IF we look for a moment at the single-celled creatures that form the present representatives of the beginning of life, we shall find that a stimulus attractive or repellant applied to any part of the cell will result in the cell reacting as a whole. There is little or no difficulty in communication within the cell from point to point. There is a single-celled creature known as Vorticella which consists of a bell attached by a spiral stalk to some neighbouring weed. A stimulus applied under the microscope to any part of the cell will occasion the stalk to contract and the bell to close up so that the creature takes up a defensive attitude.
When we come to the next grade of living creatures, those that are composed of more than one cell, a difficulty arises.
In the struggle for existence the body must work as a whole, each cell must react as a co-ordinated member if the whole mechanism of the creature is to survive the difficulties of its life. If a noxious chemical substance or another dangerous creature touches one single cell all the other cells in the body must react in an endeavour to flee and escape the danger. For this a service of communications between each body cell and every other cell is required. These communications are known as the nerves and the whole communicating system as the nervous system.
The earliest nervous system consisted in special cells which were situated in the skin of the animal and were connected by a long process running inwards to a muscle cell. Any stimulus applied to the special skin cell would produce contraction of the muscle. This, however, provided a connection between only two cells and each muscle would have to contract independently of the others, so very soon the specialised cells began to branch so that several muscle fibres became connected together. The next step came when the branches became large and numerous and it was necessary to provide a cell both to control them and to nourish them, so nerve cells were formed in the course of the network.
The primitive nervous system thus formed was composed of fibres leading from the special skin cells to nerve cells which were connected together by a branching network of communicating fibres which gave off branches to the muscle fibres. This system was found to be adequate up to a point and is still found to-day in certain lowly animals. But it presented a big disadvantage which had to be overcome before progressive evolution could take place. A stimulus applied to one part of the system might have to travel all round the system and by various devious routes before it could arrive at some point at a distance from where it was applied.
What was needed was a central clearing station or exchange which on receiving impulses could co-ordinate them and pass them on directly to the correct channel. The fibres leading into this central system are known as sensory fibres and those leading out of it to muscles or other organs which produce a reaction are known as motor fibres. The animal thus came to possess sensory organs situated in the skin which sent impulses to an exchange which relayed them by the motor fibres to the muscles or other organs. This central exchange, in order to suit the requirements of the animal,
became formed into a long cord running the whole length of the creature and so a primitive spinal cord was formed.
The path leading from the sensory organ through the nerve cells situated in the central nervous system to the motor organ is known as a reflex are, and the response of a muscle to a stimulus applied to the sensory organ is known as a reflex response. The earliest vertebrates (animals with backbones) and their predecessors, animals like the earthworm, were composed of a number of divisions or segments each of which contained a segment of the central nervous cord which controlled their movements by means of reflex arcs. Thus sensory organs in segment 6 of the animal would send messages to segment 6 of the nervous system and produce a response in the muscles of segment 6 of the animal.
In addition it was essential that each segment of the nervous
system should be in communication with its neighbours and others at a greater distance, so communicating fibres or long tracts were formed which ran the whole length of the central nervous system and co-ordinated the whole. By these means a stimulus applied to segment 6 could now produce a reflex response in one or all of the other segments.
THE EARLY BRAIN: A ‘LOOKOUT’ FOR THE BODY IN the act of locomotion it was found to be of enormous advantage to the animal if that segment which went first, namely the head end, was capable of detecting in advance changes in environment, such as enemies or food. The animal must not wait to discover an enemy until he is already within its jaws. He must know beforehand that an enemy is approaching so that he can make preparations to fight or flee before his enemy is upon him. For this reason that segment of the animal which preceded the rest in locomotion came to be endowed with special sense organs, eyes that could detect variations in light, ears that could pick up vibrations, a nose that could smell and detect food, and organs that could inform the animal of his position in space.
