Serratus anterior muscle

The serratus anterior (historically termed serratus magnus) is a thin, broad, and digitated muscle occupying the upper lateral thoracic wall, situated between the ribs and the scapula. It forms a substantial component of the medial wall of the axilla and is essential for scapulothoracic stability and coordinated upper limb movement.

Attachments and fibre arrangement

1. Origin

• Arises by fleshy digitations from the external surfaces and superior borders of ribs 1 to 8 or 1 to 9

• Additional fibres arise from the intercostal fascia (aponeuroses of intercostal muscles)

• Inferior digitations interdigitate with the upper slips of the external oblique muscle, reflecting functional continuity of the thoracoabdominal wall

2. Insertion (complex, segmental)

• Fibres pass posteriorly, closely applied to the thoracic cage, to insert on the anterior (costal) surface of the medial border of the scapula:

  • Superior fibres (1st digitation): Insert into the superior angle

  • Intermediate fibres (2nd to 3rd digitations): Spread along the medial border

  • Inferior fibres (lower 5 to 6 digitations): Converge to insert strongly into the inferior angle, forming a fan-shaped mass critical for rotation

Innervation

• Long thoracic nerve (C5 to C7). The nerve courses superficially along the muscle making it vulnerable to iatrogenic or traumatic injury

Blood Supply

• Lateral thoracic artery (primary supply)

• Contributions from the thoracodorsal artery

Functions

• Protraction of the scapula (primary action)

• Fixation of the scapula against the thoracic wall, preventing medial displacement

• Upward rotation of the scapula, particularly via the inferior fibres, enabling overhead abduction of arm (above 90°)

• Acts as an accessory muscle of respiration when the scapula is fixed

Clinical Correlation

• Long thoracic nerve injury → serratus anterior paralysis

  • Results in winged scapula (prominent medial border and inferior angle)

  • Impaired overhead abduction

• Vulnerable during axillary surgery, chest trauma, or repetitive strain