OBTURATOR IXTERXUS AXD GEMELLI 357 



by a special nerve which, after passing through the lesser sacro-sciatic notch, is 

 distrilHited to the niusclc, entering its inner surface near its upper border and close 

 to the lesser sacro-sciatic foramen. 



The gemellus superior — named from gemellus = a twin, because it is the upper 

 of the two twin muscles wliich nearly surround the tendon of the obturator internus 

 at its point of emergence from the pelvis — is a somewhat triangular sheet curved 

 upon itself to embrace the rounded tendon of the obturator internus. 



Origin. — (1) The outer surface of the spine of the ischium; (2) the upper half 

 of the outer edge of the lesser sacro-sciatic notch. 



Insertion. — The upper boixler and the anterior surface of the obturator internus 

 tendon, a sliort distance from its insertion. 



Structure. — Arising fleshy from the ])one at the margin of the lesser sacro- 

 sciatic foramen, the muscular filjres converge slightly and form a sheet which is 

 curved upon itself so as to fit round the upper border and anterior aspect of the 

 ol)turator internus tendon, with which, after a course of about two inches (5 cm. ) 

 in length, it blends. 



Nerve-supply. — The first and second nerves of the sacral plexus by a small 

 special In-anch which enters the muscle at the upper part of its anterior surface near 

 its origin. 



The gemellus inferior — the lower of these twin muscles — is also triangular in 

 sha]>e. but is somewhat broader and stouter than its fellow. 



Origin. — ( 1) The upper part of the inner border of the tuber ischii; and (2) 

 the lower half of the outer edge of the lesser sacro-sciatic notch. 



Insertion. — The lower border and anterior surface of the tendon of the 

 oljturator internus, a short distance from its attachment to the greater trochanter. 



Structure. — Its fibres, arising fleshy from the lower half of the outer border of 

 the lesser sacro-sciatic notch, converge and form a sheet which wraps round the 

 lower part of the anterior surface of the tendon of the obturator internus. The two 

 gemelli therefore together form an envelope which emljraces the whole of the tendon 

 of the ol)turator internus after its emergence from the j^elvis, Avith the exception of 

 a part of its posterior surface. 



Nerve-supply. — From the sacral plexus (through the fifth lumbar and first 

 sacral nerves), by filaments from the special nerve to this muscle and the quadratus 

 femoris, which enter the upper part of its anterior surface near its origin. 



Action. — The obturator internus with its two satellites, the gemelli, powerfully' 

 rotates the femur outwards. It should be observed that, although the fibres are 

 mostly directed backwards and downwards within the pelvis, the action of the 

 nniscle is really determined by the outward and slightly forward direction of the 

 tendon outside tlie pelvis; and the notch upon the^ bone plan's the part of a pulley 

 in changing the direction of the force. 



Besides its action as an external rotator, the muscle will l)e al tie to assist abduc- 

 tion when tlie thigh is bent through a right angle. 



Relations. — The obturator internus in the pelvis is in contact externally with 

 the innominate bone and obturator membrane; above, with the obturator artery 

 and nerve; internally, with the pelvic and obturator fascia?, the levator ani, the 

 pelvic viscera, and the pudic vessels and nerve. 



Outside the pelvis, the tendon of the obturator internus. with the gemelli, is in 

 contact, in front with the capsule of the hip-joint and the tendon of the oV)turator 

 externus; behind, with the gluteus maximus, great sacro-sciatic ligament, the sciatic 

 vessels and nen'es; above, with the pyriformis and the structures Avhich come out 

 of tlie pelvis lielow it; below, with the quadratus femoris and a branch of the 

 internal circumflex artery. 



Variations. — One or other of the gemelli may be absent, but more frequently the upper one. 

 An accessory slip to the obturator internus has been obser\'ed coming from the third jiiece of the 

 sacmm. 



