THE SENSE OF TASTE. 547 



sounds cause in many people a disagreeable feeling in the 

 teeth, or a sensation of cold trickling through the body, and, 

 in some people, intense sounds are said to make the saliva col- 

 lect. 



The sense of hearing may in its turn be affected by impres- 

 sions on many other parts of the body ; especially in diseases 

 of the abdominal viscera, and in febrile affections. Here, 

 also, it is probable that the central organs of the nervous 

 system are the media through which the impression is trans- 

 mitted. 



SENSE OF TASTE. 



The conditions for the perception of taste are : 1, the pres- 

 ence of a nerve with special endowments ; 2, the excitation of 

 the nerves by the sapid matters, which for this purpose must 

 be in a state of solution. The nerves concerned in the produc- 

 tion of the sense of taste have been already considered (pp. 

 431 and 437). 



The mode of action of the substances which excite taste 

 probably consists in the production of a change in the internal 

 condition of the gustatory nerves ; and, according to the dif- 

 ference of the substances, an infinite variety of changes of con- 

 dition, and consequently of tastes, may be induced. It is not, 

 however, necessary for the manifestation of taste that sapid 

 substances in solution should be brought into contact with its 

 nerves. For the nerves of taste, like the nerves of other 

 special senses, may have their peculiar properties excited by 

 various other kinds of irritation, such as electricity and me- 

 chanical impressions. Thus Henle observed that a small cur- 

 rent of air directed upon the tongue gives rise to a cool saline 

 taste, like that of saltpetre ; and Dr. Baly has shown that a 

 distinct sensation of taste, similar to that caused by electricity, 

 may be produced by a smart tap applied to the papillae of the 

 tongue. Moreover, the mechanical irritation of the fauces and 

 palate produces the sensation of nausea, which is probably 

 only a modification of taste. 



The matters to be tasted must either be in solution or be 

 soluble in the moisture covering the tongue ; hence insoluble 

 substances are usually tasteless, and produce merely sensations 

 of touch. Moreover, for the perfect action of a sapid, as of an 

 odorous substance, it is necessary that the sentient surface 

 should be moist. Hence, when the tongne and fauces are 

 dry, sapid substances, even in solution, are with difficulty 

 tasted. 



The principal, but not exclusive seat of the sense of taste is 

 the fauces and tongue. 



