If I move any of my limbs the reason is stated that the sensory organs, or those the same, As I now speak to you, the apparatuses which give rise to our feeldifferent tones of my voice are due to the ings when acted upon by the influences exquisitely accurate adjustment of the which produce sensation, caused a change contractions of a multitude of such por- in the sensory nerves, which he described tions of flesh; and there is no consider- as a flow of animal spirits along those able and visible movement of the animal nerves, which flow was propagated to the body which is not, as Descartes says, re- brain. If I look at this candle which I solvable into these changes in the form hold before me, the light falling on the of matter termed muscle. But Descartes retina of my eye gives rise to an affection went further, and he stated that in the of the optic nerve, which affection Desnormal and ordinary condition of things, cartes described as a flow of the animal these changes in the form of muscle in spirits to the brain. We should now the living body only occur under certain speak of it as a molecular change propaconditions; and the essential condition gated along the optic nerve to the brain; of the change is, says Descartes, the mo- but the fundamental idea is the same. tion of the matter contained within the In all our notions of the operations of nerves, which go from the central appa- nerve we are building upon Descartes' ratus to the muscle. Descartes gave foundation. Not only so, but Descartes this moving material a particular name lays down over and over again, in the the animal spirits. Nowadays we should most distinct manner, a proposition which not talk of the existence of animal spir- is of paramount importance not only for its, but we should say that a molecular physiology but for psychology. He says change takes place in the nerve, and that that when a body which is competent to that molecular change is propagated with produce a sensation touches the sensory a certain velocity, from the central appa-organs, what happens is the production ratus to the muscle. Nevertheless, the modification of the idea is not greater than that which has taken place in our view of electricity, in our change of conception of it as a fluid to our conception of it as a condition of propagated motion, there is, as everybody can find molecular change. Modern physiology by experiment for himself, something has measured the rate of the change to else which can in no way be compared to which I have referred; it has thrown motion, which is utterly unlike it, and marvellous light upon its nature; it has which is that state of consciousness which increased our knowledge of its charac- we call a sensation. Descartes insists ters, but the fundamental conception re- over and over again upon this total dismains exactly what it was in the time of parity between the agent which excites Descartes. Next, Descartes says that, the state of consciousness and the state under ordinary circumstances, this change of consciousness itself. He tells us that in the contents of a nerve, which gives our sensations are not pictures of external rise to the contraction of a muscle, is things, but that they are symbols or signs produced by a change in the central ner- of them; and in doing that he made one vous apparatus, as, for example, in the of the greatest possible revolutions, not brain. We say at the present time ex- only in physiology but in philosophy. actly the same thing. Descartes said Till his time it was conceived that visible that the animal spirits were stored up in bodies, for example, gave from themselves the brain, and flowed out along the motor a kind of film which entered the eye and nerves. We say that a molecular change so went to the brain, species intentionales takes place in the brain that is propagat- as they were called, and thus the mind ed along the motor nerve. The evidence received an actual copy or picture of of that is abundantly supplied by exper- things which were given off from it. It mental research. Further, Descartes is to Descartes we owe that complete

of a mode of motion of the sensory nerves. That mode of motion is propagated to the brain. That which takes place in the brain is still nothing but a mode of motion. But, in addition to this mode of

