In addition to these contents of the middle ear there are also two tiny
openings which, very necessary for health, are nevertheless sometimes a
pathway by which serious disease may attack the ear and destroy the
hearing. The first is a small passage-way leading from the upper part of
the middle ear cavity through the bone to the _mastoid antrum_, a hollow
space in the prominent mass of bone to be felt immediately behind the
ear projecting outward and downward from the skull.
The second passage-way opening into the middle ear cavity is that of the
Eustachian tube which leads directly to the back of the throat. The
importance of this tube is that through it air can find its way directly
into the middle ear, so that the air pressure on the two sides of the
drum is always kept the same. If it were not for some such arrangement
the pressure on the outer side of the drum would become greater than
that on its inner surface. This would, of course, push the drum inward,
and greatly reduce its mobility.