Page images

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

[merged small][graphic][ocr errors][subsumed][ocr errors][subsumed][ocr errors][ocr errors][subsumed][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors]

Diagram of the tympanum, showing the relation of the ossicles to the tympanic mem.

brane and the internal car. The tympanum is cut through nearly transversely, and the

cavity viewed from the front (left ear). (Schäfer.) Membrane, mt, of the drum to which the handle of the malleus, m, is attached at s.

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, st., the base of which closes the oval opening of the vestibule p. External auditory meatus Internal auditory meatus i au.m., where the two parts of the auditory nerve enter, a and b.

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.

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.

EUSTACHIAN TUBE. The tympanum is connected with the pharynx by means of the Eustachian tube, which, though habitually closed, is opened for a moment by swallowing and other motions of the pharynx. On these occasions air can pass in or out of the tympanum, so that the pressure on both sides of the membrane of the drum is equalized. When there is too much or too little air in the tympanic cavity, the tympanic movements are impeded. This difficulty is felt during a cold in the head, when the tube is occluded, and the oxygen being absorbed, the pressure in the tympanic cavity is reduced. Or in performing what is known as Valsalva's experiment, i.e., holding the nose and blowing air into it, whereby the Eustachian tubes are opened, and too much air is often retained in the tympanum, so that the pressure from within is higher than that from without, and hearing becomes dull. If the act of swallowing be then performed, the feeling of tension leaves the ears as the excess of air escapes, and hearing becomes as acute as before.

The Eustachian tube also acts as a way of escape for any fuid that may be secreted by the epithelial lining of the tympanic cavity. The amount of fluid is so small, that the occasional opening of the tube suffices, under ordinary circumstances, for its complete escape. When increased by disease, it may collect in the tympanum, and require catheterization.

If the tubes were permanently open, we should suffer from two great disadvantages. At every breath, during ordinary respiration, some change in tension of the air contained in the cavity of the drum would occur and impair hearing; the vibrations of the air in the pharynx, produced by the voice, would

enter the drum directly, and give rise to an exaggerated shouting noise.

CONDUCTION THROUGH THE LABYRINTH. Every motion of the oval base of the stapes causes a wave to pass along the liquid in the labyrinth. The bony case of the internal ear being firm, the wave travels through all parts of the internal ear. Through the cochlea it arrives at the inner tympanic membrane which closes the fenestra rotunda, and separates the cavity of the tympanum from the scala tympani of the cochlea. The waves have a very complex route in passing from the fenestra ovalis closed by the stapes to the membrane closing the cochlea. By means of the liquid lying around the mem

[merged small][ocr errors][merged small]

Diagram of the membranous labyrinth, all of which is filled with endolymph and sur

jounded by perilymph. a, b, c, semicircular canals opening into the ventricle d; e, the saccule from which the uniting canal, s, leads into the membranous canal of the cochlea, &. (Cleland.)

branous labyrinth-perilymphthe waves pass up the vestibular spiral of the cochlea, and arriving at its summit, they descend by the tympanic spiral to the fenestra rotunda. In this course they pass over and under the fluid-endolymph-contained in the membranous canal of the cochlea in which the special nerve terminations are placed.

For the construction of the labyrinth the student is referred to the text-books of anatomy, as space only admits of a brief account of the special arrangements of the nerve ending.

TERMINALS OF THE AUDITORY NERVE. The nervous mechanisms which are most important for the appreciation of tones are those situated in the cochlea.

The nerve endings found in the membranous sacs in the vestibule are connected with peculiar epitheloid cells, to which are attached fine bristle-like processes. These processes lie in the endolymph, and are related to calcareous masses called otoliths. Waves in this endolymph possibly bring the otoliths into collision with the hairs, and thus give a stimulus to the nerve endings. Noises may be heard from this, but no impressions of tone can be appreciated. The use of the nerves going to the other parts of the labyrinth-ampulla of the semicircular canals-is doubtful, and probably not immediately connected with hearing. * The coils of the cochlea are, throughout their entire length, partially divided by a bony shelf projecting from the central axis into the spiral cavity. This is called the osseous spiral lamina. In the fresh state the separation of the spiral canal into an upper (vestibular) and a lower (tympanic) coil is completed by a membranous partition, which stretches from the bony spiral lamina to the opposite side of the spiral canal. This is called the membranous spiral lamina, and forms the base upon which the special nerve endings of the organ of hearing are placed. An extremely delicate membrane called the membrane of Reissner stretches from the upper side of the spiral partition obliquely upward to the outer wall of the spiral cavity, so as to form a canal and cover the special organ, shutting off a portion of the vestibular coil which lies over the membranous spiral lamina. The canal of the cochlea thus formed is triangular in section. Its floor is made up chiefly of the membranous spiral lamina, particularly the part called the basilar membrane, while the oblique roof is composed of only the thin membrane of Reissner. The canal follows the turns of the cochlea, lying between the vestibular coil and that leading to the tympanum, and is filled with a fluid (endolymph) which is quite separate and distinct from that in the vestibular or tympanic coils (perilymph).

The cochlear division of the auditory nerve passes into little tunnels in the central bony column around which the coils of the cochlea turn, and gives off a series of spiral branches which run

* Compare equilibration, in connection with which they will be described.

« PreviousContinue »