The pectoralis major (large pectoral) is a large muscle on the front of the upper chest and arm.
There are 2 “heads” – the clavicular and sterno-costal.
The clavicular head originates from the clavicle (collar bone) and runs obliquely downwards and laterally to the upper arm (humerus). The sterno-costal runs more horizontally slightly behind the clavicular head and originates from the sternum (breast bone) and ribs. The 2 heads blend to form one tendon in two layers with the sterno-costal head slightly deeper to the clavicular head.
Fig 1. Outside view of the large muscles around the shoulder