226 



accordingly endeavours to establish, by arguments derived from the 

 structure of tbe ear, tbat aerial rarefactions are alone capable of 

 stimulating that organ in man. These arguments are briefly as 

 follows : 



1 . The tympanal membrane being convex inwards, a condensation 

 could only affect the air in the tympanal cavity by stretching the 

 membrane, which would cause an expenditure of force ; whereas a 

 rarefaction would produce the effect by a simple flexure of the 

 membrane. 



2. The sense of hearing being certainly produced by the motion 

 of the fluid in the labyrinth, which is a closed vessel filled with an 

 incompressible fluid, the requisite motion could not be produced by 

 a compression of the atmosphere. 



3. The disposition of the muscles of the ear is such as is calculated 

 to assist and regulate the impressions produced by rarefactions rather 

 than those produced by condensations. 



4. The existence of the Eustachian tube is indispensable to the 

 action of the organ (when all its other parts are in a normal state), 

 on the supposition that sound is occasioned by rarefaction ; whereas 

 its uses are not satisfactorily predicable on the contrary hypothesis. 



The author observes, that if rarefactions alone produce sound, it 

 follows that a simple contraction of the muscles of the ear will ren- 

 der sounds inaudible. It follows also, on the same hypothesis, that 

 a more delicate exercise of the same muscles will render the organ 

 minutely susceptible to the influence of certain sounds, to the exclu- 

 sion of others. It is urged also, that, admitting the action of these 

 muscles to be to a large extent involuntary, there can be no doubt 

 that by practice a great degree of command may be acquired over 

 them. The author conceives that we may in this way account for 

 the facility acquired by many persons of reading and writing, and of 

 carrying on intricate trains of thought, without being disturbed by, 

 or being conscious of, the noises around them. He conceives also 

 that the same mode of explanation may be applied to account for the 

 power of appreciating and analysing the most complex harmonies 

 possessed by persons having a fine musical ear ; which the author 

 considers to be as certainly the result of specific mental and muscular 

 training, as the faculty of vocalization, or the art of playing on a 

 keyed instrument. 



