THE KIDNEYS AND URINE. 371 



ried by the portal vein to the liver, and it is there that the change in all 

 probability takes place; as when the functions of the organ are irrnvolv 

 interfered with, as in the case of acute yellow atrophy, the amount of uiva 

 is distinctly diminished, and its place appears to be taken by leucin and 

 tyrosin. It has been found by experiment, too, that if these substances 

 be introduced into the alimentary canal, the introduction is followed by 

 a corresponding increase in the amount of urea, but not by the presence 

 of the bodies themselves in the urine. 



(2.) From the Nitrogenous metabolism of the Tissues. This second ori- 

 gin of urea 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-nitrogenous 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. 



Turning to the muscles, however, as the most actively metabolic 

 tissue, we find as a result of their activity not urea, but kreatin; and 

 although it may be supposed that some of this latter body appears natur- 

 ally as kreatinin, yet it is not in sufficient quantity to represent the large 

 amount of it formed by the muscles, and, indeed, by others of the tissues. 

 It is assumed that kreatin therefore is the nitrogenous antecedent of urea; 

 where its conversion into urea takes place is doubtful, but very likely the 

 liver, and possibly the spleen, may be the seats of the change. It may be, 

 however, that part but if so, a small part reaches the kidneys without 

 previous change, leaving it to the cells of the renal tubules to complete the 

 action. In speaking of kreatin as the antecedent of urea, it should be 

 recollected that other nitrogenous products, such as xanthin (C 6 H 4 N 4 2 ), 

 appear in conjunction with it, and that these may also be converted into 

 urea. 



It was formerly 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 concerning the relation of this excretory product to the de- 

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

 undergone considerable modification. There is no doubt, of course, that 

 like all parts of the body, the muscles have but a limited term of exist- 

 ence, and are being constantly although very slowly 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 was formerly 

 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 must 

 without doubt be given up. 



