CHAPTER V. 



TASTE AND SMELL. 



SECTION 1. OLFACTORY SENSATIONS. 



§ 636. Particles of odoriferous matters present in the in- 

 spired 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. If we assume that the 

 rod cells, and not the cylinder cells, are the special functional 

 elements, we may suppose that the sensory impulses are orig- 

 inated by the contact of the odoriferous particles with the 

 free endings of the rod cells ; but they may be due to contact 

 with the cylinder cells ; in either case we are wholly in the dark 

 as to the manner in which the contact of the particles with the 

 cells brings about the molecular changes constituting a nervous 

 impulse. We cannot even say whether we ought to speak of 

 the first step by which the contact of the particle begins the 

 series of changes as a chemical or as a physical process. 



In nearly all cases the odoriferous particles are conveyed to 

 the membrane in a gaseous medium, namely, the atmosphere ; 

 but before they can gain access to the cells they must become 

 dissolved or at least suspended in fluid ; for the whole olfactory 

 membrane is kept moist by a layer of fluid, secreted by the 

 glands, and the odoriferous particles must pass into this layer 

 of fluid before they can gain access to the cells. Indeed, the 

 proper condition of this layer of fluid is one of the essential 

 conditions of the exercise of the sense. If on the one hand the 

 membrane be too dry, or if on the other hand the secretion be 

 too abundant or altered in quality, the power of smelling is 

 diminished or even wholly suspended. It is a matter of com- 

 mon experience that a nasal catarrh interferes with smell. 

 When the nostril is filled with rose-water the odour of roses is 

 not perceived; and simply filling the nostrils with distilled 

 water suspends for a time all smell, the sense gradually return- 

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