630 HUMAN ANATOMY. 



smooth surface of bone immediately behind the foramen, corresponding to the 

 acetabulum externally, and (3) from the whole of the inner surface of the obturator 

 membrane. Its fibres, passing downward and backward, converge to a strong 

 tendon, which gains the lesser sacro-sciatic foramen, and there, bending around the 

 margin of the foramen, a bursa (bursa m. obturatoris interni ) intervening between 

 the tendon and the bone, passes outward through the foramen to be inserted into 

 a facet on the inner surface of the greater trochanter of the femur just above the 

 digital fossa. 



Nerve-Supply. — By a special nerve from the tirst, second, and third sacral 

 nerves. 



Action. — To rotate the thigh outward. 



• II. Gemelli (Fig. 602). 



Attachments. — The gemelli are two slender muscles which lie one on either 

 side of the tendon of the obturator internus. The gemellus S2ipe7'ior arises from the 

 spine of the ischium and the gemellus inferior from the upper part of the tuber ischii. 

 Both muscles are inserted into the inner surface of the greater trochanter of the femur 

 along with the obturator internus. 



Nerve-Supply. — The superior gemellus by the nerve to the internal obturator 

 from the fifth lumbar and first and second sacral nerves ; the inferior by the nerve to 

 the quadratus femoris from the fourth and fifth lumbar and the first sacral nerves. 



Action. — To assist in rotating the thigh outward. 



Variations. — One or other of the gemelli, usually the superior, is occasionally wanting. 

 This is very probably due to fusion with adjacent muscles, the gemellus superior with the 

 pyriformis and the inferior with the quadratus femoris. 



{b) THE POST-AXIAL MUSCLES. 



1. Gluteus maximus. 3. Gluteus medius. 



2. Tensor fasciae latae. 4. Gluteus minimus. 



I. Gluteus Maximus (Figs. 604, 607). 



Attachments. — The gluteus maximus is an exceedingly thick, coarse muscle 

 which forms the principal mass of the buttock. It arises from the lateral surface of 

 the posterior portion of the ilium, behind the superior gluteal line, from the pos- 

 terior surface of the sacrum and coccyx, and from the posterior sacro-iliac and greater 

 sacro-sciatic ligaments. The fibres pass laterally and downward, the upper ones 

 curving over the lateral surface of the greater trochanter of the femur and the lower 

 ones over the tuberosity of the ischium, and are inserted by a broad tendon partly 

 into the ilio-tibial band of the fascia lata and partly into the gluteal tuberosity of the 

 femur. 



Nerve-Supply. — By the inferior gluteal nerve from the fifth lumbar and first 

 and second sacral nerves. 



Action. — To draw the thigh backward and rotate it slightly outward. Acting 

 from below, it extends the trunk. 



Relations. — -The gluteus maximus is covered by the upper posterior portion 

 of the fascia lata. It covers the gluteus medius, pyriformis, obturator internus, 

 gemelli, quadratus femoris and the origin of the hamstring muscles, and also the 

 gluteal, sciatic, and pudic vessels and nerves. 



It is separated from the lateral surface of the trochanter major by a large bursa 

 (bursa trochanterica m. glutaei maximi), two or three additional small bursae (bursae 

 glutacofeniorales) separating the lower portion of the muscle from the shaft of the 

 femur. A bursa is also frequently present beneath the muscle where it passes over 

 the ischial tuberosity (bursa ischiadica 111. jjlutaei maximi). 



Variations. — The lower border of the gluteus maximus is occasionally separated from the 

 rest of the muscle, forming wliat may be termed the coccy,s:eo-femoraIis, and it occasionally 

 receives a slip from the ischial tuberosity, which has been named the ischio-femoralis. 



