Pectoral Region and Axilla

Pectoral fascia: thin fascial sheet covering the anterior surface of pectoralis major, continuous with the axillary fascia laterally and the deep fascia of the abdomen inferiorly.

• Clavipectoral fascia: deep fascial sheet extending from the clavicle to enclose subclavius and pectoralis minor, continuing inferiorly as the suspensory ligament of the axilla (attaches to axillary fascia, responsible for the natural concavity of the armpit).
• Pierced by: lateral pectoral nerve, cephalic vein, thoracoacromial vessels, lymphatics.

• Origin: Clavicular head, anterior surface, medial half of clavicle. Sternocostal head, anterior surface of sternum, upper 6 costal cartilages, aponeurosis of external oblique.
• Insertion: Lateral lip of intertubercular sulcus of humerus (fibres twist so clavicular head inserts lowest, sternocostal head highest).
• Innervation: Lateral and medial pectoral nerves (C5–T1).
• Blood supply: Pectoral branch of thoracoacromial artery; also lateral thoracic artery.
• Action: Adduction and medial rotation of the arm; clavicular head flexes the arm, sternocostal head extends the flexed arm back to the side.
• Relations: Forms the anterior axillary fold; overlies pectoralis minor, clavipectoral fascia, axillary vessels and brachial plexus.
• Clinical relevance: Absence (Poland syndrome); used as a landmark for intramuscular injection avoidance; tendon rupture in weightlifters.

• Origin: Outer surfaces of ribs 3–5, near their costal cartilages.
• Insertion: Medial border and superior surface of the coracoid process of scapula.
• Innervation: Medial pectoral nerve (C8, T1), pierces the muscle to reach pectoralis major.
• Blood supply: Pectoral branch of thoracoacromial artery.
• Action: Depresses and protracts the scapula (draws it forward and down); stabilises scapula against the thoracic wall; raises ribs in forced inspiration.
• Relations: Divides the axillary artery into three parts (used as the key landmark for axillary artery description); clavipectoral fascia attaches to its upper border.
• Clinical relevance: Site for surgical division to access axillary vessels; hyperabduction can compress the neurovascular bundle beneath it (thoracic outlet syndrome variant).

• Origin: Junction of 1st rib and its costal cartilage.
• Insertion: Subclavian groove on inferior surface of clavicle.
• Innervation: Nerve to subclavius (C5, C6), from the upper trunk of the brachial plexus.
• Blood supply: Clavicular branch of thoracoacromial artery.
• Action: Depresses and stabilises the clavicle; anchors clavicle during movement.
• Relations: Lies between clavicle and subclavian vessels/brachial plexus, providing a protective cushion.
• Clinical relevance: May protect the subclavian vein from injury in clavicle fractures.

• Origin: Outer surfaces of ribs 1–8 (fleshy digitations).
• Insertion: Costal (anterior) surface of the medial border of the scapula (all three parts (superior, intermediate, inferior) converge, with the inferior part being strongest, inserting near the inferior angle).
• Innervation: Long thoracic nerve (C5, C6, C7), running on the muscle's superficial surface.
• Blood supply: Lateral thoracic artery; also branches of thoracodorsal and posterior intercostal arteries.
• Action: Protracts the scapula (as in a punching motion); rotates the scapula for full arm abduction above the horizontal; holds the medial border of scapula against the thoracic wall.
• Relations: Forms the medial wall of the axilla; long thoracic nerve is vulnerable during axillary surgery due to its superficial course.
• Clinical relevance: Winged scapula: long thoracic nerve injury (e.g., axillary lymph node dissection, penetrating trauma) causes loss of scapular protraction and stabilisation; medial border lifts off the thoracic wall, most evident on pushing against a wall.

• Deltopectoral groove: narrow groove between deltoid (laterally) and pectoralis major (medially); transmits the cephalic vein.
• Deltopectoral triangle (clavipectoral/infraclavicular triangle): bounded by deltoid, pectoralis major, and the clavicle; the cephalic vein drains through this space into the axillary vein.

Shape: Pyramidal space between the upper arm and the thoracic wall.

