ANATOMY OF THE HORSE. 
247 
Eustachian Tube.— The cavity of the tympanum holds a com¬ 
munication with the external air through a canal named, after 
its discoverer (Eustachius), the Eustachian tube. This tube 
communicates with the cavity of the tympanum by what (in the 
dried bone) appears to be nothing more than a fissure, from 
which, having passed through the petrous portion of the tempo¬ 
ral bone, it becomes cartilaginous in its composition, and pro¬ 
ceeds for some distance gradually expanding in calibre, until ul¬ 
timately it opens into the guttural sac , formed at the back of the 
fauces. One side of the tube is clothed by the levator palati and 
stylo-pharyngeus musics*. 
Labyrinth. 
The labyrinth, in which are deposited the organs more imme¬ 
diately concerned in the function of hearing, is an exceedingly 
irregular cavity, comprising the vestibule, semicircular canals, 
and cochlea. 
The cavity of the tympanum we found contained air, having 
a communication with the atmosphere without; but within the 
labyrinth we find a quantity of aqueous fluid, bedewing the ex¬ 
pansion of the auditory nerve f. 
The stapes, resting as it does flat against the membrane closing the 
fenestra ovalis, and receiving these full and distinct impulses, imparts the 
benefit of them to the membrane, and thus the sensation becomes most 
impressive and perfect. 
* The design of the Eustachian tube appears to be to admit of a free 
circulation of air in and out of the cavity of the tympanum. Air, from be¬ 
ing retained within the cavity, must necessarily become heated and rare¬ 
fied, a condition in which it is less suitable to transmit sounds with full 
intensity; and therefore a renewal of it takes place through the Eustachian 
tube. 
f This cavity is provided with a watery (in place of an aeriform) fluid, 
for three sufficient reasons. In the first place, water adds to the intensity 
of a vibration in a very much greater degree than air. Formerly it was 
imagined that sound could not be conducted through so dense a medium as 
water; but the Abbe Nollct overturned this hypothesis by direct and sim¬ 
ple experiment. Every schoolboy knows that two stones struck together 
under w ater emit a sound, so far greater than the one created in air, as to 
be in a degree insupportable to the hearer. Consequently, by the water in 
the labyrinth the impression made upon the auditory nerve is so much the 
more intense. 
A second reason w hy the labyrinth should contain w ater (and not air) is, 
that sound is so much more quickly propagated by one medium than the 
other. Through air it vibrates at the rate of 1132 feet in a second; through 
water, at the rate of 4000 feet in that time. Thus the auditory impression 
is more suddenly and perfectly disseminated over a cavity filled with water 
than it would be through one that contained air. 
A third reason for placing water here, consists in its not being nearly 
so expansible a fluid as air, and consequently not subject to that rarefac¬ 
tion and increase of volume that air is ; and which might be attended with 
serious consequences in a cavity so confined as the labyrinth is. 
