198 THE HUMAN BODY. 



differences in the physical forces exciting them. This is 

 still further shown by the fact that different physical forces 

 acting upon the same nerve arouse the same kind of sensa- 

 tion. Light reaching the eye causes a sight sensation, but if 

 the optic nerve be irritated by a blow on the eyeball a sensa- 

 tion of light is felt just as if actual light had stimulated the 

 nerve-ends; and a similar result follows if an electric shock be 

 sent through the eyeball and optic nerve. Different nerves 

 excited by the same stimulus produce different results, and 

 the same nerve excited by different stimuli gives the same 

 result. How are these facts to be explained ? 



The first explanation which suggests itself is that the 

 various nerves differ in their properties : that electricity ap- 

 plied to a motor nerve causes a muscle to contract, and 

 to the optic nerve a visual sensation, and to the lingual 

 nerve a sensation of taste, because nervous impulses in 

 the motor, optic, and lingual nerves differ from one an- 

 other. This was the view held by the older physiologists; 

 and that supposed peculiarity of a muscular nerve by which 

 its irritation caused a muscular contraction, and that of 

 of the optic nerve in consequence of which its excitation 

 caused a sensation of sight, and so on, they called the specific 

 energy of the nerve. Seeing further that when a pure motor 

 nerve was cut and its peripheral stump pinched the muscles 

 connected with it contracted, but that when its central end 

 was pinched no sensation or other recognizable change fol- 

 lowed, while exactly the reverse was true of a sensory nerve, 

 they believed that afferent nerves differed essentially from 

 efferent nerves, inasmuch as the latter could only propagate 

 impulses centrifu gaily and the former only centripetally. 

 Now, however, we have much reason to believe that this view 

 is wrong, and that all nerve-fibres, though perhaps exhibiting 

 some minor differences, are essentially alike in their physio- 

 logical properties, and can carry nervous impulses either way* 

 The differences observed depend in fact not on any differ- 

 ences in the nerve-fibres, but on the different parts connected 

 with their ends; that is to say, on the different terminal 

 organs excited by the impulses conveyed by the fibre. A 

 motor fibre is one which has at its peripheral end a muscular 

 fibre, and a centrifugally travelling impulse reaching this will 

 cause it to contract: but the cells connected with its central 

 end are not of such a nature as to give rise to sensations 



