THE EAR

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THE EAR
Let's look at the overview of the Ear
The Ear is the organ of hearing and also involved in balance. With exception of the pinna (outer Ear) the structures that form the Ear are encased within the petrous portion of the temporal bone.



ANATOMY OF THE EAR PROPER
The Ear is divided into 3 distinct parts;
The Outer Ear which consists of the auricle (pinna) and external acoustic meatus (auditory canal).
The auricle (pinna) is the visible part of the Ear that projects from the side if the head. It is composed of fibroelastic cartilage covered with skin. It is deeply grooved and ridged; the most prominent outer ridge is the helix
External acoustic meatus (auditory canal) this is a slightly S-shaped tube 25cm long extending from the auricle to the tympanic membrane(eardrum). The meatus is lined with skin continuous with that of the auricle and it has a ceruminous glands and hair follicles with associated sebaceous glands.
The Tympanic membrane (Eardrum) completely separate the external acoustic meatus from the middle Ear upwards and is formed with slightly broader edge upwards and is formed by 3 types of tissue; the outer covering of hairless skin, the middle layer of fibrous tissue and the inner lining of the mucous membrane.
Middle Ear (Tympanic cavity) this is an irregular shaped air-filled cavity within the petrous portion of the temporal bone the cavity, its contents and the air sac which open out of it are lined with either simple squamous or cuboidal epithelium
·        the lateral wall of the middle Ear is formed by the tympanic membrane
·        the roof and floor are formed by the temporal bone with openings leading to the mastoid antrum through which air passes to the air cells within the mastoid process
·        the medial wall is a thin layer of temporal bone in which there are two openings
·        Oval Window
·        Round Window
·        the oval Window is occluded by part of a small bone called the stapes and round Window by a fine sheet of fibrous tissue

 Pharyngotympanic (Auditory or Eustochian) tube, which links the nasopharynx and middle Ear is about 4cm long and lined with ciliated columnar epithelium
Auditory Ossicles: these are 3 very small bones only a few millimeters in size that extends across the middle Ear from the tympanic Membrane to the oval membrane to the oval Window. They are held in place by a fine ligament and are named according to their shapes.
Malleus: this is the lateral hammer shaped bone. The handle is in contact with the tympanic membrane and the head forms a moveable joint with the incus.
Incus: This is the middle anvil shaped bone. It's body articulates with the malleus, the long process with the stapes and it is stabilized by the short process fixed by fibrous tissue to the posterior wall of the tympanic cavity
Stapes: this is the medial stirrup shaped bone. Its head articulates with the incus and its footplate fits into the oval Window

INNER EAR
The inner Ear or Labyrinth (meaning maze) contains the organ of hearing and balance.  It is described in 2 parts the bony Labyrinth and the membranous Labyrinth and is divided into 3 main regions
·        The vestibule, containing the utricle and saccule
·        Three semicircular canals
·        The cochlea

The bony Labyrinth is lined with periosteum. Within the bony Labyrinth the membranous Labyrinth is suspended in a watery fluid called perilymph

The membranous Labyrinth is filled with endolymph

Vestibule: this is the expanded part nearest the middle Ear. The oval round windows are located in its lateral wall. It contains two membranous sacs, the utricle and saccule very important in balance
Semicircular canals: these are three tube arranged so that the one is situated in each of the three planes of space. They are continuous with the vestibule and are also important in balance
Cochlea: this resembles a snail's shell. It has a broad base where it is contained with the vestibule and a narrow apex and it spiral round a central bony column. A cross section of the cochlea contains three compartments;

·        Scala vestibule
·        Scala media or cochlear duct
·        Scala tympani
PHYSIOLOGY OF HEARING
Every sound produces sound waves or vibrations in the air, which travel at about 332 meters per second. The auricle, because of its shape, collects and concentrates the waves and directs them along the auditory canal causing the tympanic membrane to vibrate. Tympanic membrane vibrations are transmitted and amplified through the middle Ear by movement of the Ossicles. At their medial end the footplate of the stapes rocks to and fro in the oval Window, setting up fluid waves in the perilymph of the scala vestibuli. Some of the force of these waves is transmitted along the length of the scala vestibuli and scala tympanic but most of the pressure is transmitted into cochlear duct. This causes a corresponding wave motion in the endolymph, resulting in vibration of the basilar membrane and stimulation of the auditory receptors in the hair cells of the spiral organ. The nerve impulses generated pass to the brain in the cochlear portion of the vestibulocochlear nerve the fluid wave is finally expended into the middle Ear by vibration of the membrane of the round Window
BALANCE AND EAR
The semicircular canals have no auditory function although they are closely association with the cochlea instead they provide information about the position of the head in space contributing to maintenance of posture and balance
There are 3 semicircular canals, one lying in each of the three planes of space. They are situated above, beside and behind the vestibule of the inner ear and open into it.
The semicircular canals, like the cochlea are composed of an outer bony wall and inner membranous tubes or ducts. The membranous ducts contain endolymph and are separated from bony perilymph
Utricle is a membranous sac which is part of vestibule and the three membranous duct open into it at their dilated ends, the ampulla. The saccule is a part of the vestibule and communicates with the utricle and the cochlea
In the wall of the utricle Saccule and ampullae are fine specialized epithelial cells with minute projections called hair cells. Amongst the hair cells there are receptors on sensory nerve endings which combine forming the vestibulocochlear nerve.

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