244 ANATOMY OF THE HORSE. 
it contains no fluid; and exhibits several foramina or apertures. 
At one part it has a communication, through the Eustachian 
tube, with the external air; but the entrance into it through the 
meatus auditorius is closed by the membrana tympani. This 
semi-transparent membrane is stretched tightly across the termi¬ 
nation of the meatus; not, however, after the manner of the 
parchment of a drum, but funnel-like, a shape into which it is 
drawn from the attachment to its centre of a lengthened process 
belonging to one of the little bones contained within the cavity. 
Two layers enter into its composition—an outer of skin, an inner 
of lining membrane; which last exhibits so high a state of vas¬ 
cularity as to have been assimilated to the iris # . 
Fenestra?. —The side of the cavity facing the meatus externus 
exhibits a very irregular aspect, and is perforated by two aper¬ 
tures—the fenestra ovalis and the fenestra rotunda. Between 
these two perforations is an osseous eminence called the tubercle. 
The fenestra ovalis (also known as the foramen ovale ) 
is an aperture bearing an approach to the fissure of an oval, 
looking from the cavity of the tympanum into the labyrinth. 
The fenestra rotunda (also called the foramen rotun- 
dum) is likewise an ovoid opening, situated by the side of the 
tubercle, and communicating (not with the labyrinth, as in the 
case of the former opening, but) with the scales of the cochlea. 
Bones. —Arranged along the interval between, and connected 
with, the membranes closing the external meatus and the oval 
fenestra are four little bones ( ossicula ), by whose intervention 
vibrations of sound agitating the membrana tympani are trans¬ 
mitted to the membrane opposite—that closing the fenestra 
ovalis : for these bones are connected to each other by ligaments, 
and are so disposed in relation to one another as to offer the 
greatest mechanical advantages in this chain of communication 
for the purpose of augmenting the intensity of the vibration. 
The names of the ossicula are, malleus , incus , os orbiculare, and 
stapes. 
The malleus (so called on account of its similitude to a 
hammer or mallet) is the one most outwardly situated. Its long 
process, manubrium or handle, is attached to the central part of 
the membrana tympani, which is drawn inward by it into the 
shape of a funnel. The head of the malleus stands out from its 
body in an oblique direction, very similar to that of the os femo- 
* In llic foetus, tbe membrana tympani (and, indeed, the tympanum alto¬ 
gether) is very superficially placed ; for that which forms the osseous part 
of the meatus externus in the adult, is a mere bony ring at this period, 
across which the membrane is extended. This is beautifully illustrated in 
the cranium of a foetal puppy now before the writer. 