Apex: Cervico-axillary canal, bounded by the clavicle (anteriorly), scapula (posteriorly), and 1st rib (medially), the route by which vessels and nerves enter the axilla from the neck.

Base: Skin and axillary fascia, forming the floor of the armpit (visible hollow), reinforced by the suspensory ligament of the axilla.

Walls:

Contents:

• Axillary artery and its branches.
• Axillary vein and tributaries.
• Brachial plexus: cords and their branches (infraclavicular part).
• Axillary lymph nodes and lymphatics.
• Long thoracic nerve (on serratus anterior).
• Thoracodorsal nerve and vessels (on subscapularis/latissimus dorsi, posterior wall).
• Intercostobrachial nerve (lateral cutaneous branch of 2nd intercostal nerve; crosses axilla to supply skin of medial arm, often sacrificed in axillary clearance).
• Axillary fat and loose connective tissue.

Beginning: Continuation of the subclavian artery at the outer border of the 1st rib.

Termination: Becomes the brachial artery at the lower border of teres major.

Three parts (relative to pectoralis minor):

Mnemonic for branches (1 branch : 2 branches : 3 branches): "Students (Superior thoracic) Take (Thoracoacromial) Long (Lateral thoracic) Subway (Subscapular) Cars (anterior Circumflex humeral) Post-exam (Posterior circumflex humeral)."

• Superior thoracic artery: small, supplies 1st/2nd intercostal spaces and serratus anterior.
• Thoracoacromial artery: short trunk, divides into pectoral, deltoid, acromial, and clavicular branches.
• Lateral thoracic artery: descends along the lateral thoracic wall on serratus anterior; supplies breast (lateral part), serratus anterior, pectoral muscles.
• Subscapular artery: largest branch; divides into circumflex scapular artery (passes through triangular space to the posterior scapular region) and thoracodorsal artery (supplies latissimus dorsi).
• Anterior circumflex humeral artery: smaller, winds around the surgical neck anteriorly.
• Posterior circumflex humeral artery: larger, passes through the quadrangular space with the axillary nerve, winds around the surgical neck posteriorly.

Important relations:

• Related throughout its course to the cords and branches of the brachial plexus, which surround the 2nd part (lateral, medial, posterior cords named by their relation to this part of the artery).
• Axillary vein lies medial to the artery throughout.

Scapular anastomosis: Formed by the connection between the suprascapular artery and transverse cervical artery (branches of the subclavian, entering above), and the circumflex scapular and dorsal scapular arteries: creates collateral flow around the scapula, clinically important if the axillary artery is ligated or occluded proximally (e.g., in axillary aneurysm or trauma), allowing retrograde filling of the distal axillary/brachial artery.

Clinical relevance: Surface marking follows a line from the midpoint of the clavicle to the midpoint of the cubital fossa, with the arm abducted; site of axillary artery compression proximal to severe limb haemorrhage; vulnerable in anterior shoulder dislocation and proximal humeral fractures.

Formation: Union of the basilic vein and the venae comitantes of the brachial artery, beginning at the lower border of teres major.

Course: Ascends medial and superficial to the axillary artery, through the axilla to the outer border of the 1st rib.

Major tributaries: Cephalic vein (via deltopectoral triangle), veins corresponding to the arterial branches (thoracoacromial, lateral thoracic, subscapular, circumflex humeral veins).

Termination: Continues as the subclavian vein at the outer border of the 1st rib.

Important relations: Lies anteromedial to the axillary artery and the brachial plexus; closely related to the axillary lymph nodes, making it vulnerable during axillary lymph node dissection.

Approximately 20–30 nodes arranged in five groups:

Drainage pattern: Pectoral, subscapular, and humeral groups → central nodes → apical nodes → subclavian lymphatic trunk → venous system (via junction of subclavian and internal jugular veins, or thoracic duct on the left).

Clinical relevance: Axillary lymph node status is central to breast cancer staging; sentinel lymph node biopsy typically targets nodes near the lower border of pectoralis minor; axillary clearance risks injury to the long thoracic nerve (winged scapula), thoracodorsal nerve (weak arm adduction/extension), and intercostobrachial nerve (medial arm/axillary numbness), and can cause lymphoedema of the upper limb.