This is a broad sheet of thick muscle which clothes the side wall of the thorax and forms the medial wall of the axilla. It arises by a series of digitations from the upper eight ribs. The first digitation, which appears in the posterior triangle, arises from the outer border of the first rib and also from the rough impression which characterizes the second rib. Over its upper border passes the neurovascular bundle from the posterior triangle to the axilla. The second digitation arises from the second rib and is inserted, together with the first, into the upper angle of the scapula. (This part of the muscle is innervated from C5.) The third and fourth digitations arise from the third and fourth ribs and form a thin sheet of muscle which spreads out to be inserted into the length of the costal surface of the scapula along a narrow strip at its vertebral border. (This part is innervated from C6.) The lower four digitations arise from the fifth, sixth, seventh and eighth ribs, interdigitating with the slips of origin of external oblique at the anterior angles of the ribs. Thick, fleshy muscles, they converge strongly on the inferior angle of the scapula. (They are supplied by C7.) The muscle is covered by a strong well-developed fascia.
Nerve supply of serratus anterior:
By the long thoracic nerve (nerve to serratus anterior), which arises from the roots of the brachial plexus (C5, 6, 7). The branches from C5 and 6 join in the scalenus medius muscle and emerge from its lateral border as a single trunk which enters the axilla by passing over the first digitation of serratus anterior. The contribution from C7 also passes over the first digitation of serratus anterior and joins the former nerve on the medial wall of the axilla (i.e. on the surface of serratus anterior) to form the nerve to serratus anterior. The nerve lies behind the midaxillary line (i.e. behind the lateral branches of the intercostal arteries) on the surface of the muscle, deep to the fascia, and is thus protected in operations on the axilla. The muscle is supplied segmentally; C5 into the upper two digitations, C6 into the next two, and C7 into the lower four digitations.
Action of serratus anterior:
The whole muscle contracting en masse protracts the scapula (punching and pushing), thus effectively elongating the upper limb. A further highly important action is that of the lower four digitations, which powerfully assist the trapezius in rotating the scapula laterally and upwards in raising the arm above the level of the shoulder. In this action it is a more powerful rotator than trapezius. In all positions the muscle keeps the vertebral border of the scapula in firm apposition with the chest wall.