168 PHYSIOLOGY CHAP. 



agree with the above. He caused a fine magnesia powder to be 

 aspirated, and then, with the rhinoscope, investigated the parts of 

 the mucous membrane to which it specially adhered. 



Since then it is proved that the inspiratory current as such 

 does not reach the olfactory region, it is clear that the odorous 

 molecules cannot be carried thither by the stream of air, but must 

 reach it by some other means. We know from Bloch's work 

 that the temperature inside the nose is above 30, so that 

 Zwaardemaker's hypothesis that odours penetrate to the sensory 

 end-organs of smell by a process of gaseous diffusion remains the 

 most probable. The act of sniffing which draws the air-current 

 higher into the nasal cavities must undoubtedly facilitate the 

 diffusion and penetration of odours into the olfactory region. 



Bidder assumed that smells can only be perceived during 

 inspiration, and denied that they can be carried to the olfactory 

 region during expiration as well. But Paulsen's experiments 

 showed that the expiratory current takes the same curved path 

 as the inspiratory, only running somewhat lower, which may 

 impede, but cannot hinder, the diffusion of odours in the olfactory 

 region. On the other hand, it is easy to show that odorous 

 substances breathed in through the mouth and breathed out 

 through the nose may give rise to distinct olfactory sensations. 

 That these are weaker than those excited by inspiration through 

 the nose is sufficiently explained by the fact that the odorous 

 substances inhaled through the mouth must pass through all the 

 air-passages, where they may be partially absorbed before being 

 brought into contact by the expired air with the olfactory region. 

 Again, during mastication of solid food and particularly during 

 the deglutition of alimentary boluses and of fluid, the vapours and 

 odoriferous particles exhaled by the foods and beverages may on 

 passing through the choanae above the soft palate reach and 

 excite the olfactory surface during expiration. This fact is 

 important, because it establishes the intimate relations and 

 associations between the senses of taste and smell which we dis- 

 cussed in the last chapter. The mechanism by which the olfactory 

 sense is excited during a meal depends principally on the fact 

 that at each act of swallowing the soft palate is suddenly raised, 

 on which the air saturated with odorous exhalations is driven from 

 the choanae towards the olfactory region at a pressure, according 

 to Tick, of about 30 cm. water. When deglutition is completed, 

 there is a deeper expiration than usual, and the air of the 

 pharynx charged with the odours exhaled by the food is driven 

 through the choanae. The olfactory sensations thus aroused can, 

 as Chevreul showed, be easily eliminated if the nostrils are kept 

 closed with the finger. 



Nagel rightly pointed out that the appreciation of smell 

 through the choanae is of higher biological importance, particularly 



