518 THE MUSCLES AND FASCIAE. 



The Obturator externus (Fig. 327) is a flat, triangular muscle which covers the 

 outer surface of the anterior wall of the pelvis. It arises from the margin of bone 

 which forms the inner boundary of the obturator foramen viz. from the body and 

 descending ramus of the os pubis and the ramus of the ischium ; it also arises from 

 the inner two-thirds of the outer surface of the obturator membrane, and from the 

 tendinous arch which completes the canal for the passage of the obturator vessels 

 and nerves. The fibres converging pass backward, outward, and upAvard, and 

 terminate in a tendon which runs under and across the back part of the hip- 

 joint and is inserted into the digital fossa of the femur. 



Relations. By its anterior surface, with the Psoas, Iliacus, Pectineus, 

 Adductor magnus, Adductor brevis, and Gracilis, and more externally, with the 

 neck of the femur and capsule of the hip-joint; by its posterior surface, with the 

 obturatoV membrane and Quadratus femoris. 



Nerves. The Gluteus maximus is supplied by the inferior gluteal nerve from 

 the sacral plexus ; the Gluteus medius and minimus, by the superior gluteal ; the 

 Pyriformis, Gemelli, Obturator internus, and Quadratus femoris, by branches from 

 the sacral plexus ; and the Obturator externus, by the obturator nerve. 



Actions. The Gluteus maximus, when it takes its fixed point from the pelvis, 

 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 Avhole 

 trunk upon the head of the femur, which 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, Semitendinosus, and Semimembranosus. The Gluteus maximus is a 

 tensor of the fascia lata, and by its connection with the ilio-tibial band it steadies 

 the femur on the articular surface of the tibia during standing, when the extensor 

 muscles are relaxed. The lower part of the muscle also acts as an abductor and 

 external rotator of the limb. The Gluteus 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 vaginae femoris. 

 Their anterior fibres, by drawing the great trochanter forward, rotate the 

 thigh inward, in which action they are also assisted by the Tensor vaginge 

 femoris. Their posterior fibres rotate the thigh outward. The remaining muscles 

 are powerful rotators of the thigh outward. In the sitting posture, when the thigh 

 is flexed upon the pelvis, their action as rotator ceases, and they become abductors, 

 with the exception of the Obturator externus, which still rotates the femur out- 

 ward. When the femur is fixed, the Pyriformis and Obturator muscles serve to 

 draw the pelvis forward if it has been inclined backward, and assist in steadying 

 it upon the head of the femur. 



Posterior Femoral Region. 



Biceps. Semitendinosus. Semimembranosus. 



Dissection (Fig. 328). Make a vertical incision along the middle of the thigh, from the 

 lower fold of the nates to about three inches below the back of the knee-joint, and there 

 connect it with a transverse incision carried from the inner to the outer side of the leg. Make 

 a third incision transversely at the junction of the middle with the lower third of the thigh. 

 The integument having been removed from the back of the knee and the boundaries of 

 the popliteal space examined, the removal of the integument from the remaining part of the 

 thigh should be continued, when the fascia and muscles of this region will be exposed. 



The Biceps (Biceps flexor cruris) is a large muscle, of considerable length, 

 situated on the posterior and outer aspect of the thigh (Fig. 329). It arises by 

 two heads. One, the long head, arises from the lower and inner facet on the 

 back part of the tuberosity of the ischium by a tendon common to it and the 

 Semitendinosus. The femoral, or short head, arises from the outer lip of the 

 linea aspera, between the Adductor magnus and Vastus externus, extending up 

 almost as high as the insertion of the Gluteus maximus, and from the external supra- 

 condylar line to within two inches of the outer condyle ; it also arises from the 



