In the antechamber of the olfactory organ, on the other hand, the amount of modification necessary is evidently but slight, as the olfactory region of the nasal chamber is merely a narrow, open passage. As far as we know, all that takes place is that the incoming stimulus, the odorous molecule, is warmed and received by the nasal mucus. Thus the very complexity of the structure both of the eye and of the ear helps us to comprehend their function. But what can we deduce from a flat surface in which all we can see is a collection of cells with minute protoplasmic hairs projecting from their distal ends? Obviously, little or nothing. We are, in fact, confounded by simplicity. It may be that we are here dealing with one of the essential properties of all living matter, little, if at all, altered from its primitive condition. To the physiologist, then, olfaction is the most mysterious of all the senses. It still retains its secrets, and therein lies the fascination of its study. Of late years, the exploration of this dark region of physiology has been, and is still being, vigorously pushed, and we shall now proceed to give what, however, can only be a brief and superficial account of the progress made and of the opinions held. Even so we shall be compelled to make an incursion into the high and dry realms of modern chemical and physical theory. That may not be good hearing, but what is still worse is that almost every single point we shall be discussing is a matter of controversy. Let us commence with a few of the details, mostly unimportant, upon which there is general agreement. Consider, first of all, the variety, the almost infinite variety, of odours. We have, for example, all the odours of the world of Nature, the emanations of inorganic matter, of the earth itself, its soil and its minerals; to these we must add the multitudinous perfumes of the vegetable kingdom, of barks, roots, leaves, flowers and fruits, including those of growing herbaceous plants, which differ so widely from one another that it is said of Rousseau, whose myopia was compensated for by an unusually acute sense of smell, and who was, moreover, no mean botanist, that he could have classified the plants according to their smell had there been a sufficiency of olfactory terms for the purpose; then we have the thousand effluvia, some pleasant and others not so pleasant, of living animals, including the various races of mankind; next come the—mostly repulsive—odours of decaying vegetable and putrefying animal matter; and finally the products of man’s own proud ingenuity and skill, such as the artificial perfumes and flavours on the one hand and on the other coal-gas, acetylene, carbon disulphide, and the like.