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. 
Asu.—The ash ingredients of the body form a well-defined 
* Neue Beitriige, ete., p. vii. 
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