Serratus anterior muscle

At a glance

English

Serratus anterior muscle

Latin (TA2)

Musculus serratus anterior

System

Muscle

Region

Trunk

Side

Paired, left and right

Inserts on

The eighth rib, fifth rib, fourth rib, ninth rib, second rib, seventh rib, sixth rib and third rib

About the serratus anterior muscle

Serratus anterior covers much of the side wall of the chest, lying between the ribs and the scapula. Its name comes from its saw-toothed front edge, formed by a row of fleshy slips that arise from the upper ribs. In a lean, muscular person these slips show on the side of the chest below the armpit and interlock with the slips of the external oblique lower down.

The muscle starts from the outer surfaces of the first eight or nine ribs, usually by nine or ten slips because the second rib often gives two. From there the fibres wrap backward around the chest wall, passing under the scapula, and attach along the whole costal (front) surface of its medial border. It is often divided into three parts: an upper part from the first two ribs to the superior angle, a thin middle part fanning to the length of the medial border, and a strong lower part that converges on the inferior angle. A bursa lies between the muscle and the chest wall, and another between it and subscapularis, letting the scapula glide.

Serratus anterior is the chief protractor of the scapula, pulling it forward around the chest, as in punching or pushing, which is why it is called the boxer's muscle. Its lower fibres, working with the upper and lower trapezius, rotate the scapula upward so that the glenoid faces up, which is needed to raise the arm above the head. The whole muscle holds the medial border flat against the ribs, opposing the rhomboids in protraction but cooperating with them in this. When the shoulder is fixed, it can help to lift the ribs in forced breathing.

It is supplied by the long thoracic nerve, from the fifth to seventh cervical roots, which runs down on its outer surface, and receives blood from the lateral thoracic, thoracodorsal and superior thoracic arteries. The nerve is exposed to injury in surgery to the axilla, in heavy carrying on the shoulder and in some sports. Its paralysis causes winging of the scapula: the medial border lifts away from the chest when the patient pushes against a wall, and raising the arm overhead becomes difficult.

Attachments, action and supply

Origin

Outer surfaces of the first eight or nine ribs by fleshy slips.

Insertion

Scapula's medial border, on its costal surface, from superior to inferior angle.

Action

Protracts the scapula and holds it against the chest wall; its lower part rotates the scapula upward for overhead reach; raises the ribs when the shoulder is fixed.

Nerve supply

Long thoracic nerve (C5-C7).

Blood supply

Lateral thoracic, thoracodorsal and superior thoracic arteries.