ODORS AND LIFE. 181 
ing 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 con- 
trary, we wish to smell as little as possible, the organ be- 
comes 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 con- 
nection 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 combina- 
tion of olfactory sensations with a small number of gusta- 
tory 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 sub- 
