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.

Figure 1-3 Anatomical features of the left external ear of the dog.

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).

Figure 1-4 Relationships of the tubus auris to the parotid salivary gland and auricular vessels. The facial nerve runs deep to the parotid salivary gland, immediately ventral to the annular cartilage, and gives off motor branches to the facial muscles.

Figure 1-5 Structure of the ear canal.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(Redrawn from Bojrab MJ: Current techniques in small animal surgery I, Philadelphia, 1975, Lea & Febiger.)

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.

Annular Cartilage

The annular cartilage is part of the external ear canal. The pinna, formed by the cone-shaped auricular cartilage, articulates with the annular cartilage. The annular cartilage is a ring-shaped structure attached to the bony orbit of the external acoustic meatus of the temporal bone. A tubular cartilage piece, the annular cartilage surrounds the osseous external acoustic meatus (see Figure 1-7). It is attached to the bony rim of the external acoustic meatus by fibrous tissue that permits some degree of movement of the external ear. The annular cartilage encloses the horizontal part of the external ear canal.

Figure 1-7 Transverse section through the head of the dog at the level of the tympanic bulla. A, Osseous external acoustic meatus covered by tympanic membrane. Note that the tympanic membrane is placed at an approximately 45-degree angle in relation to the central axis of the horizontal part of the ear canal. 1, Horizontal part of the ear canal; 2, vertical part of the ear canal; 3, annular cartilage; 4, auricular cartilage; 5, parotid salivary gland; 6, epitympanic recess; 7, carotid canal accommodating the internal carotid artery, postganglionic sympathetic nerves, and the ventral petrosal venous sinus (the caudal continuation of cavernous venous sinus), which drains into the internal jugular and vertebral veins; 8, cavity of the middle ear; 9, osseous labyrinth accommodating the internal ear.

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.

Blood Supply to the External Ear

The external ear is generously vascularized by branches of the external carotid artery. A large caudal auricular artery arises from the external carotid artery at the base of the annular cartilage, medial to the parotid salivary gland and deep to the caudal auricular muscles. This artery gives off the lateral, intermediate, and medial auricular arteries, which pass along the convex surface of the pinna, wrapping around the helicene margins as well as penetrating the scapha and supplying the skin covering the cavum conchae. In addition to providing nourishment to the tissues of the external ear, the vascular supply to the pinna may also play a minor thermoregulatory role. Venous drainage occurs via the caudal auricular and superficial temporal veins into the maxillary vein.

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.

Figure 1-6 Schematic drawing of sensory branches of the facial nerve that supply the external ear. Other important motor branches of the facial nerve are also shown.

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.

Figure 1-11 Nerves in relation to the middle ear cavity, ventral view, with most of the tympanic bulla removed.

(From Evans HE, ed: Miller’s Anatomy of the dog, ed 3, Philadelphia, 1993, WB Saunders.)

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