426 DURATION AND LOCALIZATION OF ODORS. 



supplies from the fifth pair. The upper part of the nasal cavity is there- 

 fore devoted to the proper sense of smell, the lower portion to general 

 sensation. 



In one respect there is a striking difference between this sense and 

 Duration of vision and hearing. We can perceive many luminous im- 

 odors. pressions at the same time, or hear many sounds in rapid suc- 

 cession ; but not so with odors. We can smell only one thing at a time, 

 or, at all events, the impression remains long upon the olfactory appara- 

 tus, perhaps because the odoriferous substance remains dissolved in the 

 attached moisture. The identification of substances by their odor nec- 

 essarily implies a resort to recollection or memory, and sometimes we 

 have to apply the fragrant object again and again to the nose, before we 

 can recall with satisfactory precision its name. 



In the lower animals the sense of smell is probably localized in some 

 parts of the skin ; many of them display instincts which seem 

 anatomy of to imply the possession of such a sense. Insects also, by 

 smell, are often led to their food or to one another. 



The variable current of air introduced by respiration compensates in 

 some degree for the want of mobility of the nose, which may be regard- 

 ed, in air-breathing vertebrated animals, as consisting of a diverticulum 

 from the respiratory passages. In fishes, however, the olfactory cavity is 

 not connected with the respiratory passages : there are no posterior nares. 

 'The circumstance of their living under water disables them from appreci- 

 ating the odorous peculiarities of gases and vapors. In the whale the or- 

 gan is altogether absent, being replaced by the mechanism for receiving 

 air and blowing out water. In other tribes the acuteness of the sense 

 is in proportion to the development of the olfactory ganglia : in reptiles 

 it is feeble ; in birds, more developed ; in carnivorous animals, still more. 

 But here again it exhibits a special restriction, since there is reason for 

 supposing that carnivorous animals are insensible to the perfume of flow- 

 ers, while herbivorous ones distinguish them perfectly. In man, as we 

 have said, the sense is less developed, but it has a wider range. 



The localization of odors is effected in a much less perfect manner than 

 Localization the localization of sounds. The principle by which it is ac- 

 of odors. complished is obviously that of determining the direction, of 

 maximum intensity, and this involves necessarily the constant exercise 

 of memory and comparison. The surprising manner in which this can 

 be accomplished by animals whose sense of smell is acute, as, for exam- 

 ple, by the dog, is extremely interesting. From the different manner in 

 which various odors affect different individuals, there is no general stand- 

 ard of comparison to which they may be referred, as there is in the case 

 of colors and of sound. Scents which may be highly disagreeable to one 

 are acceptable to another person. By constant exposure, the faculty may 



