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inwards, the malleus, incus, and stapes. The first is attached to the medial surface of the membrana tympani; the last is fixed within the circumference of the fenestra vestibuli.
The malleus (Fig. 714, B, D), the largest of the three ossicles, has a length of 8 to 9 mm., and consists of a head, a neck, a manubrium, and two processes, viz. :
FIG. 714.-AUDITORY OSSICLES OF LEFT EAR (enlarged about three times).
A, Incus, seen from the front; B, Malleus, viewed from behind; C, Incus, and D, Malleus, seen from medial
aspect; E, Stapes.
7. Processus lateralis.
9. Body of incus.
10. Crus breve.
11. Crus longum.
14. Facet for incus.
18. Crus anterius.
(a) processus anterior, (b) processus lateralis. The head and neck are situated in the epitympanic recess; the processus lateralis and manubrium are fixed to the medial surface of the membrana tympani; while the processus anterior is directed forwards, towards the petro-tympanic fissure, to which, in the adult, it is connected by ligamentous fibres. The head, somewhat rounded, is smooth and convex above and in front, and presents, on its posterior surface, a facet for articulation with the body
FIG. 715.-LEFT MEMBRANA TYMPANI AND CHAIN OF AUDITORY OSSICLES (seen from the medial aspect). x 3. of the incus. This facet is directed obliquely downwards and medialwards, and is more or less elliptical in form. It is constricted near the middle so as to resemble, somewhat, the figure 8; an oblique ridge, corresponding with the constriction, divides the facet into two parts an upper and larger, directed backwards, and a lower and lesser, directed medialwards. Opposite the lower part of the constriction the inferior edge of the facet is very prominent, and is continued upwards into the oblique ridge just referred to; it forms a tooth-like process, the spur or cog-tooth of
the malleus. On the back of the head, below this spur, is an oblique crest, the crista mallei, to which is attached the lateral ligament of the malleus. The neck is the slightly constricted portion immediately below the head. Flattened from before backwards, its lateral surface is directed towards the membrana flaccida, whilst its medial surface is crossed by the chorda tympani nerve. The manubrium or handle is directed downwards, backwards, and medialwards from the neck, forming with the long axis of the head an angle of 126° to 150°. Its upper part is flattened from before backwards, but towards the lower end it is twisted on itself, so that its surfaces look laterally and medially; moreover, the lower end is slightly curved, the concavity being directed forwards and laterally. It is fixed, along its entire length, to the membrana propria of the tympanic membrane by its periosteum and by a layer of cartilage (Gruber). The cartilage intervenes between the manubrium and the membrane, and must be regarded as a residue of that stage of development when the entire malleus was cartilaginous. On the medial surface of the manubrium, near its upper extremity, is a slight projection for the attachment of the tendon of the tensor tympani muscle. The processus anterior, a slender spicule, springs from the front of the neck and is directed forwards, towards the petro-tympanic fissure. In the foetus it is the longest process of the malleus, but in the adult it usually assumes the form of a small projection, since its anterior part is replaced by ligamentous tissue. The processus lateralis may be looked upon as the upper extremity of the manubrium projected laterally; it is fixed to the upper part of the membrana tympani by the cartilaginous layer already referred to, and to the extremities of the notch of Rivinus by the anterior and posterior malleolar plicæ.
The incus (Fig. 714, A, C) may be likened to a præmolar tooth with widely divergent roots. It consists of a body, a crus longum, and a crus breve; the crura form with each other an angle of 90° to 100°. The body and crus breve are situated in the recessus epitympanicus. The body presents a more or less saddle-shaped surface for articulation with the head of the malleus. This surface is directed forwards, and its lower part is hollowed out for the accommodation of the cog-tooth of the malleus; in front of this hollow it is prominent and spur-like. The crus breve is thick, triangular in shape, and projects horizontally backwards; its conical extremity, covered with cartilage, is received into the fossa incudis in the posteroinferior part of the epitympanic recess. The crus longum projects, almost perpendicularly, downwards from the body into the tympanic cavity, where it lies parallel with, but 1.25 mm. behind and medial to, the manubrium mallei. Its lower end is bent medialwards and narrowed to form a short neck, on the end of which is a small knob of bone, the processus lenticularis, for articulation with the head of the stapes. Until the sixth month of fœtal life this process exists as a separate ossicle, termed the os orbiculare.
