e 
202 PHILOSOPHY OF ZOOLOGY. 
These bones are susceptible of motion, and have muscles 
attached to them, by which they are supposed able to 
stretch the drum, to compress the fluids in the labyrinth, 
and communicate the vibrations of the drum to the pulp of 
the canals and cochlea. 
In Birds, the tympanum bears a close resemblance to 
that of quadrupeds. Its cellular openings, however, are 
more numerous, and the cells themselves of greater dimen- 
sions. ‘The fenestra ovalis and rotunda, are placed, the 
former above the latter, and separated by an osseous bar. 
The Eustachian tube is osseous throughout its whole 
length. In the barrel there is but one bone connecting the 
drum with the labyrinth, branched where united with the 
drum, proceeding by slender stalks through the cavity of 
the barrel, and forming a plate which closes the fenestra 
ovalis. 
In Reptiles, the tympanum exhibits very remarkable 
differences. In some, it can scarcely be said to exist, while 
in others, it is membranaceous, exhibiting the Eustachian 
tube, and one or two bones in the barrel. The membrane, 
or drum, is wanting in some cases, its place being supplied 
by the-common skin, while in others, it exhibits its usual 
characters of dryness, transparency, and elasticity. 
In Fishes, particularly those with free branchize, there is 
no tympanum. In those with fixed branchia, the rudi- 
ment of a tympanum may be observed in the form of a 
minute tube leading from the labyrinth to the skin. 
Between the drum and the surrounding element in Qua- 
drupeds, the Eavternal Ear is situated. It consists of a 
tube continued from the drum to the surface of the body. 
It varies greatly in the length, direction, and texture of its 
walls. When arrived at the surface, this meatus audito- 
rius caternus, as it is called, either terminates in a simple 
hole, or it is expanded into a cartilagmous arch, termed the 
