472 THE HUMAN BODY 



cells from the cavity of the capsule and especially adapted for 

 filtration and dialysis; the other represented by the contorted por- 

 tions of the tubules, with their large granular cells, which clearly 

 have some more active part to play than that of a mere passive 

 transudation membrane. And we find in the urine substances 

 which like the water and mineral salts may easily be accounted for 

 by mere physical processes, and others, urea especially, which are 

 present in such proportion as must be due to some active physio- 

 logical work of the kidney. More direct evidence does, in fact, 

 justify us in saying that in general the glomcruli are transudation 

 organs, the contorted portions of the tubuli secretory organs, 

 while the collecting and discharging tubules are merely passive 

 channels for the gathering and transmission of liquid. In calling 

 the capsules transudation organs we do not intend to assert that 

 the passage of water and salts through them is necessarily a phys- 

 ical process pure and simple. Although many physiologists have 

 supposed it to be nothing more, there is abundant evidence that 

 here, as elsewhere in the Body where the passage of liquids is 

 through membranes composed of living cells, the cells of the cap- 

 sule exercise a controlling function over the passage of the water 

 and salts through them. 



Several lines of evidence indicate that the organic constituents 

 of urine are excreted through the secretory portions of the tubules. 

 One of the best of these has come from work on frogs. Urea, the 

 most important and most abundant of the characteristic ingre- 

 dients of urine, has a very marked influence on kidney activity, the 

 injection of some of it into blood causing a greatly increased se- 

 cretion of urine, in which the injected urea is quickly passed out. 

 In amphibia the blood carried to the kidney, like that supply- 

 ing the mammalian liver, has two sources, one venous and one 

 arterial; the arterial supply comes from the renal arteries, the 

 venous from the veins of the leg by the* reniportal vein. Both 

 bloods leave the organ by the renal veins, but their distribution 

 in it is in great part distinct; the arteries supply the glomeruli, 

 the reniportal vein the tubules of the cortex, though mixed there 

 with blood from the efferent vessels of the glomeruli. On tying the 

 renal arteries of one of these animals urinary secretion ceases, there 

 being then no blood-pressure in the glomeruli to cause' the tran- 

 sudation of liquid; but if some urea be now injected into the blood 



