Smell

SMELL The Olfactory Organ.—The organ of smell in man is located in the nose. The delicate membranous lining of the uppermost part of the nasal chamber is different in colour from that of the rest of the chamber and is the true olfactory organ. The area covered by this organ in each nasal chamber is approximately one square inch. From this membrane nerve fibres gather to gether, thus constituting the first or olfactory nerve, and make their way directly into the brain. Each nerve fibre comes from a sense cell which forms a part of the olfactory organ and all such cells reach through this organ to its outer or exposed face. Here each cell terminates in six to eight relatively long fila ments, the so-called olfactory hairs which float in the thin layer of aqueous mucus that covers the olfactory surface. This mucus is in direct contact with the air of the nasal cavity in which is carried the odorous particles.

In ordinary breathing the respiratory air passes in and out through the lower part of the nasal chamber without passing up into the region of the olfactory organ. Hence under such cir cumstances we are not as a rule cognizant of odours. If, however, our suspicions are aroused we ordinarily sniff, an operation that draws the passing air into the upper part of the nasal chamber and thus brings it into contact with the olfactory surface. Here the odorous particles may be caught on the moist mucous surface to make their way ultimately to the olfactory hairs.

Two Kinds of Response.

As may be inferred from this ac count the lower portion of the nasal chamber is respiratory and only its uppermost part is olfactory. Nevertheless the whole surface of the chamber is in a way sensitive, for the fifth or tri geminal nerve is distributed throughout it. This nerve, once sup posed to have to do with smell, is now definitely known to be concerned with irritants. If the olfactory nerve in a dog is des troyed, the animal will still sneeze when vapour of ammonia enters its nostrils. The same is true of human beings who through disease have lost the olfactory nerve but who still retain the tri geminal nerve. Many persons who fail to recognize the more delicate odours of flowers, food, and the like are nevertheless readily cognizant of even minute quantities of chlorine, ammonia and other like substances. These materials are not odorous ma

terials in the proper sense of the word but are irritants and affect the terminals of the fifth nerve, not those of the first nerve. True smell has to do with relatively delicate perfumes and odours and is mediated by the first nerve in contrast with irritants which stimulate the fifth nerve. It is therefore of considerable im portance in the study of olfaction to distinguish between these two nervous activities.

No one has succeeded in classifying odours with anything like the precision that has been attained for tastes.

Exceeding Delicacy of Olfactory

sensa tions are called forth by an almost immeasurably small amount of material. A musk bean can give out the odour of musk for years without appreciably changing in weight. One of the most strongly odorous substances known is mercaptan, a liquid with the pene trating smell of garlic. When this liquid is evaporated in the atmosphere so that 5o cubic centimetres of air, enough for a sin gle whiff, contain only 4 6 0,0 0 0.0 0 0 of a milligram of mercaptan, the presence of this material can still be detected by smell. Al though mercaptan from the standpoint of olfaction is an excep tionally effective agent, it is nevertheless true that in all other instances the actual amount of a given substance needed to excite smell is almost unbelievably small.

In consequence of this immense efficiency of the olfactory sense animals rely upon it in many of their most delicate re sponses. See SMELL AND TASTE, SENSES OF, below.

Material Basis of

artistry of smell finds its ex pression in perfumes. From time immemorial man has supple mented the natural odours of his body by numerous extractives mostly from the stores of nature. These perfumes as a rule in crease in delicacy with the advancement of civilization. They are used either as a means of counteracting innate objectionable odours or of adding attractiveness to the individual carrier Odours have also played an important part in the refinements of life as seen in the aroma of good tobacco and the bouquet of fine wine.