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flow in such a case than he ever saw lost in a surgical operation.
The Ossicula Auditus are from without inwards, the malleus, incus, and stapes. The malleus or hammer consists of a head, neck, and handle, which is attached to the membrana tympani. Its short and long processes give attachment respectively to the tensor and laxator tympani. It articulates by its head and neck with the next bone. The incus or anvil more resembles a molar tooth; it consists of a body joined to the malleus, a short process running to the mastoid cells, and a long one articulated to the head of the stapes. The end of this process is often an epiphysis, and being detached, is named the " os orbiculare." The stapes is a beautiful little object, precisely like a stirrup, and consists of a head joined to the incus, a neck into which the stapedius is inserted, two branches running to the base, which is set into the fenestra ovalis by a membrane. This bone is the most important, being ossified and nearly of its full size before birth; and if it be lost, the fluids of the vestibule flow out and complete deafness ensues. The irregular course and free joints of these three bones allow free motion, which is always towards the internal ear, both the muscles producing tension of the membrana tympani. For this reason they have been compared to the iris in function. Even when the muscles are absent, as in the frog, the bones are separate; but in birds but one bone exists, the columella.
The air in the tympanic cavity insulates or prevents the dispersion of sound from a better conductor, and keeps a due elastic pressure on the membranes, which cannot vary as the temperature is always the same; echoes are said to be obviated by its free exit through the Eustachian tube. In descending in a diving-bell, or by making a very full exspiratory effort with the nose and mouth closed, the pressure from within is increased, and hearing is rendered very indistinct, distant low sounds not being perceived, though near, and shrill ones are.
The internal ear is named, from its intricacy, the labyrinth, and consists of three parts—the vestibule, semicircular canals, and cochlea.
The Vestibule is a cavity intermediate between the tympanum and the canals and cochlea. Its openings are, the fenestra ovalis, one for the cochlea, five for the semicircular canals, the aqueduct—a small venous passage lined by dura mater, and several minute perforations (the macula cribrosa), through which the auditory nerve enters from the internal meatus.
The Semicircular Canals each form more than half a circle, and are a little flattened laterally, and about A in width. They open into the vestibule by each end;
The Labyrinth—Vestibule, Semicircular Canals, and Cochlea—the bony and membraneous portions of the first and second being displayed.
but two of them, the superior and inferior, previously
joining, only 5 openings exist. One end, more dilated than the other, is named the ampulla. The superior canal is anterior to the others, and rises so high as to raise the upper surface of the petrous portion, or to project free from it in the foetus. The inferior canal is behind and at right angels to the last, and the horizontal leads backwards and outwards, and is the shortest.
The two parts of the osseous labyrinth we have now described are lined by the membraneous labyrinth. This is much smaller—but 4 in the canals, as a fluid, the perilymph or liquor Cotunnii, intervenes, save where the nerves enter. Another fluid, the endolymph, discovered by Scarpa, fills the membraneous labyrinth, and being nearly incompressible, carries impressions from the fenestra. Deafness follows its escapes by puncture. Three coats constitute the membrane—an outer, soft and stained by pigment cells, almost identical with those of the choroid; a middle, tough and transparent-like fibro-cartilage ; and an inner, consisting of nucleated epithelium. The acoustic artery forms a plexus, very like that of the retina, on the outer surface. Particles of carbonate of lime are always found in the endolymph, consolidated into earstones or otoliths in fishes, and an ear-dust or otoconia in other animals. In man the particles are not loose in the fluid, but bound in masses by some delicate tissue.
The Cochlea, so named from its likeness to a shell, is an astonishing complex apparatus. It is conical, with its base towards the internal meatus, from which it proceeds forwards and outwards, almost horizontally. An axis, or modiolus, runs up the centre from base to apex, and round this is wound, for 2£ turns, the spiral canal, about 1£ inches in length, and & in width. There are 8 openings at the beginning of the canal—one into the vestibule; another, closed by membrane, into the tympanum—the fenestra rotunda; and the aqueduct, which carries a vein to the jugular fossa. The canal is divided by the spiral lamina into two passages—a superior, the scala vestibuli, communicating with the vestibule; and the scala tympani, which begins at the fenestra rotunda. . The spiral lamina consists of bone towards the modiolus, then of a membrane like the capsule of the lens; a large capillary runs along its under surface ; and, lastly, of a structure which Kolliker describes as the spiral ligament, but which Bowman named the cochlearis muscle. In tissue it is very similar to the ciliary muscle, and as it can make tense the spiral lamina, its function may be analogous. On the upper surface of the bony part of the lamina, Bowman discovered a beautiful structure, which, from having a toothed edge, he calls the denticulate lamina. Its texture is like cartilage, with cells remarkably arranged. Its teeth project into the scala vestibuli, and in a groove below them are found columnar cells, not unlike those of Jacob's membrane. Below the membraneous part the Marquis Corti discovered another layer, and the space between them may be called the scala media. The bony lamina ends above by a hook or hamula, the concavity of which is named the helicotrema, as the scalae communicate through it. The spiral canal dilates into a summit-like dome, the cupola, which the modiolus includes somewhat by spreading into an infundibulum.
The Auditory Nerve, or portio mollis of 7th, arises in the medulla oblongata by fibres at the back of the calamus scriptorius. In its softness and absence of neurilemma it resembles the brain substance more than nerves, and it contains many bipolar cells. A ganglion, very distinct in the carp, is found at its origin, and some regard the pneumogastric lobule in this light. Together with the facial nerve and some other fibres which Wrisberg named the portio intermedia, it enters the internal meatus and divides into one portion for the vestibule and semicircular canals, and another which becomes the cochlear nerve. The vestibular nerves tie closely together the osseous and membraneous surfaces, and some seem to end by running towards the otoconia, others by spreading out on the inner side of the membrane, having lost their white substance. In the semicircular canals the nerves only extend as far as the ampullae, where they end in a forked or looped manner. The cochlear nerve passes up through the modiolus and sends branches to the under surface of the spiral lamina, where they form a most intricate plexus.
The Laws of Sound are epitomised as follows, by Todd and Bowman, in their admirable article on the organ of hearing:
"The following points respecting the laws of sound should be borne in mind in considering the offices of the various parts of that complex acoustic apparatus, the human ear:
"1. Any irregular impulse communicated to the air will produce a noise; a succession of impulses occurring . at exactly equal intervals of time, and exactly similar in duration and intensity, constitute a musical sound.
"2. The frequency of repetition necessary for the production of a continued sound from single impulses is, probably, generally not less than sixteen times in a second; but Savart thinks that some ears may distinguish a sound resulting from only ten or eight vibrations in a second. On the other hand, sounds are audible which consist of 24,000 vibrations in a second.
"8. Sound maybe propagated or conducted by air, gases, liquids, and solids, with various degrees of rapidity.
"4. Sound travels through air at the temperature of 62° Fahr. at the rate of 1,125 feet in a second.
"5. Sound is incapable of transmission through a vacuum.
"6. The propagation of sound is the more effectively performed as the medium of transmission is more dense. Rarefied air, gases of low density, and soft solids, are less perfect conductors of sound than much denser materials of the same kind.