HEARING. 



299 



rious elevations and depressions, which form a series of pa- 

 rabolic curves, apparently for the purpose of collecting the 

 sonorous undulations of the air, and of directing them into 

 a funnel-shaped canal (m,) termed the meatus auditoriiis, 



which leads to the internal ear. This canal is composed 

 partly of cartilage, and partly of bone; and the integument 

 lining it is furnished with numerous small glands, which 

 supply a thick oily fluid, of an acrid quality, apparently de- 

 signed to prevent the intrusion of insects: the passage is also 

 guarded by hairs, which appear intended for a similar pur- 

 pose. 



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 of- 

 fice corresponding to its name; for the sonorous undulations 

 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 



* The Inner surface of the ear-drum is shown in this figure, the cavity of 

 the tympanum, which is behind it, being- laid open. 



