Axilla

The axilla, commonly known as the armpit, is a pyramid-shaped space located beneath the shoulder joint. It serves as a passageway for important nerves, blood vessels, and lymphatics that supply the upper limb.^[2] The axilla is lined with skin, subcutaneous tissue, and contains fat that cushions and protects its vital structures.^[6]

Location

The axilla is located at the junction of the upper limb and the thorax. It is positioned inferior to the glenohumeral joint (shoulder joint) and lateral to the upper chest wall. It lies between the anterior axillary fold (formed by the pectoralis major muscle) and the posterior axillary fold (formed by the latissimus dorsi and teres major muscles).^[4] The base of the axilla is the skin of the armpit.

Anatomy

The axilla, commonly referred to as the armpit, is a pyramid-shaped space located beneath the shoulder joint. It acts as a critical conduit for nerves, blood vessels, and lymphatics that travel between the neck, thorax, and upper limb.^[3] The axilla is surrounded by muscles and bones, forming its boundaries, with a base and an apex that allow the passage of these structures. Below is a detailed description of the anatomy of the axilla:

Boundaries of the Axilla

The axilla has four walls (anterior, posterior, medial, and lateral), a base, and an apex:

Apex (Cervico-Axillary Canal)

The apex is a triangular opening that connects the axilla to the root of the neck.
It is bounded by:
- Clavicle (anteriorly)
- 1st rib (medially)
- Superior border of the scapula (posteriorly)
Structures such as the brachial plexus, axillary artery, and axillary vein pass through the apex.

Base

The base is formed by:
- Axillary fascia: A thick layer of connective tissue.^[1]
- Overlying skin and subcutaneous tissue of the armpit.
The axillary hair and sweat glands are present here.

Anterior Wall

The anterior wall is composed of muscles and fascia:

Pectoralis Major: A large, superficial muscle extending from the clavicle and sternum to the humerus.
Pectoralis Minor: A deeper, triangular muscle lying beneath the pectoralis major, arising from ribs 3-5 and attaching to the coracoid process of the scapula.^[5]
Clavipectoral Fascia: A sheet of connective tissue that encloses the pectoralis minor muscle and extends from the clavicle to the axillary fascia.

Posterior Wall

The posterior wall is formed by:

Subscapularis Muscle: Covers the anterior surface of the scapula.
Teres Major Muscle: Extends from the inferior angle of the scapula to the humerus.
Latissimus Dorsi Muscle: A large, broad muscle that contributes to the posterior axillary fold.

Medial Wall

The medial wall consists of:

Serratus Anterior Muscle: Originates from the upper eight ribs and attaches to the medial border of the scapula.
The thoracic wall (ribs 1–4) and intercostal muscles.

Lateral Wall

The lateral wall is the narrowest wall of the axilla and is formed by:

The intertubercular sulcus (bicipital groove) of the humerus, which houses the long tendon of the biceps brachii muscle.^[7]

Contents of the Axilla

The axilla serves as a passageway for vital structures, including nerves, arteries, veins, lymph nodes, and fat:

Axillary Artery

The axillary artery is a continuation of the subclavian artery and supplies blood to the axilla and upper limb. It is divided into three parts by the pectoralis minor muscle:

1st Part: Proximal to the pectoralis minor.
- Superior Thoracic Artery: Supplies the upper thorax.
2nd Part: Posterior to the pectoralis minor.
- Thoracoacromial Artery: Divides into branches supplying the pectoral, acromial, deltoid, and clavicular regions.^[8]
- Lateral Thoracic Artery: Supplies the lateral thoracic wall and serratus anterior.
3rd Part: Distal to the pectoralis minor.
- Subscapular Artery: Largest branch, supplying the posterior wall.
- Anterior and Posterior Circumflex Humeral Arteries: Encircle the humerus and supply the shoulder joint and surrounding muscles.

