OF HEARING, 167 



perfectly correct, that the membrana 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 ; some adjustment is demonstrably neces- 

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

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

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



* Bocrhaave, Pralect. Acad. T. iv. p. 360 sq. 



