478 ANIMAL PHYSIOLOGY. 
urea: uric acid, however, does not seem to be such, nor is it to 
be regarded as a. body that has some of it escaped complete 
oxidation, but rather as a result of a distinct departure of the 
metabolism; and there are facts which seem to indicate that 
the uric-acid metabolism is the older, from an evolutionary 
point of view, and that in mammals, and especially in man, as 
the results of certain errors there may be a physiological (or 
pathological) reversion. Hippuric acid, as replacing uric acid 
in the herbivora, may be regarded in a similar light. 
Our knowledge of the metabolism of the spleen, beyond its 
relations to the formation of blood-cells and their disintegra- 
tion, isin the suggestive rather than the positive stage. It . 
seems highly probable that this organ plays a very important 
part, the exact nature of which is as yet unknown. 
When an animal starves, it may be considered as feeding on 
its own tissues, the more active and important utilizing the 
others. Notwithstanding, organs with a very active metabo- 
lism, as the muscles and glands, lose weight to a large extent, 
The presence of urea to an amount not very greatly below the 
average in health, shows that.there is an active proteid metab- 
olism then as at all times in progress. 
General experience and exact experiments prove that, while 
an animal’s diet may be supplied with special regard to fatten- 
ing, to increase working power, or simply to maintain it in 
health, as evidenced by breeding capacity, form, etc., in all cases 
there must be at least a certain minimum quantity of each of 
the food-stuffs. No one food can be said to be exclusively 
fattening, heat-forming, or muscle-forming. 
A carbohydrate diet tends to production of fat; flesh, and 
other proteid food to supply muscular energy, but the latter 
also produces fat, and a diet. of flesh mixed with fat or gelatin 
will serve the purposes of the economy better than one contain- 
ing a very much larger quantity of proteid alone. Muscular 
energy, as is to be inferred from the excreta, is not the result 
of nitrogenous metabolism alone; and in arranging any diet 
for man or beast the race and the individual must be consid- 
ered. Animals can not be treated as machines, like engines 
using similar quantities of fuel; though this holds far more of 
man than the lower animals—i. e., the results may be predicted 
from the diet with far less certainty in the case of man than of 
other mammals. 
Food is related to excreta in a definite way, so that all that 
enters as food must sooner or later appear as urea, salts, car- 
