experiments (p. 533), by the same method, we
have for the lighter ration the following:

Per Head,

Grms.

Per 500 Kgs.

Live Weight,

Grms.

Horse II

2732
2935

3324
3328

" III

Average

3326

Finally, for the series of experiments by Wolff, just discussed,
upon the relative value of the digested matter of oats and of hay,
and from which the conclusion as to the lack of value of the crude
fiber was drawn, by computing backwards, we get figures for the
fiber-free nutrients required for maintenance which not only agree
with each other, as they necessarily must, but also with those of
the earlier experiments. The results are:

Per Head, Grms.

Per 500 Kgs Live
Weight Grms.

Period I-III

3175
3275
3180
3196

3342
3429
3329
3364

IV

V

" VI

Average

3366

THE UTILIZATION OF ENERGY. 537

Wolff's conclusions from these results* are —

1. The digested crude fiber is apparently valueless, both for
maintenance and for work production.

2. The remaining nutrients may b§- regarded as of equal value
whether derived from grain or coarse fodder.

3. The maintenance of a 500-kg. horse requires approximately
3350 grams per day of fiber-free nutrients.

Wolff's subsequent experiments up to 1891 f gave results con-
firmatory in general of the above conclusions. Particularly was
this the case when the work of locomotion was computed by Kell-

1 /W\

ner's formula and not by the formula — ( — )v^. The work done

2 W/

(expressed in number of revolutions of the dynamometer) per 100
grams of fiber-free nutrients was reasonably uniform and agreed
well with the results previously obtained, while the fiber-free
nutrients required for maintenance likewise agreed with the results
given above. On the other hand, the inclusion of the digested
crude fiber in the computations gave in many cases strikingly
discordant results. In view of the unreliability of the measurement
of the work no conclusions can be drawn as to the percentage
utilization of the energy of the food, and it seems unnecessary to
describe the individual experiments.

A discussion by Wolff J of the results of some of the experi-
ments by Grandeau in which work was done, although rendered
uncertain by the difficulty in estimating the work of locomotion at
varying velocities, and by the changes in live weight of the animals,
is to indicate that they also confirm Wolff's conclusions.

Significance of the Results. — In drawing his conclusions
Wolff is careful to say that the digested crude fiber is appan ntly
valueless, and while calling attention to Tappeiner's then recent
q the fermentation of cellulose in the digestive tract as
probably explaining its low nutritive value he points out that
ingredients of the food may also undergo fermentation. He
therefore holds fast to the fact actually observed, viz., the lower
nutritive value of the digested matter of coarse fodder compared

* Loc. tit., p. 95.

t Lanthv Jahrb., 24, 125-192.

tlbid., 16, Supp III, 110-126.

538 PRINCIPLES OF ANIMAL NUTRITION.

with that of grain, and virtually regards the amount of crude
fiber as furnishing a convenient empirical measure of the difference.
In the light of our present knowledge this reserve seems amply
justified. The difference in the value, of coarse fodder and grain
we should now regard as arising largely from the difference in the
amounts of energy consumed in digestion and assimilation. Kell-
ner's experiments on extracted straw discussed in the previous
section have shown, however, that with cattle this difference
is by no means determined by the simple presence of more or less
crude fiber, but is related rather to the physical properties of the
feeding-stuff, while Zuntz (see p. 392) has shown that the same
factor largely affects the work of mastication in the horse. That
the nutritive value of the rations in Wolff's experiments was pro-
portional to the amount of fiber-free nutrients which they contained,
or, in other words, that the energy expended in digestion, etc., was
proportional to the digested crude fiber, is explained by the limited
variety of feeding-stuffs employed. The coarse fodder was meadow
hay with, in some cases, an addition (usually relatively small) of
straw, while the grain was commonly oats, part of which was in some
instances replaced by maize, beans, barley, flaxseed, or oil-meal,
while starch was added to the ration in a number of trials. The
larger part of the work of digestion, under these circumstances, was
probably caused by the coarse fodders, viz., hay and straw, while the
digested crude fiber was likewise derived chiefly or entirely from
these substances. Such being the case, it follows that the loss
of energy through digestive work would be in general proportional
to the amount of crude fiber in the ration. The essential point in
Wolff's experiments is that the omission of crude fiber renders the
results concordant, and this is as well explained in the manner just
indicated- as by the estimate of Zuntz & Hagemann that the work
of digesting and assimilating crude fiber consumes the equivalent
of its metabolizable energy.

Experiments of 1891-94. — In the dynamometer employed by
Wolff the resistance was produced by the friction of metallic sur-
faces. A copy of his dynamometer was employed by Grandeau &
LeClerc in their investigations at Paris, and these experimenters
found* that the measurement of the work was subject to large errors,

* Fourth Memoir, p. 49.

THE UTILIZATION OF ENERGY. 539

particularly in experiments at a trot, owing to the continual changes
in the friction. Wolff believes that in his experiments, all made
at a rather slow walk, the errors are less, but admits that they are
sufficient to deprive his computations of utilization of all val e.

Grandeau tv LeClcrc, however, were successful in improving
the dynamometer, by the addition of an integrating apparatus,* so
that its measurements of the total work were satisfactory, and this
apparatus was added to Wolff's dynamometer in 1891. Before that
date, therefore, Wolff's experiments, while of great value in many
other respects, afford no trustworthy direct data as to the utili-
zation of the energy of the food for work production, although, as
we have just seen, they afford some information on subsidiary
points. From 1891, however, we may regard the measurements of
the work done on the dynamometer as reasonably accurate.

Corrections. — Unfortunately, in the light of subsequent
investigation, the same is not true of some of the other factors
entering into the comparison, particularly the work of locomotion
and the metabolizable energy of the food.

In all his later experiments Wolff computes the work of hori-

1 (W\

zontal locomotion per second by means of the formula — I — \v2,

in which W equals the weight of the animal, g the force of gravity,
and v the velocity per second. Zuntz's experiments, however,
appear to show that this formula gives too high results, the error
increasing with the velocity, and Wolff t himself recognizes the truth
of this for higher speeds. According to Zuntz's determinations
(p. 512), Kellner's method of computation gives results agree-
ing quite closely with those computed from his respiration experi-
ments. Under the conditions of Wolff's experiments this corre-
sponds quite closely to 50,000 kgm. per 100 revolutions of the
dynamometer, and in the comparisons which follow this amount
has been substituted for that computed by Wolff, thus reducing
materially the figures for the total work performed.

Wolff estimates the metabolizable energy of the food, on the
basis of Rubner's results, by multiplying the digested fat by 2.4,
adding the remaining digested nutrients, and reckoning the total

* Ann. Sci. Agron., 1881, I, 464.

f Landw. Jahrb., 16, Supp. Ill, 119.

54° PRINCIPLES OF ANIMAL NUTRITION.

at 4.1 Cals. per gram. As we have seen, however (Chapter X),
this figure is probably too high for herbivora, although exact figures
for the horse are not yet fully available. Approximately, however,
we may estimate the metabolizable energy of the several digested
nutrients as follows (p. 332):

Protein 3 . 228 Cals per gram

Crude fiber 3.523 " " "

Nitrogen-free extract 4. 1S5 " " "

Ether extract 8.572 " " "

Zuntz * estimates the metabolizable energy of the total nutri-
ents (including fat X 2.4) at 3.96 Cals. per gram. This figure is
probably somewhat high, especially for rations containing much
crude fiber or ether extract, but may serve the purpose of approxi-
mate calculations.

Experiments on Single Feeding-stuffs. — Comparatively few
of the experiments admit of a direct computation of the utiliza-
tion for a single feeding-stuff, since in most cases the amounts of
two or more feeding-stuffs were varied simultaneously. As an
example of the former class we may take Periods I and II of the
experiments of 1892-93. In Period I the ration consisted of 7.5 kgs.
of hay and 4 kgs. of oats per day, while in Period II the oats were
increased to 5.5 kgs. The quantities of nutrients digested and the
metabolizable energy of the difference between the two rations
(computed by the use of the factors just given) were —

Protein,
Grms.

Crude
Fiber.
Grms.

Nitrogen-
free
Extract,
Grms.

Ether

Extract,

Grms.

Total

Nutrients,

Grms.

Period II

" I

1022.4

847.8

849.6
819.9

4152.8
3598.4

175.8
137.1

6446.6
5595.3

Difference ....
Equiv. energy . . .

174.6

Cals.
564

29.7

Cals.

105

554.4

Cals.

2320

38.7

Cals.
332

851.3

Cals.

3321

In Period I (20 days) the daily work consisted of 300 revolutions
of the dynamometer. With this amount of work the live weight
of the horse underwent very little change, but there was a material

* Landw. Jahrb., 27, Supp. Ill, 418.

THE UTILIZATION OF ENERGY. 541

gain of nitrogen, so that Wolff estimates that the work might have
been increased to 350 revolutions. In Period II (23 days) the
daily work was increased to 450 revolutions and the same behavior
was observed, while a further increase to 500 revolutions during
the last ten days checked the gain of nitrogen without causing a
decrease in live weight. Taking 350 and 500 revolutions respec-
tively as representing the maximum amount of work that could be
done on the two rations, the equivalent of the oats added may be
computed as follows:

Revolutions.

Equivalent
Work,
Kgm.

Period II

500
350

1 030 687

" I

722,678

Difference

150

308 009

Work of locomotion for 150 revolutions

75,000

Total difference

383,009

903 Cals.

Equal to. .

The percentage utilization was therefore 903^3321 = 27.2 per cent.

The above figures serve to exemplify the general method of
computation and likewise to illustrate the weak points in Wolff's
experiments, viz., the uncertainty in the determination of the work
of locomotion and the impossibility of demonstrating the equilib-
rium of food and work without the use of the respiration apparatus
or calorimeter.

Out of the whole number of experiments between 1891 and 1894,
seven admit of a comparison of this sort, viz., four on oats, two on
straw, and one on beans. Upon making the computations, how-
ever, the results are found to be so exceedingly variable (the range
for oats, e.g., being from 1G.89 to 63.96 per cent.) as to demonstrate
that the data of Wolff's experiments are not sufficiently exact to be
used in this way, and that the apparently reasonable result just
computed is purely accidental.

Utilization of Fiber-free Nutrients. — But although Wolff's
results do not enable us to compute the percentage utilization of
single feeding-stuffs, if we accept provisionally his conclusions re-
gar* ling the non-availability of the crude fiber they afford data for
numerous computations of the utilization of the fiber-free nutrients,

542

PRINCIPLES OF ANIMAL NUTRITION.

and these computations in turn supply a check upon the hypoth-
esis of the non-availability of crude fiber.

Wolff makes the comparison by deducting from the total fiber-
free nutrients 3300 grams per 500 kgs. live weight for maintenance
and comparing the energy of the remainder with the amount of
work done. In the following tabulation of his results this method
has been pursued. For the energy of the fiber-free nutrients,
Zuntz's figure (3.96 Cals. per gram) has been used and the work of
locomotion has been estimated at 50,000 kgm. per 100 revolutions
of the dynamometer (compare p. 539).

Period.

He
III

IV.

la-d.
lib. .

III. .

IV. .
V...

I. ..
II..
III.
IV6.
V. . .
Vic.

I. .

III.

v..

VI.

Ration.

1891.

Hay, 7.0 kgs.; oats, 4.5 kgs.

" 7.0 " " 5.5 " .

" 4.5 " " 7.0 " .

Average

1892.

Hay, 7 . 5 kgs. ; oats, 4 . 0 kgs. ;

" 4.5 " " 5.5 " straw, 1 kg
" 4.5 " grain, 5.0 kgs.; '
" 4.5 " " 5.0 "
" 7.5 " oats, 4.5 " ....
Average

5 kgs
5 "

Hay.

1892-93.
5 kgs.; oats, 4.0 kgs.
5 " " 5.5 " .
0 " " 5.5 "
0 " " 5.5 "
0 " " 7.5 "
0 " " 7.5 "

Average

, 1 kg. .

1 " .

2 kgs

1893-94.
Hay, 6.5 kgs.; oats. 4.0 kgs.; straw, 1.0 kg.
" 3 0 " " 7.0 " " 2. 5 kgs.
" 3.0 " grain, 7.0 " " 2.5 " .
" 3.0 " " 6.5 " " 2.5 " .
Average

Fiher-free

Nutrients

Minus 3300

Grras.

Grms. Cals.

