Mind and Motion and Monism

by George John Romanes

That the grey matter of the cerebral hemispheres is the exclusive seat of mind is proved in two ways. In the first place, if we look to the animal kingdom as a whole, we find that, speaking generally, the intelligence of species varies with the mass of this grey matter. Or, in other words, we find that the process of mental evolution, on its physical side, has consisted in the progressive development of this grey matter superimposed upon the pre-existing nervous machinery, until it has attained its latest and maximum growth in man.

In the second place, we find that when the grey matter is experimentally removed from the brain of animals, the animals continue to live; but are completely deprived of intelligence. All the lower nerve-centres continue to perform their mechanical adjustments in response to suitable stimulation; but they are no longer under the government of the mind. Thus, for instance, when a bird is mutilated in this way, it will continue to perform all its reflex adjustments—such as sitting on a perch, using its wings when thrown into the air, and so forth; but it no longer remembers its nest or its young, and will starve to death in the midst of its food, unless it be fed artificially.

Again, if the grey matter of only one hemisphere be removed, the mind is taken away from the corresponding (i. e. the opposite) side of the body, while it remains intact on the other side. For example, if a dog be deprived of one hemisphere, the eye which was supplied from it with nerve-fibres continues able to see, or to transmit impressions to the lower nerve-centre called the optic ganglion; for this eye will then mechanically follow the hand waved in front of it. But if the hand should hold a piece of meat, the dog will show no mental recognition of the meat, which of course it will immediately seize if exposed to the view of its other eye. The same thing is found to happen in the case of birds: on the injured side sensation, or the power of responding to a stimulus, remains intact; while perception, or the power of mental recognition, is destroyed.

This description applies to the grey matter of the cerebral hemispheres as a whole. But of course the question next arises whether it only acts as a whole, or whether there is any localization of different intellectual faculties in different parts of it. Now, in answer to this question, it has long been known that the faculty of speech is definitely localized in a part of the grey matter lying just behind the forehead; for, when this part is injured, a man loses all power of expressing even the most simple ideas in words, while the ideas themselves remain as clear as ever. It is remarkable that in each individual only this part of one hemisphere appears to be used; and there is some evidence to show that left-handed persons use the opposite side from right-handed. Moreover, when the side which is habitually in use is destroyed, the corresponding part of the other hemisphere begins to learn its work, so that the patient may in time recover his use of language.

Within the last few years the important discovery has been made, that by stimulating with electricity the surface of the grey matter of the hemispheres, muscular movements are evoked; and that certain patches of the grey matter, when thus stimulated, always throw into action the same groups of muscles. In other words, there are definite local areas of grey matter, which, when stimulated, throw into action definite groups of muscles. The surface of the cerebral hemispheres has now been in large measure explored and mapped out with reference to these so-called motor-centres; and thus our knowledge of the neuro-muscular machinery of the higher animals (including man) has been very greatly furthered. Here I may observe parenthetically that, as the brain is insentient to injuries inflicted upon its own substance, none of the experiments to which I have alluded entail any suffering to the animals experimented upon; and it is evident that the important information which has thus been gained could not have been gained by any other method. I may also observe that as these motor-centres occur in the grey matter of the hemispheres, a strong probability arises that they are not only the motor-centres, but also the volitional centres which originate the intellectual commands for the contraction of this and that group of muscles. Unfortunately we cannot interrogate an animal whether, when we stimulate a motor-centre, we arouse in the animal's mind an act of will to throw the corresponding group of muscles into action; but that these motor-centres are really centres of volition is pointed to by the fact, that electrical stimuli have no longer any effect upon them when the mental faculties of the animal are suspended by anæsthetics, nor in the case of young animals where the mental faculties have not yet been sufficiently developed to admit of voluntary co-ordination among the muscles which are concerned. On the whole, then, it is not improbable that on stimulating artificially these motor-centres of the brain, a physiologist is actually playing from without, and at his own pleasure, upon the volitions of the animal.

Turning, now, from this brief description of the structure and leading functions of the principal parts of the nervous system, I propose to consider what we know about the molecular movements which go on in different parts of this system, and which are concerned in all the processes of reflex adjustment, sensation, perception, emotion, instinct, thought, and volition.

