660 MAMMALIAN ANATOMY 



the true anterior margin of the bone, and is continued below trans- 

 versely along the pubes. The ilio-pectineal line is gently emarginate, 

 and presents above the level of the acetabulum a small more or less 

 well-developed projection, known as the ilio-pectineal eminence. This 

 small anterior surface faces outward and downward, and its upper half 

 gives origin to part of the iliacus muscle ; the ilio-pectineal eminence 

 and the adjoining region receive the insertion of the psoas parvus 

 muscle. 



The internal surface of the ilium (Fig. 519) has approximately the 

 same size and shape as the external surface. It presents two distinct 

 portions : (1) an upper roughened portion, which comprises more than 

 half of the entire surface and is involved directly and indirectly in 

 the articulation with the vertebral column ; and (2) a lower smooth 

 portion, forming the anterior dorsal region of the lateral wall of the 

 pelvic cavity. 



(1) The upper portion is oblong, sometimes twice as long as wide. 

 Its anterior and posterior margins are parallel ; its upper margin is the 

 arcuate crest of the ilium. The lower margin is also the lower arcuate 

 margin of a flattened, crescentic elevation, called, from its resemblance 

 to the human ear, the auricular surface, which articulates with a 

 corresponding auricular surface., on the lateral mass of the sacrum. 

 Above the auricular surface is a roughened area where the sacro-iliac 

 ligaments are attached. The smooth space which lies above this region 

 and which slopes outward and forward to the crest gives origin to the 

 lower end of the complex system of spinal muscles. 



(2) The lower portion, rather less than half of the internal surface, 

 is wider below than above. It is limited in front by the ilio-pectineal 

 line, and behind by the emarginate posterior border. The emarginate 

 upper margin is the sharp prominent lower edge of the auricular 

 surface. The lower boundary is an imaginary transverse line about 

 seven millimetres above the highest point of the obturator foramen. 

 This part is flat or gently convex above, and faintly concave below, 

 where it is continuous with the surfaces of the pubes and ischium. 

 It faces directly toward the middle of the pelvic cavity, and gives 

 attachment only to the iliacus muscle along the ilio-pectineal line, and 

 to the ilio-caudal muscle by a narrow area of origin posterior to this 

 line. 



The crest of the ilium is the arched upper border of the bone, 



