100 SKELETON. 



Os Ischium, (Ischion.) This bone forms the posterior infe- 

 rior portion of the os innominatum, and is the next in size to 

 the ilium. It is of a triangular form, and has the anterior 

 extremity bent upwards to join with the pubes. The latter 

 part is its crus or rarnus, and the remainder is its body. 



The body of the ischium is a triangular pyramid, the inter- 

 nal side of which is smooth and uniform, but the posterior and 

 the external sides are very unequal. The internal side is broad 

 above and narrow below ; at the middle of its posterior margin 

 is the spinous process, a projection of considerable magnitude, 

 and sharp-pointed, for attaching the lesser sacro-sciatic liga- 

 ment. Immediately below the spinous process is a smooth 

 concave surface, forming a trochlea, over which the obturator 

 interims muscle plays. Below this trochlea, and forming the 

 most inferior internal margin of the bone, is a long ridge, 

 somewhat more elevated behind than in front, into which the 

 great sacro-sciatic ligament is inserted. The internal face of 

 the ischium, though technically called its plane, departs from 

 the perfect regularity implied in that name, by participating in 

 the general concavity of the pelvis. 



The posterior face of the ischium is swollen out, above, into 

 a rounded surface, for the strengthening the posterior parietes 

 of the acetabulum. This swell is bounded, below, by a trans- 

 verse depression or fossa; immediately below which, is the 

 tuberosity of the ischium, a large rough surface extending 

 from the fossa to the beginning of the crus. This rough sur- 

 face is subdivided into four, two above, and two below. The 

 one above, which is external, and nearest to the acetabulum, 

 gives origin to the semi-membranosus muscle; the other, 

 which is internal, gives origin to the semi-tendinosus, and to 

 the long head of the biceps flexor cruris. Of the two flat sur- 

 faces below, the one which borders on the ridge for the inser- 

 tion of the great sacro-sciatic ligament, is the part on which 

 we sit, and the last surface, which is exterior again to this, 

 gives origin to a part of the adductor magnus muscle. 



The exterior face of the ischium, above, forms the lower part 

 of the acetabulum, and is, therefore, very much excavated; 

 below this the surface is flat, and sufficiently uniform to dis- 

 pense with a particular description. 



The crus of the ischium is flattened internally and externally, 



