4 OBSTETRICAL AX ATOMY. 



and prismatic in shape. Posteriorly, it offers a concave articular sur- 

 face (acetabular), and above this cavity is the supra-cotyloid ridge or 

 crista ilii — an elongated eminence passing backwards, sharp at its 

 summit, smooth internally, and roughened externally, which is continu- 

 ous anteriorly with the inner border of the bone, divides its iliac and 

 articular surfaces, becomes lost on the anterior margin of the pubis, and 

 laterally limits the anterior circumference or border of the pelvic cavity 

 ■ — the linea ilio-pectinea. 



The ilium is therefore united to, or in contact with, the sacrum 

 superiorly, and inferiorly and posteriorly with the two bones next to be 

 described. 



Ischium. — The ischium {os sedentarum of man) is next in size to the 

 ilium, and the most posterior of the three bones. In form it is flat and 

 irregularly quadrilateral ; and is composed of a thick solid portion — the 

 body, and a narrow part — the neck. It has tivo faces, four borders, and 

 four angles. 



The upi^er face is smooth and nearly flat, and forms part of the floor 

 of the pelvic cavity. The loiver or external face bears some rugged 

 imprints, particularly towards the symphysis. 



The anterior border is thick and concave, and posteriorly circumscribes 

 the obturator or oval foramen. The loostcrior border is straight and 

 directed obliquely forward and inward, composing, with the corre- 

 sponding border of the opposite bone, a large notch — ihejnibic or ischiatic 

 arch. Throughout its extent it exhibits a roughened depi'essed lip, 

 which projects on the lower face — this is the spine. The external border 

 is thick and concave, affords attachment to the sacro-sciatic ligament, 

 and forms the small sciatic notch. The internal border joins its homo- 

 logue of the opposite side, to complete the ischio-puhic symphysis. 



The external or cotyloid angle is the most voluminous, and shows an 

 excavated diarthrodial facet, forming part of the acetabulum, and the 

 posterior extremity of the crista ilii, limited by a small transverse fissure 

 which separates it from the external border of the bone. The antero- 

 internal angle joins the posterior angle of the pubis. The postero- 

 external angle is the most important from an obstetrical point of view, as it 

 constitutes what is commonly named the point of the hip ; with the 

 corresponding bone of the opposite side, it forms the ischiatic tuberosity 

 {tuber ischii) — a thick, up-curved prismatic mass which is continued by 

 a prominent ridge elongated from before to behind, the thin margin 

 of which is curved outwards and downwards. The distance between 

 the external tuberosities of the two ischii gives the width of the ischial 

 arch, and allows an estimate to be formed of the transverse diameter 

 of the posterior opening of the pelvis. The postero-internal angle con- 

 stitutes, with that of the other ischium, the summit of the triangular 

 notch or space named the ' ischial,' or ' pubic arch ' in some species. 



Pubis. — The pubis {pecten or share-bone) is the smaller of the three, and 

 is situated between the ilium and ischium. It is irregularly triangular, 

 and is described as having two faces, three borders, and three angles. 



The upper face is concave and smooth, and concurs in forming the 

 floor of the pelvis. In the pelvis of many Mares it has a more or less 

 marked depression, apparently produced at the expense of the anterior 

 portion of the bone, which is thin ; while in the Horse it is generally 

 much thicker at this part, and instead of a depression the surface may 

 be plain or even convex. The loiver face is rough, and traversed on its 