revolution in our ideas, which has led us | being affected by the present danger, to see that we have really no knowledge cause some change in his brain, which whatever of the causes of those phenom- determines the animal spirits to pass ena which we term external things, and thence into the nerves in such a manner that the only certainty we possess is as is required to produce this motion, in that they cannot be like those phenom- the same way as in a machine, and withena. In laying down that proposition out the mind being able to hinder it." upon what I imagine to be a perfectly I know in no modern treatise of a more irrefragable basis, Descartes laid the clear and precise statement, of a more foundation of that form of philosophy perfect illustration than this of what we which is termed idealism, which was sub- understand by the automatic action of sequently expanded to its uttermost by the brain. And what is very remarkable, Berkeley, and has since taken very vari- in speaking of these movements which ous shapes. arise by a sensation being as it were reBut Descartes noticed not only that flected from the central apparatus into a under certain conditions an impulse made limb- as, for example, when one's finger by the sensory organ may give rise to a is pricked and the arm is suddenly drawn sensation, but that under certain other up, the motion of the sensory nerve conditions it may give rise to motion, and travels to the spine and is again reflected that this motion may be effected without down to the muscles of the arm Dessensation, and not only without volition, cartes uses the very phrase that we at but even contrary to it. I trouble you this present time employ; he speaks of with as little reading as I can, because it the "esprits réfléchis," the reflected spiroccupies so much time; but I must askits; and that this was no mere happy your patience for one very remarkable phrase lost upon his contemporaries will passage which is contained in the answer be obvious if you consult the famous that Descartes gave to the objections work of Willis, the Oxford professor, raised by the famous Port Royalist Ar-"De Anima Brutorum," which was pubnauld to his Fourth Meditation. Des- lished about 1672. In giving an account cartes says: "It appears to me to be a of Descartes' views he borrows this very very remarkable circumstance that no phrase from him, and speaks of this removement can take place either in the flection of the motion of a sensory nerve bodies of beasts or even in our own, if into the motion of a motor nerve," sicut these bodies have not in themselves all undulatione reflexa," as if it were a wave the organs and instruments by means of thrown back; so that we have not only which the very same movement would be the thing reflex action described, but we accomplished in a machine, so that, even have the phrase "reflex" recognized in in us, the spirit or the soul does not its full significance. directly move the limb, but only determines the course of that very subtle liquid which is called the animal spirits, which, running continually from the heart by the brain into the muscles, is the cause of all the movements of our limbs, and often may cause many different motions, one as easily as the other. And it does not even always exert this determination, for, among the movements which take place in us, there are many which do not depend upon the mind at all, such as the beating of the heart, the digestion of food, the nutrition, the respiration of those who sleep, and, even in those who are awake, walking, singing, and other similar actions when they are performed without the mind thinking about them. And when one who falls from a height throws his hands forward to save his head, it is in virtue of no ratiocination that he performs this action; it does not depend upon his mind, but takes place merely because his senses,

And the last great service to the physiology of the nervous system which I have to mention as rendered by Descartes was this, that he first, so far as I know, sketched out a physical theory of memory. What he tells you in substance is this, that when a sensation takes place, the animal spirits travel up the sensory nerve, pass to the appropriate part of the brain, and there, as it were, find their way through the pores of the substance of the brain. And he says that when this has once taken place, when the particles of the brain have themselves been, as it were, shoved aside a little by a single passage of the animal spirits, the passage is made easier in the same direction for any subsequent flow of animal spirits; and that the repetition of this action makes it easier still, until, at length, it becomes very easy for the animal spirits to move these particular particles of the brain, the motion of which gives rise to the appropriate sensation;

and, finally, the passage is so easy that that the whole of man's physical actions almost any impulse which stirs the an- are mechanical, his mind living apart as imal spirits causes them to flow into these it were, and only occasionally interfering already open pores more easily than they by means of volition?" And it so hapwould flow in any other direction; and pened that Descartes was led by some the flow of the animal spirits recalls the of his speculations to believe that beasts image, the state of consciousness called had no souls, and consequently could have into existence by a former sensory im- no consciousness; and thus, his two ideas pression. This view is essentially at one harmonizing together, he developed that with all our present physical theories of famous hypothesis of the automatism memory. That memory is dependent of brutes, which is the main subject of upon a physical process stands beyond my present discourse. What Descartes question. The results of the study of meant by this was that animals are absodisease, the results of the action of poi- lute machines, as if they were mills or sonous substances, all conclusively point barrel-organs; that they have no feelto the fact that memory is inseparably ings; that a dog does not see, and does connected with the integrity of certain ma- not hear, and does not smell, but that terial parts of the brain and dependent the impressions which would produce upon them, and I know of no hypothe- those states of consciousness in oursis by which this fact can be accounted for except by one which is essentially similar to the notion of Descartes, a notion that the impression once made makes subsequent impressions easier and therefore allows almost any indirect disturbance of the brain to call up this particular image.

selves, give rise in the dog, by a mechanical reflex process, to actions which correspond to those which we perform when we do smell, and do taste, and do see. On the face of it this appears to be a surprising hypothesis, and I do not wonder that it proved to be a stumbling-block even to such acute and subtle men as Henry More, who was one of Descartes' correspondents; and yet it is a very singular thing that this, the boldest and most paradoxical notion which Descartes broached, has received as much and as strong support from modern physiological research as any other of his hypothe ses. I will endeavour to explain to you in as few words as possible, what is the nature of that support, and why it is that Descartes' hypothesis, although I am bound to say I do not agree with it, nevertheless, remains at this present time not only quite as defensible as it was in his own time, but I should say, upon the whole, a little more defensible.

