456 ANIMAL PHYSIOLOGY. 
where in the fluids of the body, as opposed to the former as 
constituting organized tissues, undergoes changes of a retro- 
grade kind without ever becoming organ albumin, while the 
term luxus consumption was applied to the metabolism of pro- 
teids in the blood. The latter is not now believed to occur, 
But whether a portion of the urea that represents, in the main, 
the results of proteid metabolism is not derived from the 
metabolism of the material in the interspaces of the tissues 
‘(circulating proteids on which the cells are supposed to act 
and in which they effect changes without making these pro- 
teids a part of themselves), is uncertain. 
Nitrogenous Equilibrium.—tIt is possible to so feed an animal, 
say a dog, that the total nitrogen of the ingesta and egesta 
shall be equal; and this may be accomplished without the ani- 
mal losing or gaining weight appreciably or again while he is 
gaining. If there be a gain,it can usually be traced to the 
formation of fat, so that the proteid, we may suppose, has 
been split up into a part that is constructed into fat and a 
part which is represented by the urea, the fat being either used 
up or stored in the body. Moreover, an analysis of a pig that 
had been fed on a fixed diet, and a comparison made with one 
of the same litter killed at the commencement of the experi- 
ment, showed that of the dry nitrogenous food only about 
seven per cent in this animal and four per cent in the sheep 
had been laid away as:dry proteid. It is perfectly plain, then, 
that proteid diet does not involve only proteid construction 
within the body. 
Comparative.—The amount of flesh which a dog, being a car- 
nivorous animal, can digest and use for the maintenance of his 
metabolic processes is enormous; though it has been learned 
that ill-nourished dogs can not even at the outset of a feeding 
experiment of this kind maintain the equilibrium of their body 
weight on a purely flesh diet (fat being excluded), They at 
once commence to lose weight—i. e., they draw upon their own 
limited store of fat. 
The digestion of herbivora being essentially adapted. to a 
vegetable diet, they can not live at all upon flesh, while a dog 
can consume for a time without manifest harm 7s to g, of its 
body-weight of this food. 
Man, when fed exclusively on meat soon shows failure, he 
being unable to digest enough to supply the needed carbohy- 
drates, etc. But the large amount of urea in the urine of car- 
nivorous animals generally, and the excess found in the urine 
