Clinical Anatomy, Physiology and Examination of the External and Middle Ear EXTERNAL EAR The external ear includes the auricle and the external auditory meatus. The auricle has a complex configuration and is subdivided into two parts: the lobule, which is a skin duplicature containing adipose tissue, and the skin-covered cartilage. The skin on the posterior surface of the auricle can be folded, while on the anterior surface it adheres tightly to the perichondrium. Several projections are found on the auricle, namely, the helix, the antihelix, the tragus, and the antitragns. The tragus projects anterior to the external auditory meatus (Fig. 27). Pressure on the tragus causes pain in acute otitis externa and also in acute otitis media in infants because their external auditory meatus lacks the osseous part and is therefore shorter than in adults. Applying pressure on. the tragus actually means pressure on the inflamed tympanic membrane, which is necessary for a more accurate location of abnormality in this area (haematoma of the triangular fossa, lobular abscess, etc.). Fig. 27. Auricle 1-helix; 2-entry to the exteraal auditory meatus; 3-antihelix; 4-tragus; 5-lobule; 6-antitragus The normal height of the auricle corresponds to the length of the nasal bridge. Deviations from this norm to either side are microtia or macrotia. The auricle stands out prominently from the head and has a specific blood supply (the vessels of the anterior auricular surface lack a protective layer of fat). Therefore, when exposed to cold, vascular spasm develops, and the ear can easily be affected by frostbite. The auricle plays on important role in ototopia (the ability to locate a sound source) and performs a protective function. The complex configuration of a normal auricle facilitates retaining of dust particles in the outer portion of the auditory meatus. In patients with a deformed or completely lost auricle, dust reaches the tympanic membrane and, being deposited on it can induce inflammation. The auricle is to a certain degree important for the hearing acuity, and sometimes a person holds a cupped palm to the auricle to increase its area for letter perceiving of 1 weak sounds. The auricle narrows like a funnel to become continuous with the external auditory meatus. Its diameter varies, but this does not affect hearing (Fig. 28). Fig. 28. External auditory meatuas In infants of the first year of life the osseous part of the external auditory meatus is absent; only its cartilaginous part exists- The meatus of a child is 0.5-0,7 and of an adult 3 cm. The cartilaginous part of the auditory meatus is partly composed of the cartilaginous tissue; inferiorly it borders, on the capsule of the parotid gland. Inflammation can spread to the parotid gland through the transverse tissures (Santorini’s fissures) in the cartilaginous part of the inferior meatal wall. The cartilaginous part contains numerous glands that produce cerumen and fine hair follicles which may get infected in penetration of the pathogenic flora and cause a furuncle of the external auditory meatus. The cartilaginous part of the external auditory meatus forms a groove which is free from cartilage in the region of the posterosuperror wall. Therefore, this wall is usually incised during surgical manipulations to avoid perichondritis. The anterior wall of the external auditory meatus borders on the temporomandibular joint and during mastication this wall changes its position. If a furuncle occurs on this wall, every chewing movement intensifies pain, Due to close contact of the external auditory meatus with the temporomandibular joint, the anterior wall of the meatus fractures from a blow on the chin attended by skin rupture with possible consequent cicatricial obliteration of the meatus. Besides, this intimate anatomical relationship accounts for the development of some syndromes in otorhinolaryngology and dentistry. The bony portion of the external auditory meatus is lined with thin skin. There is a narrowing at the site where it becomes continuous with the cartilaginous part. If a foreign body passes beyond this narrowing, its removal is difficult. The superior wall of the bony portion borders on the middle cranial fossa; the posterior wall borders on air-cells of the mastoid process and, particularly, on the antrum. That makes 2 understandable the development of a pathognomonic sign of acute inflammation in the mastoid process (mastoiditis), that is, sagging of the postero-superior wall in the bony portion of the meatus, which narrows its lumen due to developing periostitis. The skin of the external auditory meatus and the auricle are innervated by the mandibular nerve (the third division of the trigeminal nerve), and by branches of the vagus and the glossopharyngeal nurves. This explains the Fig. 29, Innervatitm of the teeth and ear by the trigeminal nerve 1—inferior dental nerve; 2—mandibular nerve (the think brunch of the trigeminal nerve); 3— area of the temporomandibulur joint; 4 - tympanic plexus; 