CHAP, v.] OF SENSIBILITY IN GENERAL. 403 



paradoxical facts, admit, with some physiologists, the existence 

 of an amorphous nervous substance, impregnating as it were the 

 entire bodies of these animals 1 There is nothing to authorise such 

 hypotheses. In the whole animal kingdom we see the precision of 

 the sensitive perceptions correspond to clearly denned nervous ap- 

 paratus, aided by special organs called organs of the senses, having 

 the office of choosing amongst the shocks, the movements, the 

 vibrations, &c., in short, the physical impressions of the exterior 

 medium, all that can awaken in the nervous centres of the conscious 

 being the tactile, olfactory, gustatory, auditive, and visual sensa- 

 tions. How admit that special sensations, which, to be produced in 

 the superior animals, require diverse and complex apparatus, may, 

 nevertheless, be experienced in a state of organic confusion and 

 indivision 1 It is much more simple here to accuse the imperfec- 

 tion of our means of investigation, and to reserve till we have 

 more ample information those exceptional cases, destined pro- 

 bably to be included one day in the general law. 



With the aid of this restriction we can now sketch the picture 

 of nervous sensibility. This sensibility has its seat in the nervous 

 cells and fibres, of which we have already, given a brief descrip- 

 tion. That there may be nervous cells specially sensitive having 

 consciousness of impressions exercised upon the terminal extre- 

 mities of their fibres, no one has ever attempted to deny. 



It has not been the same with the nervous fibres, properly 

 so-called, and upon the authority of some experiments, either 

 erroneous or insufficient, the indifference of the nervous fibres 

 was for some time believed in. According to this theory, the 

 nervous fibres are all identical. It would be necessary to regard 

 them as simple conducting threads, indifferent to the kind of 

 excitations which they transmit. Sensitive, when they bring an 

 organ of sense into communication with the sensitive central 

 cells, the nervous fibres become motory by simple displacement, 

 by the single fact of connecting a muscle with the motory cells. 

 No trustworthy experiment comes to the support of this theory, 

 which is brilliantly refuted by the evident speciality of the optic, 



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