xvi. 30 EAR 489 



developed in owls and in parrots (Fig. 300). In this there is a basilar 

 membrane, with fibres increasing in length towards the tip and carry- 

 ing an organ of Corti with hair-cells in contact with a tectorial mem- 

 brane (Fig. 301), as in mammals. At the tip of the cochlea is a special 

 sensory region, the lagena, similar to that of lower vertebrates and 

 perhaps responsible for reaction to lower notes, the basilar membrane 



Bind 



Mammal 



Fig. 300. Labyrinths of various vertebrates, to show 



varying development of cochlea (c) and lagena (l). 



s, saccule; u. utricle. (From v. Frisch.) 



responding to the higher frequencies. Birds are known to be more 

 sensitive to distant gunfire and other low-frequency vibrations than is 

 man. Transmission of vibration from the tympanum to the inner ear 

 is effected by the columella auris, derived from the cartilages of the 

 hyoid arch. The inner portion of the columella is rod-like (stapes), but 

 the outer end makes contact with the tympanum by means of three 

 processes, of somewhat irregular shape. 



Hearing is, of course, acute and the song-birds must be able to dis- 

 criminate between simple tunes; some of them are surprisingly good 

 mimics. Ability to localize sound is high and owls and other night- 

 birds probably find their prey largely by ear. For the purpose of 

 direction-finding they have developed an asymmetrical arrangement 

 of the ear cavities (Strix) or asymmetrical external ears (Asio). A few 

 birds that live in caves have the power of avoiding obstacles by echo- 

 location (Steatornis, the oil bird, Collocalia, swiftlet). They emit up 

 to 5 to 6 clicks a second at 4 to 5 Kc. The rate varies inversely with 

 the amount of light and increases when obstacles are met. 



