PHYSIOLOGY OF OLFACTION 69 



possibility that odorous material may form with the ol- 

 factory mucous an emulsion rather than a true solution, 

 but this suggestion did not seem even to Henning to be 

 of much significance, for in other parts of his work he 

 refers repeatedly to the state of the stimulating material 

 as that of a solution and there appears to be no good 

 ground for assuming that such is not the case. 



6. Fatigue and Exhaustion. It is well known that 

 the olfactory organs in man are quickly and easily fa- 

 tigued by continuous exposure to odorous materials. 

 Persons whose occupations lead them to work among 

 disagreeable odors soon become insensitive to these and 

 it has long been recognized that invalids are not affected 

 by the malodors that may come from their own bodies. 

 Although these conditions of irresponsiveness may be 

 due in part to central nervous states such as lack of at- 

 tention and the like, they are also dependent in part 

 on peripheral exhaustion. The effects of unpleasant 

 smells on the growth of guinea pigs has been tested very 

 recently by Winslow and Greenberg (1918). These in- 

 vestigators employed a pair of air-proof cages through 

 which were passed 1.5 cubic feet of air per minute 

 amounting to 4 liters of air per minute for each animal 

 in the test. Through one of these cages pure air was 

 circulated; through the other, air that had passed over 

 fresh moist faeces and that in consequence was impreg- 

 nated with a strong faecal odor. A total of 15 sets of 

 growing guinea pigs, including 261 animals, were sub- 

 jected to these conditions. In the first week of the tests 

 the animals supplied with faecal air did not grow as much 

 as the controls did, but in the second week they caught 

 up in weight with the controls and were thereafter in- 



