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.
[Illustration:
FIG. 62.—OSSA COXARUM OF STALLION, DORSAL VIEW. (After
Ellenberger-Baum, Anat. für Künstler.)
]
=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.