Axilla anatomy: the walls, apex, base and contents of the armpit

The axilla is a pyramidal region below the glenohumeral (shoulder) joint. Seen in three dimensions it looks roughly like a pyramid, with four walls, an apex and a base. (TeachMeAnatomy: The Axilla.)

Kenhub defines the axillary region as the area between the upper part of the upper limb and the thorax, and notes that the space changes shape as the arm moves. (Kenhub: Axillary region.)

Its main job is as a passageway: Lecturio describes the axilla as a pyramid-shaped space below the shoulder joint that is the passageway for nerves and vessels. (Lecturio: Axilla and Brachial Plexus.)

In everyday terms Cleveland Clinic describes the axilla as the area between the chest and the upper arm. Its skin carries hair and two kinds of sweat glands: eccrine glands that release a clear, odourless fluid and apocrine glands whose thicker secretion produces odour when broken down by bacteria. (Cleveland Clinic: Axilla (Armpit).)

OpenStax confirms that apocrine sweat glands are usually associated with hair follicles in densely hairy areas such as the armpits and genital regions, and that their sweat contains organic compounds that bacteria decompose, causing the smell. (OpenStax Anatomy and Physiology 2e, 5.2: Accessory Structures of the Skin.)

The whole upper limb whose vessels, nerves and lymph pass through the axilla. Arm anatomy.

The apex, or axillary inlet, is bounded by the clavicle in front, the first rib medially and the superior border of the scapula behind. (TeachMeAnatomy: The Axilla.)

The anterior wall is formed by pectoralis major with pectoralis minor and subclavius beneath it. The posterior wall is formed by subscapularis, teres major and latissimus dorsi. The medial wall is serratus anterior over the thoracic wall (ribs and intercostal muscles). The narrow lateral wall is the intertubercular groove of the humerus, with coracobrachialis and the short head of biceps brachii. (TeachMeAnatomy: The Axilla.)

Kenhub and Lecturio give the lateral wall simply as the intertubercular sulcus of the humerus, without the two muscles, so the difference is one of detail. Kenhub notes that the lower edges of the anterior and posterior walls form the folds of the armpit: the anterior axillary fold is formed by the pectoral muscles, and the posterior fold by latissimus dorsi and teres major, with subscapularis above them. (Kenhub: Axillary region.)

Kenhub describes the apex as bounded above by the root of the neck, and the base, or floor, as the axillary fascia and the skin over it. (Kenhub: Axillary region.)

Lecturio describes the base as the hair-bearing and sweat gland-bearing skin of the armpit. (Lecturio: Axilla and Brachial Plexus.)

The axillary fascia is often called the suspensory ligament of the axilla. It tethers the skin, which gives the armpit its hollow, bowl-like shape. (Kenhub: Fascias and spaces of the shoulder girdle region.)

Pectoralis major, pectoralis minor, subclavius and serratus anterior, the front and inner walls of the axilla. Chest muscles.

Embedded in axillary fat and enclosed within the axillary sheath, the contents of the axilla are the axillary artery and its branches, the axillary vein and its tributaries, the branches of the brachial plexus and the axillary lymph nodes. (Lecturio: Axilla and Brachial Plexus.)

The axillary sheath is a fibrous layer that wraps the axillary artery and the three cords of the brachial plexus. (TeachMeAnatomy: Arterial Supply to the Upper Limb.)

Radiopaedia explains where the sheath comes from: the prevertebral fascia of the neck extends down to ensheath the axillary artery and the cords of the plexus. (Radiopaedia: Brachial plexus.)

Kenhub describes the axilla as filled with adipose tissue that allows the passage of vessels, the nerve plexus, lymphatics and muscles, and points out that the axillary vein runs medial to the artery along its whole length. (Kenhub: Axillary region.)

At the lateral border of the first rib the subclavian artery enters the axilla and becomes the axillary artery. At the lower border of teres major it leaves the axilla and is renamed the brachial artery. (TeachMeAnatomy: Arterial Supply to the Upper Limb.)

Pectoralis minor crosses in front of the artery and divides it into three parts: the first part above (proximal to) pectoralis minor, the second part behind it and the third part below (distal to) it. Radiopaedia notes that the artery descends posterior to the axillary vein and that its branches are highly variable. (Radiopaedia: Axillary artery.)

The number of branches matches the number of the part: one from the first part (superior thoracic artery), two from the second (thoraco-acromial and lateral thoracic arteries) and three from the third (subscapular, anterior circumflex humeral and posterior circumflex humeral arteries). (Lecturio: Axilla and Brachial Plexus.)

The superior thoracic artery supplies the first and second intercostal spaces. (Kenhub: Axillary region.)

The full course, branches and variants of the main artery of the axilla. Axillary artery.

The axillary vein forms at the lower border of teres major from the union of the paired brachial veins and the basilic vein. It runs up through the axilla on the medial side of the artery and becomes the subclavian vein at the lateral border of the first rib. (Radiopaedia: Axillary vein.)

