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:
Medial Wall
The medial wall consists of:
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.
- 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:
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:
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.