Chap, hi.] ELIMINATION OF WASTE PRODUCTS. 541 



while another may have so little colour and such a low specific 

 gravity as to appear hardly more than water. The reason of 

 these extreme differences lies in the fact that the kidney is not 

 only the channel by which waste solids leave the body but also 

 an important outlet for the discharge of the stream of water 

 which, in order that the various processes of the body may be 

 duly carried on, is continually passing through the system. It 

 is frequently of advantage to the body to discharge through the 

 kidney a large amount of water, more or less irrespective of the 

 solid matters which are so to speak washed away with it ; and 

 hence the advantage of the glomerular mechanism so specially 

 adapted for the special discharge of water. 



As we shall see presently, to the skin also falls the duty of 

 discharging large quantities of water. The respiratory organs 

 also, as we have seen, serve for the discharge of water ; but the 

 amount which the latter put out can only be varied by the incon- 

 venient method of increasing or diminishing the whole act of 

 breathing. Hence we find special relations between the skin 

 and the kidneys correlating the work of the one to that of the 

 other as regards this particular work of the discharge of water. 

 When the body is exposed to cold the discharge of water 

 from the skin in the form of sweat is checked, and the cutaneous 

 vessels are constricted. At the same time the blood vessels of 

 the abdominal viscera, including the kidneys, are dilated, but 

 not out of proportion to the constriction of the cutaneous vessels, 

 for the general blood-pressure does not fall but if anything rises 

 somewhat. Thus there is established just the state of things 

 which is favourable to a full and rapid stream of blood through 

 the renal glomeruli ; and an increased flow of urine results. 



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 

 constricted, so that a slow and small stream of blood trickles 

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

 § 341. 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 veiy large quan- 

 tity of fluid does not, as we have seen, by its mere quantity 

 (§ 164), increase the general blood-pressure, and therefore can- 

 not in this way produce what it undoubtedly does produce, an 

 increased flow of urine. 



Since a kidney, all the nerves of which have been severed, 

 dilates, as shewn by the oncometer, that is has a fuller supply 



