It is, of course, an essential condition of the exercise of this sense, that the whole nervous apparatus, which forms the essential part of its organ, should be in a state of integrity ; and that a free circulation of blood shall take place through the olfactive portion of the pituitary membrane. But. in addition, it is requisite that the epithelial and mucous covering of the membrane be in a normal state. If the surface be too dry, the odorous particles cannot undergo that solution in the fluid in contact with the sentient extremities of the nerves, which seems necessary for the production of an impression on them. On the other hand, when the secretion is too abundant, it interferes with its contact in the opposite manner. And thus it happens that the sense of smell is blunted, both in the primary and secondary stages of an ordinary cold, by the disorder of the secreting surface, indepen dently of the effect which the disturbance of the circulation may have upon the functional power of the olfactive nerve.
Purposes of the sense.—When we take a comprehensive survey of the animal kingdom, we at once perceive that the most general, and therefore the most essential purpose of the sense of smell, is to make known the pre sence of food, to indicate its direction and thus to guide the animal towards it, and to aid in the discrimination of its qualities. We always find the olfactive organ placed in the neighbourhood of the mouth ; its connection with the respiratory apparatus is by no means so constant. In air-breathing vertebrata, whose olfactive cavity opens into the pharynx, the sense of smell largely participates in that of taste (see TASTE), being the means by which we take cognisance of the flavours of sapid bodies introduced into the mouth. Of the importance of this sense in directing ani mals to their food, it is needless to multiply instances ; but we may remark that, from ob servation of the actions of the human infant, we are well convinced that it is rendered cognisant by smell of the neighbourhood of its nurse, long before it recognises her by sight, and that this sense is its guide in seek ing the source of its nutriment. How purely instinctive this action is, — that is, how com pletely independent of all experience, and en tirely dependent upon the provocative sens ation,—is well shown by the .experiment of Galen, who placed a kicl, just dropped, near three vessels, one filled with milk, another with honey, and another with wine ; after smelling at all three, it presently began to drink the milk. It would seem to be hy the information conveyed through their smell, that bees are induced to fly to pastures at a great distance from their hive ; and it would not seem improbable that the sense of direc tion, which is so remarkably displayed by many animals, is the result of the acuteness of their olfactive power. Whilst the chief use of
smell to the carnivorous tribes is to guide them to their prey, the herbivorous races, whose food is constantly within their reach, are warned by its means of the neighbourhood of their enemies. The sense of smell is sub servient to defence in another way; being the means by which the fcetid scents, emitted by many animals under the influence of alarm, deter their enemies from further pursuit. In nearly all animals, the sexual secretions are more or less odorous ; and these would seern to be intended, not merely to contribute to make the sexes aware of each other's prox imity, through the sense of smell, but also, in many instances, to serve as a provocative to sexual desire. The odours which are attrac tive to animals are usually related either to their food or their sexual instinct ; but there are cases in which animals seem to delight in odours which have no such relation : thus, cats seem to revel, as it were, in the odour of Nepeta (catmint) or Valerian.
In the air-breathing vertebrata, the sense of smell is, as it were, the sentinel of the respi ratory organs, having for its office to take cognisance of the aeriform fluids which enter them, and to give warning of such as are in jurious. The contact of irritating matters, however, is perceived (as already stated) through the general sense of feeling, not the special sense of smelling; and it is through the fifth pair that the act of sneezing is ex cited, the purpose of which is to expel such particles from the nasal cavity. The distinc tion is well seen in some air-breathing inverte brata, whose organ of smell is seated in the head, whilst the impression of irritants on the respiratory surface, exciting reflex movements for the purpose of avoiding or expelling them, is made through the stigmata. Thus M. Duges relates* that if the stigmata on one side of a decapitated Scolopendra be exposed to an irritating.vapour, the body will be imme diately flexed in the opposite direction ; arid that if the stigmata on the opposite side be then similarly irritated, a contrary movement will occur ; whilst by exposing the anterior stigmata on one side, and the posterior on the other, to the same irritation, the body will be bent into the form of the letter S.