Pectoralis major m.

The pectoralis major is a thick, fan-shaped muscle situated on the anterior chest. It makes up the bulk of the chest muscles in the male and lies under the breast in the female. 

The pectoralis major originates in two sets of fibers, often referred to as "heads":

1. The sternal head arises from  the lateral edge of the upper part of the sternum and manubrium.

2. The clavicular head arises from most of the anterior border and upper surface of the medial two thirds of the clavicle.

The pectoralis major receives dual motor innervation from the medial pectoral nerve and the lateral pectoral nerve. The pectoralis major flexes and adducts the humerus and rotates the humerus medially.