1,424
1 .990

2,2.39

1,775
1.873
1,521
1.860
1 903

1,475
2.297
1.670
2,036

2,577
2,692

5,639

7,881
S.945

7,026
7.416
6,023
7,365
7,537

5.841
9.095
6.613
8.063
10,210
10,660

Work Done.

Kgm.

Cals

931.676 2.197
1,129,568 2,663
1,094,328 2,581

1,074,802 2,535

1,153,813 2,720

912,454 2,152

1,186,577
1,188,388

897,678
1.280,687

905.568
1,167,127
1,421,285
1.549,620

1,607 6,362 900,26
2,580 10,220 1,519,262
2,560| 10.140 1,545,702
2,880 11,420 1,673,786

2.799
2,803

38.95
33.79
28.86
33.16

36.07
36.68
35 . 73
38.00
37.18
36.77

2,116 36.24
3.024 33.20
2,135 32.28
2.752 34.14
3,352 32.85
3,655 34.28
33 . 74

2.122 33.36
3.653 35.76
3.645 35.95
3,948 34.61
,35.05

In every instance but one the utilization as thus computed
exceeds 31.3 per cent. In other words, the energy of the body
material which, according to Zuntz & Hagemann's results, must
have been metabolized to produce the amount of work done exceeds
considerably the amount computed to be available from the food.
There being no reason to question the substantial accuracy of Zuntz
& Hagemann's factor, this means, of course, that if the food and
work were in equilibrium our estimates of the energy available from.

THE UTILIZATION OF ENERGY.

543

The food are too low. Either 3300 grams of fiber-free nutrients
(13,068 Cals.) is too large an allowance for maintenance, or the
assumption that the energy of the digested crude fiber is substan-
tial ly equivalent to the work of digestion and assimilation is erro-
neous, or, finally, the figure of 3.96 Cals. per gram of digested nutri-
ents is too small. As regards the latter possibility, while it may
be conceded that the energy per gram of digested matter will vary
somewhat in different experiments, the difference will be too small
to materially affect the result. The uncertainty regarding the
maintenance requirement may be readily eliminated by a computa-
tion based on the differences between the several periods, thus afford-
ing, to a degree at least, a test of the correctness of Wolff's hypothe-
sis regarding the crude fiber. The following table contains the
results of such comparisons. In each series the period with the
least amount of digested food (fiber-free) has been compared with
the other periods of the same series.

Metabolizable

Energy of

Fiber-free

Nutrients.

Cals.

Work,
Cals.

Utilization,
Per Cent.

Period HI

1891.

" lie

Period IV

" He

1892
Period \a-d

" III

Period 116

" III

Period IV

" III

20,949
18.707

2,242

22,013

18,707

3,306

20,094
19,091

1,003

20,484
19,091

1,393

20 433
19,091

1,342

2663
2197

466

2581
2197

384

2535

2152

383

2720
2152

568

2799
2152

647

20.79

11.62

38.19

40.77

4S.21

544

PRINCIPLES OF ANIMAL NUTRITION.

Metabolizable

Energy of

Fiber-free

Nutrients,

Cals.

Work,
Cals.

Utilization,
Per Cent.

1892.
Period V

20,605
19,091

2803
2152

" III

1892-93.
Period II

1,514

22,163
19,295

651

3024
2126

43.00

" I and III

Period TVb

2,868

21,131
19,295

898

2752
2126

31.31

" I and III

Period V

1,836

23.278
19,295

626

3352
2126

24.10

" I and III

Period Vic

3,983

23,728
19,295

1226

3655
2126

30.78

" I and III

1893-94.
Period III

4,433

23,288
19,430

1529

3653

2122

34.48

" I

Period V

3,858

23,208
19,430

1531

3645
2122

39.68

" I

Period VI

3,778

24,488
19,430

1523

3948
2122

0.31

I

Totals and averages, ex-
cluding 1891-92

5,058
31,066

1826
11,408

36.10
36.73

With the exception of the experiments of 1891-92, which were
the first with the new form of dynamometer and which Wolff con-
siders unsatisfactory, we have but two cases in which the apparent
utilization does not cxc?ed 31.3 per cent. Having eliminated the
uncertainty as to the maintenance ration, and the figures for the
energy of the food being regarded as substantially correct, this can

THE UTILIZATION OF ENERGY. 545

mean only one of two things, viz., that the figures for the work done
are too high or that the deduction on account of the crude fiber is
too great.

That a determination of the equivalence of food and work by
Wolff's method is subject to considerable uncertainty in an indi-
vidual case is obvious, but there seems to be no apparent reason
why it should be uniformly overestimated. The measurement of
the work was made with great care, and while the work of locomo-
tion is an estimate, its close agreement with the results of Zuntz &
Hagemann (p. 539) renders it unlikely that it is seriously in error.

It would appear, then, that with the rations used in these ex-
periments the energy required for digestion and assimilation was
less than the energy of the digested crude fiber. How much less it
was, however, unfortunately does not appear, and we are obliged
to content ourselves for the present with this negative conclusion.

Zuntz & Hagemann's Computations. — These investigators *
have recalculated Wolff's results in a still different maimer. In-
stead of taking for the amount of work equivalent to the ration the
figures given by Wolff, which, as already explained, are to a certain
extent estimates, they take the amount of work actually performed
in each case and correct for the observed gain or loss of live weight.
This method is in conception more scientific than Wolff's, pro-
vided the requisite correction can be accurately estimated. As the
basis for such an estimate, Zuntz & Hagemann take an early experi-
ment by Wolff, f from which they compute that one gram loss of
live weight is equivalent to one half revolution of the dynamometer
(at 76 kgs. draft). From the same experiment they compute the
mechanical equivalent of one revolution as 2694 kgm. This, how-
ever, aside from the fact that it is the result of a single series of
experiments, was obtained with the old form of dynamometer,
whose indications, as we have seen, were too high, but the later
experiments unfortunately are not reported in a way to permit of
an estimate of the difference.

Taking the correction, then, as estimated, Zuntz & Hagemann
divide Wolff's experiments into two groups, viz., those in which the
work was 400 or less revolutions and those in which it was more

* hoc tit, pp 412-422.
f Grundlagen, etc., d. 80.

546

PRINCIPLES OF ANIMAL NUTRITION.

than 400 revolutions. Comparing the averages of these two groups,
they obtain the following:

Total
Digested
Nutrients,

Grms.

Work, Kgm.

Loss of

Live

Weight,

Grms.

Heavier work (18 experiments)

Lighter " (13 " )

6236
5851

1,415,755
995,225

179.5
7.3

385

420,530
231,922

172.2

Correction for loss of weight

188,608

According to this computation, the 385 grams of added nutrients
enabled 188,608 kgm. of work to be performed. At 3.96 Cals. per
gram the metabolizable energy of the added nutrients equals 1524
Cals. From this, according to Zuntz & Hagemann, is to be de-
ducted 9 per cent, for the work of digestion and also 2.65 Cals.
for each gram of total crude fiber in the added food. On this
basis we have the following:

Weight,
Grms.

Energy,
Cals.

Digested nutrients.
Average crude fiber fed:

Heavier work

Lighter work

Difference ,

Equivalent energy ,

Work of digestion (1524 X 0.9),

Deduction

Available energy

Work done (188,608 -4- 424).

385

2338
2356

-18

1524

-48
137

89

1435
445

The work done is 31 per cent, of the computed available energy of
the food, a figure corresponding very closely with the 31.3 per
cent, found by Zuntz & Hagemann.

The difference in the average amount of crude fiber fed in the
two groups of experiments is so small that the estimate for the

THE UTILIZATION OF ENERGY. 547

energy required by its digestion hardly affects the computation.
What the result appears to show is that the estimate of 9 per
cent, for the digestion and assimilation of the fiber-free nutrients
is approximately correct.

The difference in the amount of digested crude fiber was some-
what greater than that in the total amount. If we make the com-
parison of the two averages on the basis of the fiber-free nutrients
in the same manner as in previous cases we have —

Fiber-free nutrients :

Heavier work 5524 grams

Lighter work 5086 "

Difference 438 "

Equivalent energy 1735 Cals.

Energy of work 445 "

Utilization 25. 65 per cent.

Apparently a considerable amount of energy was required for
the work of digestion and assimilation in addition to that equiva-
lent to the digested crude fiber, a result which seems to conflict
with the conclusions drawn from a discussion of the same experi-
ments in the preceding paragraph. The apparent discrepancy lies
in the determination of the amount of external work equivalent
to the added nutrients. Wolff, as we have seen, after securing
an approximate constancy of live weight, corrects the measured
amount of work in accordance with his judgment of the amount
which would have been equivalent to the ration given and relies on
the "might of averages" to overcome the inherent uncertainties of
his method. Zuntz & Hagemann, on the other hand, reckon with
the measured amount of work, but are then compelled to correct
their final result for the loss of live weight, and unfortunately this
correction is relatively a very large one (over 50 per cent.) and rests
upon a rather uncertain basis. While it would perhaps be pre-
sumptuous to attempt to decide the relative value of the two methods
and the probability-of the divergent conclusions based on them, one
can hardly avoid filing that the trained judgment of the actual
experimenter is a safer reliance than such a relatively large cor-
rection computed by a critic.

548 PRINCIPLES OF ANIMAL NUTRITION.

In any case it is obvious that while the extensive researches
of Zuntz and his associates afford very reliable data as to the
ratio between the energy liberated in muscular work and the
amount of external work accomplished, or, in other words, as to
the utilization of the net available energy of the food, we have as
yet, notwithstanding the vast amount of work done by Wolff and
his co-laborers and others, but very fragmentary and uncertain
data as to the utilization of the metabolizable energy of the food for
work production.

APPENDIX.

TABLE I. METABOLIZABLE ENERGY OF COARSE FODDERS.

'a

<

o

'u

■-

-

Organic

Matter
Eaten.

Energy of

Metabolizable
Energy.

Feed Added.

OD

B

u

O

o
H

—
a>

-. a
~ B

a

Food,
Cals.

Feces,
Cals.

Urine
(Cor-
rected),
Cals.

Methane,
Cals.

Total,
Cals.

Per
Grm.
i >r-
ganic
Mat-
ter,
Cals.

Meadow hay V -

F

F

G
G

TI
Jl

B

H

1

3

1

2
4

7
1

0175
6630

6024

3175

44821.2
31327.8

16323.7
9599 . 2

2113.3
1530.0

3250.6
2560.7

23133.6
17637.9

Difference. . . .
Correction ....

2845

9405
6651

2849

5950

13493.4
+ 19.0

6724.5
+ 5.9

583.3
+ 0.9

689.9
+ 1.5

5495 . 7
+ 10.7

Percentage . . .
Meadow hay V -

13512.4
100.00

43811.3
30750.7

6730.4
49.81

15336.3
9491.5

584.2
4.32

1916.1
1359.6

691.4
5.12

3432 . 1

2524 . 7

5506.4
40.75

23126.8
17374.9

1.933

Difference ....
Correction.. . .

2754

9527

6402

2744

13060.6
-45.4

5844.8
-14.0

556.5
-2.0

907.4
-3.7

5751.9
-25.7

Percentage .. .
Meadow hay VI -,

13015.2

100.00

45255.8
30338 . 1

5830 . 8
44.80

14103.7

8574.9

554 . 5
4.26

2576 . 3
1795.0

903.7
6.94

3306.6
2579.4

5726 . 2

44.00

25269 . 2
17388.8

2.087

Difference. . . .
Correction... .

3125

9743

6402

1 1 25

6495
3198

14917.7
-8.9

5528 . 8
-2.5

781.3
-0.5

727.2 7880.4
-0.8 -5.1

Percentage . . .

Meadow hay VI •!

14908.8
100.00

46275.0
30338 . 1

5526.3
37.07

14104.8
8574.9

780.8 ! 726.4
5.24 4.87

2593 . 0 3564 . 2
1 795 . 0 2579 . 4

7875 . 3
52.82

26013.0
17388.8

2.520

Difference. . . .
Correction.. . .

3341

:;_'07

15936.9
-208.3

5529.9
-58.9

798.0 984.8
-12.3 -17.7

8624.2
119.4

Percentage . . .

15728.6
100.00

5471.0
34.78

785.7 . 967.1
5.00 6.15

8504.8

5 1 07

2.580

549

550

APPENDIX.

TABLE I (Continued).

Feed Added.

Meadow hay VI |

Difference
Correction. . . .

Percentage . . .

Oat straw II . . <

Difference . . .
Correction.. . .

Percentage . . .

