Solid bones are extremely hard. Its structural unit is called the Haversian system. The body of the femur and humerus. The solid unstable matrix is arranged in several layers. The layers are called lamellae. The lamellae are arranged in a circle around a clear duct. This central duct is called Haversian duct. Each Haversian duct and the lamellae that surround it form a Haversian system. In each lamella there are several small cavities called lacuna. From each lacuna surrounds a number of fine ducts called canaliculi. With the help of these different lacuna maintains communication with each other.