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The osteon is the primary structure that makes up the hard outer layer of bones, which is generally known as “cortical bone.” Cortical bone, or compact bone, is dense, mature bone found in vertebrates such as mammals, amphibians, reptiles, and birds. It is this dense hard layer that gives a person or animal strength. Approximately 80% of a vertebrate skeleton is composed of cortical bone, and the outer layer is made up of many individual osteons that together create a dense outside barrier. Each unit is somewhat complex from a technical standpoint and has many different parts, but it is generally made up of the Haversian canal, Volkmann's canals, osteocytes, and canaliculi. It is most prominent in adults and mature animals; children and growing adolescents often have less of it since it often comes with age.
Human skeletons, like the skeletons of most other vertebrates, are made of “living” bone. Though it may look tough and dense, in most cases it is actually a living tissue that grows and changes, both in density and actual composition, throughout a person’s life. The structure of a bone from a scientific standpoint is rather complicated. Different layers are filled with different tissues and canals that shuttle nutrients and some fluids from one place to another. Each osteon makes up an important part of the outer layer of most bones, but they don’t occur everywhere.
These bone units, which are sometimes described by analogy to “building blocks,” are typically found only in cortical bone and are not present in trabecular bone. Trabecular bone is also called spongy bone or cancellous bone and is usually found at the ends of long bones. Cortical bone is often most prominent in the human femur and tibia, while the trabecular bone is normally surrounded by cortical bone at the end of joints and the vertebrae.
Osteons are cylindrical in shape and typically run parallel to the long axis of the cortical bone. Each one contains a central canal, called the Haversian canal, along with concentric layers of bone called interstitial lamellae. In most cases the Haversian canal is actually surrounded with rings of lamellae. Lamellae are made up of bone matrix, collagen fibers, and mineral crystals. Interstitial lamellae are contained in the spaces between osteons, and their main job is to provide stability to the long bones.
Within the Haversian canal is a layer of endosteum, which is basically connective tissue that is very rich in nerve fibers and blood vessels. Blood cells within the canal carry nutrients and waste to and away from the outer layer of the bone. In many cases these systems of canals and lamellae are also and collectively called osteons, and the term is often used synonymously with Haversian system. The larger Haversian system was named for the 17th century English physician Clopton Havers, who is widely considered to be the first person to discover this central canal. He discussed the structure of bone in his book Osteologia Nova.
A different series of canals known as “Volkmann's canals” are also an important part of this hard outer layer. They are situated perpendicular to the long axis of the bone and connect the blood supply and nerves to the periosteum, which covers the surface of bones, to the Haversian canals. Particles known as “osteocytes” are found within spaces of this canal called lacunae, and their main job is to sense strain or stress in bones. They also maintain and support the bone matrix found within the lamellae.
Osteocytes are usually held together by canaliculi, which are very narrow canals that help facilitate the distribution of nutrients and the elimination of waste through the bone. Canaliculi radiate outward from the center, like the spokes of a wheel, and they connect the Haversian canal. Most osteons don’t themselves move, but a lot of movement happens within them.