![]() This epithelial structure separates the external from the middle ear. The auriculo-temporal branch of the mandibular portion of the trigeminal nerve (CN V) and branches of the facial nerve pass rostral to the vertical ear canal. The facial nerve exits the skull through the stylomastoid foramen behind the osseous bulla it passes below the rostroventral aspect of the horizontal ear canal. ![]() Laterally is the parotid salivary gland and also a branch of the facial nerve, external maxillary vein and branches of the external carotid artery. In the tissues medial to the ear canal lie the auricular and superficial temporal arteries. The more aqueous secretions of the ceruminous glands, present proximally, allow easier upward migration, whereas the higher lipid content of the distal secretions offers better barrier protection. The upward/distal migration of epithelial cells clears desquamated cells, secretions, debris and microbes. The epithelial cells in the stratum granulosum of the tympanic membrane migrate centripetally, and this migration comprises the natural cleaning mechanism keeping the tympanum free of debris. Immunoglobulins IgA, IgG and IgM have been identified in canine cerumen, predominantly IgG. The secretions of the ceruminous glands contain mucopolysaccharides and phospholipids. The density of ceruminous glands increases proximally, towards the tympanic membrane. Sebaceous glands predominate distally and are largely responsible for the lipid component of cerumen. The material secreted into the ear canal, cerumen (or wax), is compromised of exfoliated epithelial cells (squames) and glandular secretions. The apocrine glands in the ear canal are modified and known as ceruminous glands. The nature of the secretions alters, with a decrease in the lipid content of cerumen. Otitis externa is associated with an increase in temperature, relative humidity and a rise in pH within the external ear canal. The mean relative humidity in the ear canal is 80.4%, with mean pH of 6.1-6.2. The temperature in the healthy ear canal is between 38.2 and 38.4 C, and 0.6 C below rectal temperature. The microclimate of the external ear canal remains surprisingly stable in spite of marked changes in the ambient temperature and humidity of the surrounding environment. Image by Rachael Wallace Microclimate of the Ear Canal The auricular cartilages of the left canine ear. Moves the ear caudo-laterally, so the pinna is facing backwards Moves the ear cranio-laterally, so the pinna is facing forwards Muscles around the base of the ear that are attached to the skull allow movement of the pinna, so the ear can be directed to the source of sound: Some species are devoid of hair in the lower (proximal) ear canal (eg horse) hair follicle density in the dog is variable – usually simple but in some breeds compound hair follicles.įoreign bodies can become lodged in the external auditory meatus, but glands are present that produce wax which can trap these. The density of hair follicles on the concave pinna is variable, as is the density and distribution of hair follicles in the ear canal itself. The skin of the normal ear has a thin stratified keratinising epidermis and a thin dermis containing adnexal structures – hair follicles, sebaceous and apocrine glands. The cartilage and bony process of the external ear and the tympanic membrane are covered by skin. The vertical ear canal lies in a rostroventral orientation before bending medially to become the horizontal canal. The annular cartilage lies between the proximal horizontal ear canal and the bony acoustic process of the tympanic bulla, connected by ligaments. The scutiform cartilage lies rostromedially in the lower ear canal and provides support. Proximally, the auricular cartilage is rolled into a funnel shape, known as the concha. There are also irregularities on the concave surface with ridges and prominences, the medial and lateral crus of the helix on the medial aspect of the opening of the external ear canal, opposite the rectangular tragus on the lateral aspect. The hair covering on the convex side is usually similar to the rest of the body but the hair covering on the concave or medial aspect is variable. The auricular cartilage is flared distally into a flattened cone shape, covered by skin on both sides - more tightly on the medial or concave side than the convex side. The pinna consists of the auricular cartilage, and skin which allows for flexibility and elasticity.
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