MECHANISM OF OLFACTION 635 



The delicacy of the sense of smell, measured by the possible tenuity 

 of odorous particles that can be recognized through the olfactory organs, 

 is very great, even in man, in whom olfaction is much inferior to this 

 sense in some of the lower animals. Fischer and Penzoldt, experi- 

 menting with ethyl mercaptan (C 2 H 5 SH) in 1885, were able to recog- 

 nize the peculiar and disgusting odor of this substance in a dilution 

 of one part in fifty billions of air, the weight of mercaptan distributed 

 in three cubic inches (about 50 cubic centimeters) being estimated at 

 !H7,<n)o,h(),ooo of a S ram (*6 <r,o<n),o oo of a milligram), containing 4.7 

 trillion molecules. It is said that the odor of iodoform can be recog- 

 nized in quantities as small as e\iro o,Tro~o/oo~o f a ram > or tne one hun- 

 dred billionth of a gram (Berthelot). 



Mechanism of Olfaction. In experimenting on the sense of smell 

 it has been found difficult to draw an 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 that overpower 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, aro- 

 matic, 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, that are easily recognizable 

 by the sense of smell, while they make no impression on the ordinary 

 sensory nerves. The marked individual differences in the delicacy of 

 the olfactory organs in the human subject and in different animals are 

 evidence of this fact. Hunting-dogs recognize odors to which most 

 persons are insensible ; and certain races of men are said to possess 

 a remarkable delicacy of the sense of smell. Like the other special 

 senses, olfaction may be cultivated by attention and practice, as is 

 exemplified in the delicate discrimination of wines, qualities of drugs 

 etc., by experts. 



After what has been said concerning the situation of the olfactory 

 organ in the upper part of the nasal fossae and the necessity of particles 

 impinging upon this membrane 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 divi- 

 sion 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 is an illustration of the mechanism by which the odorous particles 

 may be brought at will in contact with the olfactory membrane. 



