THE METABOLISM OF THX BODY. 441 
body, there would remain for disposal otherwise a large amount 
of carbon, for there is nearly three times as much of this ele- 
ment in proteid as in urea; so that it is possible, from a chemi- 
cal point of view, to understand the origin of fat from the pro- 
teid food; but too much importance must not be attached to 
such speculations. 
That fat is a real formation, dependent for its composition 
on the work of living tissues, is clear from the well-known fact 
that the fat of one animal differs from that of another, and that 
it preserves its identity, no matter what the food»may be, or in 
what form fat itself may be provided. Certain constituents of 
the animal’s fat may be wholly absent from the fat of its food, 
yet they appear just the same in the fat produced under such 
diet. Even bees can construct their wax from proteid, or use 
unlike substances, as sealing-wax. 
But histological examination of forming adipose tissue itself 
throws much light upon the subject. Fat-cells are those in 
which the protoplasm has been largely replaced by fat. The 
latter is seen to arise in the former as very small globules 
Fic. 334.—Mammary gland of human female (after Liégeois’. 1, sinus, or dilatation of one of 
lactiferous ducts ; 2, extremities of the ducts ; 3, lobules of gland; 4, nipple, retracted in 
renter: A areola 
