PELVIS 241 a a in figs. 1 and 2 ; the ischium is the lower division, between g and r, the lower part of which, marked , is called its tuberosity, and is the point upon which the body rests in sit- ting. The pubis is the anterior division li Tc. These divisions meet in the socket (acetabu- FIG. 1. Male Pelvis, European. lum) of the thigh bone, the dotted line showing the three divisions, which are developed from three primary (eight individual) centres, and which do not become perfectly consolidated till the age of puberty. The sacrum is the wedge-shaped bone a in the posterior part of the pelvis, between the articular surfaces of the ossa innominata. Its base is directed up- ward, having in its middle an oval articular surface corresponding to the articular surfaces of the vertebrse. Behind this is the central canal, which forms the lower portion of the spinal canal. The pelvis is divided by a promi- nent line ii, fig. 1, called the linea ileo-pectinea, into the false and the true pelvis. The false pelvis is that portion above the line, embraced by most of the ilium, and the upper portion of the ischium. In front the false pelvis is FIG. 2. Female Pelvis, European. incomplete, and behind there is also a small interval in the middle of the sacrum. The false pelvis is bounded laterally by the crest of the ilium c c, which is a favorable point of at- tachment for muscles, and its broad wings help to sustain the contents of the abdomen. The true pelvis is all that portion below the linea ileo-pectinea, being mainly embraced by the ischia and pubes. It has a superior circum- ference, brim, or inlet, and an inferior circum- ference, or outlet. The brim is formed later- ally by the linea ileo-pectinea, anteriorly by the upper margin of the pubes, and posterior- ly by the anterior margin of the base of the sacrum, and the sacro-vertebral angle. (See SKELETON.) The inlet of the pelvis is some- what heart-shaped, with its obtuse apex in front. It has three principal diameters, an antero-posterior (sacro-pubic), a transverse, and an oblique. The antero-posterior diam- eter extends from the sacro-vertebral angle to the symphysis pubis. The transverse diam- eter extends from the middle of the brim on one side to the same point on the other. The oblique diameter extends from the sacro-iliac symphysis or union on one side to the promi- nence at the junction of the ileum and the pubes (ileo-pectineal eminence) on the other. The outlet of the pelvis is irregular, and is bounded by three prominent eminences, viz. : the point of the coccyx, #, posteriorly, and the tuberosities of the ischia laterally. These emi- nences are separa- ted by three notch- es: one in front be- tween the pubes and the ascending por- tions (ramus) of the ischia, and called the pubic arch ; and one on either side of the sacrum and coccyx, called the sacro-sciatic notch- es. In the natural state the two latter are converted into foramina by the lesser and greater sacro-sci- atic ligaments, m n, fig. 3. These ligaments E ractically lessen the irregularity of the out- it. The two large openings contained be- tween the rami of the pubes and ischia, and situated below and a little in front of the socket for the head of the thigh bone, are called the obturator foramina, and in the nat- ural state are closed by a ligament called ob- turator ligament, which gives attachment to muscles of the same name. The diameters^of the outlet are two : an antero-posterior, which extends from the tip of the coccyx to the lower part of the symphysis pubis ; and a transverse, extending from the posterior part of one ischiatic tuberosity to the same point on the opposite side. The cavity of the true pelvis is contained between the inlet and the outlet, and has an average depth posteriorly (o x, figs. 1 and 2) of 4'5 in., being the length of the sacrum and coccyx. Its depth at the middle is 3'5 in., and anteriorly, at the sym- physis pubis, it is 1-5 in. The diameters of both the brim and the outlet differ in the two sexes, as do the characteristics of the whole FIG. 8. Side View of Eight Os Innominatum.
