42 THE THEORY OF IONS 



decidedly exercises a beneficent influence upon the 

 process of construction, which apparently has its 

 ne plus ultra in the production of cellular protoplasm 

 endowed with powers of reproduction, and those 

 higher functions which are the characteristics of 

 brain and mind. 



Animals are quite unable to assimilate unorganised 

 nitrogen ; and we do not know exactly how they 

 assimilate or construct out of organised nitrogenous 

 material the proteins of the body. We know, how- 

 ever, that each animal has its own specific proteins ; 

 and that the protein molecules of its food, including 

 edestin, gliadin, hordeolin, zein, legumin, hsematin, 

 globulin, casein, egg-albumin and serum-albumin, 

 are not assimilated in these forms, but are foreign 

 to the organisation of the consumer and could not 

 be directly used for cell metabolism. The proteins 

 in the composition of each animal maintain a con- 

 stancy of constitution and composition, no matter 

 what kind of protein is consumed. If serum-protein 

 of one animal were injected into the blood of another 

 animal it would lead to the formation of precipitines, 

 and would not amalgamate directly with the serum 

 of the animal into which it was injected.* Digestion 

 is necessary therefore not merely to render protein 

 absorbable, but to break down the foreign protein 

 brought to the organism as food, during which 

 process a number of amino-acids appear. The 

 proteins of each animal and tissue are peculiar to it ; 

 there must therefore be some means by which the 

 organism is rendered independent of the kind of 

 protein supplied. A child fed on milk constructs 



* Barker's address : Brit. Med. Jour., 1906, ii., 1093-1110. 



