400 MANUAL OF PHYSIOLOGY. 



urine of mammalia, but in birds, reptiles, and insects it forms 

 the chief ingredient of the renal secretion. It is sparingly soluble 

 in water, and insoluble in alcohol and ether. However, in solu- 

 tions of the neutral phosphates and carbonates of the alkalies 

 it combines with some of the base so as to form acid salts, and 

 at the same time converts the neutral into acid phosphates, to 

 which, as has been already stated, the urine owes its acid reac- 

 tion. These salts are more soluble in warm than in cold water, 

 and hence generally fall as a sediment when the urine cools. 

 Uric acid is readily converted into urea by oxidation, and is 

 probably one of the steps in the formation of urea which com- 

 monly occurs in the body during the gradual oxidation of the 

 proteid bodies. 



The presence of uric acid may be recognized by the murexide 

 test. The substance to be tested is gently heated in a flat cap- 

 sule with some nitric acid. A decomposition occurs, N and CO 2 

 going off, urea and alloxan remaining as a layer of yellow fluid. 

 If this be cautiously evaporated, and a drop of ammonia added, 

 a striking purple-red color is produced, which the addition of 

 potash turns blue. 



The amount of uric acid normally follows pretty closely the 

 variations in urea, but is usually only about 8 grains (.5 gramme) 

 per diem. In certain diseases the quantity may be much in- 

 creased. For the quantitative estimation, which is seldom de- 

 cided by the practitioner, the student must consult the text-books 

 of physiological chemistry. 



Kreatinin (C 4 H 7 N S O) is always present in urine, probably being 

 formed from kreatiu by the loss of one molecule of water. About 

 15 grains (1 gramme) is excreted per diem. 



Xanthin (CgH^N^) also occurs in urine, but in extremely small 

 quantities. 



Hippuric acid (C 9 H 9 NO 2 ) is a normal constituent of human 

 urine, occurring, however, in very small quantities. On the other 

 hand, it is one of the most important nitrogenous constituents of 

 the urine of the herbivora, where it takes the place of uric acid. 

 Its presence depends on the existence of certain ingredients (ben- 

 zoic acid, etc.) in the food, which are capable of combining with 



