Pectoralis Major

Thorax · Trunk

Located in the Thorax of the Trunk, the Pectoralis Major is a key muscle for anatomy study. Its origin is Clavicular head: medial clavicle; Sternocostal head: sternum and costal cartilages 1–6; Abdominal part: anterior rectus sheath, and it inserts onto Lateral lip (crest) of intertubercular groove of humerus. It is responsible for several movements: Adduction and medial rotation of arm; clavicular head flexes arm; sternocostal head extends arm from flexed position. Its nerve supply is the Lateral and medial pectoral nerves (C5–T1).

Origin, Insertion, Action & Nerve

Origin

Clavicular head: medial clavicle; Sternocostal head: sternum and costal cartilages 1–6; Abdominal part: anterior rectus sheath

Insertion

Lateral lip (crest) of intertubercular groove of humerus

Action

Adduction and medial rotation of arm; clavicular head flexes arm; sternocostal head extends arm from flexed position

Nerve

Lateral and medial pectoral nerves (C5–T1)

Attachments explained

The Pectoralis Major is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.

Origin

Clavicular head: medial clavicle; Sternocostal head: sternum and costal cartilages 1–6; Abdominal part: anterior rectus sheath

Insertion

Lateral lip (crest) of intertubercular groove of humerus

Action & function

When the Pectoralis Major contracts, it produces the following movements: Adduction and medial rotation of arm; clavicular head flexes arm; sternocostal head extends arm from flexed position.

As part of the Thorax group of the Trunk, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.

Nerve supply & clinical relevance

The Pectoralis Major receives its nerve supply from the Lateral and medial pectoral nerves (C5–T1). This nerve carries fibres from spinal segment(s) C5, T1.

Because a muscle can only contract when its nerve is intact, injury to the Lateral and medial pectoral nerves or to its spinal roots (C5, T1) can weaken or paralyse the Pectoralis Major, impairing the movements it normally produces (adduction and medial rotation of arm). This is why knowing the innervation is central to localising nerve lesions in clinical practice.

How to study the Pectoralis Major (the OIANS method)

OIANS stands for Origin, Insertion, Action and Nerve, the four facts that uniquely define every skeletal muscle. To learn the Pectoralis Major, work through them in order: picture its origin, trace the muscle to its insertion, reason out the action that shortening between those two points must create, then add the nerve that drives it.

Most students remember the Pectoralis Major fastest by linking its action back to its attachments rather than memorising each fact in isolation. Once the origin and insertion make sense, the action usually follows logically.