Sound waves entering the external auditory canal hit the eardrum also known as the

External Ear

Theauricle is a funnel-shaped cartilaginous structure that is continuous with the meatus and the EAC (Fig. 126.6). Intricate ridges and depressions formed by the auricular cartilage and cutaneous envelope are shown inFig. 126.5. The blood supply of the external ear originates from the external division of the carotid artery via the posterior auricular and superficial temporal vessels. The EAC (Fig. 126.7) is approximately 2.5 cm in length and comprises a lateral cartilaginous (membranous) portion and a medial bony portion.1,10 The membranous portion accounts for the lateral third of the EAC, whereas the bony portion forms the medial two-thirds. The skin that lines the membranous canal is thicker and more mobile, and it is endowed with sebaceous and apocrine (ceruminous) glands and hair follicles. Both sebaceous and apocrine ducts empty into a follicular canal that surrounds each hair follicle.8,10 The bony portion of the canal is lined by thin, immobile skin that lacks hair and glands and is continuous with the epithelium of the tympanic membrane (TM). The bony-cartilaginous junction is the narrowest point, or isthmus, of the EAC; here a fibrous interface serves as a potential pathway for spread of malignant disease beyond the ear. For this reason, en bloc resection of the lateral temporal bone is necessary to eradicate primary malignancies of the EAC. The bony-cartilaginous junction is also the site at which underlying osteomyelitis will manifest as mounds of granulation tissue in the EAC, a finding pathognomonic for malignant otitis externa.11 The incomplete ossification of the anterior bony canal produces an opening into the infratemporal region, known as theforamen of Huschke, which may also serve as a means for extension of malignant tumors from the EAC to the deep lobe of the parotid gland. Naturally occurring defects in the cartilaginous portion of the EAC, known as thefissures of Santorini, also provide avenues of spread to the superficial lobe of the gland.

The external ear is developed from ectodermal and mesodermal components of the first and second branchial arches and the intervening first branchial groove.12,13 Distinct condensations of tissue, known as thehillocks of His, give rise to the tragus and most of the helix—from the first branchial arch and to the antihelix, antitragus, and lobule and the inferior helix from the second branchial arch. Sensory innervation is provided by the corresponding first branchial nerve, the auriculotemporal branch of the trigeminal nerve, and a cutaneous branch of the facial nerve. The EAC develops from the dorsal portion of the first branchial cleft, which extends toward and eventually makes contact with endoderm of the expanding tubotympanic recess. A transient obstruction of the medial canal is created by proliferating epithelial cells to form a meatal plug that eventually dissolves to leave a patent canal.

Embryology

The external ear appears only in mammals. The internal, middle, and external ear development is a beautiful and complex embryologic orchestration. The classic description by His is as follows. The external ear forms from tissues of the first and second pharyngeal arches, which can be seen at 5 weeks (Fig. 8.1). Six tissue elevations termed auricular hillocks of His become apparent at 6 weeks in utero (Fig. 8.2). Three hillocks form from the first arch and three from the second arch. Each of the auricular hillocks forms a distinctive portion of the definitive external ear. For example, hillock 1 forms the tragus and hillock 6 forms the antitragus, as well as part of the helix (Fig. 8.3). The lobule of the ear is not derived from the hillocks. The developing external ears are initially more caudal than the lower jaw. Growth of the lower jaw places the external ear in a relatively higher and more vertical orientation.

The classical embryologic tenets have been challenged. In this description, the first branchial arch contribution is limited to the tragus.

Anatomy of the External Ear

The external ear includes the auricle and the EAC. The auricle consists of keratinizing squamous epithelium covering a framework of elastic cartilage with perichondrium tightly bound to its lateral surface and more loosely bound to its medial surface. Sebaceous glands and hair follicles are found in the subcutaneous layer, with adipose tissue restricted to the cartilage-free lobule.

