CHAPTER X

THE NERVOUS SYSTEM AND SIMPLE NERVOUS ACTIONS

In the second chapter we saw that to make our muscles act in accordance with the information brought in by the sense organs some means of communication between them is necessary; we saw, also, that this means consists of the nervous system. Now that we have learned something about both muscles and sense organs we are ready to look into the way in which communication between them is carried on. First of all, we must realize that living protoplasm does this. The nerve cells are alive and have their basic metabolism just as do all other living cells. They also have their functional metabolism; but in them, instead of taking the form of forcible motion, as in muscles, or of the manufacture of special materials, as in gland cells, it takes the form of the transmission of a disturbance from one part of the cell to another. An interesting and important fact about this transmission of disturbances is that the actual amount of functional metabolism required by it is very small. Only by the most careful measurements has it been shown that nerve cells that are functioning have a greater metabolism than those that are at rest. For a long time it was thought that a nerve cell acted very much like a telephone or a telegraph wire, transmitting some kind of a disturbance which was set up in it, but not having any active part itself in the process. We now know that the special activity of nerve cells is a form of functional metabolism, just as is the special activity of muscle cells or gland cells.

The nerve cells have to make communication between sense organs and muscles, and, as we have already seen, these are often quite a distance apart. It is necessary, therefore, that the nerve cells be long enough to reach over these distances. As a matter of fact, it is not necessary for single cells to have this great length, because it is possible for them to be arranged end to end, making a path of living protoplasm consisting not of one cell but of a chain of them. If we look at a nerve cell under the microscope we see that it is made up of a little central mass of protoplasm to which has been given the name of “cell body.” From this cell body extends a tiny thread of living protoplasm. This thread is called the _axon_. It is so very slender that it cannot be seen except under a powerful microscope, and yet in our own bodies and the bodies of all large animals many of these are three feet or more in length without a break. This tiny protoplasmic thread, the axon, was formed originally by growing out from the cell body. As it grew it became surrounded by a sheath, which probably gives it strength and decreases the danger of its being broken. Another thing which helps to keep the axons from being injured is that they are always in bundles. Instead of one of these very slender axons lying all by itself, it will be bound up with several hundred others; the arrangement is similar to that in a telephone cable, where a great many single wires are bound together in the large and very strong cable. The living protoplasm of the nerve cell has a gray color, so that wherever this shows we have what is commonly called gray matter. We saw a moment ago that every axon is inclosed in a sheath. Some of these sheaths are transparent, so that the gray color can be seen underneath, but most of them have a layer of white material, which makes them look white instead of gray. The bundles of axons corresponding to the telephone cables make up what we call the nerves. Nearly all nerves are white in color because of the white material in the sheaths.

The sense organs, as we have seen in Chapters VIII and IX, are some of them inside the body, others spread over the surface of the skin, and the rest in the special sense organs, like the eyes or the ears. A very complex organ, like the eye or the ear, has thousands of axons leading from it. In the case of the eye these are grouped into a large nerve leading away from it at the back, which is called the optic nerve. There is a similar large nerve leading from the ear. When any sense organ is acted upon, as when light falls in the eye or sound on the ear, it starts a disturbance in some or all of the axons leading away from it.

As we have said over and over, the purpose of the nervous system is to arouse the muscles to activity, and to guide them in that activity. We must ask next, then, how the nerves are distributed to the muscles. If we dissect the body of any animal or bird, we can find nerves passing to various parts. For example, a large nerve goes down each leg; this nerve subdivides here and there. Since we know that the nerve consists of a great many axons bundled together, we will realize that this subdivision is not a real branching, but simply a passing of some of the axons away from the main trunk along the smaller stem. Some of these smaller stems can be traced to endings in the skin; these contain the axons connecting with sense organs. Others lead directly into muscles. Some of these axons may also connect with sense organs, since, as we have already seen, every muscle has embedded in it the organs of muscle sense, but in addition any nerve that leads to a muscle contains a great many axons which pass directly to the muscle fibers. These are the axons by which the muscles are aroused to activity. It is a general rule of the nervous system that no nerve cell extends without a break from any sense organ to any muscle fiber. The axon which communicates with the sense organ belongs to one nerve cell; the axon which connects with the muscle fiber belongs to a different nerve cell. The first is called a _sensory_ nerve cell, the second a _motor_ nerve cell. Some idea of the appearance of these cells can be gotten from the figures on page 126.

