Medical Definition of pectoralis
: either of the muscles that connect the ventral walls of the chest with the bones of the upper arm and shoulder of which in humans there are two on each side:a: a larger one that arises from the clavicle, the sternum, the cartilages of most or all of the ribs, and the aponeurosis of the external oblique muscle and is inserted by a strong flat tendon into the posterior bicipital ridge of the humerus—called also pectoralis majorb: a smaller one that lies beneath the larger, arises from the third, fourth, and fifth ribs, and is inserted by a flat tendon into the coracoid process of the scapula—called also pectoralis minor
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