JOINTS AND LIGAMENTS OP^ THE PELVIS. 



337 



border rises from the surface of the bone distinctly below and to a less degree behind 

 and above. 



The thyroid or obturator foramen ' is a large oval opening, with the larger 

 end above and the long axis running downward and outward, bounded by the 

 pubis and ischium. A little tubercle, seen best from the inner side, marks the 

 upper limit of the ischium. Above is the obturator groove under the ramus of the 

 pubis. It is closed by a membrane, except under the groove. 



Structure. — The innominate bone is, as a whole, very strong. The two thin 

 places are in the middle of the ilium and of the cotyloid cavity. It is very thick 

 round the joint wherever pressure may be transmitted through the head of the 

 femur. Sections- show radiating trabeculae from the socket connected by concentric 

 lines. The bone is very thick in a line from the socket to the outer expansion of 

 the iliac crest, which runs nearly vertically in the upright position. It is very 

 strong also at and behind the auricular surface. 



Development. — A centre for the ilium appears early in the third foetal 

 month above the acetabulum and spreads quickly through the upper part of the 

 bone. One for the ischium appears below the socket, usually before the end of the 

 same month. One for the pubis comes decidedly later in the iliac ramus. It is 

 said to appear from the fourth to the fifth month, but it may not be present till the 

 sixth. At birth there is still much cartilage around and between the bony expan- 

 sions from these centres. The rami of the pubis and ischium unite at about eight 

 years or earlier, but the suture may be visible on the inside at eighteen. Ossifica- 

 tion commences by several centres in the Y-shaped cartilage separating the bones in 



Fig. 357. 



Ossification of innominate bone. A, at third foetal month ; B, at birth ; C during first year ; Z?, at six years ; 

 E, at about fifteen years, a, chief centre for ilium; b, chief centre for ischium ; c, for pubis; rf, for tuberosity of 

 ischium ; e, for iliac crest ; /, for anterior inferior spine. 



the socket at an uncertain date, probably before ten years. One of these centres, 

 much larger than the rest, the os acetabuli, persists at the front of the cavity be- 

 tween the pubis and ilium till perhaps fifteen, when union has made much progress 

 between the various parts of the acetabulum. The lines of junction may be seen on 

 the inside at seventeen or eighteen, that between the pubis and ischium persisting 

 longest. Secondary centres come about puberty for the crest of the ilium, the an- 

 terior inferior iliac spine, the symphysis pubis, and the ischial tuberosity. They 

 are fused at twenty, excepting, perhaps, that for the crest of the ilium, the union of 

 which may be delayed ; the suture marking its presence is one of the last in the 

 body to disappear. » 



JOINTS AND LIGAMENTS OF THE PELVIS. 

 These may be divided into ( i ) those connecting the ilium with the sacrum and 

 last lumbar vertebra, (2) those connecting the pubic bones at the symphysis, and (3) 

 the ligaments forming the lateral walls, — the sacro-sciatic ligaments and the obtu- 

 rator membrane. 



' Foramen obturatmn. 

 22 



