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tra rotunda seu cochlearis, which forms a communication between the middle ear and the external scala of the cochlea. This foramen is closed by a membrane, similar to that of the foramen ovale; not, like it, parallel, or nearly so, to that of the tympanum,-but situate obliquely. There is no communication by a chain of bones between it and the membrana tympani.

The small bones or ossicles are four in number, so connected with each other as to form a bent lever; one extremity of which is attached to the tympanic surface of the membrana tympani,-the other to the membrane of the foramen ovale. These bones are usually termed, from their shape-beginning with the most external, and following their order-malleus, incus, os orbiculare, (by some not considered a distinct bone, but a process of the incus,) and stapes. A small muscular apparatus, consisting of three muscles, anterior muscle of the malleus; internal muscle of the same bone; and muscle of the stapes,-is attached to the chain, which it can stretch or relax; and, of course, it produces a similar effect upon the membranes to which the chain is attached. Bellingeri1 thinks, that the fifth pair regulates altogether the involuntary motions of the middle ear.

At the anterior and inferior part of the cavity is the tympanic extremity of a canal, through which the drum receives the air it contains. This canal, called Eustachian tube, is about two inches long, and proceeds obliquely forwards and inwards from the middle ear to the lateral and superior part of the pharynx, into which it opens behind the posterior nares. It is partly osseous, partly fibro-cartilaginous and membranous; and, towards its pharyngeal extremity, expands, terminating by an oval aperture resembling a cleft. Throughout its course it is lined by a mucous membrane, which appears to be a prolongation of that of the nasal fossæ, and is capable of being more or less contracted and expanded by the muscles, which compose and move the velum palati. The cavity of the tympanum communicates, by a short and ragged canal, with numerous cells contained in the mastoid process. These cells open into each other, and vary in number, size, and arrangement in different individuals, and animals. They are called mastoid cells. The cavity of the tympanum is larger in animals whose sense of hearing is most acute. In man, it is about a quarter of an inch deep, and half an inch broad, and is lined by a prolongation of the same membrane as that which lines the Eustachian tube. This membrane, as we have seen, covers the membrana tympani, and the membranes of the foramen ovale, and foramen rotundum. It likewise lines the mastoid cells, and is reflected over the small bones.

The middle ear does not exist in every animal endowed with hearing. It does not begin to appear lower in the scale than reptiles; and is by no means equally complex in all. Frequently, the chain of bones is entirely wanting; and at other times we find one bone only.

3. The internal ear or labyrinth is the most important part of the apparatus. It consists of several irregular cavities in the pars petrosa of the temporal bone, in which the nerve of audition is distributed. It is,

'Edinb. Medical and Surgical Journal, July, 1834, p. 128.

A

Fig. 68.

consequently, here that the physical part of audition terminates, and the nervous begins. The labyrinth comprises the vestibule, semicircular canals, and cochlea. The vestibuleas its name imports-is the hall, that communicates with all the other cavities of the labyrinth. It would appear to be the most essential part of the organ, as it often exists alone. At its inner surface are numerous small foramina, which communicate with the bottom of the meatus auditorius internus, and through which the filaments of the auditory nerve reach the labyrinth. Externally, it communicates with the cavity of the tympanum by the foramen ovale. Posteriorly, it opens into the semicircular canals by five foramina; and anteriorly, by a single foramen, into the internal scala of the cochlea. There is, also, posteriorly and inferiorly, near the common orifice of the two vertical semicircular canals, the opening of a small, bony duct, which terminates internally at the posterior surface of

[graphic]

Labyrinth separated from the solid bone in which it lies embedded. V. Vestibule. X, Y, Z. Semicirnestra ovalis. R. Fenestra rotunda.

cular canals. K. Cochlea. O. Fe

Fig. 69.

Cochlea divided parallel with its axis, through the centre of the modiolus. (Breschet.)

the petrous portion of the temporal bone. This duct is called aquæductus seu diverticulum vestibuli. The semicircular canals are three in number, and occupy the hinder part of the labyrinth. They are called superior vertical, posterior vertical, and horizontal. They are cylindrical cavities, curved semicircularly, and are more expanded at their vestibular origin, which has been, therefore, called ampulla. They are constituted of a plate of bone, situate in the spongy tissue of the pars petrosa, and all of them communicate with the vestibule. The cochlea is the most anterior portion of the labyrinth. nerve, sending its filaments through centre It is so called in consequence of its reof modiolus. 4, 4. Scala tympani of first turn of cochlea. 5, 5. Scala vestibuli of semblance-in man and mammalia-to a first turn. 6. Section of lamina spiralis, its zonula ossea; one of the filaments of the snail's shell; hence, also, its French and cochlear nerve is seen passing between the German names, limaçon, and Schnecke. tributed upon the membrane which invests It is the most intricate part of the organ lamina spiralis. 8. Loops formed by fila- of hearing, and does not admit of easy ments of cochlear nerve. 9,9. Scala tym description. It is a conoidal canal, spirally convoluted, making two turns upon itself, and resting on a bony nucleus or pillar, called modiolus. The base of the nucleus is concave; corresponds to the bottom of the meatus auditorius inter

[graphic]

1. Modiolus. 2. Infundibulum in which

the modiolus terminates. 3, 3. Cochlear

two layers of the lamina spiralis to be dis

the lamina. 7. Membranous portion of the

pani of second turn of cochlea. 10, 10.