Axillary Vein

The axillary vein is the main venous drainage of the upper limb and axilla. It runs alongside the axillary artery and becomes the subclavian vein at the first rib. It receives tributaries such as:

Cephalic Vein
Basilic Vein

Brachial Plexus

The brachial plexus is a network of nerves formed by the ventral rami of spinal nerves C5–T1. It supplies motor and sensory innervation to the upper limb.

The brachial plexus passes through the apex of the axilla.
It is organized into roots, trunks, divisions, cords, and branches.
Major branches within the axilla include:
- Musculocutaneous Nerve
- Median Nerve
- Ulnar Nerve
- Radial Nerve
- Axillary Nerve

Axillary Lymph Nodes

The axilla contains five groups of lymph nodes that drain lymph from the upper limb, thorax, and breast:

Pectoral (Anterior) Nodes: Along the lateral thoracic vessels; drain the anterior chest wall and breast.
Subscapular (Posterior) Nodes: Along the subscapular vessels; drain the posterior thoracic wall and scapular region.
Humeral (Lateral) Nodes: Located near the axillary vein; drain the upper limb.
Central Nodes: Located deep within the axillary fat; receive lymph from the pectoral, subscapular, and humeral nodes.
Apical Nodes: Located at the apex of the axilla; receive lymph from all other axillary nodes.

These nodes eventually drain into the subclavian lymph trunk and into the venous system via the right lymphatic duct or thoracic duct.

Fat and Connective Tissue

The axilla is filled with adipose tissue that provides protection and cushioning for its vital contents.

Surface Landmarks of the Axilla

Anterior Axillary Fold: Formed by the pectoralis major muscle.
Posterior Axillary Fold: Formed by the latissimus dorsi and teres major muscles.
Axillary Hair: Found on the skin of the axillary base.
Axillary Fascia: The floor of the axilla, where the skin meets the thoracic wall.

Function

The axilla, commonly referred to as the armpit, serves as a vital anatomical passageway that allows for the transmission of critical structures between the neck, thorax, and upper limb. It provides protection, support, and conduction of neurovascular and lymphatic components, while also serving as an anchoring point for key muscles that assist in upper limb movements. Below is a detailed explanation of the functions of the axilla:

Passageway for Neurovascular Structures

The axilla acts as a conduit for critical structures, including nerves, arteries, veins, and lymphatics, that travel between the neck and upper limb.

Nerves:

The brachial plexus passes through the axilla, providing motor and sensory innervation to the entire upper limb.
The plexus is composed of roots, trunks, divisions, cords, and terminal branches, including:
- Radial Nerve: Supplies the extensor muscles of the arm, forearm, and hand.
- Median Nerve: Supplies flexor muscles of the forearm and hand.
- Ulnar Nerve: Innervates intrinsic hand muscles.
- Axillary Nerve: Supplies the deltoid and teres minor muscles.
- Musculocutaneous Nerve: Innervates the flexors of the arm (biceps, brachialis).

The axilla ensures that these nerves safely and efficiently reach their targets in the arm, forearm, and hand, enabling motor control and sensory feedback.

Blood Vessels:

The axillary artery, a continuation of the subclavian artery, provides blood supply to the shoulder, axilla, and upper limb.
The axillary vein drains blood from the upper limb back to the heart via the subclavian vein.
These vessels supply oxygen and nutrients to the muscles, skin, and bones of the upper limb while removing metabolic waste products.

Lymphatic Drainage

The axilla contains axillary lymph nodes, which play a critical role in lymphatic drainage and immune defense. The lymph nodes in the axilla drain lymph from the following areas:

Upper Limb: Receives lymphatic fluid from the arm, forearm, and hand.
Thoracic Wall: Including the lateral breast and anterior chest wall.
Back: The upper back and scapular region.

