SENSES OF SMELL, TASTE, HUNGER AND THIRST 743 



effected by pressure or by the electrical current, a fact which is fre- 

 quently cited in proof of the law of the specificity of nerve energy. 

 Under ordinary conditions, these receptors are exposed to a constant 

 temperature by reason of the steady stream of heat which escapes from 

 the blood and permeates the tissues. The escape of this heat may be 

 retarded by warm and increased by cold applications. Consequently, 

 these thermal stimulations can only arise if the heat stagnation or 

 dissipation surpasses the physiological minimum. This fact explains 

 the sensation of cold, experienced whenever the circulation of a part is 

 impeded or when especially good conductors of heat are applied to the 

 body-surface. Since a more rapid fall in temperature is effected by 

 the latter procedure, the nerve-terminals for cold are more vuickly 

 reduced below the point of minimal thermal stimulation. In other 

 words, the chief factor in the production of the sensations of heat and 

 cold is the temperature of the nerve-terminals mediating these senses.^ 

 Rather difficult to explain are the so-called paradoxical temperature 

 reactions. Menthgl applied to the skin, gives rise to a sensation of 

 cold, while carbon dioxid elicits a sensation of warmth. A sensation 

 of cold may also be evoked by stimulating a cold spot with an object 

 possessing a temperature of 45-50° C. Very peculiar sensations of 

 contrast arise in consequence of the adaptation of these sense-organs 

 to certain temperatures. If the index finger of one hand is placed in 

 water of 10° C, the primary sensation of cold eventually gives way to 

 one of indifference. If this finger is then quickly transferred into 

 water of 11° C, a distinct sensation of warmth will be obtained. Quite 

 similarly, a sensation of cold may be evoked by transferring the finger 

 from water of 39° C. into water of 38° C. Furthermore, having adapted 

 the fingers of one hand to water of 35° C, and those of the other to water 

 of 25° C, their simultaneous transfer into water of 30° C. produces a 

 sensation of cold in the former and a sensation of warmth in the latter. 

 If a warm coin is applied to the skin for some time, its removal gives 

 rise to a sensation of cold, and even when the temperature of the sur- 

 rounding medium is only very slightly below that of the skin. 



CHAPTER LXI 



THE SENSES OF SMELL, TASTE, HUNGER AND THIRST 



A. SPECIAL INTEROCEPTORS. SMELL AND TASTE 



The Structure of the Olfactory Organ. — The mucous membrane 

 of the nose consists of ciliated reticular cells which are augmented, in 

 the so-called olfactory area, by cells possessing all the characteristics of 



1 Hering, Sitzungsber., Akad. zu Wien, Ixxv, 1877, 101. 