5—external auditory meatus; 6— trigeminal ganglion; 7—branches innervating the tongue and oral mucosa radiation of pain, e.g., parodontal inflammation of the eighth lower tooth causes severe pain in the ear on the involved side (Fig. 29). MIDDLE EAR The external auditory meatus ends in a tympanic membrane that separates it from the tympanic cavity. Changes in the tympanic membrane (eardrum or a drumhead) are informative of the pathological changes occurring beyond the membrane, in the middle ear, because the eardrum is part of the middle ear, and its mucous membrane is continuous with the mucosa of other parts of the middle ear. Therefore, the present or past disease leaves marks on the drumhead: scars, perforations, calcareous deposits, retraction, etc., that can sometimes persist throughout the patient's life. The eardrum is a thin, sometimes semi transparent membrane consisting of two parts; the greater tense (pars tensa) and the smaller flaccid part (pars flaccida) ( Fig. 30). There are three layers in pars tensa: the outer squamous epithelium, the inner of the middle ear), and the middle fibrous layer 3 consisting of radial and circular fibres closely interwoven with one another. The flaccid part is composed of two layers; the middle fibrous layer is absent. Fig. 30. Right tympanic membrane 1—antero-inferior quadrant; 2—postero-inferior quadrant; 3 – anterosuperior quadrant; 4 - posterosuperior quadrant; 5 – pars flaccida; 6 – short process of the malleus; 7 – manubrium; 8 - light reflex The position of the drumhead in an adult is oblique (450) with respect to the interior wall of the auditory meatus. This angle is more pronounced in children (20°). The auricle should therefore be pulled down and back during examination of the tympanic membrane in children. The eardrum is rounded, the diameter being approximately 0.9 cm. The normal eardrum is bluish-grey and slightly retracted into the tympanic cavity forming a depression in the centre called umbo. Different parts of the eardrum are positioned in a different way in relation to the long axis of the auditory meatus. The antero-inferior portion is more perpendicular to it; therefore, the cone of light reflected from this quadrant has its specific position when the drumhead is normal. This light reflex is of topographic and diagnostic importance. A structure extending downward and posteriorly is the manubrium (the handle of the malleus). The angle formed by the manubrium and the cone of light opens anteriorly, which helps to distinguish between the left and the right eardrums. At the upper end of the manubrium a small eminence, the size of a millet grain, stands out. This is a short process of the malleus from which the anterior and posterior malleolar folds extend anteriorly and posteriorly. The latter separate pars tensa and pars flaccida. The tympanic membrane is divided into four quadrants for the convenience of topology: anterosuporior, antero-inferior, posterosuperior, and postero-inferior (see Fig. 30). These quadrates are conventionally distinguished by drawing a line through the manubrium and a line perpendicular to the first one passing through umbo. The middle ear consists of three interconnected parts: the eustachian (auditory) tube, the tympanic cavity, and the mastoid air-cells. Their mucosa is continuous and inflammation, produces 4 corresponding changes in all of them. The tympanic cavity (or the tympanum) is the central portion of the middle ear and has a complex structure. Though, the tympanum is small (the volume of about 1 cm3), it performs important functions. There ore six walls in the tympanic cavity: the membranous (lateral) wall is the inner surface of the tympanic membrane except for its upper bony part called the epitympanic recess or attic; the anterior wall is called carotid because it contains a bony canal through which the internal carotid artery passes; the upper part of this wall has a foramen, leading into the pharyngotympanic (eustachian) tube, and a groove where the body of the tensor tympani muscle is embedded; the inferior (jugular) wall (or the floor) borders on the jugular bulb that sometimes protrudes markedly into the tympanic cavity; the posterior (mastoid) wall has a mastoid foramen in its upper part which leads to a short canal connecting the tympanic cavity with the mastoid antrum, the largest, and permanent mastoid cell; the medial (labyrinthine) wall is occupied mainly with a rounded prominence called the promontory corresponding to the basal cochlear turn (Fig. 31). Posteriorly and slightly above the promontory there is an oval window (fenestra vestibuli) and posteriorly and below it, a round window (fenestra cochleae). The canal for the facial nerve is found along the superior margin of the wall. As the canal turns back, it borders on the superior edge of the oval window niche. Than it turns down, runs in the thickness of the posterior tympanum wall, and ends in the stylomastoid foramen; the tegmenlal wall (the roof) adjoins the middle cranial fossa.