Anatomy · Muscles

Pectoralis minor

The thin triangular muscle that divides the axillary artery into three parts

Chest · deep to the pectoral muscle, moves the shoulder blade

A thin, triangular muscle lying under the big pectoral muscle on the upper chest wall. It runs from the third to fifth ribs up and outwards to the coracoid process of the scapula, and it pulls the shoulder blade forwards and down against the chest. Anatomists use its edges to divide the axillary artery and the axillary lymph nodes into three parts.

How to spot them

▸A small triangle deep to pectoralis major, with its apex on the coracoid process
▸Arises from the outer surfaces of ribs 3 to 5 near their cartilages
▸Crossed by the axillary artery and the cords of the brachial plexus, which pass behind it
▸Supplied by the medial pectoral nerve

What to know

A thin triangle on the chest wall, arising from the third to fifth ribs and ending on the coracoid process of the scapula

Pectoralis minor — deep to pectoralis major.

Its edges mark the first, second and third parts of the axillary artery, which pass medial to it, behind it and lateral to it

Pectoralis minor — the landmark for dividing the axillary artery.

Axillary lymph nodes are described as levels I, II and III according to whether they lie lateral to, behind or medial to it

Pectoralis minor — the surgical levels of axillary nodes in breast cancer.

Pulls the shoulder blade forwards and down, holding it against the ribs; when the shoulder girdle is fixed it can lift the upper ribs

Pectoralis minor — protraction and depression of the scapula.

Tightness may squeeze the brachial plexus and the axillary vessels where they pass beneath the coracoid process

Pectoralis minor — pectoralis minor syndrome, a form of thoracic outlet syndrome.

Enclosed in a split of the deep fascia beneath the collarbone, which the cephalic vein and the lateral pectoral nerve pierce above it

Pectoralis minor — the clavipectoral fascia splits to enclose it.

Key ideas

It lies like a bridge across the neurovascular bundle of the arm, so a tight or short muscle can pinch the plexus and vessels; a slumped, rounded-shoulder posture keeps it shortened.
It is enclosed by the clavipectoral fascia, which stretches down from the clavicle; the cephalic vein, the lateral pectoral nerve and the thoracoacromial vessels pierce that fascia above the muscle.

Quick-fire facts

•Origin: outer surfaces of the third to fifth ribs (sometimes second to fourth), near the costal cartilages
•Insertion: the medial border and upper surface of the coracoid process of the scapula
•Action: draws the scapula forwards and downwards (protraction and depression); elevates ribs 3 to 5 in forced inspiration when the scapula is fixed
•Nerve supply: the medial pectoral nerve (C8, T1), with a branch from the lateral pectoral nerve passing through or round it
•Blood supply: the pectoral branch of the thoracoacromial artery, and branches of the lateral thoracic artery
•Divides the axillary artery into three parts: part 1 medial to it, part 2 behind it, part 3 lateral to it
•Berg's levels of axillary lymph nodes: level I lateral to it, level II behind it, level III medial to it
•The medial cord of the brachial plexus lies behind it; the lateral and medial pectoral nerves are named for their cords of origin
•Of the three muscles attached to the coracoid process, biceps and coracobrachialis run to the arm and this one to the chest wall

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Sources

TeachMeAnatomy — Pectoralis minor