CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 701 



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 thirL 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 constituents 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 ditfusibility 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 

 secretion of urine slowly returns, and the urine is then found to 

 be albuminous, remaining so for some little time. The serum- 

 albumin and globulin which could not pass through the intact 

 epithelium, can pass through when the epithelium is damaged by 

 interference with its nutrition. The appearance of albumin in the 

 urine (albuminuria) is a not infrequent symptom of kidney disease, 

 and its presence in other than minute quantities indicates imper- 



