ner
ear.

The _middle ear_, it will be seen, is a drum with its stretched
membrane like any other drum, and it too has a communication with the
exterior air through a tube, the _Eustachian tube_, which leads from
the drum into the back part of the throat. When one has a cold, the
mucous membrane which lines this tube may become swollen or even
catarrhal, and be so closed that no air can enter from the throat; the
air already within the drum being absorbed, the outer air presses
unduly against the drum-head, with the result that the whole
conducting apparatus is put more or less out of condition, and a
certain degree of deafness naturally results. The tension of the
drum-head is regulated by a muscle attached to the bone which is
connected with the inner part of this membrane.

It is now easy to understand how any unfavorable condition of the
throat may affect the ear, or that of the ear influence the throat.

In the hearing mechanism of man, the _inner ear_, or _labyrinth_, well
so named because of its complexity, is really situated in the inner
hardest portion of the "temporal" bone. It consists of a membrane and
a bony portion, the former containing the essential mechanism of
hearing, the latter being chiefly protective to it. The membranous
portion consists of a series of canals communicating with some
similarly membranous sacs, the whole being surrounded by and filled
with fluid. These latter communicate with an extension termed the
_cochlea_, which contains a central canal in which that collection of
cells is found which constitutes the _end-organ_, among them the
hair-cells, about which the nerve ends.

This end-organ in the cochlea may be compared very fitly to the
telephone which receives the message, and that portion of the brain
where the auditory tract ends, to the telephone at the distant end of
the path, the listener there representing consciousness. The auditory
path within the brain is long and complicated, there being, in fact,
many way-stations through which the message passes before it reaches
the final one.

The auditory nerve proceeds first to the lowest or hindermost portion
of the brain, known as the _bulb_, or _medulla oblongata_; thence a
continuation of the nerve tract passes forward to a central region,
the _posterior corpora quadrigemina_, then, by a new relay of
nerve-fibres, to the highest and most important part of the brain,
that most closely associated with consciousness, the _cort