irregular: it may be divided into four parts— superior, inferior, anterior and posterior.
Superior superior edge, or iliac crest, is limited in front by the anterior and superior iliac spine, behind by the posterior and superior iliac spine. It is shaped like an italic S, presenting two lips and an in terstice to which are attached the oblique, and transversalis muscles of the abdomen.
Inferior this is formed vertically by an articular facet, which unites with a similar facet on the opposite side, to form the pubic symphysis. Laterally, it is formed by the edge of the ascending ramus of the ischium, and below, by the tuberosity of the ischium. All these parts together form the pubic arch.
The anterior or inguinal edge is concave. Above and on the outside are the anterior and superior iliac spines; below this a notch, then the anterior and inferior iliac spine, a groove for the tendons of the psoas and iliacus muscles, a round eminence, the ileo-pectinal, the superior face of the horizontal ramus of the pubes, the spine and angle of the pubes, and the point of meeting of the superior and inferior edges.
Posterior Edge.—This is concave from below downward. We find: first, the postero-superior iliac spine; below this a shallow notch, the postero inferior iliac spine; below this a large notch, which is converted into a canal when the pelvis is complete in its soft parts; then the sciatic spine, the groove for the tendon of the internal obturator muscle, and finally the tuberosity of the ischium.
Articulations of the Pelvis.—The bones which constitute the pelvis are united by means of articulations to which the name of symphyses is given. They are four in number: 1st. The pubic symphysis.
2d. The two sacroiliac synchondroses.
3d. The sacro-coccygeal symphysis.
4th. The sacro-vertebral symphysis,or the articulation of the sacrum with the vertebral column.
Symphysis Pubis.
This is a true amphiarthrosis or symphysis, and is composed of two articular surfaces, an interosseous fibro-cartilage, and four peripheral liga ments. (Fig. 7.) Articular Surfaces.—Oval and rough, they present a posterior ellipsoid, the edge of which projects slightly backward, and an anterior part run ning obliquely from in front backwards, and from without inwards.
Between these two articular surfaces, and covering them, is an inter osseous fibro-cartilage. This is analogous to the inter-vertebral discs, and like them is formed of two parts, the one, outer and unyielding, the other central and soft. It is thicker in front and from above downwards, form ing behind a projection which extends beyond the bones. The soft part is so close to the posterior part that it seems to reach the limit of the bone. It has a cavity which has long been believed to be covered with synovial membrane.
Peripheral Ligaments.—There are four of these: 1st. An anterior ligament, formed by fibres which, in consequence of the spines of the pubes, cross in front of the symphysis throughout its whole height, and adhere strongly to the interosseous fibro-cartilage.
2d. A posterior ligament, which seems to be nothing but a prolongation of the periosteum of the posterior face of the pubes.
3d. A superior ligament, which fills the upper triangular space.
4th. An inferior, the sub-pubic ligament, triangular in shape, thicker than the former ones, filling the lower triangular space, and inserted on the inter-pubic cartilage, laterally on the descending rami of the pubes. It has below a cavity which forms the top of the pubic arch.
The height of the symphysis varies from 1.5 to 1.9 inches. If in some women it seems higher, this is because it has a special inclination to the pelvis.
The Sacro-Iliac Synchondroses.
Considered by Boyer as a synarthrosis, by Blandin as a diarthrosis, they are to-day ranked by most authors as among the amphiarthroses.
According to Sappey they do not belong to either of these classes. In termediate between movable and semi-movable articulations, they unite them, he says, thus forming an unbroken series.
Articular Surfaces.—These are formed by the articular facets of the os innominatum and sacrum. They lie obliquely from the base toward the apex of that bone, and from its anterior towards its posterior surface. Slightly sinuous, they are covered by a cartilage, thicker throughdut on the sacrum than on the iliac bone. That which covers the iliac is a fibro cartilage, that of the sacrum is both a true cartilage and a fibro-cartilage.