SENSATION OF SMELL. 915 



inspiration. In the act of inhalation, the air passes along the septum, 

 upward beneath the bridge of the nose, and under the roof of the nasal 

 cavity, and it then curves backward and downward. But little air passes 

 through the meatuses, especially through the superior; most passes 

 through the middle meatus. Odorous substances received through 

 the mouth and then expired through the choanae may also be smelled, 

 although not so well. 



The first moment of contact of the odorous substance with the 

 olfactory cells seems to be the most effectual for the sensation ; conse- 

 quently it is customary to repeat these inspiratory acts with closed 

 mouth when it is desired to smell accurately: sniffing. By this means 

 the air in the accessory cavities is rarefied, and as the air-pressure 

 gradually becomes equalized, the odorous fumes are capable of diffusing 

 over the entire region. Nothing is practically known as to the nature of 

 the action of odorous substances, but many odorous vapors have a 

 decided power of absorbing heat. 



There is as yet no criterion for a special classification of odorous 

 substances. The observation that certain categories of olfactory sensa- 

 tions can be abolished, while others remain intact, would seem to indi- 

 cate that there are qualitatively different forms of olfactory nerves or 

 end-organs. 



The strength of the sensation depends: (i) Upon the extent of 

 the surface affected ; hence animals with great acuteness of smell (for ex- 

 ample the seal) are found often to have exceedingly complex tur- 

 binates, which are covered with the olfactory membrane. (2) Upon 

 the frequency with which the fumes are conducted to the olfactory cells 

 (sniffing). (3) Upon the concentration of the odorous air-mixture; 

 many substances, however, can be detected even in remarkable dilution. 

 (4) There are many connections between smell and taste; chloroform 

 has an ethereal odor and a sweet taste at the same time. Moreover, 

 it excites the pain-producing and cold-perceiving nerves. Ether has a 

 similar action, but it has a bitter taste. 



Bromin may be detected by its odor in a dilution of jo^-s', hydrogen sulphid 

 in a dilution of 15^^77 mgm. when contained in i cu. cm. of air. The odor of 

 T6<r<T<n><T mgm. of chlorphenol, and of <j<j^ oo<j mgm. of mercaptan can be 

 detected. 



Odorous substances dissolved in indifferent solutions (for example 0.73 per 

 cent, sodium chlorid solution) and introduced into the nose excite a feeble smell. 

 The olfactory nerve is exhausted by olfactory sensations that persist for more 

 than a few minutes; the exhausted nerve may recover, however in the course of 

 a minute. The sensation is impaired by fever, and also by cocain. Mechanical 

 and thermal stimuli do not excite olfactory sensations. 



Variations of the olfactory sensation are described on p. 679. If both nos- 

 trils are filled with substances of different odors, some individuals do not ^appreciate 

 a mixture of the odors, but at times one and at other times the other prevails ; 

 in some, however, there is a mixture of odors. Many odors cause others to dis- 

 appear, when they act upon the nose at the same time, for example bitter almonds 

 and musk, caoutchouc and wax. Under such circumstances both odors may be 

 taken either into both nostrils, or into one and the same nostril. 



The extremely sensitive sensory nerves of the nasal cavity are painfully 

 irritated by some pungent fumes, for example of ammonia and of acetic acid; the 

 latter act upon the olfactory nerves even in great dilution. The nose is important 

 as a sentinel to guard against the introduction of bad air an4 food. The sense of 

 smell frequently assists the sensations of taste, and cpnversely. Earlier ar.d 

 recent investigators speak of a connection between the nose and sexual activity. 



To test the olfactory acuity Zwaardemaker makes use of the olfactometer, 

 that is a hollow cylinder of an odorous substance (for example vulcanized caout- 



