8 OBSTETRICAL ANATOMY. 



internally, and roughened externally, which is continuous 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 supe- 

 riorly, 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 two faces, four borders, 2iV\dfour 

 angles. 



The upper face is smooth and nearly flat, and forms part of the floor 

 of the pelvic cavity. The lower or exter?ial 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 posterior border is straight and di- 

 rected obliquely forward and inward, composing, with the corresponding 

 border of the opposite bone, a large notch — the pubic or ischiatic arch. 

 Throughout its extent it exhibits a roughened depressed lip which pro- 

 jects 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 homologue of the 

 opposite side to complete the ischio-pubic 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 antcro- 

 internal angle ]o\ns the posterior angle of the pubis. The postero-external 

 angle is the most important in an obstetrical point of view, as it consti- 

 tutes what is commonly named the point of the hip ; with the correspond- 

 ing bone of the opposite side, it forms the ischiatic tuberosity {tuber 

 ischii) : a thick, up-curved prismatic mass which is continued by a prom- 

 inent 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 al- 

 lows an estimate to be formed of the transverse diameter, of the posterior 

 opening of the pehds. The postero-internal angle constitutes, 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 tliree a?igles. 



The tipper 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 plane or even 

 convex. The lower face is rough and traversed on its whole length by a 

 wide groove which reaches the bottom of the acetabulum, and lodges the 

 pubio-femoral ligament and a very large vein. 