Oat straw II . ,

Difference. . .
Correction. . .

Percentage...

Wheat straw I -

Difference. . .
Correction. . .

Percentage . .

Wheat straw I ■<

Difference. . .
Correction . . .

Percentage . .

Extracted rye j
straw j

Difference . .
Correction . .

Percentage . .

Extracted rye J
straw I

Difference. . .
Correction. . .

Percentage . .

Organic
Matter
Eaten.

9539
0458

3081

2 9819

3 6630

3189

9740
6651

:;u,vi

1 9611
4 6402

3209

1 9583
4 6458

Energy of

Food,
Cals.

6340 45239.
3239 30548.

3101 14691
101

14792
100.

3170 46690.
0 31327.

3170 15362.
-94.

15268
100.

3115 45626
0 30750.

3115 14875.
126

15001.
100,

3195 45570.
0 30338.

3195 15232
-76

3125

5 9114

4 6402

2712

5 9142
4 6458

15155
100

3188 45365
0 30548.

3188 14817.
+ 302

2005

15119
100

41900
0 30338

2684

2665| 11562
-232

11329
100

2659 41962
0 30548

2659 11414
i — 113

11300.
100.

Feces,
Cals.

13218.1
8171.2

5046.9
+ 27.2

5074 . 1
34.30

18296.3
9599 . 2

8697 . 1
-28.9

8668.2
56.77

17983.1
9491.5

8491.6
+ 39.0

8530.6
56.86

17751.7
8574 . 9

9176.8
-21.7

9155.1
60.41

16562.1
8171.2

8390.9
-r 80 . 9

8471.8
56.03

9926.4
8574 . 9

1351.5
-65.8

1285.7
11.35

9799 . 0
8171.2

1627.8
-30.3

1597.5
14.14

Urine
(Cor-
rected),
Cals.

2755.1
1824.6

930.5
6.1

936.6
6.33

1884.2
1529.8

354.4
-4.6

349.8
2.29

1633.6
1359.6

274.0
+ 5.6

279.6
1.86

2084.7
1795.0

289.7
-4.5

285.2
1.88

2237.8
1824.6

413.2
+ 18.1

431.3
2.85

1756.5
1795.0

•38.5
■13.8

-52.3
-0.46

1705.8
1824.6

Methane

Cals.

3620.2
2722.2

80S (I

9.1

907.1
6.13

3239.9
2560 . 7

679.2

-7.7

671.5
4.40

3448.1
2524.7

923.4
+ 10.4

933.8
6.23

3792.4
2579.4

1213.0
-6.5

1206.5
7.96

4003 . 2
2722.2

1281.0
+ 26.9

1307.9
8.65

4004.5
2579.4

1425.1
-19.8

118.8
-6.8

125.6
-1.11

1405.3
12.40

4147.4
2722.2

1425.2
- 10.1

Metabolizable
Energy.

Total,
Cals.

25646.2
17830.5

7815.7
59.4

7875.1
53.24

23269 . 7
17638.1

5631.6
-53.1

5578.5
36.54

22561.3
17374.9

5186.4
+ 71.5

5257.9
35.05

21941.3
17388.8

4552.5
-43.9

4508.6
29.75

22562 . 8
17830.5

4732.3
+ 176.5

4908.8
32.47

26213.3

17388.8

8824.5
-133.3

8691.2
76.71

26310.4
17830.5

8479.9
-66.1

1415.1
12.52

8413.8
74.45

Per
Grm.
Or-
ganic
Mat-
ter,
Cals.

2.540

1.760

1.688

1 411

1.540

3.261

3.164

APPENDIX.

551

TABLE

II.

METABOLIZABLE ENERGY

OF BEET MOLASSES.

"3
E
d
•<

•6
0

'u
<a
Ph

6
3

6

4

6
4

Organic

Matter
Eaten.

Energy of

Apparent

Metabolizable

Energy.

Feed Added.

S
O

0

-a
0

-Sg

a

Food,
Cals.

Feces,
Cals.

Urine
(Cor-
rected),
Cals.

Methane,
Cals.

Total,
Cals.

Per

Gram
Or-
ganic
Mat-
ter,
Cals.

Beet mol'ses 1 j

F
F

H
H

J
J

8262
6630

1702 37946.2
0 31327.8

11365.8
9599.2

1786.1
1530.0

2397 . 9
2560 . 7

22396.4
17637.9

Difference ....
Correction ....

1632

8110
6402

1702

1611
0

6618.4
+ 330.8

1766.6
+ 101.3

256.1
+ 16.2

-162.8 1 4758.5
+27.0 +186.3

Percentage . . .
Beet mol'ses II -

6949.2
100.00

37544.4
30338.1

1867.9
26.87

9070.0
8574.9

272.3
39.2

2035.2
1795.0

-135.8
-1.95

3458.8
2579 . 4

4944.8
71.16

22980.4
17388.8

2.905

Difference. . . .
Correction.. . .

1708

8104
6458

1611

1595
0

7206.3
-459.4

495.1

-129.8

240.2
-27.2

879.4
-39.1

5591.6
- 263 . 3

Percentage . . .
Beet mol'ses II \

6746.9
100.00

37461 . 1
30548.5

365.3
5.40

9198.7
8171.2

213.0
3.16

2017.2
1824.6

840.3
12.44

3422.7
2722 . 2

5328.3
79.00

22822 . 5
17830.5

3.308

Difference. . . .
Correction. . . .

1646

1595

6912.6
-234.3

1027.5
-62.7

192.6
-14.0

700.5 1 4992.0
-20.9 —136.7

Percentage . . .

6678.3
100.00

964.8
14.45

178.6
2.67

679.6
10.18

4855 . 3
72.70

3.044

55:

APPENDIX.

TABLE III. METABOLIZABLE ENERGY OF STARCH.
EXPERIMENTS.

KUHN S

"3

£
'S
<

III

III

IV

IV

V
V

V

V

VI

VI

VI

\ 1

V

Ph

Organic
Matter
Eaten.

Energy of

Apparent

Metabolizable

Energy.

Feed Added.

s

u

a

O
H

■a

a
a

fe .

-Z u

<~

a

1651
0

Food,
Cals.

Feces
Cals.

Urine*
(Cor-
rected),

Cals.

Methane,
Cals.

Total,
Cals.

Per
Grm.

Or-
ganic
Mat-
ter,
Cals.

Starch I

2
1

2
la&b

2a
1

2b
1

26
1

3

1

8839
732S

40964 . 5
34603 . 2

16615.5
15505.1

1430.3

1549.6

3225 . 3
2670.1

19593.4
14878.4

Difference. .
Correction. .

1511

8787
7074

1651

1608
0

6361.3
+ 658.0

1110.4
+ 294.8

-119.3
+ 29 . 5

655 . 2
+ 50.8

4715.0
282 . 9

Percentage.

Starch I . . . -|

7019.3
100.00

40725.6

33405 . 1

1405.2
20.02

17202.1
15250.6

-89.8
- 1 . 29

1434.9
1481.5

706.0
10.06

3348.0
2491.3

4997.9

71.21

18740.6

14181.7

3.029

Difference .
Correction .

1713

8767
7199

1608

1621
0

7320 . 5
-492.0

1951.5
-224.6

-46.6
-21.8

856.7
-36.7

4558.9

-208.9

Percentage .
Starch II.. . -j

6828 . 5
100.00

40827 . 5
34211.5

1726.9
25 29

15804.1
15312.2

-68.4
-1.01

1618.3
1559 . 3

820.0
12.01

3021 . 1
2268.5

4350.0
63.71

20384.0

15071.5

2.705

Difference. .
Correction. .

1568

8792
7199

1621

1663
0

6616.0
+ 255.0

491.9
+ 114.2

59.0
+ 11.6

752.6
+ 16.9

5312.5

+ 112.3

Percentage .
Starch II.. . |

6871.0
100.00

40917.4
34211.5

606.1

8.82

16270 0
15312.2

70.6
1.03

1524.8
1559.3

769.5
11.20

2941.0
2268 . 5

5424 . 8
78.95

20181.6
15071.5

3.347

Difference. .
Correction. .

1593

8861
7125

1663

1669
0

6705 . 9
+ 334 . 1

957.8
+ 149.5

- 34 . 5
+ 15.2

672.5

+ 22.2

5110.1

+ 147.2

Percentage .
Starch II ... -j

7040.0
100.00

41245.9
33855 . 4

1107.3
15.73

15485.9
13765.2

-19.3
-0.27

1569.6
1737.9

694.7
9.86

3130.5
2480.6

5257.3
74.68

21059.9
15871.7

3.161

Difference. .
Correction. .

17:iC
7125

1669

2788
0

7390 . 5
-320.9

1720.7
-130.5

-168.3
L6.5

649.9
-23.5

5188.2
-150.4

Percentage .
Starch 11... j

7269.6
100.00

45859.6
33855 . 4

1590.2
22.49

16091.4
13765.2

-184.8
-2.61

1643.9
1737.9

cl'i; i
8.86

3897.8
2480.6

5037.8
71.26

24226.5
15871.7

3.018

Difference. .
Correction . .

2828

2788

12004.2
-193.4

2326.2
-78.6

-94.0
-9.9

1417.2
-14.2

8354.8
-90.7

Percentage .

11810.8
100.00

2247.6
19.03

-103.9
-0.88

1403.0
11.87

8264.1
09.98

2.964

* Computed from carbon content.

APPENDIX.

553

TABLE IV

METABOLIZABLE ENERGY OF STARCH.
EXPERIMENTS.

kellxer's

"3
=
'8

<

B

B

C
C

D
D

F

I

G
G

H

li

J
J

—
o
"C

CD

Ah

2

4

2
1

2
1

4
3

4
3

3

4

3
4

Organic
Matter
Eaten.

Energy of

Apparent

Metabolizahle

Energy.

Feed Added.

00

0
5

"3

o

£ u
-rO

73 -

<~

Food,
Cals.

Feces,
Cals.

Urine
(Cor-
rected),

Cals.

Methane,
Cals.

Total,

Cals.

Per
Grm.
Or-
ganic

Mat-
ter,
Cals.

Starch I and!
II. 1

11698
10067

3231
1607

52928 . 6
46129.1

15915.8

11S74.4

1740.1
1958.5

3382 . 7
3716.3

31890.0
28579 . 9

Difference . .
Correction . .

1631

11980
10407

1624

3193
1602

6799.5
-30.9

4041.4
-8.0

-218.4
-1.3

-333.6
-2.5

3310.1
-19.1

Percentage . .

Starch I and i
II 1

6768.6
100.00

54016.5
47458 . 0

4033.4
59.60

19185.6
15746. S

-219.7
-3.25

1723.7
1 785 . 7

- 336 . 1
-4.97

3250.6
32.55 . 9

3291.0
48.62

29856.6
26669.6

2.027

Difference . .
Correction . .

1573

11636
9974

1591

1583
0

6558.5
+ 70.8

3438.8
+ 23.5

-62.0

+ 2.7

-5.3
+ 4.9

3187.0
+ 39.7

Percentage . .
Starch III . . ]

6629.3 I 3462.3 1 -59.3
100.00 52.22 -0.89

53902.2 '17817.9 2211.2
46945.4 15718.3 : 2407.0

-0.4
-0.01

3381.4
2957.0

3226 . 7
48.68

30491.7

25863.1

2.028

Difference . .
Correction . .

1662

8374
6630

1583

1687
0

6956.8
-379.5

2099 . 6
-127.1

-195.8

-19.5

424.4
- 23 . 9

4628.6
-209.0

Percentage . .
Starch III . . -j

6577.3 i 1972. 5
100.00 29.99

38608.3 .10833.9
31327.8 j 9599.2

-215.3
-3.27

1594.3
1530.0

400.5
6.08

3382.7
2560.7

4419.6
67.20

22797.4
17637.9

2.792

Difference . .
Correction . .

1744

8380
6651

1687

1676
0

7280.5 | 1234.7
-268.3 ' -82.5

64.3
-13.2

822.0
-22.0

5159.5
-150.6

Percentage . .
Starch III . . -[

7012.2 1152.2
100.00! 16.42

37963.6 10497.1

30750.7 9491.5

51.1
0.73

1394.7
13.59.6

800.0
11.41

3170.5
2524 . 7

5008.9

71.44

22901.3
17374.9

2.969

Difference . .
Correction . . .

