piriformis /pir′ifôr″mis/ [L, pirum + forma] , a flat pyramidal muscle lying almost parallel to the posterior margin of the gluteus medius. It is partly within the pelvis and partly at the back of the hip joint. It is innervated by branches of the first and second sacral nerves and functions to rotate the thigh laterally and to abduct and to help extend it. Compare obturator externus, obturator internus.