So far, the ideas started by Descartes have simply been expanded, enlarged, and defined by modern research; they are the keystones of the modern physiology of the nervous system. But in one respect Descartes proceeded further than any of his contemporaries, and has been followed by very few of his successors in later days, although his views were for the best part of a century largely dominant over the intellectual mind of Europe. Descartes reasoned thus: "I can account for many of the actions of living beings mechanically, since reflex actions take place without the intervention of consciousness, and even in opposition to the will." As, for example, when a man If it should happen to a man that by in falling mechanically puts out his hand accident his spinal cord is divided, he to save himself, or when a person, to use would become paralyzed below the point another of Descartes' illustrations, strikes of injury. In such case his limbs would at his friend's eye, and although the be absolutely paralyzed; he would have friend knows he does not mean to hit no control over them, and they would be him, he nevertheless cannot prevent the devoid of sensation. You might prick muscles of his eye from winking. "In his feet, or burn them, or do anything these cases," Descartes said, "I have else you like with them, and they would clear evidence that the nervous system be absolutely insensible. Consciousacts mechanically without the interven- ness, therefore, so far as we can have any tion of consciousness and without the in- knowledge of it, would be entirely aboĺtervention of the will, or, it may be, in ished in that part of the central nervous opposition to it. Why, then, may I not apparatus which lies below the injury. extend this idea further? As actions of But although the man under these cira certain amount of complexity are cumstances is paralyzed in the sense of brought about in this way, why may not not being able to move his own limbs, he actions of still greater complexity be so is not paralyzed in the sense of their beproduced? Why, in fact, may it not being deprived of motion, for if you tickle

the soles of his feet with a feather the most complete assurance that in this

stimulus which sets going a certain machinery by which all the muscles of swimming are brought into play in due order and succession. Moreover, if the

limbs will be drawn up just as vigorously, case the frog is not acting from purpose, perhaps a little more vigorously, than has no consciousness, is a mere automatwhen he was in full possession of the ic machine. But now suppose that inconsciousness of what happened to him. stead of making your section of the cord Now, that is a reflex action. The im- in the middle of the body, you had made pression is transmitted from the skin to it in such a manner as to divide the the spinal cord, it is reflected from the hindermost part of the brain from the spinal cord, and passes down into the foremost part of the brain, and suppose muscles of the limbs, and they are the foremost two-thirds of the brain endragged up in this manner-dragged tirely taken away, the frog is then absoaway from the sources of irritation, lutely devoid of any spontaneity; it will though the action, you will observe, is a remain forever where you leave it; it purely automatic or mechanical action. will not stir unless it is touched; it sits Suppose we deal with a frog in the same upright in the condition in which a frog way, and cut across the spinal cord. The habitually does sit; but it differs from the frog falls into precisely the same condi- frog which I have just described in this, tion. So far as the frog is concerned, that if you throw it into the water it behis limbs are useless; but you have gins to swim — swims just as well as the merely to apply the slightest irritation to perfect frog does. Now, swimming, you the skin of the foot, and the limb is in- know, requires the combination, and instantly drawn away. Now, if we have deed the very careful and delicate comany ground for argument at all, we have bination, of a great number of muscular a right to assume that, under these cir- actions, and the only way we can account cumstances, the lower half of the frog's for this is, that the impression made body is as devoid of consciousness as is upon the sensory nerves of the skin of the lower half of the man's body; and the frog by the contact of the water conthat the body of the frog below the in-veys to the central nervous apparatus a jury is in this case absolutely devoid of consciousness, is a mere machine like a musical box or a barrel-organ, or a watch. You will remark, moreover, that the movement of the limbs is purposive-frog be stimulated, be touched by some that is to say, that when you irritate the skin of the foot, the foot is drawn away from the danger, just as it would be if the frog were conscious and rational, and could act in accordance with rational Suppose yet one other experiment. consciousness. But you may say it is Suppose that all that is taken away of easy enough to understand how so sim-the brain is what we call the cerebral ple an action might take place mechan-hemispheres, the most anterior part of ically. the brain. If that operation is properly Let us consider another experiment. performed, the frog may be kept in a state Take this creature, which certainly can- of full bodily vigour for months, or it may not feel, and touch the skin of the side be for years; but it will sit forever in of the body with a little acetic acid, a the same spot. It sees nothing; it hears little vinegar, which in a frog that could nothing. It will starve sooner than feed feel would give rise to great pain. In itself, although if food is put into its this case there can be no pain, because mouth it swallows it. On irritation it the application is made below the point jumps or walks; if thrown into the water of section; nevertheless, the frog lifts it swims. But the most remarkable thing up the limb of the same side and applies that it does is this-you put it in the the foot to rubbing off the acetic acid; flat of your hand; it sits there, crouched, and, what is still more remarkable, if you perfectly quiet, and would sit there forhold down the limb so that the frog can-ever. Then if you incline your hand, not use it, he will, by and by, take the limb of the other side and turn it across the body, and use it for the same rubbing process. It is impossible that the frog, if it were in its entirety and were reasoning, could perform actions more purposive than these, and yet we have

irritating body, although we are quite certain it cannot feel, it jumps or walks as well as the complete frog can do. But it cannot do more than this.