1729

8373
6402

1676

2013
0

7212.9 1005.6
-246.2 -76.0

35.1
-10.9

645.8
-20.2

5526.4
-139.1

Percentage . .
Starch IV... j

6966 . 7
100 . 00

38562.4

30338.1

929.6
13.35

9843.8
8574 . 9

24.2
0.35

1588.4
1795.0

625.6
8.98

31 S3. 9
2579.4

5387.3
77.32

23946 . 3
17388.8

3.214

Difference . .
Correction . .

1971

8004
6458

2013

1600
0

8224 . 3
+ 193.2

1268.9
+ 54.6

-206.6
+ 11.4

604.5 65.57.5
+ 16.4 i+110.8

Percentage . .
Starch IV. . . |

8417.5
100.00

rif.as:? r,
30548 •".

1323.5
15.72

9096.8

si 7 i . _'

-195.2
-2.32

1885.4
1824 6

620.9
7 38

3492 . 1

2722 . 2

6668 . 3
79.22

22.508.3

3.313

Difference . .
Correction . .

1546,

1600

6434 . 1

+ 25 1 . 1

925.6
+ 68.0

60.8
+ 15.2

769.9

+ 22.6

4677.8
+ 148.3

Percentage . .

-
100.00

993.6

14.85

76.0
1.14

792.5
1 1 . 85

4826.1
72.16

3.017

554

APPENDIX.

TABLE V. METABOLIZABLE ENERGY OF WHEAT GLUTEN.

Feed Added.

Wheat gluten -,

Difference . . .
Correction. . .

Percentage. .

Wheat gluten -

Difference. . .
Correction. . .

Percentage

Wheat gluten

Difference. .
Correction .

Percentage . .

Wheat gluten j
I 1

Difference . .
Correction . .

Percentage . .

Wheat gluten J
I I

Difference . .
Correction . .

Percentage . .

Wheat gluten I
II 1

Difference . .
Correction . .

Percentage . .

Wheat gluten \
II 1

Difference . .
Correction . .

Percentage .

Organic
Matter
Eaten.

— .

Energy of

Food,
Cals.

9311 576 44025.3
8839 0 40964.5

472

10037
8839

1198

9483

8787

11636
KKI67

1569

576 3060.8
+ 502.9

1157
0

1157

3563 . 7
100.00

4S293 . 6
40964 . 5

7329 . 1
-171.2

7157.9
100.00

582 44860 . 3
0 40725.6

582 4134.7
-534.4

3600 . 3
100.00

1746 54939.3
261 46129.1

1485 8810.2
-380.5

8429 . 7
100.00

1153311742 54469.0
10067 261 46129.1

1466 1481 8339.9
+ 62.3

11994

10407

1587

11578 1407
9974 0

8402 . 2
100.00

Feces,
Cals.

16041.5
16615.5

-574.0
+ 204.0

-370.0
-10.38

16593.3
16615.5

-22.2
-69.4

-91.6

-1.28

16845.6
17202.1

- 356 . 5
-225.7

-582.2
-16.17

14514.7
11874.4

2640 . 3
-97.9

2542.4
30.16

13753.4

11874.4

1879.0
+ 16.0

1895.0
22.55

1746 56293.6 17643.2
261 47458.0 115746.8

1604 1407

8835.6 1896.4
-411.4 -136.5

8424.2 1759.9
100.00 20.89

56053.6 17322.9
16945.4 15718.3

9108.2
-934.1

8174.1
100.00

1 604 . 6
-312.7

1291.9
15.80

Urine
(Cor-
rected),
Cals.

2048.8*
1430.3*

618.5
+ 17.6

636.1
17.85

2990 . 4*
1430.3*

1560.1
-6.0

1554.1
21.71

2036 . 3*
1434.9*

601.4
-18.8

582.6
16.18

3372 . 3
1958.5

1413.8
-16.2

1397.6
16.58

3092 . 1
1958.5

1133.6
+ 2.6

Methane
Cals.

3669 . 6
3325 . 3

344 . 3
+ 40.8

385.1
10.81

3703 . 0
3325 . 3

377.7
-13.9

363.8
5.08

3346.7
3348 . 0

-1.3
-43.9

-45.2
-1.26

Apparent

Metabolizable

Energy.

Total,
Cals.

22265.4
19593.4

2672.0
+ 240.5

Per
Grin.
Or-
ganic
Mat-
ter,
Cals.

2912.5 5.057
81.72

25006.9
19593.4

5413.5
-81.9

5331.6
74.49

22631 . 7
18740.6

4.608

3891.1
-246.0

3645.1
101.25

3753.7 33298.6
3716.3 28579.9

37.4
-30.7

6.7

0.08

3574.9
3716.3

141.4
+ 5.0

1136.2
13.52

2744.1
1785.7

958.4
-15.5

136.4
-1.62

2973.0
3255.9

- 282 . 9
-28.2

942.9 1-311.1
11.19 -3.69

3468 . 0
2407.0

1061.0
-47.9

1013.1
12.39

4718.7
-235.7

4483.0
53.18

34048.6
28579.9

5468.7
+ 38.7

5507.4
65.55

32933.3
26669.6

6263.7
-231.2

3171.9

2957.0 25863.1

6032.5
71.61

32090 . 8

214.9 I 6227.7
-58.8 -514.7

156.1
1.91

5713.0
69.90

6.263

3.019

3.719

4.062

4.061

* Estimated from carbon content.

APPENDIX.

555

TABLE VI. METABOLIZABLE ENERGY OF PEANUT OIL.

Feed Added.

Peanut oil I. . -.

Difference. . .
Correction. . .

Percentage . .

Peanut oil II . -j

Difference . . .
Correction. . .

Percentage . .

Peanut oil II. ■

Difference. . .
Correction.. .

Percentage . .

Organic

Matter
Eaten.

i c

■oO

<

10752
9974

Energy of

T'rine

Food,

Feces.

(Cor-

Methane

Cals.

Cals.

rected),

Cals.

Cals.

709 54007.3 17467 5 2351.2
0 46945.4 !l5718.3 2407.0

778

7491
6630

709

79S

7061.9 1749.2 -55.8
-331.1 1-110.9 -17.0

6730.8 1638.3
100.00 24.34

39185.9 14585.7
0131327.8 9599.2

861

7396

6651

745

79S

798
0

7858 . 1
-302.0

7556 . 1
100.00

4986 . 5
-92.5

-72.8
-1.08

1455.0
1530.0

-75.0
-14. S

4894.0

64.77 -1.19

3S057.3 12512.9
30750.7 ' 9491.5

7306.6
+ 249.5

3021.4
+ 77.0

1452.1

1359.6

92.5
+ 11.0

r556.1 | 3098.4
100.00. 41.00

103.5
1.37,

2909.0
2957.0

Apparent

Metaholizable

Energy.

Total,
Cals.

31279.6
25863 . 1

-48.0
-20.8

-68.8
-1.02

1369.1
2560.7

-1191.6
-24.7

1216.3
-16.10

2371.2
2524.7

-153.5
+20. 5

-133.0
-1.76

5416.5
-182.4

5234.1
77.76

21776.1
17637.9

4138.2
-170.0

3968.2
52.52

21721.1
17374.9

4346.2
+ 141.0

4487.2
59.39

Per
Gram
Or-
ganic
Mat-
ter,
Cals.

7.382

4.973

5.623

556

APPENDIX.

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PAGE

Acid, acetic, effect of, on proteid metabolism 123

total metabolism 160

replacement value of 160

aspartic, formed from proteids 39

oxidized in body 52

benzoic, formation of hippuric acid from 44

butyric, effect of, on total metabolism 158

replacement value of 158

glutaminic, formed from proteids 39

hippuric, effect of proteids on formation of 463

formation of, from benzoic acid 44

glycocol 44

in urine 44

loss of energy in 313, 322

non-nitrogenous nutrients as source of 45

origin of 44

pentose carbohydrates as source of 46

source of benzoyl radicle of 44, 45

lactic, effect of, on proteid metabolism 123

total metabolism 158

production of, in metabolism of carbohydrates 23

replacement value of 158

Acids, organic, absent from excreta 27

effect of, on proteid metabolism 123

total metabolism 157

net available energy of 425

metabolism of 26

oxidized in body 27

produced by fermentation of carbohydrates 13, 26

replacement values of 157

Acid, uric, in perspiration 48

in urine 43

origin of 43

573

574 INDEX.

PAGB

Activity, muscular, general features of 185

Adipose tissue 29

Alanine oxidized in body 53

Albuminoids 7

compound 7

composition of 62

derived 7

modified 7

simple 7

simple, composition of 62

Albumins 7

Albumoses formed from proteids 38, 39

Amido-acids 7

Amides 7

formed from proteids 7, 39, 52

influence of, on digestibility 54

of carbohydrates 57

crude fiber 57, 58

nitrogen-free extract 57

fermentation of carbohydrates 55

in digestive tract 54

metabolism of 52

not synthesized to proteids 53

oxidized in body 52

replacement of proteids by 53

urea formed from 39

Ammonium acetate, influence of, on digestibility of carbohydrates, ... 57, 58

crude fiber 57

nitrogen-free extract . 57

carbonate as antecedent of urea 43

lactate as antecedent of urea 43

salts influence of, on digestibility of carbohydrates 57

fermentation of carbohydrates 56

in digestive tract 56

in perspiration 4S

Amount of food, critical 408

influence of, on effects of muscular exertion 197

net availability of energy 430

utilization of energy 466

Anabolism 16, 1 7

absorption of energy in 17

of proteids 38. 41

Animal, efficiency of 496, 498, 51 1

as motor 498

conditions determining 511

INDEX. 575

PAGE

Animal, efficiency of, influence of fatigue on 519

gait on 513

grade on 512

individuality on 517

kind of work on 512

load on 515

size on 515

species on 515

speed on 513

training on 519

method of determining 498

Antecedents of urea 42

Aromatic compounds in urine 46

Ascent, work of, in dog, consumption of oxygen in 500

utilization of energy in 510

by dog 502

horse 506

man 503

effect of grade on 512

load on 509,510,515

Ash ingredients, balance of 79

Asparagin, influence of, on digestibility 54

of carbohydrates 57

crude fiber 57, 58

nitrogen-free extract 57

fermentation * 54

of carbohydrates 55

nutritive value of 54

oxidized in body 52

replacement of proteids by 54

typical of non-proteids 8

Aspartic acid formed from proteids 39

oxidized in body 52

Assimilation, expenditure of energy in digestion and. 372. 375

tissue building and 491

of fat, loss of energy in 35

work of 337, 372, 375

digestion and '. 80, 93, 406

above critical point 407

of bone 381

carbohydrates 379, 382, 384

fat 378, 382. 384, 385

mixed diet 382, 384

proteids 381. 382, 384

indirect utilization of heat from. . . . 406

576 INDEX.

Assimilation, work of, digestion and, in dog 378

horse 385

man 382

methods of determining 377

relat ion of, to surface 408

Availability of energy, gross 270, 395

for maintenance 396, 406, 410, 413, 427, 497

work , 497

net 394, 412

net, of energy, determination of 413, 427, 428

in carnivora 413, 427, 428

herbivora 418, 427, 428

distinction between utilization and •. . . 395

of carbohydrates 417, 419, 427, 428

crude fiber 422, 428

fat 416, 419, 427, 428

organic acids 423

pentoses 420, 428

proteids 414, 427, 428

timothy hay 424, 428

influence of amount of food on 430

character of food on 431

relation of maintenance ration to 432

Barley, utilization of energy of 483, 491

Beet molasses, metabolizable energy of 293, 297. 301

digestible protein of 318, 332

utilization of energy of 4S3. 490. 491

Benzoic acid, formation of hippuric acid from 44

Benzoyl radicle of hippuric acid, source of 44, 45

Blood, consumption of dextrose of. in muscles 22

parotid gland 22

dextrose of 17, 18

fat production from 23

percentage of 18

variations in 18

fate of dextrose of 22

laevulose in 17

peptones absent from 40

regulation of supply of dextrose to 18, 20

Body, animal, components of 1

composition of 60. 64, 66

conservation of energy in 22S, 258

liberation of energy in 1

store of energy in 1

transformations of energy in 2

INDEX. 577

PAGE

Body, schematic 60, 66

Bone, work of digestion and assimilation of 3S1

Butyric acid, effect of, on total metabolism 158

replacement value of 158

Calorie 232

Calorimeters, animal 246

Carbohydrate radicle in proteids 50

Carbohydrates 8

apparent digestibility of, influence of amides on 57

ammonium salts on. 57

asparagin on 57

non-proteids on. . . . 57

as source of energy to body. 91

consumption of, in muscular contraction 220

digestible, gross energy of 308

metabolizable energy of 324. 327, 332

utilization of energy of 475,477,490,491

disappearance of, in fasting 85

effects of, on minimum of proteids 135

proteid metabolism 115

compared with fat 127

total metabolism 146

fermentation of 12, 1 3

influence of amides on 55

ammonium salts on 56

asparagin on 55

non-proteids on 55

on nutritive value 13

organic acids from 13, 26

products of 13

formation of dextrose from, in liver 19. 20, 21

fat from 24, 30. 165

equation for 24

respiratory quotient in 17'.)

