TASTE AND SMELL 595 



substance e.g., solid quinine applied to the dry tongue evokes no 

 sensation of taste. Again, a crystal of quartz gives the sensation of 

 touch and cold, but not of taste. A crystal of common salt of the 

 same size gives not only these sensations, but that of saline taste. 

 There is no evidence to show that the sensations of taste can be elicited 

 by mechanical stimulation of the tongue. It is probable that the 

 sensations evoked by electrical stimulation are due to the electrolytic 

 decomposition of the saliva. The various taste sensations are grouped 

 in different areas of the tongue. 



The responsiveness to bitter substances is confined mainly to the 

 back and edges of the tongue. Acid is tasted all over the dorsum, 

 except a small anterior part just behind the tip. Salt and sweet are 

 likewise appreciated all over the dorsum, except for an antero-median 

 area of the dorsum, a little posterior to the insensitive area for acid. 

 ''' Sweet " is, however, in most people best appreciated at the tip of 

 the tongue. 



It has been argued that this distribution points to special receptors 

 for each form of taste, and in support of this it is stated that saccharine 

 is sweet to the tip and bitter to the back of the tongue. In like 

 manner, sulphate of magnesia is bitter at the base and sweet or acid 

 in the region of the tip. Further, the drug Gymnema sylvestre affects 

 the sensation of sweetness. After chewing the leaves of this plant, 

 sugar no longer tastes sweet nor quinine bitter. The sensations of 

 acid and salt are unaffected. 



There is apparently no relation between the chemical structure 

 and taste of substances. For example, sugar, lead acetate, and 

 chloroform, all taste sweet; magnesium sulphate, and various alka- 

 loids, like quinine and strychnine, evoke bitter sensations. 



The application of sugar and salt at the same time to opposite sides 

 of the tongue increases the effectiveness of both stimuli (cf. simul- 

 taneous contrast in vision, p. 634). Again, after tasting weak sulphuric 

 acid, distilled water has a sweetish taste (cf. successive contrast, p. 634). 

 It is to be noted that flavours are either smells or combinations of 

 taste and smell. Thus, a man with cold in the head cannot appreciate 

 the fine flavour of wine. If the nose be held, an onion, when bitten, 

 is indistinguishable from an apple. The smell of a strong cheese 

 will destroy the appreciation of the taste of fruit. 



The nerve distribution for taste is associated in the posterior 

 third with the glosso-pharyngeal nerve, and in the anterior two-thirds 

 with the lingual branch of the fifth. The course of the taste fibres is 

 complicated and still in doubt. The fibres from the anterior two- 

 thirds leave the lingual nerve and pass backward along the chorda 

 tympani nerve to join the seventh nerve in the Fallopian aqueduct. 

 They pass in the seventh nerve as far as the geniculate ganglion, 

 where some join the nerve of Wrisberg and pass to the nucleus of the 

 ninth nerve ; others are said to pass along the great superficial petrosal 

 nerve to Meckel's ganglion and to rejoin the fifth nerve. The fibres- 

 from the posterior third of the tongue pass to the brain by way of 

 the glosso-pharyngeal nerve. 



