790 NUTRITION AND HEAT REGULATION. 



sibility of the formation of toxic products in the intestines from 

 putrefactive processes, etc. It may be said, however, that although 

 these experimenters have shown that normal conditions may be 

 maintained for six months to a year upon a low proteid diet, they 

 do not demonstrate satisfactorily that a larger proteid diet is 

 actually attended by evil consequences. 



The Specific Character of the Proteid Metabolism. From 

 the fact that the nitrogen is eliminated from the body in various 

 forms ammonia compounds, urea, creatinin, purin bodies, etc. 

 we may infer that the specific metabolism varies in the different 

 tissues and under different conditions. The steps or stages of this 

 metabolism are not completely known in any case, but the follow- 

 ing suggestions have been developed by experimental work. Urea 

 arises in part in the liver by a conversion of ammonia salts, for ex- 

 ample, ammonium carbamate or carbonate (p. 754). The ammonia 

 salts in turn may arise in the tissues by a process of hydrolytic 

 cleavage giving origin to amido-bodies, leucin, glycocoll, etc., 

 which are subsequently oxidized to ammonia, carbon dioxid, and 

 water. The ammonia and carbon dioxid unite to form ammonium 



carbamate, CO\f)]sjfr > which is then dehydrated, directly or in- 



4 



directly, by the liver to form urea. Or, as previously described, 

 some of the amido-bodies formed from the proteid during digestion 

 may be carried to the liver, and the amido-group be split off to form 

 urea. The general idea, therefore, is that the nitrogen of the pro- 

 teid is by processes of hydrolysis or by hydrolysis and oxidation 

 converted to ammonia salts and then to urea. Ammonia com- 

 pounds are found in the blood, particularly in that of the portal 

 vein, and these compounds increase in quantity when the liver is 

 removed.* 



Uric acid and the other purin bodies arise probably from a 

 metabolism of the nuclein material contained in the nucleoproteids of 

 the body, but the processes involved are not known. Presumably 

 they are formed from nuclein by processes of hydrolysis caused by 

 specific enzymes or by the more direct influence of the living tissue. 

 In the laboratory the nucleins may be split by the hydrolytic action 

 of acids with the formation of purin bases. 



The creatinin of the urine is formed from the creatin found in 

 muscular tissue. Presumably this creatin is derived from a metab- 

 olism of the living muscular tissue. There is evidence to show 

 that increased muscular activity may be associated with an in- 

 creased formation of creatin (p. 759). 



* See Nencki, Pawlow, and Zaleski, " Archiv f . exp. Pathologie u. Phar- 

 makologie," 37, 26, 1895; and Nencki and Pawlow, "Archives des sciences 

 biologiques," 5, 213. 



