Serratus anterior muscle

Origin: Fleshy slips from the outer surface of upper 8 or 9 ribs

Insertion: Costal aspect of medial margin of the scapula

Artery: Lateral thoracic artery (upper part),thoracodorsal artery (lower part)

Nerve: Long thoracic nerve (from roots of brachial plexus C5, C6, C7)

Action: Protract and stabilize scapula, assists in upward rotation

Antagonist: Rhomboid major, Rhomboid minor, Trapezius

Description:
The Serratus anterior (Serratus magnus) is a thin muscular sheet, situated between the ribs and the scapula at the upper and lateral part of the chest. It arises by fleshy digitations from the outer surfaces and superior borders of the upper eight or nine ribs, and from the aponeuroses covering the intervening Intercostales. Each digitation (except the first) arises from the corresponding rib; the first springs from the first and second ribs; and from the fascia covering the first intercostal space. From this extensive attachment the fibers pass backward, closely applied to the chest-wall, and reach the vertebral border of the scapula, and are inserted into its ventral surface in the following manner. The first digitation is inserted into a triangular area on the ventral surface of the medial angle. The next two digitations spread out to form a thin, triangular sheet, the base of which is directed backward and is inserted into nearly the whole length of the ventral surface of the vertebral border. The lower five or six digitations converge to form a fan-shaped mass, the apex of which is inserted, by muscular and tendinous fibers, into a triangular impression on the ventral surface of the inferior angle. The lower four slips interdigitate at their origins with the upper five slips of the Obliquus externus abdominis.
Variations.—Attachment to tenth rib. Absence of attachments to first rib, to one or more of the lower ribs. Division into three parts; absence or defect of middle part. Union with Levator scapulae, External intercostals or External oblique.