Pectoralis major is a thick fan-shaped muscle, which is situated at the anterior part of chest. It makes the bulk of the muscular part of chest in males.
Origin and insertion of pectoralis major:
From a wide sternoclavicular origin the muscle converges on the upper humerus, folding on itself where it forms the anterior axillary wall to become attached to the humerus by means of a trilaminar tendon. A clavicular head arises from the medial half of the clavicle over a smooth flattened area which is easily seen on most clavicles. Running almost horizontally laterally the fibers of this head lie in a groove on the manubrial part of the muscle, from which they are quite separate. They are inserted by the anterior lamina of the tendon into the lateral lip of the intertubercular (bicipital) groove of the humerus, into the anterior lip of the deltoid tuberosity and, below that, into the deep fascia of the arm.
The sternocostal head arises from the lateral part of the anterior surface of the manubrium and body of sternum, and from the aponeurosis of the external oblique muscle over the upper attachment of rectus abdominis. On the deep surface of this sheet of muscle, and in continuity with it, fibers arise by a series of slips from the upper six costal cartilages. The manubrial fibers are inserted by tendon into the lateral lip of the intertubercular groove behind (deep to) the clavicular fibers as far down as the upper part of the deltoid tuberosity. They form the intermediate lamina of the insertion. The sternocostal fibers, arising from below the sternal angle, course upwards and laterally to be inserted progressively higher into the posterior lamina of the tendon, producing the rounded appearance of the anterior axillary fold. The fibers which arise lowest of all are thus inserted highest of all in the posterior leaf of the tendon. The uppermost limit of the insertion of the posterior leaf is, by a crescentic fold, into the capsule of the shoulder joint. The anterior leaf is not inserted as high as this, its uppermost limit being on a level with the surgical neck of the humerus.
Morphology of pectoralis major:
Like most limb girdle muscles, there is some doubt about how much of pectoralis major is developed from the limb bud and how much from myotomes of the trunk, with migration in either direction. On dissection 1 body in 20 shows the presence of vertical musculo- aponeurotic fibres on the surface of the pectoralis major alongside the sternum (Fig. 2.2). This is the rectus sternalis muscle, a derivative of the superficial layer of the rectus abdominis (p. 298); it is supplied seg- mentally by intercostal nerves. Its upper fibres usually fuse with the sternal tendon of sternocleidomastoid.
Nerve supply of pectoralis major:
From the brachial plexus via the medial and lateral pectoral nerves, so named because of their origins from the medial and lateral cords of the plexus, the medial pectoral pierces pectoralis minor (and therefore supplies it) and the lateral pectoral pierces the clavipectoral fascia medial to the pectoralis minor. Both nerves enter the deep surface of pectoralis major. The muscle is the only one in the upper limb to be supplied by all five segments of the brachial plexus; C5, 6 supply the clavicular head and C7, 8, T1 the sternocostal part, but the segments mainly involved are C6, 7, 8. The degree of paralysis of pectoralis major may be helpful in gauging the extent of a brachial plexus injury (p. 126).
Action of pectoralis major:
The muscle is a medial rotator of the arm and, in combination with the muscles of the posterior axillary fold, particularly the latissimus dorsi, a powerful adductor of the arm. It is especially well developed in climbing and flying animals. The sternocostal fibers are the chief adductors; the clavicular head draws the arm forward into flexion at the shoulder joint. With the upper limb fixed in abduction the muscle is a useful accessory muscle of inspiration, drawing the ribs upwards towards the humerus.