706 THE SKIN AND THE KIDNEYS. [BOOK n. 



upon the kidney not by the vaso-motor nerves alone but also by 

 nerves governing the secretory activity of the tubules ; but we 

 have no satisfactory indications of any such mechanisms, and it 

 seems more probable that the connection should be with the 

 glomerular mechanism, since the chief object at all events is to 

 get rid of water. 



Conversely, when the body is exposed to warmth the skin 

 perspires freely and the cutaneous vessels are widely dilated ; and 

 conversely also the renal and other abdominal vessels are con- 

 stricted, so that a slow and small stream of blood trickles through 

 the glomeruli, and the urine which is secreted is scanty. 



421. Even more important than its relations to the skin 

 are the relations of the kidney to the water absorbed by the ali- 

 mentary canal; this is especially seen when large quantities of 

 fluid are drunk. The whole of the water thus introduced into the 

 alimentary canal passes into the blood, for in a healthy organism 

 no amount of fluid drunk, unless it throws the economy out of 

 order, can affect the amount of water present in the faeces. But 

 the addition to the blood of even a very large quantity of fluid 

 does not, as we have seen, by its mere quantity ( 186), increase 

 the general blood-pressure, and therefore cannot in this way 

 produce what it undoubtedly does produce, an increased flow of 

 urine. 



The fluid so absorbed may act on the kidney in two ways. On 

 the one hand as we have seen ( 414), the injection of water into 

 the blood produces a local dilation of the renal vessels, as indicated 

 by the swelling of the kidney. Thus the absorption of mere water 

 from the alimentary canal may stir up to greater - activity the 

 glomerular mechanism, and in so doing may be assisted by the 

 presence of various substances absorbed from the alimentary canal 

 with the water, for some of these also may similarly lead to dilation 

 of the renal vessels. 



On the other hand, some or other of the chemical bodies thus 

 passing into the blood with the water drunk may excite the 

 secretory activity of the tubules, and that either by acting directly 

 on the epithelium as they are carried through the kidney in the 

 blood of the renal arteries, or indirectly through some intervention 

 of the central nervous system. 



Our knowledge is at present too scanty to enable us to decide 

 which of these two methods is the one usually employed by the 

 organism ; but the inordinate flow of urine, so poor in solids as to 

 be little more than water, which may be directed through the 

 kidney by means of an adequate "drinking bout," would lead us to 

 conclude that in such cases the organism, striving, though too often 

 in vain, to free itself from the evils to which it is being subjected, 

 has recourse rather to the simpler glomerular mechanism than to 

 the more expensive tissue- wasting activity of the tubules ; and the 

 urine in such cases is probably discharged chiefly by the method 



