1026 SMELL. [Book in. 



ing 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 development of olfactory 

 sensations. 



The other subsidiary 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 odoriferous particles with the 

 olfactory membrane than that supplied by mere diffusion is 

 provided for. 



§ 637. We have every reason to think that any stimulus 

 applied to the olfactory cells will produce the sensation of smell ; 

 but the proof of this is not absolutely clear ; and we have no 

 definite evidence as to what is the result of directly stimulating 

 the fibres of the olfactory nerve. The olfactory membrane how- 

 ever is certainly the only part of the body in which odours as 

 such can give rise to any sensations : and the sensations to 

 which they give rise are alwa} r s those of smell. The mucous 

 membrane of the nose is however also an instrument for the 

 development of afferent impulses other than the specific olfac- 

 tory ones. Chemical stimulation of the nasal mucous mem- 

 brane by pungent substances such as ammonia gives rise to a 

 sensation distinct from that of smell, a sensation which does not 

 afford us the same information concerning the chemical nature 

 of the stimulus, as does a real olfactory sensation, and which 

 is much more allied to the sensations produced by chemical 

 stimulation of other surfaces sensitive to chemical action. This 

 sensation moreover seems to be developed both in the non- 

 olfactory, and in the olfactory regions of the nasal mucous 

 membrane ; and it is probable that these two kinds of sensa- 

 tions, the one produced by odours, the other by pungent sub- 

 stances, thus arising in the olfactory membrane are conveyed 

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

 the fifth nerve. 



Each substance that we smell causes a specific sensation, and 

 we are not only able to recognize a multitude of distinct odours, 

 but also in certain cases to distinguish individual odours in a 

 mixed smell. And though we may recognize certain odours as 

 more like to each other than to other odours, or can even make 

 a rough classification of odours, we cannot, as we can in the case 

 of visual colour sensations, reduce our multifarious olfactory 

 sensations to a smaller number of primary sensations mixed in 

 various proportions. Nor have we at present any satisfactory 



