SUCCESSIVE AND SIMULTANEOUS CONTRAST. 1245 



effects which have been compared with the after-images in the organ of 

 vision. M. v. Vintschgau x describes the metallic bitter of the taste which 

 sometimes remains subsequent to the breaking of a current through the 

 tongue, and which, during its passage, produced a metallic acid sensation. 



Of greater interest, however, is the after-effect produced by a sapid 

 substance itself. It has long been known that after tasting common 

 salt the tongue is more sensitive to the taste of sugar ; so much so, that 

 a very weak solution of sugar, too weak to be distinguished as a sweet 

 liquid, produces a taste, if it is taken after washing out the mouth with 

 salt and water. In the same way, acid stimuli render the tongue more 

 susceptible to sweet substances, and after tasting weak sulphuric acid, 

 even distilled water appears somewhat sweet. 2 Not only does common 

 salt produce after-susceptibility to sweet, but many other substances 

 act in the same way. Thus, after gargling with chlorate of potash, tap- 

 water tastes sweet with a trace of acid (M. v. Vintschgau). A still 

 more curious case is mentioned by J. Frentzel, 3 and one that can be 

 readily verified. If the mouth be rinsed out with a weak solution 

 of sulphate of copper, and if the experimenter then smokes a pipe or 

 cigarette, a well-marked sweet taste is developed. 



Simultaneous contrasts also appear to be capable of demonstration 

 in the case of taste, as in the case of vision. If a sapid substance be 

 applied at one region of the tongue, other parts of the organ are rendered 

 more susceptible to other stimuli. Thus, if one border of the tongue be 

 rubbed with salt, the other border will give a reaction with sugar out of 

 all proportion to the strength of the stimulus employed. Salt and acid 

 give likewise simultaneous contrast, but it is questionable whether any 

 facts can be brought forward to show that bitter sensations are affected 

 by other stimuli simultaneously applied. 



Taste sensations take some time before they are generated, and the 

 reaction time for taste is comparatively slow. This can readily be 

 understood when we remember that the sapid particles have to diffuse 

 into the somewhat viscous fluids bathing the mouth, before they can gain 

 access to the end-organs of taste. We taste more quickly when we rub 

 the tongue against the roof of the mouth, or when we rub the sapid 

 solution upon the mucous membrane by means of the finger. For the 

 same reason, it is difficult at once to free the mouth from a sapid material 

 which has had free access to it ; and a clinging taste is frequently got 

 rid of only after repeated rinsings of the mouth with water or such 

 mechanical expedients as the chewing of bread-crumb. 



Our power of localising the spots at which stimuli are applied is 

 fairly developed in the case of taste, for the sensory surface is also 

 furnished with the nerves of common sensibility, so that sapid stimuli 

 are never applied without at the same time stimulating our nerves of 

 touch. It is on this account that we can learn by everyday experience 

 that bitter substances are best tasted at the root, and sweet substances 

 are best tasted at the tip, of the tongue ; for when the bitter sensation 

 develops, we know by our sense of touch that the sapid material has 

 reached the back of the tongue. Sweet sensations, on the other hand, 

 are developed almost as soon as we become conscious of the presence of 

 the sapid substance on the tip of the tongue. 



1 Arch./, d. ges. Physiol., Bonn, 1879, Bd. xx. S. 81. 



2 F. Kiesow, Phil. Stud., Leipzig, Bd. x. S. 3 and 4. 

 8 Centralbl. f. Physiol., Leipzig u. Wien, Bd. x. S. 3. 



