FOOD AND FEEDING 651 



matter in the urine, as equilibrium is soon established in the 

 adult animal of constant weight, so that the amount of nitro- 

 gen eliminated equals that ingested. In young and growing 

 animals a portion of the nitrogen does not reappear in the 

 urine, but is utilized in tissue formation. An excess of 

 circulating protein, besides being wasteful economically, is 

 harmful in causing plethora and various disordered condi- 

 tions, resulting in the formation of uric acid and other 

 products of imperfect oxidation. Nevertheless, a certain 

 surplus of protein appears to be essential for the well-being 

 of animals ; more than is necessary to make good the loss of 

 protein through tissue waste as determined by estimation 

 of the nitrogenous elimination in the urine of fasting animals. 



The vegetable proteids are transformed into bodies of 

 simpler chemical composition in the stomach and are there 

 converted in part by the gastric juice, but chiefly by pan- 

 creatic (trypsin), biliary and intestinal ferments in the intes- 

 tines, into peptone, the only form in which protein can be 

 absorbed by the digestive tract. 



The epithelial cells of the intestines possess the power 

 not only to absorb the peptone but to transmute it into 

 different isomeric compounds, as serum albumin, serum 

 globulin and fibrinogen ; special cells being employed in the 

 formation of particular compounds. 



Any peptone not so converted by the intestinal epithe- 

 lium becomes a poison when absorbed into the entero-hepatic 

 circulation, but its toxicity is destroyed by the liver cells. 

 The protein elements (as serum albumin) in the blood con- 

 sist of those absorbed unchanged from the intestines and 

 those different isomeric forms which have been used in the 

 body, and all are eliminated and oxidized by the excretory 

 glands with the production of heat and escape of the pro- 

 ducts of tissue waste, as urea, uric acid and water from the 

 kidneys ; bilirubin, taurocholic acid, etc., from the liver. 

 Heat is evolved in this oxidation of proteid substances in the 

 glands equivalent to 1,812 kilogram meters for each gramme 

 of protein consumed. ...... 



