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first, the drum of the ear, or tympanum; secondly, the long external passage, or meatus (Fig. 68).
The drum of the ear and the external meatus, which together constitute the middle ear, would form one cavity, were it not that a delicate membrane, the tympanic membrane (Ty.M. Fig. 68), is tightly stretched in an oblique direction across the passage, so as to divide the comparatively small cavity of the drum from the meatus.
FIG. 69.-The MEMBRANE OF THE DRUM OF THE EAR SEEN FROM THE
INNER SIDE, WITH THE SMALL BONES OF THE EAR; AND THE WALLS OF THE TYMPANUM, WITH THE AIR-CELLS IN THE MASTOID PART OF
THE TEMPORAL BONE. M.C. mastoid cells; Mall. malleus; Inc. incus; St. stapes; a b, lines drawn
through the horizontal axis on which the malleus and incus turn.
The membrane of the tympanum thus prevents any communication by means of the meatus, between the drum and the external air, but such a communication is provided, though in a roundabout way, by the Eustachian tube (Eu. Fig. 68), which leads directly from the fore part of the drum inwards to the roof of the pharynx, where it opens.
20. Three small bones, the auditory ossicles, lie in the cavity of the tympanum. One of these is the stapes, a small bone shaped like a stirrup. It is the foot-plate of this bone which, as already mentioned, is firmly fastened to the membrane of the fenestra ovalis, while its hoop projects outwards into the tympanic cavity (Fig. 69).
Another of these bones is the malleus (Mall. Figs. 68, 19, 70), or hammer-bone, a long process of which is similarly fastened to the inner side of the tympanic membrane (Fig. 70), and a very much smaller process, the slender process, is fastened, as is also the body of the malleus, to the bony wall of the tympanum by ligaments. The
E.M. external auditory meatus ; Ty.M. tympanitic membrane ; Ty, tym
panum; Mall. malleus; Inc. incus; Stp. stapes; F.o. fenestra ovalis; F.r. fenestra rotunda ; Eu. Eustachian tube ; M.L. membranous labyrinth, only one semicircular canal with its ampulla being represented ; Sca.V., Sca. T., Sca.M., the scalæ of the cochlea, which is supposed to be unrolled.
rounded surface of the head of the malleus fits into a corresponding pit in the end of a third bone, the incus or anvil bone, which has two processes-one, horizontal, which rests upon a support afforded to it by the walls of the tympanum ; while the other, vertical, descends almost parallel with the long process of the malleus, and articulates with the stapes, or rather unites with a little bone, the os orbiculare, which articulates with the stapes (Figs. 69 and 70).
The three bones thus form a chain between the fenestra ovalis and the tympanic membrane ; and the whole series turns upon a horizontal axis, the two ends of which, formed by the horizontal process of the incus and the slender process of the malleus, rest in the walls of the tympanum. The general direction of this axis is represented by the line a b in Fig. 69, or by a line perpendicular to the plane of the paper, passing through the head of the inalleus in Fig. 70. It follows, therefore, that whatever causes the membrane of the drum to vibrate backwards and forwards, must force the handle of the malleus to travel in the same way. This must cause a corresponding motion of the long process of the incus, the end of which must drag the stapes backwards and forwards. And, as this is fastened to the membrane of the fenestra ovalis, which is in contact with the perilymph, it must set this fluid vibrating throughout its whole extent, the thrustings in of the membrane of the fenestra ovalis being compensated by corresponding thrustings out of the membrane of the fenestra rotunda, and vice versa.
The vibrations of the perilymph thus produced will affect the endolymph, and this the otolithes, hairs, or fibres; by which, finally, the auditory nerves will be excited.
21. The membrane of the fenestra ovalis and the tympanic menibrane will necessarily vibrate the more freely the looser they are, and the reverse. But there are twa muscles--one, called the stapedius, which passes from the floor of the tympanum to the orbicular bone, and the other, the tensor tympani, from the front wall of the drum to the malleus. Each of the muscles when it contracts tightens the membranes in question, and restricts their vibrations or, in other words, tends to check the effect of any cause which sets these membranes vibrating.
