Chap, hi.] ELIMINATION OF WASTE PRODUCTS. 537 



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 adju- 

 vant, is by itself helpless. Thus when the pressure is raised by 

 venous obstruction, in which case the high pressure is accom- 

 panied 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 pre- 

 existing 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 diffu- 

 sion, in contrast to nitration (see § 253) played an important 

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

 favour diffusion. But diffusion by itself will not explain mat- 

 ters. Egg-albumin differs very slightly as regards diffusibility 

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

 tity only of the latter passes into the urine in normal circum- 

 stances, the former when injected into the blood at. once makes 

 its way into the urine, and there is evidence that it passes by the 

 glomeruli. On the other hand urea is an eminently diffusible 

 bod}% 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 for 

 some little time shut off from their blood-supply, 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 has been 

 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 quan- 

 tities indicates imperfections in the glomerular epithelium. But 

 even under unhealthy conditions that epithelium still governs to 

 a certain extent the passage of material ; for the proteids of the 



