The Pubis

The pubis (Os pubis) is the smallest of the three parts of the os coxæ. It forms the anterior part of the pelvic floor, and may be described as having two surfaces, three borders, and three angles.

The superior or pelvic surface (Facies pelvina) is convex in the young subject and the stallion, concave and smooth in the mare and usually in the gelding also. The urinary bladder rests on it.

The inferior or ventral surface (Facies externa) is convex, and in great part rough for muscular attachment. Near the anterior border it is crossed by the subpubic groove, the inner part of which is occupied by a large vein, the outer part by the pubo-femoral ligament.

The anterior border is thin in its inner part (except in the young subject and the stallion), forming the pecten ossis pubis. Externally it bears the rough ilio-pectineal eminence (Eminentia iliopectinea), beyond which it is continuous with the ilio-pectineal line.

The internal border joins the opposite bone at the symphysis pubis.

The posterior border forms the anterior margin of the obturator foramen, and is marked externally by the obturator groove.

The internal angle meets its fellow at the anterior end of the symphysis. This part is very thick in the young subject and the stallion, but in the mare, and usually in the gelding also, it becomes thin with advancing age.

The external or acetabular angle joins the ilium and ischium at the acetabulum.

The posterior angle joins the ischium, with which it forms the inner boundary of the obturator foramen.

The pubis may conveniently be regarded as consisting of two branches which meet at a right angle; these are termed the transverse or acetabular branch (Ramus acetabularis) and the longitudinal or symphyseal branch (Ramus symphysecs).

The acetabulum is a cotyloid cavity which lodges the head of the femur. It faces downward and outward, and consists of an articular and a non-articular part. The articular part (Facies lunata) is crescentic, and is cut into internally by the non-articular part, which lies at a lower level, and is termed the acetabular fossa (Fossa acetabuli). The inner part of the rim is correspondingly cut into by the acetabular notch (Incisura acetabuli), which is converted into a foramen by the transverse ligament in the fresh state, and transmits the pubo-femoral and round ligaments to the head of the femur.

The obturator foramen (Foramen obturatum) is situated between the pubis and ischium. It is oval in outline, the longer axis being directed forward and outward. Its margin is grooved antero-externally for the obturator nerve and vessels.

Development.—Each division of the os coxæ ossifies from one chief center. The center for the ilium appears first near the acetabulum, followed quickly by one for the ischium, and a little later by the pubic center. Secondary centers appear for the crest and external angle of the ilium, the tuber and posterior border of the ischium, and the acetabular part of the pubis. The pubis and ischium are united at birth or soon after, but are not fused with the ilium until the second year. The epiphyseal parts fuse with the main mass at four and a half to five years of age.

The acetabular part of the pubis ossifies from a separate center. It is most distinct in the embryo at three months, and is often called the os acetabuli. Martin says that the ilium has a center for the acetabular part, one for the shaft and wing, and a third for the crest. He also states that there is a special center for the acetabular part of the ischium, and a transitory nucleus in the symphyseal part of the pubis.