1.1 EXTERNAL EAR: PINNA AND EAR CANAL
The pinna is the most prominent portion of the external ear (Fig. 1.1). It has an inner, concave surface and an outer, convex surface. In the standing ear, the concave surface forms a conchal cavity that is directed rostrally or laterally, while the convex surface faces medially or caudally. The distal tip of the pinna is called the apex, and the lateral and medial free margins of the pinna are called the helix (Fig. 1.2). The rostrolateral boundary of the distal portion of the ear canal is called the tragus. A notch caudal to the tragus, the intertragic incisure, separates it from the antitragus, which is a thin elongated piece of cartilage that extends up to the lateral margin of the helix at the cutaneous marginal pouch.
Figure 1.1 General anatomy of the pinna. The conchal cavity of the concave surface of the ear can be directed rostrally or laterally.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
Figure 1.2 Concave surface of the right dog pinna. The antihelix and tragus form the boundaries of the ear canal opening.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
The margins of the pinna are divided into medial, or rostral, and lateral, or caudal (see Fig. 1.1). These variations in directional description can make the anatomy very confusing.
The external ear is composed of three cartilages: annular, auricular, and scutiform. The ear canal is formed proximally (near the skull) by the annular cartilage and distally (away from the skull) by the auricular cartilage, which fans out to form the pinna (Fig. 1.3).
Figure 1.3 Auricular and annular cartilage of the right ear of a dog, lateral view.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
The auricular cartilage is divided into three sections: the scapha, the concha, and the tubus auris, or conchal tube (Fig. 1.4). Whereas the scapha is distally located and flattened, the concha is rolled into a trumpet shape to form the conchal cavity (Fig. 1.5). The scapha and concha are divided on the concave surface by the antihelix, a transverse cartilaginous fold.
Figure 1.4 Auricular and annular cartilage of the right ear, caudal view. The annular cartilage is nestled within the auricular cartilage, which forms the pinna and vertical ear canal. Note that the proximal portion of the auricular cartilage spirals inward as it bends.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
Figure 1.5 Medial view of cartilage of the right ear. A portion of the auricular cartilage that forms the conchal tube has been elevated; underneath is another extension of auricular cartilage that wraps around the annular cartilage. Note that the ear canal is not a solid funnel: the auricular and annular tubes are each formed by overlapping flaps of cartilage that allow flexibility. Animals with severe otitis externa or conchal obstruction may develop periauricular abscesses from disruption of the fibrous connective tissue sheath surrounding either the tube flaps or the auricular-annular or annular-osseous junction.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
The concha forms a funnel shape that thickens proximally as it becomes the conchal tube. The conchal tube forms the vertical ear canal. This canal is up to an inch (2.5 cm) deep and, as it progresses proximally towards the head, is directed ventrally, medially, and slightly rostral, spiralling inwards. It is partially surrounded along its proximal lateral border by the parotid salivary gland.
The annular cartilage is a separate, rolled, cartilaginous band that fits inside of the base of the conchal tube. It forms the horizontal ear canal, which runs medially toward the skull. In turn, the annular cartilage overlaps the osseous external acoustic meatus. Junctions of the auricular and annular cartilages and the annular cartilage and skull are connected by a fibrous tissue sheath. Because of these moveable joints, the auditory canal can be straightened during otoscopic examination. Epithelium lining the auricular and annular cartilage contains sebaceous and ceruminous glands and hair follicles (Fig. 1.6).
Figure 1.6 (A), Cross-section of the pinna of a dog. (B), Components including hyaline cartilage, muscle, and hair follicles are easily visible.
(Courtesy, UTCVM Virtual Microscope) © 2012 The University of Tennessee.
Terminology for the ear canal varies within and amongst texts. Some authors consider the osseous extension of the skull that encompasses the tympanic membrane to be the external acoustic meatus or osseous external acoustic meatus, while others consider the external acoustic meatus to be the opening of the conchal tube at the level of the tragus and antihelix. The cartilaginous tube that extends from the meatus to the concha, which is a combination of conchal tube (auricular) and annular cartilage, is sometimes called the auditory canal.
A variety of muscles attach the ear rostrally, ventrally, or caudally to the head (Fig. 1.7); these muscles are innervated by the facial nerve. Some of these muscles are continuous with the cervical portion of the platysma. The plate-like, L-shaped scutiform cartilage, which is medial to the auricular cartilage, lies within the muscles that attach the auricular cartilage to the head (Fig. 1.8). By acting as a fulcrum, the scutiform cartilage improves mobility of the auricular cartilage.