The EAC extends from the lateral surface of the tympanic membrane (TM) to the external auditory meatus; it measures approximately 2.5 cm in adults. A bony wall surrounds the medial two-thirds of the canal, with the lateral one-third possessing a cartilaginous skeleton. The cartilaginous portion contains hair follicles along with sebaceous and apocrine glands beneath a squamous epithelial surface layer. Cerumen is found in this portion of the canal; it is a hydrophobic, slightly acidic (pH 6.0 to 6.5) substance formed by glandular secretions and sloughed epithelium. Transverse slits in the cartilaginous canal (the fissures of Santorini) allow for the spread of infection or neoplasms from the external canal to the surrounding soft tissues.

The tympanic portion of the temporal bone forms the majority of the osseous EAC. The bone is covered by a thin layer of squamous epithelium that is tightly adherent to the bone and continuous with the lateral surface of the tympanic membrane. There is no subcutaneous layer and no glands or hair follicles. The junction of the bony and cartilaginous canal is known as the isthmus and represents the narrowest portion of the canal. The foramen of Huschke is a defect in the anterior bony canal from incomplete ossification that allows for the spread of disease to the deep lobe of the parotid gland.

The ear canal possesses a unique self-cleansing mechanism. The sloughed keratinous layer of the tympanic membrane migrates in a centrifugal fashion toward the annulus and subsequently to the cartilaginous canal, where it is combined with glandular secretions and extruded as cerumen.

Normal flora isolated from the EAC and cerumen is overwhelmingly gram positive. The most common bacteria includeStaphylococcus auricularis andStaphylococcus epidermidis. Coryneform bacteria (diphtheroids), streptococci, and enterococci represent the next most common groups in descending order.Pseudomonas aeruginosa and fungal species are rare in the EAC of normal subjects.4

Development of the External Ear

The external ear (pinna) is derived from mesenchymal tissue of the first and second pharyngeal arches that flank the first (hyomandibular) pharyngeal cleft. During the second month, three nodular masses of mesenchyme (auricular hillocks) take shape along each side of the first pharyngeal cleft (Figure 7). The auricular hillocks enlarge asymmetrically and ultimately coalesce to form a recognizable external ear. During its formation, the pinna shifts from the base of the neck to its normal adult location on the side of the head.

The external auditory meatus takes shape during the end of the second month by an inward expansion of the first pharyngeal cleft. Early in the third month, the ectodermal epithelium of the forming meatus proliferates and forms a solid mass of epithelial cells called the meatal plug (see Figure 5(c)). Late during the fetal period (at 28 weeks), a channel within the meatal plug extends the existing external auditory meatus to the level of the tympanic membrane.

External Ear

The external ear is composed of the pinna and the external auditory canal. The external ear serves to funnel sound from the external environment into the ear. The peculiar shape of the pinna and the external auditory canal gives rise to specific resonant frequencies as these structures are struck by sound: the concha has a resonant frequency of around 5300 Hz, and the external auditory canal has a resonant frequency of around 3000 Hz. The external ear plays an important role in sound localization, which is achieved by two major mechanisms: interaural time difference and interaural amplitude difference. Because the left and right ears are located at the opposite sides of the head, the amount of time it takes for a sound stimulus to arrive at each individual ear is governed by the distance from the sound source to that particular ear: the farther the distance, the longer it takes for the sound stimulus to arrive. As a cue for sound localization, the differences in thearrival times of the sound stimulus between the two ears can be used, as can the differences inamplitude perceived by the two ears.1,2 This difference in amplitude is further increased by the so-called head shadow effect, in which sound coming from one side is attenuated by the head as the sound travels to the contralateral ear. The head shadow effect in binaural hearing helps to improve the signal-to-noise ratio in adverse listening environments; one ear can be closer to the source of sound or speech while the contralateral ear is exposed to the background noise. It has been shown that the interauraltime difference is important for low-frequency sound localization, whereas the interauralamplitude difference is important for higher frequencies.3

External Ear

The external ear (pinna) is a strictly mammalian innovation. Although birds and reptiles and even frogs have a tympanic membrane, it lies flat on the surface of the head. Mammals, by contrast, have prominent external ears that are specialized for collecting sound waves. Most mammalian ears are connected to a set of extrinsic muscles that can move the ears to further increase their ability to pick up sound waves. In addition, another six small intrinsic muscles can change the shape of the ear. Human ears have largely lost those capacities, although some people are able to wiggle their ears through contractions of their extrinsic ear muscles.