First of all, the rate at which these molecular movements travel through a nerve has been measured, and found to be about 100 feet per second, or somewhat more than a mile a minute, in the nerves of a frog. In the nerves of a mammal it is just about twice as fast; so that if London were connected with New York by means of a mammalian nerve instead of an electric cable, it would require nearly a whole day for a message to pass.

Next, the time has also been measured which is required by a nerve-centre to perform its part in a reflex action, where no thought or consciousness is involved. This time, in the case of the winking reflex, and apart from the time required for the passage of the molecular waves up and down the sensory and motor nerves, is about 1/20 of a second. Such is the rate at which a nerve-centre conducts its operations when no consciousness or volition is involved. But when consciousness and volition are involved, or when the cerebral hemispheres are called into play, the time required is considerably greater. For the operations on the part of the hemispheres which are comprised in perceiving a simple sensation (such as an electrical shock) and the volitional act of signalling the perception, cannot be performed in less than 1/12 of a second, which is nearly twice as long as the time required by the lower nerve-centres for the performance of a reflex action. Other experiments prove that the more complex an act of perception, the more time is required for its performance. Thus, when the experiment is made to consist, not merely in signalling a perception, but in signalling one of two or more perceptions (such as an electrical shock on one or other of the two hands, which of five letters is suddenly exposed to view, &c.), a longer time is required for the more complex process of distinguishing which of the two or more expected stimuli is perceived, and in determining which of the appropriate signals to make in response. The time consumed by the cerebral hemispheres in meeting a 'dilemma' of this kind is from 1/5 to 1/20 of a second longer than that which they consume in the case of a simpler perception. Therefore, whenever mental operations are concerned, a relatively much greater time is required for a nerve-centre to perform its adjustments than when a merely mechanical or non-mental response is needed; and the more complex the mental operation the more time is necessary. Such may be termed the physiology of deliberation.

So much, then, for the rate at which molecular movements travel through nerves, and the times which nerve-centres consume in performing their molecular adjustments. We may next consider the researches which have been made within the last few months upon the rates of these movements themselves, or the number of vibrations per second with which the particles of nervous matter oscillate.

If, by means of a suitable apparatus, a muscle is made to record its own contraction, we find that during all the time it is in contraction, it is under-going a vibratory movement at the rate of about nine pulsations per second. What is the meaning of this movement? The meaning is that the act of will in the brain, which serves as a stimulus to the contraction of the muscle, is accompanied by a vibratory movement in the grey matter of the brain; that this movement is going on at the rate of nine pulsations per second; and that the muscle is giving a separate or distinct contraction in response to every one of these nervous pulsations. That such is the true explanation of the rhythm in the muscle is proved by the fact that if, instead of contracting a muscle by an act of the will, it be contracted by means of a rapid series of electrical shocks playing upon its attached nerve, the record then furnished shows a similar trembling going on in the muscle as in the previous case; but the tremors of contraction are now no longer at the rate of nine per second: they correspond beat for beat with the interruptions of the electrical current. That is to say, the muscle is responding separately to every separate stimulus which it receives through the nerve; and further experiment shows that it is able thus to keep time with the separate shocks, even though these be made to follow one another so rapidly as 1,000 per second. Therefore we can have no doubt that the slow rhythm of nine per second under the influence of volitional stimulation, represents the rate at which the muscle is receiving so many separate impulses from the brain: the muscle is keeping time with the molecular vibrations going on in the cerebral hemispheres at the rate of nine beats per second. Careful tracings show that this rate cannot be increased by increasing the strength of the volitional stimulus; but some individuals—and those usually who are of quickest intelligence—display a somewhat quicker rate of rhythm, which may be as high as eleven per second. Moreover, it is found that by stimulating with strychnine any of the centres of reflex action, pretty nearly the same rate of rhythm is exhibited by the muscles thus thrown into contraction; so that all the nerve-cells in the body are thus shown to have in their vibrations pretty nearly the same period, and not to be able to vibrate with any other. For no matter how rapidly the electrical shocks are allowed to play upon the grey matter of the cerebral hemispheres, as distinguished from the nerve-trunks proceeding from them to the muscles, the muscles always show the same rhythm of about nine beats per second: the nerve-cells, unlike the nerve-fibres, refuse to keep time with the electric shocks, and will only respond to them by vibrating at their own intrinsic rate of nine beats per second.