Boundaries of the Axilla
The axilla is a pyramid-shaped space below the glenohumeral joint that is the passageway for nerves and vessels to pass into the upper arm.
- Base: hair and sweat gland–bearing axillary skin
- Apex (axillary inlet): between the 1st rib, scapula, and clavicle
- 4 converging walls:
- Anterior wall: pectoralis major and minor
- Medial wall: serratus anterior, thoracic wall
- Lateral wall: intertubercular sulcus of the humerus
- Posterior wall: subscapularis, latissimus dorsi, teres major muscles
Boundaries of the axilla
Image by BioDigital, edited by Lecturio
Axillary inlet
Image by Lecturio.
Apex (axillary inlet): between the 1st rib, the scapula, and the clavicle
Image by BioDigital, edited by Lecturio
The axilla viewed from the anterior anatomical position
Image by BioDigital, edited by Lecturio
Posterior and medial walls of the axilla: Anterior wall and muscles of the upper limb have been removed.
Image by BioDigital, edited by Lecturio
Anterior view of the axilla, showing the anterior wall and the biceps brachii and coracobrachialis muscles running through the axillary fossa
Image by BioDigital, edited by Lecturio
Contents of the Axilla
The contents of the axilla are structures enclosed in the axillary sheath.
Embedded in the axillary fat, enclosed within the axillary sheath, the contents of the axilla include: the axillary artery and its branches, the axillary vein and its tributaries, the branches of the brachial plexus, and the axillary lymph nodes.
Image by BioDigital, edited by LecturioAxillary artery
The axillary artery is a direct continuation of the subclavian artery and is composed of 3 parts:
- 1st part:
- Superior to the pectoralis minor muscle
- Branches: superior thoracic artery
- 2nd part:
- Deep to the pectoralis minor muscle
- Branches: thoracoacromial and lateral thoracic artery
- 3rd part:
- Inferior to the pectoralis minor muscle and extending to the lower border of the axilla
- Branches: subscapular artery, anterior, and posterior circumflex humeral artery
The axillary artery (in blue) originates at the lateral margin of the 1st rib, before which it is called the subclavian artery. After passing the lower margin of teres major, the axillary artery becomes the brachial artery.
Image by BioDigital, edited by Lecturio
First and 2nd parts of the axillary artery, giving off the superior thoracic and thoracoacromial arteries as branches.
Image by BioDigital, edited by Lecturio
Second and 3rd parts of the axillary artery, giving off the thoracoacromial, lateral thoracic, anterior, and posterior humeral circumflexes, and subscapular arteries as branches.
Image by BioDigital, edited by Lecturio
Axillary artery and its branches (humerus faded)
Image by BioDigital, edited by Lecturio
Axillary vein
The axillary vein is the primary venous drainage of the upper arm, formed by the cephalic and basilic veins.
Axillary vein: primary venous drainage of the upper arm, formed by the cephalic and basilic veins
Image by BioDigital, edited by Lecturio
Axillary lymph nodes
There are 5 groups of axillary lymph nodes draining to the apical lymph nodes:
- The anterior or pectoral group drains the anterolateral abdominal wall and lateral quadrants of the breast.
- The posterior or subscapular group drains the superficial layers of the back.
- The lateral group drains the upper limb.
- The central group drains the anterior, posterior, and lateral groups, which drain into the apical or subclavicular group at the apex of the axilla (apical → subclavian lymph trunk → thoracic duct on the left and right lymphatic trunk on the right).
- The infraclavicular or deltoid group (not found within the axilla) drains the superficial layers of the upper limb.
These lymph nodes filter lymph fluid from the arm and pectoral region, including the breast (important in the diagnosis and management of breast cancer).
Axillary lymph nodes
Image by BioDigital, edited by Lecturio
Brachial plexus
The brachial plexus is formed from the anterior rami of spinal cord segments C5–T1, which are the roots of the brachial plexus and the network of nerves that supply the upper arm.