glycogen from. 20, 21

milk fat from. . 174

hexose 8, 9

formation of glycogen from 20, 21

metaboiism of. See Metabolism

resorption of 12, 17

rate of 18

liver as reservoir of 20

metabolism of See Metabolism

mutual replacement of •'at and 151

net available energy of 417. 419. 427, 428

578 INDEX.

Carbohydrates, of crude fiber 9

food, replacement of proteids by 149

nitrogen-free extract 9

oxidized, computation of, from respiratory quotient 76

pentose 8, 9

assimilability of 25

as source of hippuric acid 46

determination of 9

digestibility of 24

effects of, on proteid metabolism 124

total metabolism 156

formation of fat from 183

glycogen from ' . . 25, 26

metabolism of. See Metabolism

of crude fiber 9

nitrogen-free extract 9

oxidized in body 25

replacement value of 156

replacement value of 1 52

resorption of 12

respiratory quotient of 74

subdivisions of 8, 9

substitution of, for body fat 146

utilization of energy of 461, 462, 473, 490, 491

value of, for maintenance 400,402

work of digestion and assimilation of 379, 382, 384

Carbon balance computation of fat from 77

heat production from nitrogen and 255

dioxide, determination of, in respiration 69, 73

produced by fermentation of carbohydrates 13

production of, in fasting 84

metabolism 14, 15

of carbohydrates 23, 27

fat 36

proteids 42

equilibrium, amount of proteids required to produce 105

factor for computation of fat from 62, 78

income and outgo of 69

determination of 69-73

of excreta, determination of 69

metabolism, effect of muscular exertion on 209

Carnivora, determination of net availability of energy in 413, 427, 428

hippuric acid in urine of 44

metabolizable energy of food of 272

utilization of energy by 448, 466

INDEX. SI 9

PAGE

Cattle, excretion of methane by 243

Cellulose, effects of, on proteid metabolism 117

total metabolism 162

fermentation of 13

formation of fat from 181

of crude fiber 9

replacement value of 162

Changes, chemical, during muscular contraction 186, 189

thermal, during muscular contraction 189

Chymosin 40

Circulation, effects of muscular exertion on 191

work of 341

Cleavage digestive of proteids 38

purpose of 38

nitrogen of proteids 98

cause of 100, 101, 103

effects of non-nitrogenous nutrients on 131

independent of total metabolism 99

of fat in digestion 12

proteids in digestion 12, 38

purpose of 38

products rebuilding of proteids from '. 40

Cleavages, influence of, on computation of heat productions 253

by an enzym 40

Coarse fodders, expenditure of energy of, in digestion, assimilation, and

tissue building 491

metabolizable energy of. . .285, 280, 287, 290, 297, 298, 300, 301

digestible protein of 320, 332

carbohydrates of.. 327, 332

non-nitrogenous matter of urine derived from 28

relative value of grain and, for maintenance.... .433, 533, 537

work production 534, 537

utilization of energy of 484, 490, 491

Coefficient of utilization of energy 444, 498

Collagens 7

composition of 62

Combustion, heats of 229

Concentrated feeding-stuffs, metabolizable energy of 289, 297, 299

digestible protein

of 315,332

utilization of energy of 472, 490, 491

Conservation of energy 228

in animal body 229, 258

Contractile substance of muscle 17

Contraction, muscular 185

58° INDEX.

PAGE

Contraction, muscular, chemical changes during 186, 189

consumption of carbohydrates in 220

dextrose in 220, 221

isometric 495

isotonic 495

oxidations in, incomplete 186

oxygen not essential to 188

respiratory quotient of muscle in 187

thermal changes during 189

transformation of energy in 495

Creatin 46

Creatimn 46

in perspiration 48

Crude fat 8

fiber 9

apparent digestibility of 12

influence of amides on 57, 58

ammonium acetate on . 57

asparagin on 57, 5S

non-proteids on 57, 58

carbohydrates of 9

cellulose of 9

digestible, gross energy of 303

metabolizable energy of 329, 332

digestive work for 389

effect of, on total metabolism 161

expenditure of energy of, in digestion, assimilation, and tissue

building 494

formation of fat from 181

furfuroids of 9

ligneous material of 9

modified in digestive tract 12

net available energy of 422, 42S

pentose carbohydrates of 9

replacement value of 161

value of, for maintenance 435, 535, 537

work production 535, 537

Descent, work of 509

influence of grade on 509

Dextrose, amount of, produced by liver 19

consumption of, in muscles 221

muscular contraction 220, 221

formation of fat from 23

from carbohydrates in liver 19, 20, 21

fat 36,385

INDEX. 581

PAC B

Dextrose, formation of, from fat during muscular exertion 223

vi |uation for 3S, 51

in liver 36, 37

proteids 19,21,49, 50

glycogen from 20, 21, 22

in muscles 23

importance of constant supply of 18, 21

liver as source of 18, 19, 21 , 49

on carbohydrate diet 19

on proteid diet 19

method of formation of, in liver 20

of blood 17,18

consumption of, in muscles 22

parotid gland 22

fate of 22

fat production from 23

percentage of IS

variations in IS

reconversion of glycogen into 20, 22, 37

regulation of supply of, to blood 18, 20

resorption of, rate of IS

storage of, in resting muscle 222

Digestibility 9

apparent 10, 11

determination of 10, 11

influence of metabolic prod-
ucts on 10

of carbohydrates, influence of amides on 57, 58

ammonium salts on 57
asparagin on. . . . 57, 58
non-proteids on. 57, 58

crude fiber 12

influence of amides on 57, 58

ammonium acetate on. 57

asparagin on 57, 58

non-proteids on 57 58

nitrogen-free extract 12

influence of amides on. . . 57
ammonium

acetate on 57
asparagin on. 57
non-proteids

on 57

significance of results on 11

determination of 9

582 INDEX.

Digestibility, determination of, influence of products of metabolism on. . 10

of pentose carbohydrates 24

real 10, 11

determination of 10

of fat 10

protein 10

Digestion, changes in proteids during 12

cleavage of fat in 12

proteids in 12, 38

purpose of 38

expenditure of energy in assimilation and 337, 372

tissue building and. 491

influence of amides on 54

asparagin on 54

non-proteids on 54

peptones produced during 12

proteoses produced during 12

saponification of fat in 12

work of 337, 372, 493

assimilation and 80, 93, 337, 372, 376, 406

above critical point 407

below critical point 406

indirect utilization of heat from. . . . 406

in the dog 378

the horse 385

man 382

methods of determining 377

of bone 384

carbohydrates 379, 382, 384

fat 378, 382, 384, 385

mixed diet 382, 384

proteids 381, 382, 384

relation of, to surface 408

factors of 374

for crude fiber 389

Digestive tract, functions of, in excretion 10

Dog, consumption of oxygen by, in locomotion 500

work of ascent 500

draft 501

expenditure of energy by, in locomotion 502

utilization of energy by, in muscular work 499

work of ascent 502

draft 502

work of ascent by, consumption of oxygen in 500

utilization of energy in 502

INDEX. 583

PAGE

Dog, work of digestion and assimilation in 378

draft by, consumption of oxygen in 501

utilization of energy in 502

Draft, work of, consumption of oxygen in, by dog 501

horse 507

utilization of energy in 502, 507, 510, 513

by dog 502

horse 507, 510

Dynamometer for experiments on horses 538, 539

Dyne 231

Efficiency of animal. See Animal.

single muscle 495

Emission of heat, rate of, influence of temperature on 350

regulation of 349

Energy 226

absorption of, in anabolism 17

available 269, 394

gross 270, 395

net 270, 395

determination of, in carnivora 413, 427, 428

herbivora 418, 427, 428

availability of, distinction between utilization and 395

for maintenance 396, 406, 410, 413, 427, 497

influence of amount of food on 430

character of food on 431

relation of maintenance ration to 432

carbohydrates as source of, to body 91

coefficient of utilization of 440

conservation of 228

in animal body 228, 258

Atwater's and Benedict's inves-
tigations 265

early experiments 261

Laulanie's experiments on. . . . 265

nature of evidence 259

Rubner's experiments on 263

expenditure of, by the body 2, 226, 336, 339

in digestion and assimilation 372, 375, 376

and tissue building. 491
method of deter-
mining 377

Energy, expenditure of, in locomotion 510

by dog 502

horse, at a trot 509, 510, 514

584 INDEX.

PAGE

Energy, expenditure of, in locomotion, by horse, at a walk 504, 506, 508, 510

533, 539

man 503

influence of gait on 513

individuality on 517

load on 509, 510, 515

size of animal on. . . . 51 6

species on 516

speed on 513

standing 499

sustaining load 508, 515

influence of individuality on. . 518

food as source of 2, 269

gross, of digestible crude fiber 303

ether extract 304

nutrients 302, 306

organic matter 309

income and expenditure of 3, 226

kinetic 226

determination of 245

liberation of, in animal body 1

loss of, in assimilation of fat 35

fermentations 374

hippuric acid 313, 322

methane 310, 325, 330, 335

tissue building 444, 447

warming ingesta 374

metabolizable 269, 270

apparent 291

factors for 279, 281, 333

Atwater's 281

Rubner's 279, 333

of coarse fodders. 285, 286,287, 290, 297, 298, 300, 301

concentrated feeding-stuffs 289, 297, 299

digestible carbohydrates 324, 332

crude fiber 329, 332

ether extract 323, 332

nutrients 310, 332, 333

organic matter 297, 307

protein. 310, 315, 317, 318, 320, 327, 332
fiber-free nutrients, utilization of, in work pro-
duction 541, 543, 547

