444 ANIMAL PHYSIOLOGY. 
does in the laboratory, seems to be too crude a conception of 
vital processes. Until it can be rendered very much clearer 
Fia. 339.—Microscopic appearances of—I, milk; II, cream: ITI, butter; IV, colostrum of 
mare ; V, colostrum of cow (after Thanhoffer). 
than at present, it is not safe to assume that their chemistry is 
our chemistry, or their methods our methods. It may be so; 
but let us not, in our desire for simple explanations or undue 
haste to get some sort of theory that apparently fits into our 
own knowledge, assume it gratuitously, in the absence of the 
clearest proofs, especially when our failures on this supposi- 
tion are so numerous. 
We may say, then, that fat is not merely selected from the 
blood, but formed in the animal tissues; that fat formation 
may take place when the food consists largely of carbohydrates, 
when it is chiefly proteid, or when proteid and fatty. In other 
words, fat results from the metabolism of certain cells, which 
is facilitated by the consumption of carbohydrate and fatty 
food, but is possible when the food is chiefly nitrogenous. We 
must, however, recognize differences both of the species and 
the individual in this respect, as to the extent to which one 
kind of food or the other most favors fat formation (excre- 
tion). The use of the adipose tissue as a packing to pre- 
vent undue escape of heat is evident; but more important 
