Olfaction is, indeed, one of the primitive senses of animal life. And in man, as it happens, while vision has constructed for itself a highly complicated camera-like end-organ, and hearing has produced an apparatus even more elaborate, the olfactory organ, on the other hand, remains primitive, its essential structure having undergone no apparent evolutionary change from the simplest and earliest type.
This, perhaps, is scarcely the proper way of expressing the situation. Evolutionary change has, as a matter of fact, occurred, but it reaches its highest development not in man, but in terrestrial mammals otherwise inferior to him—in the dog, for example.
For once, man does not occupy the apex of the evolutionary pyramid.
Olfactory development, high or low, is linked up with the natural habits of the different species. Thus, mammals which go about on all fours, whose visual outlook is restricted and whose muzzle is near the ground, are the most highly gifted; those, again, like the seals, porpoises, whales, and walruses, which have reverted from a terrestrial to an aqueous environment, where smell is of less value to them, show poorly developed olfactory organs; and finally, the apes and man, living habitually above the ground, the former in trees, the latter on his hind legs, and relying chiefly upon vision, also show a decline from the high point reached by four-footed mammalians.
The animals of this kingdom are thus divided into macrosmatic and microsmatic groups. To the latter man belongs, but we must add that his olfactory sense has not yet degenerated so completely as that of certain other species (porpoises, etc.).
It is, of course, common knowledge that in most of the animals we are closely acquainted with the sense of smell is infinitely more delicate and acute than ours, so much so, indeed, that the imagination can on occasion scarcely conceive theirs to be of the same nature. As a matter of fact, many authorities incline to the belief that not only mammalians and other vertebrates, but also insects, must be guided to their food and to their love-mates by some kind of perception, by some mysterious sense, of which we are totally devoid.
As this is a division of our subject of the highest interest, and one to which we shall have occasion to recur at intervals throughout this treatise, we shall discuss the matter as fully as the space at our disposal will permit.
The unit of the olfactory sense-organ is the olfactory cell. This, which does not vary in structure from one end of the animal kingdom to the other, is microscopically seen to consist of an elongated body like a tiny rod, bearing on its free end a small enlargement or prominence, on the surface of which is a cluster of extremely fine protoplasmic filaments, the olfactory hairs. These hairs project into and are immersed in a thin layer of mucus, at all events in air-breathing animals, an environment which is necessary for their functional activity, because, if the nose becomes desiccated, as it does in some diseases, the sense of smell is lost (anosmia). The hairs are, without doubt, the true receptive elements of the olfactory cells. It is these which come into contact with and are stimulated by odours—whatever the nature of Odour may be.
The deep (proximal) end of the rod-like olfactory cell tapers into a nerve-fibre, which passes by way of the olfactory nerve to a special lobe of the brain—the olfactory lobe—in the vertebrates, or to a nerve-ganglion in the invertebrates.
Olfactory cells in man are only found in the upper—the olfactory—region of the nose, spread over a surface of about one square inch, the olfactory area—part lying on the outer (lateral) wall of each nasal passage and part on the septum, or partition between the nasal passages. In macrosmatic animals the olfactory area is relatively greater than in man, but there is apparently no other difference between them.