Biceps and triceps muscles. Left: biceps contracted. Right: triceps contracted.
Back of the humerus with muscular attachments mapped out.
A triceps is any muscle that has three points of origin. Most commonly "triceps" refers to the triceps brachii muscle – a large extensor along the back of the upper arm in humans. It originates just below the socket of the scapula (shoulder-blade) and at two distinct areas of the humerus, the bone of the upper arm. The triceps extends downward and attaches to the upper part of the ulna, in the forearm.
Detailed anatomy of the triceps brachii
The triceps brachii muscle occupies the entire posterior osteo-fascial compartment of the arm. It arises by a long head from the scapula, and by two shorter heads, lateral and medial, from the humerus. The fleshy fibers of the three heads join a common tendon which is inserted into the upper surface of the olecranon.
The superficial part of the muscle is, for the most part, formed by the long and lateral heads. The medial head is deeply placed; but a very small portion of it appears superficially, above the elbow, on each side of the common tendon of insertion.
The long head of the triceps arises, by a flattened tendon, from the infraglenoid tubercle of the scapula.
The two humeral heads originate from the back of the humerus; and if it is borne in mind that no fibers arise from the spiral groove and that the groove intervenes between the origins of the two heads, their relations will be easily understood.
The lateral head of the triceps arises from a rough strip that descends from the back of the great tuberosity to the groove – and also from the fascial sheet that bridges across the groove for the protection of the radial nerve.
The medial head of the triceps arises from the whole of the back of the humerus below the spiral groove, and from the intermuscular septa. The upper end of the origin, which is narrow and pointed, reaches the insertion of the teres major, and is seen better on the front of the limb than on the back. The origin gradually widens as the groove passes toward the lateral border of the humerus; and in its distal third it covers the whole width of the back of the humerus. The medial head of the triceps, therefore, has very much the same origin from the back of the bone that the brachialis has from the front.
The common tendon is inserted into the back part of the upper surface of the olecranon and into the fascia that covers the anconeous muscle, which lies at the lateral side of the olecranon.Some of the short, fleshy fibers of the medial head are attached directly to the olecranon, and a few of them are inserted into the posterior part of the capsule of the elbow joint. A small bursa, which lies on the top of the olecranon, separates the common tendon from the posterior ligament of the elbow joint.
The triceps is supplied by branches from the radial nerve. It is a powerful extensor of the elbow joint and, by virtue of its long head, takes part also in the movements of the shoulder joint.