280 THE SPECIAL SENSES. 



Threshold Stimulus Delicacy of the Olfactory Sense. 



The extraordinary delicacy of the sense of smell in some of the lower 

 animals is seemingly beyond the power of objective measurement or 

 expression. The ability of a dog, for instance, to follow the trail of 

 a given person depends undoubtedly upon the recognition of the 

 individual odor, and the actual amount of olfactory material left 

 upon the ground which serves as the stimulus must be infinitesi- 

 mally small. Even in ourselves the actual amount of olfactory 

 material which suffices to give a distinct sensation is often beyond 

 our means of determination except by the aid of calculation. It 

 is recognized in chemical work, for instance, that traces of known 

 substances too small to give the ordinary chemical reactions may be 

 detected easily by the sense of smell. By taking known amounts 



Fig. 116. Zwaardemaker's olfactometer. 



of odoriferous substances and diluting them to known extents it is 

 possible to express in weights the minimal amount of each substance 

 that can cause a sensation. By this method such figures as the 

 following are obtained : Camphor is perceived in a dilution of 1 part 

 to 400,000 ; musk, 1 part to 8,000,000 ; vanillin , 1 part to 10,000,000 ; 

 while, according to the experiments of Fischer and Penzoldt, 

 mercaptan may be detected in a dilution of -^-oirVinnr f a milli " 

 gram in 1 liter of air or TSTTrlnrinnr f a milligram in 50 c.c. of air. 

 Various methods have been proposed to determine the relative 

 delicacy of the olfactory sense in different persons, and these methods 

 have some application in the clinical diagnosis of certain cases. 

 Zwaardemaker has devised a simple apparatus, the olfactometer, 

 the principle of which is illustrated in Fig. 116. It consists of an 

 outside cylinder the olfactory cylinder, whose inner surface is of 



