718 SENSATION [OH. LI. 



When the eye is excited by any other kind of stimulus than by 

 light, which is its adequate or homologous stimulus, the sensation 

 experienced is light all the same ; for instance, one sees sparks when 

 the eyeball is struck ; singing in the ears, the result of an accumula- 

 tion of wax against the membrana tympani, is a similar example. 



It has been inferred that there are separate nerve-fibres for the 

 conveyance of each kind of sensation, and Johannes Miiller expressed 

 this idea in what is known as the law of specific nerve energy. He 

 pointed out that the same nerve may be stimulated by mechanical or 

 electrical means as well as in the normal physiological manner, and 

 that in all cases the sensation light, sound, taste, contact, etc., as 

 the case might be is the same. Hence it was argued that the 

 psychical effect or sensation is independent of the nature of the 

 stimulus, but dependent on the nature of the activity of the central 

 cells among which the afferent fibres terminate. We have no observa- 

 tions which can decide whether the nerve impulses passing along 

 the optic fibres are, for instance, similar to or different from those 

 which are transmitted by the auditory fibres. The experiments of 

 Langley and others on nerve-crossing (p. 173) would seem to indicate 

 that the nervous impulse is an identical process in all nerves ; and if 

 this is so, we are obliged to infer that separate nerve-fibres convey 

 the impulses destined to give rise to different sensations. 



It is, however, possible that in the nerves of cutaneous sensation, the psychical 

 process is determined by the nature of the peripheral stimulus, and consequently 

 different branches of the same nerve-fibres may be imagined to be susceptible to 

 different forms of stimulation, and thus two different sensations follow from the 

 partial stimulation of the same nerve-fibres. Hering even argues in favour of the 

 view that the nerve impulse has different characters in different afferent nerves, and 

 further that it may be modified by the nature of the normal stimulus (e.g. in the skin, 

 heat, cold, pain, or pressure). In the absence of direct experimental proof of such 

 an idea, it is difficult to see upon what grounds it can rest. 



