668 SECRETION BY RENAL EPITHELIUM. [BOOK n. 



quantity was injected. It was 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. 



The above 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 

 concomitant 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 micro- 

 scopic tracking out of their passage, which in the case of urates 

 might otherwise be done much in the same way as with the 

 sodium sulphindigotate. Moreover other observers have main- 

 tained that the sodium sulphindigotate does like ordinary carmine 

 pass through the glomeruli. But their results may probably be 

 explained by the glomeruli having been damaged by a too rapid 

 or too abundant injection ; and 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 tubules. Without insisting too much on the value of the 

 sodium sulphindigotate experiments, they may be taken as fairly 

 supporting the view which we are considering. We may add that 

 in birds, the urine of which contains little water, urates may be 

 detected in the epithelium of the tubules but not in the capsules. 



Though much remains to be cleared up, we may, for the 

 present, conclude that the secretion of urine does consist of two 

 separate and distinct acts: secretion by the glomeruli, which we 

 may for brevity's sake speak of as glomerular secretion, and 

 secretion by the epithelium of the tubuli, which we may speak of 

 similarly as tubular secretion. Both these forms of secretion, 

 especially the former but to a certain extent the latter also, differ 

 from the secretion of such a gland as the salivary, and both 

 deserve some special consideration. 



417. The nature of glomerular secretion. We have seen 

 that the expansion of the kidney which has for its accompaniment 

 an increased flow of urine is one brought about by the renal artery 

 and its various branches becoming dilated, under such circumstances 

 that the difference between the blood-pressure in the aorta at the 

 mouth of the renal artery and the blood-pressure at the vena cava 

 at the mouth of the renal vein is at the same time increased, or at 

 all events is not diminished. We say the renal artery and its 

 various branches since our present knowledge will not enable us 

 to make a more exact statement. It is of course possible that 

 nervous impulses passing along particular nerve fibres should 

 confine their efforts to relaxing the coats of the vasa afferentia of 



