CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 535 



epithelium, and from thence into the channels of the tubules. 

 There being no stream of fluid through the tubules, owing to the 

 arrest of urinary flow by means of the preliminary operation, the 

 pigment travels very little way down the interior of the tubules, 

 and remains very much where it was cast out by the epithelium 

 cells. There are no traces whatever of the pigment having 

 passed by the glomeruli ; and the cells which appear most dis- 

 tinctly to take up and eject it, are those lining such portions of 

 the tubules (viz. the first and second convoluted tubules, zigzag 

 tubules and ascending limbs of the loops of Henle) as from their 

 microscopic features have been supposed to be the actively 

 secreting portions of the entire tubules. 



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 

 microscopic 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. Still in birds, the urine of which 

 contains little water, urates may be detected in the epithelium 

 of the tubules though not in the capsules. Without insisting 

 too much on the value of the sodium sulphindigotate experi- 

 ments, they may be taken as fairly supporting the view which 

 we are considering. 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 secre- 

 tion. 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. 



337. 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 circum- 

 stances 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. 



In dealing with the vascular system we saw that relaxation 

 of a small artery, taking place without any marked change in 

 the general blood-pressure and in neighbouring arteries, leads to 

 a fuller and more rapid stream of blood through the capillaries 

 supplied by the artery, and that at the same time the pressure in 



