« PrécédentContinuer »
Structure of the Auricula.-The greater part of the auricula consists of a lamella of yellow fibro-cartilage, the cartilago auricula; the cartilage is, however, absent from the lobule, which is composed of fat and connective tissue. When laid bare, the cartilage (Figs. 705, 706) presents, in an exaggerated condition, all the inequalities of the auricula, and is prolonged medially to form a considerable portion of the external acoustic meatus. The cartilage of the helix projects anteriorly as a conical eminence, the spina helicis, and its inferior extremity extends downwards as a tail-like process, the cauda helicis, which is separated from the lower part of the antitragus by the fissura antitragohelicina. The cartilage of the auricula is continuous with that of the meatus by a narrow isthmus, the isthmus cartilaginis auris, measuring from 8 to 9 mm. in breadth. This isthmus corresponds laterally with the deepest part of the incisura intertragica, and medially it forms the outer boundary of a deep fissure, the incisura terminalis auris, which separates the cartilage of the meatus from that of the concha. The upper edge of the tragus fits into an angle below the crus helicis. Two fissures, in addition
Incisura terminalis auris
M. antitragicus Fissura antitragohelicina Cauda helicis
FIG. 705.-LATERAL SURFACE OF CARTILAGE OF THE
to those already described, are usually present, one in the tragus and another immediately behind the spina helicis.
On the cranial surface of the cartilage (Fig. 706) the eminences produced by the concha and fossa triangularis are separated by a transverse furrow, the sulcus
antihelicis transversus, corresponding with the crus antihelicis inferior; further, the eminentia concha is crossed horizontally by a groove, the sulcus cruris helicis, and almost vertically by a slight ridge, the ponticulus: the latter indicates the attachment of the m. auricularis posterior.
Ligaments of the Auricula.-The cartilage of the auricle is attached to the temporal bone by two fibrous bands which form its extrinsic ligaments, viz. an anterior, stretching from the zygomatic process to the spina helicis and tragus; and a posterior, passing from the eminentia conchæ
wall of the meatus to the mastoid portion of the temporal bone. Small ligamentous bands pass between individual parts of the auricle, and form what are termed its intrinsic ligaments.
Muscles of the Auricula (Figs. 705, 706). The muscles of the auricle are divided into two groups, extrinsic and intrinsic. The extrinsic muscles pass to the auricula from the skull or the scalp, and are described in the section on Myology. The intrinsic muscles, on the other hand, are confined to the auricula and are six in number, four on its lateral and two on its cranial or medial surface.
(a) On the lateral surface (Fig. 705)
1. M. helicis major passes upwards from the spina helicis along the ascending part of the helix. 2. M. helicis minor covers the crus helicis. 3. M. tragicus, quadrangular in shape, consists of fibres running vertically over the greater part of the tragus. Some of its fibres are prolonged upwards to the spina helicis and constitute the m. pyramidalis. 4. M. antitragicus covers the antitragus and runs obliquely upwards and backwards as far as the antihelix and cauda helicis.
(b) On the medial surface (Fig. 706)
1. M. transversus auriculæ consists of scattered fibres, which stretch from the eminentia concha to the convexity of the helix. 2. M. obliquus auriculæ (Tod) comprises a few fasciculi, which run obliquely or vertically across the furrow corre sponding with the crus antihelicis inferior. A small muscle, the m. stylo-auricularis, sometimes extends from the root of the styloid process to the cartilage of the meatus.
Skin of the Auricula.-The skin covering the auricle is thin and smooth, and is prolonged, in the form of a tube, as a lining to the external acoustic meatus. On the lateral surface of the auricula, it adheres firmly to the subjacent perichondrium. Strong hairs are present on the tragus and antitragus, and also in the incisura intertragica, forming the barbula hirci, which guard the entrance to the concha; soft downy hairs are found over the greater part of the auricula and point towards the tuberculum auriculæ. Sebaceous glands, present on both surfaces of the auricle, are most numerous in the concha and fossa triangularis. Sudoriferous glands are found on the medial surface; few or none on the lateral surface.
Vessels of the Auricula. The arteries of the auricle are derived--(a) from the superficial temporal, which sends two or three branches to the lateral surface; and (b) from the posterior auricular, which gives three or four branches to the medial surface. From the posterior auricular artery two sets of twigs are prolonged to the lateral surface, one turning round the free margin of the helix, and the other passing through small fissures in the cartilage. The veins from the lateral surface open into the superficial temporal vein; those from the medial surface chiefly join the posterior auricular vein, but some communicate with the mastoid emissary vein. The lymph vessels take three directions, viz.: (a) forwards to the parotid lymph glands, and especially to the anterior auricular gland in front of the tragus; (b) downwards to the lymph glands which accompany the external jugular vein, and to the lymph glands under the sterno cleidomastoideus; and (c) backwards to the posterior auricular lymph glands.
Nerves of the Auricula. The muscles of the auricle are supplied by the facial nerve. The skin receives its sensory nerves from-(a) the great auricular, which supplies nearly the whole of the medial surface, and sends filaments in company with the branches of the posterior auricular artery to the lateral surface; (b) the auriculo-temporal, which supplies the tragus and ascending part of the helix; (c) the lesser occipital, which sends a branch to the upper part of the medial surface.