The cephalic vein joins the first part of the axillary vein above pectoralis minor after piercing the clavipectoral fascia, and five other tributaries correspond to the branches of the artery. About 14% of people have a double axillary vein; the lower of the two can be at risk during axillary surgery for breast cancer. (Radiopaedia: Axillary vein.)

Kenhub agrees that the vein is formed by the basilic and brachial veins. Some summaries, such as Lecturio's, describe it as formed by the cephalic and basilic veins; the cephalic vein actually joins higher up, so the Radiopaedia and Kenhub description is the one to learn. (Kenhub: Axillary region.)

The subclavian, axillary and brachial vessels and the cephalic and basilic veins. Arteries and veins of the arm.

The brachial plexus is formed by the anterior rami of spinal nerves C5 to T1. Its roots form three trunks at the base of the neck; each trunk splits into an anterior and a posterior division, and in the axilla the divisions join to form three cords. (TeachMeAnatomy: The Brachial Plexus.)

The divisions form behind the middle of the clavicle, and the cords are named for their relationship to the second part of the axillary artery, the part behind pectoralis minor: lateral, posterior and medial. (Radiopaedia: Brachial plexus.)

The lateral cord comes from the anterior divisions of the superior and middle trunks and gives the lateral pectoral nerve, the musculocutaneous nerve and the lateral root of the median nerve. The medial cord comes from the anterior division of the inferior trunk and gives the medial pectoral nerve, the medial cutaneous nerves of the arm and forearm, the ulnar nerve and the medial root of the median nerve. The posterior cord collects all three posterior divisions and gives the upper and lower subscapular nerves, the thoracodorsal nerve, and the axillary and radial nerves. (TeachMeAnatomy: The Brachial Plexus.)

The thoracodorsal nerve (C6 to C8) from the posterior cord supplies latissimus dorsi. The long thoracic nerve (C5 to C7) arises directly from the roots and supplies serratus anterior on the medial wall. (Kenhub: Axillary region.)

The long thoracic nerve runs behind the axillary vessels and the brachial plexus and then lies on the surface of serratus anterior for its whole course, sending a branch to every slip of the muscle. It has a mean length of 27 cm. (Gooding BWT et al. Scapular winging. Shoulder Elbow. 2014.)

The intercostobrachial nerve crosses the axilla to supply the skin of the axilla, the lateral chest and the medial arm. A meta-analysis of 16 studies found that it arose from the second intercostal nerve in 90.6% of cases and communicated with the brachial plexus in about 41%. (Henry BM et al. Origin, branching, and communications of the intercostobrachial nerve: a meta-analysis. Cureus. 2017.)

The brachial plexus from roots to terminal branches and the course of each nerve. Nerves of the arm.

There are about 20 to 30 axillary lymph nodes, and up to 40 have been recorded. They drain the whole upper limb, the breast and the trunk above the umbilicus. (Kenhub: Axillary lymph nodes.)

Anatomists divide them into five groups. The lateral (humeral) group of 4 to 6 nodes lies posteromedial to the axillary vein and receives most of the lymph from the upper limb. The anterior (pectoral) group lies along the lower border of pectoralis minor and drains the anterolateral abdominal wall above the umbilicus and the lateral breast. The posterior (subscapular) group of 6 to 7 nodes drains the back of the neck and upper back. The central group, at the base and centre of the axilla, receives lymph from these three. The apical group of 4 to 5 nodes, at the lateral border of the first rib, receives lymph from all the others. (Kenhub: Axillary lymph nodes.)

From the apical nodes lymph drains into the subclavian lymph trunk. The two sides differ: on the left the lymph reaches the thoracic duct, on the right the right lymphatic trunk. (Kenhub: Axillary lymph nodes.)

Radiopaedia calls the apical nodes the final common pathway for all the axillary nodes and notes lymphatic connections between the axillary and cervical nodes. (Radiopaedia: Axillary lymph nodes.)

Surgeons and radiologists use three levels instead of five groups, defined by pectoralis minor: level I nodes lie below the lower edge of pectoralis minor, level II nodes lie behind it and level III nodes lie above and medial to it. (Radiopaedia: Axillary lymph nodes.)

Lecturio adds a sixth, infraclavicular (deltopectoral) group, which lies outside the axilla and drains the superficial layers of the upper limb. (Lecturio: Axilla and Brachial Plexus.)

Lymph nodes throughout the body, how lymph flows and where the axillary nodes fit in. Lymphatic system.

Three gaps in the posterior wall let structures pass from the axilla to the back of the shoulder and arm. The quadrangular space is bounded by subscapularis (teres minor from behind) above, teres major below, the long head of triceps medially and the surgical neck of the humerus laterally. The axillary nerve and the posterior circumflex humeral artery pass through it. (Kenhub: Anatomical spaces of the pectoral region.)

