CHAPTER 36 External and middle ear
By convention, the ear is subdivided into three parts, the external, middle and inner ear. It is largely, but not entirely, contained within the temporal bone. The ears not only receive, modulate, conduct, amplify and discriminately analyse the complex pressure waves that are sound, but also contain the end organs of balance.
TEMPORAL BONE
Each temporal bone consists of four components: the squamous, petromastoid and tympanic parts and the styloid process (Fig. 36.1). The squamous part has a shallow mandibular fossa associated with the temporomandibular joint (Ch. 31). The petromastoid part is relatively large: its petrous portion houses the auditory apparatus and is formed of compact bone. In contrast, the mastoid process is trabecular and variably pneumatized. The tympanic part has the form of a thin and incomplete ring whose ends are fused with the squamous part. The styloid process gives attachment to the styloid group of muscles. Two canals are associated with the temporal bone. The external acoustic meatus, visible on the lateral surface, conveys sound waves to the tympanic membrane. The internal acoustic meatus, evident on the medial surface, conveys the facial and vestibulocochlear nerves.
Fig. 36.1 Left temporal bone. A, Lateral aspect. B, Medial aspect. C, Inferior aspect.
(From Sobotta 2006.)
Squamous part
The squamous part has a zygomatic process and a mandibular fossa.
Mandibular fossa
The mandibular fossa is limited in front by the articular eminence of the zygomatic process. It presents an anterior articular area, formed by the squamous part, and a posterior non-articular area, formed by the tympanic element. The articular surface is smooth, oval and concave, and contacts the articular disc of the temporomandibular joint (Ch. 31). Unlike most other synovial joints, it is lined by fibrous tissue rather than hyaline cartilage, reflecting its intramembranous development. The non-articular area sometimes contains part of the parotid gland. A small, conical postglenoid tubercle separates the articular surface laterally from the tympanic plate.
Petromastoid part
Mastoid part
The superior border of the mastoid part is thick and serrated for articulation with the mastoid angle of the parietal bone. The posterior border is also serrated and articulates with the inferior border of the occipital bone between its lateral angle and jugular process. The mastoid element is fused with the descending process of the squamous part: below, it appears in the posterior wall of the tympanic cavity.
Petrous part
The anterior surface contributes to the floor of the middle cranial fossa (Ch. 27) and is continuous with the cerebral surface of the squamous part (although the petrosquamosal suture often persists late in life). The whole surface is adapted to the inferior temporal gyri. Behind the apex is a trigeminal impression for the trigeminal ganglion. Bone anterolateral to this impression roofs the anterior part of the carotid canal, but is often deficient. A ridge separates the trigeminal impression from another hollow behind which partly roofs the internal acoustic meatus and cochlea. This, in turn, is limited behind by the arcuate eminence which is raised by the superior (anterior) semicircular canal. Laterally, the anterior surface roofs the vestibule and, partly, the facial canal. Between the squamous part laterally and the arcuate eminence and the hollows just described medially, the anterior surface is formed by the tegmen tympani, a thin plate of bone which forms the roof of the mastoid antrum, and extends forwards above the tympanic cavity and the canal for tensor tympani. The lateral margin of the tegmen tympani meets the squamous part at the petrosquamosal suture, turning down in front as the lateral wall of the canal for tensor tympani and the osseous part of the pharyngotympanic tube: its lower edge is in the squamotympanic fissure. Anteriorly the tegmen bears a narrow groove related to the greater petrosal nerve (which passes posterolaterally to enter the bone by a hiatus anterior to the arcuate eminence). The groove passes forwards to the foramen lacerum. A smaller and similar hiatus and groove may be found more laterally: they are related to the lesser petrosal nerve (which runs to the foramen ovale). The posterior slope of the arcuate eminence overlies the posterior and lateral semicircular canals. Lateral to the eminence, the posterior part of the tegmen tympani roofs the mastoid antrum.
External acoustic meatus
The temporal bone contains the bony (osseous) part of the external acoustic meatus (see p. 620).
