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Paries Mastoidea. The mastoid or posterior wall presents, from above downwards: (1) a rounded or triangular opening, extending backwards from the recessus epitympanicus and leading into the tympanic antrum (Fig. 709); (2) a depression, the fossa incudis, situated in the postero-inferior part of the recessus epitympanicus (Fig. 710), for the reception of the end of the short crus of the incus; (3) a minute conical bony projection, the eminentia pyramidalis (Fig. 709), the summit of which is perforated by a round aperture for the passage of the tendon of the stapedius muscle. This aperture is continued downwards and backwards as a canal in front of the facial canal, and frequently opens, by a minute orifice, on the base of the skull in front of the stylo-mastoid foramen; it communicates with the facial canal by one or two small foramina, which transmit the vessels and nerve to the stapedius muscle; a minute spicule of bone often extends from the eminentia pyramidalis to the promontory on the labyrinthic wall of the tympanum; (4) a small aperture, the apertura tympanica canaliculi chorda (Fig. 710), which is situated close to the posterior edge of the membrana tympani, nearly on a level with the upper end of the manubrium mallei; (5) a rounded eminence, the prominentia styloidea, is sometimes seen below the last, and is caused by the upward and forward prolongation of the styloid process.
Paries Carotica.-The carotid or anterior wall is narrowed in its transverse diameter by the approximation of the lateral and medial boundaries of the tympanic cavity, and in its vertical diameter by the descent of the roof and the ascent of the carotid canal. It presents (Fig. 709) two parallel semicanals, one above the other separated by a thin lamella of bone, the septum canalis musculotubarii (O.T. processus cochleariformis). These run forwards on the lateral wall of the carotid canal and open in the angle between the squama and the petrous part of the temporal bone. The higher and smaller of the two is termed the semicanalis m tensoris tympani, and lies immediately below the tegmen tympani. It has a diameter of about 2 mm., and extends on to the medial wall of the tympanic cavity above the anterior part of the fenestra vestibuli. The lower and larger semicanal gradually increases in size from before backwards, and is named the semicanalis tuba auditivæ It forms the bony part of the auditory tube and opens on the carotid wall of the tympanic cavity opposite the orifice leading into the tympanic antrum. Below the orifice of the auditory tube the anterior part of the tympanic cavity is separated from the ascending portion of the carotid canal by a thin plate of bone in which there are sometimes gaps or deficiencies. It is perforated by the carotico- tympanic canal, which transmits the carotico- tympanic nerve from the sympathetic plexus of the carotid artery to the tympanic plexus. The auditory tube is described on p. 837.
Paries Membranacea.-The membranous or lateral wall is formed almost entirely by the membrana tympani (Fig. 710), which closes the medial end of the external acoustic meatus, and is fixed throughout the greater part of its circumference in a groove, the sulcus tympanicus. The bony ring containing this sulcus is deficient superiorly, where it exhibits a distinct notch, the notch of Rivinus. On a level with the upper edge of the membrane, and in front of the ring of bone in which it is fixed, is the medial end of the petrotympanic fissure. This transmits the tympanic branch of the internal maxillary artery, and lodges the anterior process and anterior ligament of the malleus. Close to the medial end of the fissure is the iter chorda anterius through which the chorda tympani nerve leaves the tympanic cavity.
Membrana Tympani.-The tympanic membrane is an elliptical disc, its greatest diameter, 9 to 10 mm., being directed from above, downwards and forwards, whils: its least diameter is from 8 to 9 mm. It is placed very obliquely, forming an angle of about 55° with the lower and anterior walls of the external acoustic meatus; it is said to be more oblique in cretins and deaf mutes, and more perpendicular in musicians.
The circumference of that portion of the membrane which is fixed in the sulcus tympanicus is considerably thickened, and is named the annulus fibrocartilagineus It is prolonged from the anterior and posterior extremities of the notch of Rivinus to the processus lateralis of the malleus, in the form of two ligamentous bands, the
anterior and posterior malleolar plicæ. The small triangular portion of the membrane (Fig. 710) situated above these folds is thin and lax, and constitutes the pars flaccida (O.T. membrane of Shrapnell); the main portion of the membrane is, on the other hand, tightly stretched and termed the pars tensa. A small orifice, sometimes seen in the pars flaccida, is probably either a pathological condition or has been produced artificially during manipulation. The manubrium mallei is firmly fixed to the medial surface of the membrana tympani, the central portion of which is drawn towards the tympanic cavity so that its lateral surface is concave. The deepest part of this concavity corresponds with the lower end of the manubrium of the malleus, and is named the umbo membranæ tympani.
