6ib METABOLISM, NUTRITION AND BIETETICS 



other limitation of its chemical powers. While the cyclic (and hetero- 

 cyclic) compounds cannot be replaced by other ' Bausteine ' of the 

 proteins, they may to some extent replace each other. Thus it would 

 seem that tyrosin can be replaced by phenylalanin (Abderhalden) . 

 While some of the food proteins like casein are sufficient by 

 themselves to supply all the amino-bodies necessary not only for 

 the maintenance, but also for the growth of the body, and can 

 accordingly be termed adequate or complete protein food sub- 

 stances, others, like gelatin, are insufficient by themselves to supply 

 the protein required for mere maintenance, still less for growth, 



Fig. 201. Two Female Rats of the Same Age (140 days). The upper one was fed on 

 an ordinary diet, and is of the normal weight for its age. The lower one was 

 fed on a diet composed of a mixture of isolated food-stuffs. Its weight is only 

 equal to that of a normal rat thirty-six days old (Osborne and Mendel). 



and may be spoken of as inadequate or incomplete proteins. There 

 is a third intermediate group, comprising proteins which suffice 

 when given as the sole protein food to maintain the body for an 

 indefinitely long period, and to repair the tissue waste without per- 

 mitting growth of the animal to take place. Gliadin and hordein 

 (see footnote, p. 615) are representatives of this group. The ex- 

 periments of Osborne and Mendel with gliadin are of special interest, 

 since this substance is very differently constituted from the ordinary 

 food proteins, as well as from the tissue proteins of the animal 

 body. While, as already stated, it yields very large quantities of 

 glutaminic acid, prolin, and ammonia, it either contains no lysin 



