?/,. 274 \~^J^ THE SPECIAL SENSES. 



ers, each giving, when stimulated, only its own 

 quality of sensation. On the other hand, it is possible that one and 

 the same nerve fiber might give different qualities of sensation 

 according to the nature and mode of action of the sapid substances. 

 The fact, as shown by Oehrwall's experiments, that there are sensory 

 spots upon the tongue which will not react to some kinds of sapid 

 substance, but do react to, others, and perhaps only to one particular 

 kind, speaks strongly in favor of the view that there are different 

 end-organs and nerve fibers for each fundamental taste. This view 

 is still further supported by the fact that certain chemically pure sub- 

 stance s give different tastes according to the part of the tongue 

 upon which they are placed. Thus, sodium sulphate (Guyot) may 

 taste salty upon the tip of the tongue and bitter when placed upon 

 the posterior part. A better instance still is given by solutions of 

 a bromin substitution product of saccharin, the chemical name 



{CO 

 oX ^/ NH. 

 S0 2 / 



When this substance is placed upon the tip of the tongue it gives a 

 sweet sensation, while upon the posterior region it gives only a bitter 

 taste together with a sensation of astringency (Howell and Kastle). 

 Extracts of the leaves of a tropical plant, Gymnema silvestre, applied 

 to the tongue, destroy the sense of taste for sweet and bitter sub- 

 stances (Shore), and this fact may be explained most satisfactorily 

 by assuming that this substance exercises a selective action upon 

 separate terminals in the tongue, paralyzing those for the bitter 

 and the sweet substances. Finally, the fact that electrical, me- 

 chanical, or chemical stimulation of the chorda tympani, where it 

 passes through the tympanic cavity, may arouse taste sensations is 

 proof that the taste sensation in general is not due to a peculiar kind 

 of impulse that can be aroused only by the action of sapid bodies 

 upon the terminals in the tongue, but, on the contrary, that it is a 

 specific energy of these fibers, and depends for its quality, there- 

 fore, upon the specific reaction of the terminations in the brain. 



Method of Sapid Stimulation. In order that sapid substances 

 may react upon the taste terminals it is necessary, in the first place, 

 that they shall be in solution. It is impossible to taste with a dry 

 tongue. We may assume, therefore, that the stimulation consists 

 essentially in a chemical reaction between the sapid substance and 

 the terminal of the taste fiber, for instance, the hair process of 

 the sense cells in the taste .buds, and the question naturally arises 

 whether the distinctive reactions corresponding to the separate 

 taste qualities can be referred to a definite chemical structure in the 

 sapid bodies. Are there certain chemical groups which possess the 

 property of reacting specifically with the end-organs? Experience 

 shows that substances of very different chemical constitution may 



