We must not assume that the inequalities in the auditory canal have any particular influence on the conveyance of sound, for if we hear a sound through an îndiarubber tube it makes but little difference whether the tube is straight or bent, since the reflection from the sides allows the sound to pass along it with scarcely any diminution in intensity. The irregularities, therefore, cannot have any special acoustic influence; perhaps they only serve as a protection, by keeping any dust which may enter the ear from reaching the tympanic membrane. The auditory canal has been proved by Helmholtz to have a special resonant action. Since every enclosed volume of air gives its fundamental tone, when we blow across it, as with the mouth-pipe, a bottle, or a hollow globe, the auditory canal, also, must possess such a fundamental tone. The pitch of this tone is tolerably high, so that tones of the same pitch, in consequence of this strong resonance, seem shrill and unpleasant to us; as for instance, the very high tones of a violin or the cry of a bat. Perhaps this is also the cause of the unpleasant sensation produced by the screech caused by scratching glass or porcelain with certain metals. Upon the whole, the resonant action of the auditory canal strengthens the high tones in some degree, and deadens the low ones; this can be varied by placing a small tube of paper in the auditory canal, and by this artificial elongation lowering the pitch of fundamental tone.

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CHAPTER III.

The Tympanic Membrane, the Tympanic Cavity, the Ear-bones, and the Eustachian Tube-The Vibrations of the Tympanic Membrane and of the Ear-bones.

THE tympanic membrane has not an even surface, but it shows a depression, which is directed towards the tympanic cavity. This is caused by the union of the membrane with the handle of the hammer, which is firmly attached to the tympanic membrane throughout its entire length, its end corresponding with the centre of the depression. In fig. 53 this relation is roughly represented; fig. 64, however, shows us the tympanic membrane, after the view of Helmholtz, in its connection with the hammer, as seen from the tympanic cavity; fig. 65 shows the three ear-bones (ossicula auditûs). It shows that the tympanic membrane is contracted inwards into a funnel-shaped depression by the handle of the hammer, and that the centre of the depression is found beneath the middle of the handle. The tympanic membrane, therefore, has a concave form, from the edges to the central depression; but as seen from the auditory canal, it bulges inwards like a sail. The edge of the tympanic membrane is inserted into a bony groove.

Within the membrane is found a layer of muscular

fibres, part of which extend in a radiating, and part in an annular direction. In the figure other details may be observed the head of the hammer k, its long pro

cess, and a point, upon which a tendon of the Tensor Tympani muscle is fixed. We see also a ligament below the head, which fixes the hammer and the mouth of the Eustachian tube t.

In fig. 65 we have a representation of the form and connexion of the ear-bones magnified four diameters. The head of the hammer, which extends above the upper edge of the tympanic membrane, articulates with the anvil bone Am. Besides the long process 7, there is a short process close under the neck of the hammer-head, not seen in the drawing, which is directed outwards and presses against the upper edge of the tympanic membrane, and is here slightly turned up in an outward direction.

The general form of the anvil is that of a bicuspid tooth, whose upper surface articulates with the hammerhead. Two processes descend from it like roots, the

shorter of which Am. k, is directed towards the posterior side of the tyınpanic cavity, and is attached to it by means of ligaments; whilst the longer one, Am. l, projects into the interior of the tympanic cavity, and, by means of a small joint at its termination, articulates with

the head of the stirrup S. The stirrup, whose form is at once suggested by its name, has an oval-shaped footplate, which points inwards and presses against the fenestra ovalis of the tympanic cavity.

The tympanic cavity is a moderately small cavity, which extends above the upper edge of the tympanic membrane. It may be compared to a drum, which is only covered on one side with a membrane—the tympanic membrane. Still it differs from the form of a drum chiefly because it passes downwards, forwards and inwards into the Eustachian tube.

This canal, which leads to the pharynx, serves an important purpose. The tympanic cavity is a space situated within bone and filled with air. If this air were entirely shut off from the atmosphere, it would soon

change its composition, and would probably be absorbed by blood and replaced by liquid secretions, if a continuous renewal were impossible. This renewal can take place through the Eustachian tube, which, from time to time, allows of an exchange of air; but this canal performs other functions than a mere renewal of the air; it also maintains equilibrium of pressure between the air within the tympanic cavity and the atmosphere, which is of great importance for the proper action of the tympanic membrane. For, as soon as the pressure of the atmosphere is greater than the pressure within the tympanic cavity, the membrane will be compressed inwards; and if it is less than the pressure within the cavity, then the membrane will bulge outwards. Both would be injurious to the movements of the membrane, and are prevented by the Eustachian tube, which, at proper times, allows the entrance and exit of air. It does not, however, remain long open, but is generally closed, and is only opened in the act of swallowing—an act which is performed not only during eating, but frequently, at other times, for the carrying away of secreted saliva, and thus its function as an aid to the ear is sufficiently fulfilled.

Valsalva, a prominent anatomist in the seventeenth century, has shown by the following experiment how the Eustachian tube is opened during swallowing. The nose is closed, and the cheeks blown violently out with the mouth shut. If, now, under these conditions, we go through the act of swallowing, without allowing air to escape by the mouth-which is easily done after some practice-a peculiar pressure is immediately felt in both ears, which is caused by air having been forced through the Eustachian tube into the tympanic cavity, and by the