752 SPECIAL SOMATIC AND VISCERAL RECEPTORS 



It is also to be noted that the tongue is not equally sensitive to all 

 four primary tastes. By far the greatest acuity for sweet prevails 

 upon the tip of tongue, while bitter is most clearly perceived upon its 

 posterior aspect in the vicinity of the circumvallate papillse. The 

 acuity for sweet decreases gradually from before backward and that 

 for bitter in the opposite direction. The sour taste is most highly 

 developed in the central fields of the marginal regions of the tongue, 

 and the salty taste in its antero-lateral regions. Peculiarly enough, 

 these different sensibilities may be varied by means of certain drugs. 

 Thus, we find that the application of a solution of cocain to the sur- 

 face of the tongue first of all diminishes our acuity for the compound 

 impressions, so that acids produce merely a sour taste without any 

 astringent or burning admixture. 1 Next in order follow the fundamen- 

 tal sensations, namely, bitter, sweet, sour and salty. A very similar 

 effect may be produced by chewing the leaves of gymnemna sylvestre. 

 In this case, the sensations of sweet and bitter are destroyed, while the 

 acid and salty tastes, as well as the general sensibility, are not impaired. 2 



These facts recall to our minds the interesting question regarding 

 the specificity of the taste buds, it being entirely probable that the 

 four fundamental qualities of taste are mediated by four different 

 types of end-organs. Thus, Ohrwall has shown that certain papillse 

 react only to particular kinds of sapid substances. Of the total num- 

 ber of 125 examined, 98 could be activated by different substances. 

 Of this number, 60 yielded three modalities of taste sensations, while 

 12 gave sweet and acid, 12 only acid, 7 bitter and acid, 4 sweet and 

 bitter, and 3 only sweet. In addition, it has been pointed out that 

 parabrombenzoic sulphinid gives rise to a sensation of sweet when 

 placed upon the tip of the tongue, and to a sensation of bitter when 

 applied to its posterior surface. Quite similarly, sodium sulphate 

 tastes salty upon the tip of the tongue and bitter upon its posterior 

 region. 



B. GENERAL INTEROCEPTORS, APPETITE, HUNGER AND THIRST 



Appetite. It has been mentioned above that in the lower forms 

 the sensations of smell and taste occupy the position of exteroceptors, 

 while in the higher animals, they assume more especially the function 

 of interoceptors. In addition, the latter group also embraces a large 

 number of peculiar internal sensations, chief among which are the 

 sensations of appetite, hunger and thirst. Regarding the first, it 

 has commonly been held that it is merely a mild form of hunger and is 

 not mediated by separate receptors. Cannon and Washburn, 3 

 on the other hand, seem to differentiate sharply between these sensa- 



1 v. Amrep, Pfltiger's Archiv, xxi, 1880, and Knapp, Archiv fur Augenheilk., 

 1885. 



2 Edgeworth and Hooper, Nature, xxxv, 1887, 565. 



3 Amer. Jour, of Physiol., xxix, 1912, 441. 



