set in motion by its articulation with the hammer-head, indeed just as if the joint formed quite a solid connection. The latter is of a peculiar form, which, from Helmholtz's researches, may be compared in the mechanical action to toothed-wheels. These work so closely into each other, that the body of the anvil moves with the hammer-head; the long process, however, swings upwards and inwards in the same direction as the hammer-handle. This gives,

h

am

Fig. 66.

as we see, an impulse inwards to the stirrup also, whose foot-plate strikes against the fenestra ovalis, and by this means the sound-wave finds its entrance to the labyrinth.

It is evident that in the backward swing all movements must follow in the reverse direction. We have only to mention that the teeth of the hammer and anviljoint work into each other. When, therefore, under certain circumstances-for instance, a strong pressure of air in the tympanic cavity--the membrane is pressed outwards, it communicates this motion to the hammer-handle, the hammer-head is moved forcibly inwards, and the articulated surfaces separate, which prevents the stirrup from being detached from the fenestra ovalis.

The system of the ear-bones, therefore, performs a

simultaneous motion round a common axis. Such a motion may be compared to that of a lever; indeed we have. here to do with a two-armed lever, such as the bent lever used in bell-work, in order to pull the wire in a different direction. One arm of the lever, upon which the power of the vibrating tympanic membrane acts, is the hammer-handle; the other is the hammer-head, with the anvil and the stirrup, which set the entire labyrinthine fluid in vibration.

The vibrations of the ear-bones are therefore, in reality, transversal, although they are in no way analogous to the vibrations of a stretched cord, or a fixed pen; for the ear-bones are by no means firmly fixed, and do not vibrate by reason of their elasticity, but resemble very light and movable little levers, which are set in simultaneous motion by the vibration of the tympanic membrane.

In a human ear which has been dissected it has also been observed that the ear-bones do really vibrate in the above manner. If a small bright point in them is observed under the microscope, while a sounding organpipe is connected with the auditory canal, it appears to be elongated in the direction of the motion into a small bright streak.

The ear-bones, as we have seen, act as a lever which connects the tympanic membrane and the fenestra ovalis, and transfers the force from the former to the latter. While they are set in motion by the membrane, they strike like drum-sticks against the membrane of the fenestra ovalis, only with this difference, that they are as firmly attached to this membrane as they are to the tympanic membrane, and cannot be detached from it.

CHAPTER IV.

The Function of the Tympanic Membrane-The Telephon-Flame Manometer-The Importance of the Funnel-shaped Form of the Ear-The Muscles in the Tympanic Cavity.

If we strike a membrane, which is stretched over the mouth of a tube or a ring, a tone is produced-its fundamental tone—which increases in acuteness with the tension of the membrane, and diminishes with an increase in the size of the membrane. If we sound the same tone in the neighbourhood of the membrane, which may be done most simply with the voice, the membrane is set in vibration, just as a window-pane rattles when its fundamental tone is sounded loudly. If, however, a different tone is sounded, the membrane is unaffected, and only when we approach its fundamental tone is it set in vibration, or in other words, the resonance begins to increase in strength. The resonance of such a membrane may very readily be recognised by strewing upon it some fine sand, which is thrown into a dancing motion by the vibrations.

It might be supposed that the tympanic membrane also possessed such a fundamental tone. It would, however, be very injurious to our hearing, especially to our

perception of music, if the tympanic membrane behaved like an evenly stretched membrane. For, in this case, we should hear the fundamental tone of our tympanic membrane sounding with great intensity, and other tones with rapidly decreasing intensity, so that we should only hear the greater number of tones very faintly.

Thus the tympanic membrane, as we learn from experiment, has the very remarkable property of answering equally well to tones of any pitch, in the range of tones perceptible to our ear, of about 60 to 4,000 vibrations a second, which is impossible with an ordinary stretched membrane.

There are some artificial means by which membranes can be made to vibrate with a large series of tones, and which therefore show some similarity with the tympanic membrane. We have here to mention a very interesting instrument, the Telephon of Reiss. It is able to transmit tones by telegraphic means, so that we are able to telegraph a melody quite correctly. The construction is as follows:-A box, A (fig. 67), has a tube, R, through which we can sing into the box. On the cover there is a membrane made of dry bladder, which has only a slight tension, and which, on that account, has a very deep fundamental tone. At one spot there is a strip of sheet tin which extends from the edge to the middle, where a light metallic plate, P, rests upon it, which leads to the conducting wire d. On the other side a wire leads to the battery, B, and from the battery · another wire leads to the apparatus K, which can be placed at a distance. This apparatus consists of a sounding-box K, in which a thin electro-magnet, E, lies surrounded by a coil of wire, through which the

current passes. Such an electro-magnet has the property of producing tones, the pitch of which depends upon the number of currents which follow each other with extraordinary rapidity. The number of vibrations of the tone is then equal to the number of currents transmitted. When we sing into the box A, the membrane is set in vibration. The little plate, P, dances up and down on

the strip of tin, and, therefore, contact is made and broken just as many times in a second as the tone sung makes vibrations. This produces exactly the same number of currents of brief duration in a second, and the electromagnet gives the tone sung, which is strengthened by the sounding-box.

Now this apparatus works tolerably well within the compass of an ordinary voice; the membrane, therefore, vibrates with a large number of tones, similarly to our tympanic membrane. This is explained by the fact