SENSES. 607 



II. Taste. 



Conditions necessary. The conditions for the perceptions of taste 

 are: 1, the presence of a nerve and nerve-centre with special endow- 

 ments; 2, the excitation of the nerve by the sapid matters, which for 

 this purpose must be in a state of solution; 3, a temperature of about 37 

 to 40 C. (98 to 100 F.). The nerves concerned in the production of 

 the sense of taste have been already considered (p. 349 et seq.) The mode 

 of action of the substances which excite taste consists in the production 

 of a change in the condition of the gustatory nerves, and the conduction 

 of the stimulus thus produced to the nerve-centre; and, according to 

 the difference of the susbtances, an infinite variety of changes of condi- 

 tion of the nerves, and consequently of stimulations of the gustatory 

 centre, may be induced. The matters to be tasted must either be in 

 solution or be soluble in the moisture covering the tongue; hence insolu- 

 ble substances are usually tasteless, and produce merely sensations of 

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

 stance, it is necessary that the sentient surface should be moist. Hence, 

 when the tongue and fauces are dry, sapid substances, even in solution, 

 are with difficulty tasted. 



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 elec- 

 tricity and mechanical impressions. Thus, a small current of air directed 

 upon the tongue gives rise to a cool saline taste, like that of saltpetre ; and a 

 distinct sensation of taste similar to that caused by electricity, may be pro- 

 duced 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. 



Seat. The principal seat (apparent seat, that is, to our senses) of 

 the sense of taste is the tongue. But the result of experiments as well 

 as ordinary experience show that the soft palate and its arches, the uvula, 

 tonsils, and probably the upper part of the pharynx, are also endowed 

 with taste. These parts, together with the base and posterior parts of 

 the tongue, are supplied with branches of the glosso-pharyngeal nerve, 

 and evidence has been already adduced that the sense of taste is conferred 

 upon them by this nerve. In most, though not in all persons, the an- 

 terior parts of the tongue, especially the edges and tip, are endowed 

 with the sense of taste. The middle of the dorsum is only feebly en- 

 dowed with this sense, probably because of the density and thickness of 

 the epithelium covering the filiform papillae of this part of the tongue, 

 which will prevent the sapid substances from penetrating to their sensi- 

 tive parts. 



Other Functions. Beside the sense of taste, the tongue, by means 



