The os coxae or hip bone consists of three flat bones, ilium, ischium and pubis, which fuse together to form the acetabulum.

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1 The os coxae The os coxae or hip bone consists of three flat bones, ilium, ischium and pubis, which fuse together to form the acetabulum. The ilium extends from the acetabulum upwards forming the lateral wall of the pelvic cavity. The pubis and ischium extend medially and backward respectively and their medial borders fuse with those of the opposite side to form the pelvic / ischio-pubic symphysis. The pubis and ischium form the anterior and posterior parts respectively of the floor of the bony pelvis and enclose between them on each side, a large obturator foramen. Ilium 1- The ilium is the largest of the three parts. 2- It is irregularly triangular being wide above( wing ),& narrow and prismatic at the middle and slightly expanded below(body). 3- It presents two surfaces, three borders and three angles. 4- The lateral or gluteal surface is directed dorso-laterally and backward. The inferior third of this surface presents rough lines for the origin of the gluteus profundus. This surface is traversed by the very faint (gluteal line )running nearly from a little below the tuber coxae to become continuous with the ischiatic spine. This surface serves for the origin of the gluteus medius. 5- The medial or pelvic surface presents a rough triangular medial part-the sacral surface and a smooth quadrilateral part -the iliac surface. The former presents an irregular facet, the articular surface for the sacrum. The ilio-pectineal line, which separates these two surfaces, begins below the articular surface and joins the anterior border of the pubis and forms the lateral boundary of the pelvic inlet. It bears about the middle the psoas tubercle for the psoas minor. 6- The lateral or (cotyloid) border leads to the acetabulum, little above and infront of which are two depressions (the lateral one is faint) for the origin of the rectus femoris. 7- The medial or (ischiatic)border is concave and forms the greater ischiatic notch. The notch forms the greater ischiatic foramen which is covered by the sacro-sciatic ligament in life and serves for the passage of gluteal nerves and anterior gluteal vessels. In its lower part, it is convex, rough and is continuous with the ischiatic spine, which gives attachment to the sacro-sciatic ligament at its free edge and to the gluteus profundus on its lateral aspect. 8- The dorsal border or the crest of the ilium is concave thick and rough for the attachment of the muscles of the loin.

2 9- The medial angle or sacral tuber is separated from its fellow and forms with it and the sacral spines, the point of the croup. 10- The lateral angle or tuber coxae is large and compounded four tuberosities arranged in pairs, and serves for the attachment of the iliacus, obliquus abdominis internus, tensor fasciae latae, gluteus medius etc. 11- The inferior or acetabular angle is thick and meets the other two parts at the acetabulum. Ischium The ischium is smaller than ilium. It is irregularly quadrilateral and placed behind the ilium and the pubis. It has two surfaces and four borders. The dorsal pelvic surface is slightly concave transversely and forms the posterior part of the pelvic floor. The ventral surface presents about its middle a rough ridge for the biceps femoris. It is roughened for the origin of the adductor muscles of the thigh. The anterior border is concave and forms the posterior boundary of the obturator foramen. The posterior border slopes forward and downward and meets the same borders of its fellow to form the ischiatic arch, which constitutes the inferior boundary of the pelvic outlet. The medial border with its fellow form the ischiatic symphysis, The lateral border is concave and forms the lesser ischiatic notch and is continuous with the ischiatic spine. The notch forms the lower boundary of the lesser ischiatic foramen bordered above by the sacro-ischiatic ligament (in life), which is for the passage of the posterior gluteal vessels. The antero-lateral angle joins the ilium and the pubis at the acetabulum. The postero lateral angle-tuber ischiatic serves for the origin of the biceps femoris, semitendinosus and semimembranosus. Pubis The pubis is the smallest of the three parts. It is irregularly triangular and has two surfaces and three borders.

3 The dorsal or pelvic surface forms the anterior part of the pelvic floor and the urinary bladder rests on it in life. The ventral surface is rough for muscular attachment. The anterior border is thick. Laterally it bears the ilio-pectineal eminence and curves for the attachment of the prepubic tendon. The posterior border forms the anterior margin of the obturator foramen. The medial border meets the same border of its fellow at the pubic symphysis. The acetabular angle joins the ilium and the ischium at the acetabulum. The medial borders of the pubis and the ischium meet the corresponding borders of their fellows to form the pelvic symphysis / Ischia-pubic symphysis and the pelvic floor is basin like. Acetabulum Acetabulum is a cotyloid cavity formed on the ventro-lateral aspect of the os coxae by the meeting of its three components. It consists of an articular and a non-articular part. The former is nearly circular and articulates with the head of the femur. The rim of the cavity presents on its postero-medial spect the acetabular notch, which transmits the round ligament of the hip joint. The non-articular part, the acetabular fossa is situated in the depth of the acetabulum. Another small notch may be seen antero-medially, though sometimes it is replaced by a foramen or is absent. Obturator foramen The obturator foramen is a large, elliptical opening on the floor of the pelvis and is circumscribed by the ischium and the pubis. It is covered in life by the obturator muscles. Pelvic cavity The pelvic cavity is the smallest and the most posterior of the three visceral cavities of the body.

4 The bony pelvis composed of the os coxae laterally and ventrally. The sacrum and the first three coccygeal vertebrae dorsally. The lateral vacuities are closed up by the ischiatic ligaments in life. The pelvic inlet is bounded by the terminal line or brim which is composed of the base of the sacrum dorsally, ilio-pectineal lines laterally and the anterior borders of pubis ventrally. The pelvic outlet is much smaller and is very incomplete in the skeleton. The third coccygeal vertebra bound it dorsally and ventrally by the ischiatic arch and the sacro-ischiatic ligament and semimembranosus muscle completes it laterally. Sexual differences The ischiatic arch is wider and the outlet is larger in the female than in the male. The conjugate (vertical) and transverse diameters are greater in the female so that the cavity is roomier. The pubis and the ischium of the opposite sides meet at a more open angle in the female than in the male. Sheep and Goat The long axis of the ilium is almost in straight line with that of ischium. Greater ischiatic spine is low. Ischiatic symphysis is not ossified even in old animals. Acetabulum is large and deep. The pelvic brim is elliptical in outline. Dog The ilium is nearly in a vertical plane. The gluteal surface is concave. The crest of the ilium is strongly convex. The ischium has a twisted appearance. The lesser ischiatic notch is absent. The acetabulum is deep. The symphyseal part of pubis is thick and fuses late with the opposite bone\

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