OBTURATOR MUSCLES QUADRATUS. 269 



The obturator internus muscle, in great part lodged within the pelvis, 

 arises from the deep surface of the obturator membrane ; from the fibrous 

 arch which completes the canal for the obturator vessels and nerves ; from 

 a naiTow strip of the bone internal to the obturator foramen ; from the 

 posterior surface of the membrane occupying that aperture, extending on the 

 bone downwards to the outlet, backwards as far as the sciatic notch, and 

 upwards to the brim of the pelvis ; it also arises to some extent 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, turns over on 

 the trochlear surface of the ischium, and is directed outwards, to be inserted, 

 in connection with the gemelli, into the upper part of the digital fossa of 

 the great trochanter. The tendon of the muscle is formed by the union of 

 four or five bands, occupying the surface of the muscle which is towards 

 the bone, and lying side by side as they turn over the trochlear groove : 

 their motion in that groove is facilitated by a synovial bursa, which sends 

 in processes between them, and by a thin coating of cartilage on the trochlear 

 surface of the bone. Another bursa, of much smaller size, elongated and 

 narrow, is placed between the tendon and the fibrous capsule of the hip- 

 joint. These bursse are sometimes continuous with one another. 



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

 fleshy fibres extended horizontally at each side of the tendon of the obturator 

 internus ; and they are named from their position above and below that tendon. 

 The gemellus superior, which is usually the smaller muscle, arises from the 

 ischial spine ; the gemellus inferior takes origin from the upper and back 

 part of the tuberosity of the ischium. Passing outwards, they join the 

 tendon of the internal obturator muscle, which is placed between them, 

 covering and concealing it more or less, and along with it are inserted into 

 the digital fossa of the trochanter. Some of their fibres, especially from 

 the superior gemellus, run obliquely into the tendon of the obturator. 



The superior gemellus is placed immediately below the pyriformis ; the inferior 

 gemellus is above the quadratus femoris, and at its insertion is close to the tendon of 

 the obturator externus muscle. These muscles may be regarded as portions of the 

 obturator internus arising externally to the pelvis. The gemellus superior is often 

 very small, and in some cases is altogether absent. 



The quadratus femoris, of an oblong figure, arises from the external border 

 of the tuber ischii, and, proceeding horizontally outwards, is inserted into 

 the greater part of the linea quadrati on the posterior surface of the great 

 trochanter of the femur. 



Superiorly this muscle is close to the inferior gemellus. Its inferior border is in 

 contact, at its origin, with the descending fibres of the adductor magnus, and at its 

 insertion with the superior or horizontal fibres. It conceals the outer part of the 

 obturator externus, and also the lesser trochanter, which is separated from it by a 

 small bursa. 



The obturator externus, arising from the anterior two- thirds of the surface 

 of the obturator membrane, and from the outer surface of the rami of the 

 pubes and ischium as far as the margin of the thyroid foramen, encroaching 

 also a little upon the body of the pubes, and spreading towards the tuberosity 

 of the ischium, extends horizontally outwards and backwards, converging to 

 a tendon which is directed along the under and hinder surface of the neck 



