EXCRETION AND THE EXCRETORY ORGANS 527 



the more important are albumens in albuminuria or nephritis; 

 grape sugar or glucose in diabetes; bile salts; bile pigments. 



The Secretory Actions of Different Parts of a Uriniferous 

 Tubule. The microscopic structure of the kidneys is such as to 

 suggest that in those organs we have to do with two essentially 

 distinct secretory apparatuses: one represented by the glomeruli, 

 with their capillaries separated only by a single layer of flat epi- 

 thelial cells from the cavity of the capsule and especially adapted 

 for filtration and dialysis; the other represented by the contorted 

 portions 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 sub- 

 stances which like the water and mineral salts may easily be ac- 

 counted for by mere physical processes, and others, urea especially, 

 which are present in such proportion as must be due to some active 

 physiological work of the kidney. More direct evidence does, in 

 fact, justify us in saying that in general the glomeruli are transuda- 

 tion 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 

 the cells of the capsule exercise a controlling function over the 

 passage of the salts through them if not of the water. 



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, 



