DOCSLIB.ORG

Chapter 2. Structure of the Middle Ear in Mammals

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Chapter 2. Structure of the Middle Ear in Mammals CHAPTER 2. STRUCTURE OF THE MIDDLE EAR IN MAMMALS The middle ear structure of mammals has been attracting the attention of various scientists for a long time, so at present there are numerous funda- mental works devoted to a comparative and anatomical study of middle ear elements in the representatives of different orders [Hyrtl, 1844, 1845, 1848; Claudius, 1867; Doran, 1878; Denker, 1899; Bondy, 1907; Frey, 1911; Crowe et al., 1931; Wassif, 1948, 1950; Yamada, 1953 a, b; Webster, 1960, 1966; Werner, 1960; Kobrak, 1963; Hentzen, 1970; Solntseva, 1972b, 1973b, d, f]. Based on the available literature and his own data, German researcher G. Fleischer [1973 a, b] provided a comparative anatomical study of the tym- panic bulla in representatives of different orders. Fleischer’s data, along with the evidence obtained by other researchers mentioned above as well as my- self [Solntseva, 1975a, b], form the basis of this study. The bulla tympanica of mammals represents a periotic-tympanic com- plex (Fig. 8a-i). The tympanic bone is composed of thin osseous walls, form- ing the tympanum, in which the elements of the middle ear are localized. The inner ear is situated in periotic bone. The middle ear of mammals contains a tympanum (cavum tympani), which by means of an auditory tube is connected with the gullet cavity, tympanic mem- brane and a chain of the auditory ossicles (ossicula auditus): malleus, incus and stapes. Ligaments and two muscles of the middle ear (m. tensor tympani, m. stapedius) are bound to the auditory ossicles. Muscles of the tympanum regulate the transition of auditory energy and preserve the inner ear from su- per-intensive sounds. Ligaments keep auditory ossicles in a definite position. The middle ear is separated from the outer ear by a tympanic membrane, stretched onto a tympanic ring (anulus tympanicus) (Fig. 9a-c). The tympanic membrane is formed by three layers. Its basis is composed of radially and circu- larly directed connective-tissue fibers, which grow together with the periosteum of the malleus handle (manubrium mallei), set into the tympanic membrane. From inside, the tympanic membrane is covered by a mucous membrane (stra- tum mucosum), formed by a pavement epithelium; from the surface, the tym- panic membrane is covered by a lacking hair, glands and a papillary layer tegu- CHAPTER 2. STRUCTURE OF THE MIDDLE EAR IN MAMMALS 45 ment (stratum cutaneum). The tympanic membrane has the shape of a cone, with the top directed into the tympanic cavity. A stretched part (pars tensa) and a small, unstretched part (pars flacida), which is more flexible due to the lack of the basis, are discernible on the tympanic membrane (Fig. 10a, b). The tympanum is underlaid by a mucous membrane, in the middle layer of which there are blood vessels which regulate the degree of the stratum’s thickness. The tympanum is located between the external auditory meatus and the inner ear labyrinth, and is separated from these formations by thin membranes. Six walls are discernible in the tympanum: an anterior wall, where the orifice of the Eustachian tube is situated; a posterior wall, having a foramen leading into the antrum mastoideum; medial, upper, and lateral walls, formed by the tympanic membrane; and a bottom wall. The medial wall is formed by a labyrinth capsule and separates the middle ear from the inner ear. This wall contains oval and round windows. The upper wall has a form of osseous partition [Kobrak, 1963]. The auditory ossicles of mammals are more complicated than in represen- tatives of other classes of terrestrial vertebrates. The malleus is differentiated into the head of the malleus (capitulum mallei), neck of the malleus (collum mallei) and handle of the malleus. The head of the malleus bears an articular surface for a junction with the incus. At the medial side of the handle of the malleus, near the neck of the malleus, a small muscular arm (pr. muscularis) is located, which serves for the fastening of a tensor muscle of the tympanic mem- brane. At the dorsal edge of the handle of the malleus, a short arm (pr. brevis) is fastened; and at the dorsal edge of the neck of the malleus, a long arm (pr. longus), which is reduced in most mammals to a small, sharp pr. gracilis, is connected with the help of a ligament within the tympanum’s wall. In other species, the long arm is large in size and connects rigidly with the wall of the tympanum. The m. tensor tympani is attached to the handle of the malleus, under whose contraction the tympanic membrane is pulled inside and, through a system of auditory ossicles, presses the stapes into an oval window. The malleus is fixed in a definite position with the help of three ligaments: a fore bundle that reaches the fissura Glasseri and gets attached to the pr. anterior of the malleus; a ligament located in the area opposite the neck of the malleus; and an axial ligament representing the axis, around which the malleus rotates. The incus has a body and two components: the long arm (crus longum) and the short arm (crus breve). The long arm is stretched parallel to the handle of the malleus. Its lower end is curved, forming a junction with the stapes with the help of a lenticular arm. The short arm is located in the deep- ening of the osseous tympanum, where it is bound by a ligament. 46 PART I. PERIPHERAL PART OF THE AUDITORY SYSTEM OF MAMMALS IN POSTNATAL ONTOGENY.
