168 HAXD-BOOK OF PHYSIOLOGY. 



indeed able to distinguish in this manner between temperatures, experi- 

 enced one after the other, better than between temperatures to which the 

 two hands were simultaneously subjected. This power of comparing 

 present with past sensations diminishes, however, in proportion to the 

 time which has elapsed between them. After-sensations left by impres- 

 sions on nerves of common sensibility or touch are very vivid and durable. 

 As long as the condition into which the stimulus has thrown the organ 

 endures, the sensation also remains, though the exciting cause 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 sensa- 

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

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

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

 the sexual organs occurring during sleep, afford striking examples of sub- 

 jective sensations. The mind has a remarkable power of exciting sensa- 

 tions in the nerves of common sensibility; just as the thought of the nau- 

 seous 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. 



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 pro- 

 duction of the sense of taste have been already considered (pp. 142 and 146, 

 Vol. II.). The mode of action of the substances which excite taste con- 

 sists 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 variety of 

 changes of condition 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 insoluble substances are usually tasteless, and produce merely sen- 

 sations 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 



