The three heads of this muscle are named long, lateral and medial — unfortunate names, since the medial would be far better called the deep head, and the long head is in fact medial. The long head arises from the infraglenoid tubercle at the upper end of the axillary border of the scapula. The lateral head has a linear origin from the back of the humerus extending from the surgical neck to the top of the deltoid insertion. The long and lateral heads converge and fuse into a flattened tendon which lies superficially on the lower part of the muscle, and is inserted into the posterior part of the upper surface of the olecranon. The medial head arises on the medial side of the radial groove of the humerus. Thus in its upper part it lies medial to the lateral head, with radial nerve and profunda brachii vessels between them. Below the radial groove, however, the origin widens to include the whole posterior surface of the humerus and of each intermuscular septum. The fibers of the medial head that arise from the lateral intermuscular septum actually occupy a more lateral position, during contraction, than those of the lateral head. The fibers of the medial head are inserted partly into the olecranon and partly into the deep part of the flattened tendon described above. A few fibers are inserted into the posterior part of the capsule of the elbow joint to prevent its being nipped in extension of the forearm.

Nerve supply of triceps brachii:

By the radial nerve (C7 and 8), by four branches; the medial head receives two. In order from above down­wards the branches go to the long, medial, lateral and medial heads, and they arise before the radial nerve makes contact with the bone of the radial groove (see below). Fractures of the middle of the shaft of the humerus, even though they may damage the radial nerve, are not likely to cause paralysis of triceps because of the high origin of the branches.

Action of triceps brachii:

The muscle is the extensor of the elbow joint. The long head is an important factor in stabilizing the abducted shoulder joint, and it aids in extending the shoulder joint.