THE PELVIS 355 



THE PELVIS. 



The Pelvis Minor (True Pelvis). The walls of the 

 pelvis consist of three layers : (i) a bony and ligamentous 

 stratum, (2) a muscular stratum, and (3) a fascial stratum. 



1. The osseous circle, formed by the sacrum, the coccyx, 

 and the hip bones, is strengthened by the sacro-tuberous and 

 sacro-spinous ligaments (great and small sacro-sciatic), which 

 connect the sacrum and coccyx with the ischium. In the pubic 

 arch this layer is represented by the inferior fascia of the 

 urogenital diaphragm (anterior layer of triangular ligament). 



2. The muscular layer consists of : (a) the obturator internus, 

 which arises from the side wall of the pelvis and passes through 

 the lesser sciatic foramen (p. 419) ; (b) the piriformis, which arises 

 from the front of the sacrum and passes through the greater 

 sciatic foramen ; and (c) the sphincter of the membranous 

 urethra (compressor urethrae), which assists in rilling the gap 

 between the inferior rami of the pubes. 



3. The fascia which covers these muscles is termed the 

 parietal pelvic fascia. It is continuous above with the fascia 

 lining the abdomen and is attached below to the margins of 

 the inferior pelvic aperture. 



The large nerves of the lumbo-sacral and sacro-coccygeal 

 plexuses lie between the muscular and fascial strata, but the 

 large blood-vessels -lie under the peritoneum internal to the 

 fascia, and with the exception of the obturator artery (p. 411), 

 pierce it as they leave the pelvis through the various foramina. 

 The points at which they pass through the fascia are the sites 

 of pelvic hernise. 



Fractures of the Pelvis are usually caused by severe 

 crushing, and they occur at the weakest areas of the bony ring, 

 namely, the pubic and ischial rami, where they bound the 

 obturator foramen, and the sacrum, along the line of the sacral 

 foramina. Rectal or vaginal examination will often disclose 

 the extent of the fracture and may assist the surgeon to obtain 

 the correct alignment of the displaced fragments. Serious 

 complications arise if the sharp broken edges wound any of the 

 pelvic viscera. 



The Parietal Pelvic Fascia covers the piriformis, the ob- 

 turator internus, and the sphincter of the membranous urethra. 



