Posterior wall of axilla is formed by -
Axilla (armpit) 
o The axilla is a pyramidal space situated between the upper part of the arm and the chest wall. It resembles a four sided pyramid, and has following : (i) an apex (ii) a base (iii) four walls (anterior, posterior, medial and lateral).
1. Anterior (pectoral) wall Formed by (i) Pectoralis major; (ii) Pectoralis minor, and (iii) Subclavius. The . laer two muscles enclosed by clavipectoral fascia.
2. Posterior (subscapular) wallFormed by (i) Subcapularis. (ii) Teres major, and (iii) Lassimus dorsi.
3. Medial (thoracic) wallFormed by (i) Upper four ribs (with their intercostal muscles), and (ii) Upper part of serratus anterior.
4. Lateral (humeral) wallFormed by (i) Upper part of humerus with bicipital groove lodging the tendon of long head of biceps, and (ii) Corachobrachialis and short head of biceps -
5. Base Formed by Skin, superficial fascia and deep (axillary) fascia. It is directed downwards.
6. Apex It is directed upwards and medially towards the root of neck. It communicates with supraclavicular triangle of neck, hence referred to as Cervicoaxillary canal. It is triangular in shape and is bounded anteriorly by clavicle, posteriorly by upper part of scapula and medially by outer border of first rib. The axillary artery and brachial plaxus enter the axilla through this canal.
Pectoralis major
Subclavius
Subscapularis
Intercostal muscles