GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 195 



subjacent bone, and in addition to irritating the nerves of 

 the skin at the point struck, starts nervous impulses which 

 make themselves known by severe tingling pain referred to 

 the little and ring fingers to which the ulnar nerve is dis- 

 tributed. This shows not only that the nerve-fibres can be 

 irritated in their course as well as at their ends, but also that 

 sensations do not directly tell us where a nerve-fibre has been 

 excited. No matter where in its course an impulse causing 

 sensation has been started, we irresistibly refer its origin to 

 the peripheral end of the afferent nerve-fibre affected. 



General and Special Nerve-stimuli. Certain external 

 forces excite all nerve-fibres, and in any part of their course. 

 These are known as general nerve-stimuli ; others act only 

 on the end-organs of nerve-fibres, and often only on one kind 

 of end-organ, and hence cannot be made to excite all nerves: 

 these latter are commonly known as special nerve-stimuli, 

 In reality they are not properly nerve-stimuli at all; but 

 only things which so affect the irritable tissues attached to 

 the ends of certain nerve-fibres as to make these tissues in 

 turn excite the nerves. For example, light itself will not 

 stimulate any nerve, not even the optic: but in the eye it 

 effects changes (perhaps of a, chemical nature) by which 

 nerve-stimuli are produced and these stimulate the optic 

 nerve-fibres. The ends of the nerves in the skin are not 

 accessible to light nor are the proper end organs on which 

 the light acts present there, so light does not lead to the pro- 

 duction of nervous impulses in them: but the optic nerve 

 without 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 

 auditory 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 does not stimulate it. 

 (2) Mechanical stimuli. Any sudden pressure or traction, as 



