172 SMELL, TASTE, ALLIED SENSES 



vomero-nasal organs with high efficiency and the common 

 chemical receptors and organs of taste with relatively 

 low efficiency. These two sets of organs might in this 

 respect be compared with scales, the organs of taste and 

 of the common chemical sense resembling ordinary scales 

 on which only gross amounts are weighed and the organs 

 of smell and the vomero-nasal organs resembling chemi- 

 cal balances on which small weights may be determined. 



As olfaction deals effectively with very minute 

 amounts of substance and gustation only with much 

 greater amounts, it follows that materials that have be- 

 come highly attenuated by being broadly spread from 

 their sources either in water or in air may nevertheless 

 still be concentrated enough to stimulate the organs of 

 smell though they can have no possible effect upon those 

 of taste. Such faint odors are the means whereby ani- 

 mals scent their food, find their mates, or avoid their 

 enemies. Hence the olfactory organ has been appropri- 

 ately classed as a distance receptor or exteroceptor, to 

 use a convenient term from Sherrington (1906), in that 

 the impulses to which it gives rise commonly direct the 

 animal toward distant points or away from them. 



Taste and, in the higher vertebrates at least, the com- 

 mon chemical sense are stimulated only by relatively con- 

 centrated solutions such as occur in connection with the 

 food. Hence the responses that these organs call forth 

 are concerned with the swallowing of food, with the re- 

 jection of material taken into the mouth, with mastication 

 and saliva and the like. These receptors are, therefore, 

 rightly classed as interoceptors though it must be re- 

 membered, as Herrick (1918) has pointed out, that in 

 some fishes, such as the catfishes, taste-buds serve in the 



