THE PHYSIOLOGY OF THE OLFACTORY SENSE On the physiology of the olfactory sense in invertebrates, we are directed chiefly by the works of von Frisch, which deal with the olfactory sense of bees. He trained the bees to a particular odour, and, by this means, was able to establish the fact that they distinguish oil of orange-peel with certainty from 43 other ethereal oils. It may be concluded from this that, in nature, they are largely directed by the scent of the honey-bearing flowers. The most remarkable point is the extraordinary similarity shown by the olfactory sense of the bee to that of man. Substances which smell alike to man and are confused by him one with an other, bees, in general, also are unable to distinguish.
We must conclude from this that the similarity or difference of the odours is conditioned by the particular character of the odorous molecule. According to Henning's hypothesis the quality of an odour depends not so much on the chemical composition of the odorous molecule as on the intramolecular combination of the groups of atoms, and, therefore, for example, mankind and bees confuse nitrobenzol with oil of bitter almonds.
Para- and meta-cresolmethyl ether, which are alike in their chemical composition, have a different kind of atomic structure, and are clearly distinguished by both man and bees. On the basis of this hypothesis it may, perhaps, be possible later to give an exact classification of the odorous substances for the whole ani mal kingdom.
At present, we do know very little about the nature of the chemical substances to which the other invertebrates react. Spiegel showed that for the shrimp, Crangon, the water-soluble constituents of fish muscle have a far greater attraction than those which are ether-soluble, which to the human nose are much the stronger. Particularly remarkable is the surely established fact that Crangon can smell glycogen, although, to human beings, this substance has neither taste nor smell.