HEARING. 395 



in the Turkey, is of large size, and slightly curved. 

 In the cavity of the tympanum (t) is seen the 

 columella, which extends to the fenestra ovalis; 

 and beyond it, the semicircular canals (s), the bony 

 cells (b) which communicate with the tympanum, 

 the OS quadratum (q), the zygomatic process (z), 

 and the lower jaw (j). The ear-drum is now no 

 longer met with at the surface, but lies concealed 

 at the bottom of a short meatus, the orifice of which 

 is surrounded with feathers, arranged so as to serve 

 as a kind of imperfect concha, or external ear. In 

 Owls these feathers are a prominent and charac- 

 teristic 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 internal 

 ears of Mammalia is the great developement of the 

 cochlea, the tubes of which are convoluted, turn- 

 ing in a spiral, and assuming the figure of a turbi- 

 nated shell. From an extensive comparison of the 

 relative size of the cochlea in different tribes of 

 quadrupeds, it has been inferred that it bears a 

 tolerably constant proportion 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 com- 

 pletely developed only in the Mammalia.* It is 



* These tympanic ossicula are regarded by Spix and by Geoffroy 

 St. Hilaire as corresponding to the opercular bones of fishes, where, 

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

 developement. But Weber discovered the existence of the ossicula 

 iiudilus in some Hshes, quite apart from the opercula. 



