188 THE HUMAN BODY. 



extent. If, for example, the sciatic of a frog be exposed in 

 the thigh and divided, it will be found that electric shocks 

 applied at the point of division to the outer half of the 

 nerve stimulate the motor fibres in it, and cause the mus- 

 cular fibres of the leg to contract: and similarly such shocks 

 applied to the cut end of the central half irritate the affe- 

 rent fibres in it, as shown by the signs of feeling exhibited 

 by the animal. In ourselves, too, we often have the oppor- 

 tunity of observing that the sensory fibres can be stimulated 

 in their course at some distance from their ends. A blow 

 at the back of the elbow, at the point commonly known as 

 the "funny bone" or the "crazy bone," compresses the 

 ulnar nerve there against the subjacent bone, and starts 

 nervous impulses which make themselves known by severe 

 tingling pain referred to the little and ring fingers to which 

 the nerve is distributed. 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 the impulse has been started we unconsciously 

 refer its origin to the peripheral end of the afferent nerve. 

 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 aifect the irritable tissues at- 

 tached 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 (apparently of a chemical nature) 

 by which substances of the nature of general 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 there present, so light does not lead to the production 

 of nervous impulses in them: but the optic nerve without 



