ployed in setting the chain of ear-bones in motion, the excursions decrease in height. This diminution of the excursions is of great importance; for it is evident that the membrane of the fenestra ovalis cannot execute such large vibrations as the tympanic membrane, since it is twenty times less. On these grounds the vibrations of the tympanic membrane decrease in the membrane itself, during their passage towards the centre and the point of the hammer-handle. At the same time this causes an increase in the force of the sound-waves in the funnel of the tympanic membrane, just as the rapidity of a stream of liquid increases as it passes down a funnel.

In this manner the sound-waves are transferred from the tympanic membrane to a membrane twenty times less; by the ear-bones the entire force of the vibration of the tympanic membrane is concentrated upon a much smaller surface, and its intensity thereby increased twenty times. Moreover, the chain of the ear-bones acts as a lever ; and when both arms of the lever are fully extended, we then find that the arm attached to the tympanic membrane is half as long again as the arm attached to the membrane of the fenestra ovalis, and, therefore, the force of the sound-waves at the fenestra ovalis is increased thirty-fold. In a consideration of the wonderful delicacy with which the apparatus of the tympanic cavity is constructed, we must not omit the action of two small muscles, which are attached to the ear-bones. One of them is united by a.long thin fibre to the handle of the hammer, near the neck (fig. 64), so that it pulls perpendicularly to the tympanic membrane. There can be no doubt that the contraction of this muscle is intended to draw the tympanic

membrane more or less inwards, and thus to increase its tension. The question arises, therefore, what function is to be attributed to these muscles?

A greater or less expansion of the tympanic membrane must evidently have an influence upon its vibration. Since its tension is by no means great, an increase in its tension by means of the tensor tympani muscle must have the power of producing considerable changes. It has, therefore, been imagined that the function of this muscle is to adjust the tympanic membrane for tones of different pitch, so that the ear possesses a power of adjustment similar to that of the eye. It might, however, be argued in opposition to this view that it would be impossible for the tension of the muscle to alter with sufficient rapidity to enable us to perceive the great number of consecutive tones in music, which it is really in our power to do. For the contraction of the muscle would, in a shake, for instance, evidently lag behind the tone heard, and soon fall exactly upon the wrong tone. It is improbable, therefore, that this muscle can act continuously, but we may well imagine it to be set in action when the ear is listening attentively to a certain tone of long duration. The greater its contraction, and the greater the tension of the tympanic membrane, the better shall we perceive high tones, while the deep tones will be less intense, so that in this manner an adjustment of the ear can be accomplished.

The function of a damper has also been ascribed to this muscle. When the contraction of the muscle stretches the tympanic membrane strongly inwards, it might be supposed to damp the vibrations of the mem

brane, just as a vibrating wire is damped when pressed by the finger. It is, therefore, quite possible that this muscle may be set in action when a very deafening sound reaches the ear, since it diminishes the amplitude of the excursions of the tympanic membrane. At the same time it will press the stirrup more strongly into the fenestra ovalis, and will prevent it from executing vibrations of an excessive amplitude. The damping action will be more active for deep than for high tones, since an increase in the tension of the tympanic membrane increases the pitch of its fundamental tone.

Very violent concussions, the firing of a cannon, for instance, are known to have the power of bursting the tympanic membrane. This muscle cannot afford any protection against such a violent and sudden action, since it cannot contract with sufficient rapidity, although it is possible that in such cases it may modify the after-vibrations of the tympanic membrane, which are not inconsiderable.

The function of the second muscle, the stapedius, which is attached to the stirrup, is, as yet, unexplained. It arises from the posterior wall of the tympanic cavity, and is inserted at right angles into the head of the stirrup near its articulation with the anvil-process. It has been supposed that it is intended to act as a damper to the sound-vibrations, since it places the foot-plate of the stirrup obliquely against the fenestra ovalis. This action. might also consist in its pulling at right-angles to the movement of the stirrup, and thereby diminishing its excursions. For instance, if to a vibrating spring we attach a thread, which pulls at right angles to the plane of the vibrations, the latter will be checked.

It is a very curious fact that a nerve passes through the tympanic cavity, called by anatomists the chorda tympani, which has nothing to do with the hearing at all. It passes out again through a crevice in the bone towards the tongue.

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CHAPTER V.

The Labyrinth-The Organs of Corti-The Transmission of Sound in the Labyrinth-Presence of Sympathetic Vibratory Apparatus.

As we pass to the Labyrinth we penetrate into the deepest and most secret part of the auditory organ, in which the physiological action of hearing takes place. In order to follow correctly the passages of this complicated structure, we will first consider it from the exterior. It is represented, in fig. 70, from different sides, after a drawing by Helmholtz. In fig. A we are looking at the Labyrinth from the left. The oval aperture (fenestra ovalis), Fv, leads into the broad intermediate space which is called the vestibule. On both sides of it there are curiously coiled formations; on the one side the cochlea, S, and on the other the semicircular canals, C. The shape of the cochlea is exactly similar to that of an ordinary snail-shell, and consists of two coils and a half. The vestibule has a second opening, the circular aperture (fenestra rotunda), Fc, which is situated at the commencement of the cochlea. Both apertures are covered with a membrane, and open, as already mentioned, into the tympanic cavity. The plate of the stirrup presses against the fenestra ovalis, but the

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ne of the circular aperture is free. The outer