Pectoralis major - Musculus pectoralis major

Description

Origin: Anterior surface of the medial half of the clavicle. Sternocostal head: anterior surface of the sternum, the superior six costal cartilages

Insertion: Intertubercular groove of the humerus

Artery: Pectoral branch of the thoracoacromial trunk

Nerve: Lateral pectoral nerve and medial pectoral nerve - Clavicular head: C5 and C6 - Sternocostal head: C7, C8 and T1

Action: Clavicular head: flexes the humerus Sternocostal head: extends the humerus As a whole, adducts and medially rotates the humerus. It also draws the scapula anteriorly and inferiorly.

Description:
The Pectoralis major is a thick, fan-shaped muscle, situated at the upper and forepart of the chest.

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 of the pectoralis muscle converge toward their insertion;

  • those arising from the clavicle (clavicular head) 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, (sternocostal head) run upward and laterally, while the middle fibers pass horizontally.
  • those from the sheath of the straight abdominal muscle (abdominal part) run more upward and laterrally.

They all end in a flat tendon, about 5 cm. broad, which is insertedinto the crest of the greater tubercle of the humerus. This tendon consists of two laminae, 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 Deltoideus 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.

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.
Costocoracoideus is a muscular band occasionally found arising from the ribs or aponeurosis of the External oblique between the Pectoralis major and Latissimus dorsi and inserted into the coracoid process.
Chondro-epitrochlearis is a muscular slip occasionally found arising from the costal cartilages or from the aponeurosis of the External oblique below the Pectoralis major or from the Pectoralis major itself. The insertion is variable on the inner side of the arm to fascia, intermuscular septum or internal condyle.
Sternalis, in front of the sternal end of the Pectoralis major parallel to the margin of the sternum. It is supplied by the anterior thoracic nerves and is probably a misplaced part of the pectoralis.


This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from http://www.bartleby.com/107/).

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