432 COMPARATIVE PHYSIOLOGY. 



THE CONSTRUCTION OP PAT. 



It is a well-known fact that, speaking generally, a diet rich 

 in carbohydrates favors fat formation, both in man and other 

 animals; though it is not to be forgotten that many persons 

 seem to be unable to digest such food, or, at all events, to as- 

 similate it so as to form fat to any great extent. Persons given 

 to excessive fat production are as frequently as not spariag 

 users of fat itself. 



It is possible in man and probable in ruminants that fer- 

 mentations may occur in the intestines giving rise to fatty acids 

 which are possibly converted into fats by the cells of the villi 

 or elsewhere. Certain feeding experiments favor the view 

 that carbohydrates may be converted into fat or in some way 

 give rise to an increase in this substance ; for it is to be borne 

 in mind that fat may arise from a certain diet in various 

 ways other than its direct transformation into this substance 

 itself. 



There are certain facts that make it clear that fat can be 

 formed from proteids : 1. A cow wiU produce more butter than 

 can be accounted for by the fat in her food alone. 2. A bitch 

 which had been fed on meat produced more fat in her milk 

 than could have been derived directly from her food, and this, 

 when the animal was gaining in weight, which is usually to be 

 traced to the addition of fat ; so that the fat of the milk was 

 not, in all probability, derived from, that of the dog's body ; 

 and, as will be seen presently, can be accounted for without 

 such a supposition. 3. It has been shown by analysis that 473 

 parts of fat were deposited in the body of a pig for every 100 in 

 its food. 



These facts of themselves suffice to show that fat can be 

 formed from proteid, or at least that proteid food can of itself 

 give rise to a metabolism, resulting in fat formation; and the 

 latter is probably the better way to state the case in the present 

 condition of knowledge. 



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 



