424 THE ELIMINATION OF WASTE PRODUCTS. 



the kidney is not to provide a fluid, urine, which can be made use of for the 

 needs of the body, but to cast out waste matters from the body. Hence its 

 secretory activity is limited largely to the mere discharge of matters which 

 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. 



356. 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 pro- 

 portion 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 color 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 con- 

 tinually 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 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 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 bloodvessels of the abdominal viscera, including the kid- 

 neys are dilated, but not out of proportion to the constriction of the cuta- 

 neous 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 

 favorable 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 upon the kid- 

 ney, not by the vasornotor 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 connections 

 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 constricted, so that a slow and small stream 

 of blood trickles through the glomeruli, and the urine which is secreted is 

 scanty. 



357. Even more important than its relations to the skin are the rela- 



