Bone design

The design and structure of individual bones is as intricate as any engineering structure. The skull, for instance, is made up of 30 different bones: eight in the cranium (the ‘brainbox’), 16 in the face, and three in each ear. The only moveable structure is the lower jaw, which hinges to the upper jaw. There is a large hole at the base of the skull, the foramen magnum, through which the spinal cord passes, and other much smaller holes allow sets of nerve fibres to pass out from the brain. Thus the skull encloses the brain and protects it from damage, but allows communication via nerves to other parts of the body. The backbone is an extremely complex structure. It has 24 separate moveable bones called vertebrae, plus two sets of fused bones at its base. Each vertebra has a thicker rounded body at the front, a hole through which the spinal cord passes, and several bony pro- cesses that link with other bones and provide anchoring flanges for muscles.

The neck is made up of seven cervical vertebrae. At the top is the atlas, a simple ring-shaped bone on which the skull rocks and tilts. The second cervical vertebrae, the axis, has an upward projecting process that allows the head to swivel from side to side. These two vertebrae therefore allow the head a great range of movements.

Discs separating the vertebrae, or the vertebrae themselves, may be damaged by a whiplash fracture. This most often occurs in car accidents in which the head, in the absence of a head-rest, is flung sharply back and forwards again as the car stops. In severe cases the spinal cord itself may be damaged; in less severe capfs it is usually one of the intervertebral discs that is injured. Patients may have to wear a collar to support the neck and allow this kind of injury to heal. In the chest region there are 12 thoracic vertebrae. Their main role is to protect the spinal cord, and to form a hinge for the ribs. Twelve pairs of ribs link with them at the back; at the front of the body the first seven pairs attach to the breastbone (the sternum) through a soft band of costal cartilage. The next three pairs of ribs link to the breastbone indirectly through cartilage extending down from the bone. The final two pairs of ribs are ‘floating’ ribs. The rib cage, together with the backbone, protects the heart and lungs. As we breath, the whole of the rib cage moves upwards and outwards to allow the lungs to expand. The sternum is a flat bone about 15cm long. It lies very close to the wall of the chest and can easily be felt. Bone marrow in the sternum is relatively easy to extract and doctors use this bone to retrieve small amounts of marrow for tests. The process is called sternal puncture.

The bones of the legs, particularly the femur, are very large and strong to allow us to stand upright. The bones of the arms are much lighter and shorter, because the evolution of our upright posture means they are no longer needed to support our body weight. The pelvis (hipbone) is made up of three fused bones. This bowl-shaped structure protects lower abdominal organs and gives a firm base from which to anchor the powerful muscles that move the legs.