Ossification
The four temporal components ossify independently (Fig. 36.2). The squamous part is ossified in a sheet of condensed mesenchyme from a single centre near the zygomatic roots, which appear in the seventh or eighth week in utero. The petromastoid part has several centres which appear in the cartilaginous otic capsule during the fifth month: as many as 14 have been described. These centres vary in order of appearance. Several are small and inconstant, soon fusing with others. The otic capsule is almost fully ossified by the end of the sixth month. The tympanic part is also ossified in mesenchyme from a centre identifiable about the third month; at birth it is an incomplete tympanic ring, deficient above, its concavity grooved by a tympanic sulcus for the tympanic membrane. The malleolar sulcus for the anterior malleolar process, chorda tympani and anterior tympanic artery inclines obliquely downwards and forwards across the medial aspect of the anterior part of the ring. The styloid process develops from two centres at the cranial end of cartilage in the second visceral or hyoid arch: a proximal centre for the tympanohyal appears before birth, and another, for the distal stylohyal, appears after birth. The tympanic ring unites with the squamous part shortly before birth, and the petromastoid fuses with it and the tympanohyal during the first year. The stylohyal does not unite with the rest of the process until after puberty and may never do so.
Once ossified, the tympanic cavity, mastoid antrum and the posterior end of the pharyngotympanic tube become surrounded by bone. The petrous part forms the roof, floor and medial wall of the cavity, while the squamous and tympanic parts, together with the tympanic membrane, form its lateral wall. At birth the middle and inner ears are adult size, and the tympanic cavity, mastoid antrum, tympanic membrane and auditory ossicles are all almost adult size. The anterior process does not join the malleus until 6 months later. The internal acoustic meatus is approximately 6 mm in horizontal diameter, 4 mm in vertical diameter and 7 mm in length at birth, and the adult diameters are 7.7 mm and 11 mm respectively.
EXTERNAL EAR
AURICLE (PINNA)
The lateral surface of the auricle is irregularly concave, faces slightly forwards, and displays numerous eminences and depressions (Fig. 36.3). It has a prominent curved rim, the helix. This usually bears a small tubercle posterosuperiorly, Darwin’s tubercle, which is quite pronounced around the sixth month of intrauterine life. The antihelix is a curved prominence, parallel and anterior to the posterior part of the helix: it divides above into two crura which flank a depressed triangular fossa. The curved depression between the helix and antihelix is the scaphoid fossa. The antihelix encircles the deep, capacious concha of the auricle, which is incompletely divided by the crus or anterior end of the helix. The conchal area above this, the cymba conchae, overlies the suprameatal triangle of the temporal bone, which can be felt through it, and which overlies the mastoid antrum. The tragus is a small curved flap below the crus of the helix and in front of the concha: it projects posteriorly, partly overlapping the meatal orifice. The antitragus is a small tubercle opposite the tragus and separated from it by the intertragic incisure or notch. Below it is the lobule, composed of fibrous and adipose tissues. It is soft, unlike the majority of the auricle which is supported by elastic cartilage and is firm. The cranial surface of the auricle presents elevations which correspond to the depressions on its lateral surface, and after which they are named (e.g. eminentia conchae, eminentia fossae triangularis).
Fig. 36.3 Lateral surface of the left auricle.
(By permission from Berkovitz BKB, Moxham BJ 2002 Head and Neck Anatomy. London: Martin Dunitz.)
A number of common abnormalities have been recognized and carry descriptive names or eponyms (Porter & Tan 2005).
Stahl’s bar (also known as Satiro’s ears) is a common congenital deformity of the auricle where the helix is flattened and the upper crus of the antihelix is duplicated, producing a ridge of cartilage running from the antihelix to the rim of the helix. This causes a pointing of the ear and a reversal of the normal concavity of the scaphoid fossa. Occasionally, the upper part of the pinna flops over to produce an appearance known as ‘lop ear’. A variety of surgical procedures have been designed to correct this deformity using external moulds etc. Attempts at correction are made in the neonatal period while the cartilage is soft and pliable: delay until such time as the cartilage has become stiff may jeopardize a good outcome.
