the Mode in which it administers to the Perception of Sound. 121 



The human car may be divided into three principal regions, viz. 



(1) The external ear, of which the only portion which here 

 concerns us is the meatus externus terminating in the tympanal 

 membrane. 



(2) The tympanal cavity, which in the normal state is kept 

 filled with air through the intervention of the Eustachian tube 

 communicating with the throat ; which tube is considered to be 

 ordinarily closed, and from time to time opened, during the act 

 of deglutition. 



(3) The internal ear or labyrinth, consisting of a chamber or 

 system of mutually communicating chambers enclosed in the solid 

 bone of the skull. 



Omitting details unnecessary for our present purpose, the la- 

 byrinth may be described as filled with a liquid in which are 

 immersed the nerves through whose agitation the sensation of 

 hearing is produced. 



The fluid in the labyrinth is everywhere surrounded by the 

 solid bone, with the following exceptions : — 



(a) Two small apertures, denominated respectively fenestra 

 ovalis and fenestra rotunda, where in place of the bone as a boun- 

 dary are substituted membranes, by which the labyrinth is sepa- 

 rated from the tympanal cavity, and by which the liquid in the 

 former is prevented from flowing into the latter. 



(b) Certain foramina or (so-called) aqueducts, through which 

 the nerves with their attendant blood-vessels which supply the 

 labyrinth communicate with the general nervous and circulating 

 systems. 



The sensation of hearing may be occasioned by means of vi- 

 brations transmitted through the bone of the skull to the laby- 

 rinth ; but all articulate sounds, and in general all sounds which 

 are conveyed by the air, are transmitted to the labyrinth through 

 the two fenestra (ovalis and rotunda) above spoken of. 



When the ear is in its normal state (that is, when the tympa- 

 num is perfect), all aereally conveyed sounds become incident on 

 the tympanal membrane in the first instance, and are thence 

 transmitted to one or both of the tympanal fenestra by a machi- 

 nery or agency which will be described hereafter. But the agi- 

 tation of the tympanal membrane is a sine qud non as regards the 

 transmission to the sensitive system of articulate or other aereally 

 conveyed sounds. 



And here it may be observed that if the human tympanum 

 were, as its name implies, a drum (that is, a stretched flat mem- 

 brane whose movements are restrained solely by the circular 

 frame upon which it is fixed), no such simultaneous transmission 

 of waves of rarefaction and suppression of waves of condensation 

 as has above been spoken of could possibly take place. 