With the development of all these special organs in the head it was necessary that the head should contain a very much larger and more efficient central nervous system than the remaining body segments. This specially enlarged central nervous system of the first segments is known as the brain, and upon its efficiency in warning and producing reactions in the remainder of the body depended the whole survival of the animal in the struggle for existence. The more usefid the brain became the more chance had the animal of surviving. This is well seen if we look at the disaster that overtook the vast cumbersome reptiles that lived in the period following the emergence of animals from the sea. For a time their size and weight enabled them to overcome their adversaries and devour them. Their reaction to environment was to produce enormous muscles in their limbs which would give them power in fighting, and that part of their central nervous system which controlled these muscles—for example, the lower segments of the spinal cord supplying their legs—grew to be enormously larger than the brain itself I
They developed enormous power but their brains failed to develop the skill to use it. Their opponents, on the other hand, left their muscles to look after themselves and developed
their brains, which brought with them new skill and cunning so that the great cumbersome reptiles were wiped off the face of the earth. Man’s salvation in the struggle for existence has been his conservatism in leaving his body to look after itself and concentrating his attention on the development of his brain.
NERVES THAT BRING MESSAGES TO THE SPINAL CORD IN the discussion of the anatomy and physiology of the nervous system that follows, we will start with the parts which are simplest to understand, and lead up gradually to the more complete and more difficult. Situated in the skin are many different kinds of sense organs and still others for receiving sensations of heat and cold. In the muscles are organs that detect deep pressure and tension and in the joints are organs that give information of the position of the bones in relation to one another. From all these sources nerve fibres collect information, one fibre from each organ, and these are later bound together into nerve trunks which travel to the spinal cord and there relay their information.
Just before the sensory nerve fibres enter the spinal cord each fibre gives off a small branch which passes to the cell which looks after the nourishment of the fibre and enables it to live. This cell is an integral part of the nerve and the fibres which reach it from the sense organs and those which enter the spinal cord are really only prolongations of this cell. There is thus a collection of cells which form a slight lump or ganglion on the nerve trunk just before it enters the cord. This is known as the posterior root ganglion, because the sensory roots entering the spinal cord do so in its posterior part (as opposed to the motor root which leaves the cord anteriorly).
Once having entered the spinal cord the sensory nerves do several things. The spinal cord consists of a central mass of grey matter which contains large numbers of nerve cells and a surrounding sheath of white matter that consists of fibres running up and down the cord, some travelling right up to, and down from, the brain. Some of the sensory fibres entering pass immediately into the grey matter and form connections with nerve cells in the front of the grey matter of the same segment. From these, fibres arise which leave by the anterior motor roots and pass into the nerves to innervate the muscles directly.
This is the simplest reflex path. Other sensory fibres run up or down and pass to neighbouring segments, some of them crossing over to the opposite side, forming reflex arcs with other segments. Other sensory fibres, some of which make connections with cells in the posterior grey matter and some of which go direct, turn upwards and run right through the length of the cord to various parts of the brain, carrying impulses of touch, pain, heat and cold, which eventually reach consciousness in the great sensory area of the brain cortex, which we shall talk of later.
LIFE WITHOUT THE BRAIN : THE SPINE’S ACTIVITIES HAVING seen something of the anatomy of the cord we can now say something of what it does, but in order to study this experimentally we must destroy the brain, for this modifies the reflexes produced by the spinal cord by its controlling influence. An animal such as a frog, with its brain destroyed, is known as a spinal animal because its spinal cord only is now controlling it, and as it will live painlessly for a considerable lime we can use it to excellent purpose for studying the functions of the spinal cord.
Such an animal exhibits reflex activity. Thus if the foot is pinched or if a hot instrument is applied to the foot, it will be drawn away, the whole limb being thrown into an attitude of flexion. Thus, the toes and foot are bent upwards, the knee and the hip are flexed. This reflex is known as the flexor reflex and represents a primitive activity of the nervous system in protecting the lower limb from damage. It is the simplest form of reflex, the paths used being those that we have mentioned.
