CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 711 



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, but more especially, as we 

 have already said ( 415), by the rhythmically repeated 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 peri- 

 staltic contraction may be seen to pass in both directions from 

 the spot stimulated, upwards towards the kidney and downwards 

 towards the bladder. In the absence of artificial stimulation 

 spontaneous waves of contraction make their appearance, some- 

 times repeated with tolerable regularity (about every 20 seconds 

 in the rabbit), sometimes occurring in groups with longer pauses 

 between. These spontaneous contractions invariably pass in one 

 direction, from the kidney to the bladder; and their frequency 

 and vigour 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 kidney and ureter removed from the body, or even in an 

 isolated excised piece of the ureter. 



The rhythmically repeated contractions arise spontaneously in 

 the muscular coat of the ureter much in the same way as the 

 similar cardiac contractions arise in the muscular substance of 

 the heart ; and it may here be mentioned, in support of what was 

 urged in 155 with regard to the heart-beats not being started 

 by nerve-cells, that rhythmically repeated spontaneous peristaltic 

 contractions have been observed in isolated pieces of ureter taken 

 from the middle of its course, in which no nerve-cells could be 

 observed. 



In the living body these spontaneous movements, beats they 

 might be called, are subordinated to the flow of urine into the 

 pelvis ; the more active the secretion of urine the more frequent 

 and vigorous are the beats of the pelvis and ureter ; but the exact 

 mechanism by which the secretion and the movements are main- 

 tained in harmony has not yet been cleared up. 



Micturition. 



427. In the urinary bladder, the urine is collected, its return 

 into the ureters being prevented by the oblique entrance into 

 the bladder and valvular nature of the orifices of those tubes ; 

 and its discharge from thence in considerable quantity is effected 

 from time to time by a somewhat complex muscular mechanism, 

 of the nature and working of which the following is a brief 

 account. 



