54-2 SMELL. 



" No sensations can be remembered in so lively a manner as 

 those which are recalled by peculiar odours." k 



Haller mentions that less than the two billionth part of a grain 

 of camphor has been distinctly odorous. l 



The causes of the sensation of smelling are, as yet, unknown, 

 and in the absence of positive knowledge on this subject philo- 

 sophers have either avowed their ignorance or contented them- 

 selves with hypotheses destitute of proof. Among the opinions 

 respecting these recondite phenomena which have at various 

 times been advanced, three may merit our consideration. The 

 advocates of the first designate by spiritus rector, or aroma, a 

 principle independent of the substances which contain it, very 

 volatile and expansible, imponderable, and imperceptible to every 

 sense excepting that of smell : and to the various modifications of 

 this immaterial substance they attribute the varieties of odour. 

 The second, and most generally received theory, is that odours 

 are particles which evaporate from the odorous substance itself, 

 and that the cause of the sensation of smell is therefore inherent 

 in, and inseparable from, the odorous body. The third opinion, 

 which is maintained by Professor Walther, is, that olfaction is 

 independent of the emanations of material particles and is a simple 

 dynamic action of the odorous body upon the organs of smelling, 

 similar to the action of sound on the hearing. 



However this may be, odours, to become objects of sensation, 

 must pass the pituitary expansion of the olfactory nerve during 

 the respiratory process. When the breath is held, the most 

 odorous substances may be spread in the interior of the nostrils 

 without their perfume being perceived ; this observation was first 

 made by Galen. It has been frequently remarked that odours are 

 smelt only during inspiration, the same air when returned through 

 the nostrils always proving inodorous. But this is true only when 

 the odour has been admitted from without by the nostrils, for, 

 when it is admitted by the mouth, as in combination with 

 articles of nutrition, it can be perceived only during expir- 

 ation. A proof of this may be readily obtained by placing 

 the open neck of a small phial, containing an essential oil, in 



* " Respecting the power of smell over morals and propensities, consult Benj. 

 Rush, Medical Inquiries and Observations, vol. H. p. 34." 

 1 El Pkysiol., vol. v. p. 157. 



