556 HANDBOOK OF PHYSIOLOGY. 



should have long ceased to act. Both painful and pleasurable sensations 

 afford many examples of this fact. 



Subjective sensations, or sensations dependent on internal causes, are 

 in no sense more frequent than in the sense of touch. All the sensations 

 of pleasure and pain, of heat and cold, of lightness and weight, of 

 fatigue, etc., may be produced by internal causes. Neuralgic pain, the 

 sensation of rigor, formication or the creeping of ants, and the states of 

 the sexual organs occurring during sleep, afford striking examples of 

 subjective sensations. The mind has a remarkable power of exciting 

 sensations in the nerves of common sensibility; just as the thought of 

 the nauseous excites sometimes the sensation of nausea, so the idea of 

 pain gives rise to the actual sensation of pain in a part predisposed to it; 

 numerous examples of this influence might be quoted. 



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. The nerves concerned in 

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

 537 and 540). 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 substances, an infinite va- 

 riety of changes of condition of the nerves, and consequently of stimula- 

 tions 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 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 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 electricity 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 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. 



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

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

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



