Brachialis arises from the front of the lower two-thirds of the humerus and the medial intermuscular septum. Its upper fibers clasp the deltoid insertion; some fibers arise from the lower part of the spiral groove. The broad muscle flattens to cover the anterior part of the elbow joint and is inserted by mixed tendon and muscle fibers into the coronoid process and tuberosity of the ulna.
Nerve supply of brachialis:
By the musculocutaneous nerve (C5, 6). Some of the lateral part of the muscle is innervated by a branch of the radial nerve, an indication that it was developed in the extensor compartment of the fetal limb (p. 23).
Action of brachialis:
Principally a flexor of the elbow joint, the muscle, together with biceps, is used just as frequently as an extensor of the elbow, ‘paying out rope’ against gravity. ‘It picks up the drink and puts down the empty glass.’