HEARING. 423 



bony process (h), presently to be described.* 

 This muscular structure appears designed to 

 vary the degree of tension of the ear-drum, and 

 thus adapt the rate of its vibrations to those 

 communicated to it by the air. There is also a 

 slender muscle, situated internally, which by 

 acting on this delicate process of bone, as on a 

 lever, puts the whole membrane on the stretch, 

 and enables its radiating fibres to effect the 

 nicer adjustments required for tuning, as it may 

 be called, this part of the organ. t 



Immediately behind the membrane of the ear- 

 drum, there is a hollow space (r), called the 

 cavity of the tympanum^ of an irregular shape, 

 scooped out of the most solid part of the tem- 

 poral bone, which is here of great density and 

 hardness. This cavity is always tilled with air ; 

 but it would obviously defeat the purpose of the 

 organ if the air were confined in this space; 

 because unless it were allowed occasionally to 

 expand or contract, it could not long remain in 

 equilibrium with the pressure exerted by the 

 atmosphere on the external surface of the ear- 

 drum ; a pressure which, as is well known, is 

 subject to great variations, indicated by the rise 

 and fall of the barometer. These variations 



* In many quadrupeds their insertion into this process is at 

 some distance from the centre of the membrane. These mus- 

 cular fibres are delineated in Fig. 45, vol. i, p. 136. 



t Home, Lectures, &c., iii, 268. 



