As has been said above, the whole of the body of the higher vertebrates is under the direction and control of the nervous system. Quite apart from the relationship of the body to the environment, the heart, lungs, digestive system and the rest, take their time from, and owe their activity to the action of this system, in such a way that the organization of bodily function entirely depends on it. Destruction of those nervous centres which are responsible for the regulation of these organs, seriously interferes with their action and, in the case of the heart and lungs, results in death. Similarly all activity of muscles, which are responsible for the movements of the body, depend on the influence of the nervous system and are entirely subservient to it. On the other hand, every activity of the muscles and internal organs results in the stimulus of nerves, whose business it is to convey messages back to the nervous centres. In this way there is a continuous interchange of messages and influences from nervous system to organs and from organs to nervous system, by the intervention of the peripheral nerves. At the same time, the various organs produce chemical substances which circulate in the blood and influence all other organs and especially the activity of the nervous system. With this question of “temperament” we shall deal in the next chapter. For the present, it is proposed to discuss the organization of the nervous system itself, since upon it depends the organization of the whole body. This system consists of a central part concerned with the reception and transmission of messages, analogous to the exchange in a telephone system, and of a peripheral part composed of the nerves which convey messages between the nervous system and the end organs. These nerves are analogous to the wires of the telephone. The end organs are the structures by means of
which messages are received from the organs, or in the case of the special senses, from the environment, and transmitted to the central exchange, and are analogous to the instruments belonging to the various subscribers to the telephone system.