196 THE HUMAN BODY. 



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 cooling of a nerve, as for instance 

 bringing a hot wire close to it, will stimulate; slow changes 

 of temperature will not. (4) Chemical stimuli. Manv sub- 

 stances which chemically alter the nerve-fibre stimulate 

 before killing it; thus dipping the cut end of a nerve into 

 a strong solution of common salt will excite it; 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 .con- 

 siderable suddenness. On the other hand too transient in- 

 fluences have no eifect. An electric shock sent for only 

 0.0015 of a second through a nerve does not stimulate it: ap- 

 parently the inertia of the nerve molecules is too great to be 

 overcome by so brief an action. So, also, strong sulphuric 

 acid and some other liquids kill nerves immediately, altering 

 them so rapidly that they die before being stimulated. 



Special Nerve-stimuli. These as already explained act 

 only on particular nerves, not because one nerve is essen- 

 tially different from another, but because their influence is 

 excited through special end-organs which are attached to some 

 nerves. These stimuli are: (1) Changes occurring in central 

 organs, of whose nature we know next to nothing, but which 

 excite the efferent nerve-fibres connected with them. The 

 remaining special stimuli act on afferent fibres through the 

 sense-organs. They are: (2) Light, which by the interven- 

 tion of organs in the eye excites the optic nerve. (3) Sound, 

 which by the intervention of organs in the ear excites the 

 auditory nerve. (4) Heat, which through end-organs in 

 the skin is able, by very slight changes, to excite certain 

 nerve-fibres: such slight changes of temperature being 

 efficient as would be quite incapable of acting as general 

 nerve-stimuli without the proper end-organs. (5) Chemical 

 agencies, which when extremely feeble and incapable of 

 acting as general stimuli can act as special stimuli through 

 special end-organs in the mouth and nose (as in taste and 

 smell) and probably in other parts of the alimentary tract, 

 where very feeble acids and alkalies seem able to excite cer- 

 tain nerves, and reflexly through them excite movements or 

 render active the cells concerned in making the digestive 



