B

The ear is not a single organ, but two, being the peripheral receptor site both for stimuli derived from sound waves and for changes in posture. The structures subserving both of those functions are developed from an invagination of the ectoderm early in embryonic life - the otocyst - to produce the epithelia of the membranous labyrinth of the inner ear. Superimposed upon, and developing slightly later, the first and second branchial arch systems provide structures that augment the hearing function. The endodermal component of the first branchial system, the branchial pouch, gives rise to the Eustachian tube and middle ear epithelia and the corresponding ectodermal outgrowth, the first branchial cleft, to the external ear epidermis. The connective tissue part of the local branchial cranial (auditory vestibular) nerve outflow from the central nervous system, both its vestibular and cochlear branches, grows to link up with the sensory epithelia lining the otocyst-derived cochlear and vestibular endolymph-containing cavities; there is recent evidence that terminal ganglion cells, e.g. spiral ganglion cells, may also be otocyst-derived. Cartilaginous, bony and muscular configurations of the ear are developed from the mesenchyme surrounding these early epithelia. The seventh cranial (facial) nerve develops in close relation to the structures of the ear for much of its course.

The anatomy of the ear may be considered by reference to its functions in hearing and balance (Fig. 8.1).

The pinna and external canal conduct sound waves in the air to the tympanic membrane, which transmits them by very delicate vibrations. The middle ear enhances this sound energy transmission by conveying vibrations from the larger area of the tympanic membrane through the ossicular chain (malleus, incus and stapes) to the much smaller area of the footplate of the stapes, which lies in the oval window of the vestibule in contact with the perilymph. In this way vibrations representing sound are conducted to the fluids of the inner ear. The air space of the middle ear cavity is magnified by the mastoid air cells, which are complex expansions into the mastoid bone. There is a connection of the middle ear space with the nasopharynx and so with the external air through the Eustachian tube, by which air pressure can be adjusted.

From the vestibular perilymph, vibrations derived from sound waves pass directly via the scala vestibuli into the spirally coiled perilymphatic spaces of the cochlea, where it forms an upper compartment ascending from the vestibule and oval window. A lower compartment, the scala tympani, descends spirally to the round window membrane, a connective tissue disc separating the perilymph compartment from the middle ear. Between the scalae vestibuli and tympani there is an en-dolymph-containing coiled middle compartment, the cochlear duct (scala media), which houses the senso

Helix

Semicircular canals

Helix

Semicircular canals

Pinna

Ear lobe

Tensor Eustachian, typani muscle tube

Tensor Eustachian, typani muscle tube

Pinna

Fig. 8.1. Anatomical diagram of the ear. Reproduced from Michaels and Hellquist [68]

Ear lobe

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