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as soon as the stapedius muscle is left quiet. A second action of 

 the stapedius muscle is to act as a laxator of the memhrana tympani, 

 and it thus appears that the stapedius muscle is the antagonist of 

 the tensor tympani ; and it seems to be brought into action during the 

 act of listening, while the tensor tympani, on the contrary, contracts 

 when the ear has to be protected from any loud vibrations. 



The next part of the paper is devoted to the consideration of the 

 functions of the memhrana tympani ; which, besides the one usually 

 ascribed to it, viz. of receiving the sonorous vibrations from the air 

 and of conducting them to the chain of bones and thence to the 

 labyrinth, the author considers to be as follows : — 



1. To act in conjunction with the ossicles and muscles of the 

 tympanum as the analogue of the iris in the eye, and to exclude 

 from reception by the labyrinth, of such strong vibrations as would 

 be injurious to its integrity ; also, in exactly opposite circumstances, 

 to receive the most faint undulations, which would not be perceived 

 unless the memhrana tympani were rendered less tense than is the 

 case in ordinary circumstances. The former of the two duties is 

 performed by the tensor tympani muscle, the latter by the stapedius. 

 The memhrana tympani is not only of use in preventing powerful 

 sonorous vibrations from compressing too forcibly the expansion of 

 the auditory nerve, but it also protects the labyrinth from the forci- 

 ble pressure of air or of a foreign substance in the meatus, during a 

 blow on the ear or the introduction of a solid body which presses 

 against the memhrana tympani. 



2. The memhrana tympani shuts out the air in the meatus from 

 that in the tympanic cavity, and by this means an atmosphere of 

 certain physical conditions is constantly kept in contact with the 

 membranes between the tympanum and labyrinth. 



The second part of the paper is devoted to the examination of the 

 Eustachian tube in man and animals. Anatomists seem to have in- 

 ferred that the Eustachian tubes in their natural state are constantly 

 open, and that the air of the tympanic cavities is always continuous 

 with that in the fauces. An examination of the guttural portion of 

 the Eustachian tube in man and animals has led the author to con- 

 clude, that excepting during muscular effort, this orifice is always 

 closed, and that the tympanum is a cavity distinct from the outer 

 air. The agents whereby the Eustachian tubes are opened in the 

 human subject, are the muscles of the palate, and it is by their 

 action during the process of swallowing that the tubes are ordinarily 

 opened. That the act of swallowing is the means whereby the 

 Eustachian tubes are opened, is shown by some experiments of which 

 the following may be cited. If the mouth and nose be closed du- 

 ring the act of swallowing, a sensation of fulness and distension is 

 produced in the ears ; this arises from the air, which is slightly com- 

 pressed in the fauces, passing into and filling the tympanic cavities ; 

 upon removing the hand from the nose, it will be observed that this 

 feeling of distension does not disappear, but remains until the act of 

 deglutition is again performed, but while the nose is not closed ; in 

 this experiment the Eustachian tubes were opened during each act 



