CHAP, iv.] RENAL SECRETION. 407 



quent inspection, Heidenhain was able to trace the material step 

 by step into the cells, out of the cells into the interior of the 

 tubules, and for some little distance along the tubules. The 

 advantage of the absence of a large flow of urine is obvious ; had 

 this been present, the pigment immediately that it issued from the 

 cells would have been rapidly washed away down the channels of the 

 tubules. One observation he made of a peculiarly interesting 

 character. After injecting a certain quantity of pigment, and 

 allowing such a time to elapse as he knew from previous experi- 

 ments would suffice for the passage of the material through the 

 epithelium to be pretty well completed, he injected a second 

 quantity. He found that the excretion of this second quantity 

 was most incomplete and imperfect. It seemed as if the cells were 

 exhausted by their previous efforts, just as a muscle which has 

 been severely tetanized will not respond to a renewed stimulation. 



This observation may be objected to on the ground that this 

 colouring matter does not occur as a constituent of the blood either 

 in health or disease, and especially that the absence of any con- 

 comitant discharge of fluid from the cells excites suspicion that the 

 process observed was not really one of secretion ; for the injection 

 of such substances as urea or urates into the blood does cause 

 a copious flow of fluid, and indeed thus prevents the microscopic 

 tracking out of their passage, which in the case of urates might be done 

 much in the same way as with the sodium sulphindigotate. Moreover 

 other observers have maintained that the sodium sulphindigotate 

 does like ordinary carmine pass through the glomeruli ; but in the 

 case of the amphibian kidney when sodium sulphindigotate is injected 

 after ligature of renal arteries, no urine is found in the bladder, 

 but the pigment can be traced, through the epithelium of the 

 secreting portions of the tubuli. Without insisting too much on 

 the value of the sodium sulphindigotate experiments, they may 

 be taken as fairly supporting the view we are considering. 



Experimental evidence then justifies the conception which the 

 structure of the kidney led us to adopt. The secretion of urine by 

 the kidney is a double process. It is partly a process of filtration, 

 whose object is to remove as rapidly as possible a quantity of 

 water from the body, and this part of the work of the kidney is 

 directly dependent on blood-pressure. It is also however a process 

 of active secretion by the epithelium of the tubuli. and this part of 

 the work of the kidney is, in an indirect manner only, dependent 

 on blood-pressure. Both processes may give rise to a discharge of 

 water from the blood, and both may give rise to the presence 

 of the solid constituents of the urine, in solution in that water. In 

 the first process the discharge of water is the primary object, and 

 the solid matters which escape at the same time are of secondary 

 importance; in the second process the excretion of the solid 

 substance is the primary object, and the accompanying water 

 of secondary importance. The first process is governed (mainly at 



