Physiology: The Science of the Body

by Ernest G. Martin

sight is at the back of the cerebrum down at its lower margin; the arrival platform for the senses of touch, pain, and the like are just about at the top of the brain. The arrival platform for the sense of hearing is down at the side. The location of these arrival platforms has been worked out by studying the effects of brain diseases. It has been found, for example, that persons who are blind, although there is nothing the matter with their eyes, are so because of a disease of the lower back part of the cerebrum, and persons who are deaf, even though their ears are perfect, are so because of diseases in the parts of the cerebrum at the sides. The only way to explain these findings is by supposing that the arrival platforms of these senses are located in the parts that are found diseased. The cells which stop the nervous disturbances coming in from the sense organs and so serve as the seat of memories of sensations are in the arrival platforms, so that when we recall how something looked, for example, the nerve cells which are active in this recollection are those at the lower back part of the cerebrum. All these arrival platforms taken together occupy only a very small fraction of the whole of the cerebrum, so that evidently it has more to do than simply to register these memories.

One additional thing we shall look for in the cerebrum is a pathway from the brain to the muscles whereby the memories that are stored in the brain can make themselves effective in arousing the muscles to activity. There is such a pathway, and it is one of the best marked of all the nerve pathways of the body. It is made up of nerve cells which start just about at the top of the cerebrum, a little in front of the middle and pass down into the brain stem; these cross over from the side in which they started to the opposite side; and go on down the spinal cord to make connection by nerve junctions directly with the motor nerve cells. Thus from the brain to the muscles the nerve pathway is made up of just two nerve cells. Since this pathway crosses over from one side to the other in the brain stem, the whole right half of the body, both sensory and motor, connects with the left half of the brain, and vice versa. The part of the cerebrum in which this nerve pathway starts is called the motor area of the brain.

We have now seen how nervous disturbances from the various sense organs can come into the brain and be registered there as memories. We have also seen that there is a nerve pathway by which nervous disturbances starting in the motor area can pass out to the muscles, and arouse them to activity. We have left to see how the connections are made between the incoming and the outgoing disturbances, or in

other words to see how the memories that are registered in the brain act upon the outgoing pathways. We shall expect to find connecting nerve cells reaching across from the various arrival platforms to the motor area, and such connecting cells exist, but they are not simple and direct connections for a reason which we shall now try to make clear. We all know from our own experience that our memories are never the pure registering of a single sense. What we mean is that the sight of something by itself or the sound of something by itself never remains as a separate memory, but is always worked in with some other memories from some of the other sense organs. This putting together of the simple sensory memories began in earliest childhood, long before we were old enough to think about our mental processes, and see how they are carried on, so that unless our attention has been called to it, we have probably failed to realize how complicated our simplest memories are. A good example of this is in the experience of a baby with its mother’s voice. So far as the baby is concerned, the voice is an influence affecting the organ of hearing, arousing nervous disturbances which pass to the arrival platform for hearing in the brain and are registered there as memories, but it does not take the infant long to learn that some other memories that have come to it by way of other sense organs belong with this particular memory and always go with it. For example there is the sight of the mother, or the feel of the mother’s face and hands, all these are influences affecting different sense organs and registering in the child’s brain in different arrival platforms. Yet within his brain they become fused into a single composite memory of the mother, and after this fusion has once occurred, the arousing of any part of the memory brings up the whole of it, so that the child may hear the mother’s voice from the next room, but the memory that will be aroused as the result will be not simply of the voice but of the mother as a whole. This is an illustration of how our pure sense memories are fused into complex memories. After one of these memory complexes is once started, we add to it any time any sense organ is acted upon by anything that has relationship to the complex. For example, the child’s idea of the mother at first is a very simple one made up of a few sensory impressions, but as time goes on and more and more sensory impressions of the mother are received, the child’s idea of her becomes more and more complex. This process of memory fusion is called association or sometimes association of ideas. Strictly it is an association of memories and this is the method by which all our mental activities are carried on. The thought process consists of putting together various memories in various ways and so building up associations of different kinds. Of course adults of wide experience can form associations which are made of literally hundreds or perhaps thousands of separate memories of sense impressions. We receive sense impressions at the rate of hundreds every day, and very many of these, perhaps all of them, are registered as memories and are fitted into their proper associations.

The nervous machinery for carrying on different processes of association consists of nerve cells of the brain. These are all of the kind known as connecting cells. It will be remembered that the connecting cells may have many nerve junctions leading into them and they in turn are much branched so that many nerve junctions can lead out from them. The connecting cells of the brain are richer in these respects than those in any other part of the body, and the cells of the human brain than the brain of any of the lower animals. We suppose that the associations are formed by the passage of nervous disturbances from the arrival platforms where the sense memories are registered over various connecting nerve cells to a common meeting point in some cell or in some group of cells, where the associated memories are all brought together into a single memory complex. It is supposed that the large areas of the brain which are not taken up either by the arrival platforms or by the motor areas are the regions in which these associations take place.

There is one particular group of associations that are so interesting as to call for special mention. These are the associations concerned with language. We are so in the habit of using language that we are likely never to have thought of it in its real meaning as a part of our mental activity. We know that we have two kinds of language, spoken and written, and that spoken language consists of certain sounds, and written language of certain visual symbols. Two of the distance senses then are concerned, sight for written language, hearing for spoken, and language itself consists simply of sense impressions coming in through one or the other of these sensory channels. The important feature of language is that mankind has selected arbitrarily certain sounds or certain written symbols to stand for particular things. When a child is learning to understand what is said, what he is really doing is fitting a particular set of arbitrary sense impressions into their proper places in his associations. We can illustrate this by the same example that was used a moment ago. The child becomes thoroughly familiar with its mother, so far as sense impressions are concerned that come directly from her. In course of time it adds to the associations thus formed an additional one made up of the sound of the word “mother.” Of course there is no particular reason why this sound should have that meaning rather than any other. The proof is that different languages have different sounds which stand for the same thing. After this sound has once been selected and learned, it becomes as much a part of the idea of mother as any other of the associations concerned, and thereafter, whenever that particular sound strikes upon the child’s ear, the association of mother is aroused. Precisely the same sort of thing is true of written language. Arbitrarily selected symbols act through the sense of sight to arouse nervous disturbances, which are built into particular associations, and here again it makes no particular difference what the symbol is; one will do as well as another, provided a number of people have agreed to use that symbol to stand for the same thing. One very useful feature of language is that we can make a single word stand for a complex group of sensory impressions or even associations. To illustrate, the word physiology, whether spoken or written, is in itself simply a sound or a visible symbol, but it stands for a highly complex group of associations in the human mind. In this respect language is a kind of shorthand.