SIMILARITY OF ALL NERVE-FIBRES. 195 



nerves, than that afferent fibres become efferent, or efferent 

 afferent, under such conditions. This argument, as one 

 favoring the belief in the essential identity of all nerve- 

 fibres, must therefore be given up. 



Afferent and efferent nerve-fibres differ in no ob- 

 servable property. They are alike in faculty, and their 

 different names simply imply that they have different ter- 

 minal organs. Just as all muscles are alike in general 

 physiological properties, and differ in special function 

 according to the parts on which they act, so are all nerve- 

 fibres alike in general physiological properties, and differ 

 in special function only because they are attached to spe- 

 cial things. The special physiology of various nerves will 

 hereafter be considered in connection with the working of 

 yarious mechanisms in the Body. If it be true that the 

 great subdivisions of afferent and efferent fibres have 

 identical properties, it follows that this is a fortiori true 

 of the minor subdivisions of each, and that auditory, gus- 

 tatory, and optic nerve-fibres are all alike, and all identi- 

 cal with motor and vaso-motor and secretory nerve-fibres; 

 and that the nervous impulse is in all cases the same thing, 

 varying in intensity in different cases and in the rate at 

 which others follow it in the same fibre, but the same in 

 kind. To put the case more definitely: Light outside the 

 eye exists as ethereal vibrations, sound outside the ear as 

 vibrations of the air (commonly). Each kind of vibration 

 acts on a particular end organ in eye or ear which is 

 adapted to be acted upon by it, and in turn these end 

 organs excite the optic and auditory nerve-fibres; these in 

 consequence transmit impulses, which reaching different 

 parts of the brain excite them; the excitement of one of 

 these brain-centres is associated with sonorous and of the 

 other with visual sensations. The nervous impulse in the 

 two cases is quite alike, at least as to quality (though it may 

 differ in quantity and rhythm) and the resulting difference 

 in quality of the sensations cannot depend on it. The 

 quality differences in these cases must be products of the 

 central nervous system. If we had a set of copper wires we 

 might by sending precisely similar electric currents through 