The triangular space carries the circumflex scapular artery to the scapular anastomoses. The triangular interval lies outside the axilla, below the lower border of teres major, and carries the radial nerve and the profunda brachii artery from the front to the back of the arm. (Kenhub: Anatomical spaces of the pectoral region.)

Their borders: the triangular space lies between teres minor above, teres major below and the long head of triceps; the triangular interval lies between teres major above and the long head of triceps medially and the lateral head laterally. (Kenhub: Fascias and spaces of the shoulder girdle region.)

Compression of the axillary nerve and posterior circumflex humeral artery in the quadrangular space causes quadrilateral space syndrome, an uncommon condition that mostly affects athletes. (Kenhub: Anatomical spaces of the pectoral region.)

The shoulder muscle supplied by the axillary nerve after it leaves the quadrangular space. Deltoid.

About 75% of the lymph from the breast drains into the axillary nodes, which is why they are examined and sampled in breast cancer. During removal of the axillary nodes (axillary clearance) the long thoracic nerve may be damaged, leading to a winged scapula. (TeachMeAnatomy: The Axilla.)

In breast cancer surgery the sentinel node, the first node that drains the breast, is identified and checked; if it contains cancer, the surrounding nodes may be removed. Removing axillary nodes carries a risk of lymphoedema, swelling of the arm and breast. (Kenhub: Axillary lymph nodes.)

A 2022 meta-analysis of 67 studies found arm lymphoedema in about 24% of patients more than two years after axillary lymph node dissection, compared with about 6% after sentinel node biopsy alone. (Che Bakri NA et al. Impact of axillary lymph node dissection and sentinel lymph node biopsy on upper limb morbidity in breast cancer patients. Ann Surg. 2023.)

Injury to the intercostobrachial nerve during axillary surgery is linked with numbness and nerve pain in the armpit and inner arm, and an estimated 10 to 60% of breast cancer survivors who had surgery report persistent pain. (Henry BM et al. Origin, branching, and communications of the intercostobrachial nerve: a meta-analysis. Cureus. 2017.)

Prolonged pressure on the axilla can compress the radial nerve and cause wrist drop. Injury to the long thoracic nerve leaves serratus anterior unable to hold the scapula against the chest wall, so the scapula wings. (Kenhub: Axillary region.)

At the apex, the vessels and nerves can be compressed between the bones that bound it, a cause of thoracic outlet syndrome. (TeachMeAnatomy: The Axilla.)

How lymph drains from the breast to the axillary nodes, and the axillary tail of the breast. Breast anatomy.

The model opens from the front and highlights the axillary artery and vein with the thoraco-acromial trunk, the lateral thoracic, subscapular, thoracodorsal, circumflex scapular and both circumflex humeral arteries, and the matching veins, together with the muscles of the walls: pectoralis minor, subscapularis, teres major, serratus anterior, coracobrachialis and the short head of biceps. Pectoralis major is left unhighlighted so that you can see in. Rotate the model to look up into the armpit from below and the pyramid shape becomes clear. The model, BodyParts3D from one adult male, has no brachial plexus, lymph nodes or latissimus dorsi, so use the diagrams for those.

Follow the artery from the first rib to teres major and find pectoralis minor crossing in front of its middle: that crossing defines the three parts of the artery. (Radiopaedia: Axillary artery.)

Anterior circumflex humeral artery

The anterior circumflex humeral artery is a small branch of the axillary artery that wraps around the front of the surgical neck of the humerus.

Axillary artery

The axillary artery is the main artery of the armpit, continuing the subclavian artery.

Axillary vein

The axillary vein is the main vein of the armpit and carries blood from the upper limb.

Circumflex scapular artery

The circumflex scapular artery is the large branch of the subscapular artery that passes round the lateral border of the scapula.

Coracobrachialis

A slender muscle on the medial side of the upper arm.

Lateral thoracic artery

A branch of the axillary artery that runs along the lateral chest wall.

Lateral thoracic vein

A vein draining the lateral chest wall into the axillary vein.

Pectoralis minor

A thin triangular muscle lying deep to pectoralis major on the upper chest wall.

Posterior circumflex humeral artery

A branch of the axillary artery that wraps around the surgical neck of the humerus.

Serratus anterior

A broad muscle on the side of the thorax that holds the scapula against the chest wall.

Short head of biceps brachii

The medial head of biceps brachii at the front of the arm.

Subscapular artery

The largest branch of the axillary artery.

Subscapular vein

A vein that drains the scapular region into the axillary vein.

Subscapularis

The largest rotator cuff muscle, filling the subscapular fossa.

Teres major

A thick muscle of the posterior shoulder, sometimes called the lat's little helper.

Thoracodorsal artery

A branch of the subscapular artery to latissimus dorsi.

Thoracodorsal vein

A vein accompanying the thoracodorsal artery.

Trunk of thoraco-acromial artery

The main stem of the thoraco-acromial artery.