SMELL. 807 



plasmic cells, which have a cylindrical body terminating at its free extremity 

 as a squared truncated surface ; the other extremity of the body is stretched 

 out as a filamentous prolongation, which expands into a triangular plate as 

 it approaches the submucous tissue. From the base of this plate a number 

 of filaments are given off, which are prolonged into the submucous tissue. The 

 second variety of epithelial cells (c) is found at the borders of the regio 

 otfactoria. They are similar to those just described, excepting that their 

 free surface is covered with cilia. Between the epithelial cells the olfac- 

 tory nerves terminate. These terminal filaments (&,/) are long, delicate 

 structures, which have a number of fusiform expansions along their course; 

 in the largest expansion is found an oval nucleus. The terminal filaments 

 are called the olfactory cells. As yet no connection between the subepi- 

 thelial and interepithelial nerve-filaments has been demonstrated. The 

 epithelial cells (d and e) in the above figure are shown connected with 

 the subepithelial tissue. The fifth nerve supplies the fossae with sensory 

 filaments.] 



727. Odorous particles present in the inspired air passing through 

 the lower nasal chambers diffuse into the upper nasal chambers, and falling 

 on the olfactory epithelium produce sensory impulses which, ascending to 

 the brain, give rise to sensations of smell. We may presume that the sen- 

 sory impulses are originated by the contact of the odorous particles with 

 the peculiar rod-shaped olfactory cells described by Max Schultze ; but we 

 are as much in the dark about this matter as about the development of 

 visual sensory impulses in the rods and cones or of auditory sensory im- 

 pulses in the organ of Corti. 



The susidiary apparatus of smell is exceedingly meagre. By the forced 

 nasal inspiration, called sniffing, we draw air so forcibly through the nostrils 

 that currents pass up into the upper as well as the lower nasal chambers; 

 and thus a more complete contact of the odorous particles with the olfactory 

 membrane than that supplied by mere diffusion is provided for. 



We have every reason to think that any stimulus applied to the olfactory 

 nerve will produce the sensation of smell ; but the proof of this is not so 

 clear as in the case of the optic and auditory nerves. We are, however, sub- 

 ject to sensations of smell not caused by objective odors. The olfactory 

 membrane is the only part of the body in which odors as such can give rise 

 to any sensations ; and the sensations to which they give rise are always 

 those of smell. The mucous membrane of the nose is, however, also an in- 

 strument for the development of afferent impulses other than the specific 

 olfactory ones. Chemical stimulation of the olfactory membrane by pungent 

 substances such as ammonia gives rise to a sensation distinct from that of 

 smell, a sensation which affords us no information concerning the chemical 

 nature of the stimulus, and which is indistinguishable from the sensations 

 produced by chemical stimulation of other parts of the nasal membrane as 

 well as of other surfaces equally sensitive to chemical action. It is probable 

 that these two kinds of sensations thus arising in the olfactory membrane 

 are conveyed by different nerves, the former by the olfactory, the latter by 

 the fifth nerve. 



728. For the development of smell it appears necessary that the odor- 

 ous particles should be conveyed to the nasal membrane in a gaseous medium, 

 or, at least, that the surface of the membrane should not be exposed at the 

 same time to the action of fluids. Thus, when the nostril is filled with rose- 

 water, the odor of roses is not perceived ; and simply filling the nostrils with 

 distilled water suspends for a time all smell, the sense returning gradually 

 after the water has been removed ; the water apparently acts injuriously on 

 fche delicate olfactorv cells. 



