150 NUTRITIONAL PHYSIOLOGY 



teins or the proteins of the living cells from its cleavage 

 products. It is useless to increase the quantity of the 

 amino-acids if the variety is not great enough to supply the 

 details of the molecular pattern to be wrought. What is 

 true of gelatin is true of a number of proteins from vege- 

 table sources. They do not give all the groupings needed 

 in the constitution of the more elaborate animal proto- 

 plasm. 



In some cases a single protein may be adequate for nutri- 

 tion, supplying a complete assortment of amino-acids. 

 But it will be seen that successful nutrition is more cer- 

 tainly to be secured by using proteins from various foods. 

 This is our practice, save in the important case of the milk- 

 fed infant. Milk actually contains proteins of more than 

 one order, so that the exclusive use of this food does not 

 narrow the selection of building units so greatly as might 

 be supposed. The chief protein of milk contains the ele- 

 ment phosphorus and is perhaps of somewhat . unusual 

 complexity. 



In an introductory chapter the protein molecule was 

 likened to a watch with its many dissimilar parts associ- 

 ated in the one possible way to secure a desired result. 

 One or more of these parts might be missing without their 

 absence being apparent to the untrained person as he 

 looked into the works. He could nevertheless observe the 

 fact that the watch would not go. This is quite parallel 

 with our progress toward an understanding of the failure 

 of gelatin and other proteins to serve all purposes in nutri- 

 tion. Just as the watch-maker, with his special knowledge, 

 easily detects what is wanting, so the physiologic chemist 

 is now able to say with much accuracy what particular 

 amino-acids are lacking in his feeding experiments. As 

 the defective watch may be made serviceable by the addi- 

 tion of certain bits of mechanism, so in a measure an in- 

 sufficient diet may be made adequate when extra amino- 

 acids are supplied. 



It is necessary now to approach a subject of some 

 difficulty. We must attempt to show why a given quan- 



