The Pelvis is important surgically, not merely on account of its contained organs, but from its mechanical position. The centre of gravity of the adult body is just above the sacro-lumbar angle, and over the midpoint of a line drawn between the heads of the femora. In the erect posture the brim of the true pelvis forms an angle of 60 degrees with the horizon, while the base of the sacrum is 3¾ inches above the upper border of the symphysis, and the tip of the coccyx just above its lower border. This obliquity of the pelvis has an effect in modifying shocks transmitted to it, which are further distributed by certain arches.

Thus, when in the erect posture, the arch along which force is transmitted is composed of the sacrum, sacro-iliac joints, acetabula, and intervening bone, while in sitting it is composed of sacrum, sacro-iliac joints, ischial tuberosities, and intervening bone. In these arches the sacrum is compared to a keystone, but, as Cleland has pointed out, the sacrum is really suspended between the innominate bones by its ligaments, and is not really a keystone. Morris describes a counter-arch for each of these mentioned, which, by converting the arch into a ring, 'ties ' it, and greatly strengthens it.

Thus the counter-arch in the erect posture is composed of the body and horizontal rami of the pubes, and, in the sitting posture, of the rami of pubes and ischium, the arch being completed, in both cases, by the symphysis, which therefore has to stand a very considerable strain. Thus in rickets the symphysis is frequently pushed forward, and sometimes the anterior arch yields almost entirely, while disease of the symphysis is generally accompanied by pain on standing or sitting. As the sacro-iliac joint and symphysis are surrounded by powerful ligaments, the joints seldom give way, but fracture generally takes place to one side of them. Thus the most common fracture of the pelvis is through the rami of both pubes and ischium, and this is often associated with tearing of the sacro-iliac ligaments or fracture of the bone on either side. Where the pelvis is crushed by laterally applied force (indirect violence), the sides of the pelvis tend to be driven together, and thus the posterior sacro-iliac ligaments are torn, whereas when the force is applied antero-posteriorly (direct violence), the sides tend to be driven apart, and so the anterior ligaments are torn. Fractures of the pelvis are very apt to cause rupture of the urethra, rectum, or bladder. The three anatomical portions of the pelvis fuse about the seventeenth year. The symphysis has been divided to give more room in labour where the pelvis is contracted (Sigaultean operation), and separation has occurred as the result even of muscular violence. Sacro-iliac disease, which is frequently tubercular, but may be rheumatoid, is generally associated with much pain both in standing and sitting, the pain being referred to the sacrum along the upper sacral nerves, the buttock along the gluteal nerve, and the hip or knee-joint along the obturator nerve, and occasionally to the back of the thigh and calf along the lumbo-sacral cord and connection with the great sciatic. (The lumbo-sacral cord and the obturator nerve pass over the front of the articulation.) In his operation for ectopia vesicae, Trendelenburg divides the sacro-iliac joints in order to get the gaping symphysis to come together. Congenital tumours, dermoids, and teratomata, are sometimes found occupying the sacrococcygeal region, the dermoids frequently arising from remnants of the post-anal gut, which, like the allantois, is an outgrowth from the neurenteric canal, and normally entirely disappears before birth. The sacro-coccygeal region is also the common seat of attachment between attached foetuses. The sacro-coccygeal joint and surrounding parts are frequently the seat of severe pain (coccygodynia), which may necessitate removal of the coccyx. The parts are supplied by the posterior divisions of the second, third, and fourth sacral, and anterior and posterior divisions of the fifth sacral, and coccygeal nerves. Dislocation may also occur at this joint, or it may be diseased, the coccyx sometimes projecting into the rectum. In old age the coccyx becomes ossified to the sacrum.

The pelvis is divisible into two portions-an upper, the false pelvis, bounded by the iliac blades and above the level of the ilio-pectineal line, and a lower, the true pelvis, bounded behind by the sacrum, coccyx, and pelvic portions of the pyri-form muscles ; laterally by the innominate bone, covered by the obturator internus ; in front by the pubic bones and symphysis ; above by the ilio-pectineal lines ; and below by the pelvic diaphragm, formed by the pyriformis, sacro-sciatic ligament, coccygeus, levator ani, and triangular ligament. Within the pelvis, but below the pelvic diaphragm, are the ischio-rectal fossae.

The pelvic cavity is lined by the pelvic fascia, which is continuous with that of the deep surface of the abdominal wall-namely, the transversalis fascia in front and laterally, and that of the ilio-psoas and quadrat us lumborum behind. It consists of parietal and visceral layers, the former running down over the brim of the true pelvis to which it is attached, and then covering the obturator internus and pyriformis, to be inserted below into the rami of the pubes and ischium and tuber ischii. The portion of this parietal layer lining the true pelvis is called the obturator fascia. In front it forms the posterior layer of the triangular ligament. At the level of the origin of the levator ani muscles laterally, and in a line from the back of the symphysis to the ischial spine (the white line), this parietal pelvic fascia gives off a visceral layer, which runs on the abdominal surface of the levator ani to meet its neighbour of the other side, giving off processes to the bladder and rectum in both sexes, and, in addition, to the prostate and vesiculi seminales in the male and the vagina in the female. This visceral layer is frequently called the recto-vesical fascia.