PHYSIOLOGY OF OLFACTION 49 



of digestive reflexes, muscular, secretory, and so forth, 

 has long been recognized. 



3. Minimum Stimulus. The common belief that the 

 olfactory stimulus consists of minute material particles 

 suspended in the air current of the olfactory organ is 

 supported by the observation that odors may be carried 

 on the wind in a definite direction many miles. Odors 

 do not emanate from a given center and disperse in all 

 directions as sound and light do. Moreover many sub- 

 stances, such as arsenic, that are odorless under ordinary 

 circumstances, give out an odor after they have been 

 heated sufficiently to volatilize. The fact, discovered in 

 1917 by Woodrow and Karpman, that the adaptation time 

 for olfaction the time needed for an olfactory sensation 

 to wane completely is directly proportional to the vapor 

 tension of the odorous material shows that olfactory 

 stimulation is due to the activity of gaseous particles. 

 These and other like observations have led to the conclu- 

 sion, now generally accepted, that the olfactory organs 

 are normally stimulated by material particles, and not 

 by disturbances of a non-material character. 



Some odorous bodies such as musk are well known 

 to give out these material particles for a very considerable 

 time without appreciably changing weight. From the 

 standpoint of the receptor this indicates that olfaction 

 is called forth by an infinite simally small amount of sub- 

 stance, and measurements directed toward testing this 

 question justify the conclusion. These measurements 

 have been made in a variety of ways. 



One method of procedure is that of evaporating a 

 given weight of odorous material in a known volume of air 

 and then testing the air by sniffing it. This method lends 



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