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24                      BAND-BOOK OF ACOUSTICS.
membrane. The lower of these two apertures, which is about the size of a large pin's head, is called from its shape the Fenestra Rotunda, or round window (F.R., fig. 16); the upper and rather larger one, the Fenestra Ovalis, or oval window (F. 0., fig. 16).
A chain of three small bones, stretches between the Tympanum and the Fenestra Ovalis. One of these, the Malleus or Hammer Bone (Mall., fig. 16) is firmly fastened by one of its processes or arms to the Tympanum, while the other process projects upwards into the cavity, and is articulated to the second bone—the Incus or Anvil (Inc., fig. 16). The lower part of this last again, is articulated to the third bone—the Stapes or Stirrup Bone (St., fig. 16), which in its turn is firmly fastened by the flat end of the Stirrup to the membrane which closes the Fenestra Ovalis. These three bones are suspended in the tympanic cavity in such a way, that they are capable of turning as a whole upon a horizontal axis, which is formed by processes of the Incus and Malleus, which processes fit into depressions in the side walls of the cavity. This axis in fig. 16 is perpendicular to the plane of the paper, and would pass through the head or upper part of the Malleus.
There are also two small muscles in the tympanic cavity, by the contractions and relaxations of which, the membrane of the Fenestra Ovalis and the tympanic membrane can be rendered more or less tense. One, called the Stapedius, passes from the floor of the tympanic cavity to the Stapes, and the other—the Tensor Tympani— from the wall of the Eustachian tube to the tympanum.
Before going further, let us consider the functions of these various parts. The sound waves which enter the External Meatus, pass down it, assisted in their passage by its configuration, and strike against the Tympanum. Now, as the air in the cavity of the tympanum is in communication with the outer air, as long as the latter is at rest, they will be of the same density, and hence the Tympanum will sustain an exactly equal pressure from both sides. When, however, the condensed part of a sound wave comes into contact with it, this equilibrium will no longer exist; the air on the outer side will exert a greater pressure than that on the inner, and the Tympanum will bulge inwards in consequence. The condensed part of the wave will be immediately followed by the rarefied part, and now the state of things is reversed; there will be a greater pressure from the inside, and consequently the Tympanum will move outward, a movement assisted by its own elasticity. We see therefore, that the Tympanum will execute one complete vibration