Physiology of Smell

The stimuli that excite the olfactory cells are certain chemical substances termed odoriferous. They must be presented in a gaseous or finely-divided form, and they reach the olfactory region in the inspired air or by diffusion. They are then dissolved in the fluid that bathes the olfactory cells, and act as chemical stimuli.

That liquids cannot act as olfactory stimuli unless they are volatile is shown by the fact that if the nasal cavities be filled with an odourous fluid such as rose-water no smell is perceived. At the same time, unless there is sufficient moisture round the hair cells to dissolve the odourous substances, the sense of smell is greatly impaired, as is evidenced in cases of dryness of the lining membrane of the interior of the nose.

The sense of smell is very delicate in man but much more so in many animals. The human olfactory organ can, for example, recognise camphor in a dilution of 1/400000, musk in a dilution of 1/800000, etc. The organ is, however, quickly fatigued for any particular odour, especially if it be a faint one; this, however, does not affect its sensitiveness to other odours.

Several attempts have been made to classify odours, but all are unsatisfactory. The best is that of Zwaardemaker, viz. :

I. Pure odours.
II. Odours mixed with sensations of common sensibility.
III. Odours mixed or confused with tastes

Pure odours are further divided into:

1. Ethereal.
2. Aromatic.
3. Fragrant.
4. Ambrosial.
5. Garlic.
6. Burning.
7. Goat.
8. Repulsive.
9. Nauseating or Foetid.

When a mixture of odours is submitted to the olfactory organ the separate odours can generally be recognised. In some cases, however, the combination of odours gives no reaction; thus if iodoform and Peruvian balsam be mixed in suitable quantities the mixture is odourless, or, alternatively, if iodoform be presented to one nostril and Peruvian balsam to the other no sensation of smell may be perceived.