FOLIATION OF THE URINE. 165 



lymph, and are merely separated from the blood in the kid- 

 neys ; and it has consequently been pretty generally assumed 

 that nearly, if not all, the constituents of the urine preexist 

 in the circulating fluid. There is, indeed, no well-defined 

 principle in the urine that has not been actually demon- 

 strated in the blood. As an additional argument in favor 

 of this view of the mechanism of the urinary excretion, it has 

 been ascertained that when the kidneys are interrupted in 

 their function, there is a tendency to the elimination of the 

 excrementitious principles of the urine by the lungs, skin, 

 and alimentary canal ; and that these matters only accumu- 

 late in the blood after this vicarious effort has failed to effect 

 their complete discharge. 



These ideas have seemed to be so completely justified by 

 facts, that they have been applied to the mechanism of ex- 

 cretion by other organs, such as the skin and the liver ; but 

 within a few years, the older observations with regard to 

 nephrotomized animals have been discredited; and it has 

 been asserted, as the result of experiment, that urea and the 

 urates do not accumulate in the blood after removal of the 

 kidneys, .but that this result only follows when both ureters 

 have been tied. The experiments on which this idea is based 

 have been applied mainly to the pathology of uraemic intoxi- 

 cation, but it is evident that they bear directly upon the 

 mechanism of excretion. It is not assumed, however, that 

 excrementitious principles are not formed by the disassimi- 

 lation of the tissues ; but it is asserted that urea and the 

 urates are produced in the kidneys by a transformation of 

 the excrementitious matters, creatine, creatinine, etc., which 

 exist in the blood. It is foreign to our purpose to discuss in 

 exten-so the pathological conditions produced by the retention 

 of the urinary principles in the blood ; and we shall consider 

 this question only so far as it bears upon the physiology of 

 excretion. 



The original experiments of Prevost and Dumas are very 

 strong arguments in favor of the view that has been so long 



