404 ACTIVITY OF THE EPITHELIUM. [BOOK n. 



kidney to other abdominal organs. On the other hand, stimulation 

 of the splanchnic nerves is able to arrest the flow of urine by 

 producing constriction of the renal arteries. 



The experimental phenomena recorded above are thus seen to 

 receive a fairly satisfactory explanation when they are referred 

 exclusively to variations in blood-pressure. And many of the 

 natural variations in the flow of urine may be interpreted in the 

 same way. No fact in the animal economy is oftener or more 

 strikingly brought home to us than the correlation of the skin and 

 the kidney as far as their secretions are concerned ; and this seems 

 to be, in part at least, maintained by means of the vaso-motor 

 nervous mechanism. Thus when the skin is cold, its blood-vessels 

 are, as we know, constricted. This, by causing an increase of 

 general blood-pressure, will augment the flow through the kid- 

 neys, and conversely, the dilated condition of the arteries of a 

 warm skin, with the consequent diminution of general blood- 

 pressure, will give rise to a diminished renal discharge. It is 

 probable, however, that a more direct connection exists between 

 the skin and the kidneys, so that a warm skin leads to constriction 

 and a cold skin to dilation of the renal vessels ; and it is further 

 possible that the one may react on the other in another way, viz., 

 by changes induced in the blood; Tjbe effects of emotions may 

 possibly also be explained as essentially vaso-motor phenomena. 



Secretion by the Renal Epithelium. 



While thus recognising the importance of the relations of the 

 flow of urine to blood-pressure, we must not be led into the error 

 of supposing that the work of the kidney is wholly a matter of 

 filtration. The glomerular mechanism, so specially fitted for filtra- 

 tion, is after all a small portion only of the whole kidney, and the 

 epithelium over a large part of the course of the tubuli uriniferi 

 bears most distinctly the characters of an active secreting epi- 

 thelium. These facts would lead us a priori to suppose that the 

 flow of urine is in part the result of an active secretion comparable 

 to that of the salivary or other glands which we have already 

 studied. And we have experimental and other evidence that such 

 is the case. 



In the first place a flow of urine may be artificially excited 

 even when the natural flow has been arrested by diminution of 

 blood-pressure. Thus if, when the urine has ceased to flow in 

 consequence of a section of the medulla oblongata, certain sub- 

 stances, such as urea, sodium acetate, &c., be injected into the 

 blood, a more or less copious secretion is at once set up. This 

 secretion is, or at least may be, unaccompanied by any rise of 



