The Pelvis 5 



manner as to render mechanical injur}' to them during parturi- 

 tion very improbable, while others, especially the anterior gluteal 

 and obturator nerves from the last lumbar pair. Fig. i , have to 

 pass over a promontory marking the lumbo-sacral articulation at 

 A, in such a manner that they may be, and not rarely are, 

 crushed between the bony ridge and some unyielding part of the 

 fetus during its passage through the pelvic canal. The ob- 

 turator nerve may also be injured in a somewhat similar manner 

 as it enters the obturator foramen, as shown in Fig. 140. (See 

 Contusions of Lumbo-sacral Nerves. J 



The two coxae or ossa innominata are elongated, flattened 

 bones, widely expanded at each extremity, where they are com- 

 posed chiefly of cancellated tissue, and constricted in the center, 

 where they partake more of the structure of long bones. Above 

 and anteriorly they are briefly separated from each other by the 

 intervening 'sacrum, with which they articulate by means of an 

 almost immovable joint ; behind or below they converge to form 

 the pelvic floor, in which, on either side of the median line and 

 about midway from behind to before, occur two large oval open- 

 ings, the foramina ovale. They unite on the median line to con- 

 stitute the ischio-pubic symphysis and, becoming anchylosed early, 

 serve, with the aid of the well-nigh immovable sacro-iliac articlt- 

 lation, to complete the pelvic girdle. Near the middle of each 

 coxa, on the infero-external face, occurs the acetabulum or coty- 

 loid cavity for articulation with the femur. 



Each coxa is composed of an ilium, ischium and pubis, which 

 constitute separate bones during early fetal life, but become fused 

 together prior to birth to constitute a single bone. 



The general direction of the ossa innominata is obliquely back- 

 wards and downwards from the sacrum, the two coxae curving 

 ' at first outwards and later inwards to unite on the median line 

 and, with the sacrum, to complete the oval pelvic inlet. 



The ilium, the largest of the three, is flat and triangular in 

 outline, its supero-external face being concave and occupied by 

 the gluteal muscles, the infero-internal face being occupied largely 

 by the articulation with the sacrum inwardly and by muscular 

 insertions laterally. The postero-inferior angle of the bone is 

 contracted and rounded to form the iliac shaft and ends by con- 

 curring with the ischium and pubis in the formation of the ace- 

 tabulum. The two ilia extend obliquely downwards and back- 



