THE OS INNOMINATUM. 12S 



rator foramen; it affords attachment to the obturator 

 membrane. The inferior extremity of the ramus is con- 

 tinuous with the ascending ramus of the ischium. 



The ischium is the most inferior part of the os in- 

 nominatum. It consists of two parts, the body and the 

 ascending ramus. The body, placed vertically, is a 

 wedge-like mass of bone, presenting three surfaces, ex- 

 ternal, internal, and posterior; a superior portion, which 

 enters into the formation of the acetabulum; and an in- 

 ferior, broad, expanded part, called the tuberosity. The 

 external surface is rough, for the attachment of muscles; 

 at its upper part is the prominent rim of the acetabulum, 

 of which the ischium makes two-fifths. The internal 

 surface is a smooth plane of bone, which descends verti- 

 cally from the ilio-pectineal line, and forms the lateral 

 wall of the pelvis ; it is called the plane of the ischium. 

 The posterior surface begins practically at the posterior 

 inferior spine of the ilium, beneath which is a deep 

 notch, the greater sacro-sciatic, formed into a foramen by 

 the lesser sacro-sciatic ligament, which is attached to the 

 spine of the ischium, a spur-like projection from the 

 posterior border of the bone, about three and one-half 

 inches below the posterior inferior spine. The spine of 

 the ischium projects directly backward, as a rule, in the 

 female, while in the pelvis of the male sex it not unfre- 

 quently curves strongly inward; it varies in length from 

 one-half to three-fourths of an inch. Below the spine of 

 the ischium is the lesser sacro-sciatic notch, converted 

 into a foramen by the greater sacro-sciatic ligament. The 

 foramen thus formed is about an inch in diameter, and 

 transmits the tendon of the internal obturator muscle, 

 the internal pudic vessels and nerve. Below the lesser 

 sacro-sciatic notch the posterior surface presents two de- 

 pressions for the origin of the ham-string muscles. The 



