In the life of most of the lower mammals, olfaction – or the sense of smell – plays a very important role in providing information about the “outside” world, particularly as regards sources of food and the presence of danger. Sharks and dogs, for example, have such head structures in which large portions of certain specific parts (the brain in sharks and the nasal cavity in the snout in dogs) appear to be dedicated to reactions based upon olfaction.
In human life, however, olfaction is the least among all the body senses in terms of supplying definitive information, especially since there are very few reflexes that are built around it. Compared to the other human body senses – vision, hearing, taste, and touch – olfaction involves the least complex receptor structures.
There is about three-fourths square inch of olfactory epithelium (a membranous tissue) in each of the two nasal cavities in man, which consists of layers of ciliated cells. These ciliated cells act as the end organs of nerve fibers which penetrate the skull and pass back to the brain as the olfactory nerve. This olfactory nerve, while made up of a series of neurons, is not entirely a true nerve; rather, it is in part a fiber tract of the brain.
The olfactory epithelium is soaked in fluid, and the solution of the chemicals in this fluid prompts stimulation. In the upper portion of the nasal cavity lie the moist receptors where they do not get in the way of the flow of inhaled and exhaled air. Such structure is necessary to prevent drying of the surface; it is also important in encouraging sniffing to bring the greatest possible concentration of chemicals in the air into contact with the sensitive cells.
It is quite easy to classify the quality of objects involved where the four other human body senses are concerned: the sense of vision is able to distinguish between colors, shapes, and sizes; the sense of hearing is able to discriminate between soft and loud sounds, or between music and noise; the sense of taste is able to discern which food is sweet, salty, sour, or bitter; and the sense of touch is able to recognize between hot and cold, or between rough and smooth.
But it is entirely different in the case of olfaction where odors are rather difficult to classify. Of course, olfaction is able to differentiate between a pleasant smell and an unpleasant one. Still, there is a need to refer to the name of a specific thing, object, or substance when describing the quality of a scent, as when we say “let me take another sniff at that thing, it has the pleasant smell of a rose,” or “keep that thing away from me, it’s got the stench of a rotten egg.”
Also in olfaction, experiencing the intenseness of an odor is subjective. For instance, the prevailing odor in a room usually goes completely unnoticed to a person staying in that room, while the same is often quite discernible to someone just entering the room. This example suggests that olfaction involves several different individual odor receptors that make it possible for a person to get used to one odor while remaining sensitive to another.