that impeding the passage of air into the nostrils is quite as effectual a way of making any sort of olfactory sensation impossible. Let us add, that the region most sensitive to odors is that of the upper part of the nasal cavities. There are, as we shall notice in proceeding, considerable differences as regards the degree of sensitiveness in this sense of smell, comparing one man with another. But it is a still more singular fact that sometimes, without apparent cause, the sense is utterly wanting. In other cases it is unaffected by the action of certain odors only, an analogous infirmity to that which students of the eye call daltonism, and which consists in the perception of certain colors only. We find in scientific annals the case of a priest who was insensible to all odors except that of a manure-heap, or that of decayed cabbage; and another, of a person to whom vanilla was entirely without scent. Blumenbach speaks, too of an Englishman, with all his senses very acute, who perceived no perfume in mignonette.
Olfaction is sometimes voluntary, sometimes involuntary. In the former case, by an act which is called scenting something, and is resorted to for the sake of a keener sensation, we first close the mouth, and then sometimes draw in a full breath, sometimes a succession of short, quick inspirations. Then the muscular apparatus edging the opening of the nostrils comes into play, to contract that orifice, and point it downward, so as to increase the intensity of the current of inhaled air. When, on the contrary, we wish to smell as little as possible, the organ becomes passive. We effect strong expirations by the nose to drive out the air that produces scent, and inhalation, instead of being performed by the nostrils, instinctively takes place through the mouth.
Scents and the sense of smell have an important share in the phenomena of gustation, that is, there is a close connection between the perception of odors and that of tastes. Physiological analysis has clearly brought out the fact that most of the tastes we perceive proceed from the combination of olfactory sensations with a small number of gustatory sensations. In reality, there are but four primitive and radical tastes—sweet, sour, salt, and bitter. A very simple experiment will convince us of this fact. If we keep the nostrils closed when tasting a certain number of sapid substances, so as to neutralize the sense of smell, the taste perceived is invariably reduced to one of the four simple savors we have just named. Then, whenever the pituitary membrane is out of order, the taste of food is no longer the same; the tongue distinguishes nothing but sweet, sour, salt, or bitter.
It is time now to begin the study of the. physiological and chemical conditions of smell, and for this we must first inquire how odorous substances behave with regard to the medium which separates them from our organs. Prévost, in an essay published in 1799 on the means of making emanations from odorous bodies perceptible to sight, was the first to bring to view the fact that certain odorous substances,
