316 HUMAN PHYSIOLOGY 



consumption of proteins, but, on the other hand, since 

 there are bound to be amino-acids not needed for syn- 

 thetic uses it is clearly better to make fuel of them than to 

 reject them wholly. If a man pulls down an old house 

 and builds a new one out of the timbers he is pretty sure 

 to have many misfit pieces. It is the part of common 

 sense to throw these into the cellar to be burned when 

 desired. The body is operated according to the very 

 same principle. The analogy may even be made to sug- 

 gest the rejection of urea in the preparation of fuel. We 

 have only to picture the householder as detaching the 

 bolts, angle-irons, and other metal from the fragments 

 which he throws upon his wood-pile. 



We have distinguished between adequate and inade- 

 quate proteins. Recent investigations have shown that 

 a three-fold classification is more precise. Some proteins 

 are obviously insufficient, gelatin being an example. 

 Some answer for the maintenance of weight but not for 

 growth. The proteins of the highest order are those 

 which will promote the growth of young animals. 



