ed at the inner end
of the canal and separates it from the tympanum or middle ear. It is
placed like the membrane in the telephone. It is pearly gray in color.
This membrane not only serves as a protection to the delicate structures
within the tympanum, but also receives the sound vibrations from without
and transmits them to the ossicular (bony) chain of the middle ear.

The Tympanum or Middle Ear.--This cavity just beyond the drum, which forms
the greater part of its outer wall, is an irregular cavity, compressed
from without inward and situated in the petrous bone. The mastoid cells
lie behind. It is filled with air and communicates with the nose-pharynx
(naso-pharynx) by the eustachian tube. The upper portion of this cavity,
the attic, lies immediately below the middle lobe of the brain, separated
from it by a thin layer of bone, which forms the roof of the cavity. This
cavity is separated from the internal ear.

[EYE AND EAR 359]

The Eustachian tube.--This is the channel through which the middle ear
communicates with the pharynx. With an opening in the anterior of the
middle ear, a bony canal passes from this point, inward, forward, and
downward through the petrous bone, when it merges into a cartilaginous
canal, which terminates in a funnel-shaped protuberance, with a slit-like
orifice, located in the nose pharynx. This is the eustachian tube. It is
lined with mucous membrane like the throat. The air goes up from the
throat, through this canal to the middle ear. The mucous membrane of the
middle ear is continuous with that of the nose-pharynx through the
eustachian tube. So you can readily understand how easy it is for an
inflammation of the throat to extend to the middle ear through the
eustachian tube.

The posterior wall which has the greatest height, reveals in its upper
portion a passage (antrum) through which the vault of the tympanum (attic)
communicates with the cells of the mastoid process, situated posteriorly.
From this description you see how near to each other these parts are
placed and when one becomes diseased the disease can extend to the other
part or parts. The brain is separated from some of these cavities by a
very thin shell of bone, and the disease can soon affect the brain through
infection or breaking through the thin structures that separates the
parts.

Diseases of the middle ear and the mastoid are always to be considered
serious, and should be very closely watched. A child with a