food of carnivora 272

herbivora 281

man 277, 280, 282

INDEX. 585

PAGH

Energy, metabolizable, of nutrients, utilization of, in work production. . . 545

proteids 272, 276, 277

total organic matter 284, 285

real 291

utilization of, in work production 525, 540

methods of de-
termina-
tion. ... 526,528
Wolff's investi-
gations 528

muscular, fat as source of 200, 223

proteids as source of 201, 207

source of 196

starch as source of 199

nature of demands for 340

net available 394

determination of 413, 427, 428

for maintenance 396, 406, 410, 413, 427, 497

work 497

of carbohydrates 417, 419, 427, 428

crude fiber 422, 428

fat 416, 419, 427, 428

organic acids 423

pentoses 420, 428

proteids 414, 427, 428

timothy hay 424, 428

utilization of, in work 497

of food 2

protein, losses of, in methane 310

urine 312

potential 226

determination oi 235

of combustible gases 243

excreta, computation of 241

determination of 240

feces, computation of 242

food, determination of 235

gain of fat 244

protein 244

tissue 244

perspiration 244

urine 272, 275. 278, 312

computation of 241. 277, 312

store of, in animal body 1

transformation of, in animal body 2

586 INDEX.

PAGE

Energy, transformation of, in muscular contraction 495

units of measurement of 231, 233

utilization of, in tissue building 444, 447, 448, 461

by carnivora 448, 466

man 451

ruminants 455, 461, 467

swine 452, 466

earlier experiments on 460

effect of amount of food on 466

character of food on . . . 472
differences in live

weight on 457

thermal environment on 471

work 444, 447, 494

by dog 494

horse 502

at a trot 509

walk 504

man 502

influence of fatigue on 519

individuality on 517

kind of work on 512

load on 508

size of animal on 515

species on 515

speed on 507, 513, 514

training on 519

of ascent 502, 503, 506, 510

by dog 502

horse 506

man 503

effect of grade on 512

load on 509,510,515

draft 502, 510, 513

by dog 502

horse 507

locomotion, computed. 51 3

of barley 483. 491

beet molasses 483, 490. 491

carbohydrates 461, 462, 473, 490, 491

coarse fodders 484, 490, 491

concentrated feeding-stuffs 472, 490, 491

digestible carbohydrates 475, 477, 490, 491

protein 481, 491

extracted straw : 488, 490, 491

INDEX. 587

PAGE

Energy, utilization of, of meadow hay 484, 490, 491

mixed grains 483, 491

oat straw 485, 490, 491

oil 478, 490, 491

proteids 463, 482, 491

rice 483, 491

starch 473, 490, 491

wheat gluten 480, 490, 491

straw 487, 490. 491

Environment, thermal, critical 358

influence of, on heat production in fasting 347

maintenance ration 435

utilization of energy 471

Enzym, rebuilding of proteids from cleavage products by 40

Epidermis, composition of 63

Ether extract 8

digestible, gross energy of 304

metabolizable energy of 323, 332

Exchange, gaseous, computation of heat production from 249

effect of load on 509

muscular exertion on 209

respiratory, determination of 73

in fasting 84, 85

intermediary metabolism 405

Excreta, determination of carbon of 69

dioxide in (9

hydrocarbons in 69, 72

methane in 69, 72

hydrogen in 72

organic acids absent from 27

percentage of oxygen in 15

potential energy of, computation of 241

determination of 240

total, computation of heat production from 252

Excretion, functions of digestive- tract in 10

nitrogen, proteid metabolism and 97

rate of 98

effect of non-nitrogenous nutrients on 130

of free nitrogen 42

methane by cattle 243

Exertion, muscular (see also Work) • • 185

effects of, influence of amount of food on 197

on carbon metabolism 209

circulation 191

gaseous exchange 209

588 INDEX.

PAGB

Exertion, muscular, effects of, on metabolism 185, 193

proteid metabolism 194, 206

respiration 192

respiratory quotient 211, 212, 216

work of heart 192

formation of dextrose from fat during 223

functions of proteids in 207

gain of proteids caused by 204

general features of 185

intermediary metabolism during 219

nature of non-nitrogenous material metabolized in. . 218

respiratory quotient during, conclusions from 218

secondary effects of 191

Expenditure of energy. See Energij.

Extracted straw, metabolizable energy of 290, 297, 300, 301

digestible, carbohydrates of . 327, 332

utilization of energy of 488, 490, 491

Extractives 7

of muscle 8

Factor for computation of fat from carbon 62, 78

non-proteids from nitrogen 8

protein from nitrogen 67, 68, 77

Factors for metabolizable energy of digestible nutrients 302, 332, 333

human food 279, 281

protein in human foods 6

of proteid metabolism in fasting 81, 90

work of digestion 374

Fasting, constant loss of tissue in 83

disappearance of carbohydrates in. 85

glycogen from liver in 21

heat production in 344

constancy of 345

influence of size of animal on 359

thermal environment on 347

is a minimum 347, 356

measure of internal work . . 344

metabolism in 80, 90, 340

effect of body fat on 88, 90

loss of protein on 90

ratio of proteid to total 86, 88, 89, 90, 93

total 83, 90

proportional to active tissue 86, 93

of fat in 85, 88, 90

proteids in 81 , 90

minimum of proteids less than 136

INDEX. 589

PAGE

Fasting, metabolism of proteids in, tends to become constant 81, 90

two factors of 81, 90

minimum of proteids in 82, 83, 90, 94

oxygen consumption in 84

production of carbon dioxide in 84

ratio of fat to protein in body in 88, 89, 90

respiratory exchange in 84, 85

Fat 8

assimilation of, loss of energy in 35

as source of muscular energy 200, 223

body, effect of, on fasting metabolism 88, 90

formation of, from food fat 164

proteids substituted for 104

replacement of proteids by 149

substitution of non-nitrogenous nutrients for 144

carbohydrates for 146

fat for 144

cleavage of, in digestion 12

composition of, constancy of 35

from different animals %. . 61

parts of animal 33

influence of feeding on 32

computation of, from carbon balance 77

crude 8

deposition of iodine addition products of 31

digestibility of, real 10

effects of, on proteid metabolism 114

compared with carbohydrates. ... 127

factor for computation of, from carbon 62, 78

food, formation of body fat from 164

quantitative relation of, to fat production 34

replacement of proteids by 149

foreign, deposition of 30

formation of dextrose from 23, 385

during muscular exertion 223

equation for 38. 51

in liver 36,37

from carbohydrates 24, 30, 165

equation for 24

respirator}- quotient in 179

cellulose 181

crude fiber 181

dextrose of blood 23'

food fat 164

non-nitrogenous nutrients of feeding-stuffs 180

59° INDEX.

PAGE

Fat, formation of, from other ingredients of food 163

pentose carbohydrates 183

proteids 30, 50, 98, 107

difficulty of proof 113

equations for 51

later experiments Ill

Pettenkofer and Voit's experiments. . . . 108

Pfliiger's recalculations. 109

functions of food 30

gain or loss of, determination of 69, 77

influence of glycogen on computation of . . .66, 78

potential energy of 244

influence of, on minimum of proteids 135

in muscular tissue 63, 64

katabolism of 35

loss of energy in assimilation of 35

manufactured in body 29, 30, 163

metabolism of. See Metabolism.

mutual replacement of carbohydrates and 151

net availability of energy of 416, 419, 427, 428

of plant, nature of 8

oxidized, computation of, from respiratory quotient 76

production, quantitative relation of food fat to 34

ratio of, to protein in body in fasting 88, 89, 90

resorption of 12, 30

respiratory quotient of 74

saponification of, in digestion 12

sources of animal 29, 30, 163

substitution of, for body fat 144

value of, for maintenance 400, 402

work production 522

work of digestion and assimilation of 378, 382, 384, 385

Fatigue, influence of, on utilization of energy in work 519

Fattening, influence of, on maintenance ration 441, 458

Feces, computation of potential energy of 242

metabolic nitrogen in 47

products in 10, 47

determination of 10

influence of, on determination of digesti-
bility 10

nature of 47

nitrogenous 42

Feeding, influence of, on composition of fat 32

Feeding-stuffs, concentrated, expenditure of energy of, in digestion, assim-
ilation, and tissue building 492

INDEX. 591

PAOH

Feeding-stuffs, concentrated, metabolizable energy of 2S9, 297, 299

digestible protein

of 315,332

utilization of energy of 472, 490, 491

metabolizable energy of, utilization of, in work production 540
non-nitrogenous nutrients of, effects of, on total metab-
olism 154

formation of fat from. . . . ISO
mutual replacement of. . . 154

non-proteids in G

protein in 5

Fermentation of carbohydrates 12, 13

influence of amides on 55

ammonium salts on 56

asparagin on 55

non-proteids on 55

organic acids from 13, 26

products of 13

cellulose 13

Fermentations in digestive tract 12, 13

influence of amides on 55

ammonium salts on 5G

asparagin on 54, 55

non-proteids on 55

Fermentations, influence of, on nutritive value of carbohydrates 13

loss of energy in 374

Fiber, crude. See Crude Fiber.

Flesh bases 8

Flesh, proteid metabolism expressed in terms of 68

Food 5

amount of, critical 408

influence of, on effects of muscular exertion 197

net availability of energy 430

utilization of energy 466

as source of energy 269

character of, influence of, on net availability of energy 431

utilization of energy 472

composition of 5

digested 12

consumption, influence of, on heat production 338, 372, 387

metabolism 387

energ}' of 2

fund ions of 3

fat, functions of 30

592 INDEX.

PAGE

Food, increases metabolism 372

ingredients, heats of combustion of 236

metabolizable energy of. See Energy.

nature of 2

potential energy of, determination of 235

purposes to which applied 80

Foods, heats of combustion of 236, 237

Food-supply, relation of metabolism to 93

Foot-pound 231

Force 226

Furfuroids 9

of crude fiber 9

nitrogen-free extract . 9

Gain of fat, determination of 69, 77

influence of glycogen on computation of 66, 78

potential energy of 244

nitrogen by body 66, 67

protein by body 66

during work 204

potential energy of 244

tissue. 59

determination of 60

potential energy of 244

Gait, influence of, on expenditure of energy in locomotion 513

Gases, combustible, composition of 243

potential energy of 243

Gelatinoids 7

composition of 62

Globulin 7

Glutaminic acid formed from proteids 39

an intermediate product of proteid metabolism 44

Glycocol, formation of hippuric acid from 44

oxid' zed in body 52, 53

Glycogen, amount of, in body 66, 78

disappearance of, from liver in fasting 21

formation of, from artificial hexoses. 20

carbohydrates, hexose 20, 21

pentose 25, 26

dextrose 20, 21, 22, 23

hexoses. . 20, 21

pentoses 25, 26

proteids 21, 98

in liver. 20, 21, 22

muscles 23

identical, from different hexoses 20

INDEX. 593

PAGE

Glycogen, identical, from hexoses and pentoses 26

influence of, on computation of gain or loss of fat 66, 78

in muscular tissue 64

muscular, disappearance of, in work 23

functions of 222, 223

reappearance of, in rest 23

reconversion of, into dextrose 20, 22, 37

Grade, influence of, on efficiency of animal 512

utilization of energy in work of ascent 512

work of descent 509

Grain, relative effects of hay and, on metabolism 388

value of coarse fodder and, for maintenance 433, 533, 537

work production 533

Gram-meter 231

Gravity, force of 231

Hair, composition of '. . 63

Hay, relative effects of grain and, on metabolism 388

Heart, work of, influence of muscular exertion on 192

Heat, animal source of 261

determination of 245

emission and heat production 256

influence of insolation on 357

relative humidity on 358

wind on 357

method of, above critical temperature 355

rate of, influence of temperature on 350

regulation of 349

from digestive work, indirect utilization of 406

production 338

and heat emission 256

computation of 249

from carbon and nitrogen balance .... 255

gaseous exchange 249

total excreta 252

influence of cleavages on 253

hydrations on 253

determination of 245

in fasting 344

constancy of 345

influence of size of animal on 359

thermal environment on 347

is a measure of internal work 314

minimum 347, 356

influence of consumption of food on 338, 372, 387

water on 438

594 INDEX.

PAGE

Heat production, influence of muscular tonus on 191

species on 3G9

temperature on 351

thermal environment on 358

time element on 439

in intermediary metabolism 405

on maintenance ration 436, 437

relation of, to mass of tissue 370

surface 359

variations in 351

causes of 363

mechanism of 352

regulation of rate of emission of 349

Heats of combustion 229

computation of 239

of foods 236, 237

food ingredients 236, 237

organic substances 237

Heat, units of 232

Hexosans 8

Hexoses 8

artificial, formation of glycogen from 20

formation of glycogen from 20, 21

Herbivora, determination of net availability of energy in 418, 427, 428

hippuric acid in urine of 44

metabolizable energy of food of 281

minimum of proteids for 140

Hippuric acid 44

composition of 44

formation of, from benzoic acid 44

glycocol 44

in urine 44

loss of energy in 313, 322

non-nitrogenous nutrients as source of 45

origin of 44

pentose carbohydrates as source of 46

source of benzoyl radicle of 44, 45

Hoof, composition of 63

Horn, composition of 63

Horse, consumption of oxygen in locomotion by 504, 506, 507

maintenance requirement of 531, 537

utilization of energy by, in work 502

at a trot 509

walk 504

of ascent 506

INDEX. 595

PAGE

Horse, utilization of energy by, in work, of draft 507

work of digestion and assimilation in 385

locomotion in 504, 506, 508, 509, 510, 514, 535, 539

Human food, metabolizable energy of 277, 280, 282

foods, protein factors for 6

Humidity, relative, influence of, on heat emission 358

Hydrations, influence of, on computation of heat production 253

Hydrocarbons of excreta, determination of 69, 72

Hydrogen balance 78

in excreta 72

Income and expenditure of energy 3, 226

matter 3, 5

Individuality, influence of, on expenditure of energy in locomotion 517

sustaining load.. . 518

utilization of energy in work 517

Indol in urine 46

Ingesta, warming, loss of energy in 374

Insolation, influence of, on heat emission 357

Investigation, methods of 59, 234

Katabolism 16, 17

of fat 35

proteids 41

excretory nitrogen, measure of 42

final products of 41

Keratin, composition of 62

Kilogram-meter 231

Kilojoule 231, 232

Lactic acid, effect of, on proteid metabolism 123

total metabolism 158

production of, in metabolism of carbohydrates 23

replacement value of 158

La?vulose in blood 17

Leucin formed from proteids 39

oxidized in body 52

Ligneous material of crude fiber 9

Liver as reservoir of carbohydrates 20

source of dextrose 18, 19, 21 , 49

on carboydrate diet 19

proteid diet 19, 49

disappearance of glycogen from, in fasting 21

formation of dextrose in, from carbohydrates 19, 20, 21

fat 21,36,37

proteids 19, 21, 45, 50

method of 20

glycogen in 20, 21, 22

596 INDEX,

PAGE

Liver, formation of glycogen in, from dextrose 20, 21, 22

. proteids 21

sugar in 18, 19, 21, 49, 50

functions of 18

in work production 220

glycogenic function of 21

reconversion of glycogen to dextrose in 20, 22, 37

Live weight, influence of, on maintenance ration. . 458

utilization of energy 457

Load, effect of, on expenditure of energy in locomotion 509, 510, 515

total metabolism 509, 515

utilization of energy 508

in work of ascent 509, 510, 515

expenditure of energy in sustaining 508, 51 5

influence of individuality on. . . 518

Locomotion, consumption of oxygen in, by dog 500

horse 504, 506, 507

expenditure of energy in 499, 510

by dog 502

horse at a trot 509, 510, 514

walk.. 504, 506, 508, 510,
533, 539

man 503

influence of gait on 513

individuality on 517

load on 59, 510, 515

size of animal on 516

species on 516

speed on ... 507,508,513
work of. See Work of Locomotion.

