ANATOMY AND PHYSIOLOGY OF THE EAR

EXTERNAL, MIDDLE AND THE INTERNAL EAR 1 - external auditory meatus; 2 - Pinna; 3 – ossicles (malleus; incus; stapes) and tympanic membrane; 4 – labyrinth (vestibule semicircular canals cochlea); 5 – tympanic cavity; 6 - auditory (acoustic) nerve; 7 – arteria carotis; 8 - Eustachian tube

Is composed of a skin-covered cartilaginous lamella whose posterior surface is evenly convex and smooth, while its anterior surface is concave, with semilunar folds and hollows between them. The free anteroexternal margin of the pinna is known as the helix; towards the bottom the pinna gradually turns into the lobe devoid of cartilage and consisting of well-developed fat and cellular tissue with a small number of vessels and nerves. The small protube­rance of cartilage projecting over the external auditory meatus is named the tragus. In front of the helix and parallel to it is a ridge known as the an thelix, with the antitragus at its posterior end. The skin on the anterior surface of the pinna adheres directly to the perichondrium; on the posterior surface, however, it may form folds owing to the presence of a small layer of loose cellular tissue. PINNA

Tympanic Membrane 1 – umbo; 2 - handle of malleus; (3) (6) anterior and posterior malleus folds; 4 - Pars flaccida or Shrapnell's membrane; 5 - short process of malleus; 7 – pars tensa; 8 – light reflex

Interconnected Auditory Ossicles 1 - malleus handle; 2 - head and neck of malleus; 3- Malleus-incudal joint; 4- body of incus 5 - short process of incus; 6 - long process of incus; 7 - head of stapes; 8 – base of stapes; 9 - cruses of stapes

Middle Ear The tympanic cavity is a small chamber, about 1 cm in size, lying in the depth of the temporal bone, between the tympanic membrane and the internal ear. In front, through the Eustachian tube, the tympanic cavity communicates with the nasopharynx; behind, through the entrance into the mastoid antrum (aditus ad antrum mastoideum), communicates with the latter and the cells of the mastoid Process.

TYMPANIC CAVITY It is customary to divide the into three parts: the middle and biggest part, mesotympanum, corresponding to the pars tensa of the drum the upper part, epitympanum, lying above the former and also known as the epitympanic recess or attic; the lower part, hypotympanum, lying below the drum level.

TYMPANIC CAVITY HAS SIX WALLS The roof of the tympanic cavity is a thin plate of bone separating the tympanic cavity from the middle cranial fossa where the temporal lobe is situated. This plate often has congenital fissures through which vessels pass from the middle cranial fossa. These anatomical features may account for the meningeal symptoms frequently observed in young children with acute otitis media. The inferior wall or floor of the tympanic cavity is separated from the jugular bulb by a fairly thick bony plate. Bone fissures in this wall are rarely found. The Eustachian tube begins with an opening in the anterior wall separating the tympanic cavity from the internal carotid canal. An opening in the upper part of the posterior wall leads to the mastoid antrum (aditus ad antrum mastoideum). The internal wall separates the tympanic cavity from the internal ear. It is marked by a gentle eminence, the promontory (promontorium), corresponding to the basal turn of the cochlea. Above and behind the promontory is an oval window or the fenestra vestibuli which leads into the vestibule and is closed by the foot plate of the stapes. Behind and below the promontory in a niche is a round window or the fenestra cochlea which leads into the cochlea, and is filled with a thin membrane, the secondary tympanic membrane. External wall - tympanic membrane

TYMPANIC MUSCLES There are two muscles in the tympanic cavity: (1) The tensor tympani muscle which stretches the tympanic membrane. It lies in the bony ca­nal above the Eustachian tube, and is attached to the handle of the malleus. (2) The stapedius muscle which arises from the posterior wall of the tympanic cavity and is attached to the head of the stapes by a slender tendon. The tensor tympani is innervated by a branch of the trigem­inal nerve, and the stapedius muscle by a branch of the facial nerve.

