THE PROTEINS AND THEIR METABOLISM 



127 



thrives well on unvaried diets and maintains its health even though con- 

 stantly confined to a cage. As the longevity of the white rat is ahout 

 three years, they were able to study the influence of certain diets practically 

 throughout the whole life time of the animal. 



From hundreds of experiments published, four are selected here to 

 illustrate the physiological value of some of the amino acids. 



DAYS 



Diagram II illustrates graphically the result of Osborne and Mendel's experiments 



Rat Xo. 710 was kept under observation from May 9, 1913, to Sep- 

 tember 5, 1913, a period of 120 days. During that period the animal lived 

 on the following food mixtures: zein, 18.0 grams; protein-free-milk, 28.0 

 grams; starch, 27.0 grams; butter fat and lard, 27.0 grams; water, 15 c.c. 

 The influence of this diet on the animal's body weight is presented in 

 Table IX. Every one of the rats that was kept on this diet lost in weight. 

 Rat 710 lost 31) per cent of its body weight in 120 days. 



The experiment on Rat 1519 started on May 9, 1913, and ended Nov. 

 7, 1913. Between May 9 and August 8 it was kept on a mixture of zein, 

 10.92 grams, tryptophan, 0.54 gram, the rest as above. During this 

 period the animal lost weight steadily, reaching the lowest level of 100.0 

 grams on August 8 ; 0..">4 gram of lysin was then added to the diet. 

 There followed an immediate gain in body weight, reaching the highest 



