The Axilla Region

Borders

The overall 3D shape of the axilla looks slightly like a pyramid. It consists of four walls, an apex, and a base:

• Apex (axillary inlet) – it is bounded by the clavicle anteriorly, the first rib medially, and the superior border of the scapula posteriorly.
• Lateral wall – formed by the intertubercular groove of the humerus, coracobrachialis muscle and short head of biceps brachii.
• Medial wall – consists of the serratus anterior and the thoracic wall (ribs and intercostal muscles).
• Anterior wall – contains the pectoralis major and the underlying pectoralis minor and the subclavius muscles.
• Posterior wall – formed by the subscapularis, teres major and latissimus dorsi.

The size and shape of the axilla varies with arm position. Overhead abduction can contribute to narrowing of neurovascular passageways in susceptible individuals, increasing the risk of compression.

By TeachMeSeries Ltd (2026)

Fig 2
Transverse section of the axilla region.

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Contents

The contents of the axilla region include muscles, nerves, vessels, and lymphatics:

• Axillary artery – the main artery supplying the upper limb. It is commonly referred as having three parts; one medial to the pectoralis minor, one posterior to pectoralis minor, and one lateral to pectoralis minor.
• Axillary vein – the main vein draining the upper limb. It is formed by the brachial veins and basilic vein, and receives the cephalic vein as a major tributary.
• Brachial plexus – a network of spinal nerves (C5–T1) that gives rise to the peripheral nerves supplying the upper limb.
• Axillary lymph nodes – a group of lymph nodes that filter lymph drained from the upper limb, breast, and pectoral region.

By TeachMeSeries Ltd (2026)

Fig 3
Contents of the axilla region.

Passageways Exiting the Axilla

There are three main routes by which structures leave the axilla.

The main route of exit is immediately inferiorly and laterally, into the upper limb. The majority of contents of the axilla region leave by this method.

Another pathway is via the quadrangular space. This is a gap in the posterior wall of the axilla, allowing access to the posterior arm and shoulder area. Structures passing through include the axillary nerve and posterior circumflex humeral artery (a branch of the axillary artery.

The last passageway is the clavipectoral triangle, which is an opening in the anterior wall of the axilla. It is bounded by the pectoralis major, deltoid, and clavicle. The cephalic vein enters the axilla via this triangle.

By TeachMeSeries Ltd (2026)

Fig 4
Boundaries and contents of the clavipectoral triangle

By TeachMeSeries Ltd (2026)

Fig 5
Posterior view of the shoulder region, showing the quadrangular space. The subscapularis muscle lies anteriorly, and so cannot be seen.