THE SENSATION OF SIGHT. 205 



different kinds of nerves. Very lately tbe French physiologists 

 Philippeau and Vulpian, after dividing the motor and sensitive 

 nerves of the tongue, succeeded in getting the upper half of the 

 sensitive nerve to unite with the lower half of the motor. 

 After the wound had healed, they found that irritation of the 

 upper half, which in normal conditions would have been felt as 

 a sensation, now excited the motor branches below, and thus 

 caused the muscles of the tongue to move. We conclude from 

 these facts that all the difference which is seen in the excitation 

 of different nerves depends only upon the difference of the 

 organs to which the nerve is united, and to which it transmits 

 the state of excitation. 



The nerve fibres have been often compared with telegraphic 

 wires traversing a country, and the comparison is well fitted to 

 illustrate this striking and important peculiarity of their mode 

 of action. In the network of telegraphs we find everywhere 

 the same copper or iron wires carrying the same kind of move- 

 ment, a stream of electricity, but producing the most different 

 results in the various stations according to the auxiliary apparatus 

 with which they are connected. At one station the effect is the 

 ringing of a bell, at another a signal is moved, and at a third a 

 recording instrument is set to work. Chemical decompositions 

 may be produced which will serve to spell out the messages, and 

 even the human arm may be moved by electricity so as to 

 convey telegraphic signals. When the Atlantic cable was being 

 laid, Sir William Thomson found that the slightest signals 

 could be recognised by the sense of taste, if the wire was laid 

 upon the tongue. Or, again, a strong electric current may be 

 transmitted by telegraphic wires in order to ignite gunpowder 

 for blasting rocks. In short, every one of the hundred different 

 actions which electricity is capable of producing may be called 

 forth by a telegraphic wire laid to whatever spot we please, and 

 it is always the same process in the wire itself which leads to 

 these diverse consequences. Nerve fibres and telegraphic wires 

 are equally striking examples to illustrate the doctrine that the 

 same causes may, under different conditions, produce different 

 results. However commonplace this may now sound, mankind 



