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CHAPTER 1
ANATOMY OF THE EAR
Karen Tobias
1.1 External Ear: Pinna and Ear Canal
1.2 Middle Ear of the Dog
1.3 Middle Ear of the Cat
1.4 Inner Ear
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.
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).
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.
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).
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.
