The end of the auditory canal is closed by the membrana tym-pani, which slopes obliquely from above downward and inward, in which direction its size is greater than if it were straight across the canal. This membrane is not flat, for the central point is drawn in by the handle of the malleus, which is firmly attached to it. The membrane is thus held in the shape of a very blunt cone, somewhat like a Japanese umbrella, the apex of which points inward toward the cavity of the drum. The peculiar form of the membrane of the drum is of great importance for distinct hearing.

As every confined volume of air has a certain proper tone to which it resonates readily, so a membrane of a given size and tension has a proper tone {self-tone), the vibration period of which it follows naturally. This tone varies with the tension, as may be seen in a common drum, the note of which can be changed with the tension of its parchment; the tenser the membrane, the higher the pitch. If the membrane of the drum of our ears were set to one tone, our hearing would be imperfect and unpleasant, for we should be wearied by the reiteration and persistence of the one note. This does not occur; the tympanic membrane has no marked self-tone, and no succession of vibrations follows the first effect of the sound waves.

Any self-tone is prevented by the conical shape of the membrane, which is partly due to the traction of the handle of the malleus. If a stretched membrane, such as that of a drum, be drawn out at its centre, so that it is no longer a flat surface, its tension is different at the centre and the periphery, being greatest at that point at which it is drawn, and gradually decreasing toward the margin. Since the existence of a tone of a definite pitch depends upon a certain degree of tension, if no two parts of the membrane are similarly tense, no one tone can be more conspicuous than another. This is the case with the tympanic membrane.

Diagram of the tympanum, showing the relation of the ossicles to the tympanic membrane and the internal ear.

Fig. 237. Diagram of the tympanum, showing the relation of the ossicles to the tympanic membrane and the internal ear. The tympanum is cut through nearly transversely, and the cavity viewed from the front (left ear). (Schafer).

Membrane, m t, of the drum to which the handle of the malleus, m, is attached at u. Head of malleus, m, which is held in position by its suspensory ligament, s.l.m., and external ligament, l.e.m; long process of incus, i., connecting malleus and stapes, s.t., the base of which closes the oval opening of the vestibule p. External auditory meatus e.au.m. Internal auditory meatus i.au.m., where the two parts of the auditory nerve enter, a and b.

The independent vibrations of the membrane are further prevented by the tympanic ossicles. These little bones do not vibrate molecularly, but move en masse in time with the sound vibrations which they deaden. If a substance incapable of vibrating be attached to the membrane of a common drum, it ceases to vibrate. A touch of the finger to the membrane suffices to check the sound produced by a drum. The handle of the malleus, which is joined to the other bones, being fixed to the membrane, acts in this way as a damper, and checks the continuance of any vibration in the membrana tympani.

A small muscle, called the tensor tympani, is attached to the malleus, so as to draw it toward the cavity of the tympanum.

The motions occurring in the membrane of the drum are conveyed across the tympanic cavity by means of the three small bones known as the malleus, the incus, and the stapes. These ossicles form an angular lever, one arm of which (the handle of the malleus) is attached to the centre of the tympanic membrane, and the other shorter arm (the long limb of the incus) unites with the stapes, the base of which is held by the secondary tympanic membrane in the oval opening leading into the vestibule. The stapes is attached at right angles to the extremity of the inner arm of the lever, being jointed to the long arm of the incus. This little angular lever works round an axis which passes from before backward through the head of the malleus, and lies above the membrane of the drum, the two points which act as the bearings or pivots of the motion being the slender process of the malleus in front, and the short limb of the incus behind.

When the tympanic membrane vibrates in response to the sound waves of the air, it moves, and the handle of the malleus moves in and out with it. The body of the incus, being fixed by a firm joint to the head of the malleus, must follow these movements, and cause the oval base of the stapes to press in or draw out the membrane which separates the tympanum from the vestibule. Thus, the vibrations of the air communicated to the tympanic membrane are conveyed across the tympanic cavity to the liquid in the labyrinth.

A small muscle - the stapedius - is attached to the stapes near its junction with the incus, and pulls upon it in such a direction that the bone is drawn out of the direct line of motion. This action, possibly, reduces the more ample vibrations of the tympanic membrane, which might injure the delicate mechanism of the labyrinth.