ANATOMY OF THE HORSE. 
246 
Laxator Tympani. —A very minute muscle, arising from 
the outside wall, close to the attachment of the membrana tym¬ 
pani, and inserted into the handle of the malleus, near its root. 
M. externus mallei vel M. Processus Minoris, is by 
some denied altogether—at least, to be of the nature of muscle. 
It arises from the upper part of the tympanum, and is attached 
by a small tendon to the shorter process of the malleus. 
Tensor tympani arises from the side of the Eustachian 
tube, and is inserted into the handle of the malleus, upon its up¬ 
per side. 
Stapedius. —In the horse this muscle is developed in a re¬ 
markable degree. It takes its rise from a little eminence within 
the tympanum called th q pyramid, and is fixed to the head of 
the stapes*. 
confused audition, some contrivance was found necessary to put a stop to 
one vibration before another was communicated. This explains, in general 
terms, the use of the muscles of the tympanum. 
* It may not, perhaps, be considered altogether out of place to make a 
remark or two here on the motions of the bones of the ear, and on the me¬ 
chanical advantages derived from their arrangement and relative position. 
The manubrium of the malleus is, as we have seen, extended downward 
to be attached to the tympanum, whose every vibration, in course, affects it. 
The other slender process, issuing from the neck of the bone and abutting 
against the wall of the tympanum, being much nearer the point at which the 
impression is received, than the one where the power resides, becomes the 
centre of motion—the fulcrum; so that the bone is set into action upon 
the principle of the lever. The moving power is applied to the manu¬ 
brium, the process from the neck becomes the fulcrum, and the head of 
the bone is the part on which the effect is to be produced ; it being a law 
in mechanics, that in proportion as the distance of the power from the ful¬ 
crum or prop exceeds that of the weight or resistance to be overcome, so 
will prove the mechanical advantage. In the example before us, this dis¬ 
tance is twice as great; and, consequently, vibrations communicated by 
the membrana tympani to the extremity of the manubrium of the malleus, 
will be transmitted by the head of that bone to the incus with twofold 
intensity. 
Again, we perceive the same principle brought into operation in the mo¬ 
tions of the incus. One of the processes of this bone is received into a de¬ 
pression in the wall of the tympanum in such manner that the centre of 
motion proves to be in the direction of a line drawn through the middle of 
the body of the bone; so that the extremity of the other—its long • pro¬ 
cess (to which the orbicular bone is attached)—performs a greater sphere 
of motion than the part receiving the impression from the head of the mal¬ 
leus; the consequence of which is, that but a trifling degree of motion 
given to the body of the incus must become very much more perceptible 
from augmentation before it reaches the orbicular bone. 
The os orbiculare appears to have been interposed in the manner it is, 
in order that an accurate perpendicular impulse might be communicated to 
the stapes: had this bone not been where it is, the vibration from the long 
process of the incus must have been transmitted to the stapes in an oblique 
direction, the result of which would have been confusedness and indistinct¬ 
ness in audition. 
