1. Describe the anatomy of the outer, middle and inner ear; list the components
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Question
1. Describe the anatomy of the outer, middle and inner ear; list the components and functions of each. 2. Describe the role of the ear ossicles in amplification of sound vibrations waves. rt / structure of the middle ear presses against a flexible membrane separating the midle ear from the fluid filled tubes and chambers of the inner ear? 4. What is the function of Eustachian tube? 5. Describe the anatomy of the semicircular canals, vestibule, and cochlear apparatus. What are endolymph and perilymph? What is their importance in hearing and balance?Explanation / Answer
1. ANATOMY OF OUTER, MIDDLE AND INNER EAR :
The ear is divided into three distinct parts.
OUTER EAR :
The outer ear consits of the auricle (pinna) and the external acoustic meatus (auditory canal).
The auricle (Pinna): The auricle is the visible part of the ear that projects from the side of the head. It is composed of fibroelastic cartilage covered with skin. The most prominent outer ridge is the helix.
The lobule (earlobe) is the soft pliable part at the lower extremity, composed of fibrous and adipose tissue richly supplied with blood.
External acoustic meatus (auditory canal): It is slightly 'S' shaped tube which is about 2.5 cm long extending from the auricle to the tympanic membrane (ear drum).The lateral third is cartilaginous and the remainder is the canal in the temporal bone. The meatus is lined with skin continous with that of the auricle.
MIDDLE EAR :
Middle ear is usually an irregular shaped air - filled cavity withing the petrous portion of the temporal bone. The cavity, its contents and air sacs 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. The medial wall is a thin layer of temporal bone in which there are two openings:
Oval window and round window. The oval window is occluded by part of a small bone called the stapes and the round window, by a fine sheet of fibrous sheet.
INNER EAR :
The inner (internal) ear or labyrinth (meaning 'maze') contains the organs of hearing and balance. It is described in two parts, the bony labrinth and the membranous labrinth.
Bony labyrinth : This is a cavity which lies in the temporal bone lined withperi osteum. It is larger than, and encloses, the membranous labyrinth of the same shape and fits into it, like a tube within a tube. Between the bony and mebranous labyrinth there is layer of watery fluid called perilymph and within the membranous labyrinth there is sa similar watery fluid, endolymph.
The bony lymph consists of:-
The vestibule : This is the expanded part nearest to the middle ear. The ovaland round windows are located in its lateral wall. It contains two membranous sacs, the utricle and saccule, which are important in balance.
The cochlea : This resembles a snail's shell. It has a broad base where it is continous with the vestibule and a narrow apex and it spirals round a central bony column.
The semicircular canals : There are three tubes arranged so that one is situated in each of the three planes of space. They are continuous with the vestibule.
Membranous Labyrinth : This is a network of delicate tubes, filled with endolymph. It comprises:
The cochlea : A cross section of the cochlea contains three compartments:
2. ROLE OF EAR OSSICLES IN AMPLIFICATION OF SOUND VIBRATIONS / WAVES :
Auditory ossicles : There are three very small bones only a few milimetres in size that extend across the middle ear from the tympanic membrane to the oval window. They form a series of movable joints with each other and with the medial wall of the cavity at the oval window. They are named according to their stapes:
Every sound produces soudn waves or vibrations in the air, which travel at about 332 metres per second. The auricle, because of its shape, collects and concentrates the wave and diercts them along the auditory canal causing the tympanic membrane to vibrate. Tympanic membranes vibrations are transmitted and amplified through the middle ear by movement of the ossicles.
3. Part structure of the middle ear which separates the middle ear from the fluid filled tubes and chambers of the inner ear : Eustachian tube which is a narrow passage and extends from the nasopharynx. It is about 4 cm long and is lined with ciliated columnar epithelium.
4. Function of eustachian tube : The vital function of eustachial tube is clear out the middle ear space, assuring the pressure is maintained at normal environmental pressure. To remove any dead cells or debris from the ear and to clear out any excessive fluid.
5.ANATOMY OF SEMICIRCULAR CANALS, VESTIBULE AND COCHLEAR APPARATUS:
The semicircular canals have no auditory function although they are closely associated with the cochlea. Instead they provide information about the position of the head in space, contributing to maintainance of posture and balance.
There are three semicircular canals, one lying on 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 canal 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 the bony wall of perilymph.
The utricle is a membranous sac which is part of the vestibule and the three membranous duct open into it at their dilated ends, the ampullae. The saccule is a part of the vestibule and communicates with the utricle and the cochlea.
The membranous labyrinth which is found within the bony labyrinth have a fluid called endolymph and in between the outer wall of the membranous labyrinth where a perilymph is found.
Any change in the position of th head causes movement in the perilymph and endolymph, which bends the hair cells and stimulates the sensory receptors in the utricle, saccule and ampullae.
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