380 HANDBOOK OF PHYS1OLOY. 



vein to the liver, and it is there that the change in all probability takes 

 place, as when the functions of the organ are gravely interfered with, as 

 in the case of acute yellow atrophy, the amount of urea 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 correspond- 

 ing 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 

 source of urea is shown by the fact that that body continues to be ex- 

 creted, though in smaller quantity than usual, when all nitrogenous sub- 

 stances are strictly excluded from the food, as, for example, when the 

 diet consists for several days of sugar, starch, gum, oil, and similar non- 

 nitrogenous substances. It is excreted also, even though no food at all 

 is 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 tis- 

 sue, we find as a result of their activity not urea, but Kreatin; and al- 

 though it may be supposed that some of this latter body appears natu- 

 rally in the urine, as Kreatinin, or hydrated Kreatin, 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 krea- 

 tin, therefore, is the nitrogenous antecedent of urea; where its conver- 

 sion into urea takes place is doubtful, but very likely the liver, and 

 possibly the spleen, may be the seat of the change. It is possible, how- 

 ever, 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, suchasxanthin (C 5 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 circum- 

 stances; and our notions concerning the relation of this excretory pro- 

 duct to the destruction 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 existence, and are being constantly, although very slowly, re- 

 newed, at the same time that a part of the products of their disintegra- 

 tion 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 



