TASTE 



243 



special end-organs at the tij) of the tongue, if it contains both a 

 ghiciphoric and anxoghic grouping. The classification is not quite 

 satisfactory. Progress will be made when the problem is attacked 

 by physical chemists from the aspect of permeability, much in the 

 same way as we shall see has been done for sour tastes. 



Sourness. This sensation is produced when acids penetrate 

 certain cells on the sides of the tongue. The threshold value for 

 sourness does not depend on the strength of the acid alone. For 

 example, acetic acid, a weak acid, is able to affect the cells at a 

 H ion concentration less than that necessary for strong acids like 

 HCl, HNO3, etc. This is probably due to the greater penetrating 

 power of the weaker acid in an undissociated state. It then 

 dissociates in the cell, liberating H ions to act on the end-organs. 

 In the following table, XXXVI., is given a list of some organic 

 acids with their minimum concentration just to be appreciated as 

 sour, i.e. threshold value (from Taylor), and with the concentration 

 gradient necessary to produce a pYi of 5-6 in Ckromodoris tissue 

 in 20 minutes (from Crozier, 1916). 



TABLE XXXVI. 



It will be seen from this table (1) that weak acids, like acetic, 

 butyric and succinic, are effective at a remarkably low H^ con- 

 centration, both in causing taste and in penetrating tissue ; and 

 (2) that the introduction of a hydroxy group generally (not always) 

 decreases the power of stimulation and penetration, e.g. lactic 

 acid (hydroxy-propionic) is about half as active as propionic acid, 

 and salicylic about one-third as penetrating as benzoic acid. 

 Carbonic acid, which may be considered as hydroxy-formic acid, 

 apparently is an exception to this generalisation, as it is about 

 fifty times as active as the fatty acid. This is probably due to its 

 passage into the tissues not as HgCO.,, but as COo? ^vhich is soluble 



both in water and in fats. 



ic— 2 



