THE SECRETION OF URINE. 425 



tions 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 feces. But the 

 addition to the blood of even a very large quantity of fluid does not, as we 

 have seen, by its mere quantity ( 172), 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 ( 350), the injection of water into the blood produces 

 a local dilatation 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 alimen- 

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

 dilatation 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 car- 

 ried 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 " drink- 

 ing bout," would lead us to conclude that in such cases the organism, striv- 

 ing, 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 of dilating the renal 

 vessels and thus throwing the poisoned blood into the glomeruli. 



When, however, fluid is taken simply as a proper accompaniment of solid 

 food, the increase of urine which results has probably another origin. As 

 we have already said, and as we shall point out more fully later on, the 

 absorption of proteid material, which is a constituent and generally a con- 

 spicuous constituent of every meal, leads to a formation of urea ; and urea, 

 as we have seen reason to believe, directly stimulates the epithelium of the 

 tubules to secretory activity. And what seems prominently true of urea is 

 probably true of many other products of digestion ; so that the increased 

 flow of urine which follows an ordinary meal accompanied by not more 

 than the ordinary amount of fluid, is the result of the labors of the epithe- 

 lium of the tubules as well as of the fuller stream of blood through the 

 glomeruli. 



358. What has just been said concerning the influence on the kidney 

 of food and water may be applied also to the action of substances which 

 being especially efficacious in promoting a flow of urine when taken into the 

 body are called "diuretics." The several actions of various diuretics are 

 very varied, and it would be out of place to discuss them fully. We may, 

 however, say that while the action of some appears simple that of others is 

 complex. 



Such agents as sodium acetate and potassium nitrate probably produce 

 their effect chiefly by acting directly on the kidney, including, as we have 

 seen ( 350), local vascular dilatation and so working on the glomeruli, but 

 probably at the same time also stirring up, after the fashion of urea, the 



