330 NATURAL THEOLOGY. 



the parchment that gives the loud and sudden 

 sound ; while the chords alter and prolong the 

 sound. The three muscles which are attached to 

 the malleus and through it to the membrane of 

 the tympanum we must suppose may either brace 

 or throw loose the membrane and its cords ; as 

 the drum is braced or muffled : and in this way 

 the small muscles of the tympanum may have a 

 resemblance in function to the fibres of the iris ; 

 they may guard the nerve of hearing as the latter 

 docs the nerve of vision. We had occasion to 

 observe, that when a person is deaf from the dis- 

 order of this apparatus, and when he substitutes 

 the ear-trumpet, he may hear ; for the ear-trum- 

 pet, by its expanded mouth, collects the undula- 

 tions of sound and concentrates them ; but there 

 is this imperfection, that the ear w^ants its power 

 of adjustment ; and the person is accordingly often 

 timid in the use of his instrument with those who 

 are not accustomed to speak to him, the sound of 

 some voices being painfully harsh. Further, we 

 may not hear a sound when called upon to listen 

 to it, and yet when the particular sound is des- 

 cribed, we do hear it ; now it remains to be de- 

 termined whether this be a power of adjustment 

 in the ear, or owing to the effect of association in 

 the mind. 



It is, supposed by some that there are two 

 tracts by which sound is communicated to the 

 labyrinth ; that it passes both through the chain 

 of bones and through the air in the tympanum. 



