THE LOWER EXTREMITY, POSTERIOR. 607 



From the upper and back part of the tuberosity of the 

 ischium, along the lower border of the groove for the ten- 

 don of the obturator externus. The tendon of the obtura- 

 tor internus leaves the pelvis through the small sacrosciatic 

 foramen; after coming through the foramen it receives the two 

 gemelli muscles, the superior above and the inferior below, 

 on either side. These small muscles are really portions of 

 the obturator internus arising external to the pelvic cavity. 



Insertion. The common tendon of the three muscles is 

 attached to the front of the upper margin of the great tro- 

 chanter (in front of the pyriformis as explained. See Pyri- 

 formis). 



Between the tendon of the obturator internus and the 

 margin of the small sacrosciatic foramen is a large bursa ; 

 another one may exist between the tendon and the capsule 

 of the hip joint. 



Nerve Supply. By muscular branches from the sacral 

 plexus. The nerve to the obturator internus gives a branch 

 to the gemellus superior, while the gemellus inferior 

 receives its supply from the nerve to the quadratus femoris. 

 See pages 610, 612. 



Action. To rotate the thigh outward (external rotators) 

 when the limb is extended. If the thigh be flexed the 

 muscles act as abductors of it. 



The femur being the fixed point, the opposite side of the 

 pelvis will be carried backward in extension or flexion. 



Quadratus Femoris. Figs. 126, 127. 



Origin. From the outer lip of the tuberosity of the 

 ischium. 



Insertion. Into the linea quadrati, which extends verti- 

 cally downward from the middle of the posterior intertro- 

 chanteric line for about one and one-half inches. 



