FROM A VEGETABLE. 195 



The nerves are the medium of sensation whether active 

 or passive, whether of volition or pain; for every fleshy 

 fibre or muscle receives a nervous filament, and when the 

 communication of this with the rest of the system is inter- 

 rupted, we perceive that the fibre thus insulated ceases 

 as well to obey the will acting upon the centre or centres 

 of the nervous system as to communicate any sensation 

 to the source of that will. But it may be going too far to 

 say that the fibre ceases in this case to feel, for we observe 

 certain external agents to act upon the nerve, and cause con- 

 tractions of the muscular fibre even after its separation 

 from the rest of the body. Irritation of the nerve therefore, 

 though we know from experience that it can be produced 

 by an operative principle like the human mind, seems also 

 to be a mechanical process, as is certainly the action of the 

 nerve on the fibre. It is possible, then, that both these 

 effects may have an unintelligent mediate cause, such as 

 electricity for instance, and this is one of the most impor- 

 tant circumstances to be borne in mind while we investi- 

 gate the metaphysical nature of pain in irrational animals. 



We observe also on comparing different sorts of nervous 

 systems, that the contraction and irritability of the sepa- 

 rated fibre is greater in those animals whose medullary 

 substance is less concentrated ; which in some degree proves 

 that the irritability of the muscular fibre depends on the 

 proportion of nerve remaining in it after separation from the 

 rest of the body. Now it is possible that all the difference 

 between the most simple class of vegetables and the least 

 organized of animals is, that the homogeneous gelatinous 

 substance' of which the latter are composed, possesses, di- 

 spersed throughout the mass, those nervous molecules 

 which when united in the more perfect animals form the 



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