Smell

It has been supposed by some investigators that smell was in duced not by material particles from the odorous substances but by exceptional and peculiar emanations from these sources. Evi dence on the whole does not favour this view, but it seems more probable that what excites smell are the material particles them selves from the odorous substance. Even in very high dilutions enough of these particles seem always to be present to account for smell. Thus in the 4 6 0,00 0,0 0 0 of a milligram of mercaptan, the amount necessary for a single whiff of this substance, there are estimated to be not fewer than 200,000,000,00o molecules of the substance concerned. It is these molecules dissolved in the mucous covering of the olfactory organ that attack the olfactory hairs and thus initiate nervous changes leading to the sensation of smell. Smell, like taste, is a chemical sense in that odorous materials in solution activate the living olfactory cells.

Difference Between Smell and

smell as well as taste is a chemical sense, why are these two senses so different? This is by no means an easy question, but perhaps the most ready answer to it can be found in the peculiarities of the stimulating materia:s. Most substances that we smell we do not taste and most substances that we taste we do not smell. Thus sugar has a sweet taste but no odour. If we mince onion and, holding the nose, place it in the mouth we experience a sweetish taste indis tinguishable from that of minced sweet apple. Apple and onion both contain sugar which has a sweet taste but no odour. The characteristic odours of onion and of apple are smells which do not affect taste. This is true of many pure substances ; they are either tasted or smelled but not both. There are substances, however, that have both taste and smell. Ethyl alcohol has a

sweetish taste and a characteristic ethereal odour. Again sub stances that we taste are predominantly soluble in water ; those that we smell are more generally soluble in oil, in fact the essen tial oils are among the most characteristic sources of odours. But here, too, the distinction is not without exceptions.

Perhaps the most important difference between the two senses is a quantitative one; we taste only relatively strong solutions; we smell very dilute ones. This can best be demonstrated with substances that have both taste and smell. Thus ethyl alcohol can just be tasted at a concentration that is 24,000 times greater than that at which it can just be smelled, and this relation is true of most other such combinations. Quantitatively we smell extremely minute amounts of material; we taste only relatively large amounts.

As a consequence of this quantitative difference between taste and smell these two senses are commonly used in very unlike ways. In taste the stimulating material is in the mouth and we ordinarily locate it there. Again, in smell the stimulating ma terial is likewise in the cavity harbouring the sense organ but we do not think of it as there; we project it into the exterior to the object from which the odorous particles come. In this respect smell is like sight ; we do not see things in our eyes but in the exterior. Taste and other like senses are therefore called inner senses and their organs interoceptors, while the organs of smell are classed as distance sense organs or exteroceptors. What is true of the sense organs of man in this respect is true of those in other animals. These lower creatures react to taste as an affair of the mouth and to smell as something in the external environment.