312 THE SENSORIAL FUNCTIONS. 



which has been regarded as the earliest rudiment of a coch- 

 lea; and there are two folds of the skin, resembling eye-lids, 

 at the external orifice of the organ, which appear like the 

 first step towards the development of an external ear. 



The structure of the ear in the Crocodile is but an ap- 

 proximation to that which we find prevailing in Birds, 

 where the organ is of large size compared with that of the 

 head. The rudimental cochlea, as seen at K in Fig. 405, 

 which represents these organs 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 characteristic 

 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 development of the cochlea, the 

 tubes of which are convoluted, turning in a spiral, and as- 

 suming 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 inferred that it bears a to- 

 lerably 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 coch- 

 lea 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 



* These tympanic ossicula are regarded by Geoffrey St. Hilaire as cor- 

 responding to the opercular bones of fishes, where, according to his theory, 

 they have attained the highest degree of development. 



