CAVITY AND MEMBRANE OF THE TYMPANUM.

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sideration a roof and a floor, an outer and an inner wall, and an anterior and a posterior boundary.

The roof of the tympanum is formed by a thin plate of bone, which may be easily broken through so as to obtain a view of the tympanic cavity from above; it is situated on the upper surface of the petrous portion of the temporal bone, near the angle of union with the squamous portion, from which in its development it is derived.

The floor is narrow, in consequence of the outer and inner boundaries being inclined towards each other.

The outer wall is mainly formed by a thin semitransparent membranemembrana tympani, which closes the inner end of the external auditory meatus; and, to a small extent, by bone. Immediately in front of the ring of bone into which the membrana tympani is inserted, is the inner extremity of the fissure of Glasser, which gives passage to the laxator tympani muзcle, and attachment to the processus gracilis of the malleus. Close to the back of this fissure is the opening of a small canal (named by Cruveilhier the canal of Huguier), through which the chorda tympani nerve usually escapes from the cavity of the tympanum and the skull.

The membrana tympani is a nearly circular disc, slightly concave on its outer surface. It is inserted into the groove already noticed at the end of the meatus externus, and so obliquely that the membrane inclines towards the anterior and lower part of the canal at an angle of about 45°. The handle of the malleus, one of the small bones of the tympanum, descends between the middle and inuer layers of the membrana tympani to a little below the centre, where it is firmly fixed; and as the direction of this process of the bone is slightly inwards, the outer surface of the membrane is thereby rendered concave, being held inwards in the shape of a shallow cone.

Though very thin, the membrana tympani is composed of three distinct structures. A prolongation of the skin of the external meatus forms the outer layer; the mucous membrane lining the cavity of the tympanum furnishes an inner layer; and between those two is the proper substance of the membrane, made up of fine fibrous and elastic tissues with vessels and nerves. The greater number of the fibres radiate from near the centre at the attachment of the handle of the malleus; but close to the circumference are some circular fibres, which form a dense, almost ligamentous ring.

The inner wall of the tympanum, which separates it from the internal ear, is very uneven, presenting several elevations and foramina. Near its upper part is an ovoid, or nearly kidney-shaped opening-fenestra ovalis, which leads into the cavity of the vestibule. This opening, the long diameter of

which is from before backwards, with a slight inclination downwards in front, is occupied in the recent state by the base of the stapes, and the annular ligament connected with that process of bone. Above the fenestra ovalis, and between it and the roof of the tympanum, a ridge indicates the position of the aqueduct of Fallopius, as it passes backwards, containing the portio dura of the seventh nerve. Below it is a larger and more rounded elevation, caused by the projection outwards of the first turn of the cochlea, and named the promontory, or tuber cochleæ; it is marked by grooves, in which lie the nerves of the tympanic plexus.

Fig. 501.-INNER WALL OF THE OSSEOUS TYMPANUM AS EXPOSED BY A LONGITUDINAL SECTION OF THE PETROUS AND MASTOID BONE (from Gordon).

1, opening of the tympanum into the mastoid cells; 2, fenestra ovalis; 3, fenestra rotunda; 4, promontory; 5, aqueduct of Fallopius, or canal of the facial nerve; 6, junction of the canal for the chorda tympani with the aqueduct; 7, processus cochleariformis; 8, groove above it for the tensor tympani muscle; 9, Eustachian tube; 10, anterior orifice of the carotid canal.

Below and behind the promontory, and somewhat hidden by it, is a slightly oval aperture named fenestra rotunda, which lies within a funnelshaped depression. In the macerated and dried bone the fenestra rotunda opens into the scala tympani of the cochlea; but, in the recent state it is closed by a thin membrane.

The membrane closing the fenestra rotunda-the secondary membrane of the tympanum (Scarpa)-is rather concave towards the tympanic cavity, and is composed of three strata like the membrana tympani ; the middle layer being fibrous, and the outer and inner derived from the membranes lining the cavities between which it is interposed, viz., the tympanum and the cochlea.

