Thoracic muscles are divided into large and small. The large pectoral muscle has a triangular shape and is weaved by one of the bases in the shoulder.
Due to which the muscles of the chest and shoulders are connected. The small pectoral muscle is located under the large and is rather stabilizing, rather than working.
In fact, that the muscles of the breast can not work in isolation. When performing any exercise on the chest, muscles of the upper zone of the press, hands (biceps and triceps), deltoid muscles and even back muscles will be involved. That is why the upper body is recommended to train in one workout.