202 THE HUMAN BODY. 



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 trans- 

 mit 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 sensa- 

 tions. 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 sensa- 

 tions 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 them produce very different 

 results if different things were interposed in their course. 

 In one case the current might be sent through water and 

 decompose it, doing chemical work; in another, through the 

 coil of an electro-magnet and raise a weight; in a third, 

 through a thin platinum wire and develop light and heat; 

 and so on, the result depending on the terminal organs, as we 

 may call them, of each wire. Or, on the other hand, we 

 might generate the current in each wire differently, in one 

 oy a Darnell's cell, in a second by a thermo-electric machine, 

 iti a third by the rotation of a magnet inside a coil, but the 

 currents in the wires would be essentially the same, as the 

 nervous impulses are in a nerve-fibre. No matter how they 

 have been started, provided their amount is the same, 

 whether they shall produce similar or dissimilar results, de- 

 pends only on whether they are connected with similar or 

 dissimilar end-organs. 



To sum up : Afferent and efferent nerve-fibres differ in no 

 fundamental physiological property; they are simple trans- 

 mitters, everywhere alike in faculty. We may extend this 

 statement to the subdivisions of each kind, and say that 

 motor, vasomotor and secretory efferent fibres, and tactile, 

 auditory and visual afferent fibres are in all essentials like one 

 another; and that a nervous impulse is alike in every nerve, 

 varying it may be in intensity and in the rate at which 

 others succeed it, in different cases, but the same in kind. 

 Just as all muscles are alike in general physiological proper- 

 ties, and differ in special function according to the parts on 

 which they act, so are all nerve-fibres alike in general physio- 

 logical properties, and differ in special function only because 



