iv THE SENSE OF SMELL 169 



for man, than that which takes place through the nostrils. Man, 

 in fact, does not snuff up his food while he eats it, like the 

 animals that have an elongated nasal aperture placed near the 

 buccal orifice. Man uses the sense of smell (in combination with 

 taste) much more during mastication and deglutition than during 

 the act of putting the food into his mouth. 



The popular theory that smell is the sentinel of the respiratory 

 apparatus, as taste is of the digestive apparatus, cannot be accepted 

 unreservedly in the light of physiological experience. We are 

 protected from breathing noxious air by the nasal branches of the 

 trigeminal, and not by the olfactory nerve. Irritating gases, even 

 when they are capable of arousing olfactory sensations, are 

 specially perceived by the sensory branches of the nasal mucosa. 

 The chief importance of smell is, in association with taste, to 

 perceive the quality of foods, to influence their selection, to excite 

 appetite, and reflexly to promote the digestive secretions. The 

 suppression of smell is dangerous to man because it disturbs all 

 these functions, and not because he becomes incapable of enjoying 

 the perfume of flowers or the aphrodisiac exhalations of certain 

 secretions. 



III. As in the sense of taste, the adequate stimuli for smell are 

 chemical in their nature, and the odoriferous substances must come 

 into direct contact with the olfactory surface before the olfactory 

 end-organs can be excited. The earlier opinion that odours may 

 act at a distance upon the olfactory organ, by special aerial or 

 ethereal undulations, as do sound and light, is now wholly 

 abandoned, and has no foundation. The fact, as pointed out by 

 Longet, that odours can be carried by the wind to a distance of 

 several miles, in itself, according to Zwaardemaker, proves the 

 corpuscular theory of smell, and excludes the possibility that they 

 can be due merely to vibrations. 



The number of substances capable of exciting smell, that is of 

 giving out odours, is certainly very great, and many bodies that 

 seem to us to have no smell are odorous for certain animals ; this 

 is due to the limited development of our sensibility. Even if all 

 the substances that are volatile, or dissociable into the finest 

 particles, are not odoriferous, at any rate for man, it may still be 

 said that the most penetrating and characteristic odours we know 

 are given off by volatile substances. 



Certain other substances that are normally non-volatile and 

 inodorous give off odours under certain conditions. Arsenic, e.g.,. 

 which at an ordinary temperature has no smell, gives off a strong 

 smell of garlic on heating ; resin and many metals become odorous 

 under friction. Accordingly, it is often held that both under 

 ordinary conditions and under special physical influences an 

 atmosphere of minute particles emanates from the surface of many 

 bodies, and that these may be perceived by their scent, if not by 



