1206 PHYSIOLOGY 



urerhra, 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 



Ureter- -'\^f / urethra (the trigonum) by the differen- 



tiation here of fibres which serve as a 

 . -... sphincter and prevent the escape of 



^^ISL^LBB,^^ urine. Over the trigonum the mucous 

 Memb.-' VBpB^^S^ membrane of the bladder is smooth 

 Bulb ^and closely adherent to the subjacent 



Fro. 550. 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 considered 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. 550). 



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 

 urethrcB, 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-cavernosus 

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



