Anatomy of the Canine and Feline Ear

Chapter 1 Anatomy of the Canine and Feline Ear

The basic anatomical components of the dog and cat ear are as follows:

Structure of the External Ear

The external ear is composed of three elastic cartilages: annular, scutiform, and auricular (Figure 1-1). The annular and auricular cartilages form the external ear canal, and the auricular cartilage expands to form the pinna. The scutiform cartilage lies medial to the auricular cartilage within the auricular muscles that attach to the head (Figure 1-2).

Pinna, or Auricle

The pinna, or auricle, is a highly visible structure. Carriage of the pinna is breed-specific in the dog but mostly upright in the cat. It is designed to localize and collect sound waves and transmit them to the tympanic membrane (eardrum). The ear is moved by three sets of muscles (rostral, ventral, and caudal) that are innervated by branches of the facial nerve (cranial nerve VII).

The leaf-shaped pinna of the external ear is broad with medial (rostral) and lateral (caudal) margins. The caudal margin of the pinna exhibits a cutaneous pouch called the marginal pouch (Figure 1-3). This pouch has no obvious function. The skin on the concave surface of the pinna is very tightly connected to the underlying auricular cartilage, accentuating all the auricular prominences (see Figure 1-3). The skin covering the auricular cartilage may show breed-specific pigmentation. The shape and size of the external ear vary greatly among different breeds of dogs, mainly owing to the auricular cartilage that forms the skeleton of the pinna. It is the largest cartilage of the external ear. The broad auricular cartilage has numerous holes (see Figure 1-1), which are traversed by branches arising from the caudal auricular artery.

The auricular cartilage is broad dorsally and funnels to a narrow tubelike structure, the tubus auris, which fits around the annular cartilage ring. The parotid salivary gland occupies the base of the external ear, partially surrounding the tubus auris (Figures 1-4 and 1-5). The tubus auris encloses the vertical part of the external canal and, together with the tragal, antitragal, and antihelicene borders, forms the external acoustic meatus (see Figure 1-3).

Usually, the entrance to the external ear canal is guarded by a few fine hairs. Certain breeds, such as Airedales and Old English sheepdogs, exhibit hairy external ear canals. The external ear canal of the cat is devoid of hairs, and the ear canal is well ventilated. This may be a significant factor contributing to the lower incidence of external ear canal infections in cats. A hairy ear canal interferes with proper drainage and aeration of the canal in chronic otitis externa complicated by granulomatous lesions, leading to exacerbation of the condition.

Scutiform Cartilage

The scutiform cartilage is an L-shaped structure located over the temporalis muscle. It does not contribute to the formation of the external ear or its canal. The scutiform cartilage is attached to the midline raphe of the head and neck by numerous muscles (see Figure 1-2). Muscles also extend from the scutiform cartilage to the auricular cartilage. The scutiform cartilage functions like a fulcrum, providing for efficient movement of the auricle. It can be considered to function like a sesamoid cartilage. It lies over a fat cushion (corpus adiposum auriculae) on the dorsal surface of the temporalis muscle.

Structure of the Ear Canal

The external ear canal in the dog is 5 to 10 cm long and 4 to 5 mm wide (see Figure 1-5). The ear canal consists of an initial vertical part, which may extend an inch. The vertical canal runs ventrally and slightly rostrally before bending to a shorter horizontal canal that runs medially and forms the horizontal part of external ear canal. Because the external ear is elastic, the ear canal can be straightened enough to permit otoscopic examination.

The vertical part and most of the horizontal part of the canal are cartilaginous, but the deepest part is osseous. The ear canal is lined by skin containing sebaceous and ceruminous glands and hair follicles. The ceruminous glands are modified apocrine tubular sweat glands. The combined secretions of sebaceous and ceruminous glands constitute ear wax (cerumen). Cerumen (1) protects the external ear canal by immobilizing foreign objects and (2) keeps the tympanic membrane moist and pliable. The external ear canal is separated from the middle ear cavity by the semitransparent tympanic membrane.

Nerves of the External Ear

Sensory innervation of the pinna and external ear canal is provided by four nerves: the trigeminal, facial, vagus, and second cervical.

The auriculotemporal branch of the trigeminal nerve provides sensory innervation to the skin lining the horizontal part of the ear canal and to the tympanic membrane itself. This nerve also provides sensory innervation to the rostral margin of the pinna and the concave surface of the pinna close to the rostral margin and the skin over the tragus.

The facial nerve is related to the ventral surface of the annular cartilage, close to the osseous external acoustic meatus. The facial nerve provides substantial sensory innervation to the concave surface of the scapha and part of the cavum conchae via the rostral, middle, and caudal internal auricular branches (Figure 1-6). Most of the vertical along with part of the horizontal ear canal lining is supplied by the lateral internal auricular branch of the facial nerve, which may contain predominantly vagal fibers.

Communication between the facial nerve and the vagal nerve takes place as the facial nerve exits the stylomastoid foramen (see Figure 1-11). It is believed that these vagal branches are given off as the lateral internal auricular branch to the skin of the external ear canal. Reflex gastric vomiting may be triggered if the sensory endings of the vagus nerve are stimulated by mild ear canal irritation.

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Anatomy of the Canine and Feline Ear

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