axillae

The axillae, also known as the axillary fossa or armpits, are bilateral, pyramidal spaces that connect the upper limb to the thorax. They lie inferior to the glenohumeral joints and serve as a passage for neurovascular structures between the neck, thorax, and arm. The apex of each axilla is the cervicoaxillary canal bounded by the clavicle, first rib, and superior edge of the scapula; the base is formed by the skin and fascia of the armpit. The anterior wall is mainly the pectoralis major and minor; the posterior wall includes subscapularis, teres major, and latissimus dorsi; the medial wall consists of the upper thoracic wall and serratus anterior; the lateral wall is the intertubercular groove of the humerus.

Contents within the axillae include the axillary artery and vein with their branches, the brachial plexus cords and branches, and a group of axillary lymph nodes (typically categorized as humeral, pectoral, subscapular, and central). The axillary sheath encloses parts of the artery, vein, and brachial plexus. Fat and lymphatic vessels fill much of the remaining space. Nerves associated with the region give rise to the main sensory and motor branches that innervate the upper limb, while the lymph nodes drain lymph from the upper limb and portions of the breast.

Clinical relevance includes the axilla’s role in lymphatic drainage and metastasis, notably in breast cancer, where sentinel lymph node biopsy or axillary dissection may be performed. The region is also at risk during breast and shoulder surgeries, and knowledge of its boundaries and contents is important for assessing trauma and vascular or neural injuries. Variations in anatomy of the walls and vessels are recognized but typically follow the described framework.