608 



SMELL. 



to solids, from air to solids, from solids to 

 air, &c. ; arid many such changes usually take 

 place, before the vibrations originating in a 

 sonorous body are communicated to the sen- 

 tient extremities of the auditory nerve. Tl.e 

 same is the case with heat, light, electricity, 

 and other agents whose transmission is be- 

 lieved to be dynamical : that it is not the case 

 in regard to odorous emanations must be 

 regarded, therefore, as a powerful argument 

 against the idea of their dynamical nature. 

 Another argument may be derived from the 

 well-known fact, that odorous emanations re- 

 quire such a time for their propagation, as 

 corresponds rather with the diffusion of the 

 odoriferous medium itself, than with the mere 

 conduction of vibrations. Thus, in a house in 

 which free communication is established 

 throughout by passages, staircases, &c., but in 

 which the course of air is not very direct from 

 one part to another, any strong odour set 

 free in one spot will be gradually diffused 

 through the whole house, tiie rapidity being 

 governed by the circumstances which favour 

 or obstruct the movement of air. On the 

 other hand, the transmission of sonorous un- 

 dulations, which merely throw the air into 

 vibration, is not dependent upon its move- 

 ment, and is, indeed, but little influenced by- 

 it. Tiiis argument is, perhaps, yet more 

 cogent than the former, and may be regarded 

 as conclusive against the dynamical theory of 

 odours. 



It is not difficult to explain many of the 

 apparent inconsistencies which attend the 

 material theory. The varieties of the olfac- 

 tive power among human beings are quite 

 sufficient to prove, that a substance which is 

 strongly odorous to one individual may not 

 produce any impression on the smell of an- 

 other, whose scent for other substances 

 may nevertheless be very acute. And there 

 is strong reason to believe that there is a 

 great diversity in this respect amongst dif- 

 ferent species of animals, some appearing en- 

 tirely insensible to odours which strongly 

 affect others. That we do not appreciate an 

 odour, therefore, is no proof of its non-exist- 

 ence ; and we have no right to say of any 

 volatile or gaseous substances, that they are 

 not odorous, but simply that they are not 

 odorous to us. Again, the sense of smell, 

 like the other senses, is rather relative than 

 positive -, that is to say, it rather estimates a 

 change in the condition of the surrounding 

 medium, than its actual permanent state. This 

 is fully proved by the tact that persons who 

 habitually dwell amongst odours of any one 

 kind, become, in time, entirely insensible to 

 them, although their olfactive sense may re- 

 main of its full acutencss in regard to any 

 different scent. This being the case, we at 

 once perceive that water, oxygen, nitrogen, 

 ami carbonic acid could not, in accordance 

 with the general laws of sensation, possess 

 any odour to animals whose organs of smell 

 are constantly imbued with them. We shall 

 presently find that the moisture of the olfac- 

 tory membrane is a necessary condition of its 



functional power ; and thus neither fishes, 

 which have their olfactory surface con- 

 stantly bathed in water, nor air-breathing ani- 

 mals, whose pituitary membrane is lubricated 

 with it, could take cognisance of any odorous 

 properties which it might really possess. In 

 like manner, the nasal cavities of animals 

 being continually filled with a mixture of 

 oxygen, hydrogen, and carbonic acid, these 

 gases cannot excite the olfactive sense ; whilst 

 on the other hand, we can easily imagine that 

 if animals were adapted to breathe hydrogen 

 or its strongly odorous compounds, they 

 would be insensible to the latter, whilst they 

 might distinguish oxygen, nitrogen, or carbo- 

 nic acid by their respective odours, just as 

 readily as we distinguish phosphuretted, sul- 

 phuretted, or carburetted hydrogen. 



Although it is through the atmosphere 

 that odorous emanations are most readily 

 conveyed, yet there can be no reasonable 

 doubt that they may be transmitted through 

 water also. Thus we find fishes provided 

 with a complex organ of smell, which seems 

 to be of considerable importance in directing 

 them towards their prey. This may be infer- 

 red, not merely from the fact that the olfactive 

 ganglia and nerves are of large size relatively 

 to the rest of the encephalon, but also from 

 the circumstance, well known to fishermen, 

 that many fish are particularly attracted by 

 odorous bait. Some anglers are even in the 

 habit of scenting their bait with essential 

 oils, in order to render it more alluring. 



The general structure of the organ of 

 smell in man has already been described 

 (Nose ) ; but some particulars recently ascer- 

 tained by Messrs. Todd and Bowman re- 

 specting the minute anatomy of the pituitary 

 membrane, and the structure and distribution 

 of the olfactory nerve, are too important to 

 be passed by. That the true sense of smell 

 is specially, if not exclusively, the endowment 

 of the upper portion of the organ, has been 

 inferred by anatomists from the limited dis- 

 tribution of the olfactory nerve, and by phy- 

 siologists from the fact that odours are only 

 perceived strongly when the odoriferous air 

 is drawn into the upper part of the cavity. 

 The lower part of the nasal cavity is pro- 

 perly to be regarded as the orifice of the 

 respiratory passages : it is extremely sensitive 

 to irritants, but it does not participate in the 

 discrimination of odours properly so called ; 

 and its mucous membrane is covered with a 

 ciliated columnar epithelium. On the other 

 hand, the limits of the olfactive region " are 

 distinctly marked by a more or less rich 

 sienna-brown tint of the epithelium, and by a 

 remarkable increase in the thickness of this 

 structure compared with the ciliated region 

 below ; so much so, that it forms an opaque 

 soft pulp upon the surface of the membrane, 

 very different from the delicate, very trans- 

 parent film of the sinuses and lower spongy 

 bones. The epithelium, indeed, here quite 

 alters its character, being no longer ciliated, 

 but composed of an aggregation of superposed 

 nucleated particles, oT pretty uniform appear- 



