CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 705 



reach it preexistent in the blood, though in several cases it gives 

 the final shape to the excreted substance before it passes into the 

 ureter. 



420. We may illustrate the preceding discussions by briefly 

 passing in review some of the more usual ways in which the 

 secretion of urine is in ordinary life modified. 



In the preceding section the composition of urine was illustrated 

 by the daily output of the several constituents rather than by a 

 percentage account of any specimen of urine, for the reason that 

 the composition of urine varies within extremely wide limits. 

 This is especially the case as regards the proportion of water to 

 solids. One urine may be of high specific gravity with a small 

 amount of water relatively to the solids, 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 dis- 

 charge 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 inconvenient 

 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. It is possible, we may 

 perhaps say probable, that the nervous system affords a special tie 

 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 



