300 THE SENSORIAL FUNCTIONS. 



composed of an external membrane, derived from the cuti- 

 cle whicli lines the meatus; an internal layer, which is con- 

 tinuous witli that of the cavity beyond it; and a middle 

 layer, wiiich consists of radiating muscular fibres, proceed- 

 ing from the circumference towards the centre, where they 

 are inserted into the extremity of a minute bony process 

 (h,) presently to be described.* This muscular structure 

 apj)ears designed to vary the degree of tension in the ear- 

 drum, and tlius adapt the rate of its vibrations to those com- 

 municated 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 (t,) called the cavity of the tympanum^ 

 of an irregular shape, scooped out of the most solid part of 

 the temporal bone, which is here of great density and hard- 

 ness. This cavity is always filled 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 occa- 

 sionally 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 

 w^ell known, is subject to great variations, indicated by the 

 rise and fall of the barometer. These variations would ex- 

 pose the membrane of the ear-drum to great inequalities of 

 pressure at its outer and inner surfaces, and endanger its 

 being forced, according to the state of the weather, either 

 outwards or inwards, which would completely interfere with 

 the delicacy of its vibrations. Nature has guarded against 



• In many quadrupeds their insertion into this process is at some distance 

 from the centre of the membrane. These muscular fibres arc dcUncated in 

 Fig. 45, vol. i. p. 105. 



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



