Underarm

The underarm, scientifically termed the axilla, is the hollow area beneath the shoulder joint where the upper limb connects to the trunk. It forms a pyramid-shaped space bordered anteriorly by the pectoral muscles, posteriorly by the latissimus dorsi and teres major, medially by the serratus anterior, and inferiorly by the skin and subcutaneous tissue. The apex is near the clavicle and first rib. The axilla contains fat, sweat glands, hair follicles, sebaceous glands, lymph nodes, and neurovascular structures.

Glands and hair are prominent features. Eccrine sweat glands are distributed across the skin, while apocrine glands are concentrated in the axilla and become active after puberty. Apocrine secretions themselves are odorless but can produce odor after bacterial breakdown. Hair follicles are common in many individuals’ axillary skin and can influence grooming practices.

Lymphatics and immune function are a key role of the axilla. Axillary lymph nodes exist in several groups and levels and drain lymph from the upper limb, chest wall, and breast. They are an important site for cancer staging, particularly for breast cancer, and may reflect metastatic spread.

Nerves and blood vessels pass through the axilla. The brachial plexus and its cords supply the upper limb, with the intercostobrachial nerve providing sensation to the medial arm. The axillary artery and vein supply and drain the upper limb, supported by surrounding fascia and muscles.

Clinical relevance includes conditions such as hyperhidrosis (excessive sweating), hidradenitis suppurativa, infections, and irritation from shaving or deodorants. Management focuses on hygiene, antimicrobial measures, and, for sweating, topical or medical therapies.