Situated on the inner aspect of the arm. It is broad above, and becomes narrower as it descends. In front it presents a thick fleshy mass, while its posterior part is flat and thin. This muscle forms the prominence so conspicuous in front of the breast.

Origin. - From the lower border of the sternum or breast-bone, where its fibres are interwoven with its fellow on the opposite side.

Insertion.-Into the ridge in front of the humerus bounding the musculo-spiral groove, and into the fascia covering the inner surface of the arm.

Action. - To adduct or draw the limb inward towards the trunk, and to tighten the fascia and brace the muscles of the arm.

By some anatomists this muscle is divided into two parts, termed the anterior and posterior superficial pectoral.