Arm and leg bones
Press the fingers of one hand into the front of one of your shoulders. If you move your finger tips around, your should be able to feel a bone running out from your neck to the end of your shoulder. This bone is called a collarbone. Now slide your fingers over the top of your shoulder and a little way down your back. If you move your fingertips around now you should find a broad bone called the shoulder blade. Move your arm up and down as you grip your shoulder blade. You should feel your shoulder blade moving.

Your collarbone and shoulder blade share a task in your skeleton. They make a cup-shaped hollow at the end of your shoulder. Into this hollow fits the top of the upper arm bone. The end of this bone is ball-shaped so that it fits snugly into the hollow. The joint made by the three bones is called a ball and socket joint. Put your hand over the top of your arm and move your arm around. You can feel the ball of the arm bone move in the socket. Notice that your arm can go backwards and forwards, up and down and round and round. A ball and socket joint allows movement in many directions.

Straighten your arm, grip your elbow with your hand and try and move the elbow from side to side. Notice that it will not move in either direction but remains firm. Let your forearm lift and lower and notice how easily the bones move now. The type of joint at your elbow is a hinge joint. It allows the bone to move up and down like the hinges on a box lid.

Hold out your palm as if asking for some pocket money then move your hand in towards your stomach. Notice that the hinge joint at the elbow does not change. The movement is due to the ball and socket joint in your shoulder. If you do not believe it just hold your upper arm as you move your hand out and in and feel the upper arm bone turn. The two joints are working together to move your hand in a particular way.

At the top of your leg is another ball and socket joint. The socket is made by a hollow in the hipbone and the ball is made at the top of the thighbone. Check the movement that this joint gives you in the following way. Stand on one leg, move the other leg backwards and forwards then from side to side. Finally straighten your leg and pretend that you are stirring up some water in a puddle with the tip of your big toe. You should feel that the joint lets the leg move in a circular motion.

If you sit down and feel your knee as you straighten and bend it you will find that the knee joint only allows the lower leg to move backwards and forwards and not from side to side. This kind of movement is the same that is allowed at the elbow. The joints at the elbow and the knee are the same. They are hinge joints.

Finally grip your right thigh and cross your right leg over your left leg. Feel the thigh bone rise and turn. Now slide your right ankle up until it is resting on the top of your left knee and feel how the thighbone moves again.

These are just a few examples of how the bones are arranged in your skeleton to provide joints for movement. Next time you pick up a pen and write a sentence, look at your arm and see how the bones are moving at your shoulder and elbow. You should see that you are moving a little like a machine.

Are the bones in the arms and legs similar?
Yes. In the upper arm and the upper leg there is one long bone. In the forearm and the lower leg there are two long bones. Both the wrist and ankle have the same number of bones and the hand and foot have the same number of bones. The main difference is that the kneecap is found at the hinge joint in the leg and there is not a similar bone at the hinge joint in the arm.

What is the purpose of the kneecap?
It provides protection to the front of the large knee joint and connects the muscles of the upper leg to the lower leg. This connection allows some of the muscles in the upper leg to raise the lower leg.

Are the long bones solid?
No. Inside the ends of the bones it looks like a honeycomb. Along the main length of the bone, called the shaft, the bone forms a hollow tube. Both the honeycomb and the tube are filled with marrow. The reason why the bone has both these structures is to reduce its weight. If the bones were solid like metal bars they would be much heavier. This would mean that it would be more difficult to move them, like carrying round a heavy weight inside your leg or the body would need larger muscles which in turn would need you to eat much more food to make them.

Are hollow bones weaker than solid bones?
No. The tube shape of the long bone makes it stronger than a solid bone of the same size. By having hollow bones the skeleton saves weight and is actually stronger than if the bones were solid.

Do other animals have bones with a similar structure to ours?
Yes, they do. They have hollow tubular limb bones which save weight and give extra strength.

Do other animals have shoulders and hips?
You can find shoulder and hip bones in fish. A fish has two pairs of fins. One pair is at the front near the head and the other pair is nearer the tail The shoulder bones support the front pair of fins and the hipbones support the pair of fins nearer the tail. Shoulders and hips are also found in frogs, toads, newts, lizards, crocodiles, tortoises, birds and mammals. In animals that walk on four legs the hips provide a strong support for the muscles of the back legs, which provide most of the power for movement. The shoulders are less strongly attached to the backbone in animals such as frogs and rabbits, which leap into the air. When these animals land the front legs receive a shock or strong push from the ground. The shoulders help to absorb some of this shock like buffers on a train so the legs and backbone are not damaged.

Do other animals have five fingers and toes?
Yes, many do, but some have fewer. The horse, for example, stands on one toe. It does not have a toenail but a hoof. The cow, pig and sheep stand on two toes. Birds have only three fingers in their wings and four toes on their feet.

Do any animals have more than five fingers and toes?
The mole is unusual in that it has an extra bone in its hand, which acts like an extra finger to make the hand stronger for digging in the soil.

Do other animals have long bones in their arms and legs like ours?
Yes, they do. In each limb they have one long bone in the upper limb and two in the lower limb.

Why do all animals with backbones have limbs which seem very similar?
Many scientists believe that land animals developed from fish. These first land animals had a pair of limbs where the fish had a pair of fins. Each limb had one long bone in the upper part, two in the lower part and five fingers and toes. In time some of these animals developed into frogs, reptiles, birds and mammals.

What about snakes? They don't have any limbs.
This is true but the python is a snake, which has a small hipbone under its scales. This suggests that snakes once had limbs like lizards and then lost them.