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The axilla, or armpit, is a pyramidal space between the shoulder girdle and thorax. It is bordered by the serratus anterior and the thoracic wall medially, a part of the humerus laterally, the pectoralis muscles anteriorly, and the latissimus dorsi, subscapularis, and teres major muscles posteriorly. The axilla contains several muscles, including the coracobrachialis, pectoralis minor, and the biceps brachii. The anterior axillary fold is formed by the lower border of the pectoralis major, and the posterior axillary fold is formed by the subscapularis, teres major, and latissimus dorsi muscles.
| Characteristics | Values |
|---|---|
| Area | Between the superior portion of the upper limb and the thorax |
| Borders | Serratus anterior and the thoracic wall medially, a part of the humerus laterally, the pectoralis muscles anteriorly, and the latissimus dorsi, subscapularis, and teres major posteriorly |
| Contents | Axillary artery, axillary vein, brachial plexus, axillary lymph nodes, coracobrachialis, pectoralis minor, and biceps brachii muscles |
| Functions | Allows passage of muscles, blood vessels, and nerves to the upper extremity from the neck |
| Blood Supply | Axillary artery (a continuation of the subclavian artery) and its branches: superior thoracic, thoracoacromial, lateral thoracic, subscapular, anterior humeral circumflex, and posterior humeral circumflex arteries |
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What You'll Learn
The axillary region and its anatomy
The axillary region, also known as the armpit, underarm or oxter, is the area on the human body directly under the shoulder joint. It is a pyramid-shaped anatomical space within the shoulder girdle between the arm and the thoracic cage. The axilla is an important region in terms of both neurovasculature and clinical relevance.
The axillary region is bordered by the serratus anterior and the thoracic wall medially, a part of the humerus laterally, the pectoralis muscles anteriorly, and the latissimus dorsi, subscapularis, and teres major muscles posteriorly. The axilla is filled with adipose (fat) and allows passage for vessels, nerve plexus, lymphatics, and muscles. The muscles include the coracobrachialis, pectoralis minor, and the biceps brachii. The axillary artery is an extension of the subclavian artery, and is called so after passing the first rib. It is the main artery supplying the upper limb. The axillary artery is also a major blood supplier to the shoulder and upper arm. The axillary artery has three parts: the region proximal to pectoralis minor, the region beneath it, and the region distal to it. The axillary vein runs medial to the artery along its length. The axillary vein drains the skin while the deep vessels drain the bones and muscles.
The axillary walls are used as landmarks by surgeons to prevent damage to the neurovascular structures within the axilla during surgery. The axilla contains five groups of axillary nodes: pectoral, lateral, posterior (subscapular), central, and apical. The axillary lymph nodes filter lymphatic fluid that has drained from the upper limb and pectoral region. Approximately 75% of lymph from the breast drains into the axillary lymph nodes, so they can be biopsied if breast cancer is suspected. If breast cancer is confirmed, the axillary nodes may need to be removed to prevent the cancer from spreading. This procedure is known as axillary clearance.
The axillary region also contains the short head of the biceps brachii muscle, coracobrachialis muscle, fascia, and adipose tissue. Mammary tissue is also possibly present. The axilla is a three-dimensional pyramidal space that changes shape due to its location and arm movement. The size and shape of the axilla region vary with arm abduction. The apex decreases in size most markedly when the arm is fully abducted, leaving the contents of the axilla at risk of compression.
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Muscles that pass through the axilla
The axilla, or axillary region, is the area between the superior portion of the upper limb and the thorax. It is an important region in terms of neurovasculature and clinically. The axilla allows the passage of several muscles, blood vessels, nerves, and lymphatics.
The axilla is bordered by the serratus anterior and the thoracic wall medially, a part of the humerus laterally, the pectoralis muscles anteriorly, and the latissimus dorsi posteriorly. The axilla contains the axillary artery, which is the main blood supplier to the shoulder and upper limb. The axillary artery is a continuation of the subclavian artery, which lies just beneath the clavicles, and it branches into three parts: the region proximal to pectoralis minor, the region beneath it, and the region distal to it. The subscapular artery, the largest branch of the axillary artery, supplies the intercostal muscles, latissimus dorsi, and the serratus anterior muscles.
The axilla also contains the axillary vein, which runs medial to the artery along its length. The cephalic vein enters the axilla via the clavipectoral triangle, an opening in the anterior wall of the axilla. The axilla is further supplied by the brachial plexus, which is the principal nervous structure in the axillary area. It innervates the muscles around the axilla, the shoulder, and the upper limbs.
The muscles that pass through the axilla include the coracobrachialis, pectoralis minor, and the biceps brachii. The biceps brachii (short head) and coracobrachialis are muscle tendons that move through the axilla, attaching to the coracoid process of the scapula. The pectoralis major and minor muscles make up the anterior wall of the axilla.
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The axillary artery and its branches
The axillary artery is a large blood vessel that conveys oxygenated blood to the lateral aspect of the thorax, the axilla (armpit), and the upper limb. It is a continuation of the subclavian artery, which begins at the outer border of the first rib. The axillary artery is often referred to as having three parts, with these divisions based on its location relative to the pectoralis minor muscle, which is superficial to the artery.
The first part of the axillary artery is the region proximal to the pectoralis minor. This part gives rise to the superior thoracic artery, which supplies the first and second intercostal spaces.
