ears

The ear's most noted function is, of course, hearing. It is also extremely important as an organ of balance, however.

Ear anatomy

Pinna: Some dogs have ear flaps, or pinnas that stand erect, while others have long, floppy types. The ear flap serves as a partial covering of the ear canal, while at the same time directing sound towards the

eardrum. The flap has an inner core of cartilage to give it strength. Both outer and inner surfaces of the skin are covered by hair, although hair follicles are much less prevalent on the inner areas.

Ear Canal: The ear canal is a long, tube-like structure that travels diagonally down the side of the head, then moves horizontally into the head. The total length of the ear canal is at least two inches, even in small breeds. It is about as wide as a pencil. The length and size of the canal vary in relation to the animal's overall body size. As the ear canal passes into the head, it ends at a thin tissue called the tympanic membrane or eardrum.

Outer Ear: This outer ear in the dog is considered to include all structures, such as the canal and ear flap, from the eardrum outward.

Middle Ear: Internally, from the eardrum comes the middle ear, which connects to the throat area by the eustachian tube. This tube allows air to enter the middle ear to balance the pressure against the eardrum.

Inner Ear: Farther in from the middle ear is the inner ear. One responsibility of the

  

inner ear is the maintenance of the dog's equilibrium or balance. This structure contains fluid-filled canals, which, as the fluid shifts, tells the brain the body's exact position. If a dog's head is tilted the fluid shifts, and the brain detects the tilting.

Eardrum: The eardrum picks up sound waves through air vibration. The eardrum vibrates and stimulates the bones within the middle ear. The vibrating bones pass the sound vibrations to an area with tiny hairs. As the hair moves, sound waves are transformed to electrical impulses and then passed to the inner ear where they are transmitted by the auditory nerve to the brain where they are detected as sound. This is how hearing is created.

The parts of the ear, namely the ear flap, canal, eardrum, and middle and inner ear, all play important roles. These structures are complex and can become diseased, thus impairing their function. Disorders of the ear are frequently very painful and can affect both hearing and equilibrium.

Hearing development

Puppies are born unable to hear. They are unresponsive to even loud noises. The ear canals described above remain closed, unable to carry sound to the eardrum until the puppy is about ten days of age. In some individuals, the ear canals may open slightly sooner or later but it averages about ten days. The canals become fully open by three weeks of age. As a result of the ear canals 'opening up,' most puppies will begin to hear sounds at about fourteen days of age, with functional hearing by twenty-one days of age. It is very difficult to assess possible hearing impairment until the puppy is at least four weeks of age, at which time deafness, if present, may be noticed and evaluated.

  Ear canal The tube that connects the external ear with the ear drum.Follicle The group of cells in the skin in which a hair or feather develops.Tissue A group of specialized cells that together perform a particular function, e.g., muscle tissue, nerve tissue, bone.

The ears are composed of external pinna (outside portion of the ear) and a skin-lined canal (the external ear canal) descending (vertical and horizontal portions) to the ear drum (the tympanic membrane). This portion comprises the external ear. Other compartments of the ear, beginning after the tympanic membrane, are sequentially named the middle ear and the inner ear.

The horizontal ear canal is lined by specialized skin approximately 1mm thick, rich in sebaceous glands associated with hair follicles (oily secretions) and cerumen producing glands, which secrete a mixture of degenerating epithelial cells in a fatty [lipid] base and are deeper into the skin. Cerumen secreting gland are found in increasing numbers closest to the ear drum (the tympanic membrane). Glands secrete either "dryish" or "moist" cerumen.

Cerumen resists moisture and thus, under normal conditions functions to keep the ear canals relatively dry. In addition, it is believed that these secretions contain enzymes involved with antimicrobial activity.

The middle ear begins on the other side of the tympanic membrane (ear drum). It contains boney elements...the ossicles which transmit vibrations ...and a large, air-filled boney chamber, the bulla through which low-pitch sound is also transmitted. The bulla is lined with a mucus membrane (not skin). Reflex contraction of the muscles attached to the auditory ossicicles in response to extremely loud noises actually dampens the movement of the ossicles and the transmission of potentially damaging sounds. Aside from ear-related structures, some nerves traverse the bulla and inflammation or other pathology within the middle ear can result in neurologic signs not directly related to hearing or balance. Information from the middle ear is transmitted through the oval and round windows to the structures of the inner ear.

The inner ear, housed in the petrosal temporal bone, is responsible for receiving auditory signals and for maintaining balance. A portion of the petrosal bone is in close proximity to the cerebrum of the brain and another portion is juxtapost to the cerebellum. The middle portion of the cerebella portion houses the internal acoustic meatus containing, in turn, portions of the osseous facial canal, (in which the facial (seventh cranial) nerve traverses), a component for the vestibular (balance-related) part of the eighth cranial nerve and a third area housing the cochlear component of the eighth cranial nerve, the nerve which transmits auditory signals to the auditory centers of the brain. The coclear is a winding, fluid-filled structure containing hairs connected to the cochlear nerve (the portion of the eighth cranial nerve concerned with hearing). The vibration of fluid as a result of sound bends specific hairs (certain hairs bend in response to certain frequencies); this hair-bending transmits the nerve impulses via the connected coclear nerve endings to the brain where they are converted to and perceived as sound.

The actual neurological network and the physics of sound and balance control involving the boney labryinths, coclear apparatus, utricles and saccules is extremely complex and beyond a more detailed elucidation here.

Terminology

(please review anatomy, above)

Aural: refers to the ear.

Aural Hematoma: This is manifest when a bloody fluid accumulates between the skin and cartilage of the ear pinna, usually the concave surface and usually the result of self-trauma (scratching, head-shaking)

Cerumen: described above, below moving diagrams . Secretion by special glands in the vertical ear canal which functions to repel moisture and as antimicrobial

Ear Canal: As described above (anatomy),

Epithelial Migration: a clearance mechanism of the skin lining the vertical ear canal. Keratinocytes (surface epidemis cells) originating from the central portion of the tympanic membrane (the ear drum) slide along the superficial basal epithelial surface toward the outside opening of the ear. This process clears the vertical ear canal of wax and ceruminous debris

Otic: refers to the ear

Otitis: refers to inflammation of the ear; Otitis externa refers to inflammation of the external ear canal, including the horizontal and vertical portions; otitis media refers to inflammation of the middle ear and otitis interna refers to inflammation of the inner ear containing the auditory and vestibular portions of the ear that communicate with the brain. When inflammation is confined to the vertical and horizontal ear canals it is referred to as otitis externa. Inflammtion of the middle and inner ear compartments are called otitis media and otitis interna, respectively.

Stenosis: the narrowing of the ear canal or opening due to swelling, hyperplasia of skin and, eventually, thickening and mineralization of cartilage. Most often a response to chronic inflammation. And example of severe stenosis of the external ear opening is seen here.

Vestibular: refers to the portions of the inner ear and centers of the brain involved in the maintenance of body orientation and balance

Ear pinna: this is the external, most visual portion of the canine ear. In some dogs it is of the "floppy" variety and in others it is "straight" or "upright". It is composed of a cartilage core and skin. The portion of the pinna that is contiguous with the ear canal is called the concave surface and the other side...with fur on it...is the convex surface. The plural of pinna is "pinnae".