Living bones
Hold out your right hand with the palm facing upwards. Place the first two fingers of your left hand on your right thumb. Move the fingers along the skin to the wrist. Pull them in to the middle of the wrist a little and feel a cord under your skin. It should feel tight like the string on a bow that fires arrows. It is really a stout thread called a tendon. It is one of many that connect muscles in your forearm to the bones in your wrist and hand. If you press the flesh to the right of the tendon you may feel a throbbing. It is called your pulse and doctors and nurses take a person's pulse to check their health or to see if they are still alive.

The pulse is caused by the beating heart as it pumps blood around the body. The blood travels in tubes around the body. They are called blood vessels. When you press on the flesh near your wrist you touch the sides of the blood vessel and feel the pulse made as the blood is pumped along. The blood vessel in your wrist is a large one. If you move your fingers away from the pulse towards the right, you will feel something harder in the flesh. This is a bone in the forearm and this contains blood vessels too.

It may seem strange to think of blood going into bones. Most people think of bones being dead because when they are seen, the animal or person that had them is no longer alive. Just think of seeing an animal skeleton in a desert or a human skeleton being dug up by archaeologists at the site of old ruined buildings. The blood enters the bones in a living body because the bones are just as alive as other parts of the body and need materials to maintain them and help them grow.

Inside the bone, the blood passes into very tiny blood vessels. They carry the blood to all parts of the bone. In the blood is food. The bone uses the food to make more bony substances. It also uses some of the food to create energy to make the changes. There is oxygen in the blood too. This is used to help release energy from the food. When the energy is released and new bone material is being made, a waste substance is also made. It is called carbon dioxide and it is carried away in the blood to the lungs. Part of the carbon dioxide you are breathing out now has been made by your bones as they grow and maintain themselves.

The bone uses some of the body-building foods in the blood. These foods are called protein. The bone uses protein to make a substance which is similar to the material in your ear. If you grab your ears with your fingers and thumbs you can easily bend them. Bones do not bend like this because they also take something else from the blood. They take a mineral called calcium. This mineral is used to make the stony substance in the bone that makes it strong and rigid.

The bone does not take up protein and calcium and make only a solid rod. At the ends of the bone a honeycomb structure is made which makes the ends large yet light. The largeness of the end helps it to make a firm joint. The lightness of the bone makes it easier to move. The long part between the ends of the bone is called the shaft. Here the protein and calcium pack closely together around the sides to make a hollow tube of bone. This structure is stronger and lighter than solid bone.

The bone is not only a living part of the body but it also helps other parts to live. Inside the hollow parts of the bone is marrow. Some of this is red marrow. It is this colour because it makes the red part of the blood. This part of the blood is made of millions of tiny structures called red blood cells. They flow round the body and when they enter the lungs they pick up oxygen. They carry the oxygen around the body and deliver it to any part that needs it. In fact the oxygen your body is taking in now will be carried around in the blood by cells that were made in your bones during the last four months.

Why do dead animals leave their skeletons behind?
A dead animal leaves all the parts of its body behind but they soon form food for other living things. Some large animals like vultures and crows feed on the flesh of dead animals. Flesh is soft and easy to tear up, eat and digest. Bone is hard and is often left behind. In the soil microbes feed on the bone but this takes a long time. Microbes need moisture in order to live so if the bones are in a dry place like a cave they may last for thousands of years. An animal does not really leave its bones behind: it just takes other living things a long time to break them down like flesh.

What would happen if the bone could not get enough material to make it strong?
The bone would be weak. If the bone had to carry a weight it would bend. There is a disease called rickets, which is due to the body not getting enough vitamin D. This vitamin helps the bone take calcium from the blood. If vitamin D is lacking from the diet of a child the leg bones become so weak that they bend. Rickets can be cured by taking in foods such as cheese, eggs and milk which are rich in vitamin D. Most people avoid getting rickets by eating enough foods rich in vitamin D in a balanced diet.

What is a joint?
It is a place where two bones meet. Some bones meet and join firmly together. They do not allow any movement but give strength. These joints are found in the skull and the
hip. In the spine, many of the bones are separated from each other by a disc of cartilage. This allows some movement between the bones and makes the spine flexible. In many joints the bones that meet have ends coated in cartilage and covered in a slippery liquid, rather like egg white, which allows the bones to move easily.

How are bones held together in a joint?
They are held together by ligaments. These are made from tough fibres which do not stretch. Ligaments can be torn however. When this happens we say that the joint has been sprained. A sprain also occurs if a tendon is damaged.

What is the difference between a ligament and a tendon?
A ligament holds bones together; a tendon connects a muscle to a bone. The tendons form the cords that you can feel in some joints. If you bend your leg and feel behind your knee you will feel your hamstring tendons.

Why can people get aches in their bones?
They can get aches because bones are living parts of the body and contain nerves. At the ends of these nerves are parts that are sensitive to pain called pain sensors. If a joint is damaged and one part of a bone rubs on another, the pain sensors send messages to the brain so that you will try and rest the damaged part until it recovers.