end of the Eustachian tube and the adjoining surface of the sphenoid bone, and from the sides of the upper compartment of the Eustachian orifice. In Fig. 504. Fig. 504.-VIEW OF THE CAVITY The cavity of the tympanum and some parts of the labyrinth have been exposed by a horizontal section removing the upper part of the temporal bone. 1, posterior semicircular canal opened; 2, the cavity of the cochlea opened; 3, osseous part of the Eustachian tube; 4, head of the malleus; 5, incus; 6, stapes, with its base set in the fenestra ovalis; 7, tensor tympani muscle; 8, stapedius. this canal the muscle is conducted nearly horizontally backwards to the cavity of the tympanum. Immediately in front of the fenestra ovalis the tendon of the muscle bends at nearly a right angle over the end of the processus cochleariformis as through a pulley, and, contained in a fibrous sheath, passes outwards to be inserted into the inner part of the handle of the malleus, near its root. The laxator tympani (laxator tympani major of Sömmerring) is generally believed to be distinctly muscular, but being partly concealed by a band of fibrous tissue, doubts are still entertained by some observers as to whether the structure known under this name is of a muscular or ligamentous nature. Arising from the spinous process of the sphenoid bone, and slightly from the cartilaginous part of the Eustachian tube, it is directed backwards, passes through the Glasserian fissure, and is inserted into the neck of the malleus, just above the root of the processus gracilis. The laxator tympani minor of Sömmerring (posterior ligament of the malleus, Lincke) is made up of reddish fibres, which are fixed at one end to the upper and back part of the external auditory meatus, pass forwards and inwards between the middle and inner layers of the membrana tympani, and are inserted into the outer border of the handle of the malleus, and the short process near it. Sömmerring. Icones Organi Auditûs Humani, 1801. The stapedius is a very distinct muscle, but is hid within the bone, being lodged in the descending part of the aqueductus Fallopii and in the hollow of the pyramid. The tendon issues from the aperture at the apex of that little elevation, and passing forwards, surrounded by a fibrous sheath, is inserted into the neck of the stapes posteriorly, close to the articulation of that bone with the lenticular process of the incus. A very slender spine of bone has been found occasionally in the tendon of the stapedius in man: and a similar piece of bone, though of a rounder shape, exists con ACTION OF THE MUSCLES.-LINING MEMBRANE. 751 stantly in the horse, the ox, and other animals. This circumstance is the more interesting when it is remembered that cartilage occupies the position of the stapedius before the muscle is developed. (P. 66 and fig. 528.) Actions. The malleus and incus move together round an axis extending backwards from the attachment of the processus gracilis of the malleus in the Glasserian fissure to the attachment of the short process of the incus posteriorly. The tendon of the tensor tympani muscle passing from within to be inserted below that line, pulls the handle of the malleus inwards, while the laxator tympani inserted above that line, by pulling the head of the bone inwards, moves the handle outwards. The incus, moving along with the malleus, pushes the stapes inwards towards the internal ear when the membrana tympani is made tight, and withdraws that bone from the fenestra ovalis, when the membrana tympani is relaxed. But the cavity of the inner ear is full of a, a', the Fig. 505. g m m This figure is designed to illustrate the effect of the action of the tensor and laxator muscles of the tympanic membrane in connection with their relation to the axis of rotation of the malleus. malleus; b, the incus seen behind it; c, the stapes; m, m', the inner part of the meatus externus closed by the tympanic membrane, of which the posterior half is represented; the axis of rotation of the malleus being supposed to pass through a point at the root of the processus gracilis, g; the line t, indicates the direction and position of the tendon of the tensor tympani pulling the lower part of the malleus inwards, the line l, that of the laxator tympani pulling inwards the upper half of the malleus. liquid; and its walls are unyielding, except at the fenestra rotunda; when, therefore, the stapes is pushed inwards the secondary membrane of the tympanum, which blocks up the fenestra rotunda, must be made tense by pressure from within. The attachment of the handle of the malleus, however, to the membrana tympani allows greater freedom of movement to that process than is allowed to the stapes by the ligament of its base, and when the movement of the stapes ceases, it is plain that the malleus in any movement must rotate on the head of the incus; and hence, probably, the necessity of a moveable articulation between those bones. The action of the stapedius muscle is obviously to draw the head of the stapes backwards, in doing which the hinder end of the base of that bone will be pressed against the margin of the fenestra ovalis, while the forepart will