COMPARATIVE PHYSIOLOGY. 



trogenous Crystalline Bodies.— These are the derivatives 

 01 me metabolism of the body, and not in any appreciable de- 

 gree drawn from the food itself. Besides urea, and of much less 

 importance, occurring in small quantities, are bodies that may 

 be regarded as less oxidized forms of nitrogenous metabolism, 

 such as creatinin, xanthin, hypoxanthin (sarkin), hippuric acid, 



ammonium oxalurate, and urea, CO [ i^tt' The latter was 

 first prepared artificially from ammonium cyanate, -h^tt S O, 



with which it is isomeric. The quantity of urea is generally 

 in inverse proportion to that of hippuric acid, and varies much 

 with the diet in the herbivora. The richer in proteids the diet, 

 as when oats are fed, the greater the quantity of urea. In the 

 horse this proportion varies with the ordinary diet between 

 2'5 and 4'0 per cent. 



When air has free access to urine for some time in a warm 

 room, the urea becomes ammonium carbonate by hydration, 

 probably owing to the influence of micro-organisms, thus: 

 CO (NH2)2-H 2 H2O = (NHi), COa; hence the strong ammoniacal 

 smell of old urine, urinals, etc. 



Uric acid (CsHiNiOs) occurs sparingly (see table), combined 

 with sodium and ammonium chiefly as acid salts. 



Ifon-nitrogenous Organic Bodies.— A series of well-known 

 aromatic bodies occurs in urine, especially in that of the horse, 

 cow, etc. These are phenol, cresol, pyrocatechin, etc., which 

 occur not free, but united with sulphuric acid. 



Inorganic Salts. — These are mostly in simple solution, in 

 urine, and not as in some other fluids of the body, united with 

 proteid bodies. The salts are chlorides, phosphates, sulphates, 

 nitrates, and carbonates ; the bases being sodium, potassium, 

 calcium, magnesium. The phosphates are to be traced to the 

 food, to the phosphorus of proteids, and to phosphorized fats 

 (lecithin). The sulphates are derived from those of the food 

 and from the sulphur of the proteids of the body. The greater 

 part of the carbonates is supplied directly by the food. In the 

 horse the salts of potassium and calcium (GaCOa), are abundant; 

 while in the dog magnesium and calcium salts abound as sul- 

 phates and phosphates. 



Doubtless many bodies appear either regularly or occasion- 

 ally in urine that have so far escaped detection, which are, like 

 the poisonous exhalations of the lungs, not the less important 

 because unknown to science. 