The lymph nodes are divided into five groups:

Pectoral (Anterior) Nodes: Drain the breast and anterior chest wall.
Subscapular (Posterior) Nodes: Drain the back and scapular region.
Humeral (Lateral) Nodes: Drain the upper limb.
Central Nodes: Receive lymph from the anterior, posterior, and lateral groups.
Apical Nodes: Located at the apex of the axilla; drain lymph into the subclavian trunk.

The axillary lymph nodes act as filters, trapping pathogens, foreign particles, and cancer cells. This is especially important for the immune response and the early detection of conditions like breast cancer.

Muscle Attachment and Support

The axilla provides a site of muscle attachment for key muscles that facilitate movement of the shoulder girdle and upper limb:

Pectoralis Major:
- Anchors to the anterior wall of the axilla.
- Responsible for adduction and internal rotation of the arm.
Pectoralis Minor: Lies beneath the pectoralis major, stabilizing the scapula by pulling it anteriorly.
Serratus Anterior: Forms the medial wall of the axilla, assisting in protraction and upward rotation of the scapula.
Latissimus Dorsi and Teres Major: Contribute to the posterior wall of the axilla and assist in extension, adduction, and internal rotation of the arm.

By anchoring these muscles, the axilla plays a key role in shoulder movement and stability.

Fat Storage and Protection

The axilla contains adipose tissue (fat) that serves two main functions:

Cushioning and Protection: The fat cushions vital neurovascular structures (nerves, arteries, veins, and lymph nodes) within the axilla, protecting them from external trauma or compression.
Energy Storage: Adipose tissue acts as a reserve of energy in the form of stored lipids, which can be metabolized during periods of need.

Thermoregulation

The axilla plays a role in thermoregulation due to the presence of numerous sweat glands (apocrine and eccrine glands):

Apocrine Sweat Glands: Active after puberty, they secrete sweat containing proteins and lipids.
Eccrine Sweat Glands: Regulate body temperature through the secretion of water-based sweat that evaporates to cool the skin.

These glands are activated during heat, stress, or exercise, maintaining optimal body temperature.

Passage of Vital Structures During Movement

The axilla ensures the smooth passage of its vital contents during upper limb movements:

Movements such as abduction, adduction, and rotation of the arm alter the shape of the axilla, allowing the neurovascular structures to glide without compression.
The axillary fascia provides flexibility and prevents excessive stretching or injury of the structures within the axilla.

This flexibility ensures uninterrupted blood flow, nerve conduction, and lymphatic drainage, even during vigorous physical activities.

Clinical Access Point

The axilla serves as an access point for medical procedures involving the upper limb:

The axillary artery can be used to measure blood flow or access blood vessels during procedures like catheterization.
Lymph nodes in the axilla are often examined for conditions like breast cancer metastasis or infections in the upper limb.
Nerve blocks can be administered in the axilla (e.g., brachial plexus block) for regional anesthesia during upper limb surgeries.

Clinical Significance

The axilla is a clinically significant anatomical region due to its role as a conduit for major neurovascular structures and its involvement in various medical conditions.

Lymph Node Involvement

Axillary lymph nodes are critical for lymphatic drainage of the upper limb, breast, and thoracic wall.
Enlargement of these nodes may indicate breast cancer metastasis, infections, or systemic conditions like lymphoma.
Axillary node dissection is often performed in breast cancer surgery to assess cancer spread.

Brachial Plexus Injuries

Trauma to the axilla can damage the brachial plexus, leading to motor and sensory deficits in the upper limb (e.g., Erb's palsy, Klumpke's palsy).

Vascular Injuries

The axillary artery and vein are susceptible to trauma during fractures or penetrating injuries, causing severe bleeding and compromising blood supply to the upper limb.

Infections and Abscesses

The axilla is prone to infections like hidradenitis suppurativa, which affects sweat glands, causing painful abscesses.
Axillary cellulitis can result from bacterial infections of the skin.

Clinical Procedures

The axilla provides access for medical procedures such as axillary artery catheterization and brachial plexus nerve blocks for anesthesia.