The membrana tympani consists of three layers: (1) a lateral, the stratum cutaneum; (2) an intermediate, the membrana propria; (3) a medial, the stratum
The stratum cutaneum is continuous with the skin of the meatus, and consists of a thin layer of cutis covered with epidermis. The cutis is thickest near the circumference; the epidermis, on the other hand, is thickest near the centre of the membrane.
The membrana propria consists of two sets of fibres: (a) a lateral, the stratum radiatum, situated immediately under the stratum cutaneum, and radiating from the manubrium of the malleus to the annulus fibrocartilagineus; (b) a medial, the stratum circulare, the fibres of which are numerous near the circumference, but scattered and few in number near the centre of the membrane (Fig. 710). Both radial and circular fibres are absent from the pars flaccida, which consists only of the cutaneous and mucous strata. Gruber pointed out that, in addition to the radial and circular fibres, there exists, next the stratum mucosum, a series of dendritic or branched fibres, which are best developed in the posterior part of the
The stratum mucosum is continuous with the mucous lining of the tympanic - cavity. It is thicker over the upper part of the membrane than near its centre, and is covered with pavement epithelium.
Otoscopic Examination of the Membrana Tympani (Fig. 711).-The membrana tympani, in the living, is of a "pearl-gray" colour, but may present a reddish or yellowish tinge, depending upon the condition of its mucous lining and on the condition of the cutaneous lining of the meatus; the posterior segment is usually clearer than the anterior. At the antero-superior part, close to its = periphery, a whitish point appears as if projecting towards the meatus; this is the processus lateralis of the Anterior malleolar malleus. Passing downwards and backwards from this point to the
umbo is a ridge caused by the Manubrium mallei manubrium mallei, the lower extremity of which appears rounded. Two ridges, corresponding with the malleolar plica, extend from the processus lateralis of the malleus, one forwards and upwards, the other backwards and upwards. Behind, FIG. 711.-LEFT TYMPANIC MEMBRANE (as viewed from the external and near the lower extremity of the acoustic meatus). manubrium mallei, is a reddish or yellowish spot, due to the promontory of the medial wall of the tympanic cavity shining through. If the membrane is very transparent, the long crus of the incus may be visible behind the upper part of the manubrium mallei, and reaching downwards as far as its middle. From the lower end of the manubrium mallei, the "cone of light" or "luminous triangle" extends downwards and forwards, its apex being directed towards the handle; this triangle varies in size in different people. A line prolonging the manubrium downwards divides the membrane into two parts, while another, drawn at right angles to this through the umbo, will subdivide it into quadrants, viz., postero-superior, postero-inferior, antero-superior, and antero-inferior; this subdivision is useful in enabling the otologist to localise and describe accurately the seat of lesions in the
Lateral process of malleus
Vascular and Nervous Supply of the Membrana Tympani. The arteries are arranged in two sets, one on the cutaneous and another on the mucous surface; they anastomose by means of small branches which pierce the membrane, especially near its periphery. The first set is derived chiefly from the deep auricular branch of the internal maxillary, whilst those on the
mucous surface are small and proceed from the anterior tympanic branch of the internal maxillary, and from the stylo-mastoid branch of the posterior auricular. The veins from the cutaneous surface open into the external jugular; those from the mucous surface partly into the venous plexus on the auditory tube, and partly into the transverse sinus and veins of the dura mater. The lymph vessels, like the blood-vessels, are arranged in two sets, cutaneous and mucous, which, however, communicate freely with each other. Kessel has described as lymphatics the spaces between the branches of Gruber's dendritic fibres. The lateral surface of the membrane receives its nerves from the auriculo-temporal branch of the trigeminal and from the auricular branch of the vagus; the medial surface, from the tympanic branch of the glossopharyngeal.
ANTRUM TYMPANICUM ET CELLULE MASTOIDEA.
