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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 fluid 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 tynpanic 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

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Diagram of the membranous labyrinth, all of which is filled with endolymph and sur rounded by perilymph. a, b, c, semicircular canals opening into the ventricle d; e, the saccule from which the uniting canal, f, leads into the membranous canal of the cochlea, g. (Cleland.)

branous labyrinth-perilymph-the 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-ampullæ 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.

through the osseous spiral lamina to reach the membranous portion. A collection of ganglion cells connected with the radiating nerve fibres is found lying in the spiral canal of the osseous

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Transverse section through the membranous canal of the cochlea. Striated zone of basilar membrane, a.

Pectinate zone of the basilar membrane, b.

Perforated zone

of basilar membrane through which the nerves pass, c. Nerve fibres from spiral ganglion, d. Spiral ganglion, e. Limbus, f. Reissner's membrane, g. Tectorial membrane, h. Internal rod of Corti, i. External rod of Corti, m. Special cells receiving nerve terminals, o, p, p'. Epithelial cells covering the basilar membrane, q. Nerve fibres, s. Spiral ligament, t. (Cudiat.)

lamina. Passing through the bony spiral the nerves reach the basilar membrane, which, as before mentioned, forms a great part of the membranous spiral lamina, and upon which the organ of Corti is placed.

The organ of Corti, placed upon the basilar membrane within the membranous canal of the cochlea, is made up of a series of peculiarly curved bars or fibres, called the rods of Corti, and some epitheloid cells provided with short, bristle-like processes. The rods of Corti are fixed by their broad bases upon the basilar membrane, and unite above in such a way that the outer and inner rods form a bow or arch. The spiral series of rods thus propped up against each other leave a small space or tunnel under them, which runs the entire length of the basilar membrane. Beside these rods of Corti are placed rows of cells of an epithelial type into which the nerve endings pass. From the upper surface of these cells, on a level with the apex or junction of the rods, a number of hair-like processes project. A delicate reticulated membrane lies over the rods and the cells, and seems to be lightly attached to their surface, while the hairs pass through its meshes.

The basilar membrane is made up of fibrous bands held together by a delicate membrane. The fibres pass transversely across the spiral canal of the cochlea, so as to subtend the bases of the outer and inner rods. The basilar membrane gradually becomes wider as it passes from the base to the summit of the cochlea. The length of the rods also increases toward the summit of the organ, their bases being more widely separated from one another and their point of junction nearer to the basilar membrane, this forming a lower and wider tunnel. The number of rods of Corti has been estimated at 6000 inner and 4500 outer.

STIMULATION OF THE AUDITORY NERVE.

The stimulation of the nerve of hearing by sound vibrations of the air is less difficult to understand than the excitation of the optic nerve by light waves which are conveyed by an imponderable medium. The motions of the membrane of the drum, being conveyed in the manner already indicated to the liquids within

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