MODALITY AND QUALITY. 1243 



irritation, if capable of producing sensations, produce those of taste, not 

 those of smell or hearing. 1 



Since Miiller's time the law of specific energy of the senses has come 

 to mean far more than he intended, and although it is still called Miiller's 

 law it is quite contrary to his teaching. 2 It has become a law regard- 

 ing quality instead of modality; of nerve fibre rather than of nerve. 

 It is maintained by many recent writers that each nerve fibre with its 

 end-organ has a specific energy, which means that it can, when 

 stimulated, give rise to a certain definite sensation, and no other. If, 

 then, we have more than one kind of sensation, of light, taste, smell, 

 etc., we must have more than one kind of nerve fibre of light, taste, 

 smell, etc., and as many nerve fibres as we have sensations. This law 

 is at the basis of the Young-Helmholtz theory of vision, and Clerk- 

 Maxwell puts it as follows : 3 



" All the evidence we have on the nature of nervous action goes to 

 prove that, whatever be the nature of the agent which excites the 

 nerve, the sensations will differ only in being more or less acute. Now 

 we may perceive with our eyes a faint red light, which may be made 

 stronger and stronger until our eyes are dazzled. We may then perform 

 the same experiment with a green light or a blue light. We shall then 

 see that our sensations of colour may differ in other ways, besides in 

 being stronger and fainter. The sensations of colour cannot be due 

 to one nerve only." 



The facts which we have just reviewed in relation to taste lend 

 support to this neo-Miillerian law of specific energy of the senses, for 

 there appear to be, at any rate in certain cases, special end-organs and 

 nerves associated with the production of each specific taste. Whether 

 this law is really a general law of sensation can only be determined 

 when it is shown to be true in the case of other senses ; as to this the 

 evidence is as yet very meagre. 



As we have indicated, the end-organs of taste can be stimulated by 

 agencies other than sapid bodies. Mechanical blows and friction act as 

 stimuli ; a pencil or glass rod rubbed against the side of the root of the 

 tongue produces a distinctly bitter taste, and acid and sweet tastes are 

 alleged to have been produced by tapping the front of the tongue. 

 Electricity is a very powerful stimulant, for a current too weak to pro- 

 duce an effect upon the skin or the eye will produce a distinct sensation 

 of taste upon the tongue. 



It was once thought that the action of the electric current was 

 electrolytic, and that the ions were tasted, but this is negatived by the 

 fact that distinct tastes are produced by slight alternating induction 

 shocks, a condition under which there can be little electrolytic change. 

 The electric currents act probably upon the end-organs, and do not 

 directly stimulate the nerves, for cocaine and gymnema abolish the 

 electric taste when applied to the surface of the tongue. 



In experimenting with electrical stimulation of the tongue, one 

 electrode, preferably a non-polarisable one, is placed on the dorsum of 



1 Such stimuli generally, of course, stimulate the nerves of tactile sensibility as well, 

 and produce the specific sensations associated with their stimulation. 



2 Many quotations might be given from Miiller's "Physiology," in support of this 

 statement; the following has been selected, as bearing directly upon taste, p. 1066 : "The 

 nerves of taste and smell are capable of being excited to an infinite variety of sensations by 

 external causes ; but each taste is due to a determinate condition of the nerve excited by 

 the external cause." 3 "Collected Papers," vol. i. pp. 446-447. 