Antrum Tympanicum.-The tympanic antrum (O.T. mastoid antrum) is an airspace situated above and behind the tympanic cavity. It is nearly as large in the new-born child as in the adult. In the adult its measurements are-length from
Preparation to display the position and relations of the tympanic antrum. The greater part of the posterior wall of the external acoustic meatus has been removed, leaving only a bridge of bone at its medial extremity; under this a bristle is displayed, passing from the tympanic antrum through the iter to the cavity of the tympanum.
12 to 15 mm., height from 8 to 10 mm., and width from 6 to 8 mm. It is roofed in by the tegmen tympani, and its floor and medial wall are formed by the pars petrosa and pars mastoidea of the temporal bone, while laterally it is closed by the junction of the thin outer part of the squama with the pars mastoidea. It communicates with the epitympanic recess by a triangular or rounded opening, on the medial wall of which, immediately above and behind the canalis facialis, is a smooth, convex area of bone indicating the position of the ampullated extremities of the superior and lateral semicircular canals. At birth the lateral wall of the antrum has a thickness of only 1-2 mm., but by the ninth year this has increased to about 10 mm. Coincident with the growth of the mastoid process the mastoid air-cells are developed downwards and backwards as diverticula from the antrum, and present the greatest possible variation in different skulls.
Cellulæ Mastoideæ.-The mastoid air-cells may be large, comparatively few in number, and involve the whole mastoid process, in which case the compact bone surrounding them is extremely thin, and the innermost cells are separated
from the transverse sinus by a transparent lamella which, in some instances, is partly deficient. In other cases the cells may be small and numerous, invading only a portion of the process, the remainder consisting of diploëtic tissue; occasionally a solid mastoid is met with. No definite conclusion can be formed as to their condition by external percussion or examination. The air-cells are not limited to the mastoid portion of the temporal bone, but extend forwards over the roof of the meatus, upwards towards the squama temporalis, and medially towards the temporo-occipital suture; occasionally they invade the pars jugularis of the occipital bone. The tympanic antrum and the mastoid air-cells are lined with thin mucous membrane, continuous with that of the tympanic cavity; the deep surface of the mucous membrane is fixed to the periosteum; its free surface is I covered with a layer of flattened, non-ciliated epithelium.
The auditory tube (O.T. Eustachian tube) leads from the tympanic cavity to the nasal part of the pharynx, and transmits air to the former, in order that the pressures on the medial and lateral surfaces of the membrana tympani may be equalised; it may also serve to convey mucous secretion from the tympanic cavity. Its tympanic orifice (Fig. 709) opens into the anterior part of the tympanic cavity, below the semicanal for the tensor tympani muscle. Directed downwards and medialwards, the tube ends on the upper part of the nasal part of the pharynx by a wide pharyngeal orifice (Fig. 674, p. 803). It measures about 35 mm. in length, and forms with the horizontal plane an angle of 30° to 40°, with the sagittal plane an angle of about 45°, and with the bony part of the external acoustic meatus an angle of 135° to 140°. It consists of two portions: (a) an antero-medial, the pars cartilaginea tubæ auditivæ, having a length of about 25 mm.; and (b) a postero-lateral, the pars ossea tubæ auditivæ, measuring about 10 mm. in length. The two portions are not in the same plane, the cartilaginous part inclining downwards a little more than the osseous portion, and forming with it a wide angle. The lumen of the tube is widest at the pharyngeal orifice, narrowest at the junction of the bony and cartilaginous portions, forming here the isthmus, and again expanding towards the tympanic cavity; hence it presents, on longitudinal section, somewhat the appearance of an hour-glass. The pars ossea occupies the angle between the tympanic part and the petrous part of the temporal bone, and is separated by the septum canalis musculotubarii from the semicanal containing the tensor tympani muscle; medial to it is the carotid canal. The pars cartilaginea consists partly of cartilage and partly of fibrous membrane. The cartilage of the pharyngea auditory tube is an elongated triangular plate, of which the apex is firmly attached to the medial end of the pars ossea, while the base is free, and forms a projection on the upper and posterior aspects of the pharyngeal orifice. The upper edge of this cartilaginous plate is bent laterally in the form of a hook, and so produces a furrow open below, the furrow being converted into a complete canal by the membranous lamina of the tube. On transverse section (Fig. 713) the cartilage presents two laminæ continuous with each other superiorly: (a) lamina medialis, broad and thick; and (b) lamina lateralis, thin and hook-shaped. At the pharyngeal orifice the lamina medialis forms the entire medial wall of the tube, but it gradually diminishes in breadth on approaching the isthmus tube. Fissures are often seen
Lumen of tube
FIG. 713.-TRANSVERSE SECTION OF THE CARTILAGINOUS PART OF THE AUDITORY TUBE.
in the cartilage; sometimes it is completely separated into several pieces, or accessory islands may be observed in the roof, floor, or membranous lamina.
