[Illustration: FIG. 62.--Front view of the pelvis, with its ligaments. (Dorland.) _a_, Anterior sacro-iliac ligament; _b_, iliofemoral ligament; _c_, obturator membrane; _d_, symphysis pubis; _e_, sacro-sciatic ligament.] [Illustration: FIG. 63.--The right innominate bone. (After Toldt.)] The upper, expanded portion of the os innominatum is the _ilium_, whose upper border is known as the _crest_ and which has two _spinous processes_ front and back, a superior and an inferior, the superior spine being in each case the larger. These spines, especially the anterior superior spines, and the crest give attachment to many muscles, and to the outer surface of the bone the gluteal muscles are attached. The anterior superior spine is also important in making measurements to ascertain whether both legs are of equal length. Below the ilium posteriorly is the _body of the ischium_, which has on its lower edge a tuberosity, the prominent bone on which one sits. Near the upper edge is the _spine_ of the _ischium_, between which and the posterior inferior spine of the ilium is the greater _sacro-sciatic notch_ for the passage of vessels and nerves, including the sciatic nerves. From the tuberosity the ramus extends forward below the _obturator foramen_, a large opening between the ischium and the pubes, also for the passage of vessels and nerves, to meet the pubes, the last and smallest of the three bones which go to make up the os innominatum. The anterior surface of each _pubes_ presents a crest, ending externally in a spine, and the two pubic bones join in front in the _symphysis pubis_. The bone gets its name from the growth of pubic hairs over this region at puberty. [Illustration: FIG. 64.--Diameters of the pelvis: _d_, antero-posterior; _o b_, oblique; _t r_, transverse. (de Nancrede.)] Anteriorly the ossa innominata support the external organs of generation, while within are the internal organs of generation. On the inner surface of the ilium, slightly above the level of the acetabulum, is the _ileo-pectineal line_, above which lie the _iliac fossæ_. A plane drawn through the prominence of the sacrum, the ileo-pectineal lines, and the upper margin of the symphysis pubis serves to divide the upper or false pelvis from the lower or true pelvis. The _false pelvis_, which is the larger, serves to support the intestines and to take part of the weight from the abdominal walls, while the _true pelvis_, being more surrounded by bone and so capable of affording more protection, guards the internal organs of generation. The lower circumference of the pelvis is known as the _outlet_. In the female the bones are lighter, the sacrum less curved, and the diameters greater than in the male.