Two breasts and four pecs. In fact, there are even more. The chest is formed by: - the small pectoral muscle; - the large pectoral muscle; - the serratus; - subclavian muscle; - external and internal intercostal muscles. The muscle fibers of the chest are arranged in different directions. Many fibers are arranged in the form of a fan, something unique to our body. By the way, this is why fitness trainers suggest to train the chest at different angles.