SENSATIONS OF SMELL AND TASTE 557 



sensations can affect one another if excited simultaneously. With 

 weak stimulation one taste may practically annul another. Thus a 

 dilute solution of sugar is rendered almost tasteless by the addition 

 to it of a few grains of common salt. If the primitive taste sensations 

 are more strongly excited we get a mixed sensation, in which the 

 components can still be distinguished. Thus, adding sugar to lemon 

 juice not only -diminishes its acidity but produces a mixed sensation, 

 the quality of which is pleasant and in which the components, sour and 

 sweet, can be easily distinguished. We get no such fusing of sensa- 

 tions as in the eye, where a sensation of white light may result from 

 stimulation of the retina by two complementary colours. Stimula- 

 tion of one kind of taste-organ heightens the sensibility of the other 

 taste-organs. Thus after the application of salt, distilled water may 

 taste sweet. 



That these primitive taste sensations are served by different 

 nerve-endings is shown by the following facts : 



(a) The tongue is not equally sensitive at all points to all four 

 tastes. Thus the back of the tongue is more sensitive to bitter, while 

 the tip and sides of the tongue react more easily to sweet and sour 

 substances. A difference may be detected between even the circum- 

 vallate papillae themselves ; a mixture of quinine and sugar applied 

 to one papilla may excite chiefly a bitter taste, while with an adjacent 

 papilla a sweet taste may predominate. 



(6) By certain drugs we can depress the sensibility of the taste- 

 organs, and we then find that the various tastes are affected to different 

 degrees. Thus on painting the tongue with cocaine the first effect 

 is a diminution of tactile and pain sensibility, so that the application 

 of acid evokes a very sour taste without any of the astringent or 

 stinging sensations normally aroused by the contact with the acid. 

 After this point the taste sensations are also abolished. The bitter 

 sensation disappears first, then the sweet, and then the sour, while the 

 taste of salt appears to remain unaffected. On the other hand, if the 

 leaves of Gymnema sylvestre be chewed, the sensations of bitter and 

 sweet are abolished, leaving intact the acid and salt tastes, and also the 

 general sensibility of the mucous membrane. 



There is no doubt that the stimulating effect of any chemical 

 substance on the taste-nerves has relation to its chemical constitution. 

 Thus a sour taste is determined by the presence of H ions ; the 

 alkaline taste by that of OH ions. The fact that certain acids, e.g. 

 acetic, have a stronger sour taste than would correspond to their 

 dissociation, i.e. to the number of H ions present, is due to the fact 

 that these acids penetrate more easily into the gustatory cells than the 

 mineral acids with a larger dissociation coefficient. All the a-amino- 

 acids have a sweet taste. On the other hand, the polypeptides 



