120 PHYSIOLOGY 



CH 2 OH CH 2 OH 



I I 



CHOH CHOH 



I I 



CHOH CHO 



CHOH CH 2 OH 



I I 



CHOH CHOH 



I I 



CHO CHO 



and it is easy to understand how by a process of reduction the aldehyde 

 is converted into the corresponding alcohol, namely, glycerin. The synthesis 

 of the neutral fat from glycerin and fatty acid is a change which can be 

 accomplished by many ferments. It is one involving practically no absorp- 

 tion or expenditure of energy. The change is a reversible one, and we find 

 both in plants and animals that a hydrolysis of neutral fat into fatty acid 

 and glycerin always occurs when a transport of the fat is required, while 

 the laying down of fat as a store of energy is always preceded by a resynthesis 

 of the neutral fat. We shall have occasion to deal in greater detail with these 

 questions when we have to discuss the formation and fate of the fat in the 

 animal body. 