The posterior wall of the tympanum presents at its upper part one larger, and several smaller openings, which lead into irregular cavities, the mastoid cells, in the substance of the mastoid process of the temporal bone. These cells communicate freely with one another, and are lined by mucous membrane continuous with that which clothes the tympanum. Behind the fenestra ovalis, and directed forwards, is a small conical eminence, called the pyramid, or eminentia papillaris. Its apex is pierced by a foramen, through which the tendon of the stapedius muscle emerges from a canal which turns downwards in the posterior wall of the tympanum, and joins obliquely the descending part of the aqueduct of Fallopius.

THE EUSTACHIAN TUBE.

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The anterior extremity of the tympanum is narrowed by the gradual descent of the roof, and is continued into the Eustachian orifice. The lower compartment of this orifice, lined with mucous membrane, forms the commencement of the Eustachian tube; the upper compartment, about half an inch long, lodges the tensor tympani muscle, and opens into the tympanum immediately in front of the fenestra ovalis, surrounded by the expanded and everted end of the cochleariform process, which separates it from the lower compartment.

Fig. 502.-ANTERO-POSTERIOR SECTION OF THE TEMPORAL BONE, SHOWING THE INNER WALL OF THE TYMPANUM, WITH THE EUSTACHIAN TUBE AND SMALL BONES IN THE RECENT STATE (from Arnold).

1, styloid process; 2, mastoid process; 3, upper part of the petrous bone; 4, pharyngeal end of the Eustachian tube; 5, its cartilage; 6, its mucous surface; 7, carotid canal; 8, fenestra rotunda; 9, malleus; 10, incus; 11, stapes; 12, pyramid and stapedius muscle; above 9, and behind 10, the suspensory ligaments of the malleus and incus are also seen.

The Eustachian tube is a canal, formed partly of bone, partly of cartilage and membrane, which leads from the cavity of the tympanum to the upper part of the pharynx. From the tympanum it is directed forwards and inwards, with a little inclination downwards; and its entire length is about an inch and a half. The osseous division of the Eustachian tube, already described in the Osteology, is placed in the angle of junction of the petrous portion of the temporal bone with the squamous portion. The anterior part of the tube is formed of a triangular piece of cartilage, the edges of which are slightly curled round towards each other, leaving an interval at the under side, in which the canal is completed by dense but pliable fibrous membrane. Narrow behind, the tube gradually expands till it becomes wide and trumpet-shaped in front; and the anterior part is compressed from side to side, and is fixed to the inner pterygoid process of the sphenoid bone. The anterior opening is oval in form, and is placed obliquely at the side and upper part of the pharynx, into which its prominent margin projects behind the lower meatus of the nose, and above the level of the hard palate. Through this aperture the mucous membrane of the pharynx is continuous with that which lines the tympanum, and under certain conditions air passes into and out of that cavity.

SMALL BONES OF THE EAR.

Three small bones (ossicula auditûs) are contained in the upper part of the tympanum: of these, the outermost (malleus) is attached to the membrana tympani ; the innermost (stapes) is fixed in the fenestra ovalis ; and

Fig. 503.-BONES OF THE TYMPANUM
OF THE RIGHT SIDE (from Arnold). }

A, malleus; 1, its head; 2, the handle; 3, long or slender process; 4, short process; B, incus; 1, its body; 2, the long process with the orbicular process; 3, short or posterior process; 4, articular surface receiving the head of the malleus; C, stapes; 1, head; 2, posterior crus; 3, anterior crus; 4, base; C*, base of the stapes; D, the three bones in their natural connection as seen from the outside; a, malleus; b, incus; c, stapes.

the third (incus), placed between the other two, is connected to both by articular surfaces. The malleus and incus are placed in nearly a vertical, the stapes in a horizontal direction. They form

together an angular and jointed connecting rod between the membrana tympani and the membrane which closes the fenestra ovalis.

