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.
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.
On the whole, the pelvic bones are well covered in with muscles. The anterior superior spine, however, is easily felt in front and the whole crest can be felt back to the posterior superior spine. The tuberosity of the ischium also can be felt, especially when the thigh is flexed, for it is largely uncovered of muscles. The spine of the os pubis can always be felt, on a level with the great trochanter, and the relation of its position to that of a hernia shows whether the rupture is above or below Poupart’s ligament, that is, whether it is inguinal or femoral.
Occasionally there is lack of development of the pubic bones for two or three inches and the bladder is exposed. Fracture of the pelvis may occur, perhaps with injury to the viscera. The acetabulum may be fractured or the sacrum broken, with injury to the sacral plexus of nerves, causing paralysis of the lower extremities and of the sphincters, with resultant involuntary passage of urine and feces, and in childbirth the coccyx is often broken. In rickets there may be great deformity of the pelvis, causing trouble in childbirth later in life. Osteomalacia is a disease of adults, in which the bones are soft and the weight pushes the promontory of the sacrum forward and approximates the sides of the pelvis.