EXCRETION OF MATTER FEOM THE BODY 427 



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 constricted the pressure 

 behind the constriction rises, and may distend the ureters and 

 the pelvis of the kidney, but 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 rhythmic contraction of the non-s-triped muscle 

 becomes more and more powerful. These contractions are 

 chiefly excited by the fibres of the nervi erigentes of the second 

 and third sacral nerves, although fibres passing down from the 

 inferior mesenteric ganglion also probably act either in 

 exciting or inhibiting them in certain animals. The backward 

 passage of the urine into the ureters is prevented by the 

 oblique manner in which these tubes pass through the muscular 

 coat of the bladder. 



The passage of urine into the urethra is at first prevented 

 either by the oblique manner in which the urethra leaves the 

 bladder, or more probably by the contraction of a strong band 

 of non-striped muscle, the sphincter trigonalis. This muscle or 

 the striped fibres which surround the membranous part of the 

 urethra are under the control of a centre in the lumbar 

 enlargement of the spinal cord, and the expulsion of urine 

 must be preceded by their relaxation. In some cases of 

 inflammation of the spinal cord the increased activity of the 

 centre may prevent the expulsion of urine, while later in the 

 disease, when the nerve structures have been destroyed, the 

 urine is not retained and dribbles away on account of the 

 absence of the tonic contraction of the muscles. 



The expulsion of the last drops of urine is carried out by 

 the rhythmic contraction of the bulbo-cavernous muscle, while 

 the peristaltic contraction of the bladder wall is assisted by 

 the various muscles which press upon the contents of the 

 abdomen and of the bladder. 



