Smell, the special sense by which we take cognizance of the odoriferous qualities of foreign bodies. The main peculiarity of this sense is that it gives us intelligence of the physical properties of substances in a gaseous or vaporous condition. An odoriferous body gives off emanations which diffuse themselves through the atmosphere, and we thus perceive its existence at a distance and when it may be concealed from sight. The actual quantity of vaporous material necessary for making an impression upon the olfactory organ is very small; and a substance like musk or attar of roses may fill an entire apartment or even a house for days or weeks with its peculiar odor, readily perceptible by all the occupants, without suffering any appreciable loss of weight. - The organ of smell is the mucous membrane of the upper part of the nasal passages, supplied by the filaments of the olfactory or first pair of cranial nerves. These nerves are endowed with the special sense of smell, but are destitute of ordinary or general sensibility. Thus they can perceive the odors of foreign substances, but not the physical contact of a solid body.

On the other hand, the lower portion of the nasal passages is supplied by filaments from the fifth pair of cranial nerves, which are nerves of general sensibility, but not susceptible to the impression of odors. Not all vapors are odoriferous; some are simply irritating or stimulating to the mucous membrane. The odors proper are generally of an organic origin, such as those of musk, asafoetida, the leaves and blossoms of plants, and the exhalations of living or decomposing animal bodies. Other gaseous emanations are simply irritating, like those of ammonia, chlorine, and acetic acid. Sometimes the two kinds of exhalations are mingled; thus pure alcohol is nearly or quite destitute of odor, but in cologne water we have the stimulating properties of the alcohol, mingled with odoriferous ingredients of a vegetable origin. Ammonia is irritating to the mucous membrane of the nose for the same reason'that it is irritating to the skin when brought in contact with it; but the skin is incapable of perceiving a true odor. The dissemination of odors is favored by the movement of the atmosphere; and when a disagreeable or noxious odor is contained in the air of an apartment, a free ventilation is the readiest method of expelling it.

When we wish to perceive more distinctly a faint or a delicate odor, we direct the air forcibly upward, by a peculiar inspiratory effort of the nostrils, through the superior part of the nasal passages. This movement is especially observable in many of the inferior animals, in whom the sense of smell is remarkably acute, and the olfactory mucous membrane unusually extensive and sensible. The dog, for instance, will not only distinguish different kinds of animals by their odor, but will recognize different individuals of the human species, or particular articles of dress belonging to them. He will even follow the track of wild game by the minute quantity of animal odor left by their footsteps upon the grass or dried leaves. - The sense of smell, like the other senses, becomes habituated to particular impressions when long continued; even disagreeable odors gradually lose in this way their offensiveness, and we become after a time more or less insensible to their presence. A disagreeable odor is not invariably injurious in itself; but it is almost always the indication or accompaniment of a gaseous emanation which is in reality noxious, or will become so if allowed to accumulate.

The offensive odor is a warning to the senses that the atmosphere is no longer pure and should be renovated; and if this warning be neglected, it at last ceases to make itself felt, and the exhalations may then imperceptibly increase until they produce serious injury.