SEC. 3. THE DISCHARGE OF URINE. 



344. 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 by pressure and gravity aided 

 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 



Eeristaltic contraction may be seen to pass in both directions 

 rom the spot stimulated, upwards towards the kidney and 

 downwards towards the bladder. In the absence of artificial 

 stimulation spontaneous waves of contraction make their appear- 

 ance, sometimes repeated with tolerable regularity (about every 

 20 seconds in the rabbit), sometimes occurring in groups with 

 longer pauses between. These spontaneous contractions inva- 

 riably 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 154 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. 



644 



