220 ELEMENTS OF ORNITHOLOGY, 
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 siphoniwm, 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 fenestra 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 membranous 
labyrinth. 
The membranous labyrinth consists of three semicircular 
canals (anterior, posterior, and external) which open into a 
cominon 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. 
