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 arc 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
