According to the University of Washington Radiology Department's muscle atlas, the pectoralis major is located in the chest and forms most of the chest wall. The pectoralis major connects to the humerus, the clavicle and the sternum.
UW Radiology details how the pectoralis major connects to the humerus (upper arm) through an insertion into the intertubercular groove of the humerus. This connection is necessary so that the upper arm can be rotated and pulled forward. The other side of the pectoralis major runs along the bottom side of the clavicle and the front side of the sternum. The clavicular head is responsible for flexing the arm, and the sternal head rotates the arm. Both heads are used to move the arm inward.