726 THE POPULAR SCIENCE MONTHLY. 



front called the biceps muscle ; it is shortened till it becomes thicker. 

 If I move any of my limbs, the reason is the same. As I now sj^eak 

 to you, the different tones of my voice are due to the exquisitely ac- 

 curate adjustments and adjusted contractions of a multitude of such 

 particles of flesh ; and there is no considerable and visible movement 

 of the animal body which is not, as Descartes says, resolvable into 

 these changes in the form of matter termed muscle. But Descartes 

 went further, and he stated that in the normal and ordinary condition 

 of things these changes in the form of muscle in the living body only 

 occur under certain conditions ; and the essential condition of the 

 change was, says Descartes, the motion of the matter contained within 

 the nerves, which go from the central apparatus to the muscle. Des- 

 cartes gave this moving material a particular name — the animal spirits. 

 Nowadays we should not say that the animal spirits existed, but we 

 should say that a molecular change takes place in the nerve, and that 

 that molecular change is propagated at a certain velocity which has been 

 measured from the central apparatus to the muscle. Modern physiol- 

 ogy has measured the rate of the change to which I have referred. 



Next, Descartes says that, under ordinary circumstances, this 

 change in the contents of a nerve, which gives rise to the contraction 

 of a muscle, is produced by a change in the central nervous apparatus, 

 as, for example, the brain. ^Ye say at the present time exactly the same 

 thing. Descartes said that the animal spirits were stored up in the 

 brain, and flowed out from the motor nerve. We say that a molecular 

 change takes place in the brain that is propagated along the motor 

 nerve. Further, Descartes stated that the sensory organs which give 

 rise to our feelings gave I'ise to a change in the sensory nerves, to a 

 flow of animal spirits along those nerves, which flow was propagated 

 to the brain. If I look at this candle before us, the light falling on the 

 retina of my eye gives rise to an afl*ection of the optic nerve, which 

 afiection Descartes described as a flow of the animal spirits to the 

 brain ; but the fundamental idea is the same. In all our notions of 

 the operations of nerve we are building upon Descartes's foundation. 

 He says that, when a body which is competent to produce a sensation 

 touches the sensory organs, what happens is the production 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 motion, there is, 

 as everybody can And by experiment for himself, something else which 

 can in no way be compared to motion, which is utterly unlike it, and 

 which is that state of consciousness which we call a sensation. Des- 

 cartes 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 revolutions, not only 