Load more

Recommended publications
  • Experimental Studies on the Function of the Stapedius Muscle Inman
    EXPERIMENTAL STUDIES ON THE FUNCTION OF THE STAPEDIUS MUSCLE INMAN AKADEMISK AVHANDLING som med vederbörligt tillstånd av Medicinska fakulteten vid Umeå Universitet för vinnande av medicine doktorsgrad offentligen försvaras i Samhällsvetarhuset, sal D, lördagen den 25 maj 1974 kl. 9.15 f.m. av JOHN-ERIK ZAKRISSON med.lic. UMEÅ 1974 UMEÀ UNIVERSITY MEDICAL DISSERTATIONS No. 18 1974 From the Department of Otorhinolaryngology, University of Umeå, Umeå, Sweden and the Division of Physiological Acoustics, Department of Physiology II, Karolinska Institutet, Stockholm, Sweden EXPERIMENTAL STUDIES ON THE FUNCTION OF THE STAPEDIUS MUSCLE IN MAN BY JOHN-ERIK ZAKRISSON UMEÂ 1974 To Karin Eva and Gunilla The present thesis is based on the following papers which will be referred to in the text by the Roman numerals: I. Zakrisson, J.-E., Borg, E. & Blom, S. The acoustic impedance change as a measure of stapedius muscle activity in man. A methodological study with electromyography. Acta Otolaryng, preprint. II. Borg, E. & Zakrisson, J.-E. Stapedius reflex and monaural masking. Acta Otolaryng, preprint. III. Zakrisson, J.-E. The role of the stapedius reflex in poststimulatory audi ­ tory fatigue. Acta Otolaryng, preprint. IV. Borg, E. & Zakrisson, J.-E. The activity of the stapedius muscle in man during vocalization. Acta Otolaryng, accepted for publication. CONTENTS ABBREVIATIONS .......................................... 8 INTRODUCTION.............................................................................................. 9 MATERIAL.....................................................................................................
    [Show full text]
  • Vestibular Neuritis and Labyrinthitis
    Vestibular Neuritis and DISORDERS Labyrinthitis: Infections of the Inner Ear By Charlotte L. Shupert, PhD with contributions from Bridget Kulick, PT and the Vestibular Disorders Association INFECTIONS Result in damage to inner ear and/or nerve. ARTICLE 079 DID THIS ARTICLE HELP YOU? SUPPORT VEDA @ VESTIBULAR.ORG Vestibular neuritis and labyrinthitis are disorders resulting from an 5018 NE 15th Ave. infection that inflames the inner ear or the nerves connecting the inner Portland, OR 97211 ear to the brain. This inflammation disrupts the transmission of sensory 1-800-837-8428 information from the ear to the brain. Vertigo, dizziness, and difficulties [email protected] with balance, vision, or hearing may result. vestibular.org Infections of the inner ear are usually viral; less commonly, the cause is bacterial. Such inner ear infections are not the same as middle ear infections, which are the type of bacterial infections common in childhood affecting the area around the eardrum. VESTIBULAR.ORG :: 079 / DISORDERS 1 INNER EAR STRUCTURE AND FUNCTION The inner ear consists of a system of fluid-filled DEFINITIONS tubes and sacs called the labyrinth. The labyrinth serves two functions: hearing and balance. Neuritis Inflamation of the nerve. The hearing function involves the cochlea, a snail- shaped tube filled with fluid and sensitive nerve Labyrinthitis Inflamation of the labyrinth. endings that transmit sound signals to the brain. Bacterial infection where The balance function involves the vestibular bacteria infect the middle organs. Fluid and hair cells in the three loop-shaped ear or the bone surrounding semicircular canals and the sac-shaped utricle and Serous the inner ear produce toxins saccule provide the brain with information about Labyrinthitis that invade the inner ear via head movement.
