Part 25

The =ischium= (Os ischii) forms the posterior part of the ventral wall or floor of the bony pelvis. It slopes a little downward and inward, but is practically horizontal in the longitudinal direction. It is irregularly quadrilateral, and may be described as having two surfaces, four borders, and four angles.

The =superior= or =pelvic surface= (Facies pelvina) is smooth and slightly concave from side to side.

The =inferior surface= (Facies externa) is nearly flat, and is in great part roughened for the attachment of the adductor muscles.

The =anterior border= forms the posterior margin of the obturator foramen.

The =posterior border= is thick and rough. It slopes obliquely inward and forward to meet the border of the other side, forming with it the =ischial arch= (Arcus ischiadicus).

The =internal border= meets the opposite bone at the symphysis.

The =external border= is thick and rounded, but concave in =its length=; it forms the =lesser sciatic notch=, the lower boundary of the lesser sciatic foramen.

The =antero-internal angle= or =symphyseal branch= (Ramus symphyseos) meets the pubis, with which it forms the inner boundary of the obturator foramen.

The =antero-external angle= or =acetabular branch= joins the other two bones at the acetabulum, of which it forms more than half. Superiorly it bears part of the =superior ischiatic spine= (Spina ischiadica), and internally it is grooved for the obturator vessels.

The =postero-external angle= is a thick three-sided mass, the =tuber ischii= (Tuber ischiadicum); its lower border is the =inferior ischiatic spine=, to which the biceps femoris and semitendinosus muscles are attached.

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.

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.

THE PELVIS

The =bony pelvis= is composed of the ossa coxarum, the sacrum, and the first three coccygeal vertebræ. The =dorsal wall= or =roof= is formed by the sacrum and first three coccygeal vertebræ, and the =ventral wall= or =floor= by the pubic and ischial bones. The =lateral walls= are formed by the ilia and the acetabular part of the ischia. The defect in the skeleton here is supplied in the fresh state by the sacro-sciatic ligaments and semimembranosus muscles.