ORGAN OF HEARING IN BIRDS. I35 



base of the columelliform stapes : the much larger external aperture 

 of the tympanic cavity is closed by the ear-drum, e. This is con- 

 vex outwardly, semitransparent and glistening : the proper ' mem- 

 brana tympani' is lined by that of the tympanic cavity, which 

 is continued into the eustachian tube ; and is covered externally 

 by an epithelial layer, continuous with that of the meatus : the 

 former is more intimately united with the proper membrane. 

 In this may be discerned an outer layer, showing more distinctly 

 a structure of radiating fibres, and one inner, thicker, and less 

 distinctly fibrous layer. 



The margin of the ear-drum is set in a groove of bone, afforded 

 by or attached to the tympanic anteriorly, the mastoid above, and 

 the paroccipital behind. One or more points of ossification may 

 be set up in the thick periphery of the drum, Avhich coalesce 

 with the above-named bones. The membrane of the vestibule, 

 passing across the foramen ovale, becomes a little thickened where 

 it adheres to the margin of the disc of the stapes : the connection 

 is such as to admit of a sli2:ht movement of the ossicle. From the 

 disk the bone is continued, of a slender form, like a pedicle, to the 

 cartilaginous bifurcation, and this is connected by a larger cartilagi- 

 nous plate, representing the ' malleus,' to the membrana tympani, 

 at c, fig. 55. To the latter cartilage, as to the ossified and coalesced 

 incus and malleus of Marsupials, is attached the chief muscle of the 

 ear-drum, a ' tensor,' fig. 55,f: it arises from a depression in the ba- 

 sisphenoid, enters the tympanic cavity above the beginning of the 

 eustachian tube, and by its insertion into and action upon the mal- 

 leus, tends to push the membrane outward : it is counteracted by 

 two small cords extended to the inner wall of the tympanum : but 

 the muscular character of them is doubtful, and the ear-drum re- 

 sumes its normal state when the tensor ceases to act. The eusta- 

 chian tube, fig. 55, e, is continued from the lower and back part of 

 the tympanic cavity, grooves the sides of the basisphenoid, as it 

 converges toward its fellow, A^th which it unites, in most Birds, 

 to terminate by a common aperture behind the posterior or palatal 

 nares. 



§ 144. Organ of Sight in Birds. — The avian peculiarities of the 

 eye chiefly relate to the extraordinary powers of locomotion in 

 this class, adjusting \dsion to a rapid change of distance in the 

 objects viewed, and facilitating their distinct perception through 

 a rare medium. 



There is no species of Bird in which the eyes are wanting, or 

 rudimentary, as occurs in the other A^ertebrate classes. 



The eyes of Birds are remarkable for their great size, both as 



