70 SMELL, TASTE, ALLIED SENSES 



distinguishable from them. Thus the guinea pig, like 

 man, though sensitive to disagreeable odors in the be- 

 ginning, appears to become in the course of time entirely 

 inert to this form of stimulation. 



To test the immediate effects of the continuous action 

 of odorous substance on the olfactory organ of man, 

 Aronsohn (1884a) determined the length of time certain 

 olfactory stimuli at full strength continued to call forth 

 sensation. Thus oil of lemon and oil of orange were 

 smelled by nine persons till the odors of these substances 

 could no longer be perceived The period necessary to 

 bring about this obliteration of sensation varied from 

 2.5 minutes to 11 minutes with an average of 3 minutes. 

 A 0.2 per cent solution of cumarin in water was smelled 

 for from 1.75 to 2.3 minutes after which it was no longer 

 sensed. Thus olfactory exhaustion under strong stimu- 

 lation is accomplished in a very few minutes. The re- 

 covery of excitability is apparently equally rapid and 

 may be accomplished in as short a time as from 1 to 3 

 minutes though complete recovery probably requires a 

 longer time. 



Zwaardemaker (1895) tested fatigue in another way 

 and determined by means of his olfactometer the in- 

 crease in minimum stimulation as the olfactory organ 

 gradually approximated exhaustion. During a continu- 

 ous stimulation of known intensity the minimum stimu- 

 lus was from time to time determined and was found to 

 increase steadily. Two substances, benzoin and rubber, 

 at two different strengths were tested (Fig. 16). Ben- 

 zoin induced fatigue more rapidly than rubber and of 

 the two concentrations employed for each substance the 



