The human skeleton is the chassis of the body, forming a support for organs and a framework for movement. The bones of the skeleton are extremely strong, matching the strength of reinforced concrete. Because of their arrangement and the structure of bone tissue itself, they also form very efficient shock-absorbers. Part of the skeleton comprises of a system of ‘levers’ to which muscles are attached. These enable us to move. Many of the bones also protect other parts of the body; for example, the skull provides a protective casing for the brain, the rib cage protects the heart and lungs, and the pelvis encloses the bladder and (in women) the womb.
Less obvious, although no less important, is the bones’ role in making new. The manufacture of occurs in the soft bone marrow where millions of new cells are made every minute. Bones are also the body’s mineral bank, storing large amounts of minerals such as calcium and phosphorus, which are important for many functions of the body. The human skeleton is an endoskeleton, meaning it is inside the body. Some animals, such as insects and tortoises, have an external or exoskeleton – an armour-plated shell – that has a more obviously protective role than an endoskeleton. In a human being, the skull is reminiscent of an exoskeleton since it almost completely encloses the brain with only a relatively thin layer of tissue over it. But the rest of the human skeleton is especially designed for us to walk upright and has undergone several changes in evolution to allow us to do this. The pelvis became stronger and more bowl-shaped to support the upper body. The spine became bent into an S-shaped curve, to absorb shocks. The thigh bones became longer and stronger, and the bones in the foot and heel have changed to allow them to support more weight in a vertical posture. The skull itself has grown in size since the time of our primitive ancestors, to enclose the larger brain.
Bones and joints
Each of us has about 206 bones in our bodies. This number is occasionally different in some people because the number of bones in the hands and feet can vary. There is outwardly little difference between a man’s and a woman’s skeleton apart from the pelvis, which is broader and more bowl-shaped in women to allow for childbirth; men’s bones may also be slightly larger and heavier, and their shoulders tend to be broader. A trained pathologist can often gauge the age and sex of a skeleton from structural details. The human skeleton can be thought of as being composed of two parts: the axial skeleton, consisting of the skull, backbone and rib cage, the two latter of which form the supportive struts for the body; and the appendicular skeleton that supports limbs. Living bones are not the dry, brittle objects we are used to seeing in museums. Bones are a living and very active tissue especially formed to combine strength and elasticity. The flexibility of bone is conferred by the spongy matrix of collagen fibres inside it. The strength is provided by the arrangement of hard mineral deposits (mainly calcium and phosphorus) in the bone. The collagen fibres in bone extend in all directions, but expecially along the lines of tensional force.
Fractures arise mainly if a disproportionate amount of force is exerted on bones, perpendicularly or radially. Bone tends to become more fragile and less flexible as we get older, gradually being less able to resist knocks, shocks, compression and bending. Consequently the bones of elderly people tend to break more easily.
The bones of the skeleton articulate with each other by joints. The skull, although it appears to be one solid structure, is actually made up of several bones fused together. The bones of the spine (the vertebrae) interlock closely so that only limited movement is possible at each joint, although the spine as a whole has great flexibility. Other joints are much more free to move: elbows and fingers bend using hinge joints; the upper arms and legs are even more mobile because they articulate through ball-and-socket joints in the shoulder and hip.
Most of the bones of your body are made up of two types of bone tissue: compact (lamellar) bone and spongy (cancellous) bone. Compact bone tissue is very dense and strong and makes up the largest part of most bones. Spongy bone tissue has a more honeycombed structure, making it more compressible than compact bone and relatively light yet strong. It is found at the ends of the long bones such as the femur (thigh bone) in the leg and the humerus in the upper arm. It allows the bone to absorb impact, for example when we land hard on the ground after jumping. Spongy bone tissue contains red bone marrow which is responsible for making blood cells, including red cells, white cells such as granulocytes, and platelets. The cells develop and mature in the marrow before entering the bloodstream. Red bone marrow is particularly active and abundant in children. As we get older most of the red bone marrow in the cavities of the long bones becomes inactive and fills up with fat. An adult, therefore, has yellow, inactive bone marrow in most of the shafts of the long bones, and red bone marrow is restricted to the ends of the bones. A layer of fibrous tissue, the periosteum, surrounds every bone. This protects the outer surface of the bone, supplies it with blood and nutrients, and also attaches tendons and ligaments to the bone. The periosteum has an extensive sensory nerve supply, and so is sensitive to touch, although the bone itself cannot ‘feel’. The periosteum covering stops just short of the ends of bones where there are joints, and is replaced by a soft protective pad of cartilage. The main purpose of the cartilage is to prevent the bone ends being worn down by continuous rubbing against each other. The cartilage is a special type called hyalin cartilage, with a smooth bluish-white appearance. It consists of a solid matrix material (mainly the protein collagen) in which cells called chondrocytes are scat- tered. These cells are normally collected in groups of two or more and as they divide and grow they continuously make new matrix material. Hyalin cartilage is also found in parts of the larynx and windpipe and at the point where the ribs attach to the breastbone.
The structure of bone tissue
Bone tissue is basically a network of fibrous tissue containing collagen together with hard mineral deposits. It has been calculated that bone contains about 99 per cent of the body’s calcium, 88 per cent of its phosphorus, 50 per cent of its magnesium, 35 per cent of its sodium supply, and 9 per cent of all the water in the body.
Bone tissue, although appearing quite smooth at first sight, in fact is made up of many hundreds of cylindrical structures lying alongside each other. These cylinders are called Haversian systems. Each one comprises a central canal supplying blood, lymph and nerves to the bone, surrounded by a series of concentric layers of bone called lamellae. These layers are separated by spaces called lacunae. The lacunae contain lymph and bone cells (osteocytes), which are responsible for making the bone matrix. Blood, carrying oxygen and nutrients, flows through the Haversian canals in the bone at a constant rate. About five per cent of the total volume of blood in the body is flowing through the bones at any one time.
Bones in the skeleton are classified according to their shape: long, short, flat and irregular. Long bones include the humerus in the upper arm, the radius and ulna in the lower arm, the femur in the thigh and the tibia and fibula in the shin. They all
Share the same basic shape: a long shaft, called the diaphysis, with two rounded ends called the epiphyses. The diaphysis of each long bone contains mainly compact bone tissue, although the open, spongy canal running through its centre, as we have seen, is filled with yellow marrow in adulthood. The epiphyses have an outer shell of compact bone tissue, with spongy bone and red bone marrow making up the interior.
The breastbone (sternum) and the skull are both flat bones. They have an outer layer of compact bone tissue covered by periosteum; inside, the bone tissue is of the spongy type.