658 MAMMALIAN ANATOMY 



straight clown to the front of the top of the acetabulum. The upper 

 margin has the curve of the crest, and is arcuate. When the bone is 

 held with the long axis vertical the posterior end of the upper margin 

 is lower than the anterior end. The posterior margin is, therefore, 

 shorter than the anterior margin. It is the outer edge of the posterior 

 border, and forms an obtuse angle with the superior margin at the 

 posterior superior spine of the ilium. Its upper half is straight, 

 and the lower half, below the posterior inferior spine of the ilium, 

 is emarginate. The emargination is the greater sacro-sciatic notch. 



The upper half of the external surface of the ilium is depressed 

 or faintly concave in all directions. It is crossed obliquely by a line 

 which begins above, near the anterior superior spine, and is continuous 

 below the posterior inferior spine with the outer edge of the posterior 

 border. This line marks approximately the boundary between the 

 areas of origin of the glutens medius muscle and the upper part of the 

 gluteus minimus. A narrow strip along the front part of the crest, 

 adjacent to the anterior border, gives origin to the tensor vaginae 

 femoris muscle. 



The lower half of the external surface is flat above, but becomes 

 wider as it approaches the acetabulum, and presents an anterior part 

 facing directly outward and a posterior part facing upward and back- 

 ward. On the anterior larger part, above the acetabulum, is a rough- 

 ened quadrate area from which spring the rectus femoris and gluteus 

 quartus muscles. On the posterior part is a portion of the area of 

 origin of the gluteus minimus. 



The part of the ilium which enters into the formation of the 

 acetabulum (Fig. 518) is a small, smooth excavation, with an arcuate 

 upper border and a straight lower border ; as it forms the top of 

 the acetabulum, it faces downward, backward, and outward. To the 

 anterior border is attached the aponeurosis of the internal oblique 

 muscle. 



It will be observed that a long, narrow, triangular area, which 

 appears on the anterior side of the lower two-thirds of this external 

 aspect of the bone, has not been included in the external surface. It 

 is rather an anterior surface, since it lies to the inner, or median, side 

 of the anterior border from the anterior inferior spine down to the 

 acetabulum, and is limited within by a line called the ilio-pectineal 

 line, which, beginning also at the anterior inferior spine, appears as. 



