612 ABDOMEN 



Dissection. Remove the levator ani, leaving small portions attached to 

 its bony origins, viz. the body of the pubis and the spine of the ischium. 

 Take away all the remains of the parietal pelvic fascia from the side-wall 

 of the pelvis, and the obturator internus will be exposed. 



M. Obturator Internus. This muscle clothes the lateral 

 wall of the pelvis on its inner aspect. It is fan-shaped and takes 

 an extensive origin, viz. (i) from the circumference of the 

 obturator foramen, except above where the obturator vessels 

 and nerves quit the pelvis; (2) from the deep surface of the 

 obturator membrane; (3) from the surface of bone behind the 

 obturator foramen as far back as the great sciatic notch. A few 

 fibres are also derived from the parietal pelvic fascia which 

 covers it. From these origins the fibres converge towards the 

 small sciatic notch, and end in a tendon which issues from the 

 pelvis through the small sciatic foramen. In the gluteal 

 region the tendon is inserted, together with the two gemelli, 

 into the medial margin of the upper border of the greater 

 trochanter of the femur. The margin of the small sciatic notch 

 over which the tendon glides is coated with smooth cartilage, 

 and this is raised into three or four parallel ridges which fit 

 into fissures in the deep surface of the tendon. A synovial 

 bursa intervenes between the tendon and the cartilage. 



The obturator internus is supplied by a special branch 

 from the front of the upper part of the sacral plexus (p. 607). 



M. Piriformis. The piriformis lies on the anterior 

 aspect of the posterior pelvic wall. It arises by three pro- 

 cesses from the anterior surface of the sacrum in the region 

 of the second, third, and fourth sacral segments, between and 

 outside the foramina and, to a slight extent, medial to the 

 foramina; it takes origin also from the upper border of the great 

 sciatic notch and from the sacro-tuberous ligament. The 

 muscle leaves the pelvis through the upper part of the great 

 sacro-sciatic foramen, and is inserted, by a rounded tendon, into 

 the middle of the upper border of the great trochanter of the 

 femur. It is supplied by branches from the first and second 

 sacral nerves. 



LlGG. ClNGULI EXTREMITATIS INFERIORIS (LIGAMENTS OF 



THE PELVIC ARTICULATIONS). 



The pelvis is attached to the last lumbar vertebra at the 

 lumbo-sacral articulation, and its several parts are held 