Human ears come in a wide variety of sizes and shapes. In fact, ear shape has been used as a personal identifier, much as fingerprints and iris patterns. The external ear arises from a series of six bumps on the first two embryonic pharyngeal arches—three on each arch (Fig. 7.18). These sites of origin are consistent with the origins of the components of the middle ear, which also arise from the first and second arches.

The collection of sound waves by the external ear plays a major role in the localization of sound. This function is highly important for survival—both in nature, where it is critical in both finding food and avoiding becoming food, and in civilization, where it used both in communication and avoiding danger from moving objects, like cars. Sound is localized along both vertical and horizontal planes, and each occurs through a different mechanism.

Vertical localization of sound depends upon the shape of the external ear. A given sound can enter the external auditory meatus (see Fig. 7.16) either directly or by reflection of part of the external ear (Fig. 7.19). Reflected sound takes slightly longer to reach the external auditory meatus, and through central analysis of the interference patterns caused by these delays, the brain is able to localize the vertical source of the sound, even with only one ear.

Horizontal localization of sound requires both ears, and it depends upon the frequency of the sound. For frequencies greater than 2000 Hz, the intensity of the sound received by each ear is the determining factor. If the sound is coming from the right side of the head, the left ear is in a sound shadow, resulting in a lesser intensity of the sound (Fig. 7.20A). This difference in intensity is recognized and processed by the brain. For wavelengths less than 2000 Hz, the wavelength is greater than the width of the head. This allows sound waves from the right to reach the left ear with essentially equal intensity because they diffract around the head, but the sound reaching the left ear is slightly delayed (Fig. 7.20B). In this case, the brain picks up and processes the delay, with the resultant localization of sound.

Once sound waves pass the external ear proper, they enter the external auditory meatus, a tube roughly 30 mm in length. Its surface is lined with skin, with some hairs, and it contains both sebaceous glands and modified apocrine glands. Cerumen (earwax) is a mixture of secretions from both the sebaceous and apocrine glands, along with desquamated epithelial cells, that collects in the ear canals. Cerumen and the hairs protect the auditory meatus against entry by insects and foreign bodies, and it has been found to protect against certain species of pathogenic bacteria. An important auditory function of the external auditory meatus is amplification of sound waves around 3000 Hz, which is in the common range of human speech. The external and middle ears amplify sound by as much as 10–15 dB.

The tympanic membrane lies between the external and middle ear. In mammals, it is largely located within the confines of the temporal bone of the skull. Because of its deep location, it considered not to be homologous with the tympanic membranes of many lower vertebrates. The tympanic membrane is well innervated by several small sensory nerves. For that reason, stretching or rupture of the tympanic membrane is very painful. Its inner surface is the attachment point for the handle of the malleus, one of the inner ear bones.

Among nonhuman mammals, large ears are often adaptations for exceptionally acute hearing. In addition they have sometimes evolved for other functions. For example, the large ears of African elephants are designed to radiate heat as a mechanism for controlling overall body temperature. Similarly, rabbits or hares living in hot deserts have much larger ears than do their Arctic counterparts.

Embryology

The external ear develops during the sixth week of gestation and is completely developed by the 20th week. Six hillocks fuse to form the basic units of the pinna. Defects in the fusion of the hillocks lead to preauricular tags and sinuses. The external auditory canal develops from the first branchial cleft. A solid epithelial plug forms during the beginning of the third month of gestation and canalizes in the seventh month to form the external auditory canal.

