344.  The Middle Ear.  At the inner end of the outer ear passage is the tympanum, known as “the drum of the ear.”  It is a thin, oval membrane, stretched at an angle across the deep end of the passage, which it completely closes.  The tympanum is thus a partition between the passage of the outer ear and the cavity of the middle ear.  On its inner side is a small air chamber in the petrous portion of the temporal bone, called the cavity of the tympanum.  Its bony walls are lined with mucous membrane similar to that lining the nose, mouth, and throat.  On the inner wall of the tympanum are two openings, the round window, or foramen rotundum, and the oval window, or foramen ovale.

The tympanic cavity communicates with the back part of the throat, by the Eustachian tube.  This tube is about one and a half inches long and lined with mucous membrane similar to that of the tympanic chamber and the throat.  This passage is usually closed, but is opened in the act of swallowing.  In health there is no communication between the chamber of the middle ear and the outside, except by the Eustachian tube.  Thus a throat cold, with redness and swelling of the mucous membrane, is usually accompanied with some degree of deafness, because the swelling may block the lumen of the tube, and thus prevent the free passage of air to and fro.

[Illustration:  Fig. 139.—­General View of the Organ of Hearing.

  A, pinna;
  B, cavity of the concha, showing the orifices of a great number of
     sebaceous glands;
  C, external auditory meatus;
  D, membrana tympani;
  F, incus;
  H, malleus;
  K, handle of malleus applied to the internal surface of the membrana
     tympani;
  L, tensor tympani muscle;
  between M and K is the tympanic cavity;
  N, Eustachian tube;
  O, P, semicircular canals;
  R, internal auditory canal;
  S, large nerve given off from the facial ganglion;
  T, facial and auditory nerves.
]

A most curious feature of the ear is the chain of tiny movable bones which stretch across the cavity of the middle ear.  They connect the tympanic membrane with the labyrinth, and serve to convey the vibrations communicated to the membrane across the cavity of the tympanum to the internal ear.  These bones are three in number, and from their shape are called the malleus, or hammer, incus, or anvil; and stapes, or stirrup.

The hammer is attached by its long handle to the inner surface of the drum of the ear.  The round head is connected with the anvil by a movable joint, while the long projection of the anvil is similarly connected with the stirrup bone.  The plate of the stirrup is fixed by a membrane into the oval window of the inner wall of the tympanic chamber.

These little bones are connected with each other and the tympanum by ligaments and moved by three tiny muscles.  Two are attached to the hammer, and tighten and relax the drum; the other is attached to the stirrup, and prevents it from being pushed too deeply into the oval window.