XIII. 



THE ANIMAL 



279 



produced. The epithelial cells must, during absorption, change the 

 peptones and albumoses back again into the original proteids. 



The length of the alimentary canal varies greatly in different animals, 

 being comparatively short in carnivora and very long in herbivora. 

 The usual length of the intestines of the various farm animals and the 

 percentage ratio of the capacity of the stomach to the total capacity 

 of stomach and intestines are as follows : 



The Urine. This is, perhaps, the most important excretion of the 

 animal body, since in it are the nitrogenous waste products, the water 

 and the soluble mineral salts derived from the food or the breaking 

 down of tissue. It is formed (or, probably more correctly, extracted) 

 from the blood by the kidneys, which contain several albuminous 

 bodies, fat, xanthine, urea, uric acid, glycogen, leucine, inosite, taurine 

 and cystine. Urine varies greatly in composition with the breed of 

 animal, the food, quantity of exercise, amount of water taken, and 

 many other circumstances. In carnivora and man, urine is usually 

 acid, while in herbivora it is neutral or alkaline. 



The specific gravity varies greatly ; its determination furnishes an 

 important means of estimating the total solids present. In the case 

 of human urine, variations from 1-002 to 1*040 have been observed. 

 The characteristic constituent of urine is urea or carbamide, CO(NH 2 ) 2 , 

 which is more abundant in the urine of carnivora than in that of her- 

 bivora. 



TVTTT ri(~\ 



Creatinine, C 4 H 7 N 3 0, or NH:<V j xanthine, C 6 H 4 N 4 O S 



\N(CH 3 ).CH 2 , 



and smaller quantities of allied substances also occur in the urine of man 

 and some mammalia. 



Uric acid, C 5 H 4 N 4 O 3 , or CO' 



-NH.CO 



C.NH\ 

 -NH.C.NH/ 



CO, 



occurs abundantly in the excrements of birds and serpents, also in the 

 urine of carnivora and man, and, to a very small extent, in that of the 

 herbivora. Uric acid is a dibasic acid, and both the acid and its acid 

 salts are very slightly soluble in water. In certain diseases, e.g., gout, 

 deposits of uric acid and urates are formed in the body. 



