CHAPTER XXI 



The sense of smell 



\V. R. A D E V \ Department nf Anatomy, University of Melbourne, Melbourne, Australia 



CHAPTER CONTENTS 



Olfactory Mucosa and Peripheral Receptor 

 Arrangement of C31factory Mucosa 



Olfactory Bulb and Its Connections with Olfactory Mucosa 

 Essential Processes Involved in Olfactory Stimulation 

 Characters of Odorous Substances 

 Enzyme Theories of Olfaction 

 Radiation Theories of Olfaction 

 Methods of Odor Measurement 



Subjective measurement techniques 

 Objective measurement techniques 

 Electrical Phenomena in Olfactory Bulb Accompanying Ol- 

 factory Stimulation 

 Patterns of Spontaneous and Induced Activity in Bulb and 

 Effects of Anesthesia 

 Spontaneous waves 



Waves and unit activity accompanying olfactory stimula- 

 tion 

 Differential Excitation of Receptors 

 Differentiation of response in area 

 Temporal differentiation of response 

 Central Connections of Olfactory Bulb 

 Efferent Pathways From Olfdctory Bulb 

 Extent of Primary Olfactory Cortex 

 Neuroanatomical investigations 

 Electrophysiological investigations 

 Higher Order Olfactory Connections 

 Behavior Studies of Olfactory Mechanisms 



At alt stages of corticat etaboratio7i an important function of 

 ttie olfactory cortex, in addition to participation in its own 

 specific way in cortical associations, is to serve as a non-specific 

 activator Jor all cortical activities. 



C. J. HERRICK, 1933 



WHILE THE IMPORTANCE of the olfactory sense is 

 greatly reduced in primates in comparison with other 

 telereceptor mechanisms, such as sight and hearing, 

 it can nevertheless provide significant information 

 about events possibly remote in space and time. In- 



deed, olfaction may provide the only warning of 

 serious environmental hazards, and in man it retains 

 its importance in feeding and sexual functions. 

 There remain many baffling aspects to even the 

 most basic phenomena in the olfactory process, par- 

 ticularly in the mechanisms ins-olved in excitation of 

 the peripheral receptor and in the physiological 

 patterning of activity through which fine differences 

 in odors are presumably perceived. 



OLF.\CTORV MUCOS.\ AND PERIPHER.XL RECEPTOR 



Arrangement nf Oljactory Mucosa 



The olfactor\- mucosa forms a restricted zone in 

 man, lying in the dorsal and posterior part of the 

 nasal cavity. To the naked eye it appears yellowish- 

 brown in comparison with the rest of the mucosa, 

 and it covers the upper parts of both the lateral wall 

 of the nasal cavity and septum, extending over a 

 total area of about 240 sq. mm (66). The olfactory 

 mucosa is a pseudostratified columnar epithelium 

 and, unlike the respiratory portion, has no distinct 

 basement meinbrane or cilia. It lines the surface of 

 nearly all the superior turbinate, a small part of the 

 middle turbinate and the upper third of the nasal 

 septum (fig. i). 



Inspired air traverses the inferior meatus and 

 partially the middle meatus during normal breath- 

 ing, and the olfactory area is thus above the main 

 air current. Since a change in breathing, as in sniffing, 

 causes adequate eddying of air into the upper olfac- 

 tory area, it is apparent that aerodynamic factors 

 may be intimately concerned in determining thresh- 

 olds of excitability (56). 



The olfactory receptors or hair cells are bipolar 



535 



