We have just described the simplest arrangement of a nerve path from a sense organ to a muscle; it consists of the sensory nerve cell, a nerve junction, and a motor nerve cell. This arrangement will answer where the sense organ and the muscle are in the same part of the body, but it may happen that the sense organ is in one part of the body and the muscle is in a distant part, as, for example, the eye and the muscles of the hand. To make these distant connections there must be additional nerve cells, and these we find in the body in the form of a kind of nerve cell that serves as a connecting link between sensory cells and motor cells. It may be called simply a connecting nerve cell. In appearance it is like the motor nerve cell, except that it has many branches which do not terminate in muscle fibers, but in fine feathering like that at the tips of the sensory nerve cell. When these connecting cells are present in the chain, the arrangement is as follows: from the sense organ the sensory nerve cell will pass just as previously described, the feathering at the tip will form a nerve junction with a connecting cell instead of with a motor cell. This connecting cell also has feathering at the tips of its branches, and these featherings will form a nerve junction with another nerve cell. This may be a motor cell, in which case the pathway is completed, or it may be another connecting cell, which in turn may lead into motor cells or connecting cells.
The nervous system is made up, then, of chains of nerve cells. Now why is this arrangement present? In very many cases sense organs and muscles are side by side or within a short distance of each other; why does not a single nerve cell reach directly from the sense organ to the muscle? The
answer is simple, if we think for a moment of how the body works. The information that comes in by way of particular sense organs cannot always be used to arouse particular muscles to activity. Things that we see will sometimes cause us to move one hand, sometimes another hand, sometimes the legs, sometimes muscles of the head, and so on. This means that the eye is able to make connection with a very large number of different muscles. The same thing is true of the other sense organs. The body could not possibly work as it does, if certain sense organs connected with certain muscles and no others. As a matter of fact, it is not an exaggeration to say that the proper working of the body requires any sense organ to be able to make connection with any muscle. It might be possible to do this by giving every sense organ as many sensory nerve cells as there are muscles, but this would be as bunglesome as to attempt to provide every business house in a large city with a separate telephone wire to every other business house.
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