MEATUS ACUSTICUS EXTERNUS.
The external acoustic meatus (Figs. 707, 708) is the passage leading from the concha to the membrana tympani. Its average length, measured from the bottom of the concha, is about 24 mm., but, if measured from the level of the tragus, about 35 mm. On account of the obliquity of the membrana tympani the anterior and inferior walls of the meatus are longer than the posterior and superior. The meatus consists of two parts, viz.: (a) an external portion, the pars cartilaginea, about 8 mm. in length; and (b) an internal portion, the pars ossea, about 16 mm. in length. The entire meatus forms a somewhat S-shaped bend (Fig. 708), and may be divided into three portions-external, intermediate and internal; each is directed medialwards, but, in addition, the external part is inclined forwards and slightly upwards; the intermediate, backwards; and the internal, the longest, forwards and slightly downwards. On transverse section the canal is seen to be elliptical, its greatest diameter having an inclination downwards and backwards. Widest at its lateral extremity, it becomes somewhat narrower at the medial end of the pars cartilaginea; once more expanding in the lateral portion of the pars ossea, it is again constricted near the medial end of the latter, where its narrowest part, or isthmus, is found at a distance of about 19 mm from the bottom of the concha. The medial extremity of the meatus is nearly circular and is closed by the membrana tympani.
Bezold gives the diameters of the meatus as follows:
At the commencement of the pars cartilaginea
The lumen of the pars cartilaginea is influenced by the movements of the mandible, being increased when that bone is depressed. This can be easily verified by inserting a finger into the meatus, and then alternately opening and shutting the mouth.
The condyle of the mandible lies
in front of the pars Ossea, while between the condyle and the pars cartilaginea a portion of the parotid gland is sometimes present. Behind the pars ossea, and separated from it by a thin plate of bone, are the mastoid air-cells.
Pars ossea of external acoustic meatus
Crus antihelicis inferior
Pars cartilaginea of external acoustic meatus
Lower boundary of incisura intertragica
FIG. 707.-FRONTAL SECTION OF RIGHT EAR; ANTERIOR HALF OF SECTION, viewed from behind (natural size).
Structure of the Meatus.-The cartilage of the meatus, directly continuous with that of the auricula, is folded on itself to form a groove, opening upwards and backwards, the margins of which are connected by fibrous tissue. The medial end of the cartilaginous tube is firmly fixed to the lateral margin of the bony meatus, whilst its lateral extremity is continuous with the cartilage of the tragus
(p. 829). Two fissures exist in the anterior portion of the pars cartilaginea, and are filled by fibrous tissue.
Membrana tympaui In the lateral part of
First turn of
FIG. 708. HORIZONTAL SECTION THROUGH RIGHT EAR; UPPER HALF OF
SECTION, seen from below (natural size).
the meatus the cartilage forms about threefourths of the circumference of the tube; but, near the medial end of the pars cartilaginea the cartilage forms merely a part of the anterior and lower boundaries of the canal.
The pars ossea of the meatus is described on p.
127; but it may be well to state here that in the new-born child it is represented only by an incomplete ring of bone, the annulus tympanicus, together with a small portion of the squama temporalis, which articulates with, and bridges over the interval between, the extremities of the ring superiorly. In the concavity of the annulus is a groove, the sulcus tympanicus, in which the circumference of the membrana tympani is fixed. On the medial surface of the anterior part of the annulus, a little below its free extremity, a groove, the sulcus malleolaris, is directed downwards and forwards. It transmits the anterior process and the anterior ligament of the malleus. the tympanic artery, and the chorda tympani nerve. It is limited above by a ridge, the crista spinarum (Henle), which ends in front and behind in a spinous process (spina tympanica major and minor). Below the sulcus malleolaris there is a second, less prominent ridge, the crista tympanica (Gruber), which subsequently unites with a process of the tegmen tympani, and so shuts off the canalis musculotubarius from the petrotympanic fissure. A fibrous tympanic plate (Symington) intervenes between the annulus tympanicus and the cartilage of the meatus, and into this plate the bony ring extends. The bony outgrowth does not, however, proceed uniformly from the whole of circumference of the annulus, but occurs mos rapidly in its anterior and posterior parts. These outgrowths fuse about the end of the seco
year of life, so as to surround a foramen (foramen of Huschke) in the floor of the meatus; this foramen is usually closed by the fifth year, but persists until adult life in some 19 per cent of skulls (Bürkner).
The lumen of the meatus in the new-born child is extremely small: its outer part is funnelshaped; its inner a mere slit, bounded below by the fibrous tympanic plate and above by the obliquely placed membrana tympani.
The skin which envelops the auricula lines the entire meatus, and covers also the outer surface of the membrana tympani. It is thick in the pars cartilaginea, and contains fine hairs and sebaceous glands, the latter extending for some distance along the postero-superior wall of the pars ossea. The sudoriferous glands are enlarged and of a brownish colour; they constitute the glandulæ ceruminosæ and secrete the ear wax or cerumen.
