192 THE HUMAN BODY. 



shock be sent through the nerve. Different nerves excited 

 by the same stimulus produce different results, and the 

 same nerve excited by different stimuli gives the same re- 

 sult. How are these facts to be explained? 



The first explanation which suggests itself is that the 

 various nerves differ in their properties: that electricity 

 -applied 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 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 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 followed, 

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

 Relieved that afferent nerves differed essentially from effe- 

 rent nerves, inasmuch as the latter could only propagate 

 impulses centrifugally and the former only centripetally. 

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

 view is Avrong and that all nerve-fibres are exactly alike in 

 their physiological properties, and can carry nervous im- 

 pulses either way. The differences observed depend in 

 fact not on any differences in the nerve-fibres, but on the 

 -different parts connected with their ends; in other words 

 on the different terminal organs excited by the impulses 

 conveyed by the fibre. A motor fibre is one which lias at 

 its peripheral end a muscular fibre, and a centrifugally trav- 

 eling 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 when centripetally traveling 

 impulses reach them, or to transmit these to other efferent 

 fibres and so cause reflex movements; and therefore when a 

 motor fibre is stimulated in the middle of its course the 

 outward-going impulse causes a movement, while the cen- 