The stapes (Fig. 714, E) consists of a head, a neck, two crura, and a base. The head, directed lateralwards, is concave for articulation with the processus lenticularis of the incus. The neck is slightly constricted, and from it the two crura spring; the tendon of the stapedius is inserted into the posterior aspect of the neck. The crus anterius is shorter and less curved than the crus posterius. Diverging from each other, the crura are directed medialwards and are attached-one near the anterior, the other near the posterior end of the base. The base almost completely fills the fenestra vestibule, and, like it, is somewhat oval or reniform, its anterior end being the more pointed. In the recent condition a membrane fills the arch formed by the crura and the base, the crura being grooved for its reception. In the child the crura of the stapes are less curved than in the adult, and the opening bounded by them and the base is nearly triangular.
Articulations of the Auditory Ossicles.-The incudo-malleolar joint between the head of the malleus and the body of the incus is diarthrodial, and may be described as one of reciprocal reception. It is surrounded by an articular capsule, and from the inner surface of the fibrous stratum a wedge-shaped meniscus projects into the joint cavity and incompletely divides it. The incudo-stapedial articulation between the processus lenticularis and the head of the stapes is of the nature of an enarthrosis and is surrounded by an articular capsule. An interarticular cartilage has been described as occurring in this joint, while some observers deny
the presence of a synovial cavity and regard the articulation as a syndesmosis, the articular surfaces being held together by fibrous tissue.
Ligamenta Ossiculorum Auditus.-The malleus is attached to the walls of the tympanic cavity by three ligaments (Fig. 715), viz.: anterior, superior, and lateral. The anterior ligament consists of two portions: (a) the band of Meckel, which is attached to the base of the processus anterior, and passes forwards, through the petrotympanic fissure, to reach the spine of the sphenoid; it represents the remnant of a portion of Meckel's cartilage, and was formerly described as the laxator tympani muscle; (b) a firm bundle of fibres, the anterior ligament of Helmholtz, which extends from the spina tympanica anterior at the anterior boundary of the notch of Rivinus to the anterior surface of the malleus, above the base of the processus anterior. The superior ligament stretches, almost vertically, from the head of the malleus to the roof of the epitympanic recess. The lateral ligament is short and fan-shaped; its fibres converge from the posterior half of the notch of Rivinus to the crista mallei. The posterior part of this ligament is strong and constitutes the posterior ligament of Helmholtz; it forms, together with the anterior ligament of the malleus, the axis around which the malleus rotates, and the two constitute what Helmholtz termed the "axis-ligament" of the malleus.
The posterior extremity of the crus breve of the incus is tipped with cartilage and fixed by means of a ligament to the fossa incudis (Fig. 715). Some observers describe this as a diarthrodial joint. The vestibular surface and the circumference of the base of the stapes are covered with hyaline cartilage, and a similar layer lines the opening of the fenestra vestibuli; that encircling the base of the stapes is joined to that lining the fenestra by a dense ring of elastic fibres, named the ligamentum annulare baseos stapedis. The posterior fibres of this ligament are thicker and shorter than the anterior, and thus during the movements of the stapes, the anterior end of its base is free to make greater excursions than the posterior.
Development of the Auditory Ossicles.-It is generally maintained that the malleus and incus are developed from the upper end of Meckel's cartilage, and that the stapes arises from the mesoderm in the region of the fenestra vestibuli, where it is developed around a small artery, the stapedial artery, which atrophies in man, but persists in many mammals. On the other hand, Gadow (Phil. Trans., London, vol. clxxix.) says "the whole system of the one to four elements of the middle ear, which have all the same function, is to be looked upon as one organ, of one common origin, viz., a modification of the hyomandibular, the proximal paramere of the second visceral arch." Ossification commences in all three bones about the third month of foetal life. The malleus is ossified from two centres, one for the head and manubrium, and one for the processus anterior; the incus from two centres, one for the body including the crura, and a second for the processus lenticularis; the stapes from one centre which appears in the base.
