Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Sense organs, organa sensuum
Vestibulocochlear organ - Middle ear

The Middle ear, auris media (Fig. 220), consists of the tympanic cavity, cavitas tympani, with its contents, the auditory tube, tuba auditiva, and the pneumatic spaces of the mastoid process of the Temporal bone, namely the mastoid antrum, antrum, and mastoid Cells, cellulae mastoideae.

The tympanic cavity is located between the external acoustic meatus and the bony labyrinth. Its volume is 1—2 cm³. The distance between its lateral and medial walls is about 3 mm in the anterior part and 6.5 mm in the posterior part. This is of practical importance when performing a paracentesis (tympanocentesis), which should be carried out in the posterior region of the membrane, as there is less risk of injuring the labyrinth with a sharp instrument.

The tympanic cavity has six walls. The lateral wall, paries membranaceus, is formed by the tympanic membrane, membrana tympani, and the surrounding bone.

The tympanic membrane is an oval-shaped membrane 1 mm thick (Fig. 221). Its longitudinal dimension, extending from superior to inferior and anteroposteriorly, is 9-10 mm, while its transverse dimension is 8-9 mm. Its inferior margin is located 5-8 mm more medially than the superior one, whereas in newborns the tympanic membrane lies almost horizontally.

Two parts are distinguished in the tympanic membrane: the pars flaccida (1/8 of its surface area), which is attached to the notch on the squama ossis temporalis, and the pars tensa (7/8 of its surface area). The pars tensa consists of three layers: 1) external — the cutaneous layer, stratum cutaneum; 2) middle — the fibrous layer, stratum fibrosum; 3) internal — the mucous layer, stratum mucosum. The middle layer is absent in the pars flaccida.

In the center, the tympanic membrane is funnel-shaped and drawn inward. This depression is called the umbo of the tympanic membrane, umbo membranae tympanicae. Superior to the tympanic membrane lies the epitympanic recess, recessus epitympanicus, which houses the HEAD of the malleus and the incus.

In otolaryngological practice, the tympanic membrane is divided into four quadrants (Fig. 221 B) by two intersecting lines. One line runs along the handle of the malleus down to the inferior margin of the membrane, while the second runs perpendicularly to the first through the umbo. These quadrants serve as landmarks when selecting a site for paracentesis.

The medial wall of the tympanic cavity (Fig. 222) separates it from the labyrinth and is called the labyrinthine wall, paries labyrinthicus. In its center lies the promontory, promontorium. Superior to it is an oval opening — the vestibular window (oval window), fenestra vestibuli, which leads into the vestibule. The vestibular window is closed by the Base of the stapes. Inferior to the promontory is the cochlear window (round window), fenestra cochleae, which leads to the beginning of the cochlea and is closed by the secondary tympanic membrane, membrana tympanica secundaria.

Superior to the vestibular window lies the tympanic part of the facial canal. The canal frequently communicates with the tympanic cavity via small clefts known as dehiscences, in the area of which the epineurium of the nerve lies directly against the mucous membrane of the tympanic cavity. This is why inflammation of the middle ear can lead to Facial Nerve palsy.

The anterior wall of the tympanic cavity is called the carotid wall, paries caroticus. Its upper part opens into the tympanic ostium of the auditory tube, ostium tympanicum tubae auditoriae. The carotid wall is perforated by 2-3 caroticotympanic canaliculi, through which pass the corresponding Branches of the carotid artery and sympathetic nerve fibers from its plexus. This wall also frequently exhibits dehiscences, which create a risk of injuring the carotid artery during paracentesis.

The posterior wall is termed the mastoid wall, paries mastoideus. It bears the pyramidal eminence, eminentia pyramidalis. Lateral to the latter, the canaliculus for the chorda tympani opens. Deep within the posterior wall, posterior to the eminentia pyramidalis, lies the mastoid segment of the facial canal, surrounded by compact bone. The posterior wall also contains the aditus to the mastoid antrum, aditus ad antrum (Fig. 223). This is a narrowed passage of the middle ear's pneumatic system situated between the epitympanic space (attic or epitympanum in clinical terminology) and the mastoid antrum. The aditus ad antrum helps maintain a constant intratympanic pressure balanced with atmospheric pressure, as well as providing drainage (O. K. Patyakina, 1982; O. I. Yashcha, 1999).

