axilla

The axilla, or armpit, is a pyramidal, transitional space between the upper thorax and the upper limb. It serves as a conduit for nerves, vessels, and lymphatics that supply the arm and shoulder, and it functions as a regional lymphatic hub for the chest wall and breast.

Boundaries and walls: The apex of the axilla is the cervico-axillary canal formed by the clavicle, first rib, and superior border of the scapula. The base is the skin and fascia of the axillary fossa. The anterior wall is formed by the pectoralis major and minor and subclavius; the posterior wall by the subscapularis, teres major, and latissimus dorsi; the medial wall by the upper thoracic wall and serratus anterior; and the lateral boundary is the intertubercular groove of the humerus.

Contents: The axilla contains the axillary artery and vein with their branches and tributaries, the cords and terminal branches of the brachial plexus, and axillary lymph nodes. The artery is described in three parts, with branches including the superior thoracic, thoracoacromial trunk, lateral thoracic, subscapular, and the anterior and posterior circumflex humeral arteries. The accompanying vein drains the upper limb and thoracic wall. Nerves include branches from the brachial plexus such as the axillary nerve, thoracodorsal nerve, and the upper and lower subscapular nerves, which innervate shoulder muscles.

Lymphatic drainage and clinical relevance: The axillary lymph nodes are grouped as pectoral, subscapular, and humeral (lateral), with central and apical nodes deeper within. They drain the upper limb, chest wall, and breast, making the axilla a key region in breast cancer staging and in sentinel lymph node biopsy.