NERVE STIMULI. 189 



its peculiar end organs would be just as insensible to light 

 as these are. Similarly the aerial vibrations which affect 

 us as sounds, do not stimulate directly the fibres of the audi- 

 tory nerve. They act on terminal organs in the ear, and 

 these then stimulate the fibres of the nerve of hearing, just 

 -as they would any other nerve which happened to be con- 

 nected with them. 



General Nerve Stimuli. Those known are (1) electric 

 currents : an electric shock passed through any part of 

 .any nerve-fibre, powerfully excites it. A steady current 

 passing through a nerve' does not stimulate it, but any 

 sudden change in this, whether an increase or a decrease, 

 does. A very gradual change in the amount of electricity 

 passing through a nerve in a unit of time will not stimii' 

 late it. (2) Mechanical stimuli. Any sudden pressurp 

 or traction, as a blow or a pull, will stimulate a nerve- 

 fibre. On the other hand steady pressure, or pressure very 

 slowly increased from a minimum, will not excite the 

 nerve. (3) Thermal stimuli. Any sudden heating or cool- 

 ing of a nerve, as for instance bringing a hot wire close to 

 it, will stimulate; slow changes of temperature will not. (4) 

 Chemical stimuli. Many substances which alter the nerve- 

 fibre chemically, stimulate before killing it; thus dipping 

 the cut end of a nerve into strong solution of common salt 

 will excite it, but very slow chemical change in a nerve 

 fails to stimulate. 



In the case of all these general stimuli it will be seen 

 that as one condition of their efficacy they must act with 

 considerable suddenness. On the other hand too transient 

 influences have no effect. An electric shock sent for only 

 0.0015 of a second through a nerve does not stimulate 

 it: apparently the inertia of the nerve molecules is too 

 .great to be overcome by so brief an action. So, also, too 

 strong sulphuric acid and many other bodies kill nerves 

 immediately, altering them so rapidly that they die without 

 being stimulated. 



Special Nerve Stimuli. These as already explained 

 act only on particular nerves, not because one nerve is es- 

 sentially different from another, but because their influence 



