Axilla
On this page
Direct answer
Every artery, vein and nerve passing from the root of the neck to the free upper limb travels through the axilla, a pyramidal space between the arm and the lateral chest wall. Its apex — the cervicoaxillary canal, framed by the first rib, clavicle and superior border of the scapula — admits the axillary artery (the subclavian's renamed continuation at the outer border of the first rib), the axillary vein and the brachial plexus cords; its base is skin and axillary fascia; its four walls are muscular. The space also holds the axillary lymph nodes, which makes it the surgical crossroads of breast cancer.
What you must remember
- Anterior wall: pectoralis major and minor with subclavius, knit by the clavipectoral fascia, pierced by the cephalic vein, thoracoacromial artery and lateral pectoral nerve.
- Posterior wall: subscapularis above, teres major and latissimus dorsi below.
- Medial and lateral walls: medial — serratus anterior over the upper four ribs, with the long thoracic nerve on its surface; lateral — the intertubercular groove of the humerus between the lips of pectoralis major and latissimus dorsi.
- Apex and base: apex (cervicoaxillary canal) — first rib, clavicle, superior border of scapula; base — skin, superficial fascia and axillary fascia.
- Contents: axillary artery with its six branches, axillary vein (medial to the artery throughout), the three cords in an axillary sheath of prevertebral fascia, lymph nodes in fat, and the intercostobrachial nerve (T2).
- Arterial parts: pectoralis minor divides the artery into three parts — one branch above the muscle, two behind it, three below it.
- Lymph node levels: level I lateral to pectoralis minor, level II behind it, level III medial and above it; groups — pectoral, subscapular, humeral, central, apical and interpectoral (Rotter's).
- Clinical anchors: the axillary tail of Spence pierces the anterior wall, so breast carcinoma spreads first to these nodes; dissection endangers the long thoracic nerve (winged scapula) and thoracodorsal nerve; dividing the intercostobrachial nerve numbs the upper medial arm.
A modified radical mastectomy, read wall by wall
The surgeon's axillary clearance is a tour of the walls. The anterior wall is raised first: pectoralis major retracted, the clavipectoral fascia opened, and the pectoral nodes — the breast's principal drainage along the lateral thoracic vessels — swept. Behind pectoralis minor lie the central nodes, the largest group and the one a palpable armpit lump usually represents; above the muscle, the apical nodes drain everything below into the subclavian lymph trunk. On the medial wall the dissection hugs serratus anterior, where the long thoracic nerve (C5–C7) runs on the muscle's surface — sight of it must be kept, or the patient wakes with a winged scapula. On the posterior wall the thoracodorsal nerve and subscapular vessels are preserved with the subscapular nodes swept from the fat; the intercostobrachial nerve (T2) crossing the space is often sacrificed, so the patient is warned of permanent numbness along the upper inner arm. Level by level — I, II, III — the pathologist's nodal count decides adjuvant treatment.
Where students slip
The cords of the brachial plexus are named by the side of the limb rather than by their relation to the second part of the axillary artery — the part behind pectoralis minor. The axillary vein is placed lateral to the artery; it lies medial throughout, which is why the artery is the pulsatile lateral structure at the fingertip. The six branches by part are muddled — recite them as 1–2–3: superior thoracic; thoracoacromial and lateral thoracic; subscapular with anterior and posterior circumflex humeral. The base is imagined as a muscular floor; it is skin and fascia, the axilla being a potential space whose dome collapses unless the arm is abducted — exactly how the axillary sheath is reached for a brachial plexus block. And the long thoracic nerve is said to lie "within the wall"; it runs on the surface of serratus anterior, precisely where the dissecting instrument travels.
Frequently asked questions
How does pectoralis minor divide the axillary artery, and what branches come from each part?
Into three parts: first (above) — superior thoracic; second (behind) — thoracoacromial and lateral thoracic; third (below) — subscapular and the anterior and posterior circumflex humeral arteries.
Which muscles form the four walls of the axilla?
Anterior — pectoralis major and minor with subclavius; posterior — subscapularis, teres major and latissimus dorsi; medial — serratus anterior; lateral — the intertubercular groove of the humerus.
What are axillary lymph node levels I, II and III?
Level I lateral to pectoralis minor, level II behind it, level III medial and superior to it; the pectoral, subscapular and humeral groups drain to central and then apical nodes — the staging basis of breast cancer surgery.
Why does breast carcinoma present with an axillary lump?
The axillary tail of Spence — breast tissue piercing the anterior wall — and the chief lymphatic drainage to the pectoral nodes mean axillary involvement is often the first sign of spread, sometimes the presenting lump itself.