Muscles of the Tympanic Cavity. These are two in number, viz., m. tensor tympani and m. stapedius.
The m. tensor tympani is the larger, and takes origin from the roof of the cartilaginous part of the auditory tube, from the adjacent part of the great wing of the sphenoid, and from the bony canal in which the muscle lies. The muscle ends in a tendon which bends laterally, nearly at a right angle to the belly of the muscle, round the pulley-like posterior extremity of the septum canalis musculotubarii. Passing across the tympanic cavity this tendon is inserted into the medial edge and anterior surface of the manubrium mallei, near its upper end. When the muscle contracts it draws the manubrium of the malleus towards the tympanic cavity, and so renders tense the membrana tympani; it probably also slightly rotates the malleus around its long axis. It receives its nerve from the motor division of the trigeminal nerve, through the otic ganglion.
The m. stapedius arises within the eminentia pyramidalis, and from the canal which prolongs the hollow of the pyramidal eminence downwards. Its tendon emerges from the apex of the eminence and is inserted into the posterior surface of the neck of the stapes. On contraction it draws back the head of the stapes, and so tilts the anterior end of the base towards the tympanic cavity and the
posterior end towards the labyrinth, thus rendering tense the ligamentum annulare -the lateral movement of the anterior end of the base being greater than the medial movement of its posterior end. The muscle is supplied by the facial nerve.
Movements of the Auditory Ossicles.-The manubrium mallei follows all the movements of the membrana tympani, while the malleus and incus move together around an axis extending forwards through the crus breve of the incus and the anterior ligament of the malleus. When the membrana tympani moves medialwards it carries with it the manubrium mallei, while the incus, moving medialwards at the same time, forces the base of the stapes towards the labyrinth. This movement is communicated to the fluid (perilymph) in the labyrinth, and causes a lateral bulging of the secondary tympanic membrane, which closes the fenestra cochleæ. These movements are reversed when the membrana tympani is relaxed, unless the lateral movement of the membrane is excessive. In such a condition the incus does not follow the full movement of the malleus, but merely glides on this bone at the incudo-malleolar joint, and thus the forcible dragging of the base of the stapes out of the fenestra vestibuli is prevented. The cog-tooth arrangement, already described, on the head of the malleus and body of the incus, causes the incudo-malleolar joint to become locked during the medial movement of manubrium mallei, the joint becoming unlocked during its lateral movement.
Tunica Mucosa Tympanica.-The mucous lining of the tympanic cavity is continuous, through the auditory tube, with that of the nasal part of the pharynx; it extends backwards also and lines the tympanic antrum and the mastoid air-cells. Thin, transparent, and closely united with the subjacent periosteum, it covers the medial surface of the membrana tympani and is reflected over the auditory ossicles and their ligaments. It also supplies sheaths for the tendons of the tensor tympani and stapedius muscles, and forms the following folds, viz.: (a) one from the roof of the recessus epitympanicus to the head of the malleus and body of the incus; (b) one enveloping the chorda tympani nerve and crus longum of the incus; (c) two extending from the processus lateralis mallei-one to the anterior, the other to the posterior margin of the notch of Rivinus. A recess, the pouch of Prussak, is situated between the membrana flaccida and the neck of the malleus. Communicating behind with the tympanic cavity, this pouch may serve as a reservoir to confine pus or other fluid, since its opening into the tympanic cavity is above the level of its floor, a condition analogous to the opening from the maxillary sinus into the nasal cavity. The fold of mucous membrane which extends downwards to envelop the chorda tympani nerve gives rise to two pouches, one in front of, and the other behind, the manubrium mallei; these are named the anterior and posterior recesses of Tröltsch. The epithelium which lines the mucous membrane is flattened over the membrana tympani, promontory, and auditory ossicles, but ciliated and columnar over the greater portion of the rest of the cavity.
