316 Cuvieriaft Si/stem of Zoology. 



combustions, detonations, &c. Hence, Cuvier infers that it 

 is by an imponderable fluid that the nerve acts on the muscu- 

 lar fibre, since it can be demonstrated, that it does not act 

 mechanically. All the external causes that excite sensations, 

 or occasion a contraction of the muscular fibre, are chemical 

 agents, capable of effecting chemical decompositions, as light, 

 caloric, salts, odorous vapours, percussion, compression, &c'. : 

 it is therefore probable, he says, that these causes act on the 

 nervous fluid chemically, and change its composition ; for their 

 action is v^eakened (s^emousse) by continuance, as if the ner- 

 vous fluid required to be restored to its primitive state {compo- 

 sition), in order to be acted upon again. The internal agents 

 which irritate the muscular fibre, and occasion it to contract, 

 probably produce the same effect upon it by the intervention 

 of the nerves, as is caused by the will. The nervous fluid is 

 changed {/ alter e) by muscular irritation, as well as by sens- 

 ation and voluntary motion, and has the same need to have its 

 composition restored. The most important involuntary mo- 

 tions are the peristaltic, or wormlike motion of the intestines, 

 which is excited by the irritation of the aliments, and the 

 pulsation of the heart and arteries, which is excited by the ir- 

 ritation of the blood. . 



The external organs of the senses are a kind of sieves 

 [cribles) that only admit those agents to the nerves, which 

 ought to affect them in each place. Thus the tongue has 

 strong papillae, which imbibe saline solutions ; the ear has a 

 gelatinous pulp, which is agitated by sonorous vibrations ; and 

 the eye has transparent lenses, that are only permeable by 

 light. I forbear to follow our author farther in his details 

 of the action of the nervous fluid; for the existence of such a 

 jftuid may be regarded as hypothetical : the nervous fluid may, 

 however, be admitted as a term, to express the cause of ner- 

 vous action, though we remain profoundly ignorant of the 

 essence of this cause ; the same may be said of the essence of 

 the electric fluid, of caloric, and of light ; and it may not be 

 improper to remark, that there are some striking analogies 

 between the nervous action, and electric or galvanic energy, 

 which might induce us to believe that they are modifications 

 of the same principle. Whatever explanation chemical or 

 electric agency may afford, respecting the motions of the 

 nerves or muscles, they entirely fail, when applied to sensation 

 itself. Cuvier justly observes that die impression of external 

 objects on the mind or individual consciousness {le moi), the 

 production of a sensation or image, is an impenetrable mystery 

 to the human understanding ; and materialism is so far from 

 offering any solution, that philosophy cannot furnish us with 