Schematic of the brachial plexus and its branches
Image by Lecturio
The Brachial Plexus
The brachial plexus is a nerve plexus formed by intercommunication of the ventral rami of C5–T1 nerves. The brachial plexus serves as the origin of all the peripheral nerves that innervate the upper limb and shoulder.
| Segment | Branch | Function |
|---|---|---|
| Roots | Dorsal scapular nerve (C5) | Supplies the rhomboid major and minor muscles and the levator scapulae muscles |
| Long thoracic nerve (C5–C7) | Supplies the serratus anterior muscles | |
| Upper trunk | Suprascapular nerve (C5–C6) | Supplies the supraspinatus and infraspinatus muscles |
| Nerve to subclavius (C5–C6) | Supplies the subclavius muscle and the sternoclavicular joint | |
| Lateral cord | Lateral pectoral nerve (C5–C7) | Supplies the pectoralis major muscle |
| Musculocutaneous nerve (C5–C7) | Supplies the coracobrachialis muscle, biceps brachii, and brachialis muscles, terminates as the lateral cutaneous nerve of forearm | |
| Lateral root of the median nerve (C5–C7) | Joins a similar branch from the medial cord to form the median nerve (opposition of the thumb and flexion of the first 3 fingers) | |
| Medial cord | Medial pectoral nerve (C8, T1) | Supplies the pectoralis major and minor muscles |
| Medial cutaneous nerve of the arm (medial brachial cutaneous nerve) (C8, T1) | Supplies the skin on the medial side of the arm | |
| Medial cutaneous nerve of the forearm (medial antebrachial cutaneous nerve) (C8, T1) | Supplies the skin on the medial side of the forearm | |
| Medial root of the median nerve (C8, T1) | Joins with the lateral root to form the median nerve | |
| Ulnar nerve (C7–T1) | Flexion of the 4th and 5th fingers | |
| Posterior cord | Upper subscapular nerve (C5–C6) | Supplies the upper part of the subscapularis muscle |
| Thoracodorsal nerve (C6–C8) | Supplies the latissimus dorsi muscle | |
| Lower subscapular nerve (C5–C6) | Supplies the lower part of the subscapularis and teres major muscles | |
| Axillary nerve (C5–C6) | Supplies both the deltoid and teres minor muscles | |
| Radial nerve (C5–T1) | Extension of the wrist and fingers |
Brachial plexus
Image by Lecturio.Axillary Spaces
The axillary spaces are anatomic spaces in the axilla where important nerves and vessels exit the axilla.
Posterior view of the axilla, featuring the scapulohumeral muscles and the axillary spaces: quadrangular and triangular spaces
Image by BioDigital, edited by Lecturio
Anterior view of the axilla, featuring the scapulohumeral muscles and the axillary spaces: quadrangular space, triangular space, and triangular interval
Image by BioDigital, edited by Lecturio
| Space | Borders | Content |
|---|---|---|
| Quadrangular space |
|
|
| Triangular space |
|
Scapular circumflex artery and vein |
| Triangular interval |
|
|
Spaces of arm using a finger arrangement
Image by Lecturio.Clinical Relevance
The following are common conditions and pathologies associated with the axilla:
- Axillary lymphadenopathy: inflammation or disease of the lymph nodes. Common causes include infection and breast cancer; lymph node dissection is often part of the staging of breast cancer disease.
- Axillary nerve injury: The axillary nerve innervates the deltoid and teres minor muscles and the skin of the upper lateral and dorsal areas of the upper arm through the superior lateral brachial cutaneous nerve. Common causes of injury include shoulder dislocation, fracture of the surgical neck of the humerus, and scapular fracture.
- Radial nerve injury: commonly injured in the upper arm at the spiral groove of the humerus after fracture, or from pressure on the axilla from a chair (also known as “Saturday night palsy”) or from crutches.
- Brachial plexus injuries:
- Erb’s palsy: a type of obstetrical palsy; obstetrical palsy is an injury due to trauma during childbirth. Erb’s palsy is a stretch injury to the brachial plexus, specifically C5–C6, during labor and delivery.
- Klumpke’s palsy: another type of obstetrical palsy that results in injury to the lower trunk of the brachial plexus.
References
- Drake, R.L., Vogl, A.W., & Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA: Churchill Livingstone.
