154 SMELL, TASTE, ALLIED SENSES 



stances are known that possess different tastes depend- 

 ing upon the part of the tongue to which they are applied. 

 Many salts have this peculiarity Herlitzka (1908). 

 Potassium nitrate and magnesium sulphate are both said 

 to be saline in taste when applied at the tip of the tongue 

 and bitter at its base. This action, however, is proba- 

 bly due not to the molecules of the salts but to their ions. 

 At the tip of the tongue the anions stimulate the organs 

 of the saline taste, which in this location are in the ascen- 

 dency, and at the base of the tongue the cations stimulate 

 the organs of the bitter taste which is here better devel- 

 oped. There is thus a kind of competition between the 

 two sets of ions, as Herlitzka has expressed it, and in one 

 locality the anions win out, in the other the cations. 



Such an explanation, however, does not apply to sub- 

 stances like parabrombenzoic sulphinide. This material, 

 according to Howell and Kastle (1887) has a distinctly 

 sweet taste when applied to the tip of the tongue and an 

 intensely bitter one at the back. Dulcamarin, the gluco- 

 side from bittersweet, is another case of the same kind; a 

 list of these is given by Sternberg (1898). In these in- 

 stances ions are probably not involved, but each substance 

 is a stimulus for both the organs of the sweet taste and 

 of the bitter taste. It seems impossible to explain double 

 tastes such as those just mentioned except on the assump- 

 tion of independent receptor systems for the tastes con- 

 cerned. Thus far no substance is known that excites 

 three categories of tastes though I know of no reason why 

 such a substance might not exist. 



12. Latency of Taste Sensations. Von "Wittich 

 (1868) appears to have been the first to attempt to meas- 

 ure the interval of time between the application of a 



