378 THE SENSORIAL FUNCTIONS. 



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

 tinuous Avith that of the cavity beyond it ; and a 

 middle layer, which consists of radiating muscular 

 fibres, proceeding from the circumference towards 

 the centre, where they are inserted into the ex- 

 tremity of a minute 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 ena- 

 bles its radiating fibres to effect the nicer adjust- 

 ments required for tuning, as it may be called, this 

 part of the organ.! 



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 hardness. 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 freely to expand or contract, it could not 

 long remain in equilibrium wdth the pressure ex- 

 erted 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 would 



* In many quadrupeds their insertion into this process is at some 

 distance from the centre of the membrane. These muscular fibres 

 are delineated in Fig. 45, vol. i. p. 125. 



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