Scala vestibuli of second turn; the septum Scala tympani of remaining half turn. 12. Remaining half turn of scala vestibuli; the dome placed over this half turn is the cupola. 13. Lamina of bone which forms

between the two is the lamina spiralis. 11.

the floor of the scala vestibuli curving spirally around to constitute the infundibulum (2). 14. The helicotrema through which a bristle is passed; its lower extre-. nus, and is pierced by small foramina,

mity issues from the scala tympani of middle turn of cochlea.

through which the filaments of the au

ditory nerve reach the cochlea. The spiral canal is divided, in its whole length, by a partition, half osseous and half membranous, called lamina spiralis; so that two distinct tubes are thus formed. These are the scale of the cochlea. At the apex of the cochlea they run into each other, by an opening termed by M. Breschet helicotrema; and at the base, one turns into the vestibule, and is hence called superior or vestibular or internal scala; the other communicates with the cavity of the tympanum by the foramen rotundum, and is called inferior, tympanic, or external scala. At this scala, near the foramen rotundum, a bony canal begins, which proceeds towards the posterior surface of the pars petrosa, on which it opens. It is aquæductus seu diverticulum cochleæ. The cochlea does not exist in all animals that hear. It is not, therefore, of essential importance. It varies, too, greatly, in complication, in different animals. In birds, whose hearing is extremely delicate, it merely consists of a short, hollow, bony process, divided into two scale but without any spiral arrangement. In reptiles, it is still more imperfect; and in many species can scarcely be said to exist. In fishes there is no trace of it.

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Osseous Labyrinth laid open to show especially the Membranous Labyrinth.

X, Y, Z. Semicircular canals. A, A, A. Ampullæ. P. Perilymph between the osseous and membranous labyrinth. U. Utricle. S. Sacculus. O, O. Cretaceous bodies. G, N. Auditory nerve. K. Branch going to the cochlea. L. Lamina spiralis. M. Apex of modiolus. D. Portio dura.

The different cavities of the internal ear are lined by an extremely delicate membrane. In many animals this membrane exists alone,

without any bony parietes. It exhales at its inner surface a limpid fluid, called liquor or lymph of Cotugno or Cotunnius, perilymph of Breschet, which, under special circumstances, can reflow into the aquæductus vestibuli and aquæductus cochleæ. This fluid is contained in all the cavities of the internal ear. Within that of the osseous labyrinth

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1. Corpora quadrigemina. 2, 2. Processus è cerebello ad testes. 3, 3. Corpora restiformia. 4. Fourth ventricle. 5. Iter a tertio ad quartum ventriculum. 6. Calamus scriptorius. 7. Posterior median columns of spinal cord forming by their divergence the point of the calamus, also called ventricle of Arantius. 8. Lines of origin of 4th ventricle, and of auditory

nerve. 9. Anterior branch distributed to cochlea. 10. Poste

semicircular canals.

are contained membranes
having nearly the shape of
the vestibule and semicir-
cular canals, but not ex-
tending into the cochlea.
These membranes, which
compose what has been
called the
the membranous
labyrinth, form a continu-
ous but close sac, contain-
ing a fluid, endolymph,-
termed by M. De Blainville
vitrine auditive, from its
supposed analogy to the
vitreous humour of the eye.
It is similar in appearance
to the perilymph, which
surrounds it on the outer
side, and intervenes be-
tween it and the sides of
the osseous labyrinth so as

[graphic]

rior or vestibular branch. 11. Utriculus communis concealing sacculus proprius from view. 12. Ampulla of oblique semi- to prevent any contact. circular canal. 13. Ampullæ of perpendicular and horizontal The form of the membranous vestibule requires special notice, as it is not an exact imitation of the osseous cavity, being composed of two distinct sacs which open into each other; one of these is termed utricle, sinus seu alveus utriculosus, sacculus vesti

Fig. 72.

buli, and median sinus; the other, sacculus. Each sac contains in its interior a small mass of white calcareous matter resembling powdered chalk, which seems to be suspended in the fluid of the sacs, by means of nervous filaments proceeding from the auditory nerves, G, N, Fig. 70. From the universal presence of these substances in the labyrinth of all the

[graphic]

Ampulla of the External Semicircular Membranous Canal, mammalia, and from their

showing the mode of termination of its Nerve.

much greater size and hard

ness in aquatic animals, it is presumable, that they perform some office of importance in audition. They are termed by M. Breschet, otolithes

and otoconies, according as they are of a hard or a soft consistence. The small square figures (Fig. 70), represent their size and appearance in the dog and the hare.

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It is in the cavities of the internal ear, and on the different parts of the membranous labyrinth, that the auditory or acoustic nerve is distributed. This nerve is the portio mollis of the seventh pair, of most anatomists. It arises, like other nerves of the senses, from the medulla oblongata; and near the anterior paries of the fourth ventricle. Thence it passes obliquely outwards, forwards, and upwards, and enters the meatus auditorius internus, the foramen of which is situate on the posterior surface of the pars petrosa. The base of this meatus corresponds to the inner surface of the vestibule, and to the base of the cochlea. Through the first foramen, near the base of the meatus, the portio dura of the seventh pair or facial nerve passes to gain the aqueduct of Fallopius; along which it proceeds, giving off filaments to different parts of the middle ear, and ultimately issuing by the stylo-mastoid foramen to be lost on the muscles of the face. Below the part of the meatus, where the facial nerve emerges, are several other foramina, through which filaments of the auditory nerve attain the labyrinth. These are distributed to the vestibule, semicircular canals, and cochlea; and terminate, by very delicate ramifications, in the tissue and at the surface of the membrane that lines the labyrinth. The precise mode in which the ramifications terminate has been a matter of dispute: some affirming, that they end in papillæ, as in the marginal figure from Treviranus (Fig. 74); others, that the fibres return by loops. The arrangement is probably analogous to that which prevails in the retina.1 Such is the apparatus concerned in the function of audition. Before proceeding to the physiology of these different parts, and the assistance

'Carpenter's Human Physiology, $ 352,, Lond., 1842.

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