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The Pectoralis major (popularly known as pecs) is a thick, fan-shaped muscle, situated at the upper front (anterior) of the chest wall. It makes up the bulk of the chest muscles in the male and lies under the breast in the female. It arises from the anterior surface of the sternal half of the clavicle; from breadth of the half of the anterior surface of the sternum, as low down as the attachment of the cartilage of the sixth or seventh rib; from the cartilages of all the true ribs, with the exception, frequently, of the first or seventh and from the aponeurosis of the abdominal external oblique muscle. From this extensive origin the fibers converge toward their insertion; those arising from the clavicle pass obliquely downward and outwards (laterally), and are usually separated from the rest by a slight interval; those from the lower part of the sternum, and the cartilages of the lower true ribs, run upward and laterally, while the middle fibers pass horizontally. They all end in a flat tendon, about 5 cm in breadth, which is inserted into the bicipital groove of the humerus.
This tendon consists of two laminae, placed one in front of the other, and usually blended together below. The anterior lamina, which is thicker, receives the clavicular and the uppermost sternal fibers. They are inserted in the same order as that in which they arise: the most lateral of the clavicular fibers are inserted at the upper part of the anterior lamina; the uppermost sternal fibers pass down to the lower part of the lamina which extends as low as the tendon of the Deltoid and joins with it. The posterior lamina of the tendon receives the attachment of the greater part of the sternal portion and the deep fibers, i. e., those from the costal cartilages. These deep fibers, and particularly those from the lower costal cartilages, ascend the higher, turning backward successively behind the superficial and upper ones, so that the tendon appears to be twisted. The posterior lamina reaches higher on the humerus than the anterior one, and from it an expansion is given off which covers the intertubercular groove of the humerus and blends with the capsule of the shoulder-joint. From the deepest fibers of this lamina at its insertion an expansion is given off which lines the intertubercular groove, while from the lower border of the tendon a third expansion passes downward to the fascia of the arm. The Pectoralis Major has three actions. It flexes the humerus, as in throwing a ball side-arm, and in lifting a child. It also adducts the humerus, as when flapping arms. It also rotates the humerus medially, as occurs when arm-wrestling.
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