Nerves are the organs which tell us what is going on in the world outside our bodies, and which take messages about this world to the brain, which translates the messages into feelings. There are four types of nerve in the body. The first is the one which picks up messages like pain, light, cold and heat. The second type of nerve receives these messages and sends others to parts of the body which cause swift action to be taken. For example, if you touch a hot iron, the first type of nerve tells you it is hot, and a split second late the second sort of nerve makes your muscles pull your hand away in what is called a reflex action. The third type of nerve takes messages over longer distances in the body, and is the type of nerve which makes our body do the things our brain orders it to do, such as moving parts of the body to perform certain actions. The final sort of nerve is that which transmits messages from the first type, called sensory nerves, to the brain where the messages are translated into feeling. This is why, having touched the iron, we may not feel the pain until just after we have pulled our hand away. We have nerves all over our body, although there are more in certain parts than in others. The central nervous system, through which all messages pass, is composed of the brain and the spinal cord, which is why we have to take special care not to damage our head or backs.