At the same time as the stimulated limb is withdrawn by flexion, the opposite limb will be reflexly extended—the crossed extensor reflex. This enables the animal when standing to preserve its balance on the other foot while it draws the affected one away. Other reflexes may be briefly mentioned such as the scratch reflex in which tickling the back of a dog produces movements of flexion and extension of the hind limb in an endeavour to scratch away the offending object. Also the stepping reflex in which pressure on the sole of the foot, especially if the limb is slightly flexed, produces extension of the stimulated limb and flexion of the other. This is the mechanism of walking. As soon as the right foot touches the ground the pressure reflexly excites
the muscles so that the right foot straightens to bear the weight, while the left flexes in preparation for the next step.
In addition to these reflexes which produce movement, many other reflexes are served by the spinal cord, the movements of the bowel producing peristalsis and the passing of faeces, the contraction of the bladder resulting in the act of passing water, various reflexes resulting in contraction or dilation of the vessels of a part, and a whole multitude of other reflexes which look after our well-being without ever reaching consciousness.
It must be understood that these muscular reflexes, such as the flexor reflex, are shown to their best advantage only when the spinal cord is working independently of the brain, when the brain has been destroyed or when the long tracts connecting it with the cord have been severed. The brain exercises a powerful control over these reflexes, modifying them and preventing them from dominating our existence. In the lower animals they were originally of immense value to the animal which possessed them, but as evolution progressed so have the spinal reflexes become more and more suppressed and brought under control of the higher centres of the brain.
An example of this is well shown in the case of the flexor reflex. In man, if the spinal cord is destroyed or if the nerve fibres running down from the brain which serve the voluntary control of the muscles are destroyed, a flexor reflex may be elicited by scratching the outer side of the sole of the foot. This action causes the great toe to be bent upwards, which, as we have seen, is one of the component parts of the protective flexor reflex. If, however, the sole of the foot is scratched in an individual in whom the tracts to the voluntary muscles are undamaged, the toe will bend downwards and the foot will, as it were, attempt to curl up.
The brain has depressed the flexor reflex and replaced it with a new reflex. It is only when the cord is ‘released ‘from the control of the brain that the flexor reflex manifests itself, although it is there, lying latent all the time. This is an example of what is known as the ‘release phenomenon ‘and is seen right throughout the central nervous system. It is found even in the higher centres of the brain which watch over such recently developed powers as the observance of social conventions, in fact the veneer of civilisation.
There are certain drugs which temporarily paralyse the highest and most recently developed functions of the brain
and release those that lie below. A well-known example of a drug of this kind is alcohol, which is not really a stimulant at all but a paralyser of the highest centres. Under its influence those centres of the brain which supply us with our critical faculties and with the finer conventions of society are temporarily paralysed, allowing our ‘baser ‘and more instinctive natures to appear. It has often been said that alcohol is one of the finest means of revealing the true nature of a man.
THE NERVE CENTRES UPON WHICH LIFE DEPENDS IF the spinal cord is followed upwards to its entry into the skull, it will be found to swell up into a wider mass which is known as the medulla oblongata. This is really only a specialised portion of the spinal cord and works in just the same way. Sensory nerves enter in its posterior part and motor nerves leave in its anterior part. Here, though, are situated the vital centres of the body—the respiratory centre which controls respiration and the cardiac centre which drives the heart. Sensory nerve fibres enter in a nerve, which is known as the vagus, and bring impulses carrying information from the heart and lungs. These fibres make connections with the cells in the medulla, and from them motor fibres leave, those to the heart travelling back to this organ in the vagus nerve, and those to the muscles of respiration passing down into the spinal cord and leaving it at different suitable levels. These nerve centres are absolutely vital for our continued existence, and their destruction brings about instantaneous death.
In this region, too, lie the centres which watch over our balance. Situated in the ear are special sense organs which supply these centres with information of the position in space which our head is occupying and of the movements which the head is making at any moment.
