1290 PHYSIOLOGY 



by the name of the detrusor urince. At the neck of the bladder these 

 bundles send some fibres to be attached to the pubes as the pubo- 

 vesical muscles. On 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 

 anastomosing 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 bladder 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 differentiation hero 

 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. 535). 



This sphincter is the most important mechanism for the retention 

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



