The arrangement of the nervous system is very much like that of a city telephone system. The sensory nerve cells all lead into a part of the nervous system to which is given the name of the _Central Nervous System_, just as the telephone wires all lead into a “central exchange.” From this central nervous system or “exchange” all the motor nerve cells extend to the muscles. There is one important difference between the arrangement of the nervous system and that of the telephone exchange; namely, that the nervous system is a “one-way” system. As we all know, a telephone instrument can be used either for sending or receiving and the same wires conduct the messages in both directions. This is not true of the nervous system. The messages from the sense organs pass into the center by way of the sensory nerves and out from the center by way of the motor nerves. The central nervous system operates as an “exchange,” connections can be made from any sensory cell to any of the motor cells. Since this ability of the central nervous system to make connections here and there within itself is about the most important of all our nervous activities, as we shall see shortly, we must try to form an idea of how it is done. If we look again at the figures of the sensory and connecting nerve cells, we shall note that both kinds are branched; the tip of every branch makes a nerve junction. This means that every sensory cell, for example, has as many outlets as it has branches. If every one of these outlets were to communicate directly with a motor nerve cell, the number of connections that [Illustration: DIAGRAM OF NERVE CELL CONNECTIONS _S C_, spinal cord; _S_, sense organ; _M_, muscle; _a_, sensory nerve cell; _b_, connecting nerve cell; _c_, motor nerve cell] this sensory cell could make would depend on the number of its branches; but, as a matter of fact, most of the branches from the sensory cells make nerve junctions with connecting cells, and not with motor cells directly. These connecting cells in turn are branched and many of these branches lead again to connecting cells, so we see that the number of connections that a single sensory cell can make quickly becomes very large. The arrangement is shown in the accompanying diagram. Not only does one sensory cell have in this way the possibility of nerve connection with all the muscles, but the reverse is also true; namely, that every muscle has the possibility of being connected with every sense organ. This means that there must be a number of nerve junctions connecting with the cell body of each of the motor nerve cells. If we look back at the diagram of the motor nerve cell, we shall see that it has a great many tiny branches leading off it. These are numerous enough to enable the feathery tips of a great many sensory or connecting nerve cells to interweave with them, and so enable any motor cell to be acted upon from a great many different directions.