220 ELEMENTS OE OENITHOLOGT, 



It is closed by the drum of the ear, tympanum, or tympanic 

 'membrane, which is invested by the general skin, and is there- 

 fore not itself to be seen. Within this membrane is a bony 

 chamber named the tympanic cavity. In some Birds a bony tube, 

 called the siphonium, passes from the tympanic cavity to the 

 lower jaw, and conveys air to the articular bone. 



In the front part of the tympanic cavity is an opening which 

 is the hinder orifice of the eustachian tube, while in the inner 

 wall of the tympanic cavity are two apertures close together. 

 The -upper and anterior of these is called the fenestra ovalis, 

 while the other is the fenestra rotunda. A delicate little bone 

 shaped like a doctor's stethoscope traverses the tympanic cavity. 

 Its expanded end is applied to the fenestra ovalis, while its 

 opposite extremity, from which various delicate processes may 

 diverge, is attached to the tympanic membrane. It is called 

 the columella. 



The two fenestrsB open into another still more internal cavity, 

 within which is the true ear or organ of hearing. This most 

 internal cavity lies within the substance of the periotic bones 

 of the side-wall of the skull *, and is the " internal ear " — 

 the tympanic cavity being the " middle ear,'' and the parts ex- 

 ternal to the tympanic membrane the "external ear." This 

 innermost cavity has a very complex shape and is therefore 

 called a bony labyrinth. Its form is determined by the mem- 

 branous parts it encloses, which constitute the memiiranous 

 labyrinth. 



The membranous labyrinth consists of three semicircular 

 canals (anterior, posterior, and external) which open into a 

 common central cavity or vestibule, from which an elongated 

 membranous bag, the cochlea, proceeds in the opposite direction. 

 This membranous labyrinth is filled with a fluid called the endo- 

 lymph, and floats in another fluid, filling the bony labyrinth 

 containing it, called the perilymph. The fenestra ovalis is set 

 in the wall of the bony cavity containing the vestibule. The 

 fenestra ovalis is set in that which encloses the cochlea. 



The nerve of hearing penetrates the periotic capsule and 

 supplies the walls of the cochlea and semicircular canals. 



The Eye. — This organ in Birds is formed in essentially the 

 same way as our own. It consists of an eyeball enclosed by a 

 strong sclerotic membrane, which is transparent in front, forming 



* See ante, p. 182, and below, p. 227. 



