The external ear of mammals consists of the auricle and external acoustic meatus. It varies greatly in size and shape between species and within domestic breeds. The external ear evolved as a sound-gathering structure, although its morphologic features in some breeds of domestic dogs appear to impede rather than enhance its function. The ears, when erect, can be directed independently to localize and collect sound. Sound is conducted via the external acoustic meatus to the tympanic membrane deep in the external acoustic meatus.
External Acoustic Meatus
The external acoustic meatus (meatus acousticus externus) is the canal from the base of the auricle to the tympanic membrane surrounded by annular cartilage and the tubular portion of the auricular cartilage. The latter is the rolled up proximal part of the auricular cartilage.
The auricle (auricula), pinna, is the externally visible part of the ear. The size and shape of the nontubular part of the auricular cartilage (cartilago auriculae) determines the appearance of the auricle, which may be upright or pendulous. In some breeds, such as the Boxer, it is often surgically trimmed The auricle is covered by skin and is moved by muscles. The auricular cartilage is pierced by many foramina that permit the passage of blood vessels and nerves from the convex surface to the concave surface. When the ear is traumatized and blood vessels rupture, large hematomas may develop between the skin and the cartilage; in such cases surgical intervention is often required.
With the ear (auricle) erect, the concave surface of the auricle is directed rostrally and the convex surface caudally. The conchal cavity (cavum conchae) is the proximal portion of the auricle where it is funnel-shaped at the entrance into the external acoustic meatus. The anthelix is a transverse fold of cartilage on the concave surface of the auricle. It is adjacent to the distal portion of the conchal cavity and separates the conchal cavity from the scapha, which is the large flat concave internal side of the auricle. The proximal portion of the auricular concha that surrounds the conchal cavity is rolled into a tube to enclose the external acoustic meatus. This opening faces dorsally. The tubular external acoustic meatus extends ventrally and then bends medially until it meets the small anular cartilage (cartilago anularis) that fits into the proximal portion of the tubular cartilage of the external acoustic meatus. The anular cartilage is approximately 2 cm long in the average size dog. The proximal portion of the anular cartilage overlaps and attaches to the osseous portion of the external acoustic meatus. This arrangement of a separate cartilaginous “joining ring” connecting these two portions of the external acoustic meatus by fibrous tissue gives the external ear flexibility.
The auricular cartilage is elastic, thin, and pliable. It thickens proximally where it rolls into a tube. The entire free margin of the auricle and passing over the apex is the helix. The spine of the helix (spina helicis) is a medial projection on the proximal part of the helix medially. On the proximal lateral portion of the helix is a fold of skin partly supported by cartilage known as the marginal cutaneous sac (saccus cutaneus marginalis) or pouch.
A number of cartilaginous projections are found at the entrance into the external acoustic meatus where the conchal portion of the auricular cartilage is rolled into a tube. The tragus is a thick blunt irregularly quadrangular plate of cartilage that projects from the rostral border of this entrance. Lateral to the tragus on this rostral border is a thin elongate projection of cartilage, the antitragus. The antitragus is separated from the tragus by a notch, the intertragic incisure (incisura intertragica). The antitragus consists of medial and lateral processes that project laterally. The apex of the lateral process ends in a sharp point laterally called the styloid process (processus styloideus). This process is separated from the lateral portion of the helix by the antitragicohelicine incisure (incisura antitragicohelicina). The marginal cutaneous sac is just distal to this incisure. Medial to the tragus on this rostral border are two crura formed from the medial portion of the helix. These medial and lateral crura (crus helicis mediale and laterale) project laterally and are separated from the tragus by the pretragic incisure (incisura pretragica). The lateral crus of the helix is close to the opening of the external acoustic meatus and caudal to the medial crus of the helix, which borders the intertragic incisure.
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