Overview

Serratus anterior arises by finger-like digitations from the outer surfaces of the upper 8–9 ribs. The lower digitations interdigitate with the origins of the external oblique. It wraps around the chest wall, passes deep to the scapula, and inserts on the anterior (costal) surface of the medial border of the scapula, from the superior to the inferior angle. Its powerful lower fibres converge on the inferior angle.

It protracts the scapula, as in punching or reaching forward, earning it the name "boxer's muscle". Its lower fibres upwardly rotate the scapula, working with the upper and lower fibres of trapezius, so the glenoid turns upward and the arm can be raised above 90°. Above all, it holds the medial border of the scapula against the thoracic wall. It is supplied by the long thoracic nerve (C5–C7), which arises from the roots of the brachial plexus and descends superficially on its outer surface in the mid-axillary line, where it is vulnerable.

Location: Lateral thoracic wall (medial wall of the axilla), deep to the scapula posteriorly.

Anatomy

Origin: External surfaces of the lateral parts of ribs 1–8 (9).
Insertion: Anterior (costal) surface of the medial border of the scapula, from the superior angle to the inferior angle.
Action: Protracts the scapula and rotates it upward; holds it against the thoracic wall; can elevate the ribs when the scapula is fixed.
Innervation: Long thoracic nerve (from the roots of the brachial plexus).
Nerve roots: C5–C7 ("C5, 6, 7 raise your wings to heaven")
Arterial supply: Lateral thoracic artery, thoracodorsal artery and superior thoracic artery.
Relations: Forms the medial wall of the axilla. The long thoracic nerve and lateral thoracic artery lie on its outer surface. Its lower digitations interdigitate with the external oblique.

Function

Protracts the scapula (punching, pushing, reaching)
Upwardly rotates the scapula for abduction above 90°
Holds the scapula against the chest wall

Clinical significance

Winged scapula (long thoracic nerve palsy)

Injury during axillary dissection or mastectomy, from stab wounds or from heavy backpacks causes medial winging. The medial border and inferior angle protrude when the patient pushes against a wall, and abduction above 90° becomes impossible.

Chest drain safe triangle

Chest drains are placed in the "safe triangle" (lateral border of pectoralis major, anterior border of latissimus dorsi, 5th intercostal space). The long thoracic nerve is avoided by staying anterior to the mid-axillary line.

Mnemonics

Long thoracic nerve roots

"C5, 6, 7 — raise your wings to heaven": the long thoracic nerve (C5–C7) prevents scapular winging.

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.