662 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the breaking down of the albuminous food-constituents which has com* 

 menced in the intestinal canal, perhaps through the formation of leucin 

 and tyrosin. . Again, it seems clearly established that increase in mus- 

 cular exercise leads to an increase in the elimination of urea. Since as 

 has been stated, the amount of kreatin formed by the muscles increases 

 with exercise, and the amount of urea eliminated by the kidneys 

 increases under the same circumstances, it would appear warrantable to 

 assume that the kreatin resulting from the breaking down of the albu- 

 minous tissue-constituents of muscles and nerves represents the main 

 source of urea. As to where this conversion — which, it should be stated, 

 can only be acknowledged to have a certain degree of probability — takes 

 place, but little positive information can be given. 



It does not occur solely in the kidney, for the suppression of urine 

 is followed bv an accumulation in the system of a large amount of urea. 

 Again, the urea, under all circumstances, is a constant constituent of tlie 

 blood, indicating that even if a part of the urea be formed in the 

 kidnej's the renal epithelium is not the sole source of the manufacture of 

 this body. As to where kreatin becomes converted into urea no data 

 can be given. 



Another possible source of urea maybe found in the products of 

 the decomposition of albuminous matter in the intestine. It has been 

 seen that the introduction of a large amount of proteid in the alimentary 

 canal is followed by a corresponding increase in the amount of urea 

 eliminated. It is further known that the excess of proteid over and 

 above that needed for nutrition in the alimentary canal breaks down 

 under the influence of pancreatic digestion into leucin and tyrosin. If 

 leucin be itself introduced into the intestinal tube the amount of urea 

 eliminated will be proportionately increased. Leucin, therefore, may 

 represent a step in the processes of conversion of albumen into urea. In 

 the latter case we can probably locate the conversion of leucin into urea 

 in the liver, for the liver, unlike other glands, normally contains large 

 quantities of urea; and if we again assume, as perhaps seems warranted, 

 that the leucin is absorbed by the portal vein, the formation of urea out 

 "of leucin would be a natural conclusion. 



Uric acid has also been found to be a constant constituent of the 

 urine of carnivora and of suckling herbivora. It is never met with in 

 the free condition, but in the form of uric acid salts. In the urine of 

 birds and reptiles it replaces urea. It also evidently results from the 

 decomposition of proteids, and, perhaps, under certain circumstances, is 

 an antecedent of urea, since by oxidation one molecule of uric acid may 

 be split up into two molecules of urea and one molecule of mesoxalic 

 acid. This is not, however, to be regarded as invariably taking place, 

 but the majority of evidence would, perhaps, point to the formation of , 



