746 PHYSIOLOGY. 



volatile, odorous particles coining into direct contact with the exposed 

 nerve-filaments during the act of inspiration. As the regio olfactoria 

 is in the highest position of the nasal cavity, it becomes necessary for 

 the individual to cause the inspired air forcibly to reach this area. 

 This is accomplished by an act ordinarily known as "sniffing." 



During ordinary respiration the inspired and expired air courses 

 along close to the septum and below the inferior turbinated bone. 

 Should the respired air be heavily charged with odorous particles, of 

 course some will find their way into the regio olfactoria, as the air 

 in this compartment is gradually changed. There will then result 

 a sensation of smell, but it will be faint and not so sharply defined 

 as when the person sniffs. By the latter process the air is changed 

 more quickly and a greater number of volatile particles irritate the 

 exposed nerve-endings, with the result of a sharply defined sensation. 

 The sensation seems to occur at the first moment of contact of the 

 odorous particles with the mucous membrane. The olfactory nerve 

 tires very quickly when an odor acts for a certain time; the effect 

 becomes weaker and weaker little by little, until the odor is finally 

 unperceived. 



Should the free movement of the air be prevented as, for 

 example, when nasal catarrh brings on a tumefaction of the mucous 

 membrane of the inferior turbinate, the odorous impression cannot 

 take place. 



In case many different odors act simultaneously upon one nasal 

 cavity, the individual receives a mixed sensation. Should but two 

 odors act, the one is perceived on the right half of the mucous mem- 

 brane of the cavity, the other upon the left half. This is not a 

 true mixture, for the person perceives slightly the one odor and 

 slightly the other. One part of vanillin to 10,000,000 can be recog- 

 nized by the sense of smell. 



SECONDAKY SENSATION. The olfactory impression having been 

 made, the secondary after-sensation often remains for a long time. 

 This is particularly the case with strong, disagreeable odors. This 

 phenomenon is explained on the supposition that the odorous parti- 

 cles remain in the cavity of the nose, even in the air. It is not 

 believed that the manifestation is due to persistence of excitation of 

 the olfactory nerve-fibers after the stimulus has been removed. 



There are subjective olfactory sensations which are true hallucina- 

 tions. They are often met with in demented, in hysterical, or in 

 pregnant women. These sensations owe their existence to some 

 material alteration of the nervous apparatus. 



