Ischium

The ischium, or the ischial bone (Os ischii) is much more extended caudally in domestic mammals than in Men. It also spreads in a tabula of ischium (Tabula ossis ischii) that constitutes the main part and holds two cranial extensions enclosing the foramen obturatum. The lateral extension constitutes the body of the ischium (Corpus ossis ischii), that takes part, by its cranial part, in the constitution of the acetabular part of the coxal bone. The medial extension is the branch of ischium (Ramus ossis ischii), that reaches the caudal angle of the pubic bone. Flatenned dorsoventrally, the tabula os ischium has two surfaces, four borders and four angles.

The dorsal (or endopelvic) surface is more or less excavated from one side to the other, and generally smooth. The ventral surface, or exopelvic, planiform or concavo-convex, is nearly entirely occupied by muscular insertions.

The cranial border is regularly concave and limitates the foramen obturatum.

The lateral border is thick and concave; it forms the lesser ischiatic notch (Incisura isciatica minor) that goes from the ischiatic spine to the ischiatic tuber.

The medial border is the shortest and extends on the branch. In domestic mammals, it enters in the constitution of the pelvic symphysis by a symphysial surface (Facies symphysialis) which is thick, rough, slightly concave, oblique in mediocranial direction; it is mingled in Men with the medial border, while in domestic mammals, it always forms a clear angle with it. It constitutes with its opposite a vast transverse arch opened on the caudal side, the ischiatic arch (Arcus ischiadicus). The bottom of this arcade reaches the pubic symphysis in Men and is therefore called pubic arch, or subpubic arch, while in domestic mammals it stays separated by a more or less long ischiatic symphysis.

Of the four angles, two are cranial: the lateral, very thick, belongs to the body of the ischial bone and the medial reaches the caudal angle of the pubic bone. Of the two caudal angles, the medial occupies the botom of the ischiatic arch; the lateral, extremely voluminous, forms the ischiatic tuber or ischial tuber (Tuber ischiadicum). Tuberous and more or less prismatic in great Ungulates, this ischiatic tuber is bicuspid in Equidae and tricuspid in Bulls. It spreads in Carnivorous as in Men in a strong and thick crest that trespass on the caudal border. Its lateral extremity lifts up with a massive tuber, that spreads into a strong crest in Equidae and converts itself in several species (small Ruminants and especially Rabbits) into a real hook.