FOOD COMPOSED OF SIMPLE SUBSTANCES 495 



supply of frogs is more easily maintained by feeding with 

 frog-meat than with the flesh of mammals ; tadpoles fed in 

 some instances with frog-liver, in others with calf -liver, are 

 said to thrive better in the former case. 



The idea that proteins of different origin are not physio- 

 logically equivalent is by no means new. Some reference to 

 this point has been made above, in connection with the sub- 

 ject of the processes of resorption in the intestine; and the 

 statement was made, too, that it has not been found possible 

 to maintain the nutrition of an animal on gelatin as the sole 

 source of nitrogen. Casein, gliadin and, too, zein (to be 

 thought of in connection with its relation to pellagra) may 

 also be regarded as examples of proteins which do not con- 

 tain certain characteristic " building stones." There are a 

 number of comparative studies upon the physiological value 

 of different proteins, among which may be especially men- 

 tioned those of Bohmann, Thomas, 28 E. Voit and Zisterer, 29 

 and, too, those of Osborne and L. B. Mendel. 30 The last 

 named writer believes that at present it is too early to come 

 to final conclusions upon this subject; that the factors of 

 variation are too numerous to be mastered in a few simple 

 equilibrium experiments of short duration. 



Food Composed of Simple Substances. In concluding, 

 the interesting question of feeding animals on a diet arti- 

 ficially constructed of elementary foodstuffs may be properly 

 discussed here. 



The basic principle of maintaining animal life by a mix- 

 ture of simple nutrient substances was long since brought 

 forward by studies by Zadik, Abderhalden and Rona, and 

 by Henriques and Hansen. Abderhalden 's recent investiga- 

 tions, which have been presented above (p. 64), indicating 

 the possibility of nourishing animals on a mixture of the 



18 K. Thomas, Arch, f . Anat. u. Physiol., 1909, 219. 



29 E. Voit and J. Zisterer, Zeitschr. f. Biol., 53, 157, 457, 1909. 



30 T. B. Osborne and L. B. Mendel, Carnegie Institution of Washington, 

 Publication, No. 156, I and IV, 1911; cf. Literature: L. B. Mendel, Ergebn. 

 d. Physiol., 11, 482, 1911. 



