THE MIDDLE EAR
The middle ear is housed in the temporal bone and is essentially the small air-filled space known as the tympanic cavity (Figure 9-24/5). It is lined with a thin mucous membrane and communicates with the nasopharynx by the auditory tube (Figure 9-24/7).
The upper part of the tympanic cavity is compressed from side to side and slanted outward. The lateral wall of the cavity incorporates the tympanic membrane (Figure 9-24/4). The medial wall is formed by the petrous part of the temporal bone, which houses the internal ear. It contains two windows (fenestrae), closed in the natural state, through which the mechanical stimuli produced by sound waves enter the internal ear for translation into nerve impulses. The more dorsal vestibular window connects the tympanic cavity with the vestibule of the internal ear. In the live animal it is occupied by the stapes, the most medial of the auditory ossicles (Figure 9-24/6). The other, the cochlear window, leads to the cavity of the cochlea (Figure 9-24/18). It is closed by the thin secondary tympanic membrane. Ventral to the two windows the medial wall bulges over the cochlea, forming the promontory.The tympanic cavity may be divided into dorsal, middle, and ventral parts. The dorsal part (epitympanic recess) is situated above the level of the tympanic
Figure 9-24 A, Transverse image of a 2-mm-thick computed tomographic slice of the canine tympanic bullae and petrous temporal bones. (Bone settings were used.) B, Schema of the right ear, caudal view. Note that the sizes of the structures shown are out of proportion to each other. I, Internal ear; II, middle ear; III, external ear. 1, Auricle; 2, external acoustic meatus; 3, annular cartilage; 4, tympanic membrane; 5, tympanic cavity; 5', epitympanic recess; 5", tympanic bulla; 6, auditory ossicles; 6', malleus; 6", base of stapes in vestibular window; 7, auditory tube; 8, nasopharynx; 9, chorda tympani; 10, facial nerve; 11, vestibule; 12, semicircular canals; 13, semicircular ducts; 14, utriculus; 15, sacculus; 16, cochlear duct; 17, endolymphatic duct; 18, cochlea; 19, perilymphatic duct; 20, internal acoustic meatus; 21, vestibulocochlear nerve in internal acoustic meatus; 22, meninges; 23, brain; 24, petrous temporal bone; 25, stylohyoid bone.
membrane.
It contains the chain of auditory ossicles and the two associated muscles. The middle part includes the tympanic membrane in its lateral wall and opens rostrally into the nasopharynx via the auditory tube. The ventral part is an enlarged bulbous extension of the temporal bone known as the tympanic bulla (Figure 9-24/5"). The bulla varies in prominence among species; in some it is subdivided into numerous bony cells. The function is not known with certainty, but it has been suggested that it may improve the perception of sounds of very low and very high frequencies.The tympanic membrane (Figure 9-26) is a thin partition separating the lumen of the external acoustic meatus from that of the tympanic cavity. Like the tympanic cavity, it is slanted so that its dorsal part is more lateral than its ventral part, and its surface area is thus considerably larger than that of the transected external acoustic meatus. The dog’s eardrum on average measures 10 ? 15 mm; its long axis is oriented rostrocau- dally. Its lateral surface is covered with an epidermis continuous with that of the meatus, its medial surface by the mucosa lining the tympanic cavity. A layer of fibrous tissue between epidermis and mucosa firmly attaches the membrane to the osseous tympanic ring of the temporal bone. The tympanic ring is interrupted dorsally by a notch that extends onto the roof of the external acoustic meatus. The part of the tympanic membrane attached to the tympanic ring is tense; the part that closes the notch is flaccid.
Figure 9-25 Left auricular cartilage of dog compared with human ear. 1, Helix; 2, apex; 3, medial crus of helix; 4, lateral crus of helix; 5, pretragic notch; 6, tragus; 7, intertragic notch; 8, antitragus; 9, annular cartilage.
Figure 9-26 Medial surface and transverse section (below) of canine tympanic membrane.
1, Tense part of tympanic membrane; 2, medial surface; 3, lateral surface; 4, handle of malleus; 5, chorda tympani; 6, m. tensor tympani; 7, head of malleus; 8, one of the ligaments associated with the malleus.The handle of the malleus (Figure 9—26/4), the most lateral of the ear ossicles, is embedded in the medial surface of the tympanic membrane. Tension in the chain of ossicles pulls the tympanic membrane medially, which hollows its lateral surface. The handle shines through the thin membrane and is visible as a light band (stria mallearis) when the eardrum is examined with an otoscope (see Figure 11-43, A-B).
Auditory Ossicles
The transmission of sound waves across the tympanic cavity is mediated by the three auditory ossicles (Figure 9-24/d) known, in lateromedial sequence, as malleus, incus, and stapes (Latin names for hammer, anvil, and stirrup, from their rather fanciful resemblance to these objects).
The handle (manubrium) of the malleus (Figure 9-27/3) is embedded in the tympanic membrane so that the head of the malleus protrudes above the membrane by a few millimeters. The head articulates with the body of the incus, and the latter articulates with the head of the stapes by means of its long crus. The base (footplate) of the stapes sits in the vestibular window in the medial wall of the tympanic cavity.
The oscillations of the tympanic membrane perceived by the handle of the malleus are magnified and transmitted to the vestibular window by lever action through the chain of ossicles. The base of the stapes is set in motion, which causes the fluid in the internal ear to vibrate. This stimulates the neuroreceptor cells in the membranous labyrinth, and sound is perceived.
The mechanism of sound transmission from the outside to the internal ear may not in fact be quite so simple. There is evidence that sound waves are also transmitted to the fluid through the walls of the tympanic cavity and directly through the cochlear window.
The auditory ossicles are attached to the wall of the epitympanic recess by several ligaments, and their relationships can be altered by two small muscles (tensor tympani and stapedius).
These are believed to tense the tympanic membrane and the chain of ossicles in an effort to decrease the amplitude of their vibrations in the lower frequencies and to protect the system from damage caused by sudden overload (see p. 317 for their innervation).
Figure 9-27 Left auditory ossicles of the horse, craniome- dial view. 1, Malleus; 2, head of malleus; 3, handle of malleus; 4, rostral process; 5, incus; 6, short crus; 7, long crus; 8, os lenticulare; 9, head of stapes; 10, base (footplate) of stapes.
Auditory Tube
This structure, often called the Eustachian tube, connects the tympanic cavity with the nasopharynx (Figure 9-24/5). It is short with a narrow lumen that is laterally compressed and usually collapsed. The tube is confined by an inverted cartilaginous trough except along its ventral border. The membranous wall of the horse’s auditory tube evaginates through this ventral defect in the cartilaginous support to form the large, thin-walled guttural pouch dorsolateral to the nasopharynx (see p. 522).
The pharyngeal openings of the auditory tubes are located in the lateral walls of the nasopharynx and are marked by accumulations of lymphoid tissue (tubal tonsils) (see Figure 18-11/5). The cartilage of the auditory tube extends into the medial wall of the pharyngeal opening and stiffens it. The auditory tubes allow equalization of the pressures on the two sides of the delicate eardrums. The pressure sometimes becomes unbalanced, for example, during a ride in an express elevator, and its sudden restoration causes our ears to pop. The auditory tubes temporarily open each time we swallow or yawn. This permits the slight secretion from the goblet cells and the glands in the lining of the tympanic cavity to escape.