22. The outer extremity of the external meatus is surrounded by the concha or external ear (Co. Fig. 68), a broad, peculiarly-shaped, and for the most part cartilaginous plate, the general plane of which is at right angles with that of the axis of the auditory opening. The concha can be moved by most animals and by some human beings in various directions by means of muscles, which pass to it from the side of the head.
23. The manner in which the complex apparatus now described intermediates between the physical agent, which is the condition of the sensation of sound, and the nervous expansion, the affection of which alone can excite that sensation, must next be considered.
All bodies which produce sound are in a state of vibration, and they communicate the vibrations of their own substance to the air with which they are in contact, and thus throw that air into waves, just as a stick waved backwards and forwards in water throws the water into waves.
The aërial waves, produced by the vibrations of sonorous bodies, in part enter the external auditory passage, and in part strike upon the concha of the external ear and the outer surface of the head. It may be that some of the latter impulses are transmitted through the solid structure of the skull to the organ of hearing ; but before they reach it they must, under ordinary circumstances, have become so scanty and weak, that they may be left out of consideration.
The aërial waves which enter the meatus all impinge upon the membrane of the drum and set it vibrating, stretched membranes taking up vibrations from the air with great readiness.
24. The vibrations thus set up in the membrane of the tympanum are communicated, in part, to the air contained in the drum of the ear, and, in part, to the malleus, and thence to the other auditory ossicles.
The vibrations communicated to the air of the drum impinge upon the inner wall of the tympanum, on the greater part of which, from its density, they can produce very little effect. Where this wall is formed by the membrane of the fenestra rotunda, however, the communication of motion must necessarily be greater.
The vibrations which are communicated to the malleus and the chain of ossicles may be of two kinds : vibrations of the particles of the bones, and vibrations of the bones as a whole. If a beam of wood, freely suspended, be very gently scratched with a pin, its particles will be thrown into a state of vibration, as will be evidenced by the sound
given out, but the beam itself will not be moved. Again, if a strong wind blow against the beam, it will swing visibly, without any vibrations of its particles among themselves. On the other hand, if the beam be sharply struck with a hammer, it will not only give out a sound, showing that its particles are vibrating, but it will also swing from the impulse given to its whole mass.
Under the last-mentioned circumstances, a blind man standing near the beam would be conscious of nothing but the sound, the product of molecular vibration, or invisible oscillation of the particles of the beam ; while a deaf man in the same position, would be aware of nothing but the visible oscillation of the beam as a whole.
25. Thus, to return to the chain of auditory ossicles, while it seems hardly to be doubted that, when the membrane of the drum vibrates, they may be set vibrating both as a whole and in their particles, it depends upon subsidiary arrangements whether the large vibrations, or the minute ones, shall make themselves obvious to the auditory nerve, which is in the position of our deaf, or blind,
The evidence at present is in favour of the conclusion, that it is the vibrations of the bones, as a whole, which are the chief agents in transmitting the impulses of the aërial
For, in the first place, the disposition of the bones and the mode of their articulation are very much against the transmission of molecular vibrations through their substance, while, on the other hand, they are extremely favourable to their vibration en masse. The long processes of the malleus and incus swing, like a pendulum, upon the axis furnished by the short processes of these bones ; while the mode of connection of the incus with the stapes, and of the latter with the edges of the fenestra ovalis, allows that bone free play, inwards and outwards. In the second place the total length of the chain of ossicles is very small coinpared with the length of the waves of audible sounds, and physical considerations teach us that in a like small rod, similarly capable of swinging en masse, the minute molecular vibrations would be inappreciable. Thirdly, it is affirmed, as the result of experiments, that the bone called columella, which, in birds, takes the place of the