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Outer ear:

The outer ear consists of three parts:

(1) A trum­pet shaped fleshy external part, the pinna;

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(2) A short funnel-shaped tube about an inch in length from the exterior to the tympanic mem­brane or eardrum, the outer auditory canal and

(3) A thin, semitrans- parent elliptical, flexible membrane, the tympanic membrane or ear­drum (about 0-4 inches in diameter), it is stretchsd across the inner end of the auditory canal and separating it from the middle ear.

Middle ear:

The middle ear is a small, hollow, air-filled chamber between the inner ear and the tympanic membrane and is enclosed within a tympanic bone of skull. In many mammals this bone has a swollen tympanic bulla.

The middle ear is connected to the pharynx by a tube, the eustachian tube and is lined with an epi­thelial membrane.

It has three tiny bones or ear ossicles which are joined together. These bones or ear ossicles are commonly called the malleus, incus and stapes (hammer, anvil, and stirrup) because of their shapes.

The “handle” end of the hammer-shaped malleus bone is attached to the inner surface of the tympanic membrane.

Its oppo­site end is connected to the curved incus (anvil) which in turn is joined to the stitrup-shaped stripes.

The foot plate of stapes fits into the so-called oval window, a membrane-covered opining leading into the inner ear.

A second opening which is also covered by a thin membrane connects the middle and inner ears and is called the round window.

Three additional openings also lead into the middle ear and these are : one from the external auditory canal covered over by the tympanic membrane ; one from a network of small irregular honey­comb spaces called the mastoid sinuses located in one of the surroun­ding bones of the head; and one from the so-called eustachian tube.

Inner ear:

In its overall structure the inner ear consists of both a bony and membranous .labyrinth. In general the bony portion of the labyrinth entirely encloses and protects the similarly shaped membranous labyrinth.

The bony and membranous labyrinth remains separated from each other by a layer of lymph-like fluid (perilymph) which serves as a protective cushion or buffer. Of the different sense organs that are present in the inner ear, only one what is known as cochlea is concerned with the sense of hearing.

The others namely the sacculus, utriculus and its attached semicircular canals have to do with the sensation of physical equilibrium and orienta­tion.

The anterior and posterior semicircular canals form a crus commune. Utriculus and sacculus are small and the sacculoutricular connection is a narrow duct.

From the sacculus arises an endolym­phatic duct which ends blindly in the duramater. Also arising from the sacculus is a long, complicated, and spirally coiled cochlear duct which does not have any macula except in monotremes.

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