ANATOMY OF THE HORSE. 
245 
ris ; the handle forming an angle with the head and neck, and 
tapering to its termination against the membrane. From the up¬ 
per part of the handle, immediately below the neck, issues an 
acute tapering process ; and this is received into a depression in 
the bone (in which it rests) in the side of the cavity. This pro¬ 
cess forms the centre of motion of the malleus, and as such it is 
worthy of remark here, that it originates near its head and at a 
distance from the extremity of the handle ; a point of consider¬ 
able import in the physiology of hearing. 
The incus, though it has received its name from its fancied 
resemblance to a blacksmith’s anvil, bears a much nearer ap¬ 
proach in figure to a molar tooth. Answering to the tooth, too, 
it has a depression upon its surface, adapted to receive the head 
of the malleus. Like the malleus, it possesses two processes—a 
superior and short one turned backwards to be let into a depres¬ 
sion in the wall of the tympanum; a longer one projected down¬ 
wards into the cavity, whose extremity is curved a little, and 
whose point is attached to the os orbiculare. 
The os orbiculare is not only the smallest of the ossicula, 
but it is the smallest bone in the whole body. Soemmering dis¬ 
believes in the existence of such a bone : but it may, with a 
little care, be found and demonstrated, although it hardly ex¬ 
ceeds in magnitude a grain of sand. It forms the medium of 
junction and communication between the incus and stapes: 
through it the joint existing between the two obtains additional 
freedom and facility of motion. 
The stapes (or stirrup-bone) seems to be the most aptly 
named of them all; since it really possesses much of the form 
and character of the common iron stirrup. Its base (which is 
not an exact oval, one side being somewhat flattened) rests 
against the membrane filling up the fenestra ovalis, to which in 
figure it precisely corresponds. By a small head at the other 
extremity it articulates with the os orbiculare. 
Muscles .—The mechanism of the internal ear is such as to re¬ 
quire in the cavity of the tympanum the presence of four small 
muscles*; and these operate on the malleus and stapes. They 
are the— 
* Sound is the effect of impression upon the portio mollis of the seventh 
pair of nerves—the true auditory nerve. This impression is produced by 
vibrations of the air upon the rnembrana tympani, communicated therefrom 
by the osseous chain extended between them to the membrane of the 
fenestra ovalis, and thence to the expanded auditory nerve. Now, these 
vibrations being once excited, do not immediately cease, but continue suc¬ 
ceeding one another in great rapidity; as, in common speaking, every syl¬ 
lable articulated produces a separate and distinct impulse or vibrating mo¬ 
tion upon the car. Consequently, to prevent confusion of sound, or rather 
