CH. LV.] THE NERVE OF SMELL 783 



Tastes may be classified into 



1. Sweet. 2. Bitter. 



3. Acid or Sour. 4. Salt. 



Whether alkaline and metallic tastes are elementary, is as yet 

 undecided. All the above affect to a varying extent the nerves of 

 tactile sense as well of those of touch proper, sweet having the least, 

 acids the most marked action upon the latter. Sweet tastes are best 

 appreciated by the tip, acid at the side, and bitter tastes at the 

 back of the tongue. 



The substance to be tasted must be dissolved; here there is a 

 striking contrast to the sense of smell ; flavours are really odours. 

 In testing the sense of taste in a patient, the tongue should be 

 protruded, and drops of the substance to be tasted applied with 

 a camel's hair brush to the different parts; the subject of the 

 experiment must signify his sensations by signs, for if he with- 

 draws the tongue to speak, the material gets widely spread. The 

 more concentrated the solution, and the larger the surface acted on, 

 the more intense is the taste; some tastes are perceived more 

 rapidly than others, saline tastes the most rapidly of all The best 

 temperature of the substance to be tasted is from 10 to 35 C. 

 Very high or very low temperatures deaden the sense. 



Individual papillae, when thus treated with various solutions, show 

 great diversity: from some only one or two tastes can be evoked, 

 from others all four. The papillae may also be stimulated electrically. 



Cocaine and gymnemic acid, prepared from the leaves of the 

 plant Gymnema sylvestre, act deleteriously, chiefly on the bitter 

 and sweet tastes ; cocaine abolishes especially the bitter, gymnemic 

 acid especially the sweet, leaving the salt and acid tastes almost 

 untouched. 



It will thus be seen that there are many facts pointing to the 

 conclusion, that the varieties of gustatory like "those of cutaneous 

 sensation are due to the stimulation of different end-organs. 



When diluted sweet and salt solutions are simultaneously applied 

 to the tongue, they tend to neutralise one another, but a true indifferent 

 point is difficult or impossible to reach. Sweet and bitter, sweet 

 and acid liquids are antagonistic to a similar but less perfect 

 extent. Contrast-effects of one taste upon another are matters of 

 common observation, but can only be experimentally investigated 

 with difficulty. 



Smell. 



The entrance to the nasal cavity is lined with a mucous membrane 

 closely resembling the skin. The greater part of the rest of the 

 cavity is lined with ciliated epitheMum; the corium is thick and 



