As to the existence or absence of the sense of smell in the lower Invertebrata, nothing can be definitely stated.
Nerve of smell.— That the first pair of cra nial nerves is the true olfactive, and that through it alone are the proper odorous emanations perceived, would seem a legitimate inference from the fact, that its development in vertebrated animals is constantly propor tionate, ceeteris paribus, to the acuteness of the sense ; and that it is chiefly distributed to that part of the nasal cavity, which is most distinguished by the possession of this endow ment. This inference is fully borne out by the facts supplied by experiment and patho logical observation. The division of the olfac tory nerves in animals evidently produces a complete destruction of the power of per ceiving odours ; although they are stAl affected.
by irritating vapours. They do not imme diately perceive these vapours, however, but seem indifferent to them at first, and then suddenly and vehemently avoid them as soon as the Schneiderian membrane becomes irri tated. It was maintained by Magendie that the fifth pair in some way furnishes conditions requisite for the enjoyment of the sense of smell ; this sense being destroyed, according to his assertion, by section of its trunk. His experiments, however, were made with irritating vapours which excite sternutation ; and he inferred the loss of the sense of smell from the absence of the automatic movements which these vapours normally excite. This in ference was altogether unjustifiable ; since the experiments in question afford no proof that the power of perceiving odours, with which the excitement of automatic movements does not appear to be in any way connected, is de stroyed by section of the fifth pair. A dimi nution in the acuteness of the true sense of smell, however, appears to be a usual result of paralysis of the fifth pair ; but this is readily accounted for by the diminution of the nor mal secretion of the pituitary membrane, by which its surface is deprived of the moisture that is necessary for the exercise of its sen sory powers. The difference in the endow ments of the first and fifth pairs of nerves, and the speciality of the former, are further marked by the result of mechanical irritation of their trunks and branches. Such irritation of the first pair excites no muscular movement, either direct or reflex, and it produces no in dication of pain. On the other hand, irrita tion of the nasal branches of the fifth pair is obviously attended with violent pain, and excites various automatic muscular move ments. Lastly, it has been found that in cases of deficiency or loss of the sense of smell, some abnormal condition of the olfac tive nerves or ganglia has existed ; and the same kind of change has been discovered in cases in which subjective sensations (i. e. sen sations not originating in external objects) had existed during life.
Conditions of the exercise of the sense.—The first condition requisite for the exercise of the sense of smell, is the contact of the odorife rous medium with the olfactive surface. This may be favoured or prevented by a variety of circumstances. Thus, odours are more rapidly transmitted by air in motion than by air at rest ; but they only proceed in the direction of the movement : and hence animals pos sessed of the keenest scent, which would be alarmed by the presence of a human or other foe a mile off' on the windward side, may be approached within a short distance on the leeward, when a fresh breeze is stirring. The odoriferous medium must not only be brought to the nose, but it must be introduced within the olfactive cavity. This is usually accom plished by the ordinary movement of inspira tion, which draws a current of air through the nose ; but as the current chiefly passes through the lower part of the nasal cavity, to which the olfactive nerve is very sparingly or not at all distributed, the full use of the sense of smell is not thus gained. It is only by making a series of short and quick inspira tions,—the effect of which seems to be, to empty the whole nasal cavity of the air it pre viously contained, and thus to cause the newly-inspired air to pass forcibly into its upper part, instead of merely streaming through the passage between the anterior and posterior nares,—that we employ our olfactive powers to the best advantage. This move ment, combined with the direction of the nostrils towards the source of the odour, and with the dilatation of their orifices by the muscles adapted for that purpose, constitutes the active exercise of the sense, which may be termed scenting. This bears the same rela tion to ordinary smelling, as feeling bears to touch, listening to hearing, or looking to see ing. The effect of the sensory impression on the mind is further heightened by the atten tion which is bestowed upon it ; and it does not seem improbable that the sensation itself is rendered more acute by an increased deter mination of blood to the olfactive surface when it is being thus actively employed. On the other hand, the use of this sense may be prevented, not merely by the closure of the nares, anterior and posterior, so as completely to exclude the odoriferous medium, but also by simply refraining from drawing air into the nasal cavity. If we breathe through the mouth only, closing the posterior nares by means of the velum palati, we may avoid being affected by odours even of the strongest and most disagreeable kind ; for the nasal cavity being already filled with air, there is no room for the entrance of the odoriferous atmosphere from without ; and it may thus be long before the odorous particles come into contact with the olfactive surface.