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suits ; whilst much of that in the drum enters the membrane, and 

 some of it finds its way along the ossicles, and noises in the head 

 are engendered. 



Now I find that the cutaneous surface of the drum-head admits 

 vibrations from air with very much greater facility than water does, 

 that is, readily ; for on filling the external meatus of the sound ear 

 with water, and then letting it leak out again, I remarked that for more 

 than half an hour afterwards septa of water were constantly forming 

 themselves across the canal and producing much deafness, and then 

 breaking again with a loud noise, and the deafness vanishing. After 

 some evaporation the following instructive effects alone took place : 

 the membrane would attract a film of water over its surface, and 

 deafness ensue ; but on a gust of air plunging into the drum through 

 the Eustachian tube, the membrane springing outwards with a smart 

 smack, would throw off the fluid, and the hearing as instantaneously 

 be restored. This would gradually wane away again by the re- 

 attraction of the water, to be instantaneously regained again, and so 

 on. But since the transition is easy between the membrane's outer 

 surface and air, what has been said above shows that it must be diffi- 

 cult between the inner surface and air, and the statement in (4.) is 

 demonstrated. 



Accordingly the external layer of the membrane is formed of skin, 

 a dry tissue of loose texture, penetrable by air, and coming into in- 

 timate relation with it ; whilst the mucous membrane of the drum 

 is, as it were, unparalleled not only for tenuity, but compactness and 

 high vascularity, though it is barely possible to verify the presence of 

 mucous exudation upon it, affording a glassy surface which is a for- 

 midable barrier to the passage of vibrations from it to air, and vice 

 versa-, and this is so reflected, that the membrane and ossicles lead- 

 ing to the labyrinth lie without it, confining useful vibrations to 

 their destined path, and excluding hurtful ones from it ; and the 

 mastoid cells help to further stifle such vibrations as by any accident 

 intrude upon the air in the drum. The membrane of the fenestra 

 rotunda, by its elasticity, protects the acoustic nerve from undue 

 compression, &c. The membrana tympani avails acoustically by its 

 area, whilst its flexibility, the joints in the ossicular chain, &c., are 

 mere machinery for conveying, under all contingencies, vibrations to 

 the fenestra ovalis, and provision against mechanical accidents. The 



