THE LOWER LIMB 429 



is separated from the great trochanter by a synovial bursa, and is 

 connected with the upper part of the capsular ligament of the hip- 

 joint by a strong arched band of fibres. 



The anterior portion of the gluteus minimus is sometimes 

 detached from the rest of the muscle, and, when this occurs, 

 the separated part represents the gluteus quartus or musculus 

 scansorius (cUmbing muscle) of certain animals. 



Pyriformis — Origin. — (i) By three fleshy shps from the anterior 

 surfaces of the second, third; and fourth sacral vertebrae, which 

 are interposed between, and lie external to, the adjacent anterior 

 sacral foramina ; (2) the deep surface of the great sacro-sciatic liga- 

 ment ; and (3) the posterior border of the iliirni immediately below 

 the posterior inferior iliac spine. 



Insertion. — The upper border of the great trochanter near its 

 centre. 



Nerve-supply. — ^Two branches from the sacral plexus, which enter 

 the intrapelvic part of the muscle. The branches come from the 

 dorsal divisions of the first and second sacral nerves. 



Action. — External rotator of the thigh. 



The pyriformis, in emerging from the pelvis through the great 

 sacro-sciatic foramen, divides that foramen into a small upper and 

 a large lower compartment. Through the upper compartment the 

 gluteal vessels and superior gluteal nerv-e pass. Through the lower 

 compartment the following structures are transmitted : the sciatic 

 and internal pudic vessels, great and smaU sciatic nerves, inferior 

 gluteal nerve, pudic nerve, nervx to obturator intemus, and common 

 nerv^e to gemellus inferior and quadratus femoris. The extra- 

 pelvic part of the pyriformis is sometimes pierced by the external 

 popliteal nerve. 



Gemellus Superior — Origin. — The lower border and adjacent 

 portion of the external surface of the spine of the ischium. 



Insertion. — The upper border of the tendon of the obturator 

 intemus. 



Nerve-supply. — Branch from the nerve to the obturator intemus, 

 which enters the muscle on its anterior or deep surface near the 

 upper border, close to the origin. 



The gemellus superior is sometimes absent. 



Obturator Intemus — Origin. — (i) The internal surface of the 

 obturator membrane ; (2) the posterior smiace of the body of the 

 OS pubis, descending pubic ramus, and ascending ramus of the 

 ischium ; (3) the inclined plane of the ischium, extending as far 

 back as the great sacro-sciatic foramen, and nearly as high as the 

 iUac portion of the ilio- pectineal line ; and (4) the parietal pelvic 

 fascia covering the muscle. 



Insertion. — The inner surface of the great trochanter, above and 

 in front of the digital fossa. 



Nerve-supply. — The nerve to the obturator intemus from the 

 sacral plexus. 



Action. — External rotator of the thigh. 