    [Show full text]
  • ANATOMY of EAR Basic Ear Anatomy
    ANATOMY OF EAR Basic Ear Anatomy • Expected outcomes • To understand the hearing mechanism • To be able to identify the structures of the ear Development of Ear 1. Pinna develops from 1st & 2nd Branchial arch (Hillocks of His). Starts at 6 Weeks & is complete by 20 weeks. 2. E.A.M. develops from dorsal end of 1st branchial arch starting at 6-8 weeks and is complete by 28 weeks. 3. Middle Ear development —Malleus & Incus develop between 6-8 weeks from 1st & 2nd branchial arch. Branchial arches & Development of Ear Dev. contd---- • T.M at 28 weeks from all 3 germinal layers . • Foot plate of stapes develops from otic capsule b/w 6- 8 weeks. • Inner ear develops from otic capsule starting at 5 weeks & is complete by 25 weeks. • Development of external/middle/inner ear is independent of each other. Development of ear External Ear • It consists of - Pinna and External auditory meatus. Pinna • It is made up of fibro elastic cartilage covered by skin and connected to the surrounding parts by ligaments and muscles. • Various landmarks on the pinna are helix, antihelix, lobule, tragus, concha, scaphoid fossa and triangular fossa • Pinna has two surfaces i.e. medial or cranial surface and a lateral surface . • Cymba concha lies between crus helix and crus antihelix. It is an important landmark for mastoid antrum. Anatomy of external ear • Landmarks of pinna Anatomy of external ear • Bat-Ear is the most common congenital anomaly of pinna in which antihelix has not developed and excessive conchal cartilage is present. • Corrections of Pinna defects are done at 6 years of age.
    [Show full text]
  • Research Reports
    ARAŞTIRMALAR (ResearchUnur, Ülger, Reports) Ekinci MORPHOMETRICAL AND MORPHOLOGICAL VARIATIONS OF MIDDLE EAR OSSICLES IN THE NEWBORN* Yeni doğanlarda orta kulak kemikciklerinin morfometrik ve morfolojik varyasyonları Erdoğan UNUR 1, Harun ÜLGER 1, Nihat EKİNCİ 2 Abstract Özet Purpose: Aim of this study was to investigate the Amaç: Yeni doğanlarda orta kulak kemikciklerinin morphometric and morphologic variations of middle ear morfometrik ve morfolojik varyasyonlarını ortaya ossicles. koymak. Materials and Methods: Middle ear of 20 newborn Gereç ve yöntem: Her iki cinse ait 20 yeni doğan cadavers from both sexes were dissected bilaterally and kadavrasının orta kulak boşluğuna girilerek elde edilen the ossicles were obtained to investigate their orta kulak kemikcikleri üzerinde morfometrik ve morphometric and morphologic characteristics. morfolojik inceleme yapıldı. Results: The average of morphometric parameters Bulgular: Morfometrik sonuçlar; malleus’un toplam showed that the malleus was 7.69 mm in total length with uzunluğu 7.69 mm, manibrium mallei’nin uzunluğu 4.70 an angle of 137 o; the manibrium mallei was 4.70 mm, mm, caput mallei ve processus lateralis arasındaki and the total length of head and neck was 4.85 mm; the uzaklık 4.85 mm, manibrium mallei’nin ekseni ve caput incus had a total length of 6.47 mm, total width of 4.88 mallei arasındaki açı 137 o, incus’un toplam uzunluğu mm , and a maximal distance of 6.12 mm between the 6.47 mm, toplam genişliği 4.88 mm, crus longum ve tops of the processes, with an angle of 99.9 o; the stapes breve’nin uçları arasındaki uzaklık 6.12 mm, cruslar had a total height of 3.22 mm, with stapedial base being arasındaki açı 99.9 o, stapesin toplam uzunluğu 2.57 mm in length and 1.29 mm in width.
