SMELL. 



701 



by irritating vapours. They do not imme- 

 diately perceive these vapours, however, but 

 seem indifferent to them at first, find 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 farther 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 ollac- 

 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 wil/tin 

 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 bv 

 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. 



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- 



