670 GLOMERULAR SECRETION. [BOOK n. 



a selection of some, of the soluble diffusible constituents of the 

 blood; for, as we have said, the presence of proteids in normal 

 urine is contested, and, at most, there is present a very small 

 quantity only (which moreover may come from the tubular epi- 

 thelium). This difference in the material which passes through 

 may be referred to the differences in the nature of the partition. 

 The transudation of lymph takes place through the capillary 

 wall ; between the blood on one side and the lymph in the lymph- 

 space on the other is only the thin film of conjoined epithelioid 

 plates. But the corresponding wall of the glomerular loop is 

 covered over and wrapped round so to speak by an adherent layer 

 of cells, which though reduced and thin are still epithelial cells ; 

 the materials which go to form urine have to pass through these 

 cells as well as through the film of epithelioid plates. It seems to 

 be this layer of cells which determines what shall pass and what 

 shall not. 



Obviously the passage through this epithelium is of a peculiar 

 nature. The necessary condition for the due accomplishment of 

 the passage is as we have seen a full and rapid stream of 

 (arterial) blood ; the high pressure which accompanies that full 

 and rapid stream, though probably under normal circumstances 

 an adjuvant, is by itself helpless. Thus when the pressure is 

 raised by venous obstruction, in which case the high pressure is 

 accompanied by a slow stream or by actual arrest of the flow, 

 even the passage of mere water is retarded. Seeing that 

 many of the constituents of urine are diffusible substances 

 certainly preexisting in the blood, inorganic salines for instance, 

 and seeing that, if we may trust the experiments on the 

 amphibian kidney spoken of above, diffusible abnormal con- 

 stituents of blood, such as peptone and sugar, pass into the urine 

 not by the tubular epithelium but by the glomeruli, we might 

 expect that diffusion, in contrast to filtration (see 312) played an 

 important part in the passage ; and a full rapid stream would 

 undoubtedly favour diffusion. But diffusion by itself will not 

 explain matters. Egg-albumin differs very slightly as regards 

 diffusibility from serum-albumin, and yet while at the most a 

 minute quantity only of the latter passes into the urine in normal 

 circumstances, the former when injected into the blood at once 

 makes its way into the urine, presumably by the glomeruli. On 

 the other hand urea is an eminently diffusible body, and yet if we 

 can trust the experiments on the amphibian kidney, the main 

 mass at all events of the urea of the urine passes by the 

 epithelium of the tubules. 



The important part played by the epithelium is shewn when 

 the epithelium is deranged. If the renal artery be temporarily 

 ligatured or otherwise obstructed, so that the glomeruli are shut 

 off from their blood-supply for some little time, the secretion of 

 urine is stopped ; on reestablishment of the circulation the 



