UREA, THE CHARACTERISTIC SALT OF URINE. 259 



accumulates in undue proportion in the blood under various 

 circumstances of no uncommon occurrence, such as by the effect 

 of a cold or damp atmosphere diminishing the natural amount 

 of the cutaneous perspiration, or by abstinence from the ac- 

 customed amount of exercise, or by the use of an unusual 

 quantity of drink. To draw off this excess of water from the 

 blood the Malpighian bodies are supposed to be especially 

 adapted, the thin-walled capillaries of which "allow the transu- 

 dation of water to take place under a certain pressure into the 

 tubuli uriniferi, and thus act the part of regulating valves, 

 permitting the passage of whatever is superfluous, while they 

 retain the liquid that is needed in the system." * The same 

 rule, however, does not hold in birds. They drink very little ; 

 so that the proportion of water in their urine is only sufficient 

 to give it a semi-fluid consistence. The urinary secretion of 

 reptiles is still more solid ; and the same seems to be the case 

 in fishes. The solid matter of the urine consists, as just hinted 

 at, of organic compounds originating within the body under 

 the disintegration of the tissues, or the decomposition of the 

 aliments taken into the stomach, or of organic salts, which, 

 under any special circumstances, may have exceeded their due 

 proportion in the blood. 



Though the organic compounds in the urine are not the same 

 in all animals, yet they bear a close relation to each other, and 

 in particular agree as to the large proportion of nitrogen that 

 they respectively contain. For the most part they assume a 

 crystalline form, which marks their affinity with inorganic sub- 

 stances, inasmuch as that form appears to be wholly incompati- 

 ble with the possession of plastic or organisable properties. 



In the urine of man the characteristic substance is urea, a 



crystalline body; neutral that is, neither acid nor alkaline ; 



very soluble in water, somewhat less soluble in spirit ; iso- 



meric that is, containing the same simple elements in the 



* Carpenter, * Comparative Physiology, ' p. 434. 



