408 ACTIVITY OF THE EPITHELIUM. [BOOK n. 



least) by the vaso-motor nervous system; the second process is 

 excited, as far as we know at present, by substances in the blood 

 acting directly as chemical stimuli to the epithelium ; but future 

 researches may disclose the existence of a secretory nervous me- 

 chanism analogous to that of other secretory glands. It must also 

 be left for further inquiries to determine exactly which of the two 

 processes, or to what relative extent each of them, is concerned in 

 bringing about the presence in the urine of its several constituents. 



In one respect the kidney as a secreting organ differs markedly 

 from such a gland as the salivary. In the case of the latter, we 

 have seen that the saliva as it flows may cause a pressure in the 

 duct greater than the mean arterial pressure ; in the case of the 

 former when a manometer is connected with a cannula tied into the 

 ureter of a dog, the mercury may rise to 60 mm.,but not much beyond, 

 and often becomes stationary at lower level, shewing that the urine 

 cannot be secreted at a pressure greater than that probably 

 obtaining in the renal vessels ; or, at least, if secretion does 

 take place it is counterbalanced by an absorption taking place at 

 the same time. But in this respect the kidney has its fellow in 

 another secreting organ, the liver, for in this, as we have seen, the 

 secretion of bile is arrested when the pressure is raised too high. 



One or two words of caution are necessary. In speaking of the 

 glomerulus as a filtering apparatus, it must not be understood that 

 it is thereby really compared to an ordinary filter made of dead 

 material, and that when filtration through it is spoken of, a process 

 exactly like that which takes place in the laboratory is meant. In 

 the glomerulus the elements of the blood have to pass through the 

 living wall of the capillary, and the covering layer of epithelium 

 cells ; and the transit must be affected by the condition of these 

 living structures. By virtue of their constitution they allow cer- 

 tain things to pass and not others; and when they become changed 

 the passage of material is changed also. The possible influence 

 of a mere layer of squamous epithelium is shewn by experiments on 

 the cornea, which acts absolutely differently as a filter according 

 as the epithelium of Descemet is retained or removed ; and in 

 speaking of the circulation we dwelt on the importance of the 

 physiological condition of the capillary walls. The nature of the 

 filtration taking place through the glomerulus will depend therefore 

 on the condition of the capillary walls and their epithelial invest- 

 ment. This is illustrated by the phenomena of albuminuria (or 

 the passage of albumin into the urine), especially as seen in the 

 following interesting experiment by Nussbaum on the artificial 

 production of albuminuria in the frog. The renal arteries being 

 tied, an injection of urea (1 cm. of a 10 p.c. solution) into the blood 

 gave rise to a flow of urine which was free from albumin. Upon 

 loosing the ligatures so as to re-establish the flow of blood through 

 the glomeruli, the urine at once became albuminous. The arrest 

 of the circulation through the glomeruli had damaged the capillary 



