

CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 



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 uratTcs 

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

 sodium sulphindigotate. In birds, however, the urine of which 

 contains little water, urates may be detected in the epithelium of 

 the tubules but not in the capsules. Again, other observers have 

 maintained that the sodium sulphindigotate does 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. Further, 

 while some observers state that carmine, which injected in 

 solution into the blood certainly passes into the urine, finds its 

 way, in a state of solution, through the glomeruli, others maintain 

 that it is taken up by the epithelium cells, appears in them in the 

 form of granules, and is discharged by them into the channels of 

 the tubules. Without insisting too much on the value of experi- 

 ments of this kind, they may be taken as fairly supporting the view 

 which we are considering. 



We may then, 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 

 the glomeruli ajid not pass to the other branches of the renal 

 artery, in which case the circulation of the glomeruli would be 

 exclusively (or nearly so) affected ; but of this at present we know 

 nothing, and the general argument remains good if we speak simply 

 of the branches of the renal artery as a whole. 



452 



