What is the large fan shaped chest muscle?

The large fan-shaped chest muscle is known as the pectoralis major. It plays a crucial role in the movement of the shoulder joint, assisting in actions such as pushing, lifting, and rotating the arm. The muscle is divided into two parts: the clavicular head, which originates from the clavicle, and the sternocostal head, which arises from the sternum and the ribs. Together, they contribute significantly to the upper body's strength and aesthetics.