In order that a movement may be carried out in an orderly fashion, it is not sufficient for a given centre only to give orders that a movement must be made. In addition sensory nerves must inform the centre concerned of the progress of the movement from start to finish, so that it may order any modifications that may be necessary. This rule applies, of course, to all movements that are made, the sensory nerves carrying information from the muscles that make them. But, in movement of the head, information comes also from the car, infon jing the centres about the balance of the head and
its position in space. In all the lower vertebrates, these balancing organs are highly efficient and well developed, but in man they have regressed considerably, their powers being taken over by the eyes. They are, however, still of great importance.
Fish have balancing organs of a high degree of efficiency and so have birds, the reason being that these animals live and move freely in space, sometimes freely in three dimensions at one moment. Man, however, has had occasion, up till now, of moving only on flat surfaces. Deprived of the use of his eyes, his balance is precarious, and it is well known that airmen flying in fogs and clouds are often quite unaware of their position in space and may actually fly upside down without knowing it.
THE GREY MATTER: SEAT OF ALL SENSATION ALL the way up the spinal cord, nerves enter from the body and limbs, carrying impulses of touch, pain, temperature and other sensations. Some of these fibres, we have seen, pass to cells in the cord and subserve local reflexes. Others, however, pass up towards the brain and, after relaying in two centres and being joined by similar nerves from the head and face, reach what is known as the sensory cortex of the brain. In their course upwards, they cross to the side of the body opposite to the one where they started, so that fibres reaching the right cortex come originally from the left.
Here we must say a few words about the structure of the brain. It is a hollow mass of nervous tissue, in the centre of which is a cavity containing fluid which circulates downwards to the medulla and then leaves by a small hole and gains the surface of the brain. The cavity is continuous with a central canal lying in the spinal cord. Surrounding the cavity is a great mass of white matter, made up of vast numbers of fibres passing in all directions and connecting together all the centres of the brain. Spread out like a thick plate over this white matter is a,layer of grey matter which contains innumerable nerve cells. This grey matter is known as the cortex and is folded upon itself in convolutions in order to increase its area. The cortex of the brain is the highest and most specialised part of all, and it is this which confers upon us the powers of consciousness.
The sensory fibres, passing into the brain from the spinal cord, spread out and eventually reach a part of the cortex
which runs vertically upwards and downwards just behind the middle of its lateral surface. This area is accurately localised and has been proved by the following experiment to be the site of conscious body sensations. During operations on the brain, this area has been stimulated in a conscious person by an electric current, with the result that sensations of pain have been felt in some part of the body at a distance, say in the hand, if the hand area was stimulated.
The sensory cortex is divided up accurately into areas which receive information from particular parts of the body, and which are the same for all persons. Owing to the fact that the sensory fibres cross over, as we have mentioned, before they reach this area, stimulation of the hand area on one side of the brain will produce a sensation in the hand on the other side of the body.
HOW THE BRAIN GIVES INSTRUCTIONS TO THE BODY JUST in front of the sensory cortex is a vertical strip of grey matter, known as the motor cortex. Here are found large cells of a triangular or pyramid shape, which give off long processes passing right through the brain and spinal cord and making connections with the motor cells in the anterior part of the grey matter of the spinal cord. These are the same cells that we mentioned when we spoke of the motor cells or nerves that the spinal cord contained.
In their course downwards, these long fibres from the motor cortex cross over to the opposite side, so that a cell in the motor cortex of the right side controls a cell in the spinal cord, and through it a muscle fibre, on the left side of the body. This is the reason why a haemorrhage in the motor area of the brain on one side (which is known popularly as a ‘stroke ‘) produces a paralysis of some part of the body on the opposite side. Just like the sensory cortex, the motor cortex has special areas for special parts of the body and stimulation of an area, say the hand area, will produce a movement in the hand of the opposite side.
