The Axilla Region

Original Author: Oliver Jones
Last Updated: March 23, 2017
Revisions: 33

The axilla is the name given to an area that lies underneath the glenohumeral joint, at the junction of the upper limb and the thorax. It is a passageway by which neurovascular and muscular structures can enter and leave the upper limb.

In this article, we shall examine the anatomy of the axilla – its borders, contents, and any clinical correlations.

Fig 1 - Anterior view of the right axilla region. Note the pyramidal shape, with 6 borders (or sides).

Fig 1 – Anterior view of the right axilla region. Note the pyramidal shape, with 6 borders (or sides).


Borders

The overall 3D shape of the axilla looks slightly like a pyramid. The borders consist of four sides and a base with an opening at the apex.

  • Apex –  Also known as the axillary inlet, this is formed by lateral border of the first rib, superior border of scapula, and the posterior border of the clavicle.
  • Lateral wall – Formed by intertubercular groove of the humerus.
  • Medial wall – Consists of the serratus anterior and the thoracic wall (ribs and intercostal muscles).
  • Anterior wall – Contains the pectoralis major and the underlying pectoralis minor and the subclavius muscles.
  • Posterior wall – Formed by the subscapularis, teres major and latissimus dorsi.

The size and shape of the axilla region varies with arm abduction. It decreases in size most markedly when the arm is fully abducted – at this point, the contents of the axilla are at most risk of injury.

Fig 2 - Transverse section of the axilla region.

Fig 2 – Transverse section of the axilla region.

Contents

The main, and clinically important contents of the axilla region include muscles, nerves, vasculature and lymphatics:

  • Axillary artery – It is the main artery supplying the upper limb. It is commonly referred as having three parts, one medial to the pectoralis minor, one posterior to pectoralis minor, and one lateral to pectoralis minor. The medial and posterior parts travel in the axilla.
  • Axillary veinThe main vein draining the upper limb, its two largest tributaries are the cephalic and basilic veins.
  • Brachial plexus – A collection of spinal nerves that form the peripheral nerves of the upper limb.
  • Biceps brachii and coracobrachialis –  These muscle tendons move through the axilla, where they attach to the coracoid process of the scapula.
  • Axillary Lymph nodes – The axillary lymph nodes filter lymph that has drained from the upper limb and pectoral region. In women, axillary lymph node enlargement is a non-specific indicator of breast cancer.
Fig 4 - Contents of the axilla region.

Fig 3 – Contents of the axilla region.


Passageways Exiting the Axilla

There are three main routes by which structures leave the axilla.

The main route of exit is immediately inferiorly and laterally, into the upper limb. The majority of contents of the axilla region leave by this method.

Another pathway is via the quadrangular space. This is a gap in the posterior wall of the axilla, allowing access to the posterior arm and shoulder area. Structures passing through include the axillary nerve and posterior circumflex humeral artery (a branch of the axillary artery.

The last passageway is the clavipectoral triangle, which is an opening in the anterior wall of the axilla. It is bounded by the pectoralis major, deltoid, and clavicle. The cephalic vein enters the axilla via this triangle, while the medial and lateral pectoral nerves leave.

Fig 3 - Boundaries and contents of the clavipectoral triangle

Fig 4 – Boundaries and contents of the clavipectoral triangle


Clinical Relevance

Thoracic Outlet Syndrome

The apex of the axilla region is an opening between the clavicle, first rib and the scapula. In this apex, the vessels and nerves may become compressed between the bones – this is called thoracic outlet syndrome.

Common causes of TOS are trauma (e.g fractured clavicle) and repetitive actions (seen commonly in occupations that require lifting of the arms) It often presents with pain in the affected limb, (where the pain is depends on what nerves are affected), tingling, muscle weakness and discolouration.

Lymph Node Biopsy

Approximately 75% of lymph from the breast drains into the axilla lymph nodes, so 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 spreading. This is known as axillary clearance. During this procedure, the long thoracic nerve may become damaged, resulting in winged scapula.

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Quiz

Question 1 / 9
Which of the following forms part of the medial wall of the axilla?

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Question 2 / 9
The subscapularis muscle forms a part of which border of the axilla?

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Question 3 / 9
What structure enters the axilla through the clavipectoral triangle?

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Question 4 / 9
Which of these muscles travels through the axilla?

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Question 5 / 9
What can be biopsied in suspected breast cancer?

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Question 6 / 9
The anterior wall of the axilla is formed by:

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Question 7 / 9
Which structure exits through the quadrangular space?

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Question 8 / 9
In which situation are the contents of the axilla most at risk of injury?

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Question 9 / 9
Which of these nerves exit the axilla via the clavipectoral triangle?

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