Loss of fat, determination of 69. 77

influence of glycogen on computation of 66, 78

nitrogen by body 66, 67

protein by body 66

tissue 59

constant, in fasting 83

determination of 60

Maintenance 394

availability of energy for 390, 406, 410, 413, 427, 497

isodynamic values for 397

isoglycosic values for 400

ration 432

heat production on 436, 437

of horse 531 . 537

influence of consumption of water on 438

INDEX. 597

PAGB

Maintenance ration, influence of fattening on 441, 458

live weight on 458

shearing on 436

size of animal on 440

thermal environment on 435

time element on 439

relation of, to net availability of energy 432

relative value of grain and coarse fodder for 433, 533, 537

value of carbohydrates for 400-402

crude fiber 435

fat for 400, 402

Man, expenditure of energy by, in locomotion 503

hippuric acid in urine of 44

metabolizable energy of food of 277, 280, 282

utilization of energy by, in muscular work 503

tissue building 451

work of ascent 503

work of digestion and assimilation in 382

Mastication, work of 391

Matter, income and expenditure of 3, 5

Meadow hay, metabolizable energy of 286, 290, 297, 300, 301

utilization of energy of 484, 490, 491

Metabolic products, nitrogenous, in feces 42

Metabolism 14

a gradual process 16

an analytic process 15

a process of oxidation 15

carbon dioxide produced in 14, 15

carbon, effects of muscular exertion on 209

consumption of oxygen in 14, 15, 16

effects of muscular exertion on 185, 193

non-nitrogenous nutrients on 114, 125

proteid supply on 94, 104

excretory products of 14

fasting 80, 90, 340

effect of body fat on 88, 90

loss of protein on 90

ratio of proteid to total 81, 88, 89, 90, 93

total 83, 90

proportional to active tissue 86, 93

fat, in fasting 85, 88, 90

food increases 372

glandular, similar to muscular 344

influence of food consumption on 387

muscular exertion upon 185. 193

598 INDEX.

PAGFT

Metabolism in muscular tonus 190

intermediary 91

during muscular exertion 219

heat production in 405

of fat 91

protein 91

respiratory exchange in 405

intermediate products in , . 16, 44

muscular, nature of 495

of amides 52

carbohydrates 15, 17

hexose 17

pentose. 24

production of carbon dioxide in 23, 27

lactic acid in 23

water in 23, 27

fat 15, 29

intermediary 91

production of carbon dioxide in 36

water in 36

non-proteids 52

organic acids 26

proteids 15, 38

products of, in feces 10, 47

determination of 10

influence of, on determination of diges-
tibility 10

nature of 47

proteid 15, 38

and nitrogen excretion 97

determined by supply 128

effects of acetic acid on 123

carbohydrates on 115

compared with fat 127

cellulose on 117

excess of proteids on 96

fat on 114

compared with carbohydrates 127

lactic acid on 123

muscular exertion on 194, 206

influence of amount

of food on 197

non-nitrogenous nutrients on 114, 125

duration of . . 128
magnitude of 128

INDEX. 599

PAGE

Metabolism, proteid, effects of organic acids on 123

pentose carbohydrates on 124

proteid supply on 94

starch on 116

sugars on 116

expressed in terms of flesh 68

glycocol intermediate product of 44

identity of, in different animals 317, 335

on different feeds 322

in fasting 81, 90

minimum of proteids less than 136

tends to become constant 81, 90

two factors of 81, 90

intermediate products in 44

intermediary 91

production of carbon dioxide in 42

phosphoric acid in 42

sulphuric acid in 42

urea in 42

water in 42

ratio of, to total, in fasting 86, 88, 89, 90, 93

urea as measure of 68

relations of, to food supply 93

relative effects of hay and grain on 388

total, computation of 78

effect of acetic acid on 160

butyric acid on 158

cellulose on 162

crude fiber on 161

lactic acid on 158

load on 509, 515

non-nitrogenous ingredients of feeding-stuffs

on 151

nutrients on 144, 1 54

organic acids on 157

pentose carbohydrates on 156

proteid supply on 104

rhamnose on 156

in fasting 83, 90

proportional to active tissue 8<>, '■<'■'>

nitrogen cleavage of proteids independent of 99

ratio of, to proteid, in fasting 86, 88, 89, 90

urea produced in 14, 15

water produced in 14, 15

Methane, excretion of, by cattle 243

6oo INDEX.

Methane in excreta, determination of 69, 72

losses of energy in 310, 325, 328, 330, 335

produced by fermentation of carbohydrates 13

Methods of investigation 234

Milk fat, formation of, from carbohydrates 174

Minimum demands of vital functions 80

of proteids 133

amount of non-nitrogenous nutrients required to

reach 139

effect of carboydrates on 136

fat on 135

non-nitrogenous nutrients on 134

on health ,. . 143

for herbivora 140

in fasting 82, 83, 90, 94

less than fasting metabolism 136

Mixed diet, work of digestion and assimilation of 382, 384

grains, utilization of energy of 483, 491

Moekern experiments 281, 455

Mot or, efficiency of animal as 498

Mucins, composition of 62

Muscle, consumption of dextrose in 22, 221

contractile substance of 17

efficiency of single 495

extractives of 8

formation of glycogen in 23

respiratory quotient of 187

resting, storage of dextrose in 222

oxygen in 222

voluntary, work of 337

Nails, composition of 63

Nitrogen-free extract, apparent digestibility of 12

influence of amides on. ... 57
ammonium

acetate on. 57
asparagin on. 5?
non-proteids

on 57

carbohydrates of 9

digestible, gross energy of 305, 306

furfuroids of 9

pentose carbohydrates of 9

Nitrogen balance, computation of heat production from carbon and 255

cleavage of proteids 98

cause of 100, 101, 103-

INDEX. 60 1

paob

Nitrogen cleavage of proteids, effects of non-nitrogenous nutrients on.. . . 131

independent of total metabolism 99

content of proteids 39

equilibrium, amount of proteids required to reach 94

estimation of protein from 5, 6

excretion, effect of proteids on 94, 96

of free 42

proteid metabolism and 97

rate of 98

effects of non-nitrogenous nutrients on 130

excretory, measure of proteid katabolism 42

factor for computation of non-proteids from 8

protein from 67, 68, 77

gain or loss of, by body 66, 67

income and outgo of 66

in perspiration 48

metabolic, in feces 47

percentage of, in body protein 62, 65

proteids 6, 7

protein 6, 62, 65

Non-proteids 7

asparagin typical of 8

determination of , 8

factor for computation of, from nitrogen 8

in feeding-stuffs 6

influence of, on digestion 54

apparent digestibility of carbohydrates ... 57
crude fiber . . . 57, 58
nitrogen-free ex-
tract 57

fermentation of carbohydrates 55

fermentations in digestive tract 54

of animal body 8

oxidized in body 52

metabolism of 52

nature of 7

not synthesized to proteids 53

produced by cleavage of proteids 7,8

replacement of proteids by 53

resorption of 12

Nutrients, available 10

digestible, energy of 302, 306

gross 302

metabolizable 310, 332, 333

factors for 302, 332, 333

602 INDEX.

Nutrients, fiber-free, utilization of, in work production .541, 543

computed 547

isodynamic replacement of 152

isoglycosic replacement of 153

metabolizable energy of, utilization of, in work production .... 545

modified in digestive tract 12

mutual replacement of 148

non-nitrogenous, amount of, required to reach minimum of

proteids 139

as source of hippuric acid 45

effects of, on metabolism 114, 125

minimum of proteids 134

nitrogen cleavage of proteids. . . 131

proteid metabolism 114, 125

magnitude

of 128

duration of. 128

rate of nitrogen excretion 130

total metabolism 144, 154

formation of fat from 162

of feeding-stuffs, formation of fat from 180

mutual replacement of 1 54

substitution of, for body fat 144

utilization of excess of 162

percentage of oxygen in 5

relative values of 152

in work production 522

replacement values of 152, 396

Nutrition, function of 2

statistics of 3

Oat straw, metabolizable energy of 290, 297, 300 301

utilization of energy of 485, 490, 491

Oil, metabolizable energy of 296, 323, 332

utilization of energy of 478, 490, 491

Organic acids, absence of, from excreta 27

net availability of energy of 423

oxidized in body 27

matter, digestible, gross energy of 309

metabolizable energy of 297, 307

utilization of energy of 490

total, metabolizable energy of 284, 285

utilization of energy of 455,461, 490

substances, heats of combustion of 237

Oxidations incomplete in muscular contraction 186

Oxygen balance 79

INDEX. 603

PAGE

Oxygen, consumption of, determination of 70, 71, 73, 79

in fasting 84

locomotion, by dog 500

horse 504, 506, 507

metabolism .. 14, 15, 16

work of ascent by dog 500

horse 506

draft by dog 501

horse 507

not essential to muscular contraction 188

percentage of, in excreta 15

nutrients 15

storage of, in resting muscle 222

Parotid gland, consumption of dextrose of blood in 22

Peanut oil, metabolizable energy of 296, 323, 332

Pentosans 8

oxidized in body 26

Pentose carbohydrates. See Carbohydrates.

Pentoses 8

formation of glycogen from 25, 26

in urine 25, 26

metabolism of 24

net availability of energy of 420, 428

oxidized in body 25

Peptones absent from blood 40

formed from proteids 12, 38, 39

produced during digestion 12

synthesis of, to proteids by an enzym 40

Perspiration, ammonium salts in 48

creatinin in 48

nitrogen in 48

nitrogenous matter in 42

potential energy of 242

proteids in 48

urea in 48

uric acid in 48

Phenols in urine 27, 46

Phosphoric acid, production of, in metabolism of proteids 42

Phosphorus balance 79

Plastein 40 s

Proteids 6

albumoses formed from 38, 39

amides formed from 7, 39, 52

not synthesized to 53

604 INDEX.

PAGB

Proteids, amount of, required to produce carbon equilibrium 105

nitrogen equilibrium 94

anabofism of 38, 41

aspartic acid formed from 39

as source of muscular energy 201, 207

body, and food proteids 40

carbohydrate radicle in 50

changes in, during digestion 12

classification of 7

cleavage of, in digestion 12, 38

purpose of 38

non-proteids produced by 7, 8

differences in , . 39

effect of excess of, on proteid metabolism 96

on formation of hippuric acid 463

metabolism 94, 104

nitrogen excretion 94, 96

proteid metabolism 94

total metabolism 104

food, and body proteids 40

formation of dextrose from, in liver 19, 21, 49, 50

fat from 30, 50, 98, 101

difficulty of proof of 113

equations for 51

later experiments Ill

Pettenkofer and Voit's experiments. . . . 108

Pfliiger's recalculations 109

glycogen from 21, 98

sugar from 19, 21, 49, 50

functions of, in muscular exertion 207

gain of, during work 204

glutaminic acid formed from 39

in perspiration 48

intermediary metabolism of 91

katabolism of 41

excretory nitrogen measure of 42

final products of 41

leucin formed from 39

metabolizable energy of 272, 276, 277

metabolism of. See Metabolism.

