OF SMELLING. 423 



proved by the simple experiment of dipping the finger into very warm 

 water, and then the whole hand into water many degrees cooler. The 

 increased extent of surface seems to overcompensate for the Dece tions 

 lower temperature, and we come to the erroneous conclusion the sense of 

 that the cooler specimen is the warmer of the two samples. 



As sounds may be heard which have no reality, but merely originate 

 in the brain, or spectral illusions may be seen, so the sense Subjective sen- 

 of touch is subject to similar hallucinations, as a sensation 

 of pressure or weight, or the crawling of insects on the skin ; ture. 

 and though we can not, by artificial irritation of the nerves, give rise to 

 impressions of heat and cold, those effects very frequently occur in this 

 interior or subjective way. 



CHAPTEE XXIII. 



OF SMELLING, AND THE MEANS OF DISTINGUISHING GASEOUS AND VA- 

 POEOUS SUBSTANCES. 



Structure of the Organ of Smell. Its proper Instrument the First Pair of Nerves. Limited Re- 

 gion of Smell. Conditions of its perfect Action. Duration of Odors. TJieir Localization. 

 Subjective Odors. 



BY the sense of smell we are able to distinguish many gaseous and 

 vaporous substances from one another. They enter the nos- gensc of gmell 

 trils with the respiratory current, and are brought in con- for gases and 

 tact with the olfactory or Schneiderian membrane. Though va P rs - 

 received at first in the elastic state, they become dissolved in the mucus 

 which moistens that membrane. It does not follow, however, that all 

 vaporous substances give rise to the perception of an odor ; for example, 

 water itself communicates no sensation whatever. Again, there are other 

 bodies, as, for instance, musk, which yield an odor far more Deiicacj-oftMs 

 powerful than corresponds to their loss of weight. Thus it perception, 

 is said that.that substance may be exposed for years in an apartment, dif- 

 fusing all the time its penetrating emanations, and yet not becoming 

 lighter. Such statements are, however, on their face, exaggerations. 

 There can be no doubt that the olfactory organs detect extremely minute 

 portions of matter. In most cases, elevation of the temperature of a 

 body increases its odorous effect. 



The primary uses of the function of smell are for a discrimination of 

 the qualities of food, or its condition, and also for enabling 



, .,, p !. -i , ic -,T T Uses of smell. 



an animal with greater lacility to provide it sen with supplies. 

 Hence the development of this structure takes place in the utmost per- 

 fection among the carnivora, which often depend almost exclusively upon 



