The stimulus that provokes the sense of smell is caused by certain substances, in solution, coming into contact with special cells in the nose. Such substances usually enter the nose in the form of vapor. While this sense has not in man the importance which it has in lower animals, it has more than a mere esthetic value, as what we call flavor in food is really a compound of taste and smell, and flavor in food is an important aid to its digestion. Moreover, the sense of smell gives warning of certain dangerous vapors and of putrefaction in food.
There would appear to be certain primary, or elementary, smells, out of which others are compounded, just as happens with colors. The sense of smell is easily fatigued, if one smells a perfume sufficiently long the sense for it is lost, although something else may be detected readily.
Pungent substances like ammonia and strong acetic acid are not smells so much as ordinary sensation, and stimulate the ordinary sensory nerve endings in the nose rather than those of the olfactory nerves. Such substances should not be used when testing the sense of smell. What is often used for this purpose is a solution of camphor, contained in test-tubes in progressive degrees of dilution, the patient being asked to identify first the most dilute.
The loss of smell is described as anosmia and may be due to many causes. The most frequent one is nasal obstruction, and smell may be restored if the obstruction is removed.
Atrophic changes in the lining of the nose also interfere with smell, and a victim of ozena is himself unconscious of the foul stench emitted from his nose. Another cause is the action of tobacco or some other poison on the olfactory nerves.
Loss of smell may also be a consequence of injuries to the brain, or of tumors or other diseases there.
A tumor towards the front of the temporal lobe may give rise to a subjective smell of some kind, but in many mental disorders there is a hallucination of some smell, with no obvious changes in the brain or elsewhere to account for it. See: Nose..