Page 2 of 8

AN10.1-13 | Axilla, Shoulder and Scapular region — Part 1

CLINICAL SCENARIO

Raise your arm and look at your armpit. It seems like a simple fold of skin — but hiding beneath that skin is one of the most important crossroads in your body. Every major blood vessel, nerve, and lymphatic channel travelling to your arm passes through this narrow space. Surgeons approach it with great respect during breast cancer surgery, because a single slip can paralyse the arm. This crowded passageway has a name: the axilla — and understanding it is your gateway to the entire upper limb.

WHY THIS MATTERS

As a doctor, the axilla matters to you in three ways. First, during a breast examination, you'll palpate axillary lymph nodes — the most common site where breast cancer spreads. Second, in the emergency department, axillary artery injuries from shoulder dislocations need urgent recognition. Third, every time you trace a nerve to the arm (brachial plexus), you'll pass through the axilla. Think of it as the Grand Central Station of the upper limb — everything passes through here.

RECALL

From your earlier study of the body wall, you know that the pectoralis major is the large chest muscle (the one you feel when you push against something). You also know that muscles have origins (fixed attachment) and insertions (moving attachment). We'll build on these concepts as we explore the axilla's walls.

The Axilla — A Pyramid-Shaped Space (AN10.1)

The axilla is not just the surface 'armpit' — it's a pyramid-shaped space between the chest wall and the upper arm. Like any pyramid, it has an apex (top), a base (bottom), and four walls.

Imagine you're looking at a cross-section through someone's armpit:

Apex — the narrow top of the pyramid, formed by the gap between the clavicle (front), scapula (back), and first rib (medially). This is the gateway through which vessels and nerves enter the axilla from the neck. It's called the cervicoaxillary canal.
Base — the floor, formed by the skin and axillary fascia (the concave skin of the armpit you can see).
Anterior wall — the front, formed by the pectoralis major and pectoralis minor muscles. These are the muscles you feel on your chest.
Posterior wall — the back, formed by the subscapularis, latissimus dorsi, and teres major muscles.
Medial wall — the inner side, formed by the upper ribs and serratus anterior muscle (the finger-like muscle on the side of the chest).
Lateral wall — the narrow outer side, formed by the intertubercular sulcus (bicipital groove) of the humerus.

Think of it this way: the axilla is like a pyramid tent. The chest muscles form the front flap, the back muscles form the rear, the ribs are one side, and the arm bone is the other. Everything going to the arm must squeeze through this tent.

IMAGE PLACEHOLDER

Cross-section through the axilla showing the pyramid shape: anterior wall (pectoralis major/minor), posterior wall (subscapularis, latissimus dorsi, teres major), medial wall (serratus anterior, ribs), lateral wall (humerus), apex (cervicoaxillary canal), and base (axillary fascia)

The Axillary Artery — Three Parts and Their Branches (AN10.4)

The main blood supply to the upper limb enters through the apex of the axilla as the axillary artery — a continuation of the subclavian artery that changes its name as it crosses the outer border of the first rib.

The pectoralis minor muscle crosses the axillary artery and divides it into three parts (this is a favourite exam question!):

1st part — above (proximal to) pectoralis minor → gives off 1 branch: the superior thoracic artery
2nd part — behind pectoralis minor → gives off 2 branches: the thoracoacromial artery and the lateral thoracic artery
3rd part — below (distal to) pectoralis minor → gives off 3 branches: the subscapular artery, the anterior circumflex humeral artery, and the posterior circumflex humeral artery

Memory trick: The number of branches matches the part number — 1, 2, 3 branches for the 1st, 2nd, and 3rd parts respectively. Simple!

The axillary artery becomes the brachial artery at the lower border of teres major — which is the artery you feel when taking blood pressure.

Diagram of axillary artery divided into three parts by pectoralis minor muscle, with anatomical labels — click to enlarge

Axillary Lymph Nodes — The Sentinel Guards (AN10.7)

The axilla contains about 20–30 lymph nodes arranged in groups. These nodes filter lymph from the upper limb, breast, and chest wall — making them critically important in breast cancer staging.

The groups are named by their location within the axilla:

Anterior (pectoral) group — along the lateral thoracic artery, drains the breast and anterior chest wall. These are the first nodes to receive breast cancer metastasis.
Posterior (subscapular) group — along the subscapular artery, drains the back and posterior shoulder.
Lateral group — along the axillary vein, drains the upper limb.
Central group — in the fat of the axilla, receives lymph from all three groups above.
Apical group — at the apex of the axilla, receives lymph from the central group and drains into the subclavian lymph trunk.

Clinical connection: During breast cancer surgery, surgeons perform a sentinel lymph node biopsy — they inject a dye near the tumour and trace which lymph node it reaches first (the 'sentinel' node). If that node is cancer-free, the remaining nodes are likely clear too, avoiding unnecessary removal of all axillary nodes (which can cause arm swelling called lymphoedema).

Anatomical diagram of axillary lymph node groups showing anterior, posterior, lateral, medial, and apical groups with anatomical landmarks and associated vessels labeled. — click to enlarge