Eustachian or auditory tube About 3.5 cm in length connects the tympanic cavity with the naso-pharynx. The upper third of this tube, adjoining the tympanic cavity, has bony walls, while the remaining lower portion leading into the nasopharynx is made up of membrane and cartilage. The movement of the cilia of the ciliated epithelium lining the Eustachian tube is towards the naso­pharynx. At rest, the Eustachian tube is in a collapsed state, but with each swallowing movement it opens by contraction of the soft palatal muscles attached to it, to let air into the tympanic cavity.

MASTOID PROCESS Is located just behind the external auditory meatus is a bony structure protruding downwards with the sternocleidomastoid muscle attached to it. In young children, the mastoid process is not fully developed and represents a bony tubercle behind the osseous tympanic ring. The antrum communicates with the tympanic cavity and the air-filled cells of the mastoid process. The superior wall or roof of the antrum separates it from the middle cranial fossa. The following types of structure are to be found in the mastoid process: the pneumatic or large-celled, diploic ". In the case of pneumatic structures, the cavity of the mastoid process is divided by thin bony partitions into a lattice of larger and smaller cells. The diploic structure has tiny cells resembling a diploetic bone; the most frequent is the mixed form of mastoid structure where smaller cells are to be found along­side bigger ones. In compact structures the bone is indurated and the cells are very few; this structure frequently occurs as a result of chronic suppurative otitis media. sclerotic

Internal Ear or Labyrinth (1; frontal semicircular canal; (4) ampulla of frontal semicircular canal; (3) apex of cochlea; (4) medial turn of cochlea; (5) apical turn of cochlea; (6) basal turn of cochlea; (7) round window; (8) oval window; (9) ampulla of sagittal semicircular canal; (10) sagittal semicircular canal; (11), (12) crura; (13) ampulla of horizontal semicircular canal; crus commune of frontal and sagittal semicir­cular canals

VESTIBULE (VESTIBULUM) The) lies in the centre of the bony labyrinth on whose external wall is the oval window; on the opposite, internal wall, there are two recesses for the two membranous sacs of the vestibule. The front sac known as the saccule (sacculus) communicates with the membranous cochlea lying before the vestibule, while the rear sac or utricle (utriculus) is connected with the three membranous semicircular canals passing behind and above the vestibule. The intercommunicating sacs of the vestibule contain the statokinetic receptors or maculae acusticae, otolithic organs made up of a highly-differentiated specific neuroepithelium covered with a membrane containing granules of carbonate and phosphorate of lime, i.e. the otoliths

SEMICIRCULAR CANALS The are set at right angles to each other and represent the three planes of space. They are three in number: the external or horizontal, the superior or frontal, and the posterior or sagittal. One end of each canal opens out into a larger space known as ampulla, the other end is even. The frontal and sagittal canals have a common even stem (crus commune). The ampulla of each membranous canal contains a ridge. the crista ampullaris, which is a receptor, i.e. a nerve ending consisting ot a highly-differentiated neuroepithelium or hair and supporting cells. The free surface of the hair cells is covered with hairs which respond to the slightest displacement or pressure of the endolymph. The receptors of the vestibule and semicircular canals are the peripheral nerve endings of the vestibular analysator.

COCHLEA Modiolus the external wall and also turning round the former, divides the tube lumen into two directions, the upper or scala vestibuli and the lower or scala tympani which communicate at the apex of the cochlea through a small opening known as the helicotrema. Both channels are filled with perilymph. The scala vestibuli communicates with the vestibule, while the scala tympani borders on the tympanic cavity through the round window covered by the secondary tym­panic membrane. The scala vestibuli of the cochlea contains the thin Reisner's membrane which extends from the osseous spirn lamina to cut off a small membranous canal of trianguli section filled with endolymph and known as the cochles duct or ductus cochlearis.

ORGAN OF CORTI (1) basilar membrane: (2) tectorial membrane; (3), (4) hair cells; (5) supporting cells; (6) nerve fibres reaching hair cells