ODORS AND LIFE. 143 



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 de- 

 cayed cabbage ; and another, of a person to whom vanilla was entirely 

 without scent. Blumenbach speaks, too of an Englishman, with al] 

 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 re- 

 sorted 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 pos- 

 sible, 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 phe- 

 nomena 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 experi- 

 ment 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. Prevost, 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, 



