EXCRETION OF URINE 581 



loins in suppression of urine. There is some evidence that 

 slight obstruction of a ureter may also cause a reflex vaso- 

 dilatation with increased secretion of urine. 



III. EXCRETION OF URINE. 



1. Passage from Kidney to Bladder. — The pressure under 

 which the urine is secreted is sufficient to drive it along the 

 ureters to the bladder. If these are obstructed, the pressure 

 behind the obstruction rises, and may distend the ureters and 

 the pelvis of the kidney, and when it reaches about 50 mm. Hg 

 in the dog, the secretion of urine is stopped. The muscular 

 walls of the ureters show a rhythmic peristaltic contraction, 

 which must also help the onward passage of the urine to the 

 bladder. 



2. Micturition — As the urine accumulates in the urinary 

 bladder, the viscus expands to accommodate it, the tone of 

 the visceral muscular fibres being adapted to the degree of 

 distension. The backward passage of the urine into the ureters 

 is prevented by the way in which these tubes pass obliquely 

 through the muscular coat of the bladder. When a certain 

 distension is reached, rhythmic contractions are produced, 

 which become more and more powerful. These are primarily 

 dependent on the muscular fibres ; but the wall of the bladder 

 is richly supplied with peripherally placed neurons, and the 

 possible action of these in controlling the contractions has not 

 been excluded. Even after section of the nerves to the bladder, 

 this peripheral mechanism is capable of controlling the act of 

 micturition. 



This involves not merely the contraction of the wall of the 

 bladder, but also the relaxation of the visceral muscular fibres 

 (the sphincter trigonalis) which surround the neck of the 

 bladder, and of the striped fibres which surround the upper 

 part of the urethra. 



It is therefore an act requiring the co-ordinated contraction 

 and relaxation of muscles, and it is presumably presided over 

 by the nervous mechanism in the wall of the bladder. 



