This large, fan - shaped muscle originates at the clavicle, ribs and sternum, and inserts into the upper portion of your humerus (upper arm bone from elbow to shoulder), and its main job is to help flex the shoulder joint and move your arm toward and across your chest. (fitnessandpower.com)
One end of your pecs is on your sterum and the other is here on upper arm bone as shown in the below diagram: (scoobysworkshop.com)
In 1996, an amateur archaeologist found a single upper arm bone sticking out of the steep riverbank — the first clue that the Tollense Valley, about 120 kilometers north of Berlin, concealed a gruesome secret. (sciencemag.org)