SMELL 



245 



membrane, but some air diffuses backways throuji^h I lie j)ostcrior 

 nares (Fig. 62, upper portion). This is important for the preserva- 

 tion of the sense. The receptor neurons have retained their 

 primitive condition of cell l)ody in the epithelium itself (Parker). 

 They are rapidly fatigued and readily destroyed. Now, by their 

 situation in a backwater they do not come directly into contact 



OLFACTORY BULB 



OLFACTORY EPITHEUUM 



OPENING OF 



EUSTACHIAN 



TUBE 



ARYTENOID 



HYOID BONE 

 THYREOID CARTILAGE-' 

 EPIGLOTTIS-'' 



VOCAL CORD-' 



' CE50PHAGUS 



TRACHEA-- 



Fig. 62. — Diagram of antero-posterior section througli nasal fossw, moutli and neclc. In 

 the upper portion of the figure tlie arrows show the direction of the air ('urrents during 

 inspiration. The soft palate should, of course, be down to allow of the passage of air. The 

 lower portion uf the diagram represents the position of the structures during the act of 

 swallowing. 



with high concentrations of odoriferous substances and, further- 

 more, air attains body temperature and moisture, and is freed from 

 suspended particles (dust, bacteria, etc.) before reaching the sensory 

 surface. The physical details of the mechanism for the perception 

 of smell, that is, for the conversion of chemical into nervous energy, 

 have not yet been brought to light. The sense is extraordinarily 

 delicate. Mercaptan, in as low a concentration as 0-0000000004 

 gram per litre of air, can be detected. Training renders the sense 

 more acute. The working chemist relies on his sense of smell to 