It will be seen that the cell body of the sensory cell appears to be off on a little side branch. As a matter of fact, the branch is double, so that when a nervous disturbance passing along from the sense organ comes to the beginning of this branch it can pass up to the cell body and then out from the cell body along the second part of the branch, and so along the other part of the axon. This part of the axon is seen in the figure to have several branches; these are really branches of protoplasm and not separate axons coming off, as in the case of the nerve trunk. The use of these branches we shall see in a moment. Also at the tips of each branch there is a tiny feathering. We shall explain this presently. Let us look first at the figure of the motor nerve cell. This has a cell body and long axon, and, besides these, has a great many short protoplasmic branches sticking out in all directions from the cell body. Since a nervous disturbance to get from a sense organ to a muscle has to pass over a sensory nerve cell, and also over a motor nerve cell, evidently there will have to be some point at which it leaves the sensory cell and gets into the motor. This is accomplished by having the tiny feathering at the tip of the sensory cell interwoven with the fine processes projecting from the body of the motor cell. This arrangement we may call a nerve junction. In the whole body there are, of course, millions of these nerve junctions.

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

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.

The central nervous system, which is the place where all these much-branched pathways are and where all the nerve junctions are located, is made up of two chief parts, the brain and the spinal cord. The brain is inside the skull and the spinal cord is an extension down the back. It lies in the tunnel made up of the arches of the bones of the vertebral column as described in Chapter VI. The cables which contain the axons of both sensory and motor nerve cells extend from the brain or from the spinal cord out to the different parts of the body where the sense organs and the muscles are located. They start as large nerve trunks which divide and subdivide as they get farther and farther away from the central nervous system. The large nerve trunks are arranged in pairs--those that spring from the brain are called _cranial_ nerves; those that spring from the spinal cord are called _spinal_ nerves. The cranial nerves, with the exception of one, lead only to points in the head or neck, and so are short; the spinal nerves on the other hand reach to the various parts of the trunk or down the arms and legs, so that some of these may be three feet long or more. An idea of the arrangement of the nervous system is given in the accompanying figure.

A good way to realize the actual working of the nervous system is to take a particular action and follow it through. Suppose a barefooted boy steps on a sharp thorn. The thorn arouses some of the sense organs in the sole of the foot. These in turn start a disturbance in the sensory nerve cells which pass up the leg to the lower end of the spinal cord in the small of the back. Within the spinal cord the sensory cells branch and the disturbance set up by the prick of the thorn spreads all over these branches to their tips. Some of the nerve junctions thus affected lead to muscles which will cause the foot to be jerked up; others communicate with connecting nerve cells which extend all the way up the spinal cord and into the brain and make the boy aware of the fact that he has stepped on the thorn; still others may make connection with muscles which would cause him to sit down and look at the bottom of his foot; still others may lead to the vocal muscles and the tear glands, causing the boy to cry. Of course these are not the only possible nerve connections; every muscle in the body might theoretically be aroused to action as the result of the stepping on the thorn. As a matter of fact, there is a condition in which this will happen. The drug strychnine has such an effect upon the nervous system that the stimulation of any sense organ actually does arouse all

the muscles in the body, giving what is called a convulsion. There are some other poisons which may act similarly. Convulsions are not at all uncommon in young children. The way in which a convulsion is produced is by the spreading of the disturbance all over from a single sense organ. What we have, then, in the nervous system is an arrangement whereby under special conditions a nervous disturbance can pass from any sense organ to any or all of the muscles, but under ordinary conditions the disturbance spreads to a particular muscle or group of muscles only. It is evident that the nervous system would be of no use at all, if this latter arrangement did not exist. In order for our muscles to serve us, they must act in obedience to information brought in by the sense organs, and this can happen only when certain groups of muscles work in accordance with the information brought in by certain sense organs or groups of sense organs. We explain the behavior of the central nervous system by saying that there are preferred pathways through it, or, to put it in a slightly different way, when a nervous disturbance spreads over any nerve cell it extends equally over all parts of it, but does not pass with equal ease over all the nerve junctions. Some of the nerve junctions allow the disturbance to pass more readily than others, and it is this difference in the ease of passing the nerve junctions that determines which pathway the disturbance shall follow. We know no other means by which this picking out of particular paths from the huge number of possible paths could be accomplished.

