QUADRATICS FEMORIS. OBTURATOR EXTERNUS. 



of the ischium, along the lower border of the groove for the obturator interims. 

 Passing outwards, they join the tendon of the internal obturator muscle, and are 

 inserted with that into the great trochanter. The two muscles usually meet at their 

 origin beneath the tendon of the obturator, while at their insertion they overlap and 

 more or less conceal it. 



Relations. The superior gemellus is placed immediately below the pyrif ormis ; 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 outside the pelvis. 



Varieties. The gemellus superior is often very small, and not unfrequently is altogether 

 absent. Absence of the gemellus inferior has also been observed, but more rarely than that 

 of the upper muscle. 



The quadratus femoris muscle, of an oblong shape, arises from an impression 

 along the outer border of the ischial tuberosity, and proceeding horizontally outwards, 

 is inserted into the tubercle of the quadratus and the back of the femur immediately 

 below this, reaching to the level of the small trochanter. 



Relations. The upper border of this muscle is in contact with the inferior gemellus ; the 

 lower with the adductor magnus, the transverse branch of the internal circumflex artery 

 passing between the two. It conceals the outer part of the obturator externus, and also the 

 lesser trochanter, which is separated from it by a small bursa. 



Varieties. The quadratus femoris may be entirely absent and replaced by an enlarged 

 gemellus inferior. 



The obturator externus muscle (fig. 248, 7 ; 251, 3) arises from the outer 

 surface of the obturator membrane for the inner half of its extent, from the femoral 

 surface of the body of the pubis, and from the conjoined rami of the pubis and 

 ischium. The fibres converge as they are directed outwards in the groove between 

 the acetabulum and the tuberosity of the ischium to the lower part of the hip-joint ; 

 then winding backwards and upwards, closely applied to the lower and hinder surfaces 

 of the neck of the femur, they end in a tendon which is inserted into the bottom 

 of the digital fossa of the great trochanter. 



Relations. The obturator externus is concealed in front by the pectineus, ilio-psoas, 

 adductor brevis and adductor magnus muscles ; near its insertion it is covered behind by the 

 quadratus femoris : its deep surface is closely connected to the capsule of the hip as it passes 

 backwards. The obturator vessels are placed between the muscle and the obturator membrane, 

 and the superficial part of the obturator nerve passes above it, while the deep part perforates 

 it near its upper border. 



Nerves. The ilio-psoas is supplied by the second and third lumbar nerves, the branches 

 for the psoas arising from the nerves as they lie in the substance of the muscle, while those 

 for the iliacus are given off from the anterior crural nerve. The gluteus maximus receives the 

 inferior gluteal nerve (51, 1, 2s) ; the gluteus medius and minimus and the tensor vaginae 

 femoris are supplied by the superior gluteal (4, 5 1, 1 s). The nerves to the pyrif ormis (2 s), 

 to the obturator internus and superior gemellus (1,2s), and to the quadratus femoris and 

 inferior gemellus (5 1, 1 s) spring from the sacral plexus ; and the obturator externus is 

 supplied by the deep part of the obturator nerve (2, 3, 4 1). 



Actions. The ilio-jjsoas muscle bends the thigh on the body, or the body on the thigh, 

 according as either of these is the more fixed. The psoas also assists in bending the lumbar 

 portion of the spine, either forwards or laterally. 



The gluteus maximus muscle is the chief extensor of the hip-joint. By its agency the bent 

 thigh is brought into a line with the body. The lower part of the muscle also acts as an 

 adductor and external rotator of the limb, while the upper part will assist in producing 

 abduction. Its most powerful action, and that in connection with which it is so largely 

 developed in the human subject, is to extend the trunk upon the thigh when bent forwards. 

 It also comes into operation in ascending an incline or a stair, in leaping, and in rising from 

 the sitting posture. But although the full contraction of the glutei maximi is required to 

 bring the body into the erect posture, it is not necessary for its maintenance when complete, 

 the trunk being then supported on the thigh-bones by the tension of ligaments, while the 

 body is so poised that its centre of gravity is placed behind the vertical plane passing through 

 the middle of the hip- joints (see p. 183). By means of its insertion into the ilio-tibial band of 



