428 BIOLOGY. [BOOK vi. 



descend in the animal series, we see the apparatus, the organs 

 of the senses simplify, and even disappear, leaving desquamated 

 the nervous terminations and their bacilla. 



As to the tactile fibres and the gustatory fibres, which are 

 only a variety thereof, their function is much more simple ; also 

 they have not usually any terminal cells. Neither do they lead to 

 true bacilla, but to corpuscles, dermic projections, appendices of 

 varied form. 



The nature of the sensations transmitted to the conscious 

 nervous centres by the sensitive fibres is, however, altogether 

 independent of the terminal appendices and apparatus. In effect 

 any sensitive nerve excited on its passage awakens in the nervous 

 centres solely and specially the kind of sensation which it has 

 the function to provoke. Every excitation, physical, mechanical, 

 chemical, and so on, any lesion, puncture, contusion, section of a 

 sensitive nerve, have as repercussion in the nervous centres 

 sensations tactile or painful, gustatory, olfactory, auditory, 

 optical, according as the nerve affected appertains to touch, to 

 taste, to smell, to hearing, or to sight. 



It remains for us now, after having passed in review the 

 peripheric organs of sensibility, the apparatus which are the 

 gatherers of sensations, to consider the receptive centres, the 

 nervous parts, in which the mechanical agitation of the terminal 

 sensitive fibres determines a physical fact, a phenomenon of con- 

 sciousness. Recently some unsuccessful efforts have been made 

 to apply here the theory of the transformation of forces. But 

 there is not here any transformation of the molecular movement 

 in the habitual sense of the expression. When a luminous undu- 

 lation comes to agitate the nerves of the retina, the central 

 sensation which results therefrom is by no means a direct trans- 

 formation of the ethereal vibration. This undulation has been 

 simply the occasional cause of a whole series of physiological 

 phenomena, the nutritive oxydation of the tissues being the real 

 cause. The mechanical agitation of the sensitive nervous ex- 

 tremities plays in sensation the part of a trigger in a firearm. 



