336 



HUMAN ANATOMY. 



The Ischium. — The ischium ' the thickest and most solid part of the bone, 

 consists of a body, chiefly concerned with the acetabulum, a tuberosity, and a ramus. 

 The body, continuous above with the ilium, forms the front of the great sciatic notch, "^ 

 below which is the sharp spine of the ischium pointing backward and inward for the 

 lesser sacro-sciatic ligament. The tuberosity ' is a great thickening of the back of 

 the lower end of the body of the ischium which bears the weight in sitting. It is 

 broad above and behind, narrowing in front as it passes into the ramus. It extends 

 but little onto the inner side of the bone, which otherwise is smooth. Its inner lip 

 receives the great sacro-sciatic ligament and its falciform prolongation. A smooth 

 surface (in life coated with cartilage) passes from the inside of the back of the 

 bone just below the spine and above the tuberosity, forming the lesser sciatic notch,^ 



occupied by the tendon of 

 Fig. 356. the obturator internus. In 



front of this, under the ace- 

 tabulum and above the tu- 

 berosity, is a groove for a 

 part of the obturator ex- 

 ternus. The upper part of 

 the tuberosity is divided into 

 an upper and front area for 

 the origin of the semimem- 

 branosus, and one behind 

 and below it for the semi- 

 tendinosus and biceps. Be- 

 low these, extending onto 

 the ramus, is a surface for 

 the adductor magnus. The 

 ramus ^ is a strip of bone 

 running forward to meet the 

 inferior ramus of the pubis. 

 The lower edge, forming 

 the margin of the subpubic 

 arch, is twisted outward and 

 rough. The border towards 

 the foramen is relatively 

 sharp. The line of junction 

 of the rami of the ischium 

 and pubes can be distin- 

 guished by the greater 

 breadth of the former. 



The acetabulum, the 

 socket for the hip, is a deep 

 hemispherical cavity with a 

 raised border, imperfect be- 

 low. The imaginary axis 

 of the cavity runs upward, 

 inward, and backward. It 

 is formed by all three bones, the ischium contributing the most and the pubes the 

 least. The lines of union are sometimes seen on the smooth posterior surface in 

 the adult. The cavity is only in part articular. In shape this portion may be com- 

 pared to a horseshoe beaten concave and fitted into the cavity with the two ends 

 pointing downward, enclosing a non-articular cavity at a somewhat deeper level, 

 which extends more than half-way up the back of the socket. The bone at the 

 bottom of the cavity is very thin. The articular strip is broadest above and behind 

 the middle and narrowest in front. Of the two ends of this articular strip the 

 posterior is the more prominent, overhanging a groove leading into the non- 

 articular hollow from below. The front one has no corresponding projection. The 

 border of the acetabulum is formed by the convexity of this horseshoe-shaped strip, 

 and consequently is wanting below. The interruption is the cotyloid notch.^ The 



'Osischii. - Incisura ischladica major. ^ Tuber ischii. '' lacis. isch. minor. ' Ramus inferior. ^ Incisura acetabuli. 



Pubes 



Ischium 



Oblique sagittal section of right innominate bone passing through bottom 

 of acetabulum ; inner surface. 



