UREA. 361 



creased by drinking large quantities of water. In various dis- 

 eases, as albuminuria, the quantity is reduced considerably be- 

 low 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 uuassimilated 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 Herbivora, inde- 

 pendent of exercise ; and in its diminution to about one-half 

 during starvation, or during the exclusion of non-nitrogenous 

 principles of food. But that it is in larger part derived from 

 the disintegration of the azotized animal tissues, is shown by 

 the fact that it continues to be excreted, though in smaller 

 quantity than usual, when all nitrogenous substances are 

 strictly excluded from the food, as when the diet consists for 

 several days of sugar, starch, gum, oil, and similar non-azotized 

 vegetable substances (Lehmann). It is excreted, also, even 

 though no food at all be taken for a considerable time ; thus 

 it is found in the urine of reptiles which have fasted for 

 months ; and in the urine of a madman, who had fasted eigh- 

 teen days, Lassaigne found both urea and all the components 

 of healthy urine. Probably all the nitrogenous tissues furnish 

 a share of urea by their decomposition. 



It has been commonly taken for granted that the quantity 

 of urea in the urine is greatly increased by active exercise ; but 

 numerous observers have failed to detect more than a slight 

 increase under such circumstances ; and our notions concern- 

 ing the relation of this excretory product to the destruction of 

 muscular fibre, consequent on the exercise of the latter, have 

 lately undergone considerable modification. There is no 

 doubt, of course, that like all parts of the body, the muscles 

 have but a limited term of existence, and are being constantly 

 renewed, at the same time that a part of the products of their 

 disintegration appears in the urine in the form of urea. But 

 the waste is not so fast as it has been frequently supposed to 

 be ; and the theory that the amount of w r ork done by the muscle 

 is expressed by the quantity of urea excreted in the urine, and 

 that each act of contraction corresponds to an equivalent waste 

 of muscle-structure, is founded on error. (See also chapter on 

 Motion.) 



Urea exists ready-formed in the blood, and is simply ab- 

 stracted therefrom by the kidneys. It may be detected in 

 small quantity in the blood, and in some other parts of the 

 body, e. g., the humors of the eye (Millon), even while the func- 

 tions of the kidneys are unimpaired : but when from any cause, 



