GLUTEUS MINIMUS 461 



special fasciculus, the ischio-femoralis, may arise from the tuberosity of the ischium and become 

 inserted into the lower border of the muscle near the great trochanter. The sacral, ischial, or 

 coccygeal origin may be lacking, or the origin of the muscle may be from the sacrum only. 



II. Second Layer 



The muscles of this layer are the gluteus medius and the piriformis. 



The gluteus medius (fig. 408). — Origin. — From (1) the ventral three-fourths of the iliac 

 crest, and the outer surface of the ihum between the anterior and posterior gluteal lines and (2) 

 the investing fascia. 



Structure and insertion. — The fibre-bundles converge upon both surfaces of a broad tendon 

 nearly to its insertion on an oblong impression on the postero-superior angle and the external 

 surface of the great trochanter. The more posterior fibre-bundles of the superficial stratum 

 of the ventral portion of the muscle cross obliquely those of the deeper dorsal portion near 

 the tendon of insertion. From the tendon an aponeurotic extension is usually continued into 

 the tendon of the vastus laterahs. 



Nerve-supply. — From the superior gluteal nerve a branch passes to the dorsal portion of the 

 muscle and one or more twigs of the branch to the tensor fasciae latse enter the ventral portion of 

 the muscle. The branches enter the middle third of the muscle between its tendons of origin 

 and insertion. The nerve-fibres arise usually from the fourth and fifth lumbar and first sacral 

 nerves. The branch to the dorsal portion of the muscle has a lower spinal origin than 

 those to the ventral portion. 



Action. — To abduct the thigh. The anterior portion of the muscle is a flexor and a medial 

 rotator, the posterior a lateral rotator and an extensor. When the muscle acts as a whole, it is 

 a medial rotator. 



Relations. — Upon the muscle he the tensor fasciae latse and gluteus maximus muscles and the 

 fascia lata; beneath it he the gluteus minimus muscle, the superior gluteal nerve and vessels, 

 and the great trochanter. 



Variations. — It may be divided into two distinct portions, or it may be fused with the 

 piriformis or the gluteus minimus or both. A special fasciculus may extend to the superior 

 portion of the great trochanter. 



The piriformis (fig. 408). — Origin. — From (1) the lateral part of the ventral surface of the 

 second, third, and fourth sacral vertebrae; (2) the posterior border of the great sciatic notch; 

 and (3) the deep surface of the sacro-tuberous (great sacro-sciatic) hgament near the sacrum. 



Structure and insertion. — The fibre-bundles converge upon a tendon which is inserted upon 

 the anterior and inner portion of the upper border of the great trochanter. The insertion of 

 fibre-bundles continues nearly to the great trochanter. An accessory shp of insertion may pass 

 to the gluteus minimus. 



Nerve-supply. — From a nerve which arises either directly from the first or second sacral 

 nerve or from a loop between them. The nerve enters the deep surface of the muscle in its 

 middle third. There may be two or more nerves. 



Action. — It is an extensor, abductor, and lateral rotator of the thigh. It causes medial rota- 

 tion when the hip is flexed. 



Relations. — Its ventral surface faces the sacral plexus, the rectum, and the hip-joint. It is 

 covered dorsally by the gluteus maximus. It hes between the gluteus medius and the superior 

 gemellus. Between the piriformis and the superior gemellus the sciatic nerve usually passes 

 into the thigh. The superior gluteal nerve and vessels pass dorsally above its superior margin; 

 the inferior nerve and vessels beneath its inferior margin. 



Variations. — It is rarely absent. The origin may extend to the first sacral or to the fifth 

 sacral vertebra and the coccyx. It may be fused with the gluteus medius or minimus or more 

 rarely with the superior gemellus. Its tendon of insertion may be fused with that of the gluteus 

 medius or the obturator internus. In about 20 per cent, of bodies it is divided partly or com- 

 pletely into two portions, between which the sciatic nerve or its peroneal (external popliteal) 

 division usually passes. Rarely the tibial instead of the peroneal portion may pass between the 

 two fasciculi, or the muscle may be divided into three or more fascicuh, between which the 

 branches of the sciatic nerve pass. 



III. Third Layer 



The gluteus minimus (fig. 409). — Origin. — From the outer surface of the ihum between 

 the anterior and inferior gluteal lines; (2) froni the septum between it and the gluteus medius 

 near the anterior superior iliac spine; and (3) from the capsule of the hip-joint. 



Structure and insertion. — The fibre-bundles converge upon a tendon which appears on the 

 middle of the ventral border and gradually spreads over the lateral surface. The muscle is 

 thickest in front, where it is usually bound by an intermuscular septum to the gluteus medius. 

 The tendon is inserted into the ventral surface of the great trochanter of the femur. 



Nerve-supply. — From twigs of the branch of the superior gluteal nerve which goes to the 

 tensor fasciae latae. These twigs enter the middle third of the muscle as the tensor branch paase.'s 

 across it. 



Action. — To abduct the thigh and rotate it medialward. The anterior part of the muscle 

 is a flexor, the posterior an extensor. . 



Relations. — It is covered by the gluteus medius and piriformis muscles. Beneath it he the 

 inferior part of the ihac ala, the hip-joint (to the capsular hgament of which it is bound), and the 

 direct tendon of the rectus femoris muscle. 



Variations. — It may be fused with the gluteus medius or the piriformis. It may send a 

 slip to the fascia lata or the vastus laterahs. It may be divided into two distinct divisions, 



