Pathology of the Head and Neck

236 L. Michaels88.1 Summary of Embryology,Anatomy and Histology8.1.1 EmbryologyThe ear is not a single organ, but two, being the peripheralreceptor site both for stimuli derived from soundwaves and for changes in posture. The structures subservingboth of those functions are developed from aninvagination of the ectoderm early in embryonic life– the otocyst – to produce the epithelia of the membranouslabyrinth of the inner ear. Superimposed upon,and developing slightly later, the first and second branchialarch systems provide structures that augment thehearing function. The endodermal component of thefirst branchial system, the branchial pouch, gives riseto the Eustachian tube and middle ear epithelia and thecorresponding ectodermal outgrowth, the first branchialcleft, to the external ear epidermis. The connectivetissue part of the local branchial cranial (auditory vestibular)nerve outflow from the central nervous system,both its vestibular and cochlear branches, grows to linkup with the sensory epithelia lining the otocyst-derivedcochlear 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 earare developed from the mesenchyme surrounding theseearly epithelia. The seventh cranial (facial) nerve developsin close relation to the structures of the ear for muchof its course.8.1.2 AnatomyThe anatomy of the ear may be considered by referenceto its functions in hearing and balance (Fig. 8.1).The pinna and external canal conduct sound wavesin the air to the tympanic membrane, which transmitsthem by very delicate vibrations. The middle ear enhancesthis sound energy transmission by conveying vibrationsfrom the larger area of the tympanic membranethrough 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 contactwith the perilymph. In this way vibrations representingsound are conducted to the fluids of the inner ear. Theair space of the middle ear cavity is magnified by themastoid air cells, which are complex expansions into themastoid bone. There is a connection of the middle earspace with the nasopharynx and so with the external airthrough the Eustachian tube, by which air pressure canbe adjusted.From the vestibular perilymph, vibrations derivedfrom sound waves pass directly via the scala vestibuliinto the spirally coiled perilymphatic spaces of the cochlea,where it forms an upper compartment ascendingfrom the vestibule and oval window. A lower compartment,the scala tympani, descends spirally to the roundwindow membrane, a connective tissue disc separatingthe perilymph compartment from the middle ear. Betweenthe scalae vestibuli and tympani there is an endolymph-containingcoiled middle compartment, thecochlear duct (scala media), which houses the senso-Fig. 8.1. Anatomical diagram of theear. Reproduced from Michaels andHellquist [68]Tympanicmembrane