260 TASTE SENSATIONS. [BOOK HI. 



these, which arc 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 general 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 way 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 substances, 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 sensation, 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 which act as 

 chemical stimuli ; and it is possible that the mechanical disturb- 

 ance 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 

 singular 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 glosso-pharyngeal nerve 

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

 evidence as to whether direct stimulation of the optic fibres 

 without the intervention of the retinal structures could produce 

 visual sensations was uncertain. We have no satisfactory evidence 

 whatever that direct stimulation of the gustatory fibres along their 



