DISEASES OF THE KIDNEYS. 743 



been taken into the system, and the condition of the intestinal 

 canal, skin, and the surrounding atmosphere. 



The amount of urine daily excreted by healthy horses varies, 

 according to the observations of Colin, from 44-033 to 61"646 

 English pints; and the amount of water in 1000 parts of 

 urine, according to the analyses of Von Bibra, Boussingault, 

 and others, from 880 to 930 parts; whilst in that of the ox, 

 Von Bibra found 91201 and 92311 ; in that of the pig from 

 980 to 990. 



Urea is the most important product of tissue change ; it has 

 the same chemical composition as the carbonate of ammonia, to 

 which it is readily converted when heated a little above the 

 temperature of boiling water, or when kept in contact with 

 decaying animal matter. When pure, it crystallizes from a 

 watery solution as a white, semi-transparent, crystalline body 

 with a bitter taste. The crystals are quadratic prisms, with 

 rectangular terminal planes; it readily combines with nitric 

 acid, forming the nitrate of urea, which speedily crystallizes ; 

 and the addition of nitric acid to urine is the method of detect- 

 ing its presence in abnormal quantities. The crystals of the 

 nitrate of urea, when rapidly formed, are flat, shining, rhom- 

 boidal plates (see fig. 26, p. 454) ; when slowly crystallized, fine 

 prisms. Nitrate of urea is soluble in w^ater and alcohol, but only 

 sparingly so when they contain an excess of nitric acid ; it is 

 insoluble in ether. 



It combines also with oxalic acid, forming the oxalate of urea 

 (Ur. Cg Og H 0), which crystallizes in prisms and quadrilateral 

 tubes. Urea forms insoluble compounds with the nitrate or 

 protoxide of mercury. 



Although found in the urine, urea is not formed in the 

 kidneys, but is excreted by them from the blood ; and, as already 

 pointed out in former chapters, it results not only from meta- 

 morphosis of tissue, but also from excess of food, as well as from 

 the decomposition of uric and hippuric acid. Its formation from 

 uric acid has been experimentally proved by ISTeubauer, who on 

 giving rabbits from 31 to 46 1 grains of uric acid with their 

 food, the quantity of urea excreted in the twenty-four hours 

 was augmented from 32'5 to 65"1 grains. Uric acid, when acted 

 upon by permanganate of potash, is transformed into several 

 substances, one of which is urea. 



