LECT. XIX. PROPAGATION OF SOUND. 355 



If we close the nose and mouth, and then enlarge, as 

 much as possible, the thoracic cavity, we produce a tem- 

 porary deafness ; and the same effect is obtained by means 

 of a strong and sustained expiration. This deafness con- 

 tinues for a few seconds, and is most effectually got rid of 

 by an act of deglutition. In the one case, the air of the 

 tympanum is less dense ; in the other, more dense than the 

 exterior air ; while the membrane undergoes a different 

 tension in the two cases, in the one being drawn inwards, 

 in the other being forced outwards. Wollaston, who first 

 observed these facts, remarked that in the first case he was 

 deaf to grave sounds ; while, in the other case, he was deaf 

 only to acute ones. Thus he states that he could not hear 

 distinctly the loud noise of a carriage at a certain distance, 

 though he readily heard the noise produced by striking 

 the end of the nail upon a table. I think I have given 

 a sufficiently plausible reason for this difference. An expe- 

 riment analogous to the preceding one, may be made by 

 means of a trumpet prepared with the membrane fixed over 

 its opening. By rendering this very tense, all loud sounds 

 were indistinctly heard, whilst, on the contrary, the ticking 

 of a watch was rendered stronger. Cases are mentioned 

 of individuals who cannot hear the ordinary voice, and who 

 can only hold conversation in the midst of a great noise. 

 In these cases it must be admitted, that the sensibility for 

 grave and strong sounds may continue. This can be un- 

 derstood only to a certain extent, by assuming that the 

 membrane is deprived of the power of varying the tension 

 under different circumstances. 



The internal muscle of the hammer (malleus) and that of 

 the stirrup (stapes) serve to modify the tension of the mem- 

 brane of the tympanum, in obedience to the will. Miiller 

 assumes that these muscles are excited to contract by a 

 reflex action, as the muscular fibres of the iris are by a 



