IX.] ANIMAL AUTOMATISM. 207 



With the progress of research, the term " animal 

 spirits" gave way to "nervous fluid," and "nervous 

 fluid" has now given way to " molecular motion of 

 nerve -substance/' Our conceptions of what takes 

 place in nerve have altered in the same way as our 

 conceptions of what takes place in a conducting wire 

 have altered, since electricity was shown to be not a 

 fluid, but a mode of molecular motion. The change 

 is of vast importance, but it does not affect Descartes 7 

 fundamental idea, that a change in the substance of a 

 motor nerve propagated towards a muscle is the ordi- 

 nary cause of muscular contraction. 



III. The sensations of animals are due to a 

 motion of the substance of the nerves which 

 connect the sensory organs with the brain. 



In La Dioptrique (Discours Quatrieme), Descartes 

 explains, more fully than in the passage cited above, 

 his hypothesis of the mode of action of sensory 

 nerves : 



" It is the little threads of which the inner substance of the 

 nerves is composed which subserve sensation. You must con- 

 ceive that these little threads, being inclosed in tubes, which are 

 always distended and kept open by the animal spirits which they 

 contain, neither press upon nor interfere with one another, and 

 are extended from the brain to the extremities of all the mem- 

 bers which are sensitive in such a manner, that the slightest 

 touch which excites the part of one of the members to which a 

 thread is attached, gives rise to a motion of the part of the brain 

 whence it arises, just as by pulling one of the ends of a stretched 

 cord, the other end is instantaneously moved. . . . And we 

 must take care not to imagine that, in order to feel, the soul 

 needs to behold certain images sent by the objects of sense to 



