556 ABDOMEN 



and anteriorly by the innominate bones. The bony wall is 

 deficient, on each side posteriorly, between the sacrum and 

 coccyx behind and the innominate bone in front. This 

 interval is partially filled in by the ligamentum sacro-tuberosum 

 (O.T. great sacro-sciatic ligament] and the ligamentum sacro- 

 spinosum (O.T. small sacro-sciatic ligament) which divide it into 

 the great and small sacro-sciatic foramina. Anteriorly, on 

 each side, the bony wall is broken by the obturator foramen, 

 which is closed by the obturator membrane, and directly in 

 front there is a gap, the pubic arch, which is occupied by the 

 uro-genital diaphragm and its fasciae (O.T. triangular ligament). 

 The area thus enclosed is separated into an upper and a lower 

 part by an imaginary plane, the plane of the pelvic brim. 

 The margins of this plane are the ilio-pectineal lines of the 

 innominate bones and the promontory of the sacrum. The 

 part above the plane is the pelvis major (O.T. false pelvis), 

 which has already been studied as part of the abdomen. The 

 part below the plane is the pelvis minor (O.T. true pelvis). 

 The inner aspect of the wall of the small pelvis is partially 

 covered by the following muscles. Posteriorly, on the front 

 of the sacrum, are the two piriformes muscles. Laterally, 

 on the inner surface of each hip bone, is the obturator internus 

 muscle. Anteriorly is the uro-genital diaphragm formed by the 

 deep transverse perineal muscles and the sphincter urethra 

 membranacece. The inner surfaces of these muscles and the 

 intervening skeletal structures are covered by a continuous 

 layer of fascia, the parietal pelvic fascia. The wall of the 

 pelvis minor may, therefore, be regarded as consisting of 

 three strata, viz. i. A bony stratum. 2. A muscular 

 stratum. 3. A membranous stratum. 



The small pelvis is separated into an upper part and a 

 lower part by the pelvic diaphragm, and the fascia covering 

 it. The pelvic diaphragm is formed by the two levatores ani 

 and the two coccygei muscles. In the following account the 

 upper part will be spoken of as the pelvis. The lower part 

 has already been called the perineum. 



The pelvis as thus defined is the smaller, basin-shaped, 

 lower part of the abdominal cavity. It communicates with 

 the upper portion of pelvis major through a somewhat 

 constricted aperture, the pelvic inlet. 



The contents of the pelvic cavity differ in the two sexes ; 

 in both, however, the bladder occupies the fore-part of the 



