438 UREA. [BOOK 11. 



ureter simply ligatured, the result in both cases is the same, an 

 accumulation in the tissues and fluids of urea, or, in the case of 

 birds and snakes, &c., of uric acid. And indeed the ligature of the 

 ureters soon leads to such trouble in the renal epithelium as to 

 arrest their functional activity, so that the distinction between ex- 

 tirpation of the kidney and ligature of the ureters is an illusory 

 one. Hence, though we need not go so far perhaps as to say 

 that no part of the urea of urine is furnished by a transformation 

 of kreatin by the kidney itself, it is obvious that we cannot speak 

 of the main mass of urea, which passes from the body, as having 

 such an origin, and as having undergone such a transformation in 

 the kidney. Clearly the great part of the urea of the urine reaches 

 the kidney either as urea existing in the blood and has simply to be 

 passed through the epithelium cells, the healthy kidneys usually 

 performing their work so well as to leave behind in the blood only 

 such a slight amount of urea as to be with difficulty detected by the 

 means at present at our command, or as some substance, which 

 though not actually urea itself, is so nearly allied as to be capable 

 of being readily transformed into urea, and accumulated as urea 

 in the tissues and fluids, when the excreting power of the kidneys 

 is lost. 



The kreatin of muscle and other tissues may, before it reaches 

 the kidney, be a source of this urea or antecedent of urea in the 

 blood, having undergone transformations whose seat and nature 

 are hidden from us or, as we have suggested, it may not be. There 

 are however other possible sources of urea besides the kreatin 

 formed in muscle and elsewhere. We have seen that one result of 

 the action of the pancreatic juice is the formation of considerable 

 quantities of leucin and tyrosin. In dealing with the statistics of 

 nutrition, our attention will be drawn to the fact that the intro- 

 duction of proteid matter into the alimentary canal is followed 

 by a large and rapid excretion of urea, suggesting the idea that a 

 certain part of the total quantity of the urea normally secreted 

 comes from a direct metabolism of the proteids of the food, without 

 these really forming a part of the tissues of the body. We do not 

 know to what extent normal pancreatic digestion has for its product 

 leucin, and its companion tyrosin; but if, especially when a meal 

 rich in proteids has been taken, a considerable quantity of leucin 

 is formed, we can perceive an easy and direct source of urea, 

 provided that the metabolism of the body is capable of converting 

 leucin into urea. That the body can effect this change is shewn by 

 the fact that leucin, when introduced into the alimentary canal in 

 even large quantities, does reappear in the urine as urea ; that is, 

 the urine contains no leucin, but its urea is proportionately in- 

 creased ; and the same is possibly the case with tyrosin, though 

 this is disputed. Now the leucin formed in the alimentary canal 

 is probably carried by the portal blood straight to the liver; and 

 the liver, unlike other glandular organs, does, even in a perfectly 



