374 Mr. R. Moon on the Structure of the Human Ear, and on 



rebound and again move outwards. A single atmospheric pulse 

 might thus throw the auditory apparatus into a state of oscilla- 

 tion for a considerable time — a circumstance which would mate- 

 rially interfere with the distinct perception of articulate sound. 

 To obviate such an effect is the special function of the tympanal 

 muscle in birds. 



It is worthy of remark that, although in the auditory apparatus 

 of birds recourse is had to the principle of elasticity to the extent 

 above explained, the principle requires to be kept in check, and 

 is kept in check in the manner above described. In the more 

 perfect organ of man and of mammals, on the other hand, the 

 uncertain and unmanageable principle of elasticity is through- 

 out excluded, the tympanal membrane, the ligamento-fibrous 

 membrane wrapped about the base of the stapes, and the mem- 

 brane of the fenestra rotunda being alike inelastic and inex- 

 tensible. 



IV. I would next remark that the success of the experiment 

 of Valsalva (which, though in general only temporary in its effects, 

 I apprehend to be of all known means for the diminution of 

 deafness the most simple and the most universal of application) 

 is confirmatory of the views with regard to the mode of action 

 of the human ear which I have endeavoured to set forth. 



For if, as I have stated, the sensation of hearing is produced 

 primarily by the tympanal membrane and the stapes being forced 

 outwards, and the cochlear membrane being drawn inwards by 

 the operation of rarefied waves, and secondarily by these parts of 

 the apparatus being restored to their former status through the 

 operation of the muscles of the ear, the first and most natural 

 step to be taken in any case of defective hearing is obviously to 

 strengthen the tendency to move outwards of the tympanal mem- 

 brane and stapes when under the influence of rarefied waves; 

 and this will clearly be effected by Valsalva's experiment*, by 

 which the density of the air in the tympanal cavity is temporarily 

 increased. The enhanced effect of the experiment, as performed 

 under the improved method introduced by Politzer, is thus also 

 strikingly accounted for. 



In the cases to which it is applicable (that is, when the tym- 

 panal membrane is wholly or in part present, and the connexion 

 between the ossicles is wholly or partially maintained) the effect 

 of Valsalva's experiment, upon the principles before explained, is 

 precisely that of raising the voice in speaking to the patient. 



On the other hand, if hearing took place through the agency 

 of condensed waves, the result of the experiment would be to 

 diminish the difference of the pressures on the two sides of the 



* By this experiment, the nose and mouth being closed, air is forced 

 through the Eustachian tube into the tympanal cavity. 



