22 



V. G. DETHIER 



icals has not yet been satisfactorily answered. The problem was first approached 

 by studying cross adaptation, odor mixtures, and cases of differential anosmia. 

 Le Gros Clark and Warwick (1946) found that small lesions in the olfactory 

 bulb lead to retrograde atrophy of receptors scattered diffusely over the entire 

 septum. As Allison and Warwick (1949) pointed out, this observation suggests 

 an absence of point-to-point localization of the projection of the olfactory epi- 

 thelium on the olfactory bulb. The projection is so diffuse that a small area of 

 the epithelium may be represented over a considerable area of the bulb. Each 

 glomerulus probably receives fibers from receptors scattered widey throughout 

 the olfactory epithelium. A glomerular system is at least automatically set up 

 to mediate responses to specific odors. 



Cross adaptation and its absence suggests different receptor groups each of 

 which is sensitive to a different group of chemicals, i.e., a receptor mosaic. But, 

 as long as the vapor pressure and hence the concentration of the stimulus is 

 not controlled, the various results obtained could equally well be a result of 

 concentration effects. The subjective sensations elicited by various mixtures 

 tend to bolster the mosaic theory in that trained experts can analyze the con- 

 stituents of mixtures. In this respect olfaction resembles an auditory analogue. 

 It should be pointed out here, however, that the auditory stimulus is of the 

 same nature throughout the auditory spectrum. By analogy, olfactory stimuli 

 which elicit different odor sensations need not necessarily operate through differ- 

 ent mechanisms. 



a 



A. Isolated olfactory cell and sustentacular cell from a frog. (After Schultze 

 1862, Plate 1, Figure 4.) 



B. A Golgi preparation of an olfactory cell from a pike (Esox), in which 

 not only the cell body and the basal nerve fiber process are well shown but also 

 the long peripheral olfactory flagellum. (After Jagodowski, 1901, Figure ic.) 



Fig. 19. Olfactory receptors from the frog (from Crozier, 1934) 



