208 ANIMAL ATTOMATISM v 



no means great, velocity, through the substance of 

 the nerve towards the muscle. 



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' fundamental idea, that a change 

 in the substance of a motor nerve propagated 

 towards a muscle is the ordinary cause of muscular 

 contraction. 



III. The sensations of animals are due to a 

 motion of the substance of the / 

 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 

 in T\VS is composed which subserve sensation. You nm 

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

 always distended and kept open by the animal spirits which they 

 ciintain, neither press upon nor interfere with one another ami 

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



