THE MUSCLES OF THE PERINEUM. 



341 



proper pelvic cavity ; while the inferior layer, continuous with the posterior part of 

 the prostatic sheath, extends across between the bladder and rectum, on the one 

 hand binding the vesiculse seminales and vasa deferentia to the base of the bladder, 

 on the other, forming the front part of the sheath of the rectum. The hinder part 

 of the fascia is similarly attached to the rectum, and sends a prolongation down- 

 wards on the lower part of the gut, which becomes gradually thinner and is lost a 

 short distance from the anus. 



In the female the vagina receives an investment from the recto_-vesical fascia, 

 corresponding to the prostatic sheath of the male. In other respects the arrange- 

 ment of the pelvic fascia is substantially the same in the two sexes. 



MUSCLES. The muscles of the perineum differ somewhat in the two sexes, and 

 must therefore be separately described in each. In both sexes they may be divided 

 into two groups, according as they are more immediately connected with the lower 



Fig. 299. DIAGRAM OP AN OBLIQUE SECTION OP THE PELVIS IN THE SITUATION INDICATED 



BY THE LINE B, B', IN PIG. 298. (G.D.T.) 



a, section of hip-bone, passing behind the centre of the acetabulum ; b, bladder ; c, vesicula semi 

 nalis and vas deferens of the right side ; d, rectum ; 1, obturator internus muscle ; 2, 2, obturator 

 fascia, in the lower part of which the pudic vessels and nerves are contained in a sheath ; 3, recto- 

 vesical fascia ; 4, its upper layer attached to the bladder ; 5, its lower layer passing across in front of 

 the rectum, and continuous with, 6, the lateral part of the investment of the rectum ; 7, levator ani, 

 the dotted line on its under surface indicates the position of the anal fascia ; 8, external sphincter ; 

 9, 9, peritoneum. 



orifice of the alimentary canal or with the genito-urinary outlet. In both groups 

 superficial and deep muscles are to be distinguished. 



A. IN THE MALE. a. ANAL MUSCLES. The internal or circular sphincter 

 is a thick ring of unstriped muscle continuous with the circular fibres of the rectum, 

 and will be referred to along with the anatomy of that organ. 



The superficial or external sphincter muscle (sphincter ani externus) is a 

 layer of fibres nearly an inch in depth on each side, placed immediately beneath the 

 skin surrounding the margin of the anus. It is elliptical in form, and is attached 

 posteriorly by a small tendon to the tip and back of the coccyx, usually receiving 

 also some fibres from the overlying integument. Passing forwards it divides into 

 two parts which enclose the anus and meet again anteriorly, where the superficial 

 fibres end in the skin, some of the innermost ones decussating across the middle line, 

 while the larger part becomes blended with the transverse and the bulbo-cavernosus 



z 2 



