Pectoralis muscle, any of the muscles that connect the front walls of the chest with the bones of the upper arm and shoulder. There are two such muscles on each side of the sternum(breastbone) in the human body: pectoralis major and pectoralis minor.
The pectoralis major, the larger and more superficial, originates at the clavicle (collarbone), the sternum, the ribs, and a tendinous extension of the external oblique abdominal muscle. The pectoralis major extends across the upper part of the chest and is attached to a ridge at the rear of the humerus (the bone of the upper arm). Its major actions are adduction, or depression, of the arm (in opposition to the action of the deltoideus muscle) and rotation of the arm forward about the axis of the body. When the raised arms are fixed (as in mountain climbing), it assists the latissimus dorsi and teres major muscles in pulling the trunk up. The pectoralis minor lies, for the most part, beneath the pectoralis major, arising from the middle ribs and inserting into (attaching to) the scapula (shoulder blade). It aids in drawing the shoulder forward and downward (in opposition to the trapezius muscle).