The air which enters the organ of smell deposits on the surface of the pituitary membrane the odoriferous principles with which it is charged, it becomes impregnated with them, and it is in its tissues that these principles come in contact with the terminal fibres of the olfactory nerve. We have already stated that this nerve is only distributed over the upper portion of the nasal fossę; in order to produce the sensation of odours, therefore, the air inspired must reach not only the inferior but also the superior parts of these cavities. The nose is contracted at the root like a funnel, and tends to guide the odoriferous effluvia towards the point where the impression is to be perceived; and the stronger the inspiration the higher up the column of air is carried, and the more it excites the filaments of the special nerve. Some physiologists have thought, with Magendie, that the nerves of the fifth pair, which ramify over the lower portion of the pituitary membrane, were designed to serve the purpose of smell; it seems to be clearly demonstrated that the sensations caused by acid or ammoniacal vapours are not olfactory, but simply painful.
The pituitary membrane in its normal condition is con-stantiy humid, and the secretion with which it is bathed is one of the indispensable conditions of the function of smell; and, therefore, we remark at the commencement of a cold in the head, when this membrane becomes dry, that the sense of smell is more or less impaired. The nose, by shielding the membrane from the immediate contact of the air, preserves its functions, and the loss of this organ diminishes, or even completely destroys, the sense of smell. Smelling is ordinarily involuntary, but it may be rendered more active by the exertion of the will. The inspirations are then stronger and more frequent, in order that the odour which we wish to perceive or enjoy may be carried in greater quantity toward the nasal arch. But if, on the contrary, we wish to avoid a disagreeable odour, a sudden expiration takes place from the nose, and we breathe instinctively through the mouth, and the soft palate closes the olfactory cavities behind. It is in this way that we are able to diminish the disagreeable impression arising from the odour in drinking sulphureous waters.
Whether odours reach the seat of smell by the nose, or through the posterior opening of the nasal fossę, the result is the same; it is by this means that we perceive the aroma of the food when eating with the mouth shut; but under these latter conditions the persistence of the impressions very soon blunts the sensibility. A man fasting immediately perceives it, if a man with whom he is speaking has taken the smallest quantity of alcohol, even though it was only a glass of red wine; after eating we distinguish much less easily in others the odour of the aliments of which we ourselves have partaken, and the odoriferous principles of which have already saturated the olfactory membrane.
The sinuses of the bones of the skull and of the face, which are in communication with the nasal fossę, take no part in the perception of odours. It has been thought that they may contribute by their secretions to moisten the pituitary membrane, or serve as receptacles for the air, which is afterward carried from their cavities to the organ of smell.