The upper and medial surfaces of the medial lamina are firmly fixed to the base of the skull, where it lies in a groove, the sulcus tubæ auditive, between the great wing of the sphenoid and the petrous part of the temporal. Extending forwards on to the root of the pterygoid process this sulcus ends at a projection, the processus tubarius, on the medial pterygoid lamina. The tensor veli palatini muscle lies on the lateral side of the tube and receives some fibres of origin from its lamina lateralis; these fibres constitute the dilatator tube muscle (Rüdinger). On the medial side of the cartilage are the levator veli palatini and the mucous membrane of the pharynx. The membranous lamina consists of a strong fibrous membrane, stretching between the two edges of the cartilage, and so completing the lower and lateral walls of the tube. Thin above, it becomes thickened below and forms the fascia salpingopharyngea of Tröltsch, which gives origin to some of the fibres of the tensor veli palatini muscle. Between this fascia and the mucous lining of the tube is a layer of adipose tissue.
The pharyngeal orifice of the auditory tube, triangular or oval in shape, is situated on the lateral wall of the nasal part of the pharynx, the centre of the opening being on a level with the posterior end of the inferior nasal concha. It is bounded above and behind by a pad, the torus tubarius, produced by the base of the cartilage, which there abuts against the mucous membrane; the posterior part of the torus is very prominent and forms the anterior boundary of the pharyngeal recess. Prolonged downwards from it is an elevation of the mucous membrane, termed the plica salpingopharyngea, which covers the small salpingopharyngeus muscle. From the upper part of the torus an indistinct fold, the plica salpingopalatina, extends to the palate.
The mucous lining of the tube is continuous behind with that of the tympanic cavity, and in front with that of the nasal part of the pharynx. It is thin in the pars ossea, contains few, if any, mucous glands, and is firmly fixed to the bony wall; in the pars cartilaginea it is loose and thrown into longitudinal folds. Numerous mucous glands open into the tube near its pharyngeal orifice, and here also exists a considerable amount of adenoid tissue, which constitutes the "tube-tonsil" of Gerlach. This adenoid tissue is continuous with that of the nasal part of the pharynx, and, like it, is especially well developed in children. The lumen of the tube is lined with ciliated columnar epithelium.
The tube is opened, during deglutition, by the dilatator tube and salpingopharyngeus muscles. The former springs superiorly from the cartilaginous hook of the tube, and blends inferiorly with the tensor veli palatini. When the dilatator tubæ contracts, the cartilaginous hook and the membranous lamina of the tube are drawn lateralwards and forwards. Some anatomists believe that the entire tensor veli palatini acts chiefly as a dilator of the tube, and Rüdinger named it the abductor tubæ. The salpingopharyngeus muscle draws downwards and backwards the medial lamina, increasing the angle between it and the lateral lamina. Some difference of opinion exists as to the precise action of the levator veli palatini; probably it assists in opening the tube.
The auditory tube receives its blood-supply from the ascending pharyngeal artery, and from the middle meningeal artery and the artery of the pterygoid canal, both of which are branches of the internal maxillary artery. Its veins form a network which drains into the pterygoid venous plexus. The sensory nerves of the tube are derived from the tympanic plexus and from the pharyngeal branch of the spheno-palatine ganglion.
The auditory tube of the child differs considerably from that of the adult; its lumen is relatively wider, its direction more horizontal, and its pars ossea relatively shorter. Kunkel states that its pharyngeal orifice is below the level of the hard palate in the fæœtus; at birth it is on the same level as the palate, whilst at the fourth year it is 3 to 4 mm., and in the adult 10 mm., above it. In the child the pharyngeal orifice forms a narrow fissure, and its cartilage projects less towards the middle line.
The auditory ossicles form an articulated column connecting the lateral with the medial wall of the tympanic cavity; they are named, from without