PELVIC LIMBS 663 



face of the ischium. The external surface is much longer than wide, 

 and its most conspicuous feature is the acetabulum on the upper end, 

 to the formation whereof the ischium contributes at least three-fifths. 

 Below and behind the acetabulum the surface is slightly convex 

 transversely and faintly concave from the anterior to the posterior 

 end. It forms a smooth surface, over which the obturator externus 

 muscle passes from within outward. Behind the middle point of the 

 posterior border is the beginning of a line which crosses the surface 

 obliquely, parallel to the edge of the obturator foramen, and is con- 

 tinued on the external surface of the ramus. This line is the anterior 

 boundary of the triangular area of origin of the quadratus femoris 

 muscle. The external surface faces downward, forward, and outward. 



The posterior surface of the ischium (Fig. 517) is a narrow strip 

 included between the posterior border on the dorsal side and the rim 

 of the acetabulnm and the external border on the ventral side. It is 

 continuous above with the dorsal part of the external surface of the 

 ilium, and ends below at the dorsal lip of the tuberosity. On the 

 posterior border, at the junction of the upper with the middle third, 

 is the projection known as the spine of the ischium, below which the 

 surface is slightly narrowed by the rolling outward of the posterior 

 border. The posterior surface is concave, and gives attachment to the 

 following muscles : above, to the lower part of the gluteus minimus ; 

 around the spine, to the gemellus superior ; and ventral to the posterior 

 border, to the gemellus inferior. In the position in which the pelvis 

 is placed for study the posterior surface faces outward and slightly 

 upward. 



The internal surface (Fig. 519) of the body of the ischium is as 

 wide as the two other surfaces combined. It is limited above and in 

 front from the inner surfaces of the ilium and pubes by imaginary 

 lines only. On the ventral side, along the obturator foramen, is the 

 inner border, whereof the continuation back to the tuberosity, parallel 

 with the posterior dorsal border, is the artificial line of division from 

 the inner surface of the ramus. 



The internal surface does not lie in one plane ; the anterior part 

 faces inward, but the posterior part slopes upward and outward from a 

 line drawn from the spine to the lowest part of the inner border ; 

 hence it faces upward, inward, and backward. The entire surface 

 forms the lower part of the lateral wall of the pelvis. It is almost 



