758 SPECIAL SENSES. 



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Mechanism of Olfaction. 



There can be no doubt at the present day with regard to the mechanism of the sense 

 of smell. Substances endowed with odorous properties give off material emanations, 

 which must come in contact with the olfactory membrane before their peculiar odor is 

 appreciated. As we have seen, this membrane is situated high up in the nostrils, is 

 peculiarly soft, is provided with numerous glands, by the secretions of which its surface 

 is kept in proper condition, and it possesses the peculiar nerve-terminations of the olfac- 

 tory filaments. 



In experimenting upon the sense of smell, it has been found quite difficult to draw the 

 exact line of distinction between impressions of general sensibility and those which attack 

 the special sense, or, in other words, between irritating and odorous emanations ; and 

 the vapors of ammonia, acetic acid, nitric acid, etc., undoubtedly possess irritating 

 properties which greatly overshadow their odorous qualities. It is unnecessary, in this 

 connection, to discuss the different varieties of odors recognized by some of the earlier 

 writers, as the fragrant, aromatic, fetid, nauseous, etc., distinctions sufficiently evident 

 from their mere enumeration ; and it is plain enough that there are emanations, like 

 those from delicately-scented flowers, which are easily recognizable by the sense of 

 smell while they make no impression upon the ordinary sensory nerves. The very 

 marked individual differences in the delicacy of the olfactory organs in the human sub- 

 ject and in different animals is an evidence of this fact. Hunting-dogs recognize odors 

 to which we are absolutely insensible ; and certain races of men are said to possess a 

 wonderful delicacy of the sense of smell. Like all of the other special senses, olfaction 

 may be cultivated by attention and practice, as is exemplified in the delicate discrimina- 

 tion of wines, qualities of drugs, etc., by experts. 



After what we have said concerning the situation of the true olfactory membrane in 

 the upper part of the nasal fossa3 and the necessity of particles impinging upon this mem- 

 brane in order that their odorous properties may be appreciated, it is almost unnecessary 

 to state that the passage of odorous emanations to this membrane by inspiring through 

 the nostrils is essential to olfaction, so that animals or men, after division of the trachea, 

 being unable to pass the air through the nostrils, are deprived of the sense of smell. The 

 act of inhalation through the nose, when we wish to appreciate a particular odor, is an 

 illustration of the mechanism by which the odorous particles may be brought at will in 

 contact with the olfactory membrane. 



It is a curious point to determine whether the sense of smell be affected by odors 

 passing from within outward through the nasal fossae. Persons who have offensive ema- 

 nations from the respiratory organs usually are not aware, from their own sensations, 

 of any disagreeable odor. This fact is explained by Longet on the supposition that the 

 olfactory membrane becomes gradually accustomed to the odorous impression, and there- 

 fore it is not appreciated. This is an apparently satisfactory explanation, for we could 

 hardly suppose that the direction of the emanations, provided they came in contact with 

 the membrane, could modify their effects. He cites a case of cancer of the stomach, 

 in which the vomited matters were exceedingly fetid. At first, the patient, when he 

 expired the gases from the stomach through the nostrils, perceived a disagreeable odor at 

 each expiration ; but little by little this impression disappeared. 



Relations of Olfaction to the Sense of Taste. The relations of the sense of smell to 

 gustation are very intimate. In the appreciation of delicate shades of flavor, it is well 

 known that the sense of olfaction plays so important a part, that it can hardly be sepa- 

 rated from gustation. The common practice of holding the nose when disagreeable 

 remedies are swallowed is another illustration of the connection between the two senses. 

 In most cases of anosmia, there is inability to distinguish delicate flavors ; and patients 

 can distinguish by the taste only sweet, saline, acid, and bitter impressions. 



