456 THE URINE. 



kept for some days after being voided, and explains the 

 ammoniacal odour then evolved. It is probable, that this 

 spontaneous decomposition is accelerated by the mucus and 

 other animal matters in the urine, which, by becoming 

 putrid, act the part of a ferment and excite a change of 

 composition in the surrounding compounds. It is chiefly 

 thus that the urea is sometimes decomposed before it 

 leaves the bladder, when the mucous membrane is diseased, 

 and the mucus secreted by it is both more abundant and, 

 probably, more prone than usual to become putrid. The 

 same occurs also in some affections of the nervous system, 

 particularly in paraplegia. 



The quantity of urea excreted is, like that of the urine 

 itself, subject to considerable variation. It is materially 

 influenced by diet, being greater when animal food is 

 exclusively used, less when the diet is mixed, and least -of 

 all with a vegetable diet. As a rule, men excrete a larger 

 quantity than women, and persons in the middle periods of 

 life a larger quantity than infants or old people (Lecanu). 

 The quantity of urea does not necessarily increase and 

 decrease with that of the urine, though on the whole it 

 would seem that whenever the amount of urine is much 

 augmented, the quantity of urea also is usually increased 

 (Becquerel) ; and it appears from observations of Genth, 

 that the quantity of urea, as of urine, may' be especially 

 increased by drinking large quantities of water. In various 

 diseases, as albuminuria, the quantity is reduced consider- 

 ably below the healthy standard, while in other affections 

 it is above it. 



The urea appears to be derived from two different sources. 

 That it is derived in part from the unassimilated elements 

 of nitrogenous food, circulating with the blood, is shown 

 in the increase which ensues on substituting an animal or 

 highly nitrogenous for a vegetable diet ; in the much larger 

 amount, nearly double, excreted by Carnivora than Her- 

 bivora, independent of exercise ; and in its diminution to 



