CHAP. IIL] ELIMINATION OF WASTE PRODUCTS. 695 



kidney of the other side ; it seems doubtful, however, whether this 

 is really the case. 



There is no satisfactory evidence that the vagus nerve of 

 either side contains any vaso-motor fibres reaching the kidney 

 (see 399). 



414. It is obvious then that by means of this vaso-motor 

 mechanism the flow of blood through the kidney is governed by 

 the central nervous system in such a way that afferent impulses, 

 started in this or that region or surface, and passing up to the 

 central nervous system, may lead either to constriction or to 

 dilation of the renal vessels ; and to such actions of this kind we 

 shall presently return. Meanwhile, we wish to call attention to 

 the fact that the volume of the kidney is remarkably sensitive to 

 chemical changes taking place in the blood. The injection into 

 the blood of even a small quantity of water causes a transient 

 shrinking of the kidney followed by a more lasting expansion. 

 The injection of urea and some other diuretics produces the same 

 effect to a more marked degree, leading especially to a swelling 

 which lasts for some considerable time, while the injection of 

 normal saline solution, and especially of such diuretics as sodium 

 acetate, causes an expansion from the very first, the primary 

 shrinking being absent. It is moreover worthy of note that these 

 effects of diuretics and of chemical changes in the blood are 

 observed even after all the renal nerves have apparently been 

 completely severed. Hence the changes in volume caused by the 

 presence of these substances in the blood must be due to the sub- 

 stances acting either upon some peripheral vaso-motor mechanism, 

 or, even more directly, on the blood vessels themselves. It may 

 be added that they will produce considerable effects in the kidney 

 itself without appreciably modifying the general blood-pressure. 



415. If, while the kidney is in the oncometer, and the 

 various experiments on section and stimulation of nerves and the 

 like are being carried on, a cannula be tied in the ureter, the 

 secretion of urine may be watched at the same time. It will then 

 be seen that the flow of urine through the end of the cannula is 

 not equable, and does not either increase or decrease in an even 

 manner. On the contrary, it will frequently be found that a sort 

 of gush of urine takes place, several drops following each other in 

 rapid succession, followed by a cessation of flow ; and if the ureter 

 be watched it will be seen that the gushes of urine are syn- 

 chronous with waves of peristaltic contraction sweeping down 

 the ureter. Obviously the urine collects to a certain extent in 

 the pelvis of the kidney and is driven thence by muscular action 

 from time to time ; to this point we shall return later on. 



Making every allowance, however, for these irregularities of 

 flow, we may take the rate of flow from the end of the cannula as 

 a measure of the rate of secretion; and it is found that as a 

 general rule increased flow of urine is coincident with swelling of 



