The pectoralis major is fan-shaped muscle that has two anatomic sections, or heads. The upper clavicular head arises from the clavicle (collarbone), and the lower sternal head arises from the sternum (breastbone). The two heads pass putward across the chest wall and merge into a single tendon that attaches to the humerus bone in the upper arm. As the muscle inserts, the tendon twists so that the upper head attaches beneath the lower head. When the pectoralis muscle contracts, movement takes place at the shoulder joint. When the pectoralis muscle contracts, movement takes place at the shoulder joint. Pectoralis major adducts, flexes, and internally rotates the arm, thus moving the arm forward and across the chest during movements such as a push-up or a bear hug. Even though the muscle has only two sections (upper, middle, and lower), depending on the angle through which the arm is moved. As the position of the shoulder joint changes, certain fibers of the chest muscle have better mechanical advantage to create motion. Other fibers of the chest muscle are still active but are not able to contract as much because of the shoulder position.
The side wall of the chest is formed by the serratus anterior. This
muscle arises f rom the scapula behind, and it passes forward around
the chest wall to attac to the upper eight ribs. The serrated edge of
this muscle emerges from beneath the outer margin of the pectoralis
muscle. The serratus anterior pulls (protracts) the scapula forward,
stabilizing it against the rib cage. The serratus anterior is active during
most chest exercises and works especially hard during the lockout
phase of a push-up or bench press. The pectoralis minor muscle lies
deep beneath the pectoralis major and is not visible. It has only a minor
function and does not contribute to the size of the chest.
Source: wikipedia.org | book – Bodybuilding Anatomy by Nick Evans