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thin membrane which closes the fenestra ovalis, or when that which 

 closes the fenestra rotunda is destroyed, deafness takes place in conse- 

 quence of the escape of the fluid which fills the cavities in which the au- 

 ditory nerve is distributed. 



The existence of this fluid appdltafc essential to the mechanism of hear- 

 ing, either from its keeping thenervfte in the soft and moist state required 

 for the purpose of sensation, or from its transmitting to them, the undu- 

 latory motion with which it is agitated. 



The deafness of old people, which, according to authors, depends on 

 the impaired sensibility of the nerves, whose excitability has been ex- 

 hausted by impressions too frequently repeated, appears sometimes to be 

 occasioned by a deficiency of this humour, and by the want of moisture 

 in the internal cavities of the ear. During the severe winter of 1798, 

 Professor Pinel opened, at the Hospital of Saltpetriere, the skulls of se- 

 veral women who died at a very advanced age, and who had been deaf 

 for several years. The cavities of the internal ear were found quite empty; 

 they contained an icicle in younger subjects who had possessed the power 

 of hearing. 



Deafness may, likewise, be produced by a palsy of the portio mollis of 

 the seventh pair, or by a morbid condition of the part of the brain from 

 which this nerve arises. The mechanical explanation applied by Willis 

 to the anomalous affections of the organ of hearing, is inadmissible, those 

 in which that organ is sensible only to the impression of weak or strong- 

 sounds acting together or separately. 



This author relates the case of a woman who could not hear, unless a 

 great noise was made near her, either by beating a drum or by ringing 

 a bell; because, says he, under such circumstances, these loud noises de- 

 termine in the membrana tympani, which he supposes in a state of re- 

 laxation, the degree of tension required to enable it to vibrate under the 

 impression of weaker sounds. This membrane, to prevent greater re- 

 sistance, must be put on the stretch by the internal muscle of the malleus, 

 or by its own contraction. The total absence of muscular fibres in the 

 membrana tympani, in man, renders very doubtful this spontaneous con- 

 traction. Mr. Home, however, has just ascertained that the membrana 

 tympani of the elephant is muscular and contractile. Admitting all these 

 suppositions, we only substitute one difficulty for another, and it remains 

 to be shown, why the more powerful sounds merely increase the tone of 

 membrana tympani; why they do not become objects of perception of 

 the organ of hearing, though they might be expected to render us insen- 

 sible to the perception of weaker sounds. 



CXXIV. Of odours. Chemists have long thought that the odoriferous 

 part of bodies formed a peculiar principle, distinct from all the other 

 substances entering into their composition; they gave it the name of 

 aroma; Fourcroy, however, has clearly shown, that this pretended ele- 

 ment consisted merely of minute particles of bodies, detached by heat 

 and dissolved in the atmosphere, which becomes loaded with them and 

 conveys them to the olfactory organs. According to this theory, all bo- 

 dies are odoriferous, since caloric may sublimate some of the particles of 

 those which are least volatile. Linnaeus and Lorry had endeavoured to 

 class odours, according to the sensations which they produce*; Four- 



* L-innzeus admits seven classes of odours; 1st class, ambrosiac odours, those of the rose 

 and cf musk belong- to this class, they are characterised by their tenacity ; 2d, fragrant , 



