THE PELVIS. 115 



ing the place of origin of the quadratus femoris muscle. The ramus of the ischium 

 is flattened like the inferior ramus of the pubis, with which it is continuous on the 

 inner side of the thyroid foramen. 



The ischial spine gives attachment posteriorly to the small sacro-sciatic ligament, exter- 

 nally to the superior gemellus, and internally to the coccygeus and the hinder fibres of 

 the levator ani. The inferior gemellus arises from the upper margin of the tuberosity. 

 below the sciatic notch ; and the inner surface of the ischium gives origin to part of 

 the obturator internus. On the conjoined ischio-pubic rami, immediately above the inner 

 margin, in the male bone, is an oval surface to which are attached the crus penis, surrounded 

 by the ischio-cavernosus muscle, and more deeply the transversus perinei and constrictor 

 urethra3 muscles. The smaller and less distinct surface in the female gives attachment to the 

 eras clitoridis and corresponding muscles. 



The acetabulum, or cotyloid cavity, is a cup-shaped hollow, looking outwards, down- 

 wards, and forwards, and surrounded in the greater part of its circumference by an 

 elevated margin, which is most prominent at the posterior and upper part ; while at 

 the opposite side, close to the obturator foramen, it is deficient, forming the cotyloid, 

 notch. Its lateral and upper parts present a broad horseshoe-shaped smooth surface, 

 which articulates with the head of the femur, and in the recent state is coated with 

 cartilage ; but the central part of the cup and the region of the notch are depressed below 

 the level of the articular surface (fossa acetabuli), lodge a mass of fat and the inter- 

 articular ligament, and have no cartilaginous coating. Rather more than two-fifths 

 of the acetabulum are formed from the ischium, less than two-fifths from the ilium, 

 and the remainder from the pubis. The iliac portion of the articular surface is 

 the largest, the pubic the smallest : the non-articular surface belongs chiefly to 

 the ischium. 



The thyroid or obturator foramen, also called foramen ovale, is internal and inferior 

 to the acetabulum. In the male it is nearly oval, with the long diameter directed 

 downwards and backwards ; in the female it is broader and more triangular. In the 

 recent state it is closed by a fibrous membrane, except in the neighbourhood of the 

 groove in its upper margin. 



The crest of the ilium is subcutaneous, and forms the boundary between the 

 abdomen and the region of the hip. In front the pubic spine is to be felt through 

 the integuments, and lower down the inner margin of the united rami of the ischium 

 and pubis can be followed to the ischial tuberosity, dividing the perineum from the 

 thigh. The remainder of the bone is thickly covered by muscles. 



The hip-bone varies greatly in thickness at different parts. The strongest portions are 

 found along the lines of greatest pressure ; these are, a very thick bar in the ilium between 

 the auricular surface and the acetabulum, through which the weight of the body is trans- 

 mitted to the thigh-bones, and a second formed by the ischium, ending in the tuberosity, 

 which supports the body in the sitting posture. The ilium has also a thick rib running from 

 the acetabulum to the most prominent portion of the iliac crest, while the bone between this 

 and the auricular surface, corresponding to the deepest part of the iliac fossa, is very thin. 

 The floor of the non-articular portion of the acetabulum is also a thin plate of bone, and this, 

 as well as the thin part of the ilium, is occasionally perforated. The chief vascular foramina 

 penetrate the bone where it is thickest, viz., in the iliac fossa near the auricular surface, 

 on the pelvic surface of the ilium near the ilio-sciatic notch, on the outer surface of the ilium 

 between the inferior gluteal line and the acetabulum, and on the ischium between the 

 acetabulum and tuberosity. 



THE PELVIS. 



The hip-bones with the sacrum and coccyx form the pelvis. 



This part of the skeleton may be considered as divided into two parts by a plane 

 passing through the sacral promontory, the ilio-pectineal lines, and the upper border 

 of the symphysis pubis. The circle thus completed constitutes the Irim or inlet of 



i 2 