minimum of 133

amount of non-nitrogenous nutrients required to

reach 139

effect of non-nitrogenous nutrients on 134

effects of, on health 143

INDEX. 605

PAGE

Proteids, minimum of, for herbivora 140

in fasting 82, 83, 90, 94

influence of carbohydrates on 136

fat on 135

less than proteid metabolism in fasting 136

molecular weight of 15

nature of 39

net availability of energy of 414, 427, 428

nitrogen cleavage of 98

cause of 100, 101, 103

effects of non-nitrogenous nutrients on. . . . 131

independent of total metabolism 97

content of 6, 7, 39

non-nitrogenous residue of 48, 98

fate of 49, 98

formation of sugar from 49, 50, 98

non-proteids not synthesized to 53

peptones formed from 12, 38, 39

percentage of nitrogen in 6,7

proteoses formed from 12, 39

putrefaction of, in intestines 44, 46

products of 44, 46

rebuilding of, from cleavage products 40

replacement of, by amides 53

asparapn 54

body fat 149

fats and carbohydrates of food 149

non-proteids 53

resorption of 12

respiratory quotient of 74, 75

synthesis of peptones to 40

substituted for body fat 104

Proteid supply, effects of, on metabolism 94, 104

proteid metabolism 94

total metabolism 104

Proteids, terminology of 5, 7

t ransitory storage of 96

tyrosin formed from 39

utilization of energy of 482,491

excess of 107

work of digestion and assimilation of 381, 382, 384

Protein, circulatory 82

composition of 62

digestibility of, real 10

digestible, gross energy of 309

606 INDEX.

PAGE

Protein, digestible, metabolizable energy of . . 310, 315, 317, 318, 320, 327, 332

utilization of energy of 481, 491

estimation of, errors in 6

from nitrogen 5, 6

factor for computation of, from nitrogen 6, 67, 68, 77

in human foods 6

gain or loss of, by body 66

potential energy of 244

in feeding-stuffs 5

loss of, in fasting, effect of, on metabolism 90

energy of, in methane 310

urine 312

nature of 5

of body, composition of 62, 65, 66

percentage of nitrogen in 62, 65

organized. 82

percentage of nitrogen in 6, 62, 65

ratio of fat to, in body in fasting 88, 89, 90

real digestibility of 10

storage, cause of 102

extent of 132

terminology of 6, 7

Proteoses, formed from proteids 8, 12, 39

produced during digestion 12

Putrefaction of proteids in intestines 44, 46

products of 44, 46

Quotient, respiratory 74

change in, caused by work 212

computation from, of carbohydrates oxidized 76

fat oxidized 76

deductions from 75

during work, conclusions from 75

effects of muscular exertion on 211

in fat-formation from carbohydrates 179

of carbohydrates 74

fat 74

muscle 187

influence of contraction on 187

proteids 74, 75

variations of 211

during work 216

Range, thermic 348

Rate of nitrogen excretion 98

Ration, maintenance. See Maintenance.

Regulation of body temperature 347, 348

INDEX. 607

PAGE

Regulation of body temperature, chemical 352

means of 348

physical 351

emission of heat 349

Rennet ferment, functions of 40, 41

Replacement, isodynamic, law of 152, 399

isoglycosic, law of 153, 399

mutual, of fat and carbohydrates 151

non-nitrogenous ingredients of feeding-stuffs ... 1 54.

nutrients 148

of proteids by amides 53

asparagin 54

body fat 149

carbohydrates and fat of food 149

non-proteids 53

value of acetic acid 1G0

butyric acid 158

carbohydrates 152

cellulose 162

crude fiber 1G1

lactic acid 158

non-nitrogenous ingredients of feeding-stuffs 154

nutrients 152

organic acids 157

pentose carbohydrates 156

rhamnose 156

Residue, non-nitrogenous, of proteids 48, 98

fate of * 49, 98

formation of sugar from 49, 50, 98

Resorption of carbohydrates 12

hexose 12, 17

rate of 18

dextrose, rate of 18

fat 12, 30

non-p/oteids 12

proteids 12

Respiration apparatus . . . 69

determination of water by 79

Pettenkofer type of 70

Regnault type of 69

Zuntz type of 72

Respiration-calorimeter 2 in, 248

Respiration, determination of products of 69, 73

effects of muscular exertion on 192

work of 193, 341

608 INDEX.

PAGE

Respiratory exchange, determination of 73

in intermediary metabolism 405

Rest, reappearance of muscular glycogen in 23

Rhamnose, effect of, on total metabolism 156

replacement value of 15G

Rice, utilization of energy of 483, 491

Ruminants, utilization of energy in 455, 461, 467

Sarkosin oxidized in body 53

Saponification of fat in digestion 12

Schematic body 60, 66

Shearing, influence of, on maintenance ration 436

Size of animal, influence of, on efficiency of animal 515

expenditure of energy in locomotion 516

heat production 359

in fasting 359

maintenance ration 440

relation of, to physiological activities 368

Species, comparison of heat production of 369

influence of, on efficiency of animal 515

expenditure of energy in locomotion 511

Speed, correction for, in work of locomotion 507, 508

influence of, on expenditure of energy in locomotion 507, 508, 513

utilization of energy in work 507, 513, 514

Standing, expenditure of energy in 343, 499

Starch, as source of muscular energy 199

digestible, gross energy of 306

metabohzable energy of 324, 332

utilization of energy of 475, 477, 490

effect of, on proteid metabolism 116

metabohzable energy of 294, 297, 301

utilization of energy of 473, 490, 491

States, initial and final, law of 228

Statistics of nutrition 3

Storage of protein, extent of 132

transitory 96

cause of 102

Straw, extracted, gross energy of carbohydrates of 308

metabohzable energy of 290, 297, 300, 301

carbohydrates of 327

utilization of energy of 488, 490, 491

oat, metabohzable energy of 290, 297, 300, 301

protein of 321

carbohydrates of 329

utilization of energy of 485, 490, 491

wheat, metabohzable energy of 290, 297, 300, 301

INDEX 609

PACE

Straw, wheat, metabolizable energy of protein of 321

carbohydrates 329

utilization of energy of 487, 490, 491

Sugar, effect of, on proteid metabolism 116

formation of, from non-nitrogenous residue of proteids 49, 50, 98

proteids 19, 21, 49, 50

in liver 18, 19, 21, 49, 50

Sulphur balance 79

Sulphuric acid, conjugated, in urine 46

production of, in metabolism of proteids 42

Surface of animal, computation of 364

relation of heat production to 359

internal work to 366

to work of digestion and assimilation 408

Swine, utilization of energy by 452, 466

Temperature, body 347

regulation of 347, 348

chemical 352

means of 348

physical 351

critical 353

method of heat emission above 355

modification of conception of 357

influence of, on heat production 351

rate of emission of heat 350

Thermal environment, critical 358

influence of, on heat production in fasting 347

maintenance ration 435

utilization of energy 471

Thermic range 348

Thermo-chemistry 228

Time element, influence of, on heat production 439

maintenance ration 439

Timothy hay, metabolizable energy of 287, 290, 297, 301

net availablity of energy of 424, 428

Tissue 59

active, fasting metabolism proportional to 86, 93

adipose 29

building, expenditure of energy in digestion, assimilation, and . . . 491

loss of energy in 44 1 , 117

utilization of energy in 444, 447, 448, 461

by carnivora 448, 466

man 451

ruminants 455, 461, 467

swine 452, 466

610 INDEX.

PAGE

Tissue, building, utilization of energy in, earlier experiments on 460

effect of amount of food on ... . 466
character of food on. . . 472
differences in live weight

on 457

thermal environment on 471

constant loss of, in fasting 83

gain of potential energy in 244

gains and losses of 59

determination of 60

mass of, relation of heat production to 370

muscular, composition of 63, 64

fat in 63, 64

glycogen in 64

heat of combustion of 63, 64

Tonus, muscular 190

influence of, on heat production 191

metabolism in 190

work of 341

Training, influence of, on utilization of energy in work 519

Transformation of energy in body 2

muscular contraction 495

Trot, expenditure of energy in locomotion at 509, 510, 514

utilization of energy in work at 509, 510

Tyrosin, formed from proteids 39

oxidized in body 52

Units of heat 232

measurement of energy 231, 233

Urea 42

antecedent of 42

ammonium carbonate as 43

lactate as 43

as measure of proteid metabolism 68

in perspiration 48

production of, from amides 52

in metabolism 14, 15

of proteids 42

Uric acid 43

origin of 43

in perspiration 48

urine 43

Urine, aromatic compounds in 46

computation of potential energy of 241, 313

conjugated sulphuric acid in 46

INDEX. 6ll

PAGE

Urine, hippuric acid in 44

indol in 46

losses of energy of protein in 312

non-nitrogenous matter of 27, 312, 320

amount of 28

derived from coarse fodders 28

non-nitrogenous matter. . 321

influence of 320

source of 27, 321

pentoses in 25, 26

phenols in 27, 46

potential energy of 272, 275, 278, 312

computation of 241, 277, 312

uric acid in 43

Utilization of energy. See Energy.

Values, isodynamic 397, 399

isoglycosic 399, 400

replacement 396

modified conception of 405

of nutrients 396

Variations in heat production, causes of 363

Walking, consumption of oxygen in, by horse 505

expenditure of energy in, by horse 504, 506, 508, 510, 533, 539

utilization of energy in, by horse 513

Water, consumption of, influence of, on heat production 438

maintenance ration 438

determination of, by respiration apparatus 79

production of, in metabolism 14, 15

of carbohydrates 23, 27

fat 36

proteids 42

Wheat gluten, digestible protein of, metabolizable energy of 310, 317

gross energy of 309

utilization of energy of 481, 491

metabolizable energy of 295, 297, 301

digestible matter of 301

utilization of energy of 480, 490, 491

straw, digestible carbohydrates of, metabolizable energy of 329

crude fiber of, metabolizable energy of 330, 332

matter of, gross energy of 310

metabolizable energy of 300, 301

utilization of energy of 487

protein of, metabolizable energy of 321 , 332

metabolizable energy of 290, 297, 300, 301

612 INDEX.

Wheat straw, utilization of energy of 461, 487, 490, 491

Wind, influence of, on heat emission 357

Wool, composition of 63

Work. (See also Exertion, muscular) 226

cellular 344

change in respiratory quotient caused by 212

coefficient of utilization in 498

disappearance of muscular glycogen in 23

gain of proteids during 204

glandular 343

internal 336, 337

fasting heat production a measure of 344

muscular '. 341

relation of, to surface 366

kind of, influence of, on efficiency of animal 512

mechanical, determination of 245

muscular, disappearance of glycogen in 23

incidental 342

net available energy for 497

of ascent, consumption of oxygen in, by dog 500

corrected 508

utilization of energy in 502, 503, 510

by dog 502

horse 506

man 503

effect of grade on 512

load on 509, 510

circulation 191, 341

descent 509

influence of grade on 509

digestion and assimilation 80, 93, 337, 372, 376, 406, 493

above critical point 407

below critical point 406

indirect utilization of heat from. . . . 406

in dog 378

horse 385

man 382

methods of determining 377

of bone 381

carbohydrates 379, 382, 384

fat 378, 382, 384, 385

mixed diet 382. 384

proteids 381, 382, 384

relation of, to surface 408

INDEX. 613

PAGE

Work of digestion, factors of 374

for crude fiber 389

draft, consumption of oxygen in, by dog 501

utilization of energy in 502, 507, 513

by dog 502

horse 507,51:5

heart 192, 341

locomotion, computation of 512

consumption of oxygen in, by dog 500

horse 505

correction for speed in 507, 508

expenditure of energy in, by dog 500

horse. . . . 504, 506, 508, 509
510, 514, 533, 539

utilization of energy in, computed 513

mastication 391

muscular tonus 341

respiration 192, 341

standing 343

voluntary muscles 337

physiological 336

production, function of liver in 206

relative value of nutrients in 522

coarse fodder and grain

for 533

value of crude fiber for 535, 537

fat for 522

utilization of energy in 444, 447, 494

by dog 499

horse 502

at a trot 509

walk 504

man 502

influence of fatigue on 519

individuality on 517

kind of work on 512

load on 508

size of animal on 515

species on 515

speed on 507, 513, 514

training on 519

metabolizable energy in 525

methods of determina-
tion 526, 528

614

INDEX.

PAGE

Work, utilization of metabolizable energy in, Wolff's investigations 528

of feeding-stuffs in 510

fiber-free nutrients in 541, 543,
545, 547

net available energy in 497

variations of respiratory quotient during 216

\

AGRICULTURE

FORESTRY
i LIBKAR.Y.

m m ■ m