CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 675 



between the skin and the kidney so that, under the circumstances 

 in question, the renal arteries are dilated even more than those 

 of the other abdominal viscera; but this has not been proved 

 experimentally. It is also possible that by another reflex 

 mechanism of the central nervous system the skin may work 

 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 alimentary 

 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 



432 