Vascular and Nervous Supply of the Meatus.-The external acoustic meatus receives its blood-supply from the posterior auricular and superficial temporal arteries, and also from the deep auricular branch of the internal maxillary artery, the last distributing some minute twigs to the membrana tympani. The veins open into the external jugular and internal maxillary veins, and also into the pterygoid plexus, while the lymph vessels have a similar mode of termination to those of the auricle. Sensory nerves are supplied to the meatus by the auriculo-temporal branch of the trigeminal and by the auricular branch of the vagus.
CAVUM TYMPANI OR MIDDLE EAR.
The tympanic cavity is a small air chamber in the temporal bone, between the membrana tympani and the lateral wall of the internal ear or labyrinth (Figs. 707, 708). Lined with mucous membrane, it contains a chain of ossicles,-malleus, incus, and stapes,—which reaches from its lateral to its medial wall, and transmits the vibrations of the membrana tympani across the cavity to the internal ear. Attached to the ossicles are several ligaments and two small muscles.
The tympanic cavity consists of two portions: (1) The tympanum proper, or atrium, lying medial to the membrana tympani; and (2) the recessus epitympanicus, or attic, lying above the level of the membrane and containing the greater par of the incus and the upper half of the malleus. Including this recess, the vertical and antero-posterior diameters of the tympanic cavity each measure about 15 mm. The distance between its lateral and medial walls is about 6 mm. above and 4 mm. below, while at its central part, owing to the bulging of the two walls towards the cavity, it measures only from 15 to 2 mm.
The tympanic cavity is enclosed by six walls, tegmental, jugular, labyrinthic. mastoid, carotid, and membranous.
Paries Tegmentalis.-The tegmental wall or roof (Fig. 709) is formed by a thin plate of bone, the tegmen tympani, constituting a portion of the anterior surface of the petrous part of the temporal. It extends backwards so as to cover in the tympanic antrum, and forwards, to roof in the semicanal for the tensor tympani
It separates the tympanic cavity and antrum from the middle fossa of the cranial cavity, and may contain a few air-cells, whilst occasionally it is partly deficient. In the child its lateral edge corresponds with the petro-squamous sutur traces of which can generally be seen in the adult bone.
Paries Jugularis.-The jugular wall or floor is narrower than the tegmental wall, and consists of a thin plate of bone which separates the tympanic cavity from: the fossa jugularis; anteriorly, it extends upwards and is continuous with the posterior wall of the carotid canal. The inner orifice of the foramen for the transmission of the tympanic nerve is seen near the junction of the jugular and labyrinthic walls.
Paries Labyrinthica. The labyrinthic or medial wall of the tympanic cavity is formed by the lateral surface of the internal ear (Fig. 709). It presents (1 a rounded eminence, the promontory, which is caused by the first coil of the cochlea. and is grooved for the tympanic plexus of nerves. (2) An oval or somewha reniform opening, the fenestra vestibuli, which is situated above and behind the promontory, with its long axis directed antero-posteriorly. It measures 3 mm. in length and 1.5 mm. in width and, in the macerated bone, leads into the vestibule of the osseous labyrinth, but is closed in the recent state by the base or foot-plat
of the stapes, surrounded by its ligamentum annulare. (3) An elevation, the prominentia canalis facialis, which is situated above the fenestra vestibuli, in the recessus epitympanicus; this elevation indicates the position of the upper part of the canalis facialis (O.T. aqueduct of Fallopius), which contains the facial nerve, and
FIG. 709.-SECTION THROUGH LEFT TEMPORAL BONE, showing labyrinthic wall of tympanic cavity, etc. is continued backwards and downwards behind the tympanic cavity, to end at the stylo-mastoid foramen. (4) The septum canalis musculotubarii (O.T. processus cochleariformis), which extends backwards, above the anterior end of the fenestra vestibuli, where it makes a sharp lateral curve, and forms a pulley over which the tendon of the tensor tympani muscle plays. (5) A funnel-shaped recess, situated behind and below the promontory, and almost hidden by its overhanging edge, leads to an irregularly oval opening, termed the fenestra cochleæ; in the macerated bone this fenestra communicates with the cochlea, but in the recent state is closed by the membrana tympani secundaria; this membrane is bent angularly along a line joining its antero-inferior two-thirds with the postero-superior third; and consists of three layers: (a)
lateral, continuous with the mucous lining of the tympanum, and containing a network of capillaries; (b) intermediate,
membrana propria, the fibres of which radiate chiefly towards the periphery of the membrane some branched, dendritic fibres are also
present; (c) medial,
Apertura tympanica canaliculi
lining of the
labyrinth. (6) Be
from within. The head and neck of the malleus have been removed to show the pars flaccida and the malleolar plicæ. x 3.
tween the fenestra vestibuli above and the fenestra cochleæ below is a small circular depression, the sinus tympani, which is perforated by one or two minute foramina for blood-vessels, and indicates the position of the ampullated end of the posterior semicircular canal.