Bone is a rigid body tissue that consists of cells embedded in a hard intercellular substance. Collagen and calcium phosphate are the two principle components of the material and distinguish bone from other hard tissues, such as chitin and enamel. The exterior layer of bone tissue, known as the cortex, is smooth, dense, and continuous. The interior of bone consists of a network of intersecting plates and spicules that vary in proportion and enclose spaces filled with blood vessels and marrow.