340 MUSCULAR MECHANISMS OF URINARY TRACT. 



verse fibres on the ventral surface of the lower half of the prostate gland, and 

 is here known as the external sphincter of Henle. Bound the membranous 

 part of the urethra the sheet is termed the constrictor urethra?, and round the 

 penile portion forms the accelerator urinoe. In the dog we find a similar 

 collection of striated muscle fibres round the small portion of the urethra which 

 intervenes between the prostatic and membranous parts. This is designated 

 Wilson's muscle. 



Mechanism of closure of the bladder. — The mechanism of closure 

 of the bladder has been the subject of much discussion. The retention 

 of urine has been variously ascribed to the elastic resistance of the 

 tissues at the neck of the bladder, or to the contraction of the internal 

 sphincter or of the external sphincter (striated muscle) of Henle. 

 Rosenthal's l view, that the closure was due to the elasticity of the 

 sphincter, which had to be forcibly overcome by the contraction 

 of the detrusor in micturition, was discredited by the statement of 

 Heidenhain and Colberg, 2 that the bladder is able to hold fluid at a 

 much greater pressure in the living than in the dead subject. These 

 authors found that the intravesical pressure necessary to overcome the 

 resistance of the sphincter amounted to between 21 and 33 cms. of water 

 in the living rabbit, and to between 2 '5 and 7*5 cms. of water in the dead 

 rabbit. In the dog, the bladder in the dead animal is able to hold fluid 

 at a pressure of 18 to 20 cms. of water, whereas in the living animal the 

 pressure may reach 100 cms. before the contents of the bladder begin to 

 trickle through the urethra. If the lumbar spinal cord be destroyed, 

 the difference in favour of the living animal is abolished, showing that 

 it depends on the existence of a tonic contraction under the influence of 

 this part of the cord. Griffiths has pointed out that there is no such 

 striking difference between the pressure necessary to open the urethra and 

 force out urine in living and dead animals. In the latter case, however, 

 if the urethra has once been forced open by raising the pressure in the 

 bladder, it remains more or less patent, so that on subsequent occasions 

 a flow of urine is obtained at a low pressure. This author finds that 

 the resistance to the escape of urine, which in the male dog varies from 

 16 to 20 in. of water, remains the same after division of the pelvic 

 nerves, of the hypogastric nerves, or even of the spinal cord in the 

 dorso-lumbar region. 



There can be no doubt that the greater part of the resistance to the 

 outflow of urine is occasioned at the urethro- vesical orifice, only a small 

 part being offered by the urethra itself. Thus, in surgical operations, 

 the urethra may be incised up to the neck of the bladder without any 

 escape of urine, and in the cat 3 the urethra between the neck of the 

 bladder and the prostate gland may be divided completely without any of 

 the urine escaping. Eeyfisch has excised the prostate in dogs without 

 producing any incontinence. Moreover, if the intravesical pressure 

 necessary to cause a flow of urine be determined, by connecting the 

 interior of the bladder through a tube in the ureter with a reservoir of 

 fluid (a catheter having previously been placed in the urethra), it is 

 found that this pressure remains the same, whatever may be the situation 

 of the catheter. It is not until the catheter is actually pushed into the 



1 "Detono cum musculorum, turn eo imprimis, qui sphinctorum tonus vocatur," Diss., 

 Konigsberg, 1857. 



2 Arch. f. Anat., Physiol, u. ivisscnsch. Med., Berlin, 1858, S. 437. 

 a Griffiths, loc. cit. 



