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. 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.