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Peripheral Hearing Analyzer

The peripheral part of the auditory analyzer, or the ear itself, anatomically consists of three parts: the outer, middle and inner ear (Fig. 4).

Fig. 4.

The structure of the hearing organ

:



1 - cartilage of the auricle; 2 - external auditory meatus: 3 - eardrum; 4 - auditory tube; 5 - auditory ossicles; 6 - a labyrinth;

7 - the inner surface of the eardrum: 8 - hammer;

9 - anvil; 10 - stirrup; 11 - semicircular canals; 12 - the vestibule;

13 - a snail; 14 - the vestibular nerve; 15 - cochlear nerve



The outer ear consists of the auricle (Fig. 5) and the external auditory meatus. The auricle is a funnel-shaped cartilaginous plate covered with skin and passing directly into the external auditory meatus. The lower part of the auricle, or lobe, is devoid of cartilage. In front of the external auditory meatus is the protrusion of the auricle - the tragus. The external auditory meatus consists of cartilaginous (external) and bone (internal) parts. The total length of the external auditory meatus in adults is about 2.5 cm. At the place of transition of the cartilaginous part into the bone, the external auditory meatus forms a bend. To straighten the axis of the external auditory canal, when examining it, one has to pull the auricle in adults and in older children back and forth, and in young children, back and down.

Fig. 5.

Auricle

:



1 - tragus; 2 - lobe



Throughout its entire length, the external auditory meatus is covered with skin. The skin lining the cartilaginous part is provided with hairs and contains glands - sebaceous and secreting earwax. The width of the lumen of the external auditory meatus is not the same everywhere: it narrows at the beginning of the bony part and secondarily at the site of attachment of the eardrum. In young children, the external auditory canal is shorter than in older children and adults, due to the fact that its bone part has not yet had time to develop and is represented only by a bone ring in which the eardrum is strengthened. The lumen of the external auditory canal in newborns and young children seems slit-like. As the child grows, the lumen of the auditory canal from the slit-like gradually becomes oval.

The eardrum separates the external auditory meatus from the middle ear and is a thin elastic plate covered on the side of the auditory meatus with a thin outer layer of the skin (epidermis), and on the side of the middle ear - the mucous membrane.



The eardrum has a rounded oval shape with the largest diameter of about 10 mm and the smallest - 8.5 mm, thickness - about 0.1 mm. It is located at an angle to the axis of the external auditory canal and is pulled toward the middle ear, forming a semblance of a very flat cone.

The size of the eardrum changes very slightly with age: in a newborn, it has almost the same dimensions as in an adult. The position of the eardrum with the development of the child undergoes noticeable changes. In a child under the age of two months, it is located almost horizontally, being, as it were, a continuation of the upper wall of the external auditory canal and forming an angle with a horizontal plane of only 10-20 °. In older children, the angle of inclination to the horizontal reaches 40–45 °.

Most of the tympanic membrane is inserted, like a watch glass, into a special bone groove located deep in the ear canal, and is called stretched, unlike the smaller, front-upper part of the tympanic membrane, which is attached to the place where the bone groove is interrupted. This part of the eardrum is called the relaxed or shrapnel membrane. The stretched part of the tympanic membrane consists of three layers: 1) external, facing the auditory meatus, consisting of the epidermis; 2) the middle, consisting of circular (circular) and radial (beam) fibrous fibers; 3) the internal formed by the mucous membrane. The circular and radial fibers of the fibrous layer of the tympanic membrane are intertwined, which gives special strength to its stretched part. With a gradual increase in air pressure, it can withstand it up to two atmospheres (atm.), Which in the modern dimension is 200,000 pascals, since 1 atm = 105 Pascals (Pa), i.e. 100,000 pascals. In the shrapnel membrane, the fibrous layer is absent.

A normal eardrum is presented upon examination (NOTE: An examination of the external auditory canal and the eardrum, or otoscopy, is performed using an ear funnel, which is inserted into the ear canal and serves to direct a beam of light reflected from the frontal reflector (a concave mirror mounted on the examining forehead) )) in the form of a rounded oval plate. The main color of the eardrum is pearl gray (Fig. 6). Against this general background, the following identification points are highlighted (Fig. 7). In the front-upper part of the eardrum, a short process of the malleus protrudes as a yellowish-white point. Grayish-white stripes extend anteriorly and posteriorly from it - these are the anterior and posterior folds that separate the stretched part of the eardrum from the relaxed (shrapnel) one. Down and backward from the short process stretches, standing out sharply in the form of a sharp scallop, the handle of a malleus; with its expanded end, it reaches the center of the eardrum, called the navel. A shiny light reflex in the shape of a narrow triangle, the top of which faces the navel and the base to the front-lower edge of the eardrum, goes down and anterior to the navel. This reflex is formed due to the reflection of light rays from the concave surface of the eardrum.

Fig. 6.

Normal eardrum



Fig. 7.

