The Middle Ear

The middle ear (Auris media) or tympanic cavity (Cavum tympani) is a space in the tympanic and petrous parts of the temporal bone situated between the membrana tympani and the internal ear. It is a pneumatic cavity, which is lined by mucous membrane, and communicates with the pharynx and the guttural pouch by the Eustachian tube. It contains a chain of auditory ossicles by which the vibrations of the membrana tympani are transmitted to the internal ear.

The cavity consists of: (1) A main part or atrium, which lies immediately to the inner side of the membrana tympani; (2) the recessus epitympanicus, situated above the level of the membrane and containing the upper part of the malleus and the greater part of the incus; (3) a relatively large ventral recess in the bulla ossea.

The external wall (Paries membranacea) is formed largely by the thin membrana tympani, which closes the inner end of the external auditory canal, and thus forms the septum between the external and middle parts of the ear. The membrane is an oval disc, which slopes obliquely downward and inward at an angle of about 30 degrees with the lower wall of the external auditory canal. The circumference is attached in a groove (Sulcus tympanicus) in the thin ring of bone (Annulus tympanicus) which almost completely surrounds it. The handle of the malleus (the outermost of the auditory ossicles) is attached to the inner surface of the membrane and draws the central part inward, producing a slight concavity of the outer surface. The periphery is thickened, forming the annulus fibrosus. The membrana tympani consists of three layers. The external cutaneous layer (Stratum cutaneum) is a prolongation of the lining of the external auditory meatus. The middle fibrous layer or membrana propria includes two chief sets of fibers; the outer stratum (Stratum radiatum) consists of fibers which radiate from the handle of the malleus, while the inner stratum (Stratum circulare) is composed of circular fibers which are best developed peripherally. There are also branched or dendritic fibers in part of the membrane. The internal mucous layer (Stratum mucosum) is a part of the general mucous membrane which lines the tympanic cavity.

The internal wall of the tympanic cavity (Paries labyrinthica) separates it from the internal ear; it presents a number of special features. The promontory (Promontorium) is a distinct eminence near the center which corresponds to the first coil of the cochlea, and is marked by a faint groove for the superficial petrosal nerve. Above this is the fenestra vestibuli (s. ovalis), a reniform opening which is closed by the foot-plate of the stapes and its annular ligament. The fenestra cochleæ (s. rotundum) is situated below and behind the preceding; it is an irregularly oval opening and is closed by a thin membrane (Membrana tympani secundaria), which separates the tympanic cavity from the scala tympani of the cochlea.

The anterior wall (Paries tubaria) is narrow, and is pierced by the slit-like tympanic opening of the Eustachian tube. Above this and incompletely separated from it by a thin plate of bone is the semicanal for the tensor tympani muscle.

The superior wall or roof (Paries tegmentalis) is crossed in its inner part by the facial nerve; here the facial canal is more or less deficient ventrally, and the nerve is covered by the mucous membrane of the tympanum.

The posterior wall (Paries mastoidea) presents nothing of importance; a tympanic antrum and mastoid cells, such as are found behind the tympanic cavity proper in man and many animals, are not present in the horse.

The inferior wall or floor (Paries tympanica) is concave and thin. It is crossed by delicate curved ridges, which radiate from the greater part of the annulus tympanicus.

The auditory ossicles (Ossicula auditus) form a chain which extends from the outer to the inner wall of the cavity. They are named, from without inward, the malleus, the incus, the os lenticulare, and the stapes. The first is attached to the inner surface of the tympanic membrane and the last is fixed in the fenestra vestibuli.

The malleus or hammer, the largest of the ossicles, consists of a head, neck, handle, and two processes. The head (Capitulum mallei) is situated in the epitympanic recess. It is smooth and convex above and in front, and presents on its postero-internal aspect a concave facet for articulation with the body of the incus. The neck (Collum mallei) is the constricted part below the head; its inner surface is crossed by the chorda tympani. The handle (Manubrium mallei) is directed downward, inward, and a little forward from the neck, and is attached along its entire length to the membrana tympani. On its inner surface, near the upper end, there is a slight projection to which the tendon of the tensor tympani muscle is attached. The long process (Processus longus) is a pointed spicule which projects forward from the neck toward the petro-tympanic fissure. The short process (Processus brevis) is a slight projection of the outer side of the neck, and is attached to the upper part of the membrana tympani.

