436 THE HUMAN BODY. 



the tubuli secretory organs, while the loops of Henle and the 

 collecting and discharging tubules are merely passive channels 

 for the gathering and transmission of liquid. Even in the 

 glomeruli, however, the renal cells provide something more 

 than a merely passive physical membrane for dialysis and 

 filtration : to a certain extent they control the passage of 

 substances through them; while they are in health no serum 

 albumen or paraglobulin passes through them into the urine, 

 though egg albumen injected into the blood of a living 

 mammal does. But when they are altered in disease or even 

 by a temporary stoppage of their blood -supply, then they 

 allow the normal blood proteids to transude: if the blood- 

 supply of a kidney be cut off for some minutes by clamping 

 the renal artery, then the urine secreted for some time after 

 the clamp is removed is albuminous. 



The structure of the glomerular epithelium and its rela- 

 tion to the blood-vessels are such as to make it almost certain 

 that when albumen appears in the urine it enters through 

 them and not through other parts of the tubule; but in some 

 amphibia we get direct evidence of the entry of substances other 

 than salts and water into the renal secretion by the path of 

 the Malpighian capsules. In amphibia the blood carried to 

 the kidney, like that supplying 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. In some small 

 amphibia it is, in fact, possible to observe the circulation in 

 the living kidney and to see that all blood-flow in the glomer- 

 uli ceases when the renal arteries are tied, though it con- 

 tinues elsewhere throughout the organ. When sugar or 

 peptone is injected into the blood of such an animal those 

 substances appear in the urine; but if the renal arteries be pre- 

 viously tied they do not. It is true that under those circum- 

 stances all secretion of urine usually ceases, but it may be 

 excited by administering certain drugs, and then is found to 

 be free from sugar and peptone. Grape-sugar when present 

 in the blood of mammals beyond a certain small percentage 

 passes out in the urine; and the same is true of peptone: 