The malleus, or hammer bone, consists of a central thicker portion, with processes of different lengths. At the upper end of the bone is a rounded head (capitulum), which presents internally and posteriorly an irregularly oval surface covered with cartilage, for articulation with the incus. Below the head is a constricted neck (cervix); and beneath this another slight enlargement of the bone, to which the processes are attached. The handle (manubrium) of the malleus is a tapering and slightly twisted process, compressed from before backwards to near its point, where it is flattened in the opposite direction: it descends with a slight inclination forwards and inwards, and is received between the middle and inner layers of the membrana tympani, to which it is closely attached. The long process (processus gracilis) is a very slender spiculum of bone, which in the adult is usually broken off in its removal from the tympanum, in consequence of its union with the temporal bone; it projects at nearly a right angle from the front of the neck of the malleus, and extends thence obliquely downwards and forwards to the Glasserian fissure. Its end is flattened and expanded, and is connected by ligamentous fibres and by bone to the sides of the fissure. The short process (processus brevis vel obtusus) is a low conical eminence springing from the root of the manubrium, beneath the cervix, and projecting outwards towards the upper part of the membrana tympani.

The incus has been compared to an anvil in form; but it resembles perhaps more nearly a tooth with two fangs widely separated. It consists of a body and two processes. The body presents in front a concavo-convex articular surface, which is directed upwards and forwards, and receives the head of the malleus. The surfaces of the joint thus formed are tipped with articular cartilage and enclosed by a synovial membrane. The shorter of

THE SMALL BONES AND THEIR MUSCLES.

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the two processes (crus breve) of the incus projects nearly horizontally backwards from the upper part of the body of the bone, and is connected by ligamentous fibres with the posterior wall of the tympanum near the entrance of the mastoid cells. The long process (crus longum) tapers rather more gradually, and descends nearly vertically behind the handle of the malleus at its extremity it is bent inwards, and is suddenly narrowed into a short neck; and upon this is set a flattened rounded tubercle (processus lenticularis), tipped with cartilage. This tubercle, which articulates with the head of the stapes, was formerly, under the name of os orbiculare seu lenticulare, described as a separate bone, which indeed it originally is in childhood.

The stapes, the third and innermost bone of the ear, is in shape remarkably like a stirrup, and is composed of a head, a base, and two crura. The head is directed outwards, and has on its end a slight depression, covered with cartilage, which articulates with the lenticular process of the incus. The base is a plate of bone placed in the fenestra ovalis, to the margin of which it is fixed by ligamentous fibres. The form of the base is irregularly oval, the upper margin being curved, while the lower is nearly straight. The crura of the stapes diverge from a constricted part (neck) of the bone, situated close to the head, and are attached to the outer surface of the base near its extremities. The anterior crus is the shorter and straighter of the two. The crura, with the base of the stapes, enclose a small triangular or arched space, which in the recent state is occupied by a thin membrane stretched across. A shallow groove runs round the opposed surfaces of the bone, and into this the membrane is received.

LIGAMENTS AND MUSCLES OF THE TYMPANUM.

Ligaments. In the articulations of the small bones of the ear with each other, the connection is strengthened by ligamentous fibres which cover the synovial membranes.

The attachment of the bones of the ear to the walls of the tympanum is effected partly by the reflections of the mucous membrane lining that cavity, but chiefly by muscles and by the following ligaments.

The suspensory ligament of the malleus consists of a small bundle of fibres, which descends perpendicularly from the roof of the tympanum to the head of the malleus.

The incus is likewise suspended by a small ligament (the posterior ligament of the incus), which extends from near the point of the short crus directly backwards towards the posterior wall of the tympanum, where it is attached near the entrance to the mastoid cells.

Arnold describes an upper ligament which attaches the incus, near its articulation with the malleus, to the roof of the tympanum. It lies close behind the suspensory ligament of the malleus.

The annular or orbicular ligament of the stapes connects the base of the bone to the margin of the fenestra ovalis, in which it is lodged.

Muscles.-There are three well-determined muscles of the tympanum. Sömmerring describes four, and some authors a larger number; but their descriptions are not confirmed by later research. Of the three muscles generally recognised, two are attached to the malleus, and one to the stapes. The tensor tympani (musculus internus mallei) is the largest of these muscles. It consists of a tapering fleshy part, about half an inch in length, and a slender tendon. The muscular fibres arise from the cartilaginous