HEARING. 377 



portions, which I shall afterwards have occasion to 

 explain by magnified representations, may be pro- 

 perly appreciated.* 



The Concha, or external ear (c), is formed of an 

 elastic plate of cartilage, covered by integument, 

 and presenting various elevations and depressions, 

 which form a series of parabolic curves; apparently 

 for the purpose of collecting the sonorous undu- 

 lations of the air, and of directing them into a 

 funnel-shaped canal (m), termed the meatus audi- 

 tor ius, 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 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 mem- 

 brane (d), which is stretched across it like the skin 

 of a drum, and has been termed, from this resem- 

 blance, the membrafie of the tympanum, or the ear- 

 drum.'\ It performs, indeed, an office correspond- 

 ing 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 composed of an ex- 

 ternal membrane, derived from the cuticle which 



* In this and all the following figures, the parts of the right ear 

 are shown, and similar parts are always indicated by the same 

 letters. 



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

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



