EXCRETION 415 



propelled along the ureters by peristaltic contractions of 

 their walls, and drops from their valve-like orifices into the 

 bladder. When this becomes distended, rhythmical peri- 

 staltic contractions are set up in it, and notice is given of its 

 condition by a characteristic sensation, which is perhaps 

 aided by the squeezing of a few drops of urine past the 

 tonically contracted circular fibres that form a sphincter 

 round the neck of the bladder, and into the first part of the 

 urethra. The desire to empty the bladder can be resisted 

 for a time, as can the desire to empty the bowel. If it is 

 yielded to, the smooth muscular fibres in the wall of the 

 viscus are thrown into contraction. This is aided by an 

 expulsive effort of the abdominal muscles. The sphincter 

 vesicae is relaxed ; and the urine is forced along the urethra, 

 its passage being facilitated by discontinuous contractions 

 of the ejaculator urinae muscle, which also serve to squeeze 

 the last drops of urine from the urethral canal at the com- 

 pletion of the act. 



Regurgitation into the ureters is to a great extent pre- 

 vented by their compression between the mucous and 

 muscular coats of the bladder, where they run for more than 

 half an inch before opening at the posterior angle of the 

 trigone. But it has been shown that a certain amount of 

 back flow can take place. Small bodies like diatoms sus- 

 pended in water and pigments dissolved in it have been 

 found in the pelvis of the kidney, the renal tubules, and even 

 the circulation after being injected into the bladder. 



The pressure in the bladder of a man may be made as high 

 as 10 cm. of mercury during the act of micturition ; about 

 half this amount is due to the contraction of the vesical walls 

 alone, the rest to the contraction of the abdominal muscles. 



Although the whole performance seems to us to be com- 

 pletely voluntary, there are facts which show that it is at 

 bottom a reflex series of co-ordinated movements, that can 

 be started by impulses passing to a centre in the spinal 

 cord from above or from below from the brain or from the 

 bladder. In dogs, with the spinal cord divided at the upper 

 level of the lumbar region, micturition takes place regularly 

 when the bladder is full, and can be excited by such slight 



