The axilla resembles a truncated four-sided pyramid and presents an apex, a base and four walls (anterior, posterior, medial, and lateral).
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Apex/cervico-axillary canal: It is a passageway between the neck and axilla. It is directed upwards and medially into the root of the neck and corresponds to the triangular space bounded in front by the clavicle, behind by the upper border of the scapula and medially by the outer border of the first rib.The axillary artery and brachial plexus enter the axilla from neck through this gap, hence it is also termed cervico-axillary canal. The axillary vein enters the neck from axilla into the neck through this canal.
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Base/floor:It is at the lower end of the axilla and directed downwards. It is formed by the axillary fascia. The base corresponds to the hollow bounded in front by the anterior axillary fold, formed by the lower border of the pectoralis major muscle, behind by the posterior axillary fold formed by the tendon of latissimus dorsi and teres major muscles, and medially by the lateral aspect of the chest wall.
SPECIAL FEATURES
Ø The perpendicular line dropped from a point midway between the anterior and posterior axillary folds is called midaxillary line – an important surface landmark.
· Anterior wall: It is formed by the pectoralis major, Subclavius, and pectoralis minor muscles. Posterior wall: It is formed by the subscapularis muscle above and lastissimus dorsi and teres major muscles below.
· Medial wall: it is formed by the upper four or five ribs, and corresponding intercostal spaces covered by the serratus anterior muscle.
· Lateral wall: It is formed by tendon of biceps brachii in the bicipital groove of humerus Coracobrachialis and short head of biceps brachii. The lateral wall is extremely narrow because anterior and posterior walls of the axilla converge at this site.