Sept. 3, 1874 1 



iSTATURE 



363 



remains exactly what it was in the time of Descartes. Next, 

 Descartes says that, under ordinary circumstances, this change 

 in tlie contents of a nerve, wliich gives rise to the contraction of 

 a muscle, is produced by a cliange in the central nervous appa- 

 ratus, as, for example, in the brain. We say at the present time 

 exactly the same thing. Descartes said that the animal spirits 

 were stored up in tlie brain, and flowed out along the motor 

 nerves. We say that a molecular cliange takes place in the 

 brain that is ])ropagated along the motor nerve. The evidence 

 of that is abundantly supplied by experimental research. Further, 

 Descartes stated that the sensory organs, or those apparatuses 

 which give rise to our feelings when acted upon by the influences 

 which produce sensation, caused a change in the sensory nerves, 

 which he described as a flow of animal spirits along those 

 nerves, which flow was propagated to the brain. If I look at 

 this candle which I hold before me, tlie light falling on the retina 

 of my eye gives rise to an affection of the optic nerve, which 

 alTection Descartes described as a flow of the animal spirits to 

 the brain. We should now speak of it as a molecular change 

 j)ropagated along the optic nerve to tlie brain ; but the funda- 

 mental idea is the .same. In all our notions of the operations of 

 nerve we are building upon Descartes' foundation. Not only 

 so, but Descartes lays down over and over again, in the most 

 distinct manner, a proposition wliich is of paramount importance 

 not only for physiology but for psychology. He says that when 

 a body which is competent to produce a sensation touches tlie 

 sensory organs, wliat happens is the production of a mode of 

 motion of the sensory nerves. That mode of motion is propa- 

 gated to the brain. Tliat which takes place in the brain is still 

 nothing but a mode of motion. But, in addition to this mode 

 of motion, there is, as everybody can find by experiment for 

 himself, something else which can in no way be compared to 

 motion, which is utterly unHke it, and which is that state of con- 

 sciousness wliich we call a sensation. Descartes insists over and 

 over again upon this total disparity between the agent which 

 excites the state of consciousness and the state of consciousness 

 itself. He tells us that our sensations are not pictures of 

 external things, but that they are symbols or signs of them ; 

 and in doing that he made one of the greatest possible re- 

 volutions, not only in physiolo;;y but in philosophy. Till 

 his time it was conceived that visible bodies, for example, 

 gave from themselves a kind of film which entered the eye 

 and so went to the brain, species inloitionales as they were 

 called, and thus the mind received an actual copy or picture of 

 things which were given off from it. It is to Descartes we owe 

 tliat complete revolution in our ideas, which has led us to see 

 that we have really no knowledge whatever of the causes of those 

 phenomena which we term external things, and that the only 

 certainty we possess is that they cannot be like those phenomena. 

 In laying down that proposition upon what I imagine to be a 

 perfectly irrefragable basis, Des artes laid the foundation of that 

 form of jihilosophy which is termed idealism, which was sub- 

 sequently expantled to its uttermost by Berkeley, and has since 

 taken very various shapes. 



But Descartes noticed not only that under certain condi- 

 tions an impulse made by the sensory organ may give rise 

 to a sensation, but th.at under certain other conditions it 

 may give rise to motion, and that this motion may be eflfected 

 without sensation, and not only without volition, but even 

 contrary to it. I trouble you with as little reading as I can, 

 because it occupies so much time ; but I must ask your patience 

 for one very remarkable passage which is contained in the 

 answer that Descartes g.ave to the objections raised by the 

 famous Port Royalist Arnauld to his Fourth Meditation. 

 Descartes says: "It apjiears to me to be a very remarkable 

 circumstance that no movement can take place either in the 

 bodies of beasts or even in our own, if these bodies have not in 

 themselves all the organs and instruments by means of which the 

 very same movement would be accomplished in a macliine, so 

 that, even in us, the spirit or the soul does not 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 

 tlie 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 wlvn th-^y are performeil wHboul llu; 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 jilace merely liecause his senses, being affected by the 

 present danger, cause some change in his brain, which deter- 

 mines the animal spirits to pass thence into the nerves in such a 

 manner as is required to produce this motion, in the same way 

 as in a machine, and without the mind being able to hinder it." 

 I know in no modern treatise of a more clear and precise state- 

 ment, of a more perfect illustration than this of what we under- 

 stand by the automatic action of the brain. And what is very 

 remarkable, in speaking of these movements which arise by a 

 sensation being as it were reflected from the central apparatus 

 into a limb — as, for example, when one's finger is pricked and 

 the arm is suddenly drawn up, the motion of the sensory nerve 

 travels to the spine and is again reflected down to the muscles of 

 the arm — Descartes uses the very phrase that we at this present 

 time employ ; he speaks of the " espnts rc/iA/iis," the reflected 

 spirits ; and that this was no mere happy phrase lost upon his 

 contemporaries will be obvious if you consult the famous work of 

 Willis, the Oxford professor, " De Anima Brutorum," which 

 was published about 1672. In giving an account of Descartes' 

 views he Ijorrows this very phrase from him, and speaks 

 of this reflection of the motion of a sensory nerve into 

 the motion of a motor nerve, "sicut undulatiune reflexa," as 

 if it were a wave thrown back ; so that we have not only 

 the thing reflex action described, but we have the phrase 

 " reflex " recognised in its full significance. 



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 stUl, 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 almost any impulse which stirs the animal spirits causes them 

 to flow into these already open pores more easily than they would 

 flow in any other direction ; and the flow of the animal spirits 

 recalls the image, the state of consciousness called into existence 

 by a former sensory impression. This view is essentially at one 

 with all our present physical theories of memory. That memory 

 is dependent upon a physical process stands beyond question. 

 The results of the study of disease, the results of the action 

 of poisonous substances, all conclusively point to the fact that 

 memory is inseparably connected with the integrity of certain 

 material parts of the brain and dependent upon them, and I know 

 of no hypothesis 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 im- 

 pressions easier and therefore allows almost any indirect 

 disturbance of the brain to call up this particular image. 



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 suc- 

 cessors 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 in fiilling mechani- 

 cally puts out his hand to save himself, or when a person, to use 

 another of Descartes' illustrations, strikes at his friend's eye, and 

 although the friend knows he does not mean to hit him, he 

 nevertheless cannot prevent the muscles of his eye from winking. 

 " In these cases," Descartes said, " I have clear evidence that 

 the nervous system acts mechanically without the intervention of 

 consciousness and without the intervention of the will, or, it may 

 be, in opposition to it. Why, then, may I not extend this idea 

 further ? As actions of a certain amount of complexity are 

 brought about in this nay, why may not actions of still greater 



