|Derivation||From pectus, Latin for the breast.|
|Origin||Medial half of the clavicle, the lateral surface of the sternum, and the top of the abdominal aponeurosis.|
|Insertion||A ridge on the front of the humerus below the head.|
|Action||Pulls the arm forward and across the body; rotates the upper arm inwardly.|
Pectoralis is familiar as the muscle of the chest.
The form of pectoralis could be likened to an oriental folding fan. The fibers of the upper portion point downward to the lower part of the ridge on the humerus. The lower fibers point upwards and cross under the upper fibers to go to the higher part of the insertion.
The upper portion makes a downward drop towards the arm from the clavicle. This portion raises the humerus upwards, or with the arm down, curls the shoulders forward. On a muscular figure it may be apparent as a distinct triangular band.
The lower portion pulls down the arm, or pulls down the shoulder when the arm is lowered. Striations following the muscle fibers may be apparent on a muscular figure.
On the male, there is a distinct horizontal, curved break below the lower portion that appears to be the border of the muscle. However, the lowest fibers extend past this curve and attach to the abdominal aponeurosis, lying over the topmost digitation of external oblique.
On the female, the attachment of the breast covers the lower half of pectoralis and a small portion of the muscles below. (See p.0. for further discussion of the breast.)