The incus or anvil is situated chiefly in the epitympanic recess. It may be said to resemble a bicuspid tooth with two divergent roots, and consists of a body and two processes. The body (Corpus incudis) articulates with the head of the hammer. The long process (Crus longum) projects downward from the body and then curves inward; its extremity has attached to it a small nodule of bone, the os lenticulare, which articulates with the head of the stapes. The short process (Crus breve) projects chiefly backward, and is attached to the wall of the recess by a small ligament.

The stapes or stirrup consists of a head, two crura, and a base. The head (Capitulum stapedis) is directed outward and articulates with the os lenticulare. The crura, anterior and posterior (Crus anterius, posterius), are directed inward from the head, and join the ends of the base. The base or foot-plate (Basis stapedis) occupies the fenestra vestibuli, to which it is attached. The space between the crura and the base is closed by a membrane.

The articulations and ligaments of the auditory ossicles comprise: (1) A diarthrodial joint between the head of the malleus and the body of the incus (Articulatio incudomalleolaris), inclosed by a capsule. (2) An enarthrosis between the os lenticulare and the head of the stapes (Articulatio incudostapedia), also surrounded by a capsule. (3) The base of the stapes is attached to the margin of the fenestra vestibuli by a ring of elastic fibers (Lig. annulare baseos stapedis). (4) Small ligaments attach the head of the malleus and the short crus of the incus to the roof of the epitympanic recess. (5) The axial ligament (of Helmholtz) attaches the neck of the malleus to a small projection (Spina tympanica anterior) above and in front of the annulus tympanicus.

The muscles of the auditory ossicles are two in number, viz., the tensor tympani and the stapedius. The tensor tympani arises from the upper wall of the osseous Eustachian tube, and ends in a delicate tendon which bends outward and is inserted into the handle of the malleus near its upper end. When it contracts, it draws the handle of the malleus inward and tenses the membrana tympani; it probably also rotates the malleus around its long axis. It is innervated by the motor part of the trigeminus through the otic ganglion. The stapedius arises from a small prominence (Eminentia pyramidalis) of the posterior wall of the tympanum, runs forward on the facial nerve, and is inserted into the neck of the stapes. Its action is to draw the head of the stapes backward and rotate the anterior end of the base outward, thus tensing the annular ligament. It is innervated by the facial nerve.

The tympanic mucous membrane (Tunica mucosa tympanica) is continuous with that of the pharynx and the guttural pouch through the Eustachian tube. It is thin, closely united with the underlying periosteum, and is reflected over the ossicles, ligaments, and muscles, the chorda tympani, and the facial nerve in the open part of the facial canal. It contains minute lymph nodules and small mucous glands (Glandulæ tympanicæ). The epithelium is in general columnar ciliated, but over the membrana tympani, ossicles, and promontory it is flattened.

The artery of the tympanum is the stylo-mastoid, a small vessel which arises from the posterior auricular branch of the internal maxillary artery. It enters the tympanum through the stylo-mastoid foramen, and forms a circle around the tympanic membrane. The nerves of the mucous membrane come from the tympanic plexus.

The Eustachian Tube

The Eustachian or auditory tube (Tuba auditiva [Eustachii]) extends from the tympanic cavity to the pharynx; it transmits air to the former and equalizes the pressure on the two surfaces of the membrana tympani. It is directed forward, downward, and slightly inward, and is four to five inches (ca. 10 to 12 cm.) in length. Its posterior extremity lies at the inner side of the root of the muscular process of the petrous temporal, and communicates with the anterior part of the tympanic cavity by the small slit-like tympanic opening (Ostium tympanicum tubæ auditivæ). For a distance of about a quarter of an inch (ca. 6 to 7 mm.) in front of this opening it is a complete tube, with a curved lumen which is little more than a capillary space. Further forward it has the form of a plate which widens anteriorly and is curved to inclose a narrow groove, that opens ventrally into an extensive diverticulum termed the guttural pouch. The pharyngeal opening (Ostium pharyngeum tubæ auditivæ) is situated on the postero-superior part of the lateral wall of the pharynx just below the level of the posterior nares. It is a slit, an inch or more (ca. 3 cm.) in length, which slopes downward and backward. It is bounded internally by the thin free edge of the tube, from the lower part of which a fold of mucous membrane (Plica salpingo-pharyngea) extends in the same direction on the lateral wall of the pharynx for a distance of a little more than an inch usually. The outer boundary of the opening is the lateral wall of the pharynx. The basis of the tube is a plate of fibro-cartilage (Cartilago tubæ auditivæ) which is firmly attached dorsally to the fibrous tissue which closes the foramen lacerum basis cranii, the temporal wing of the sphenoid, and the pterygoid bone. On cross-section the cartilage (except at its tympanic end) is seen to consist of two laminæ which are continuous with each other above. The internal lamina (Lamina medialis) gradually widens toward the pharyngeal end, where it forms a broad valvular flap. This is convex internally, and its thin anterior edge forms the inner margin of the pharyngeal opening. Behind this the lamina has a thick free edge which projects ventrally from the roof of the guttural pouch. The external lamina (Lamina lateralis) is narrow and thin, and is related externally to the levator and tensor palati muscles, which are in part attached to it; it does not exist at the pharyngeal end of the tube. The mucous membrane of the tube is continuous behind with that of the tympanum and in front with that of the pharynx. On either side it is reflected to form a large diverticulum, the guttural pouch. It is covered with ciliated epithelium, and contains mucous glands and lymph nodules.

