The nerves of the nose are the first pair or olfactory which. are .specially connected with the sense of'smell, branches of the fifth pairWhich confer ordinary sensibililty on its skin and mucous membrane, and motor filaments, from the facial nerves to the nasal muscles. The olfactory nerve on each side is connected with the inferior surface of the brain (q.v.) by an external, a middle, and an internal root, which unite and form a flat band (or, more correctly, a prism), which, on reaching the cribriforin plate of the etlimoid bone, expands into an oblong mass of grayish-white-substance, the olfactory &nlb. From the lower surface of this bulb are given off the olfactory filaments, fifteen or twenty in number, which pass through the cibriform foramina, and are distributed to the mucous membrane of the olfactory region. These filaments differ essentially from the ordinary cerebral nerves. They contain no white substance of Schwann, are not divisi ble into elementary fibulT, and resemble the gelatinous fibers in being nucleated, and of a finely granular texture. The branches of the fifth pair (or trifacial) given to the nose are the nasal nerve (derived from the ophthalmic division), which supplies the skin and MUCOUS membrane in the vicinity of the nostrils, and the naso-palatine nerve (derived front Meckel's ganglion; which is connected with the superior maxillary division), which supplies the mucous membrane on the spongy bones and on the septum. The peculiar sensation that precedes sneezing is an affection of the nasal nerve, and the flow of tears that accompanies a severe fit of sneezing is explained by the common source of this and the lachrymal nerve; while the common sensibility of the nose, generally, is due to the branches of this and of the naso-palatine nerve.
The nature of odorous emanations is so little known, that it is impossible to give a definite account of the mode in which they produce sensory impressions. From the fact that most odorous substances are volatile, and dee versa, it may he presumed that they consist of particles of extreme minuteness dissolved in the air; yet the most delicate experiments have failed to discover any loss of weight in musk, and other strongly odorous substances, after they have been freely evolving their effluvia for several years.
But whatever may be the nature of the odorous matter, it is necessary that it should be transmitted by a respiratory current through the nostrils to the true olfactory region, whose membrane must be in.a healthy condition. If it is too dry, or if there is an inor dinate excretion of fluid from its surface (both of which conditions occur in Catarrh or cold in the head), smell is impaired or lost, in consequence of the necessary penetration of the stimulating odor to the nervous filaments being prevented.
The acuteness of the sense of smell is far greater in many of the lower animals (dogs, for example) than in man, and they employ it in guiding them to their food, in,warning them of approaching danger, and for other purposes. To civilized men its utility is comparatively small; but it is occasionally much increased when other senses are deficient. In the well-known case of James Mitchell, who was deaf and blind from his birth, it was the principal means of distinguishing persons, and enabled him at once to perceive the approach of a stranger. Amongst many savage tribes the sense is almost as acute as in many of the lower mammals. For example, the Peruvian Indians are able, according to Humboldt, to distinguish, in the middle of the night, whether au approach ing stranger is a European, American Indian, or Negro, Although all poisonous gases are not odorous, and all bad odors may not be posi tively deleterious to health, there can be no doubt that one of the principal objects for which the sense of smell is given to us is to enable us to detect atmospheric impurities, many of which are of a most noxious character, and give rise to the most serious forms of fever.