We have spoken so far only of the two simplest parts of the brain cortex, where some body sensations become conscious and where volitional movement is originated. The rest of the cortex is made up of similar areas which serve their own particular function. Thus the visual area that brings to consciousness what we see with our eyes is situated on each side at the back of the brain. The hearing area is low down on the lateral
surface, and a special speech area is situated on the left side of the brain. The front part of the brain is that part which is used in the processes of intellectual thought, memory and all the higher mental functions of which man is capable. All these areas are connected together by libres which run between all the centres of both sides of the brain, so that a stimulus in affecting only one of them may produce a reaction in any one or all of the others.
THE ESSENCE OF MENTAL ACTIVITY
SO far we have hinted that for any activity of the body to take place, a stimulus must be applied to some sense organ which will initiate a reflex. We have given the impression that, like a slot machine, something must be put in before anything can be got out. That this applies to the spinal reflexes there can be no doubt, but much argument and discussion have taken place over the mechanisms which lie behind conscious and unconscious mental processes. Many of our actions and thoughts appear at any rate to arise spontaneously in the brain without any previous stimulus setting them off, but on further consideration, it will always be found that there is a sequence in our thoughts and actions each depending on one that passed before.
Thoughts that spring into our minds with apparent spontaneity will always be found to have some association, often unconscious, with something previously seen or heard, a combination of circumstances or a previous thought. What constitutes the difficulty in applying our knowledge of reflexes to all grades of mental activity is not this but memory. It is difficult to see how a stimulus applied last year can produce its results to-day. If I agree to meet a friend in a certain place at a certain date and time next year, it is difficult to see how one can ascribe the performance of the visit to ordinary reflex activity. Some say that the reflex has been delayed, others that the nerve cells store up energy which is discharged at the appointed time, but these theories only beg the question by answering it in terms of another.
The best theory on which it is explained is that of the conditioned reflex. If a dog is given food, it will secrete saliva. If a bell is rung, and a few minutes later food is given to the dog, it will again secrete saliva in simple response to the food reflex. But after this process has been repeated a number of times, a curious thing occurs, for it is found that
the dog will secrete saliva even in the absence of food, if the bell is still rung. The secretion of saliva is said to be conditioned by the ringing of the bell, and the reflex in response to which the saliva is secreted is known as a conditioned reflex. The secretion of saliva in response to the ringing of a bell is therefore a conditioned reflex.
It is known for certain that the nerve cells subserving all conditioned reflexes are situated in the brain cortex, for if this is destroyed experimentally in animals, all conditioned reflexes are abolished. It is assumed that new paths are formed between the cells of the cortex concerned and that a given stimulus—light or sound or anything else—therefore travels direct through this new path to the cells which produce the conditioned reflex. It is the laying down of new paths between the untold myriads of cells which constitute the cortex of our brain, in the course of every experience that we undergo, that is the essence of our mental and intellectual activity.
HOW THOUGHTS CARVE NEW PATHWAYS IN THE BRAIN IN the case of memory, a new path is formed down which a stimulus travels at an appointed time, directly to the cells producing the conditioned response. In the case of a prearranged visit, when the arrangement is made a new path is formed between the sensory cells which bring to consciousness the knowledge of time and the motor cells which will set in motion all the activity needed for the visit. The combination of the date and hour when they arrive will occasion impulses which will travel along this specially made path, producing the conditioned response, in this case the visit. It may be argued that we remember such things actually before the time arrives. This is true, but does not invalidate the argument, for some association will have brought the combination of date and time into our mind, producing in an indirect way the necessary conditioned response. Thus, in a chance glance at a calendar a week before the visit was to take place, our eyes might fall upon the date arranged. This stimulus would travel along the path specially laid down and would awake the consciousness of memory. This is only a crude example but serves to illustrate the theory and brings our mental processes into line with the general laws of nature, according to which we know that energy and matter never arise out of nothing.