426 THE ELIMINATION OF WASTE PRODUCTS. 



epithelium of the tubules to secretory activity, the accompanying fuller 

 stream of blood through the whole kidney being, as in the case of the 

 salivary and other glands, a useful adjuvant. 



The diuretic effect of such an agent as digitalis is probably more com- 

 plex. By increasing the cardiac stroke, and at the same time constricting 

 many small vessels, digitalis raises the general blood-pressure ; but the tend- 

 ency of the increased blood-pressure to increase the flow of urine may be 

 counterbalanced by the -constriction of the renal vessels themselves. And 

 while it is a matter of common experience that digitalis is very effective as 

 a diuretic in cardiac disease, there is great doubt whether it really acts as a 

 diuretic in health ; in cardiac disease it probably raises the blood-pressure 

 by improving the cardiac stroke and not by constriction of the bloodvessels. 

 But even in the absence of cardiac disease, digitalis has been found in 

 certain cases to act as a powerful diuretic, and in these cases either it must 

 act directly on the tubular epithelium or its effects in constricting the renal 

 arteries must be less than its effects on other small arteries or must pass off 

 before the influence of the heightened blood-pressure has disappeared. 



359. Quite removed from the intervention of chemical substances in 

 the blood and yet most striking is the influence on the kidney of the central 

 nervous system. The potent influence of emotions in promoting the secre- 

 tion of urine is proverbial, and the general features of " nervous " urine, the 

 water increased out of proportion to the solid constituents, especially seen 

 in the " urina hysterica," which is hardly more than simple water, often dis- 

 charged in enormous quantity, at once suggests the view that impulses origi- 

 nating in the brain and passing down to the kidney along the vaso-dilator 

 fibres, of whose existence evidence was given in 349, lead to dilated blood- 

 vessels and great play of glomerular activity, without perhaps producing 

 any other direct effect on the economy ; though possibly the same emotions 

 by constricting the cutaneous and, it may be, other vessels may raise the 

 general blood-pressure and so help the dilated renal vessels. 



THE DISCHARGE OF URINE. 



360. The urine, like the bile, is secreted continuously ; the flow may 

 rise and fall, but in health never absolutely ceases for any length of time. 

 The cessation of renal activity, the so-called suppression of urine, entails 

 speedy death. The minute streams passing continuously now more rapidly, 

 now more slowly along the collecting and discharging tubules, are gathered 

 into the renal pelvis, whence the fluid is carried along the ureters into the 

 bladder partly by pressure and gravity, and from time to time partly, as we 

 have already said ( 351), by the peristaltic contractions of the muscular 

 walls of the ureter. 



If in a living animal a ureter be laid bare and stimulated, mechanically 

 or otherwise, at a part of its course, waves of peristaltic contraction may be 

 seen to pass in both directions from the spot stimulated up toward the 

 kidney and down toward the bladder. In the absence of artificial stimula- 

 tion spontaneous waves of contraction make their appearance, sometimes 

 repeated with tolerable regularity (about every twenty seconds in the rabbit), 

 sometimes occurring in groups with longer pauses between. These sponta- 

 neous contractions invariably pass in one direction, from the kidney to the 

 bladder, and their frequency and vigor seem to be determined by the activity 

 of the secretion of urine. But they are not directly called forth by the 

 urine, either mechanically distending the tube or chemically stimulating the 

 inner surface, for regularly recurring contractions may be observed in a 



