1030 TASTE SENSATIONS. [BOOK in. 



we experience when various substances are placed in the mouth ; 

 and these, which are the gustatory sensations proper, may be 

 broadly classified into ' bitter,' 'sweet,' 'acid' or 'sour,' and 'salt,' 

 to which perhaps should be added 'metallic ' and 'alkaline.' The 

 sensation of bitterness, such as that produced by quinine, and 

 the sensation of sweetness, such as that produced by sugar, are 

 very definite and specific sensations ; they appear to be of an 

 order different from those of acidity or sourness and of saltness ; 

 indeed an acid ' taste ' is apt to merge into an affection of gen- 

 eral sensibility mentioned above. The characters ' metallic ' 

 and ' alkaline ' should perhaps be regarded as qualifying one or 

 other of the other sensations rather than as being independent 

 sensations. 



In the ordinary course of things these sensations are excited 

 by the contact of specific sapid substances with the mucous 

 membrane of the mouth, the substances acting in some waj^ or 

 other, by virtue of their chemical constitution, on the endings 

 of the gustatory fibres. When we taste quinine, the particles of 

 the quinine, we must suppose, set up chemical changes in the 

 cells of the taste-buds or in other parts of the epithelium, and 

 by means of those changes gustatory impulses are started. But 

 mechanical or electrical stimuli, in the absence of sapid sub- 

 stances, will give rise to gustatory sensations. When the tongue 

 is smartly tapped, in addition to the sensation of touch or the 

 more or less painful sensation which may be produced, a sensa- 

 tion, which we must call a sensation of taste, is developed and 

 often lasts for some considerable time. If a constant current 

 be applied to the tongue, sensations of taste are developed at 

 the two electrodes, that at the anode differing from that at the 

 kathode, and the exact nature of each being dependent upon 

 the region of the mouth stimulated. It is probable that in this 

 case electrolysis either of the fluids covering the epithelium or 

 of the substance of the epithelial cells themselves generates 

 bodies whicli act as chemical stimuli ; and it is possible that the 

 mechanical disturbance of the cells, when the tongue is tapped, 

 also sets free chemical stimuli. But sensations of taste may be 

 provoked by an interrupted induced current, so feeble as not to 

 be felt as an electric current, and so arranged that the make and 

 break shocks are equalized ; in this case there can be little or no 

 electrolysis, and we may infer that the current acts in some way 

 or another on the specific nerve endings. It is somewhat singu- 

 lar that heat when applied to the tongue appears not to produce 

 any sensations of taste. 



As we shall presently see, the nerve fibres concerned in taste 

 belong either to the fifth nerve or to the glossopharyngeal nerve 

 or to both nerves. We saw in dealing with vision that the evi- 

 dence as to whether direct stimulation of the optic fibres without 

 the intervention of the retinal structures could produce visual 



