THE MUSCLES OF THE PERINEUM. 



487 



Nerve-Supply. The external sphincter is supplied by the inferior haemorrhoidal branch of 

 I the pudendal nerve (S. 3. 4), by the perineal branch of the fourth sacral nerve, and by the deep 

 perineal branch of the pudendal' nerve (S. 3. 4.). 



Actions. The muscle closes the anal aperture. It is a voluntary muscle. 



M. Transversus Perinei Superficialis. The transversus perinei superficial 



is not always present. It consists of a more or less feeble bundle of fibres, which 

 arises from the inferior rarnus of the ischium and the fascia over it, and from the 

 base of the fascia inferior of the urogenital diaphragm. 



It passes obliquely over the base of the fascia inferior to be inserted into the 

 central point of the perineum. 



i Nerve -Supply. Deep perineal branch of pudendal nerve (S. 3. 4.). 

 Action. The two muscles acting together draw backwards and fix the central point of 

 the perineum. 



M. Bulbocavernosus. The bulbocavernosus (O.T. ejaculator urinse), in the 



male, surrounds the bulb, corpus cavern osum urethrae, and root of the penis. 

 It is sometimes separ- 

 ated into two parts 

 posterior (compressor 

 bulbi), and anterior 

 (compressor radicis 

 penis). It arises from 

 the central point of 

 the perineum, and from 

 a median raphe on the 

 under surface of the 

 bulb and corpus cav- 

 ernosum urethrse. 



The muscular fibres 

 pass laterally and 

 forwards and have a 

 triple insertion : from 

 behind forwards, (1) 

 into the inferior sur- 

 face of the fascia in- 

 ferior of the urogenital 

 diaphragm ; (2) into 

 the dorsal aspect of 

 the corpus cavernosum 

 urethrae ; and (3), after 

 encircling the corpora 



cavernosa penis, into 

 the fascia covering the 

 dorsum of the penis. 



The ischiobulbo- 

 SUS, not always present, 

 arises from the ischium, 

 and passes obliquely medially and forwards over the bulbocavernosus, to be inserted into 

 'the raphe superficial to that muscle. It belongs to the same stratum as the transversus 

 'perinei superficialis and ischiocavernosus. 



The compressor hemispheriorum bulbi is frequently absent. It consists of a thin 

 cap-like layer of muscular fibres surrounding the extremity of the bulb under cover of the 

 bulbocavernosus. 



ISCHIO- 

 CAVERNOSUS 



BULBO- 

 CAVERNOSUS 



ISCHIO- 

 CAVERNOSUS 



TRANSVERSUS 

 PERINEI 



SUPER- 

 FICIALIS 



LEVATOR ANI 



SPHINCTER ANI EXTERNUS 



Fia. 432. THE MUSCLES OF THE FEMALE PERINEUM 

 (after Peter Thompson). 



M. Bulbocavernosus. The bulbocavernosus, in the female (O.T. sphincter 

 jinse), is separated into lateral halves by the vaginal and urethral openings. It 

 >rms two thin lateral layers covering the bulb of the vestibule, and arises behind 

 : the vaginal orifice from the central point of the perineum. 



Anteriorly it is inserted into the root of the clitoris, some of its fibres em- 

 bracing the corpora cavernosa clitoridis so as to reach the dorsum of the clitoris. 