Vessels and Nerves of the Tympanic Cavity. The arteries supplying the tympanic cavity are: (1) The anterior tympanic artery, a branch of internal maxillary, which reaches the cavity by way of the petro-tympanic fissure. (2) The stylo-mastoid branch of posterior auricular, which passes through the stylo-mastoid foramen and the facial canal; it supplies branches to the tympanic antrum and mastoid air-cells, to the stapedius muscle, to the floor and medial wall of the tympanic cavity, and forms an anastomotic circle, around the membrana tympani, with the anterior tympanic artery. (3) The middle meningeal artery sends a branch to the tensor tympani muscle, and, after entering the skull, gives off its petrosal artery, which is conducted to the tympanic cavity along the hiatus canalis facialis; some twigs from the posterior division of the middle meningeal reach the tympanic antrum and epitympanic recess through the petrosquamous fissure. (4) The internal carotid artery, in its passage through the canal in the temporal bone, gives off one or two tympanic twigs, while (5) a branch from the ascending pharyngeal accompanies the tympanic nerve. The veins drain into the pterygoid plexus, and the superior petrosal sinus. The lymph vessels form a network in the mucous membrane and end mainly in the retro-pharyngeal and parotid lymph glands. The nerves supplying the muscles of the tympanic cavity have already been referred to (pp. 832, 834). The mucous membrane receives its nerves from the tympanic plexus, which is described on p. 786. The chorda tympani branch of the facial nerve passes from behind, upwards, and forwards through the tympanic cavity. Its course is described on p. 782.
Early Condition of Tympanic Cavity. During the greater part of intra-uterine existence the tympanic cavity is almost completely filled by a soft, reddish, jelly-like embryonic tissue in which there is a slit-like space lined with epithelium. Towards the end of fœtal life this tissue disappears and at birth the cavity is filled with fluid which becomes absorbed, after the entrance of air from the nasal part of the pharynx through the auditory tube.
The internal ear or essential part of the organ of hearing is situated in the substance of the petrous part of the temporal bone, and consists of two sets of structures, viz.: (1) a series of passages hollowed out of the bone and constituting the osseous labyrinth; these are continuous with each other, and are named
from before backwards the cochlea, vestibule, and semicircular canals (Figs. 716, 717); (2) a complex arrangement of membranous channels (Fig. 720), situated within, but not nearly filling, the bony labyrinth and forming the membranous labyrinth. These channels are named the ductus cochlearis, the utricle, the saccule, and the semicircular ducts; the utricle and saccule are lodged within the vestibule.
Vestibulum. The vestibule is the central portion of the osseous labyrinth, and communicates behind with the semicircular canals and in front with the cochlea. It is somewhat ovoid in shape, its long axis being directed forwards and lateralwards; it measures about 6 mm. antero-posteriorly, 4-5 mm. vertically, and about 3 mm. transversely. Its lateral wall is directed towards the tympanic cavity, and in it is the fenestra vestibuli, which is closed by the base of the stapes. Its medial wall corresponds with the bottom of the internal acoustic meatus, and presents, at its antero-inferior part, a rounded depression, the recessus sphæricus, which lodges the saccule. This recess is perforated by twelve or fifteen small foramina (macula cribrosa media), which transmit the filaments of the acoustic nerve to the saccule. The recessus sphæricus is limited above and behind by an oblique ridge, the crista vestibuli, the anterior extremity of which is triangular in shape and named the pyramis vestibuli. Posteriorly this crista divides into two limbs, between which is a small depression, the recessus cochlearis, perforated by about eight small foramina, which give passage to the nervous filaments supplying the vestibular end of the ductus cochlearis. Above and behind the crista vestibuli, in the roof and medial wall of the vestibule, is an oval depression, the recessus ellipticus, which lodges the utricle. The pyramis vestibuli and adjacent part of the recessus ellipticus are perforated by twenty-five or thirty small apertures (macula cribrosa superior). The foramina in the pyramis vestibuli transmit the nerves to the utricle; those in the recessus ellipticus, the nerves to the ampullæ of the superior and lateral semicircular ducts. Behind and below the recessus ellipticus is a furrow, gradually deepening to form a canal, the aquæductus vestibuli, which passes backwards through the petrous part of the temporal bone, and opens, as a slit-like fissure, about midway between the internal acoustic meatus and the groove for the transverse sinus. This aqueduct measures 8-10 mm. in length, and gives passage to the