URINE 



395 



urea, ammonia, and creatinine are the chief end-products of nitrogen- 

 ous metabolism. It is generally said that the relation existing between 

 uric acid and urea in human urine under normal conditions varies on 

 the average from 1 140 to i : 100 and is subject to wider variations under 

 pathological conditions; and further that because of the high content of 

 uric acid in the urine of newborn infants the ratio may be increased to 

 i MO or even higher. We now know that this ratio of uric acid to urea 

 is of little significance under any conditions. 



In man, uric acid probably results principally from the destruction 

 of nuclein material. It may arise from nuclein or other purine material 

 ingested as food or from the disintegrating cellular matter of the organ- 

 ism. The uric acid resulting from the first process is said to be of ex- 

 ogenous origin, whereas the product of the second form of activity is 

 said to be of endogenous origin. As a result of experimentation, Siven, 

 and Burian and Schur, and Rockwood claim that the amount of endoge- 

 nous uric acid formed in any given period is fairly constant for each 

 individual under normal conditions, and that it is entirely independent 

 of the total amount of nitrogen eliminated. Folin has taken exception 

 to the statements of these investigators and claims that, following a 

 pronounced decrease in the amount of protein metabolized, the absolute 

 quantity of uric acid is decreased but that this decrease is relatively 

 smaller than the decrease in the total nitrogen excretion and that the 

 per cent of the uric acid nitrogen, in terms of the total nitrogen, is there- 

 fore decidedly increased. According to Mares', 1 food-stuffs act to in- 

 crease the endogenous uric acid output 6y stimulating the digestive 

 glands to activity. That a portion of the endogenous uric acid may 

 arise in this way has recently been shown by Mendel and Stehle. 2 



In birds the formation of uric acid is analogous to the formation 

 of urea in man. In these organisms it is derived principally from the 

 protein material of the tissues and the food and is formed through a 

 process of synthesis which occurs for the most part in the liver; a 

 comparatively small fraction of the total uric acid excretion of birds 

 may result from nuclein material. 



When pure, uric acid may be obtained as a white, odorless, and 

 tasteless powder, which is composed principally of small, transparent, 

 crystalline, rhombic plates. Uric acid as it separates from the urine 

 is invariably pigmented, and crystallizes in a large variety of character- 

 istic forms, e.g., dumb-bells, wedges, rhombic prisms, irregular rec- 

 tangular or hexagonal plates, whetstones, prismatic rosettes, etc. Uric 

 acid is insoluble in alcohol and ether, soluble with difficulty in boiling 



\ JJare^: Arch f .d ges. Physiol., 134, 59, 1910. 



* Mendel and Stehle: Jour. Biol. Chem., 22, 215, 1915. 



