UREA. 



455 



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, 

 <rum, 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 eighteen days, 

 Lassaigne found both urea and all the components of 

 healthy urine. Probably all the nitrogenous tissues fur- 

 nish a share of urea by their decomposition. 



It has been commonly taken for granted that the quan- 

 tity 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 concerning the relation of this excretory 

 product to the destruction of muscular fibre, consequent 

 on the oxercise of the latter, have lately undergone con- 

 siderable 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 disin- 

 tegration 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 work 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 

 abstracted therefrom by the kidneys. It may be detected 

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



