248 THE MUSCLES OF THE LOWER LIMB. 



The pyriformis muscle arises within the pelvis by three fleshy digitations from 

 the second, third and fourth pieces of the sacrum between and outside the anterior 

 sacral foramina, slightly from the hinder border of the ilium immediately below the 

 posterior inferior spine, and from the great sacro-sciatic ligament. The muscle 

 passes out of the pelvis by the great sacro-sciatic foramen, and is inserted into a 

 mark on the upper border of the great trochanter by a rounded tendon which is 

 closely united for some distance before its insertion to the subjacent tendon of the 

 obturator internus and gemelli muscles. 



Relations. The anterior surface of the pyriformis is in contact, inside the pelvis, with 

 the nerves of the sacral plexus, a thin portion of the pelvic fascia, the anterior branches of the 

 internal iliac vessels, and the rectum ; while in the gluteal region it lies over the ischium and 

 the hinder part of the glutens minimus, by which it is usually separated from the hip- joint. 

 Its posterior surface is covered by the sacrum and the gluteus maximus ; and the tendon at its- 

 insertion is beneath the gluteus medius, there being 1 occasionally a bursa between the two. 

 Between the upper border of the muscle as it escapes from the pelvis and the bone the gluteal 

 vessels and superior gluteal nerve issue ; and at its lower border, between it and the superior 

 gemellus, the sciatic and pudic vessels and nerves make their appearance. 



Varieties. This muscle is frequently pierced by the external popliteal nerve, and is thus 

 divided more or less completely into two parts. It may be united with the gluteus medius, 

 or give fibres to the gluteus minimus, or receive the insertion of the superior gemellus. It 

 may have only one or two sacral attachments ; or again its tendon may be inserted into the 

 capsule of the hip- joint. Its entire absence has also been noted. 



The obturator internus muscle (fig. 251, 2 ; 254, 3), in great part lodged within 

 the pelvis, arises from the deep surface of the obturator membrane except at its lower 

 part, from the fibrous arch which completes the canal for the obturator vessels and nerve, 

 and from the pelvic surface of the hip-bone, externally between the thyroid foramen 

 and the ilio-sciatic notch, reaching up to the ilio-pectineal line, and internally between 

 the foramen and the margin of the subpubic arch ; a few fibres also arise from the 

 obturator fascia, which is in contact with the deep surface of the muscle. Its fibres 

 converging as they proceed backwards from this origin, the muscle emerges from the 

 pelvis by the small sacro-sciatic foramen, turning round the trochlear surface of the 

 ischium, and is directed outwards, to be inserted, in connection with the gemelli, into 

 an impression on the fore part of the inner surface of the great trochanter. The 

 tendon occupies the surface of the muscle which is towards the bone, and consists as 

 it passes through the foramen of four or five narrow portions which, commencing 

 independently in the substance of the pelvic portion of the muscle, receive the 

 pinnately disposed fleshy fibres. A layer of cartilage lines the trochlear groove of 

 the bone, and forms a series of ridges with intervening grooves corresponding to the 

 divisions of the tendon, while the movement of the latter is facilitated by a large syno- 

 vial bursa. Another bursa, of much smaller size, elongated and narrow, is sometimes 

 placed between the tendon and the capsule of the hip-joint. These bursae are 

 occasionally continuous with one another. 



Relations. The deep surface of the pelvic portion is in contact with the obturator 

 portion of the pelvic fascia, and near its lower border with the pudic vessels and nerve. The 

 outer surface is in contact with the bone and obturator membrane. At its upper border the 

 obturator vessels and nerve pass through the thyroid foramen. The extrapelvic portion lies 

 between the gemelli and in contact with the ischium and capsule of the hip- joint. It is 

 covered by the great sacro-sciatic ligament, the sciatic vessels and nerves, and the gluteus 

 maximus. 



The gemelli (gemini) are two small narrow muscles, consisting chiefly of fleshy 

 fibres extended horizontally on each side of the tendon of the obturator internus ; 

 and they are named from their position above and below the tendon. The gemellus 

 superior, which is usually the smaller muscle, arises from the outer and lower part of the 

 ischial spine ; the gemellus inferior takes origin from the upper part of the tuberosity 



