346 THE MUSCLES AND FASCIAE OF THE TRUNK. 



The superficial genito-urinary muscles are supplied by the perineal branch of the pudic 

 nerve, and the constrictor urethrae by the dorsal nerve of the penis. 



Actions. The sphincters of the anus cause by their contraction occlusion of that aperture. 

 The contraction of the external is usually maintained involuntarily, though it may be rendered 

 firmer by an act of the will ; that of the internal is wholly involuntary. 



The levator ani and coccygeug act principally in supporting and to a slight extent raising 

 the floor of the pelvis. They thus come into play with the muscles of the abdominal wall in 

 forcible expiratory and other expulsive efforts. The levator ani can also compress the lower 

 part of the rectum, which it thus assists in emptying, while the fibres inserted into the wall 

 of the bowel at the same time tend to raise and expand the aperture. The lower fibres of the 

 anterior division of the muscle, however, act with the sphincter in closing the anal passage. 



The transversi acting together draw backwards and fix the central point of the perineum, 

 thus assisting to give a base of support to the ejaculator muscles. 



The ischio-cavernosus serves to compress the crus penis and thus assists in producing or at 

 least in maintaining the erection of the penis. 



The bulbo-cavernosi compress the bulb and the adjoining part of the corpus spongiosum, se- 

 as to eject forcibly any fluid lodged in the urethra. They come into action at the end of the 

 process of micturition, when their contraction is mainly a voluntary act, and in the emission 

 of the semen, when it is involuntary. 



The constrictor urethree diminishes the calibre of the urethra and expels its contents ; it 

 contracts at the end of micturition, so as to assist the bulbo-cavernosi in clearing the canal. 

 According to Henle the constrictor also takes an important share in producing the erection of 

 the penis, by compressing the veins of the corpora cavernosa, which are contained between its 

 fibres. 



B. IN THE FEMALE. In this sex the anterior fibres of the levator ani embrace 

 the vagina as they do the prostate in the male. 



The transversns perinei and the external sphincter are arranged essentially 

 in the same manner as in the male. 



The erector clitoridis (ischio-cavernosus) differs from the erector penis of the 

 male by its smaller size alone. 



The sphincter vaginae (bulbo-cavernosus) is attached behind to the central 

 point of the perineum, in common with the external sphincter and transversus 



Fig. 303. MUSCLES OP THE PERINEUM IN THE FEMALE. 

 (Allen Thomson.) 



a, clitoris ; b, crus clitoridis ; c, is placed in the vesti- 

 bule above the orifice of the urethra ; d, vagina ; x , anus ; 

 e, coccyx ; 1, external sphincter of the anus ; 2, sphincter 

 vaginae ; 2', some of its fibres prolonged to the clitoris ; 3, 

 levator ani ; 4, on the left ischial tuberosity, points to the 

 transversus perinei (the inner portion of this muscle is 

 represented too far forwards in the figure) ; 5, 6, ischio- 

 cavernosus ; 7, gracilis ; 8, adductor magnus, semitendino- 

 sus, &c. ; 9, gluteus maximus. 



perinei muscles ; its fibres open out to surround 

 the vaginal orifice and vestibule, closely em- 

 bracing on the outer side the two bulbs of the 

 vestibule ; again approaching each other in front, 

 they become narrow, and are inserted mainly 

 upon the corpora cavernosa of the clitoris, a 

 fasciculus crossing over these and including the dorsal vein ; some of the inner 

 fibres end in the mucous membrane of the vestibule in front of the urethral orifice. 

 The two halves of this elliptical muscle appear to correspond strictly to the bulbo- 

 cavernosi muscles in the male. 



The constrictor urethras, or transversus perinei profundus, differs from 

 the corresponding muscle of the male in being, like the deep perineal fascia between 

 the layers of which it is contained, almost completely divided into lateral halves by 

 the vagina. The fibres spring on each side from the margin of the ischio-pubic 



