Pectoralis minor is a triangular-shaped muscle that lies deep to pectoralis major on each side of the thorax. It forms part of the anterior wall of the axilla. Anatomically, it provides a lanmark to deeper structures, notably the axillary artery and cords of the brachial plexus.
It originates from the:
- external surfaces of the third, fourth and fifth ribs; occasionally, this costal origin may commence on the second rib or finish on the sixth rib
- intervening costal fascia
After passing superolaterally, it forms a tendon which inerts into the medial and superior surfaces of the coracoid process of the scapula.
It is mainly supplied by the medial pectoral nerve which passes through it. However, it receives also a motor supply from the lateral pectoral nerve. The combined root level is C6, C7 and C8.
The main actions of pectoralis minor are:
- protraction of the scapula around the thorax; it works in conjunction with serratus anterior to keep the scapula in close proximity to the thorax as the latter draws it anteriorly. This action assists in transferring the weight of the thorax to the arm when doing 'press-ups'.
- draws scapula inferiorly down thorax
- medial rotation of scapula
- as an accessory muscle of respiration, drawing the costal origins up during inspiration, when the scapula and upper arm are fixed
- stabilisation of the scapula so that other muscles can exert an effect on it
Testing pectoralis minor is difficult as it cannot be palpated through pectoralis (and the breast in females).