342 



THE AMEBIC AN NATURALIST [Vol. XLIX 



simpler splitting before absorption is a mechanism of 

 protection against the entrance of foreign protein into 

 the blood. 



The alimentary tract to a certain extent renders the 

 individual independent of the quantitative or stereo- 

 chemical constitution of the proteins of its food, the only 

 necessity being the presence of certain amino acids in the 

 food. 



But efficient as the alimentary tract is, it does not ex- 

 clude all foreign or inutilizable substances. Cholohema- 

 tin for example, from sheeps' bile, characterized by a 

 peculiar spectrum and hence susceptible of easy identifi- 

 cation, is absorbed by the dog and at least some of it is 

 excreted in the dogs' bile without change. 



The protection due to changes in the mucous membrane 

 of the alimentary tract is well shown in the case of ar- 

 senic. It was long supposed that the arsenic-eating 

 peasants of the Austrian Tyrol had become immune to 

 the effects of a considerable concentration of arsenic in 

 the blood, since they were able to take large quantities of 

 it by mouth without suffering the usual effects of an over- 

 dose. It was shown by Cloetta 33 that the apparent im- 

 munity arose from changes in the intestinal mucosa which 

 prevented the absorption of all but a small percentage 

 of the arsenic taken. 



If the lower members of the fatty acid series be fed to 

 a dog the body fat which is stored up is softer and of a 

 lower melting point than usual. Excess fat in the food 

 may be stored up as body fat, but under certain condi- 

 tions, the animal may use as much fat as is taken in the 

 food, and the animal remains in metabolic equilibrium so 

 far as the fat is concerned. 



Although the amount of protein necessary to sustain 

 life in an adult or to provide for growth in a young animal 

 varies with the nature of the protein taken in the food, 

 the animal is able to build up its own body protein from 



33 Cloetta, Archiv fiir ErperimenteUe Pathologic unci Pharmacologic 

 1906, LIV, p. 196. 



