OBTURATOR INTERNUS 463 



2. ISCHIO-PUBO-FEMORAL MUSCULATURE OF THE HiP 



The muscles belonging to this group (the obturator internus, the two gemelli, 

 the quadrat us f em oris and the obturator externus, extend from the pubis and 

 ischium across the back of the hip-joint to the great trochanter and the neigh- 

 bouring part of the shaft of the femur. They are powerful lateral rotators of 

 the thigh. The obturator internus (fig. 409), a large, flat, triangular muscle, 

 arises from the pelvic surface of the innominate bone and from the obturator 

 membrane. At the lesser sciatic notch its tendon is joined by the two gemelli 

 (fig. 408), one of which arises on each side from the bony projections which make 

 the notch, and the combined tendon is inserted into the trochanteric (digital) 

 fossa. The quadratus femoris (fig. 408) passes from the tuber of the ischium to 

 the femur behind and below the great trochanter. These muscles are supplied 

 by special nerves which arise from the front of the sacral plexus and enter the 

 deep surfaces of the muscles. A fifth muscle, attached to the greater trochanter 

 and associated with this group, the obturator externus, is differentiated near the 

 adductor muscles of the thigh and is supplied by a branch from the obturator 

 nerve. It arises from the outer surface of the bones bounding the ventral two- 

 thirds of the obturator foramen and is inserted by a tendon into the trochanteric 

 (digital) fossa. 



These muscles seem to have no certain representatives in the arm, where the shoulder-joint 

 is entirely ensheathed by the dorsal musculature. It is possible that the pectoral group has a 

 corresponding embryonic origin. The group is represented, with marked variations, in the lower 

 extremities of amphibia and all higher vertebrates. 



FASCI.E 



Within the pelvis the obturator internus lies on the obturator membrane. It is covered 

 by the obturator fascia, which 's attached to the body of the pubis, to the ihac portion of the 

 arcuate line, to the ventral margin of the great sciatic notch, to the ischial spine, to the sacro- 

 tuberous (great sacro-sciatic) ligament, and with the falciform process of that ligament, to the 

 ischial and pubic rami. Near the upper part of the obturator foramen the fascia instead of being 

 attached to bone is reflected over the muscle and attached to the obturator membrane. It here 

 helps to bound the canal for the obtm-ator vessels and nerve. The upper part of the fascia lies 

 beneath the pelvic peritoneum and the levator ani. The lower part forms the outer boundary of 

 the ischio-rectal fossa. The fascia is continued as a thin, adherent membrane over the obturator 

 internus and the gemellus muscles to their attachment. The quadratus femoris is invested by 

 a thin adherent fascial sheet. 



MUSCLES 



The obturator internus (fig. 409). — Origin. — From (1) the pelvic surface of the pubic rami 

 near the obturator foramen; (2) the pelvic surface of the ischium between the foramen and the 

 great sciatic notch; (3) the deep surface of the obturator internus fascia; (4) the fibrous arch 

 which bounds the canal for the obturator vessels and nerve; and (5) the pelvic surface of the 

 obturator membrane except in the lower part. 



Structure and insertion. — From this extensive area of origin the fibre-bundles converge 

 toward the lesser sciatic notch and become applied to the broad tendon of insertion. At the 

 notch the muscle curves laterally and extends outward and upward to its insertion into the fore 

 part of the trochanteric fossa of the femur. The tendon is formed of five or six bands which 

 begin high in the muscle and converge into a common tendon situated on the deep surface 

 of the muscle as the latter curves about the ischium. The tendon bands at first throw the ten- 

 don into folds which run in ridges in the fibro-cartilage which lines the notch. The attachment 

 of fibre-bundles continues upon the dorsal surface of the tendon to half way between the lesser 

 sciatic notch and the great trochanter. 



Nerve-supply. — A special nerve to the obtiirator internus arises from the front of the sacral 

 plexus, usually from the lumbo-sacral cord and the first and second sacral nerves. This nerve 

 passes lateral to the sacro-spinous (lesser sciatic) Ugament, then re-enters the pelvis through 

 the lesser sciatic notch and sends out branches of distribution on the pelvic surface of the obtu- 

 rator internus. 



Action. — This muscle with its two companions, the gemelli, is a powerful lateral rotator 

 of the thigh. It is also an extensor and abductor when ttie thigh is bent at a right angle. 



Relations. — The chief pelvic relations have been described in connection with the obturator 

 fascia which completely covers the medial surface of the muscle. The muscle passes out be- 

 tween the two sacro-ischial (sacro-sciatic) ligaments. Outside the pelvis the geniellus muscles 

 run on each side of the tendon, which is here closely applied to the capsule of the joint. Dorsal 

 to it lie the gluteus maximus, the sacro-tuberous (great sacro-sciatic) ligament, the inferior 

 gluteal (sciatic) vessels, and the sciatic and posterior cutaneous nerves. The nerve of the quad- 

 ratus femoris runs beneath the obturator internus and gemellus muscles. 



Variations. — It varies in the extent of its insertions. It may be divided into two parts, 



