_Origin_ by fleshy fibres from the median lip of the obturator foramen and from both dorsal and ventral surfaces of the rami of the pubis and ischium adjacent to the lip. Also from the outer surface of the ramus of the ischium as far as the area for the quadratus femoris (Fig. 90, _f_). The fibres converge to a strong flat tendon. [Illustration: FIG. 90.--MUSCLES ON THE LATERAL SIDE OF THE LEG, AFTER REMOVAL OF THE MUSCLES SHOWN IN FIG. 68 (BICEPS, TENSOR FASCIÆ LATÆ, CAUDOFEMORALIS, AND GLUTEUS MAXIMUS). _a_, M. sartorius; _b_, M. gluteus medius; _c_, M. rectus femoris; _d_, M. vastus lateralis; _e_, M. obturator internus; _f_, M. quadratus femoris; _g_, M. tenuissimus; _h_, M. adductor femoris; _i_, M. semimembranosus; _j_, M. semitendinosus; _k_, M. vastus intermedius; _l_, M. plantaris; _m, m′, m″_, M. gastrocnemius, outer head (_m_, part from the external sesamoid bone; _m′_, from the plantaris; _m″_, from the superficial fascia); _n_, M. tibialis anterior; _o_, M. soleus; _p_, M. extensor longus digitorum; _q_, M. peroneus longus; _r_, M. peroneus tertius (_r′_, its tendon); _s_, M. peroneus brevis; _t_, tendon of Achilles; _u_, M. extensor brevis digitorum. 1, great trochanter of femur; 2, tuberosity of ischium, with cut origin of M. biceps femoris; 3, patella; 4, calcaneus; 5, transverse ligaments; 6, lateral malleolus.] _Insertion_ into the proximal portion of the bottom of the trochanteric fossa. _Relations._--Ventral surface with the adductor femoris (Fig. 92, _g_). Dorsal surface with the pubis and ischium. Caudal border with the quadratus femoris (Fig. 90, _f_). _Action._--Assists the iliopsoas. B. MUSCLES ON THE MEDIAL SURFACE OF THE HIP.--=M. obturator internus= (Fig. 90, _e_).--The obturator internus appears as a triangular muscle caudad of the gemellus superior. _Origin_ by numerous small separate heads from the dorsal surface of the ramus of the ischium along its symphysis and following its medial border from the symphysis nearly to the tuberosity. The fibres form a flat muscle which narrows and passes through the lesser sciatic notch, turns ventrad and ends in a strong flat tendon whose _Insertion_ is into the bottom of the trochanteric fossa of the femur. The tendon is continued proximad on the inner surface of the muscle and forms a smooth firm surface by which the muscle glides over the dorsal border of the ischium. Into the inner surface of this tendon near its insertion the tendon of the gemellus inferior is inserted. _Relations._--Medial surface within the pelvis with a mass of fat separating it from the pelvic organs. Dorsal or outer surface with the biceps femoris, the tenuissimus, the caudofemoralis, and the great sciatic nerve. Inner surface with the ischium and the great sciatic nerve. Caudal (or ventral) border with the quadratus femoris. _Action._--Abductor of the thigh.