    [Show full text]
  • Anatomy of the Ear ANATOMY & Glossary of Terms
    Anatomy of the Ear ANATOMY & Glossary of Terms By Vestibular Disorders Association HEARING & ANATOMY BALANCE The human inner ear contains two divisions: the hearing (auditory) The human ear contains component—the cochlea, and a balance (vestibular) component—the two components: auditory peripheral vestibular system. Peripheral in this context refers to (cochlea) & balance a system that is outside of the central nervous system (brain and (vestibular). brainstem). The peripheral vestibular system sends information to the brain and brainstem. The vestibular system in each ear consists of a complex series of passageways and chambers within the bony skull. Within these ARTICLE passageways are tubes (semicircular canals), and sacs (a utricle and saccule), filled with a fluid called endolymph. Around the outside of the tubes and sacs is a different fluid called perilymph. Both of these fluids are of precise chemical compositions, and they are different. The mechanism that regulates the amount and composition of these fluids is 04 important to the proper functioning of the inner ear. Each of the semicircular canals is located in a different spatial plane. They are located at right angles to each other and to those in the ear on the opposite side of the head. At the base of each canal is a swelling DID THIS ARTICLE (ampulla) and within each ampulla is a sensory receptor (cupula). HELP YOU? MOVEMENT AND BALANCE SUPPORT VEDA @ VESTIBULAR.ORG With head movement in the plane or angle in which a canal is positioned, the endo-lymphatic fluid within that canal, because of inertia, lags behind. When this fluid lags behind, the sensory receptor within the canal is bent.
    [Show full text]
  • Topographical Anatomy and Morphometry of the Temporal Bone of the Macaque
    Folia Morphol. Vol. 68, No. 1, pp. 13–22 Copyright © 2009 Via Medica O R I G I N A L A R T I C L E ISSN 0015–5659 www.fm.viamedica.pl Topographical anatomy and morphometry of the temporal bone of the macaque J. Wysocki 1Clinic of Otolaryngology and Rehabilitation, II Medical Faculty, Warsaw Medical University, Poland, Kajetany, Nadarzyn, Poland 2Laboratory of Clinical Anatomy of the Head and Neck, Institute of Physiology and Pathology of Hearing, Poland, Kajetany, Nadarzyn, Poland [Received 7 July 2008; Accepted 10 October 2008] Based on the dissections of 24 bones of 12 macaques (Macaca mulatta), a systematic anatomical description was made and measurements of the cho- sen size parameters of the temporal bone as well as the skull were taken. Although there is a small mastoid process, the general arrangement of the macaque’s temporal bone structures is very close to that which is observed in humans. The main differences are a different model of pneumatisation and the presence of subarcuate fossa, which possesses considerable dimensions. The main air space in the middle ear is the mesotympanum, but there are also additional air cells: the epitympanic recess containing the head of malleus and body of incus, the mastoid cavity, and several air spaces on the floor of the tympanic cavity. The vicinity of the carotid canal is also very well pneuma- tised and the walls of the canal are very thin. The semicircular canals are relatively small, very regular in shape, and characterized by almost the same dimensions. The bony walls of the labyrinth are relatively thin.
    [Show full text]
  • Hearing Loss
    Survivorship Clinic Hearing Loss Some chemotherapy drugs, other medications, or radiation needed for treatment of cancer can damage hearing. Hearing loss interferes with daily living. If you received these treatments, it is important to have your hearing checked and to obtain treatment if hearing loss is found. How do the ears work? It’s easier to understand hearing loss if you understand how the ears work. The ear is made up of three main parts, known as the outer, middle, and inner ear. Outer ear Sound waves travel through the air and first enter the body through the outer ear. The part of the ear that can be seen outside the body is called the pinna. The pinna collects and funnels sound into the auditory (ear) canal. The auditory canal is like a tunnel. It makes the sound louder and directs it toward the middle ear. Middle Ear The eardrum separates the outer ear from the middle ear, a chamber that is normally filled with air. Inside the middle ear are three tiny bones (ossicles) that form a chain connecting the eardrum to the opening of the inner ear. Sound waves cause the eardrum to vibrate. These vibrations cause the three tiny bones in the middle ear to move, transmitting the sound to the inner ear. Inner Ear The inner ear is known as the cochlea, and it is filled with fluid. The cochlea contains thousands of tiny nerve endings, known as sensory hair cells. Sounds travels in waves through the fluid of the inner ear. The sensory hair cells change the sound waves into nerve impulses that are sent to the brain by way of the auditory nerve (also known as the eighth cranial nerve).
