Pectoralis major muscle

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Template:Muscle infobox The Pectoralis major 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 half the breadth 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, or both, and from the aponeurosis of the Obliquus externus abdominis.

From this extensive origin the fibers converge toward their insertion; those arising from the clavicle pass obliquely downward and lateralward, 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 lateralward, while the middle fibers pass horizontally.

They all end in a flat tendon, about 5 cm. broad, which is inserted into the crest of the greater tubercle of the humerus.

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Laminae

Image:Pectoralis Major Matt.JPG This tendon consists of two laminæ, placed one in front of the other, and usually blended together below.

The anterior lamina, the thicker, receives the clavicular and the uppermost sternal fibers; they are inserted in the same order as that in which they arise: that is to say, 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 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.

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Variations

The more frequent variations are greater or less extent of attachment to the ribs and sternum, varying size of the abdominal part or its absence, greater or less extent of separation of sternocostal and clavicular parts, fusion of clavicular part with deltoid, decussation in front of the sternum.

Deficiency or absence of the sternocostal part is not uncommon.

Absence of the clavicular part is less frequent.

Rarely the whole muscle is wanting and this may be associated with abscence of the breast in females.

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