THE PELVIC AND PERINEAL MUSCLES. 565 



Action. — To .compress the corpus cavernosum and thus assist in producing or 

 maintaining erection of the penis (or clitoris). 



4. Bulbo-Cavernosus (Fig. 554). 



Attachments. — The bulbo-cavernosus differs somewhat in its relations in the 

 two sexes. In the male, in which it is also termed the accelerator urines, the two 

 muscles of opposite sides are united in a median fibrous raphe which extends forward 

 from the central tendon of the perineum over the bulb and corpus spongiosum. 

 Arising from this raphe, the fibres are directed laterally and forward over the bulb 

 and corpus spongiosum and are iyiserted into the under surface of the inferior layer 

 of the urogenital trigone and into the fibrous sheath of the corpus cavernosum, some 

 of the more anterior fibres being continued dorsally to insert into the fascia covering 

 the dorsum of the penis and forming what has been termed the muscle of Houston, 

 or compressor vencE dor salis penis. 



In the female, in which the muscle has been termed the sphincter vagincE (Fig. 

 1732), the two muscles of opposite sides are widely separated from each other by the 

 vagina, which they surround. They arise from the central tendon of the perineum, 

 pass forward, investing the bulbi vestibuli, and are lost in the fascia covering the 

 corpora cavernosa and the dorsal surface of the clitoris. 



Nerve-Supply. — From the perineal branches of the pudic nerve. 



Action. — To compress the bulb and corpus spongiosum and so tend to expel 

 any fluid contained in the urethra. The fibres which pass to the dorsum of the penis 

 (or clitoris) may aid slightly in the erection of that organ, either directly or by 

 compressing the dorsal vein. 



Variations.— The posterior portion of the muscle, that surrounding the bulb, is unrepre- 

 sented in the female and is frequently distinctly separable from the anterior part in the male ; 

 it has been termed the compressor bulbi. The deeper fibres of this part of the muscle are sep- 

 arated from the more superficial ones by a thin layer of areolar tissue, and have been regarded 

 as forming a distinct muscle, the compressor hcmisphericum bulbi, which closely surrounds the 

 bulb, the two muscles of either side interlacing above the bulb so as to form practically a single 

 muscle very variable in its development. Finally, fibres may arise from the ischial tuberosities 

 in common with those of the transversi superficiales and pass forward and medially to unite 

 with the bulbo-cavernosi forming what have been termed the ischio-bulbosi. 



5. Transversus Perin^i Profundus (Fig. 1629). 



Attachments. — The deep transverse perineal muscle is situated in the poste- 

 rior part of the deep perineal interspace. It arises from the medial surface of the 

 inferior ramus of the ischium and passes transversely inward to the median line, 

 where it partly unites with its fellow of the opposite side and partly iyiser.ts into the 

 central tendon of the perineum. 



Nerve-Supply. — From the perineal branches of the pudic nerve. 



Action. — To assist in fixing the central tendon of the perineum. 



6. Compressor Urethra (Fig. 1629). 



Attachments. — The compressor or constrictor of the urethra (m. sphincter 

 urethrae membranaceae) in the male is a thin sheet of muscle-tissue situated in the deep 

 perineal interspace anterior to the deep transversus perinaei. It arises from the inner 

 surface of the inferior ramus of the pubis and is inserted by passing medially to sur- 

 round the membranous portion of the urethra, its anterior fibres forming a median 

 raphe with those of the opposite side. The posterior fibres of the muscle enclose the 

 bulbo-urethral gland. 



In the female the fibres are inserted into the walls of the vagina as it traverses the 

 deep perineal interspace. , 



Nerve-Supply. — From the perineal branches of the pudic nerve. 



Action. — To constrict the membranous urethra and, in the female, also to 

 flatten the wall of the vagina. 



The tn. iscltio-pubicus is a small muscle situated at the side of the deep perineal interspace. 

 It arises from the inferior rami of the ischium and pubis and passes anteriorly to be attached to 

 the arcuate ligament of the pubis. It is frequently wanting. 



