Learning Objective
Understand the anatomy of the axilla, including its borders, contents, passageways, and clinical significance, to identify its role as a major conduit for neurovascular and muscular structures of the upper limb.
Introduction
The axilla, or 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 critical passageway for nerves, vessels, and muscles traveling to and from the upper limb.
This module explores the structure, contents, passageways, and clinical relevance of the axilla.
Structure and Borders
The axilla has four walls, a base, and an apex:
| Border | Description |
|---|---|
| Apex | Formed by the lateral border of the first rib, the superior border of the scapula, and the posterior border of the clavicle (axillary inlet). |
| Lateral Wall | Intertubercular groove of the humerus. |
| Medial Wall | Serratus anterior muscle and thoracic wall (ribs + intercostal muscles). |
| Anterior Wall | Pectoralis major, pectoralis minor, and subclavius muscles. |
| Posterior Wall | Subscapularis, teres major, and latissimus dorsi muscles. |
| Base | Skin, subcutaneous tissue, and axillary fascia. |
Key Point: Arm abduction decreases the size of the axilla, increasing the risk of compression of its contents.
Contents of the Axilla
| Structure | Description |
|---|---|
| Axillary Artery | The main upper limb artery is divided into three parts relative to the pectoralis minor. |
| Axillary Vein | Main venous drainage includes cephalic and basilic tributaries. |
| Brachial Plexus | The network of spinal nerves forms the peripheral nerves of the upper limb. |
| Axillary Lymph Nodes | Filter lymph from the upper limb and pectoral region; clinically important in breast cancer. |
| Muscles (Biceps Brachii & Coracobrachialis) | Tendons pass through the axilla and attach to the coracoid process. |
Passageways Exiting the Axilla
| Passageway | Description |
|---|---|
| Inferior and Lateral Exit | Main route for structures traveling to the upper limb. |
| Quadrangular Space | Posterior wall space transmitting the axillary nerve and posterior circumflex humoral artery. |
| Clavipectoral Triangle | Anterior opening bounded by pectoralis major, deltoid, and clavicle; passage for cephalic vein and pectoral nerves. |
Clinical Relevance
Thoracic Outlet Syndrome
- Definition: Compression of nerves and vessels at the apex of the axilla.
- Causes: Trauma (clavicle fracture), repetitive overhead movements, cervical rib.
- Symptoms: Pain, tingling, weakness, discoloration in the limb.
Axillary Lymph Node Biopsy and Clearance
- Clinical Importance: 75% of breast lymph drains into axillary nodes; key in breast cancer evaluation.
- Complication: Injury to the long thoracic nerve can cause a winged scapula.
Key Points to Remember
- The axilla is a vital conduit for upper limb neurovascular structures.
- Its pyramidal shape has six borders, and its size changes with arm movement.
- Common clinical concerns include thoracic outlet syndrome and breast cancer metastasis.
