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pectoralis major (anatomy)
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On each side of the thorax, pectoralis major is an important muscular link to the upper arm. It is thick, triangular in shape and forms the anterior wall of the axilla. In the female, it is covered by the breast which is attached to the pectoral fascia.

It originates and inserts as two main groups of fibres which usually are separated by a groove:

  • clavicular head: from the medial half of the anterior surface of the clavicle. Run horizontally to insert as the anterior lamina of the tendon into the lateral lip of the intertubercular groove of the humerus, anterior edge of the deltoid tuberosity and deep fascia of arm.
  • sternocostal head, which is larger:
    • originates from a number of sites:
      • anterior surface of manubrium and body of sternum
      • costal cartilages of upper five or six ribs
      • the sixth rib
      • aponeurosis of abdominal external oblique muscle
    • inserts as a bilaminar tendon deep to the insertion of the clavicular head's fibres:
    • manubrial fibres insert separately and more deeply into the lateral lip of the intertubercular groove
      • lower sternal and costal fibres run a lateral and superior course to form the deepest tendon; this spirals 180 degrees near its insertion so that the most superior sternal fibres insert into the inferolateral margin of the intertubercular groove and the most inferior sternal fibres invert upwards as a fold that is continuous with the shoulder joint capsule

Pectoralis major is innervated by the medial (C8, T1) and lateral (C5, C6, C7) pectoral nerves. Its key actions include medial rotation and adduction of the arm. Its accessory functions and testing are considered in the submenu.

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