|Derivation||Tri, three; ceps, head; the three-headed muscle.|
|Origin||The medial and lateral heads attach to the posterior surface of the humerus. The long head attaches to lower border of the scapula, close to the arm socket.|
|Insertion||All three heads attach to the triceps tendon, which inserts into the base of the ulna on its posterior side. This point is called the olecranon (oh-LECK-ruh-non).|
|Action||Extends arm at elbow.|
Triceps comprises all of the bulk of the upper arm in back. Its three heads and tendon are distinct enough to be treated as individual masses.
The largest mass is that of the long head. Emerging from between deltoid and teres major as it leaves the scapula, it comes down the medial side of the arm and attaches to the the triceps tendon. Its lowest point is about three-quarters of the way down the humerus.
Next largest (as it appears from the surface) is the lateral head. The bulk of the lateral head lies on the upper half of the humerus under the deltoid, although a slender edge contiunues downward along the tendon. Its lowest point is roughly halfway between the lowest point of the long head (on the other side) and the elbow.
The breaks between the tendon and the surrounding muscles are sometimes quite clear. In any case, the tendon forms a prominent cylinder on the lower half of the back of the arm, sandwiched between the long and lateral heads of the triceps.
Below the long head and beneath the tendon lies the medial head. The medial head repeats the curve of the long head and inserts right into the olecranon, rounding out the shape of the lower inner arm.
The medial head is a prominent mass on the inside of the arm. Considering again the groove between the biceps and triceps: just as coracobrachialis seems to extend this groove into the armpit, the medial head seems to extend it the other way, down and around to the medial epicondyle of the humerus. On the other side of this groove is brachialis.