288 ON THE SENSES. 



earned upon tlie upper side of the shell. The mechanical part performed by 

 theJower shell in conveying the air to the smelling region being thus established 

 with tolerable certainty, the conjecture is natural, on the other hand, that its es- 

 sential service to the sense of smell cannot consist in this simple act of convey- 

 ing, as wc have just seen that an odorous current blown directly against the 

 upper shells produces only faint sensations. There must be something peculiar 

 in that conveying operation of the lower shell ; some change of the odorous 

 current must there take place by which its faculty of affecting the nerves of 

 smell in the upper cuticle is enhanced. In what that consists we do not know; 

 all the suggestions made rest on slender foundations, and can expect belief only 

 from confirmations as yet to come. It may be discovered on some future day 

 that the current of air impregnated with an odorous substance receives on its 

 way over the cuticle of the lower shell some admixture by which its power 

 of affecting the end-organs of the nerves of smell is increased. As long as we 

 do not know the effective principle of odorous substances we have little hope 

 of obtaining full light as to the necessary conditions of their efficiency or of satis- 

 factorily explaining what has been empirically discovered to be such. 



That effective principle must be different in quality in different odorous sub- 

 stances ; it must have as many different modifications as there are distinguished 

 qualities of sensation; but its intensity is also exceedingly varying in different 

 odorous substances, as some of these, even when received in large quantities 

 from the atmospheric air, produce only faint sensations, Avhile others, even when 

 contained in the breathing air in an infinitely small quantity, in imponderable 

 particles, still intensely affect the nerve of smell. We shall mention some well- 

 known instances as evidences of the enormous power which that effective prin- 

 ciple must have in some bodies ; or, as may be differently expressed, of the 

 extraordinary sensitiveness of our organ of smell in regard to certain substances. 

 The most wonderful substance in this respect is undoubtedly musk, which, 

 with incredible tenacity, preserves its efficacy in the very minutest parts. It 

 is enough slightly to touch a musk-bag with one's fingers or a garment to have 

 the smell in the touching part for days; nay, even if we spent only a short 

 time in a musky atmosphere, our dresses will for a long time fill the rooms we 

 enter with a musky odor. In order to offer an inexact idea of the minuteness 

 of the particles of musk which are still capable of imparting some odor, we 

 state, after a well-known experimenting physiologist, that a certain liquid, 

 containing as much of an extract of spirit of musk as a^ooouVoooo^^ P*'^^'*' 

 of its whole weight, was at times still distinctly odorous. A grain's weight of a 

 liquid of which yoooUoo*^ P^^"'' '^^^^ ^^ ^^^'"^^ extract spread an intensely pene- 

 trating odor. These figures are naturally but of little value, as the other 

 moments which are to be taken into account, in ascertaining the intensity of 

 smell, cannot be precisely measured, and the intensity of the sensation not even 

 inexactly. However, they facilitate the forming of an idea, and people like to 

 see endless greatness or minuteness represented in figures, although a million- 

 fold increase by means of a solar microscope is as little apt to create a clear 

 conception as the number of miles, learned by heart, can do it in regard to the 

 distance of stars from the earth. Next after musk are to be mentioned certain 

 flower ethers, especially the oil of roses, a little drop of which is sufficient to fill 

 with odor an immense atmosphere. The same physiologist states that a certain 

 space filled with air of which, at the highest, only yoo^ooTT P''^^^ ^^^^ vapor of 

 oil of roses, still diffused a distinct odor of roses. Figures like these could easily 

 be presented in numbers, but we prefer restricting ourselves to those given. 



The intensity of a sensation of smell depends not only on the quality of the 

 odorous substance, and on the quantity in which it is carried upon the nasal 

 cuticle, but also on the sensitiveness of the latter and of its receiving apparatus. 

 This sensitiveness, it is well known, greatly differs in different persons, and still 

 more in animals of different speciea It varies also, and that considerably, in 



