486 TASTE AND SMELL 



2. SENSATIONS OF SMELL 



While the olfactory organ of man is particularly sensitive to certain odors, 

 it in general is much less sensitive than that of many other mammals. The 

 organ of smell is, in fact, much more important for the whole life of the 

 lower animals, because it is mainly by this sense that they find and select 

 their food. Among civilized men this sense plays but a small part for such 

 purposes, although it is stated that some so-called wild people are possessed 

 of a very highly developed sense of smell. Thus Humboldt relates that the 

 Peruvian Indians can follow the trail by scent with as much accuracy as a 

 hunting dog. Among civilized people the sense of smell serves to test the air 

 inhaled or furnish information as to the nature of the food to be eaten. As 

 a rule it is of no very great service even in this respect for the olfactory sense 

 very soon becomes blunted for a certain odor and then gives us no indication 

 of the presence of harmful substances in the air. This is notably true, for 

 example, of those who live in close, poorly ventilated dwellings. The report 

 made by the sense of smell as to the quality of the food is influenced largely 

 by conventional customs, and these differ according to times and places. The 

 olfactory sensations probably serve us most by arousing and promoting the 

 desire for food. 



It has long been known that only the uppermost part of the mucous mem- 

 brane lining the nasal passages is provided with the olfactory epithelium. 

 The investigations of v. Brunn have shown that the region actually supplied 

 with olfactory nerves extends over only a relatively small part of the superior 

 turbinated bone and the opposite face of the nasal septum. The epithelium 

 here covers an area somewhat smaller than a ten-cent piece each side of the 

 olfactory cleft, situated, therefore, in the roof of the nasal cavity as far 

 removed as possible from the external nares. 



Unless the act of inspiration is modified so as to carry the air directly 

 to the upper part of the nose the air current never goes higher than the 

 anterior lower edge of the superior turbinated bone (Franke) and conse- 

 quently does not pass over the olfactory region. Since, however, we experi- 

 ence olfactory sensations in ordinary respiration and since we have convincing 

 evidence that the normal excitation of the olfactory organ takes place by 

 means of material particles in the air (see below) we must suppose that the 

 odoriferous particles reach the olfactory cleft by diffusion. 



Since the nose is always in open communication with the throat, odoriferous 

 substances can of course always pass thence into the nose and so reach the 

 olfactory cleft. This is what happens when we eat. While a morsel of food is 

 being* masticated vapors pass into the nasopharynx and are then carried upward 

 into the olfactory region by the expired air. In swallowing the nasal cavity is 

 closed off from the throat but immediately afterwards communication is reestab- 

 lished and the following expiration carries the vapor of substances moistening 

 the wall of the pharynx into the nose. It is at this moment, but not so long 

 as the fluid remains in the mouth, that one " tastes " the aroma or the bouquet 

 of drinks. 



It was thought for a long time that the olfactory organ is stimulated by 

 vibrations of the odorous substances, and that the organ of smell was, there- 



