OP HEARING. 167 



perfectly correct, — that the memhrana tympani is the principal 

 instrument of distinct hearing ; — that its muscles give it various 

 degrees of tension, putting it in unison with the sounds to which 

 we are desirous of attending.* I am conscious that some adjust- 

 ment within the ear takes place when I direct my attention from 

 one sound to another j some adjustment is demonstrably neces- 

 sary 5 the membrana tympani is fitted for this adjustment by its 

 structure and by a supply of muscles j and when it is scaly and 

 rigid, every sound may be heard, but without distinctness. 



I ; ■ , "- 



* Bocrhaavc, Preelect. Acad. T. iv. p. 360 tq. 