    [Show full text]
  • Download PDF Intratemporal Course of the Facial Nerve
    Romanian Journal of Morphology and Embryology 2010, 51(2):243–248 ORIGINAL PAPER Intratemporal course of the facial nerve: morphological, topographic and morphometric features NICOLETA MĂRU1), A. C. CHEIŢĂ2), CARMEN AURELIA MOGOANTĂ3), B. PREJOIANU4) 1)Department of Anatomy, Faculty of Dental Medicine 2)PhD candidate, ENT specialist “Carol Davila” University of Medicine and Pharmacy, Bucharest 3)ENT resident, Emergency County Hospital, Craiova University of Medicine and Pharmacy of Craiova 4)Tehno Electro Medical Company, Bucharest Abstract The purpose of this study is to present some morphological and morphometric aspects of the facial nerve and especially of the tympanic and mastoid segments of this nerve. The authors follow up a mesoscopic study concerning the tract (length, angulation, width) of these segments and the anatomic relations with the important structures of the middle ear. At the same time, some anatomical variations which involve the canal of the facial nerve (dehiscences, tract deviation or other anatomical deviations) are presented. To evaluate the risk of the facial nerve injury during operations for chronic otitis media with or without cholesteatoma, stapedectomy in otosclerosis, exploratory tympanotomy, tympanoplasty, canaloplasty, osteomas surgery or other otologic surgery that involve facial nerve area. The intricate course of the facial nerve through the temporal bone is of vital concern to all otologic surgeons, since it often traverses the surgical field. Therefore, authors will review the course of the facial canal through the petrosal portion of the temporal bone from the internal auditory meatus to the stylomastoid foramen, paying particular attention to its relations to adjacent structures. Keywords: intratemporal part, facial nerve.
    [Show full text]
  • 1 Surgical Anatomy Alexander Rauchfuss
    Chapter 1 1 1 Surgical Anatomy Alexander Rauchfuss The temporal bone presents a very complex anatomy. Therefore this overview is restricted to some major points from the viewpoint of surgical anatomy. For more detailed information see “Suggested Reading”. Thetemporalboneaccordingtoitsdevelopmentalanatomyisdivisibleinto four parts: the squamous, mastoid, petrous, and tympanic portions. Points of topographical reference on the lateral surface are the external acoustic meatus with its suprameatal spine, the temporal line, and the mastoid process. Thebaseofthezygomaextendsasacrestposteriorlyandslightlyupward, forming the supramastoid crest or temporal line. The temporal line as a land- mark corresponds to the base of the medial cranial fossa/tegmen tympani, which in most cases of surgery can easily be identified. In combination with the radiological anatomy in a Schüller view it allows adequate planning of the surgical approach to the antrum via the mastoid. All figures show the anatomy of a left ear. 2 1 Surgical Anatomy Figs. 1.1–1.5. Temporal bone and sigmoid sinus Fig. 1.1. Temporal bone. The degree of pneumatization is inconstant. The extent and arrangement of air cells varies considerably from a minimal air cell system in the surroundings of the antrum to involvement of most of the tempo- ral bone. Pneumatization usually begins in late fetal life, progressing until the end of childhood. The pneumatization process starts from the antrum. In most cases one can describe the topography of the cells as follows: periantral, sino- dural, perisinual, perifacial and mastoid tip cells. According to the extension of the cells, there is only one rule: the further from the antrum, the bigger the cells Fig.
