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→Middle Ear
*The eustachian tube functions to equalise pressure on either side of the tympanic cavity, by opening while yawning or swallowing, for example.
*The eustachian tube functions to equalise pressure on either side of the tympanic cavity, by opening while yawning or swallowing, for example.
The middle ear comprises the '''tympanic cavity''' bounded by the walls of the '''tympanic bulla''' and the '''medial tympanic membrane'''.
[[image: Canine Ear Canal.jpg|thumb|250px|right|The Canine Middle Ear]]
It has 3 compartments, the smallest dorsal or '''epitympanum''', conatining 2 of the auditory ossicles – the '''malleus''' and '''incus'''; the middle or '''mesotympanum''' bounded by the tympanic membrane laterally and containing the third auditory ossicle, '''stapes''', attached to the oval window and the ventral, '''hypotympamum''', or fundic cavity, which is the largest compartment. The auditory ossicles are supported by ligaments and muscles, which alter their position and influences the tension of the tympanic membrane. The ratio of the malleus:incus in dogs and cats is 2-3 times that of man, and may explain the increased acuity of hearing. The '''round window''' is on the caudomedial aspect of the mesotympanum and the opening of the auditory '''(Eustachian)''' tube opens on rostromedial aspect of the mesotympanum. Opposite the lateral tympanic membrane on the medial aspect of the cavity is a bony promontory. Associated structures close to or in channels in the wall of the tympanic cavity (bulla) are the '''facial nerve''', '''vagus nerve''' and branches of the '''carotid and lingual arteries'''. Post-ganglionic fibres of the '''cervical sympathetic trunk''' run in the region of the dorsomedial wall of the tympanic cavity.
The feline middle ear has an incomplete '''bony septum''' dividing the ventral chamber into a large ventromedial and small dorsolateral chamber, communicating caudomedially.
The middle ear is lined by cuboidal to columnar mucosal epithelium with scattered '''goblet cells'''. The auditory tube is lined by pseudostratified, ciliated columnar epithelium with scattered goblet cells. The goblet cells are more prominent at the tympanic cavity end, contributing to the surfactant nature of the secretions- containing '''lecithin''', lipids and '''mucopolysaccharides''' - that decreases surface tension and keeps the tube patent. The density of '''cilia''' increases as the tube runs dorsolaterally to open into the nasopharynx behind the soft palate, facilitating movement and drainage of mucus and other material.
===Inner Ear===
===Inner Ear===