452 ANIMAL PHYSIOLOGY. 
that we must regard it as an especially valuable storehouse of 
energy, available as required. 
‘When we turn to the urine for information, it is found that 
in the above case 27 grammes of nitrogen were excreted and 
almost entirely, of course, in the form of urea; and since the 
loss of nitrogen from the muscles amounted to 15 grammes, it 
will appear that more than one half of the nitrogenous excreta 
is traceable to the metabolism of muscular tissue. It has been 
customary to account for the urea in two ways: first, as derived 
from the metabolism of the tissues as such, and continuously 
throughout the whole starvation period; and, secondly, from a 
stored surplus of proteid which was assumed to be used up 
rapidly during the early days of the fasting, and was the luxus 
consumption of certain investigators, 
Comparative—Hxperiment has shown that the length of 
time during which different groups of animals can endure com- 
plete withdrawal of food is very variable, and this applies to 
individuals as well as species. That such differences hold for 
the human subject is well illustrated by the history of the sur- 
vivors of wrecks. Making great allowances for such devia- 
tions from any such results as can be established by a limited 
number of experiments, it may be stated that the human being 
succumbs in from twenty-one to twenty-four days; dogs in 
good condition at the outset in from twenty-eight to thirty 
days; small mammals and birds in nine days, and frogs in 
nine months. Very much depends on whether water is allowed 
or not—life lasting much longer in the former case. The very 
young and the very old yield sooner than persons of middle 
age. It has been estimated that strong adults die when they 
lose #5: of the body weight. Well-fed animals lose weight 
more rapidly at first than afterward. 
Diet.— All experiments and observations tend to show that 
an animal can not remain in health for any considerable period 
without having in its food proteids, fats, carbohydrates, and 
salts; indeed, sooner or later deprivation of any one.of these 
will result in death. 
Estimates based on many observations have been made of 
the proportion in which these substances should enter into a 
normal diet. In the nature of the case, for a creature like 
man especially, whose adaptive power is so great that he can 
learn to live under a greater variety of conditions than any 
other animal, any figures on this subject must be interpreted 
as being but a very general statement of the case. 
