478 



MYOLOGY 



by passing in front of the muscle, anc 



obturator nerve reaches the thigh 

 posterior branch by piercing it. 



Nerves. The Glutaeus maximus is supplied by the fifth lumbar and first and second sacra 

 nerves through the inferior gluteal nerve; the Glutsei medius and minimus and the Tensor fasciae 

 latae by the fourth and fifth lumbar and first sacral nerves through the superior gluteal; the Piri- 

 formis is supplied by the first and second sacral nerves; the Gemellus inferior and Quadrat us 

 femoris by the last lumbar and first sacral nerves; the Gemellus superior and Obturator intern us 

 by the first, second, and third sacral nerves, and the Obturator externus by the third and fourth 

 lumbar nerves through the obturator. 



Obturator nerve 



Ant. inf. iliac spine 



FIG. 436. The Obturator externus. 



Actions. When the Glutseus maximus takes its fixed point from the pelvis, it extends the 

 femur and brings the bent thigh into a line with the body. Taking its fixed point from below, 

 it acts upon the pelvis, supporting it and the trunk upon the head of the femur; this is especially 

 obvious in standing on one leg. Its most powerful action is to cause the body to regain the erect 

 position after stooping, by drawing the pelvis backward, being assisted in this action by the 

 Biceps femoris, Semitendinosus, and Semimembranosus. The Glutseus maximus is a tensor of 

 the fascia lata, and by its connection with the iliotibial band steadies the femur on the articular 

 surfaces of the tibia during standing, when the Extensor muscles are relaxed. The lower part 

 of the muscle also acts as an adductor and external rotator of the limb. The Glutaei medius and 

 minimus abduct the thigh, when the limb is extended, and are principally called into action in 

 supporting the body on one limb, in conjunction with the Tensor fasciae latse. Their anterior 

 fibers, by drawing the greater trochanter forward, rotate the thigh inward, in which action they 

 are also assisted by the Tensor fasciae latae. The Tensor fasciae latse is a tensor of the fascia lata; 

 continuing its action, the oblique direction of its fibers enables it to abduct the thigh and to rotate 

 it inward. In the erect posture, acting from below, it will serve to steady the pelvis upon the head 

 of the femur; and by means of the iliotibial band it steadies the condyles of the femur on the 

 articular surfaces of the tibia, and assists the Glutseus maximus in supporting the knee in the 

 extended position. The remaining muscles are powerful external rotators of the thigh. In the 

 sitting posture, when the thigh is flexed upon the pelvis, their action as rotators ceases, and they 

 become abductors, with the exception of the Obturator externus, which still rotates the femur 

 outward. 



4. The Posterior Femoral Muscles (Hamstring Muscles) (Fig. 434). 

 Biceps femoris. Semitendinosus. Semimembranosus. 



The Biceps femoris (Biceps) is situated on the posterior and lateral aspect of the 

 thigh. It has two heads of origin ; one, the long head, arises from the lower and inner 



