This is a long fleshy muscle partly encircling the upper arm.

Origin. - It arises from the posterior part of the head of the humerus, and winds round the outer side of the bone in the furrow of torsion.

Insertion. - After crossing the elbow-joint, it becomes inserted partly into the inner side of the head of the radius and partly also into the ulna.

Action. - To flex the fore-arm on the humerus and rotate it outwards.