THE SENSE OF SMELL 693 



provided at its extremity with several hair-like processes. The central 

 pole becomes the axon process and passes directly to the olfactory bulb. 



The sustentacular cells are epithelial in character and, as their name 

 implies, support or sustain the olfactory cells. 



For the appreciation of odorous particles the air must be drawn through 

 the nasal fossae with a certain degree of velocity. If the particles are widely 

 diffused in the air, they must be drawn not only more quickly but more 

 forcibly into contact with the olfactory hairs, as in the act of sniffing, the 

 result of short energetic inspirations. To many substances the olfactory 

 apparatus is extremely sensitive. Thus, it has been shown that a particle 

 of mercaptan the actual weight of which was calculated to be 40.000 ooo 

 of a milligram gives rise to a distinct sensation. 



The Olfactory Sensations. The sensations which arise in consequence 

 of the excitation of the olfactory apparatus are very numerous and their 

 classification is extremely difficult. For this reason it is customary to divide 

 them into two groups: viz., agreeable and disagreeable, in accordance with 

 the feelings they excite in the individual. As the olfactory sensations give 

 rise to feelings rather than ideas, this sense plays in man a subordinate part 

 hi the acquisition of knowledge. In lower animals this sense is employed 

 for the purpose of discovering and securing food, for detecting enemies and 

 friends, and for sexual purposes. In land animals the entire olfactory appa- 

 ratus is well developed and the sense keen; in some aquatic animals, as the 

 dolphin, whale, and seal, the apparatus is poorly developed and the sense 

 dull. 



