422 THE SENSORIAL FUNCTIONS. 



reeling them into a funnel-shaped canal (m), 

 termed the meatus auditorius, which leads to the 

 internal ear. This canal is composed partly of 

 cartilage and partly of bone ; and the integu- 

 ment lining it is furnished with numerous small 

 glands, which supply a thick oily fluid, of an 

 acrid quality, apparently designed to prevent the 

 intrusion of insects : the passage is also guarded 

 by hairs, which appear intended for a similar 

 purpose. 



The meatus is closed at the bottom by a 

 membrane (d), which is stretched across it like 

 the skin of a drum, and has been termed, from 

 this resemblance, the membrane of the tympanum, 

 or the ear-drum.^ It performs, indeed, an office 

 corresponding to its name ; for the sonorous un- 

 dulations of the air, which have been collected, 

 and directed inwards by the grooves of the 

 concha, strike upon the ear-drum, and throw it 

 into a similar state of vibration. The ear-drum 

 is composed of an external membrane, derived 

 from the cuticle which lines the meatus ; an in- 

 ternal layer, which is continuous with 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 extremity of a minute 



* The inner surface of the ear-drum is shown in this figure ; 

 the cavity of the tympanum, which is behind it, being laid 

 open. 



