A defining aspect of human anatomy is the skeleton and the bones of which it is made up. They are often only thought of as the inert ‘scaffolding’ on which humans are made, supporting muscles and protecting the brain and internal organs. However, bones are much more complex in their form and function than being a mere supporting structure.
The average adult human has 206 bones in their body. In rare cases people may be born with an extra rib or an additional vertebra in the spine. Bones are made from cells which create a fibrous tissue framework. Within this framework, which is flexible, calcium (in the form of calcium phosphate) is deposited, giving bone its strength and familiar look. Bones begin formation within the first month of pregnancy, and to begin with are soft and very flexible, being formed from cartilage. This cartilage is gradually replaced by fibrous tissue, but contains little calcium to harden the bones. As the child grows, the bones gradually harden and only reach their maximum hardness at the end of puberty. A diet containing sufficent calcium and phosphorus is needed to retain the hardness of bone, as is vitamin D which helps in the absorption of these minerals. Deficiencies in any of these can lead to various forms of bone disease.
Adult bones are hollow, a fact that only fractionally diminishes their strength but considerably reduces the weight of the skeleton. The hollows are filled with bone marrow and this is where blood cells are made. Red blood cells carry oxygen, white blood cells serve the immune system, and platelets aid clotting and healing. Thus bones are not just for support, but are continually manufacturing the blood cells vital to our everyday existence.
Bones are usually classified according to their shape and use. Long bones are the simple cylinders of hard bone forming the limbs. Short bones, such as in the wrist, afford a greater degree of movement due to their size but have the same basic structure as the long bones. Flat bones, such as the skull or shoulder blade, are comprised of hard bone sandwiching an internal spongy layer. The skull consists of flat bones which create a protective shell around the brain. The shoulder blade is flat to create a large area for muscle attachment. The last type of bone, irregular bones, include the box like vertebrae of the spine and the minute bones of the middle-ear.
Where bone come together to form joints, they are either of the ball-and-socket or hinge type. The ends of the bones are covered with pads of cartilage and lubricated to prevent excessive wear or damage. The joints are held together by tough, fibrous ligaments. It is then the muscles, firmly anchored to the bones, which provide the power for motion. Bones are resilient structures and can mend themselves when broken. A break in the bone also results in the breaking of the blood vessels present in the bone. Clotting occurs and eventually new fibrous tissue grows into the clot from both sides of the break. The tissues intertwine, calcium is laid down and the join hardens. Initially the repair is in the form of a lump of bone, or callus, around the break, but with time remodeling occurs and a smooth join is formed. Of course, the time taken for healing depends on the severity of the break and a lot of other factors, but the basic process is the one outlined above.