The middle ear space develops from the first pharyngeal pouch. The ossicles develop from the first and second pharyngeal arches. The inner ear arises from neuroectodermal tissue within the otic placode that forms the otic pit.2

Any combination of anomalies may occur. Abnormalities of the development of the ear may create anomalies of the pinna, external auditory canal, middle ear structures, and inner ear. One of the anomalies that involves the external and middle ear is aural atresia (absence of the external auditory canal). Absence of the external canal may occur with a deformed or normal external ear. The ossicles may be deformed and are usually fused to each other as well as the bony plate representing the undeveloped tympanic membrane. The facial nerve may also be altered in its course through the temporal bone. Reconstruction of the atretic canal, removal of the bony tympanic plate, release of the fused ossicles, and reconstruction of a new eardrum is a complex surgical procedure that may improve hearing. Rarely, there is incomplete development of the inner ear structures. The most common of these is dysplasia of the cochlea, and it may vary in severity. Dysplasia is associated with sensorineural hearing loss in most cases.3,4

Gross Anatomy

The external ear is composed of the pinna (also known as the auricle) and the external acoustic meatus (external ear canal) (Figs. 22.1 and 22.2) and is designed to funnel sounds to the tympanic membrane, which separates the external and middle ear. The rodent pinna is relatively thin and less folded compared to that of humans but is otherwise similarly structured. In mice, the pinna is located on the lateral aspect of the head and is roughly cone-shaped, with few folds on the conchal surface (Fig. 22.1A), and has a greater surface area relative to body size than rats. Hairs cover both sides of the pinna, with shorter, less dense hairs found on the conchal surface and proximal to the external acoustic meatus. The human pinna has complex cartilage folds with fewer hairs than rodents (Fig. 22.1B). Rodents have a unique gland, Zymbal’s gland, located subcutaneously at the base of the ear (Fig. 22.3). Zymbal’s gland is grossly visible in rats but not mice. Rat Zymbal’s gland is 3–5 mm in diameter, comprises 3–4 lobes, and is located anteroventrally to the base of the ear (Fig. 22.3A). It has been noted that beneath Zymbal’s gland in rats are two smaller sebaceous glands, which some include as part of Zymbal’s gland. The external acoustic (or auditory) meatus (EAM) in mice is approximately 6.25 mm and has a slight rostral curve to its canal as it approaches the tympanic membrane. In rats, the EAM is approximately 1–1.5 cm long. In humans, the EAM is S-shaped and approximately 2.5 cm long.

External Ear

The external ear is composed of the pinna and the external auditory meatus. Its principal functions are to collect and funnel sound to the tympanic membrane (eardrum) and to provide cues about the location of wideband sound sources in the vertical plane. With regard to the latter, the unique convolutions of a person's pinna, combined with reflections off the upper body, filter sound such that energy is enhanced or attenuated at different frequencies when a sound source is located above, behind, below, or in front of the head. These head-related transfer functions are specific to each individual.

External ear (pinna)

The external ear is affected by inflammatory and neoplastic conditions similar to those occurring elsewhere in the skin and subcutaneous tissues. The protruding auricle is particularly liable to traumatic damage. In mice it can be used as a convenient site for mechanistic studies of skin irritancy (see Skin and Subcutaneous Tissue, Chapter 2). A number of more specific conditions may occasionally be seen in the external ear, especially in rats, notably auricular chondritis and carcinomas of the auditory sebaceous glands. Changes to cartilage within the pinna may reflect systemic alterations to cartilage homeostasis. For example, large ears characterized histologically by thickened, hypertrophic and proliferative changes in the cartilage layer were observed in C3(1)/SV40 T antigen (TAg) transgenic mice where the background strain was the C57BL/6J rather than the usual FVB/N type. It was suggested that the simian virus 40 large tumour antigen (TAg) transforming sequence, which inactivates p53 and retinoblastoma proteins and usually leads to cell transformation, may also be a cell cycle regulator of chondrocyte proliferation and differentiation. Whilst no neoplasms developed in these mice, the change in the pinna was associated with generalized cartilaginous changes in joints resembling chondromatosis in humans.373