OBTURATOR FASCIA. PERIXEAL MUSCLES. 327 



that organ, except at its base, where an angular furrovr, occupied by 

 large veins, exists between the prostate and bladder. Into this furrow 

 the incision for lithotomy ought not to extend, on account of the danger 

 from wounding the veins and from the infiltration of urine. A portion 

 of the recto- vesical fascia invests the vesiculaj seminales, and is extended 

 across between the bladder and the rectum ; continuing into the mem- 

 brane of the opposide side, it supports the bladder, and separates that 

 organ from the intestine. On the rectum the fascia is also reflected 

 upwards and downwards, gradually degenerating into a thin membrane 

 over the surface of the bowel, as it likewise does on the bladder. 



c. The obtarator fascia is a membrane stretched over the lower part 

 of the surface of the obturator internus muscle within the pelvis. It is 

 connected superiorly with the white band before referred to, which con- 

 sists indeed of its superior fibres, and it is attached in the rest of its 

 circumference to the rami of the pubis and ischium, the ischial tuber- 

 osit}^ and the greater or lesser sacro-sciatic ligaments. It lines the 

 inner surface of the obturator internus muscle and presents between its 

 fibres a canal, which contains the internal mibic artery and nerve in 

 their course to the perinteum. 



The obturator fascia is sometimes included in the description of the pelvic 

 fascia, whUe the recto-vesical is considered as an offset from it. It will be found, 

 however, on dissection, that the recto-vesical fascia is always most directly con- 

 tinuous witli the pelvic fascia, and that the obturator fascia is only loosely con- 

 nected with it. Indeed, the fibres of the levator ani muscle in most cases pass 

 upwards to some extent beyond the white line, and thus separate the obturator 

 from the pel'vic fascia. 



The iscMo-redal fossa is a pyramidal space occupied by subcutaneous 

 fat. It is bounded externally by the obturator fascia, posteriorly by 

 the gluteus maximus muscle and great sacro-sciatic ligament, and 

 internally by the recto-vesical fascia ; anteriorly, its base is limited by 

 the margins of the perineal and the subpubic fascise. 



In the female, the pelvic fascia is connected Avith the vagina in the 

 same manner as with the other pelvic organs. 



Muscles. — The muscles of the perinneum 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 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 iuternal or circular 

 sphincter is a thick ring of unstriped muscle connected with the lowest 

 circular fibres of the rectum, which will fall more naturally to be 

 described along with the anatomy of that organ. 



The superficial or external sphincter muscle is a tliin layer of 

 fibres placed immediately beneath the skin surrounding the margin of 

 the anus. It is elliptical in form, about half an inch in breadth on each 

 side of th e anus, and is attached posteriorly by a small tendon to the 

 tip and back of the coccyx ; passing forwards on each side of the anus, 

 it becomes blended anteriorly with the transverse and the bulbo-caver- 

 nosus muscles at the central point of the perinmim, a name given to the 

 depressed root situated in the male between the anus and the bulb of 

 the uiethra, and iu the female between the anus and vulva. 



