Structure of Bones
Types of tissue found in bones include periosteum, bone tissue, endosteum, bone marrow, blood vessels, and nerves. If you were to cut a cross-section through a bone, you would first come across a thin layer of dense connective tissue known as periosteum. The periosteum provides a good blood supply to the bone and a point for muscular attachment. Two types of bone tissue are cortical and cancellous bone. Under the periosteum is a thin layer of cortical bone also called compact bone, which provides the bones strength and accounts for 80% of the mass of bone in the human body. Cortical bone is solid bone with only a few small canals, which is covered by a periosteum on its outer surface, and an endosteum on its inner surface. Cancellous bone also called spongy bone is like scaffolding or a honey-comb, which accounts for roughly 20% of the total mass of the skeleton. In a long bone, cancellous bone is normally found at the ends of the bone, while in a flat or irregular bone, it is a thin layer found just inside the cortical bone. Moreover, the centre of the bone shaft is hollow and known as the medullary cavity which contains both red and yellow bone marrow. The red bone marrow produce blood cells, while yellow bone marrow is mainly a fatty tissue. In newborns, all bones are filled exclusively with red marrow, but as the child ages it is mostly replaced by yellow marrow. In adults, red marrow only remains in the vertebrae, ribs, sternum and cancellous bones of humerus and femur. Additionally, bone blood vessels mainly provide nutrition to the bones and many nerve fibers are associated with blood vessels in the bones.