The pharyngeal opening appears to be closed ordinarily, but the tube opens during deglutition. This action is apparently produced by the part of the palato-pharyngeus muscle which is attached to the tube.

The Guttural Pouches

The guttural pouches, right and left (Figs. , , ), are large mucous sacs, each of which is a ventral diverticulum of the Eustachian tube (Diverticulum tubæ auditivæ); they are not present in the domesticated animals other than the equidæ. They are situated between the base of the cranium and the atlas above and the pharynx below. Medially they are in apposition in great part, but are to some extent separated by the intervening ventral straight muscles of the head. The anterior end is a small cul-de-sac which lies below the body of the presphenoid between the Eustachian tube and the median recess of the pharynx. The posterior extremity lies near or below the atlantal attachment of the longus colli. The pouch is related dorsally to the base of the cranium, the atlanto-occipital joint capsule, and the ventral straight muscles. Ventrally it lies on the pharynx and the origin of the œsophagus. Laterally the relations are numerous and complex. They comprise the pterygoid, levator palati, tensor palati, stylo-hyoideus, and digastricus muscles; the parotid and submaxillary glands; the external carotid, internal maxillary, and external maxillary arteries; the internal maxillary and jugular veins; the pharyngeal or guttural lymph glands; the glosso-pharyngeal, hypoglossal, and superior laryngeal nerves. The vagus, accessory, and sympathetic nerves, the superior cervical ganglion, the internal carotid artery, and the inferior cerebral vein are situated in a fold of the upper wall of the pouch. The pouch is reflected around the anterior border of the great cornu of the hyoid bone so as to clothe both surfaces of the upper part of the latter. It thus forms an outer compartment, which extends backward external to the great cornu and the occipito-hyoideus muscle; it is related externally to the parotid gland, the articulation of the jaw, the ascending part of the internal maxillary artery, the superficial temporal artery, and the facial nerve; dorsally it covers the mandibular nerve and its chief branches, and is attached to the styloid process of the conchal cartilage.

Each pouch communicates with the pharynx through the pharyngeal orifice of the Eustachian tube, and is in direct continuity with the mucous membrane of the latter.

The wall of the pouch is a delicate mucous membrane which is in general rather loosely attached to the surrounding structures. It is covered with ciliated epithelium and is supplied with mucous glands.

It is worthy of note that the pharyngeal orifice of the Eustachian tube is at such a level as to provide (in the ordinary position of the head) only an overflow outlet for the escape of fluid which may accumulate in the pouch. The two pouches are often unequal in size, and variations in regard to the distance which they extend backward are not uncommon. In one case, a small aged horse, the right pouch extended along the œsophagus about five inches (ca. 12 cm.) behind the ventral tubercle of the atlas, and the left one a little more than two inches (ca. 6 cm.). No pathological changes were apparent, and the condition was not recognizable externally. Cases of extreme size—so-called tympanites—of the pouches occur, and are apparently congenital defects. In a case in a yearling colt, the head of which was 24 inches long, the left pouch extended about 12 inches (ca. 30 cm.) behind the tubercle of the atlas, and had a capacity of six quarts. The anterior end formed a cul-de-sac about two inches (ca. 5 cm.) long between the Eustachian tube and levator palati internally and the internal pterygoid muscle externally.