Eardrum Identification Points

:



a - the stretched part; b - the relaxed part (shrapnel membrane); 1 - light reflex; 2 — a short process of the malleus; 3 - the back fold of the tympanic membrane; 4 - front fold; 5 - the handle of the malleus; 6 - navel



To indicate changes that occur on the eardrum, it is conditionally divided into four parts by means of two mentally drawn lines (Fig. 8): one of them passes along the handle of the malleus and reaches the edge of the eardrum; the other crosses the first at right angles at the level of the navel. With these two lines, the eardrum is divided into four sectors, or quadrants: front-upper, front-lower, posterior-upper and posterior-lower.

Fig. 8. Quadrants of the eardrum:

1 - back-upper; 2 - back-lower; 3 - front-upper; 4 - front-bottom



The middle ear is a system of air cavities in the thickness of the temporal bone and consists of a tympanic cavity, an auditory tube and a mastoid process with its bone cells (Fig. 9).

Fig. 9.

Middle ear cavity

:

1 - auditory tube; 2 - tympanic cavity, 3 - cave; 4 - cells of the mastoid process



The drum cavity is the central part of this system and represents a narrow space in the thickness of the temporal bone with a volume of about 1 cm3. Six walls are distinguished in the tympanic cavity. The outer wall for most of its length is the eardrum. The remaining walls are bone. The inner wall separates the tympanic cavity from the inner ear. There are two openings in this wall called windows: oval, or the vestibule window (long diameter 3-4 mm) and round, or the cochlea window (1-2 mm diameter). In the oval window is inserted, as in a frame, a stirrup foot plate attached to the edges of the oval window by means of an annular ligament. The round window is tightened with an elastic thin membrane, which is called the secondary eardrum. The upper wall, or roof of the tympanic cavity, separates the tympanic cavity from the cranial cavity. The lower wall is bordered by a large blood vessel - the jugular vein bulb. In the back wall below there is a hole connecting the tympanic cavity with the cave of the mastoid process.



The upper and lower walls of the tympanic cavity are often very thin, and often, especially in early childhood, there are holes in these walls. Then the mucous membrane of the tympanic cavity adjoins directly to the cerebral membrane or to the bulb of the jugular vein, which poses a significant danger in the sense of the possible transition of the inflammatory process from the tympanic cavity to the meninges or the walls of the jugular vein. In the thickness of the inner and posterior walls of the tympanic cavity is the channel of the facial nerve. Due to the close anatomical proximity between this channel and the tympanic cavity, the facial nerve can be involved in the inflammatory process,

A chain of auditory ossicles is placed in the tympanic cavity (Fig.
10), consisting of a malleus, anvil and stapes. The malleus has a head, a handle and two processes (short and long). The anvil consists of a body, short and long processes. The stirrup consists of two arches, a head and a base plate. The handle of the malleus is rotated into the fibrous layer of the eardrum, and the lower end of the handle forms a protrusion - the navel in the center of the eardrum, and the short process forms a protrusion in the front-upper part. These protrusions determine the characteristic appearance that the eardrum has upon examination. The head of the malleus articulates with the body of the anvil, and it, with its long process, articulates with the head of the stapes. The base plate of the stapes, as was said, enters the oval window connecting the middle ear to the inner one. A certain tension of the tympanic membrane and the chain of the auditory ossicles is provided by two muscles - the tension of the tympanic membrane and the stirrup. The first of them is attached to the handle of the malleus, and the second to the head of the stapes.

The auditory or Eustachian tube is a 3.5 cm long (in adults) canal connecting the tympanic cavity with the nasopharynx. The drum mouth of the Eustachian tube is located in the anterior wall of the tympanic cavity, and the nasopharyngeal is located in the lateral wall of the nasopharynx. That part of the Eustachian tube, which is adjacent to the tympanic cavity, is bone, and the part facing the nasopharynx has cartilaginous walls. The entire Eustachian tube is lined with ciliated epithelium: the movement of its hairs is directed towards the nasopharynx. The walls of the cartilaginous part of the Eustachian tube, usually in contact with each other, at the time of swallowing (due to contraction of the pharyngeal muscles) diverge, passing air from the nasopharynx into the tympanic cavity. In young children, the Eustachian tube is shorter and its lumen wider than in older children and adults.



The mastoid process is a bone formation, similar in shape to the nipple, where its name comes from. This is a process of the temporal bone located behind the auricle. In the thickness of the mastoid process are cells communicating with each other through narrow slits. The shape, size and number of these cells are very variable, but one of them, the largest, called the cave (antrum), is constantly available. The cave communicates with the tympanic cavity through an opening in the back wall of the latter. The cave is separated from the cranial cavity by a bone plate, sometimes very thin. The cells of the mastoid sometimes reach the large venous sinus of the brain (transverse sinus) and are also separated from it by only a thin layer of bone.

In children up to about two years old, the mastoid process is not yet developed and looks like a bone tubercle. However, a cave already exists in a newborn child.

