|Derivation||Delta- (triangle-) shaped.|
|Origin||Lateral third of clavicle and spine of scapula, including the acromion process.|
|Insertion||Lateral surface of the humerus, about halfway down.|
|Action||Raises the arm at the shoulder.|
Deltoid is regarded as having three portions: anterior, middle, and posterior.
The anterior portion originates on the clavicle and borders pectoralis. Between the two there is usually a distinct break, particularly when the muscle is tensed. The anterior portion will sometimes have a furrow that is visible on the surface.
The middle portion originates on the acromion process and forms the visible bulk of the muscle's insertion on the arm. (The fibers of the anterior and posterior portions run in back of this point.) The lateral portion's fibers run in a spiralling, segmented manner, which sometimes produce irregular short grooves when the muscle is tensed.
The posterior portion orginates on the spine of the scapula. Because of the forward-looking disposition of the head of the humerus, the anterior portion of the muscle is prominent while the posterior portion sits flatter on the back. The breaks on either side of the posterior portion are comparatively slight.