Figure 1.7 Muscles of the canine ear and face: right lateral view.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
Figure 1.8 Muscles of the canine ear and head: dorsal view. The scutiform cartilage is enveloped within the dorsal group of muscles.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
The major portion of the blood supply to the external ear comes from the caudal auricular artery, which arises from the external carotid artery at the base of the annular cartilage and medial to the parotid salivary gland (Fig. 1.9). The caudal auricular and superficial temporal veins, which terminate at the maxillary vein, provide drainage of the external ear (Fig. 1.10). Perforations in the auricular cartilage permit passage of blood vessels and nerves from the convex to the concave surface.
Figure 1.9 Selected branches of the common carotid artery. The external carotid artery gives off the caudal auricular artery and then travels around the ventral and rostral aspects of the horizontal canal before terminating in the maxillary and superficial temporal arterial branches.
(Background photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
Figure 1.10 Selected tributaries of the external jugular vein. The superficial temporal vein travels ventrally around the rostral aspect of the horizontal canal and then joins the maxillary vein, which lies ventral to the canal.
(Background photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
Sensory innervation to the concave surface of the pinna is provided primarily by branches of the facial nerve (Fig. 1.11) and, at the rostral extent of the pinna, by branches of the trigeminal nerve. The lateral auricular branch of the facial nerve provides sensation to the majority of the vertical canal, along with a portion of the horizontal canal, while the auriculotemporal branch of the trigeminal nerve provides sensory innervation to the horizontal canal and tympanic membrane. The convex surface of the pinna receives sensory innervation via the second cervical nerve. Communications between vagal and facial nerve branches may also be present.
Figure 1.11 Selected branches of the facial nerve. After exiting the stylomastoid foramen, the facial nerve travels near the caudal, ventral, and rostral aspects of the horizontal ear canal.
(Background photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
1.2 MIDDLE EAR OF THE DOG
The canine middle ear (Figs 1.12 and 1.13) consists primarily of an air-filled tympanic cavity that is separated from the external ear by the tympanic membrane and from the inner ear by the vestibular and cochlear windows. The middle ear is divided into three parts: (1) a large, ventral tympanic bulla within the temporal bone; (2) a small, dorsal epitympanic recess, which sits above the level of the tympanic membrane; and (3) the tympanic cavity proper, which connects the two and is bounded on its lateral surface by the tympanic membrane (Fig. 1.14).The tympanic cavity proper is partially separated from the ventral tympanic bulla by an incomplete septum. The tympanic cavity proper contains the cochlear (round) window along its caudal aspect. The ossicles of the ear – the stapes, incus, and portions of the malleus – reside within the epitympanic recess and span the distance from the inner ear to the tympanic membrane (Fig. 1.15). The tympanic cavity is lined by simple squamous or cuboidal epithelium, except at the orifice of the auditory tube.
Figure 1.12 Left lateral view of the canine skull. In this image, the mandible has been removed and the skull has been rotated slightly. Note how the bulla is less prominent than the retroarticular and jugular processes.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
Figure 1.13 Right lateral view of the canine bulla with the mandible in place. During ventral bulla osteotomy, the position of the canine bulla is estimated by palpating the jugular and angular processes.
(Photo by Phil Snow, UTCVM) © 2012 The University of Tennessee.
Figure 1.14 Parts of the canine middle ear. In this dog, the manubrium of the malleus is visible as an L-shaped structure, and a portion of the septum can be seen along the medial wall of the bulla.
(Courtesy, UTCVM Radiology) © 2012 The University of Tennessee.
Figure 1.15 The auditory ossicles – the malleus, incus, and stapes – span the distance from the tympanic membrane to the oval window membrane.
The tympanic membrane is oval in shape and concave from an external viewpoint because of medial traction by the attached malleus (Fig. 1.16). In the dog it lies at a 45° angle to the long axis of the horizontal canal, with its ventral aspect farther from view than the dorsal portion. The largest portion of the tympanic membrane is called the pars tensa, a taut, semi-transparent, fibrous membrane. The pars tensa is firmly attached to the surrounding osseous external acoustic meatus by the annulus fibrocartilaginous, a fibrocartilage ring. The much smaller, dorsal portion of the tympanic membrane, known as the pars flaccida, is loose, opaque, and richly vascularized (Fig. 1.17).
Figure 1.16 Diagram of the tympanic membrane. The tympanic membrane curves away from the external canal because of inward tension of the malleus, which attaches to it. Epithelium regenerates from the umbo outwards; this area should be avoided during myringotomy.
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