The chief contents of the cavity of the middle ear are three tiny bones
called the _malleus_ or hammer bone, the _incus_ or anvil bone, and the
_stapes_ or stirrup bone. In addition, an important nerve called the
_chorda tympani_ passes across the middle ear chamber. The three little
bones contained in the middle ear may be looked upon as the connecting
link between the outer ear, which gathers the sounds, and the internal
ear, which transmits the effect of the sound waves to the brain, where
they are translated into what we call hearing.
From without inward the three little bones lie touching each other, end
to end, the outer end of the first bone being implanted between the
layers of the drum membrane and the inner end of the innermost bone,
fitting into a tiny opening which connects the middle ear with the
internal ear. As the result of their lying touching each other, any
movement of the ear drum caused by a sound wave striking against its
outer aspect, moves the malleus bone; this, in turn, moves the middle
incus, and this passes the movement on to the innermost part of the
stirrup. This, in turn, passes the movement onward to the fluid or
_perilymph_ in the outermost part of the internal ear, and here the
endings of the nerve of hearing receive the stimuli which we recognize
as “sounds.” (See Plates.)