CHAP, v.] TASTE AND SMELL. 251 



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. 



860. 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 however 

 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 always those of smell. The mucous membrane of the 

 nose is however also an instrument for the development of 

 afferent impulses other than the specific olfactory ones. Chemical 

 stimulation of the nasal mucous membrane 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 imformation con- 

 cerning 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 sensations, 

 the one produced by odours, the other by pungent substances, 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 clas- 

 sification 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 guide to connect the characters 

 of an olfactory sensation with the chemical constitution of the body 

 giving rise to it. 



The sensation takes some time to develope after the contact of 

 the stimulus with the olfactory membrane, and may last very long. 

 When the stimulus is repeated the sensation very soon dies out : 

 the sensory terminal organs speedily become exhausted. The 

 larger, apparently, the surface of olfactory membrane employed, 

 the more intense the sensation; animals with acute scent have 

 a proportionately large area of olfactory membrane. The greater 

 the quantity of odoriferous material brought to the membrane, the 

 more intense the sensation up to a certain limit ; and an olfacto- 



