THE PHYSIOLOGY OF MICTURITION 1153 



the dorsal surface some bundles in the male pass on to the prostate and the 

 urethra, while in the female they end in the tough connective tissue in the 

 urethro- vaginal septum. 



(2) The middle layer, which is the thickest of the three, is composed 

 of fibres arranged circularly and forming a continuous layer. 



(3) The inner layer is thin and incomplete, and is composed of anasto- 

 mosing bundles of fibres with meshes in between them which are covered 

 by the folds of the mucous membrane. The bundles of fibres run freely 

 from one layer to the other, and there is no doubt that the name of detrusor 

 ought physiologically to be applied to 



the whole of the three coats, which act 

 as one in diminishing the capacity of 

 the bladder. At the base of the blad- 

 der the structure of the wall is modified 

 over the triangular region lying between 

 the orifices of the ureters and of the 

 urethra (the trigonum) by the differen- Ureter-- 

 tiation here of fibres which serve as a 

 sphincter and prevent the escape of 

 urine. Over the trigonum the mucous 

 membrane of the bladder is smooth 

 and closely adherent to the subjacent 

 muscular fibres, which themselves are 

 much more closely packed than the rest 



of the bladder wall. From these muscular fibres the most important sphincter, 

 the sphincter trigoni, is formed. Bundles of muscle fibres pass from the 

 trigonal muscle obliquely forwards and downwards (the individual being con- 

 sidered in the erect posture), and form a loop around the orifice of the bladder, 

 lying on the ventral side of the bladder below and quite distinct from the 

 thick coat of circular fibres belonging to the bladder itself (ss, Fig. 537). 



This sphincter is the most important mechanism for the retention of 

 urine. If a catheter be passed into the urethra no urine escapes until its 

 orifice has actually entered the bladder. The wall of the urethra is sur- 

 rounded by circular muscular fibres, which, by their tonic contraction, will 

 also tend to prevent the escape of urine along the canal. This urethral 

 muscle is strengthened by two sphincter muscles which are voluntary and 

 composed of striated fibres. The chief one, which has been named by 

 Kalischer the sphincter urogenitalis, but is better known as the compressor 

 urethra, forms a flat ring around the second part of the urethra, extending 

 in the male from the prostate to the bulb, where its function is taken up 

 by the bulbo-cavernosus. 



The bladder is therefore supplied with a powerful muscular wall, the 

 contraction of which will cause its evacuation, and with sphincters of two 

 kinds, one involuntary, the sphincter trigoni, at the upper neck of the 

 bladder, and two voluntary, the sphincter urogenitalis and bulbo-cavsrnosus 

 muscles, which can empty the lower parts of the urethra. 



37 



