Physiology: The Science of the Body

by Ernest G. Martin

CHAPTER XI
THE BRAIN AND COMPLEX NERVOUS ACTIONS

WE have just seen that the underlying arrangement of the nervous system is one which makes communication possible between any sense organ in the body and any muscle; also that in the working of the nervous system there are certain paths from given sense organs to particular muscles which we may call the selected paths, and over which nervous disturbances easily pass. These paths work out in such a way that the muscles that are thrown into activity as the result of the arousing of any sense organ are those most likely to serve the needs of the animal. There are a number of respects in which a nervous system, having no more than we have thus far described, would fall short of meeting the requirements of any animal except perhaps the very simplest. The thing that is missing that is of most importance, perhaps, is the ability to perform actions in response to information received at some past time; in other words, the ability to profit by experience. As the nervous system has been described up to this point, only immediate effects upon the sense organs are transmitted to the muscles to throw them into activity. As a matter of fact, we know that we ourselves, as well as many of the higher animals, are able to profit by past experience. That is to say, we can make our muscles move in accordance with information received at some time in the past. There must be in the nervous system, then, some arrangement for doing this, and our present task is to describe it. In the chapter on the distance sense organs considerable emphasis was placed on the fact that the advantage of having distance sense organs is to allow time for a choice among the various responses that the animal might make to the information received by the sense organs. It is this opportunity for choice that lies at the foundation of the higher nervous activities that we are now beginning to describe. As soon as the element of choice enters, the nervous disturbance cannot pass from the sense organ to the muscle in the least possible time. In order for there to be a choice there must be a delay while the selection of the muscle to be aroused to activity is being made. We have, then, a new feature in connection with the operation of distance sense organs in that the nervous disturbance can come to a stop at some point in the course of its progress from sense organs to muscles. Every animal that has distance sense organs shows this feature of possible delay in the movement of the nervous disturbance over the nervous pathway, and in every one the particular part of the nervous system in which this occurs is known as the brain. In fact no other nerve cells except some of the nerve cells of the brain have this ability of stopping the nervous disturbance and holding it for a time before sending it on.

One thing about the information that comes in through the distance sense organs is that much of it does not require an immediate activity. For example, a hungry fox, seeing a fowl roosting in an accessible place, might pounce upon it at once, but if he happened at the moment to be either fully satisfied or to be carrying a fowl which he had captured a moment ago, there would be no occasion for him to seize this one; rather would it be desirable for him to set it aside in his mind to be captured in the future. In the case used in the illustration the nervous action goes on in the beginning just as we have been picturing it; that is, the sense organ is aroused and this in turn starts a disturbance over the nerve path; when it comes to the brain, however, it does not go on from there to cause immediate muscular activity, but instead is held in the brain and can cause activity at some future time, as for example if the fox, after taking the fowl which he is carrying home to his young, comes back in search of the one that he saw roosting. We are perfectly familiar with this ability to stop nervous disturbances; we know it under the name of memory, although we may not have thought of memory in exactly this sense. If we consider what memory really is, we shall see that it is just what we have been describing, namely the stopping of nervous disturbances that come in from the sense organs and holding them, so that they may in turn set up at some future time a nervous disturbance that shall produce activity. If we try to picture what actually goes on in the nerve cells where this memory is located, we can say only that when the nervous disturbance strikes upon these cells, it does not pass on through at once leaving them very much as they were before, but stops in them and brings about in them some kind of permanent change. An interesting thing about memory is its persistence. We may act upon memories that we have held for years. More than one person moving away from a particular town early in life, and going back to it in later years has remembered things he saw in childhood well enough to find his way to them again.

Since memory is registered in the brain cells as a permanent or fairly permanent change, it can become the source of nervous disturbance over and over again. This in fact is one of its greatest advantages, because when we have once learned a thing, we can make use of it a great many times, and do not have to have the sense organs freshly aroused every time it is desirable to use this particular bit of information.

In order that the brain cells may receive and store up memories, it is evidently necessary that nerve paths from the sense organs should lead into the brain; so, if we go back to the sensory nerve cells, we shall find that in nearly every one of them, if not in every one, one of the branches makes a nerve junction with a connecting cell, which either extends directly into the brain, or links with other connecting cells which do. This is true not only of the distance sense organs, but also of the contact sense organs and to some extent of those inside the body. The path by which the brain is reached from the sense organs that are located in the body is over the sensory nerve cells to the spinal cord, and along the spinal cord by way of connecting nerve cells to the brain. All the distance sense organs are located in the head, so their nerve paths lead into the brain directly over cranial nerves; for the eye, the optic nerve; for the ear, the auditory nerve; and for the organ of smell, the olfactory nerve. One very interesting fact about the connection of the sense organs with the brain is that the nerve paths in every case cross from the left-hand

side to the right-hand side, and vice versa, so that all the sense organs in the left half of the body have their connection with the right half of the brain, and those in the right half of the body with the left half of the brain. We know of no reason why this should be so. It is merely an interesting fact. A diagram showing the path from a sense organ in the body and from one of the head senses to the brain is given in the accompanying figure.