All cavities of the middle ear (tympanic cavity, Eustachian tube and cells of the mastoid process) are filled with air, and their walls are lined with the thinnest mucous membrane, which is a continuation of the mucous membrane of the nasopharynx. Air exchange in the middle ear occurs through the Eustachian tube: during swallowing movements, air from the nasopharynx enters the Eustachian tube, and from there into the tympanic cavity and partly into the cells of the mastoid process.



The inner ear, or ear labyrinth, is a system of channels and cavities in the thickness of the temporal bone. This system consists of a vestibule, semicircular canals and a cochlea (see Fig. 4). There are bone (labyrinths) and membranous labyrinths, and the bone labyrinth is, as it were, a case for membranous. The membranous labyrinth is filled with a special fluid - endolymph, and the space between the membranous and bone labyrinths is also filled with a fluid - perilymph.



Fig. eleven.

Bone Maze Cast

:

1 - vestibule; 2 - upper semicircular canal; 3 - outer semicircular canal; 4 - rear semicircular canal; 5 - snail



The vestibule forms the central part of the labyrinth and consists of two webbed sacs: the anterior (round) and posterior (oval). The front sac communicates with the cochlea, and the back with semicircular canals.

There are three semicircular canals: upper, posterior, and external. They are located in three mutually perpendicular planes. One of the ends of each channel is smooth, and the other has an extension - an ampoule. The vestibule and semicircular canals form the so-called vestibular (from lat. Vestibulum - vestibule) apparatus and are the peripheral part of the spatial analyzer, or the organ of equilibrium. In the vestibule and semicircular canals are groups of specific nerve cells that form the terminal apparatus, or receptor, of the vestibular nerve. In the vestibule sacs, such a receptor is the otolith apparatus, i.e., terminal nerve cells covered by a membrane containing special crystals - otoliths. In semicircular canals, the receptor consists of specific hair nerve cells, which form a special scallop in the ampoule of each channel. Rectilinear movements cause displacement of otoliths in the sacs of the vestibule, and rotational (angular) movements are accompanied by the movement of endolymph in the semicircular canals and entail irritation of sensitive hair cells in ampullar combs. Irritations of the terminal apparatus are transmitted through the fibers of the vestibular nerve to the central nervous system. In response to them, reflex reactions occur that contribute to maintaining equilibrium. One of these reflex reactions is labyrinthine nystagmus, that is, the rhythmic movements of the eyeballs, consisting of two components - fast abduction and slow return to its original position. The direction of nystagmus is determined by its fast component.

The cochlea is a spiral bone canal that runs around the bone column and forms 2 1/2 curls (main, middle and upper), each subsequent curl being smaller than the previous one, so that the cochlea really resembles a garden snail in shape. The cochlear canal has a length of about 22 mm.

Along its entire length, the cochlear osseous canal is divided into two floors, called stairs. The boundary between them is a spiral bone ridge (Fig. 12) and an elastic membrane extending from the edge of this ridge - the main membrane (Fig. 13). The upper floor is called the vestibule (which leads to the vestibule), and it communicates with the tympanic cavity through an oval window. The lower floor is the drum ladder, which communicates directly with the drum cavity through a round window. At the top of the cochlea, the vestibular and tympanic stairs are connected to each other through a narrow opening. The vestibule is divided by two thin membranes, the so-called racer membrane, into two channels: the vestibule proper and the cochlear membrane, or cochlear passage. The cochlear canal is filled with endolymph, the vestibule and tympanic stairs are filled with perilymph.



Fig. 12.

Vertical section through the bone cochlea

:

1 - bone column; 2 - spiral bone crest; 3- entrance door; 4 - drum ladder



Fig. thirteen.

Cross section through one of the scrolls of a snail

:

1 - the main membrane; 2 - fibers of the auditory nerve; 3 - the bone wall of the cochlea; 4 - auditory (hair) cells; 5 - supporting cells; 6 - integumentary membrane; 7 - Reiner membrane; P - antechamber; B - a drum ladder; U - cochlear passage



In the cochlear course is the corti (spiral) organ. Its main functional part is the auditory cells, ending in sensitive hairs and therefore also called hair cells. These cells are arranged in several rows and represent the specific terminal apparatus of the auditory analyzer, or auditory receptor. There are over 20,000 auditory cells. In addition to auditory cells, the structure of the Corti's organ includes a support apparatus consisting of several rows of supporting cells. Above the Corti organ and at a very close distance from it there is a special membrane, the so-called integument, or Corti's membrane.

According to the latest data, there is a direct connection between the integumentary membrane and the auditory hair cells. The integumentary membrane closely approaches hair cells, and the auditory cell hairs penetrate into the tissue of the integumentary membrane. The organ of Corti is located on the main membrane, which consists of several thousand transverse fibers of different lengths stretched between the edge of the spiral bone ridge and the opposite wall of the cochlea. These fibers are very elastic, but weakly interconnected. In shape, the main membrane is a spirally curved tape, the width of which gradually increases from the base of the cochlea to its apex.
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Peripheral Hearing Analyzer

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