OF THE HIP 523 



inserted into the upper part of the linea quadrata; that is, the line which crosses 

 the posterior intertrochanteric line. 



Relations. By its superficial surface, with the Gluteus maximus and the sciatic vessels and 

 nerves; by its deep surface, with the tendon of the Obturator externus and trochanter minor 

 and with the capsule of the hip-joint; by its upper border, with the Geniellus inferior. Its lower 

 border is separated from the Adductor magnus by the terminal branches of the internal circumflex 

 vessels. A synovial bursa is often found between the under surface of this muscle and the 

 lesser trochanter, which it covers. 



Dissection. In order to expose the next muscle (the Obturator externus) it is necessary to 

 remove the Psoas, Iliacus, Pectineus, and Adductores brevis and longus muscles from the front 

 and inner side of the thigh, and the Gluteus maximus and Quadratus femoris from the back 

 part. Its dissection should, consequently, be postponed until the muscles of the anterior and 

 internal femoral regions have been explained. 



The Obturator externus (m. obturator externus) (Figs. 388 and 389) is a flat, 

 triangular muscle, which covers the outer surface of the anterior wall of the pelvis. 

 It arises from the margin of bone immediately around the inner side of the obturator 

 foramen viz., from the body and ramus of the os pubis and the ramus of the 

 i.schium; 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 from the pubic arch extend on 

 to the inner surface of the bone, from which they obtain a narrow origin between 

 the margin of the foramen and the attachment of the membrane. The fibres- 

 converging pass backward, outward, and upward, and terminate in a tendon 

 which runs across the back part of the hip-joint, and is inserted into the digital 

 fossa of the femur. 



Relations. In front, with the Psoas, Iliacus, Pectineus, Adductor magnus, and Adductor 

 brevis; and more externally, with the neck of the femur and capsule of the hip-joint. The 

 external and internal terminal branches of the obturator artery and accompanying veins lie 

 between this muscle and the obturator membrane; the superficial part of the obturator nerve 

 lies above the muscle, and the deep branch perforates it; by its deep surface, with the obturator 

 membrane and Quadratus femoris at its insertion. 



Nerves. The Gluteus maximus is supplied by the fifth lumbar and first and second sacral 

 nerves through the inferior gluteal nerve from the sacral plexus; the Gluteus medius and mini- 

 mus, by the fourth and fifth lumbar and first sacral nerves, through the superior gluteal; the 

 Pyriformis is supplied by the first and second sacral nerves; the Gemellus inferior and Quad- 

 ratus femoris by the last lumbar and first sacral nerve; the Gemellus superior and Obturator 

 interims by the fifth lumbar and first and second sacral nerves, and the Obturator externus by 

 the second, third, and fourth lumbar nerves through 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 whole trunk upon the head of the femur, which is 

 especially obvious in standing on one leg. Its most powerful actions are to hold the head of the 

 femur in close approximation to the acetabulum in walking and 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 iliotibial band it steadies the femur on tne articular 

 surface 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 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 fasciae femoris. Their 

 anterior fibres, by drawing the great trochanter forward, rotate the thigh inward, in which 

 action they are also assisted by the Tensor fasciae femoris. The remaining muscles are power- 

 ful Rotators of the thigh outward. In the sitting posture, when the thigh is flexed upon the 

 pelvis, their action as Rotators cease, and they become Abductors, with the exception of the 

 Obturator externus, which still rotates the femur outward. When the femur is fixed, the 

 Pyriforrnis 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. 