The second part of the axillary artery is the region beneath the pectoralis minor. This part gives rise to the thoracoacromial artery, which has several branches that supply different regions of the upper limb and trunk. The clavicular branch supplies the subclavius and the sternoclavicular joint, while the pectoral branch supplies the pectoral muscles and provides oxygenated blood to the muscles and mammary glands. The deltoid branch supplies the deltoid muscle and the pectoralis major. The second part of the axillary artery also gives rise to the lateral thoracic artery, which carries oxygenated blood to the structures of the thoracic wall.
The third part of the axillary artery is the region distal to the pectoralis minor. This part gives rise to the subscapular artery, the largest branch of the axillary artery, which travels along the posterior wall of the axilla. The subscapular artery then divides into the thoracodorsal artery, supplying the latissimus dorsi muscle, and the circumflex scapular artery, which contributes to the scapular anastomosis. The third part of the axillary artery also gives off the anterior and posterior circumflex humeral arteries, with the latter passing through the posterior wall of the axilla and supplying the deltoid, triceps brachii, and glenohumeral joint.
The axillary artery is an important structure in the axilla region, supplying the upper limb with blood. It is surrounded by the brachial plexus, and its branches can be remembered with various mnemonics, such as "Screw The Lawyers Save A Patient" or "She Tastes Like Sweet Apple Pie." The right axillary artery is commonly used as an arterial cannulation site in cardiac surgery.
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Axillary nerve and its functions
The axillary nerve is one of the five peripheral nerves that run through the shoulder. It starts in the neck and extends to the shoulder, causing movement and sensation in the shoulder and the back of the upper arm. It is formed within the axilla (armpit) area of the upper limb and is a direct continuation of the posterior cord from the brachial plexus. It contains fibres from the C5 and C6 nerve roots.
The axillary nerve travels through the quadrangular space, a gap in the posterior wall of the axilla, along with the posterior circumflex humeral artery and vein. It passes behind the top of the upper arm bone (humerus) and exits the axilla at the inferior border of the subscapularis muscle. The nerve then divides into three terminal branches:
- The posterior terminal branch provides motor innervation to the posterior aspect of the deltoid muscle and the teres minor muscle, as well as the skin over the inferior part of the deltoid.
- The anterior terminal branch winds around the surgical neck of the humerus and provides motor innervation to the anterior aspect of the deltoid muscle.
- The collateral branch connects to the long head of the triceps brachii.
Injuries to the axillary nerve can affect the ability to rotate the arm or lift it. Damage can occur through trauma to the proximal humerus or shoulder girdle, such as a fracture of the humeral surgical neck, shoulder dislocation, or iatrogenic injury during shoulder surgery. Erb's palsy, for example, is a condition resulting from damage to the C5 and C6 roots of the brachial plexus, affecting the axillary nerve and causing an inability to abduct or externally rotate at the shoulder.
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Clinical relevance of the axillary region
The axillary region, also known as the armpit, is a clinically important area due to its anatomy and function. It is a pyramidal-shaped space between the shoulder girdle and thorax, which changes shape with arm movement. This region is of particular significance in the case of breast cancer, where axillary lymph node status determines the treatment approach.
The axillary region allows the passage of several muscles, blood vessels, and nerves. The muscles that pass through this region include the coracobrachialis, pectoralis minor, and the biceps brachii. The axillary artery, an extension of the subclavian artery, supplies the upper limb. It is divided into three parts, each giving rise to different arteries that supply the pectoral and deltoid muscles, as well as the subclavius and sternoclavicular joint. The axillary vein, which becomes the subclavian vein after exiting the apex of the axilla, serves as the major vessel of drainage for the upper extremity.
The axillary lymph nodes are a collection of five groups of lymph nodes found in this region. They filter lymphatic fluid that has drained from the upper limb and pectoral region. Enlargement of these nodes can be an indicator of breast cancer, and their biopsy is often performed to confirm the presence of cancer. If cancer is detected, axillary clearance, or the removal of these nodes, may be necessary to prevent cancer from spreading. However, this procedure carries the risk of damaging the long thoracic nerve, resulting in winged scapula due to denervation of the serratus anterior muscle.
The axillary region also contains crucial nerves, such as the brachial plexus, which gives rise to peripheral nerves supplying the muscles of the shoulder girdle and chest. Compression of the nerves in this region can lead to muscle wasting, fasciculations, weakness, and loss of sensation. The intercostobrachial nerves supply sensation to the skin of the axilla.
Overall, the axillary region is clinically relevant due to its role in allowing the passage of various structures, including muscles, blood vessels, and nerves. Its significance in the diagnosis and treatment of breast cancer, particularly through the evaluation and removal of axillary lymph nodes, further underscores its clinical importance.
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Frequently asked questions
The axilla (or armpit) is an area under the shoulder joint where the arm connects to the shoulder. The anterior axillary fold is formed by the lower border of the pectoralis major, and possibly the pectoralis minor. The posterior axillary fold is formed by the subscapularis, teres major, and latissimus dorsi.
The axillary region is a pyramidal space that allows the passage of several muscles, blood vessels, nerves, and lymphatics. The primary blood supply to the axilla is the axillary artery, which branches off the subclavian artery.
The medial wall is formed by the serratus anterior and the first four ribs. The lateral wall is formed by the humerus, the coracobrachialis, and the short head of the biceps. The anterior wall is formed by the pectoralis major and minor muscles.
