GENERAL DESCRIPTION AND RELATIONS.--Deeply situated immediately posterior to head of femur; part extending through obturator foramen and lying inside pelvis; extrapelvic part deep to Mm. flexor ischiofemoralis and piriformis; muscle partially divisible into four parts--pars antica, pars dorsalis, pars ventralis, and pars postica (fig. 20K); _pars postica_: mostly inside pelvis; much larger than other parts; broad (narrow anteriorly); on medial surface of ischium; composed of several fascicles; anterior end forming narrow, heavy tendon (with some fleshy fibers on posterior part of deep surface) passing through obturator foramen; anteriormost fleshy fibers of ventralmost fascicle fused with pars ventralis; _pars ventralis_: essentially extrapelvic (see origin); mostly ventral to tendon of pars postica; superficial to pars antica; fused to anterior fleshy part of pars postica; anterodorsal edge usually adjacent to, and often slightly fused with, ventral edge of pars dorsalis (deep to tendon of pars postica); _pars dorsalis_: entirely extrapelvic; mostly dorsal to tendon of pars postica; superficial to dorsal part of pars antica; _pars antica_: extremely short but relatively thick; entirely fleshy; entirely extrapelvic; between obturator foramen and head of femur; anterior surface adjacent to articular capsule; almost completely covered by other parts of muscle; proximal end of posterior surface often slightly fused with adjacent parts of pars ventralis and pars dorsalis. ORIGIN.--_Pars postica_: This arises fleshily from the medial surface of the entire ischium except the posterior end, from the dorsomedial and medial surfaces of the anterior half of the pubis as far forward as the obturator foramen, from the internal ilio-ischiatic crest, from the medial surface of the ilium for a short distance posterior to this crest, and from the iliac recess; the posteroventral corner usually arises from the medial surface of the ischiopubic membrane. _Pars ventralis_: This arises fleshily from the dorsomedial edge of the ventral border of the obturator foramen (fig. 20M) and (narrowly) from the anterior border of the foramen; this part may or may not arise from the lateral surface of the anteroventral border of the foramen and is usually adjacent along the anterior border of the foramen to pars dorsalis; _pars ventralis_ is continuous along the ventral border of the foramen with the intrapelvic origin of pars postica. _Pars dorsalis_: This arises fleshily from the lateral surface of the anterodorsal border of the foramen (fig. 20L) and may extend posteriorly along the dorsal border of the foramen. _Pars antica_: This arises fleshily from the depresssed area anterior to the obturator foramen (adjacent to pars dorsalis and pars ventralis); the posteroventral corner may arise from the lateral surface of the anteroventral border of the obturator foramen (ventral to the anterior end of pars ventralis; fig. 20L).