SEC. 2. OLFACTORY SENSATIONS. 



859. Particles of odoriferous matters 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 sen- 

 sations of smell. We may assume that the sensory impulses are 

 originated by the contact of the odoriferous particles with the free 

 endings of the rod cells ; but 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, the secretion of the glands described 

 above, 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 common 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 returning after the water has been 

 removed; the water apparently acts injuriously on the delicate 

 olfactory cells. If instead of using rose water, the rose scent be. 

 dissolved in "normal saline solution" which ( 14) more closely 

 resembles the natural secretion, the cells can perform their 

 function, and the scent is perceived. The glands of the olfactory 

 membrane form an important subsidiary apparatus for the develop- 

 ment of olfactory sensations. 