doing it very gently and slowly, so that the frog would naturally tend to slip off, you feel the creature's fore-paws getting a little slowly on to the edge of your hand until he can just hold himself there, so that he does not fall; then, if you turn your hand, he mounts up with great care

and deliberation, putting one leg in front and then another, until he balances himself with perfect precision upon the edge of your hand; then if you turn your hand over, he goes through the opposite set of operations until he comes to sit in perfect security upon the back of your hand. The doing of all this requires a delicacy of co-ordination, and an adjustment of the muscular apparatus of the body which is only comparable to that of a ropedancer among ourselves; though in truth a frog is an animal very poorly constructed for rope-dancing, and on the whole we may give him rather more credit than we should to a human dancer. These movements are performed with the utmost steadiness and precision, and you may vary the position of your hand, and the frog, so long as you are reasonably slow in your movements, will work backwards and forwards like a clock. And what is still more wonderful is, that if you put the frog on a table, and put a book between him and the light, and give him a little jog behind, he will jump take a long jump, very possibly-but he won't jump against the book; he will jump to the right or to the left, but he will get out of the way, showing that although he is absolutely insensible to ordinary impressions of light, there is still a something which passes through the sensory nerves, acts upon the machinery of his nervous system, and causes it to adapt itself to the proper action.

out of the village, fell senseless, but, after the action, was picked up and taken to the hospital, where he remained some time. When he came to himself, as usual in such cases of injury, he was paralyzed on the opposite side of the body, that is to say, the right arm and the right leg were completely paralyzed. That state of things lasted, I think, the better part of two years, but sooner or later he recovered from it, and now he is able to walk about with activity, and only by careful measurement can any difference between the two sides of his body be ascertained. The inquiry, the main results of which I shall give you, has been conducted by exceedingly competent persons, and they report that at present this man lives two lives, a normal life and an abnormal life. In his normal life he is perfectly well, cheerful, does his work as a hospital attendant, and is a respectable, well-conducted man. This normal life lasts for about seven-and-twenty days, or thereabouts, out of every month; but for a day or two in each month he passes suddenly and without any obvious change into his abnormal condition. In this state of abnormal life he is still active, goes about as usual, and is to all appearance just the same man as before, goes to bed and undresses himself, gets up, makes his cigarette and smokes it, and eats and drinks. But he neither sees, nor hears, nor tastes, nor smells, nor is he conscious of anything whatever, and Can we go further than this? I need he has only one sense organ in a state of not say that since those days of com- activity, namely, that of touch, which is mencing anatomical science when crimi- exceedingly delicate. If you put an obnals were handed over to the doctors, we stacle in his way, he knocks against it, cannot make experiments on human be- feels it and goes to the one side; if you ings, but sometimes they are made for push him in any direction, he goes us, and made in a very remarkable man- straight on until something stops him. ner. That operation called war is a great I have said that he makes his cigarettes, series of physiological experiments, and but you may supply him with shavings or sometimes it happens that these physio- with anything else instead of tobacco, and logical experiments bear very remarkable still he will go on making his cigarettes fruit. I am indebted to my friend Gen- as usual. His actions are purely meeral Strachey for bringing to my notice chanical. He feeds voraciously, but an account of a case which appeared whether you give him aloes or assafoetida, within the last four or five days in the or the nicest thing possible, it is all the scientific article of the Journal des Dé- same to him. The man is in a condition bats. A French soldier, a sergeant, was absolutely parallel to that of the frog I wounded at the battle of Bazeixes, one, have just described, and no doubt when as you recollect, of the most fiercely con- he is in this condition the functions of tested battles of the late war. The man his cerebral hemispheres are, at any rate was shot in the head, in the region of largely, annihilated. He is very nearly — what we call the left parietal bone. The I don't say wholly, but very nearly bullet fractured the bone. The sergeant in the condition of an animal in which had enough vigour left to send his bayo- the cerebral hemispheres are extirpated. net through the Prussian who shot him. And his state is wonderfully interesting Then he wandered a few hundred yards to me, for it bears on the phenomena of