164 



OS ILIUM. 



The ossa innomlnata are the two large bones which are con- 

 nected to the sacrum behind, and to each other by the interven- 

 tion of a cartilage in front. 



Each of the ossa innominata is composed of three portions, in 

 children ; and although these are united in adults, so as to form 

 but one bone, yet anatomists have generally considered the bone 

 as divided into its original parts, which are denominated os ilium, 

 OS ischium, and os pubis. 



The original separation was at the acetabulum, or cavity for 

 receiving the head of the os femorls, which is on the outside of 

 the OS innominatum. The upper and posterior part of this cavity, 

 to the amount of two-fifths, is formed by the os ilium, two-fifths 

 of the inferior portion by the os ischium, and the anterior fifth 

 by the os pubis. 



The Os Ilium 



Fig. 39.^ 



Is the largest of the three 

 portions. Its external sur- 

 face has been called its 

 dorsum, and the internal 

 concave surface its costa or 

 venter. The semicircular 

 edge at the upper part of the 

 bone, is named the spine or 

 crest; the external oblique 

 muscle of the abdomen is in- 

 serted into it, and the inter- 

 nal oblique, and the transver- 

 salis arise from it. The ends 

 of the spine are prominent, 

 and therefore are called pro- 



* The OS innominatum of the right side. 1. The ilium ; its external surface. 

 2. The ischium. 3. The os pubis. 4. The crest of the ilium. 5. The superior 

 curved line. 6. The inferior curved line. 7. The surface for the gluteus maxi- 

 mus. 8. The anterior superior spinous process. 9. The anterior inferior spinous 

 process. 10. The posterior superior spinous process. 11. The posterior inferior 

 spinous process. 12. The spine of the ischium. 13. The great sacro-ischiatic 

 notch. 14. The lesser sacro-ischialic notch. 15. The tuberosity of the ischium, 

 showing its four surfaces. IG. The ramus of the ischium. 17. The body of the 

 OS pubis. 18. The ramus of the pubis. 



