As to the function of the olfactory pigment, Ogle remarks first of all that odours are absorbed more readily by dark than by light materials.
Pigment is also present in the labyrinth of the ear as well as in the eye, and its presence in these organs seems to be essential to their activity.
It is to be noted that the pigment does not occur on the nerve structure in any of those end-organs, but external, though contiguous to it. In the eye, it lies in contact with the rods and cones of the retina; in the nose, with the olfactory hairs; in the ear, with the terminal bodies of the auditory nerve.
Hence the pigment, he supposes, must be associated with the reception of the sensory impressions.
In the eye and the ear those impressions are undulatory in character. That being so, he holds that the undulatory theory of olfaction also is probably the correct one.
Ogle finishes with the remark that the theory would be strengthened if it could be shown that pigment was specially suited for the absorption and modification of undulations.
It is interesting to us to learn that claims are now being made that pigment does possess the power necessitated by Ogle’s theory. At all events, there is a theory of vision (Castelli’s) which claims for the ocular pigment the power of absorbing and modifying light waves, and Heyninx holds that the olfactory pigment possesses a similar property.
Summing the whole matter up, then, we may say that the undulatory theory of olfaction is, that an odorivector gives off in the form of vapour (in the aerial medium) extremely attenuated portions of its substance, too minute to be weighed, and that this vapour, disseminated through the air, enters the nose in respiration, and, being wafted up into the olfactory region, is received by the mucus bathing the olfactory hairs, where, in virtue of the ultra-violet radiations which proceed from its molecules and are modified by the olfactory pigment, it acts on the hairs, setting up changes (it may be also undulatory in nature) in them and in their cells, which changes are transmitted thence by the olfactory nerves to the neurones or nerve-cells of the olfactory bulb (or lobe) of the brain.
The undulatory theory of olfaction, then, as will be evident to the reader, has a good deal in its favour. And in addition to what we have already said of it as accounting for the absorption by odorous vapours of ultra-violet rays, and as giving a hint regarding the function of pigment in the olfactory area, there are also a number of other phenomena which it seems to explain. We have seen, for example, how one odorivector, such as musk or civet, may have the property of enhancing the power of another, and this is a property which is characteristic also of certain luminous conditions (fluorescence, lumino-luminescence).
Again, there is a harmony existing between certain of the manufacturers’ primitive odours; “they go well together,” and are employed for that reason in the art of perfumery. This resembles the harmony existing in another class of undulations, the sound waves.