    [Show full text]
  • Effects of Stapedius-Muscle Contractions on Masking of Tone Responses in the Auditory Nerve
    Effects of Stapedius-Muscle Contractions on Masking of Tone Responses in the Auditory Nerve RLE Technical Report No. 544 May 1989 Xiao Dong Pang Research Laboratory of Electronics Massachusetts Institute of Technology Cambridge, MA 02139 USA a e a a -2- EFFECTS OF STAPEDIUS-MUSCLE CONTRACTIONS ON MASKING OF TONE RESPONSES IN THE AUDITORY NERVE by XIAO DONG PANG Submitted to the Department of Electrical Engineering and Computer Science on April 29, 1988 in partial fulfillment of the requirements for the Degree of Doctor of Science ABSTRACT The stapedius muscle in the mammalian middle ear contracts under various condi- tions, including vocalization, chewing, head and body movement, and sound stimulation. Contractions of the stapedius muscle' modify (mostly attenuate) transmission of acoustic signals through the middle ear, and this modification is a function of acoustic frequency. This thesis is aimed at a more comprehensive understanding of (1) the functional benefits of contractions of the stapedius muscle for information processing in the auditory system, and (2) the neuronal mechanisms of the functional benefits. The above goals were approached by investigating the effects of stapedius muscle contractions on the masking by low-frequency noise of the responses to high-frequency tones of cat auditory-nerve fibers. The following considerations led to the approach. (1) Most natural sounds have multiple spectral components; a general property of the audi- tory system is that the responsiveness of individual auditory-nerve fibers and the whole auditory system to one component can be reduced by the presence of another component, a phenomenon referred to as "masking". (2) It is known that low-frequency sounds mask auditory responses to high-frequency sounds much more than the reverse.
    [Show full text]
  • Anatomic Moment
    Anatomic Moment Hearing, I: The Cochlea David L. Daniels, Joel D. Swartz, H. Ric Harnsberger, John L. Ulmer, Katherine A. Shaffer, and Leighton Mark The purpose of the ear is to transform me- cochlear recess, which lies on the medial wall of chanical energy (sound) into electric energy. the vestibule (Fig 3). As these sound waves The external ear collects and directs the sound. enter the perilymph of the scala vestibuli, they The middle ear converts the sound to fluid mo- are transmitted through the vestibular mem- tion. The inner ear, specifically the cochlea, brane into the endolymph of the cochlear duct, transforms fluid motion into electric energy. causing displacement of the basilar membrane, The cochlea is a coiled structure consisting of which stimulates the hair cell receptors of the two and three quarter turns (Figs 1 and 2). If it organ of Corti (Figs 4–7) (4, 5). It is the move- were elongated, the cochlea would be approxi- ment of hair cells that generates the electric mately 30 mm in length. The fluid-filled spaces potentials that are converted into action poten- of the cochlea are comprised of three parallel tials in the auditory nerve fibers. The basilar canals: an outer scala vestibuli (ascending spi- membrane varies in width and tension from ral), an inner scala tympani (descending spi- base to apex. As a result, different portions of ral), and the central cochlear duct (scala media) the membrane respond to different auditory fre- (1–7). The scala vestibuli and scala tympani quencies (2, 5). These perilymphatic waves are contain perilymph, a substance similar in com- transmitted via the apex of the cochlea (helico- position to cerebrospinal fluid.
    [Show full text]
  • Let's Talk About . . . Otosclerosis
    LET’S TALK ABOUT . OTOSCLEROSIS diagnosed with otosclerosis. Pregnancy can cause Key points otosclerosis to advance more quickly. • Otosclerosis affects the bones of the middle Otosclerosis is rare, affecting about 3 in 1,000 ear that conduct sound. people. Research suggests between 25 to 50% of people with otosclerosis have a family history of the • It is one of the most common causes of conductive hearing loss in young adults. condition. • How quickly, or to what extent, hearing will The word otosclerosis comes from Greek. It means be affected is unpredictable. abnormal hardening of body tissue (sclerosis) of the ear (oto). • If otosclerosis goes into the inner ear, you may be troubled by ringing in the ears, dizziness and balance problems. How do we hear? • Hearing aids are usually the preferred first treatment choice. To understand why otosclerosis causes hearing loss, it is important to have a basic understanding of how we hear. For hearing to function normally a What is otosclerosis? sound has to travel through all three parts of the Otosclerosis (oh-toe-skler-OH-suhs) a complex ear: outer, middle and inner. The first two are air disorder of abnormal bone growth in the middle ear. filled; the latter is fluid filled. It most often happens when the tiny stapes (“STAY- The outer ear is made up of the part you can see peez”) bone knits with surrounding bone. on the side of your head (pinna) and the funnel- Otosclerosis usually results in slow, shaped external ear canal. The pinna gathers progressive conductive hearing loss. sound waves (vibrations) and channels them When the stapes is unable to vibrate, hearing through the ear canal to the eardrum (tympanic becomes impaired.
    [Show full text]