Ischium

The Ischium consists of a body and two rami-superior and inferior. The body forms the inferior and posterior two-fifths of the acetabulum, and from the posterior border of the body projects a sharp spine-the ischial spine. From the body the superior ramus passes downwards and back, and from its inferior extremity the inferior ramus passes forwards at an acute angle, to join the inferior ramus of the pubis. The angle of the two ischial rami forms a large tubercle - the tuberosity of the ischium-and between this and the ischial spine above, the edge of the bone is curved forwards to form the small sciatic notch.

Pubis

The Pubis consists of a small triangular body with two rami-superior and inferior. The superior ramus completes the remaining one-fifth of the acetabulum, and the inferior ramus passes backwards to join the inferior ramus of the ischium, thus enclosing the obturator, or thyroid foramen. The upper part of the body projects forwards to form the crest of the pubis, which externally ends in a small spine or tubercle which joins the ilio-pectineal line. The inner border of the body consists of a long, narrow, oval surface which articulates with its fellow of the opposite side by means of an intermediate disc of cartilage.

Acetabulum

The Acetabulum is practically half a sphere, and is directed downwards and forwards. It has a sharp, well-defined margin, which is interrupted inferiorly, forming the cotyloid notch.

Ossification

At birth there are three primary centres, one in each bone. Afterwards secondary centres appear for the ischial tuberosity and spine, and inner part of the body of the pubis. The whole bone is usually fused together by the twenty-fifth year.

Sacrum

The Sacrum, articulating with the innominate bones, completes the pelvis. (For description of sacrum, see the Vertebrae, in Section IV.)

Pelvis

The Pelvis is divided into two parts by the ilio-pectineal line, which extends from the sacro-iliac joint behind to the spine of the pubis in front. The part above this line is called the false pelvis; the part below, the true pelvis.

The false pelvis is bounded by the expanded wings of the ilium and the upper part of the sacrum, and supports the abdominal contents. The true pelvis is much smaller, and is bounded above by the ilio-pectineal lines and the upper margin of the pubis, and its walls are formed by the lower part of the sacrum, the bodies and rami of the ischium and pubis.

Fig. 12. - Pelvis.