232 RECENT PROGRESS OF THE THEORY OF VISION. 



ference according to their thickness, undergo the same 

 coagulation of their contents. In short, all that we can 

 ascertain of nervous structure and function, apart from the 

 action of the other organs with which they are united and 

 in which during life we see the proofs of their activity, is 

 precisely the same for all the different kinds of nerves. 

 Very lately the 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 differ- 

 ence 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 tele- 

 graphic wires traversing a country, and the comparison is 

 well fitted to illustrate this striking and important pecu- 

 liarity of their mode of action. In the net-work of tele- 

 graphs we find everywhere the same copper or iron wires 

 carrying the same kind of movement, a stream of elec- 

 tricity, 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 



