The triceps brachii is a wide and long muscle which lies on the posterior side of humerus. It is made up of three portions: a long, a lateral, and a medial head. All three heads are hidden by the deltoid muscle at their site of origin.
The long head of the triceps brachii arises by a wide tendon from the infraglenoid tubercle of scapula, traveling downwards in the space between the teres minor and major muscle. The lateral head originates from the posterior surface of humerus above the spiral groove of radial nerve, and from the medial and lateral intermuscular septa. The medial head arises also from the posterior surface of humerus but below the spiral groove.
As they extend downwards, all three heads meet to form a strong spindle-shaped belly, which continues stretching downwards by a strong tendon, which is inserted into the olecranon, which is a bony projection or process on the proximal end of ulna.
Action
Contraction of all heads of triceps brachii straighten and extend the forearm backwards at the elbow-joint; contraction of the long head pulls the arm backwards and adducts it. It works along with the pectoral and deltoid muscle when we do pushups on the floor; and, together with the latissimus dorsi, it contracts to help us do pullups on a bar.
Innervation
The triceps brachii muscle is innervated by branches of the radial nerve (C6-C7).
Blood Supply
It receives oxygenated blood from branches arising from the deep brachial and recurrent interosseous artery.
Below, schematic drawing of triceps brachii muscle, and the position it occupies on arm.