What we are describing now is the simple foundation on which all our nervous activities rest. For that reason we are not taking up at this point the working of the brain, but only the direct connections between sense organs and muscles. In some very low animals the whole nervous system is made up of such simple connections.

A nervous activity which consists of the passage of a disturbance from a sense organ to a muscle is called a _reflex_. We have many examples in ourselves; if we inadvertently touch something hot, the hand is jerked away. Tickling the soles of the feet in one who is asleep will cause them to be drawn up; irritation in the throat causes us to cough, or in the nose to sneeze; the flashing of a bright light into the eye compels us to wink; all these are examples of the direct passage of nervous disturbances from sense organs to muscles. In every case the path is from the sense organ over the sensory nerve cell to some point in the central nervous system, then either by a direct nerve junction to a motor nerve cell or over one or more connecting nerve cells to a motor nerve cell and so to the appropriate muscles. These reflex actions follow the arousing of the sense organ with no more delay than is required for the passage of the disturbance over the nervous pathway. There is a delay of a small fraction of a second at every nerve junction, which makes some reflexes slower in their action than others. For example, there is a reflex known as the knee jerk; this is an outward kick which results from a sharp blow on the front of the leg just below the knee. The kick follows so closely after the blow that there cannot be more than one or at the most two nerve junctions in the pathway. The reflex of winking, on the other hand, takes several times longer, although the eye is much nearer the central nervous system than is the place on the leg which is struck in arousing the knee jerk. Since the actual length of nerve to be passed by the disturbance is much shorter in the winking than in the knee jerk, while the time for the reflex is a good deal longer, we conclude that the nerve pathway which is used in arousing winking contains a great many more nerve junctions, and therefore includes a great many more connecting cells than does the path for the knee jerk. In this particular case the reason why the pathway of winking contains so many connecting cells is that it is a brain pathway, while that for the knee jerk includes only the lower end of the spinal cord, where the arrangement of nerve cells is very much simpler.

It is important for us to get clearly in mind the working of the reflexes in order to be able to understand the more complex nervous actions which will be described in the next chapter. We need to remember that the ordinary way of starting nervous disturbances is from the sense organs. With a few exceptions, to be described later, whenever a nervous action occurs anywhere in our bodies it can be traced back, although often very indirectly, to the sense organ from which the disturbance originally came. The part played in this by the brain and by what we call our mental processes will be described in the next chapter.

Before going on to that topic we have a word to say about nervous fatigue. We mentioned the fact in Chapter VII that much of our actual feeling of fatigue is nervous rather than muscular. Not as much is known about nervous fatigue as about the fatigue that comes on when the muscles are overworked. One thing that seems pretty evident is that the place where the fatigue actually is located is in the very delicate nerve junctions. These junctions offer some resistance to the passing of nervous disturbances over them, and if they are compelled to submit too often to this passing they appear to offer still more resistance; in other words, to become fatigued. We must remember that the nerve junctions are exceedingly delicate things, consisting, as they do, of the interweaving of the almost inconceivably tiny featherings at the tips of sensory or connecting nerve cells with the equally tiny featherlike branches from the cell bodies of connecting or motor nerve cells. It is likely, also, that on account of their delicacy they are easily affected by the waste products that may be circulating about in the blood stream from the active muscles. In either case, the way to recover from nervous fatigue is simply by resting. It is not hard for the delicate nerve junctions to throw off fatigue if given a chance. The way to give them this chance is not to use them. As we shall see in the next chapter, mental processes are made up of nervous disturbances passing here and there in the brain. If we allow ourselves to be occupied too continuously with the same lines of thought, we are evidently sending nervous disturbances over the same nerve junctions over and over again. In order to give those nerve junctions a chance to rest, what we have to do is to think about something entirely different. The word that best expresses what we have in mind is “diversion.” In the strict sense diversion means a turning aside from what we have been doing to something different, and that is the best way to allow the brain to rest. The man who takes his business home with him, and dwells on it during the hours that are supposed to be set aside for rest, may be able to achieve more for the moment than if he were really to rest, although even that is doubtful; but in the long run there is no doubt that continuous efficiency depends on allowing the fatigued nerve junctions ample opportunity to recover, which means that the thoughts must be directed into entirely different channels.