214: ANIMAL AUTOMATISM. 



locity, 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-subtance." Our conceptions of what takes place 

 in nerve have altered in the same way as our concep- 

 tions 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' fundamen- 

 tal 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 

 tJie substance of the nerves which connect the sen- 

 sory 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 conceive 

 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 members which are sen- 

 sitive 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 pull- 

 ing one of the ends of a stretched cord, the other end is instantane- 



