Contents of Axilla

Introduction

Introduction to the Contents of the Axilla

The axilla, commonly known as the armpit, is a pyramid-shaped anatomical gateway located between the upper lateral thorax and the upper limb. It serves as a vital neurovascular conduit through which all major nerves, blood vessels, and lymphatics pass from the root of the neck and mediastinum into the upper extremity. Understanding the complex arrangement of these contents is clinically crucial, as they are susceptible to compression, trauma, and surgical exploration.

Study

The Neurovascular Core: Axillary Artery and Vein

The axillary artery is the continuation of the subclavian artery, beginning at the outer border of the first rib and terminating at the inferior border of the teres major muscle, where it becomes the brachial artery. The artery is structurally divided into three parts by the pectoralis minor muscle, which crosses it anteriorly: • First Part (proximal to pectoralis minor): Gives off 1 branch — the superior thoracic artery. • Second Part (posterior to pectoralis minor): Gives off 2 branches — the thoracoacromial artery and the lateral thoracic artery. • Third Part (distal to pectoralis minor): Gives off 3 branches — the subscapular artery (which divides into the circumflex scapular and thoracodorsal arteries), the anterior circumflex humeral artery, and the posterior circumflex humeral artery. The axillary vein lies anteromedial to the axillary artery throughout its course. It is formed by the union of the brachial veins and the basilic vein at the lower border of the teres major muscle. It receives the cephalic vein near its termination at the outer border of the first rib, where it becomes the subclavian vein.

The Brachial Plexus in the Axilla

The axilla contains the cords and terminal branches of the brachial plexus, which are named according to their anatomical relationship to the second part of the axillary artery: 1. Lateral Cord: Gives rise to the lateral pectoral nerve, the musculocutaneous nerve, and the lateral root of the median nerve. 2. Medial Cord: Gives rise to the medial pectoral nerve, the medial cutaneous nerve of the arm, the medial cutaneous nerve of the forearm, the ulnar nerve, and the medial root of the median nerve. 3. Posterior Cord: Gives rise to the upper subscapular nerve, thoracodorsal nerve, lower subscapular nerve, axillary nerve, and radial nerve (recalled by the acronym 'ULTRA'). The long thoracic nerve (C5-C7) and the intercostobrachial nerve (lateral cutaneous branch of the second intercostal nerve, T2) also traverse the axillary space. The intercostobrachial nerve provides sensory innervation to the skin of the axilla and upper medial arm and is frequently at risk during axillary lymph node dissections.

Axillary Lymph Nodes and Fasces

The axillary lymph nodes drain lymph from the upper limb, the scapular region, and the extensive cutaneous area of the trunk down to the level of the umbilicus, including approximately 75% of the mammary gland. They are arranged into five distinct functional groups: 1. Pectoral (Anterior) Group: Located along the lower border of the pectoralis minor; drains the anterior thoracic wall and breast. 2. Subscapular (Posterior) Group: Located along the lower wall of the axilla and subscapular vessels; drains the posterior thoracic wall and scapular region. 3. Humeral (Lateral) Group: Located along the lateral wall of the axilla, posteromedial to the axillary vein; drains almost the entire upper limb. 4. Central Group: Located deep in the axillary fat near the base; receives lymph from the pectoral, subscapular, and humeral groups. 5. Apical Group: Located at the apex of the axilla along the medial side of the axillary vein; receives lymph from all other groups and drains into the subclavian lymph trunk. All these structures, except the axillary vein, are enclosed within the axillary sheath, a downward facial expansion of the prevertebral layer of deep cervical fascia. The axillary vein lies outside the sheath to allow for volumetric expansion during increased venous return.

Summary

Summary of Axillary Contents

The contents of the axilla form a highly organized neurovascular bundle embedded within adipose tissue. The axillary artery provides foundational vascular landmarks via its three segments defined by the pectoralis minor muscle. The cords of the brachial plexus encircle this artery, distributing vital motor and sensory innervation to the upper limb. The five groups of axillary lymph nodes provide structured clinical tracking for lymphatic drainage from the upper limb and breast tissue, making axillary anatomy indispensable during oncological and orthopedic assessments.