SENSE OF HEARING. 265 



communication, to the ear of the auditor, of the sonorous vibrations of the 

 contained air ; which are thus transmitted directly through the atmosphere, 

 instead of being weakened by transmission through the walls of the instru- 

 ment. Hence there is no real analogy in the two cases. The principal object 

 of the Eustachian tube (which is always found where there is a tympanic 

 cavity) seems to be the maintenance of the equilibrium between the air within 

 the tympanum and the external air ; so as to prevent inordinate tension of the 

 membrana tympani, which would be produced by too great or too little pres- 

 sure on either side, and the effect of which would be imperfection of hearing. 

 It also has the office of conveying away mucus secreted in the cavity of the 

 tympanum, by means of cilia vibrating on its lining membrane ; and the deaf- 

 ness, consequent on occlusion of this tube, is in part explicable by the accu- 

 mulation which will then take place in the tympanum. 



858. From what has been stated, it is evident that sonorous undulations 

 taking place in the air, will be propagated to the fluid contained in the laby- 

 rinth, through the tympanum, the chain of bones, and' the membrane of the 

 fenestra ovalis to which the stapes is attached, without any loss, but rather 

 an increase of intensity. Why water should be chosen as the medium through 

 which the impression is to be made upon the nerve, it is impossible for us to 

 say with any thing like certainty, in our present state of ignorance as to the 

 physical character of that impression. But, the problem being to communi- 

 cate to water the sonorous undulations of air, the experimental results already 

 detailed satisfactorily prove, that, whilst this may be accomplished, in a 

 degree sufficient for the wants of the inferior animals, by the simple interposi- 

 tion of a tense membrane between the air and the fluid, the tympanic appa- 



[Fig. 57. 



A view of the labyrinth of the Left Side, laid open in its whole extent so as to show its Structure; 

 these figures are all magnified ; 1, the thickness of the outer covering of the cochlea; 2, 2, the scala vesti- 

 buli or upper layer of the lamina spiralis; 3, 3, the scala tympani or lower layer of the lamina spiralis; 

 4, the hamulus cochlese ; 5, centre of the infundibulum ; 6, the foramen rotundum communicating with the 

 tympanum; 7, the thickness of the outer layer of the vestibule; 8, the foramen rotundum; 9, the fenestra 

 ovalis; 10, the orifice of the aqueduct of the vestibule ; 11, the inferior semicircular canal; 12, the superior 

 semicircular canal; 13, the external semicircular canal; 14, the ampulla of the inferior canal; 15, the am- 

 pulla of the superior canal ; 16, the common orifice of the superior and inferior canals ; 17, the ampulla of 

 the external canal.] 



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