Axilla: Anatomy, Contents, and Clinical Significance

Overview

The axilla is a pyramidal-shaped region located at the junction between the lateral thorax and the proximal arm. It serves as the principal neurovascular conduit between the neck/thorax and the upper limb, transmitting the axillary artery, axillary vein, the brachial plexus, and lymphatic channels. Clinically, it is most significant in breast cancer surgery (axillary lymph node dissection) and as the site of axillary vessel and nerve injury.

Boundaries (Walls of the Axilla)

Wall	Structure
Apex (inlet)	Bounded by clavicle, 1st rib, and scapula; communicates with posterior triangle of neck
Base	Skin and fascia between arm and chest wall
Anterior wall	Pectoralis major and minor, clavipectoral fascia
Posterior wall	Subscapularis (upper), teres major, latissimus dorsi (lower)
Medial wall	Serratus anterior over ribs 1–4
Lateral wall	Bicipital groove of humerus (narrow)

Contents of the Axilla

Axillary Artery

The axillary artery is the continuation of the subclavian artery at the lateral border of the first rib, becoming the brachial artery at the inferior border of teres major. It is divided into three parts by the pectoralis minor:

• Part 1 (medial to pectoralis minor): superior thoracic artery
• Part 2 (behind pectoralis minor): thoracoacromial artery, lateral thoracic artery
• Part 3 (lateral to pectoralis minor): subscapular artery, anterior and posterior circumflex humeral arteries

Axillary Vein

Formed by the union of the brachial veins and basilic vein at the lower border of teres major. Lies medial to the artery. Receives the cephalic vein just below the clavicle. Becomes the subclavian vein at the first rib.

Brachial Plexus (Cords)

The three cords are named by their relation to the axillary artery:

• Lateral cord (C5–C7): lateral to artery → musculocutaneous nerve + lateral root of median nerve
• Medial cord (C8–T1): medial to artery → ulnar nerve + medial root of median nerve + medial cutaneous nerves
• Posterior cord (C5–T1): posterior to artery → radial nerve + axillary nerve

Axillary Lymph Nodes (5 Groups)

1. Anterior (pectoral): along lateral thoracic vessels; drain breast (most of it), anterior thorax
2. Posterior (subscapular): along subscapular vessels; drain posterior thorax, scapular region
3. Lateral: along axillary vein; drain upper limb
4. Central: embedded in axillary fat; receive from all other groups
5. Apical: at apex, medial to pectoralis minor; drain all other groups → subclavian lymphatic trunk

Level classification (surgery):

• Level I: lateral to pectoralis minor
• Level II: behind pectoralis minor
• Level III: medial to pectoralis minor (apical)

Other Contents

• Long thoracic nerve (on serratus anterior): supplies serratus anterior; injury → scapular winging
• Thoracodorsal nerve (on subscapularis and teres major): supplies latissimus dorsi
• Intercostobrachial nerve: cutaneous nerve from T2 intercostal; passes through axilla to supply medial arm; preserved in nerve-sparing axillary dissection to prevent medial arm numbness

Clinical Relevance

• Axillary lymph node dissection (ALND): standard staging/treatment for breast cancer; removes Level I and II nodes; risks: lymphedema, serratus anterior damage (scapular winging), latissimus dorsi weakness
• Sentinel lymph node biopsy: identifies the first draining lymph node from a tumor using blue dye or radiocolloid; avoids full ALND if negative
• Axillary vein thrombosis: effort thrombosis (Paget-Schroetter syndrome) in young athletes; arm swelling
• Axillary nerve injury: surgical neck of humerus fracture or shoulder dislocation; deltoid weakness, sensory loss over "regimental badge" area
• Crutch palsy: compression of radial nerve in axilla by crutch; wrist drop

Study Points

• Name the five walls of the axilla and the structures forming each.
• Describe the three parts of the axillary artery and the branch from each part.
• Name the five groups of axillary lymph nodes and explain the surgical level classification.
• Explain why long thoracic nerve injury causes scapular winging.
• Describe the nerves at risk during axillary lymph node dissection and their consequences if injured.