Finally, we have the question of the olfactory pigment to consider, and in this matter we cannot do better than follow the exposition of William Ogle, an English physician who wrote as long ago as 1870. As will be seen, he forestalls the modern undulatory theory of olfaction in a remarkable manner.
Ogle contends that the presence of pigment must be of great importance in the function for the following reasons:
First, the epithelium of the olfactory region is pigmented, while that of the rest of the nasal chamber and sinuses is devoid of colouring matter.
Secondly, there seems to be some correspondence between the degree of pigmentation and the acuteness of smell, as the following facts suggest:—
In macrosmatic animals, such as the dog, cat, fox, sheep, and rabbit, pigmentation extends over a larger space and is darker in tint than in man. In these animals also the mucus covering the olfactory area of the nose is itself pigmented.
We have seen that human albinos are anosmic, and the same is probably true of animal albinos. But care is necessary in making observations on suspected albinos in animals, as even when they are altogether white a certain amount of black pigment remains about the face and nose.
The following reports, however, would lead us to conclude that as with man, so with the animals, a relative deficiency of pigment is associated with a dull olfactory sense.
It is by smell that the herbivora detect and avoid plants which are poisonous, and when poisoning does occur, it is usually a white animal that suffers. In some parts of Virginia the farmers will only rear black pigs, because, they say, the white ones eat and are poisoned by the roots of Lachtanthus tinctoria. For the same reason in the Tarentino only black sheep are reared.
Thirdly, the dark-skinned human races have a keener sense of smell than the lighter races.
Fourthly, the sense grows more acute as we get older, as we have already seen, and nasal pigmentation, it is said, also increases with age.