HEARING. 441 



In Owls these feathers are a prominent and cha- 

 racteristic feature ; and in these birds there is, 

 besides, a membranous flap, acting as a valve to 

 guard the passage. 



The chief peculiarity observable in the in- 

 ternal ears of Mammalia is the great develope- 

 ment of the cochlea, the tubes of which are con- 

 voluted, turning in a spiral, and assuming the 

 figure of a turbinated shell. From an extensive 

 comparison of the relative size of the cochlea in 

 different tribes of quadrupeds, it has been in- 

 ferred that it bears a tolerably constant propor- 

 tion to the degree of acuteness of hearing, and 

 that, consequently, it contributes essentially to 

 the perfection of that faculty : bats, for instance, 

 which are known to possess exquisite delicacy 

 of hearing, have a cochlea of extraordinary size, 

 compared with the other parts of the ear. The 

 tympanic ossicula are completely developed 

 only in the Mammalia.* It is also in this class 

 alone that we meet with a concha, or external 

 ear, distinctly marked ; and the utility of this 

 part, in catching and collecting the sonorous 

 undulations of the air, may be inferred from the 

 circumstance, that a large and very moveable 

 concha is generally attended with great acute- 



* These tympanic ossicula are regarded by Geoffroy St. Hilaire 

 as corresponding to the opercular bones of fishes, where, accord- 

 ing to his theory, they have attained their highest degree of 

 developement. 