Six auricular hillocks, the embryological precursors of the auricle, form round the margins of the dorsal portion of the first pharyngeal cleft, three on the caudal edge of the first pharyngeal arch and three on the cranial edge of the second pharyngeal arch (see Ch. 40). They fuse to form the auricle and surround the dorsal end of the first branchial cleft from which the external acoustic meatus arises. Sinuses and cysts are often found just anterior to the root of the helix, near to the point of fusion of the hillocks derived from the first branchial arch and those derived from the second branchial arch. There is debate as to whether the abnormalities are epithelial inclusions between the hillocks or remnants of the first branchial cleft. The sinuses may be simple pits or complex branching sinuses that occasionally extend deeply towards the external acoustic meatus so that they lie close to the facial nerve. Clinically they may become chronically infected and require surgical excision: this may be technically demanding surgery given the close proximity to the facial nerve.
Skin
The skin of the auricle continues into the external auditory meatus to cover the outer surface of the tympanic membrane. It is thin, has no dermal papillae, and is closely adherent to the cartilaginous and osseous parts of the canal (inflammation of the canal skin is very painful because of this attachment to the underlying structures). The thick subcutaneous tissue of the cartilaginous part of the meatus contains numerous ceruminous glands that secrete wax, or cerumen. Their coiled tubular structure resembles that of sweat glands. The secretory cells are columnar when active, but cuboidal when quiescent; they are covered externally by myoepithelial cells. Ducts open either onto the epithelial surface or into the nearby sebaceous gland of a hair follicle. Cerumen prevents the maceration of meatal skin by trapped water. Antibacterial properties have been attributed to cerumen, but the evidence for this is lacking (Campos et al 2000, Pata et al 2003).
Two types of wax, wet and dry, are recognized. They are genetically determined. Dry wax is common in East Asians, while the wet type is more common in other ethnic groups (Yoshiura et al 2006). Overproduction, accumulation or impaction of wax may completely occlude the meatus, thereby hindering sound from reaching the tympanic membrane and also restricting the natural vibration of the drum. Although ceruminous glands and hair follicles are largely limited to the cartilaginous meatus, a few small glands and fine hairs are also present in the roof of the lateral part of the osseous part of the canal. The warm, humid environment of the relatively enclosed meatal air aids the mechanical responses of the tympanic membrane.
Cartilaginous framework
The auricle is a single thin plate of elastic fibrocartilage covered by skin, its surface moulded by eminences and depressions (Fig. 36.4). It is connected to the surrounding parts by ligaments and muscles, and is continuous with the cartilage of the external acoustic meatus. There is no cartilage in the lobule or between the tragus and the crus of the helix, where the gap is filled by dense fibrous tissue. Anteriorly, where the helix curves upwards, there is a small cartilaginous projection, the spine of the helix. Its other extremity is prolonged inferiorly as the tail of the helix and it is separated from the antihelix by the fissura antitragohelicina. The cranial aspect of the cartilage bears the eminentia conchae and eminentia scaphae, which correspond to the depressions on the lateral surface. The two eminences are separated by a transverse furrow, the sulcus antihelicis transversus, which corresponds to the inferior crus of the antihelix on the lateral surface. The eminentia conchae is crossed by an oblique ridge, the ponticulus, for the attachment of auricularis posterior. There are two fissures in the auricular cartilage, one behind the crus of the helix and another in the tragus.
Auricular muscles
Intrinsic muscles
The intrinsic auricular muscles are helicis major and minor, tragicus, antitragicus, transversus auriculae and obliquus auriculae (Fig. 36.5). Helicis major is a narrow vertical band on the anterior margin of the helix, passing from its spine to its anterior border, where the helix is about to curve back. Helicis minor is an oblique fasciculus covering the crus of the helix. Tragicus is a short, flattened, vertical band on the lateral aspect of the tragus. Antitragicus passes from the outer part of the antitragus to the tail of the helix and the antihelix. Transversus auriculae, located on the cranial aspect of the auricle, consists of scattered fibres, partly tendinous, partly muscular, which extend between the eminentia conchae and the eminentia scaphae. Obliquus auriculae, also located on the cranial aspect of the auricle, consists of a few fibres which extend from the upper and posterior parts of the eminentia conchae to the eminentia scaphae.
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