be withdrawn from the fenestra. The object gained by this movement of the stapes is not sufficiently ascertained; but it is at least evident that if the stapes be pressed inwards by the incus in the action of the tensor tympani, the stapedius muscle, if then contracted, will modify the pressure on the internal ear. It is conceivable that the stapedius may thus protect the sensitive part of the ear to a certain extent from excessive stimulation of the auditory nerve. THE LINING MEMBRANE OF THE TYMPANUM. The mucous membrane of the tympanum is continuous with that of the pharynx through the Eustachian tube, and is further prolonged from the tympanum backwards into the mastoid cells. Two folds which cross the breadth of the cavity descend from the part of the membrane which lines the roof. The anterior fold descends to turn round the tendon of the tensor tympani muscle; the posterior fold passes round the stapes. The malleus and incus are invested by the lining of the outer wall of the cavity. The mucous membrane which lines the cartilaginous part of the Eustachian tube resembles much the membrane of the pharynx, with which it is immediately continuous; it is thick and vascular, and is covered by several layers of laminar epithelium with vibratile cilia, and is provided with many simple mucous glands which pour out a thick secretion: in the osseous part of the tube, however, this membrane becomes gradually thinner. In the tympanum and the mastoid cells it is paler, thinner and less vascular, and secretes a less viscid, but yellowish fluid. The epithelium in the tympanic cavity is also ciliated. The cilia, however, are usually absent from the part which lines the membrana tympani (Kölliker, Handbuch, p. 691). THE VESSELS AND NERVES OF THE TYMPANUM. The arteries of the tympanum, though very small, are numerous, and are derived from several branches of the external, and from the internal carotid. The fore part of the cavity is supplied chiefly by the tympanic branch of the internal maxillary (p. 356), which enters by the fissure of Glasser. The back part of the cavity including the mastoid cells, receives its arteries from the stylo-mastoid branch of the posterior auricular artery (p. 353), which is conducted to the tympanum by the aqueduct of Fallopius. These two arteries form by their anastomosis a vascular circle round the margin of the membrana tympani. The smaller arteries of the tympanum are, the petrosal branch of the middle meningeal, which enters through the hiatus Fallopii; branches through the bone from the internal carotid artery, furnished from that vessel whilst in the carotid canal; and occasionally a twig along the Eustachian tube from the ascending pharyngeal artery. The veins of the tympanum pour their contents through the middle meningeal and pharyngeal veins, and through a plexus near the articulation of the lower jaw, into the internal jugular vein. Nerves.-The tympanum contains numerous nerves; for, besides those which supply the parts of the middle ear, there are several which serve merely to connect nerves of different origin. The lining membrane of the tympanum is supplied by filaments from the plexus (tympanic plexus), which occupies the shallow grooves on the inner wall of the cavity, particularly on the surface of the promontory. The tympanic plexus is formed by the communications between, 1st, the tympanic branch (nerve of Jacobson) from the petrous ganglion of the glossopharyngeal; 2nd, a filament from the carotid plexus of the sympathetic; 3rd, a branch which joins the great superficial petrosal nerve, from the Vidian ; 4th and lastly, the small superficial petrosal nerve, from the otic ganglion. Fig. 506. Fig. 506.-VIEW OF THE TYMPANIC PLEXUS OF NERVES (after Hirschfeld and Leveillé). 6, spheno-palatine ganglion; 7, Vidian nerve; 8, great superficial petrosal nerve; 9, carotid branch of the Vidian nerve; 10, part of the sixth nerve connected by twigs with the sympathetic; 11, superior cervical ganglion of the sympathetic; 12, carotid branch; 13, facial nerve; 14, glosso-pharyngeal nerve; 15, nerve of Jacobson; 16, its twig to the sympathetic; 17, filament to the fenestra rotunda; 18, filament to the Eustachian tube; 19, filament to the fenestra ovalis; 20, union of external deep petrosal nerve with the lesser superficial petrosal; 21, internal deep petrosal twig uniting with the great superficial petrosal. 14 The nerve of Jacobson enters the tympanum by a small foramen near its floor, which forms the upper end of a short canal in the petrous portion of the temporal bone, beginning at the base of the skull between the carotid foramen and the jugular THE INTERNAL EAR. OSSEOUS LABYRINTH. 753 fossa. The nerve from the carotid plexus is above and in front of this, and passes through the bone directly from the carotid canal. The branch to the great superficial petrosal nerve is lodged in a canal which opens on the inner wall of the tympanum in front of the fenestra ovalis. The small superficial petrosal nerve also enters at the fore part of the cavity beneath the canal for the tensor tympani. Nerves to Muscles.