464 THE MUSCULATURE 



a pubic and an ischial. Fasciculi may be sent to the postero-inferior part of the ilio-pectineal 

 eminence, the tendon of the psoas minor, the tuber ischii, the sacro-tuberous (great sacro-sciatic) 

 ligament, the ischial spine, etc. 



The gemellus superior (fig. 408). — Origin. — From the outer surface of the ischial spine 

 and the neighbouring edge of the lesser sciatic notch. 



Structure and insertion. — The fibre-bundles encircle the upper border and ventral aspect of 

 the tendon of the obturator internus. They are inserted into the upper border of this tendon, 

 and sometimes also into the trochanteric fossa. 



Nerve-supply. — From a small nerve which arises either directly from the plexus or as a 

 branch of the nerve to the obturator internus or of that to the quadratus femoris. This nerve 

 usually enters the deep surface of the muscle near the junction of its ischial and middle thirds. 



Action. — It is essentially a part of the obturator internus. 



Relations. — It lies between the piriformis and the tendon of the obturator internus. Proxi- 

 maUy it adjoins its fellow beneath this tendon; distally, the two gemeUi enclose the tendon in a 

 musculo-tendinous sheath. 



Variations. — It may be wanting or may have a more extensive origin than usual. It 

 may be joined to the piriformis or to the gluteus minimus or be joined more closely than usual 

 to the obturator tendon. 



The gemellus inferior. — Origin. — From the upper part of the inner border of the tuberosity 

 of the ischium, the sacro-tuberous (great sacro-sciatic) ligament and from the neighbouring edge 

 of the lesser sciatic notch. 



Structure and insertion. — The fibre-bundles converge upon the inferior border of the tendon 

 of the obturator internus, and are inserted by tendon-fibres into this or into the great trochanter 

 below the obturator internus tendon. 



Nerve-supply. — From a branch of the nerve to the quadratus femoris. This branch enters 

 the deep surface of the muscle near the junction of the ischial with the middle third. 



Action. — It is essentially a part of the obturator internus. 



Relations. — It lies between the quadratus femoris and the tendon of the obturator internus. 



Variations. — It is rarely absent. It may be joined to the quadratus femoris. It is fre- 

 quently closely bound up with the obturator internus. It may be doubled. 



The quadratus femoris (fig. 4S)%).— Origin. — From the upper part of the outer border of the 

 tuber of the ischium. 



Structure and insertion. — The fibre-bundles take a nearly parallel course and are inserted 

 into the vertical ridge which terminates above on the inferior dorsal angle of the great trochanter. 



Nerve-supply. — From a nerve which arises usually from the lumbo-sacral cord and the first 

 sacral nerve and passes under the gemelli and the tendon of the obturator internus. The nerve 

 enters the deep surface of the muscle near the junction of the ischial and middle thirds. 



Action. — It is a powerful lateral rotator and a weak adductor of the thigh. 



Relations.— li is covered by the gluteus maximus. Between this muscle and the quadratus 

 femoris runs the sciatic nerve. The obturator externus muscle lies in front. The inferior 

 gemellus extends along its superior border. The adductor minimus adjoins it distally. 



Variations.— it is absent in from 1 to 2 per cent, of instances. (Schwalbe and Pfitzner.) 

 It may be double near its femoral insertion. It may be fused with the inferior gemellus or the 

 adductor magnus. It may send a fasciculus to the semimembranosus. 



The obturator externus (figs. 407, 409). — Origin. — From the lateral surface of the pubic 

 and ischial rami, where they bound the obturator foramen, and from the surface of the obtu- 

 rator membrane. 



Structure and insertion. — Often the muscle is distally divided into three fasciculi, a superior 

 from the superior pubic ramus, a middle from the inferior pubic ramus and the obturator mem- 

 brane, and an inferior from the ischium. The fibre-bundles converge upon a tendon which is at 

 first deeply buried, then appears on the lateral surface of the muscle and is continued as a 

 rounded tendon over the capsule of the joint to its insertion into the dorsal part of the trochan- 

 teric fossa. 



Nerve-supply. — The obturator nerve gives rise, usually in the obturator canal, to a branch 

 which bifurcates to enter the superior border, and ventral surface of the muscle in its middle 

 third. 



Action. — It is a powerful lateral rotator of the thigh and is also a weak adductor. 



Relations. — It is covered by the pectineus, the ilio-psoas, and the adductor magnus muscles 

 in front, and by the quadratus femoris behind near its insertion. It covers over the obturator 

 membrane. The obturator nerve passes either above the muscle or through its upper portion. 



Variations. — The reported variations are few. It may be joined by a slip from the ad- 

 ductor brevis. 



BuRSiE 



B. m. obturatoris interni. — A fairly large bursa constantly present between the tendon of 

 the obturator internus muscle and the les.scr sciatic notch. It may extend on each side be- 

 neath the gemellus muscles. B. m. quadrati femoris. — A small bursa frequently found between 

 this muscle and the small trochanter. B. m. obturatoris externi. — A bursa is sometimundes of 

 between the tendon of this muscle and the capsule of the joint. 



B. MUSCLES OF THE THIGH 



In the thigh three groups of muscles may be recognised, an anterior or ex- 

 ten.sor (figs. 411, 412), a nuHliai or adductor (figs. 409, 411, 412), and a posterior, 

 flexor or hamstring group (figs. 408, 413). 



