The serratus anterior is a muscle that originates on the surface of the 1st to 8th ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula. The serratus anterior acts to pull the scapula forward around the thorax. The muscle is named from Latin: serrare = to saw, referring to the shape, anterior = on the front side of the body.
Structure
Serratus anterior normally originates by nine or ten slips from either the first to ninth ribs or the first to eighth ribs. Because two slips usually arise from the second rib, the number of slips is greater than the number of ribs from which they originate. The muscle is inserted along the medial border of the scapula between the superior and inferior angles along with being inserted along the thoracic vertebrae. The muscle is divided into three named parts depending on their points of insertions:
the serratus anterior intermediate is inserted along the medial border
the serratus anterior inferior is inserted near the inferior angle.
Relations
The serratus anterior lies deep to the subscapularis, from which it is separated by the subscapularis bursa. It is separated from the rib by the scapulothoracic bursa.
Nerve supply
The serratus anterior is innervated by the long thoracic nerve, a branch of the brachial plexus. The long thoracic nerve travels inferiorly on the surface of the serratus. The nerve is especially vulnerable during certain types of surgery. Damage to this nerve is the most common cause of winged scapula.
Function
All three parts described above pull the scapula forward around the thorax, which is essential for anteversion of the arm. As such, the muscle is an antagonist to the rhomboids. However, when the inferior and superior parts act together, they keep the scapula pressed against the thorax together with the rhomboids and therefore these parts also act as synergists to the rhomboids. The inferior part can pull the lower end of the scapula laterally and forward and thus rotates the scapula to make elevation of the arm possible. Additionally, all three parts can lift the ribs when the shoulder girdle is fixed, and thus assist in respiration. The serratus anterior is occasionally called the "big swing muscle" or "boxer's muscle" because it is largely responsible for the protraction of the scapula — that is, the pulling of the scapula forward and around the rib cage that occurs when someone throws a punch. The serratus anterior also plays an important role in the upward rotation of the scapula, such as when lifting a weight overhead. It performs this in sync with the upper and lower fibers of the trapezius.
Other animals
The muscles of the shoulder can be categorized into three topographic units: the scapulohumeral, axiohumeral, and axioscapular groups. Serratus anterior forms part of the latter group together with rhomboid major, rhomboid minor, levator scapulae, and trapezius. The trapezius evolved separately, but the other three muscles in this group evolved from the first eight or ten ribs and the transverse processes of the cervical vertebrae. Functional demands have resulted in the evolution of individual muscles from the basal unit formed by the serratus anterior. In primitive life forms, the main function of the axioscapular group is to control the movements of the vertebral border of the scapula: fibers concerned with the dorsal movement of scapula evolved into the rhomboids, those with ventral motion into serratus anterior, and those with cranial movements into levator scapulae. The evolution of the serratus anterior itself has resulted in grouping of its distal and proximal fibers, size reduction of its intermediate fibers, and the insertion of its dominant superior and inferior parts onto the superior and inferior angles of the scapula. In primates, the thoracic cage is wide and the scapula is rotated onto its posterior side to have the glenoid cavity face laterally. Additionally, the clavicle takes care of medial forces. In cursorial mammals, the scapula is hanging vertically on the side of the thorax and the clavicle is absent. Therefore, in climbing animals, the serratus anterior supports the scapula against the reaction forces of the free limb and exerts high bending forces on the ribs. To sustain these forces, the ribs have a pronounced curvature and are supported by the clavicle. In cursorial animals, the thorax is hanging between the scapulae at the serratus anterior and pectoralis muscles.