-The tensor tympani muscle obtains its nerve from the otic ganglion (see fig. 410); the laxator tympani is said to be supplied by the chorda tympani: and the stapedius is figured by Sömmerring as receiving a filament from the facial nerve. The chorda tympani is invested by a tubular reflection of the lining membrane of the tympanum; its course across the cavity has already been described (p. 611). THE INTERNAL EAR, OR LABYRINTH. The inner, or sensory part of the organ of hearing, is contained in the petrous portion of the temporal bone. It consists of a cavity-the osseous labyrinth-hollowed out of the bone, and of the membranous labyrinth contained within the osseous walls. Fig. 507.-RIGHT BONY LABYRINTH, VIEWED FROM THE OUTER SIDE (after Sömmerring). 21 1 The specimen here represented is prepared by separating piecemeal the looser substance of the petrous bone from the dense walls which immediately enclose the labyrinth. 1, the vestibule; 2, fenestra ovalis ; 3, superior semicircular canal; 4, horizontal or external canal; 5, posterior canal; *, ampullæ of the semicircular canals; 6, first turn of the cochlea; 7, second turn; 8, apex; 9, fenestra rotunda. The smaller figure in outline below shows the natural size. The osseous labyrinth is incompletely divided into three parts, named the vestibule, the semicircular canals, and Fig. 507. the cochlea. They are lined throughout by a thin membrane, within which there is a clear fluid named perilymph. The membranous labyrinth is contained within the bony labyrinth, and, being smaller than it, leaves a space between the two, occupied by the perilymph just referred to. The membranous structure supports numerous minute ramifications of the auditory nerve, and encloses a fluid named the endolymph. THE OSSEOUS LABYRINTH. The vestibule forms a central chamber of the labyrinth, which communicates in front with the cochlea, behind with the semicircular canals, on the outer side with the cavity of the tympanum, and on the inner side with the meatus auditorius internus. The vestibule is irregularly ovoidal in shape from before backwards, and is slightly flattened or compressed from without inwards: except in the last-mentioned direction in which it is somewhat smaller, it measures about 1th of an inch in diameter. The outer wall, which separates it from the cavity of the tympanum, is perforated by the fenestra ovalis, which in the recent state is closed by the base of the stapes and its annular ligament. At the fore part of the inner wall is a small round pit, the fovea hemispherica, pierced with many small holes, which serve to transmit branches of the auditory nerve from the internal auditory meatus. This fossa is limited behind by a vertical ridge named crista vestibuli or eminentia pyramidalis. Behind the crest is the small oblique opening of a canal, the aqueduct of the vestibule, which extends to the posterior surface of the bone, and transmits a small vein in a tubular prolongation of membrane. In the roof is an oval depression, placed somewhat transversely, fovea hemi-elliptica, whose inner part is separated by the crest from the hemispherical fossa. At the back part of the vestibule are five round apertures, leading into the semicircular canals: and at the lower and fore part of the cavity is a larger opening, which communicates with the scala vestibuli of the cochleaapertura scala vestibuli. The semicircular canals are three bony tubes, situate above and behind the vestibule, into which they open by five apertures, the contiguous ends of Fig. 508. Fig. 508.-VIEW OF THE INTERIOR OF THE The bony wall of the labyrinth is removed superiorly and externally. 1, fovea hemielliptica; 2, fovea hemispherica; 3, common opening of the superior and posterior semicircular canals; 4, opening of the aqueduct of the vestibule; 5, the superior, 6, the posterior, and 7, the external semicircular canals; 8, spiral tube of the cochlea (scala tympani); 9, opening of the aqueduct of the cochlea; 10, placed on the lamina spiralis in the scala vestibuli. two of the canals being joined. They are unequal in length, but each tube is bent so as to form about two-thirds of a circle; and each presents, at one end, a slightly dilated part, called the ampulla. The canals are compressed laterally, and measure across about th of an inch; but in the ampulla each has a diameter of th of an inch. ΤΟ 20 The canals differ from one another in position with regard to the vestibule, in direction, and in length. The superior semicircular canal is vertical and transverse; and, rising above any other part of the labyrinth, its place is indicated by a smooth arched projection on the upper surface of the bone. The ampullary end of this canal is the anterior, and opens by a distinct orifice into the upper part of the vestibule; whilst the opposite extremity joins the non-dilated end of the posterior semicircular canal, and opens by a common aperture with it into the back part of the vestibule. The posterior semicircular canal, vertical and longitudinal in direction, is the longest of the three tubes: its ampullary end is placed at the lower and back part of the vestibule; and the opposite end joins in the common canal above described. The external semi |