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M01.03.001 Anatomy of the Axilla
The axilla, commonly referred to as the armpit, is a pyramidal-shaped space located beneath the glenohumeral (shoulder) joint at the junction of the upper limb and thorax. It serves as a crucial passageway for neurovascular and muscular structures traveling to and from the upper limb.
This article explores the anatomy of the axilla, including its borders, contents, passageways, and clinical relevance.
Structure and Borders
The axilla has a pyramidal structure with four walls, a base, and an apex:
| Border | Description |
|---|---|
| Apex | Formed by the lateral border of the first rib, superior border of the scapula, and posterior border of the clavicle. Known as the axillary inlet. |
| Lateral Wall | Formed by the intertubercular groove of the humerus. |
| Medial Wall | Composed of the serratus anterior muscle and thoracic wall (ribs and intercostal muscles). |
| Anterior Wall | Contains the pectoralis major, pectoralis minor, and subclavius muscles. |
| Posterior Wall | Formed by the subscapularis, teres major, and latissimus dorsi muscles. |
| Base | Formed by the skin, subcutaneous tissue, and axillary fascia. |
Key Note: The size of the axilla decreases during arm abduction, increasing the risk of compression of its contents.
Contents of the Axilla
The axilla contains several important structures, including muscles, nerves, vessels, and lymphatics:
| Structure | Description |
|---|---|
| Axillary Artery | Main artery of the upper limb, divided into three parts based on its relation to the pectoralis minor. |
| Axillary Vein | Main vein draining the upper limb, with tributaries including the cephalic and basilic veins. |
| Brachial Plexus | A network of spinal nerves forming peripheral nerves for the upper limb. |
| Axillary Lymph Nodes | Filter lymph from the upper limb and pectoral region; often evaluated for breast cancer. |
| Muscles (Biceps Brachii & Coracobrachialis) | Tendons of these muscles pass through the axilla, attaching to the coracoid process. |
Passageways Exiting the Axilla
There are three key routes through which structures leave the axilla:
| Passageway | Description |
|---|---|
| Inferior and Lateral Exit | Primary pathway for structures traveling into the upper limb. |
| Quadrangular Space | Located in the posterior wall; allows the axillary nerve and posterior circumflex humeral artery to pass. |
| Clavipectoral Triangle | Anterior opening bounded by the pectoralis major, deltoid, and clavicle; the cephalic vein and pectoral nerves traverse this route. |
Clinical Relevance
Thoracic Outlet Syndrome
Definition: Compression of vessels and nerves at the apex of the axilla.
| Cause | Examples |
|---|---|
| Trauma | Fractured clavicle |
| Repetitive Movements | Overhead lifting (e.g., certain occupations) |
| Cervical Rib | An extra rib from the seventh cervical vertebra |
Symptoms: Pain, tingling, muscle weakness, and discoloration in the limb.
Lymph Node Biopsy and Axillary Clearance
- Clinical Importance: Around 75% of breast lymph drains into axillary lymph nodes, which are commonly biopsied for suspected breast cancer.
- Complication: Damage to the long thoracic nerve during axillary clearance can result in a winged scapula.
Points to Remember
- The axilla is a vital anatomical passageway for upper limb structures.
- Its pyramidal shape includes six borders, with a dynamic size that changes during arm movement.
- Clinical conditions such as thoracic outlet syndrome and breast cancer metastasis often involve the axillary region.
Reference
TeachMeSeries Ltd. (2024). Anatomy of the Axilla.