The inferior wall of the tympanic cavity is called the jugular wall, paries jugularis. It lies 2-3 mm lower than the corresponding margin of the tympanic membrane. Consequently, during otitis media, a small amount of effusion can accumulate in the floor of the tympanic cavity without coming into the physician's field of view.

The superior wall, paries tegmentalis, simultaneously forms part of the floor of the middle cranial fossa. Dehiscences are frequently found in this wall, allowing the dura mater to come into contact with the mucous membrane of the tympanic cavity. In such cases, infection can spread from the middle ear into the cranial cavity, potentially leading to meningitis.

Class="center">

Fig. 221. Tympanic membrane, membrana tympani:

A - normal tympanic membrane (external view); B - tympanic membrane during otoscopy; C - division of the tympanic membrane into quadrants (diagram)

Fig. 222. Medial wall of the tympanic cavity

The auditory ossicles, ossicula auditoria (Fig. 224), are located in the upper part of the tympanic cavity and are connected by joints to form a chain extending from the tympanic membrane to the vestibular window.

The malleus consists of a long handle (manubrium), manubrium mallei, a short lateral process, processus lateralis, a long and slender anterior process, processus anterior, a neck, collum mallei, and a head, caput mallei.

The incus consists of a body, corpus incudis, and a short and long limb (crus), crus breve et crus longum. The latter features a terminal enlargement known as the lenticular process, processus lenticularis.

The stapes consists of a head, caput stapedis, anterior and posterior limbs (crura), crus anterius et crus posterius, and a base (footplate), basis stapedis.

The malleus is attached to the tympanic membrane by its handle (specifically, the tip of the handle draws the pars tensa inward, forming the umbo) and its short process. The anterior process is anchored by a ligament to the petrotympanic fissure. The head of the malleus articulates with the body of the incus via the incudomallear joint, articulatio incudomallearis. The long process of the incus articulates with the head of the stapes, forming the incudostapedial joint, articulatio incudostapedialis. The base of the stapes is secured within the vestibular window by the annular ligament, lig. anulare stapediale.

Fig. 223. Cavity of the mastoid antrum (right ear). Area — 85 mm². Axial computed tomography scan at the level of the lateral semicircular canal

The tensor tympani Muscle, m. tensor tympani, occupies the semicanal of the same name in the temporal bone. It originates from the walls of this canal and the Cartilage of the auditory tube, and inserts into the handle of the malleus near its base. The muscle's function is evident from its name.

The stapedius muscle, m. stapedius, originates from the interior of the eminentia pyramidalis and inserts into the head of the stapes. This muscle acts as an antagonist to the previous one.

The auditory tube, tuba auditoria*, connects the tympanic cavity with the nasopharyngeal cavity. The course of the auditory tube is directed forward, inward, and downward. Its tympanic opening lies 2 cm higher than its pharyngeal opening. In children, the auditory tube is shorter and wider than in adults and has a more horizontal orientation.

The auditory tube is divided into a bony part, pars ossea (lateral third), and a cartilaginous part, pars cartilaginous (medial 2/3 of its length) (see Fig. 220). The junction between the bony and cartilaginous parts, the isthmus tubae auditoriae, is the narrowest point. The pharyngeal opening of the tube and its cartilaginous part are normally collapsed, but they open during swallowing and chewing, which facilitates the passage of air into the tympanic cavity and equalizes the air pressure within it relative to the atmospheric pressure.

The mucous membrane of the auditory tube is lined with ciliated epithelium, the cilia of which beat toward the nasopharynx. The mucosa of the cartilaginous part of the tube contains numerous mucous glands, while the region of the pharyngeal opening contains lymphoid tissue that forms the tubal tonsil. These mucosal structures protect the tympanic cavity from the penetration of infection from the Pharynx. An important appendage of the tympanic cavity is the air spaces of the mastoid process—cellulae mastoideae. Their size and number vary considerably.

* Former name (Eustachian tube); hence, tubal inflammation. Bartolomeo Eustachi (Eustachio) (1510–1547) was a professor of anatomy and medicine in Rome. The auditory tube is named after him.

Fig. 224. Auditory ossicles, ossicula auditoria

In the newborn, a single air Cell—the mastoid antrum, antrum mastoideum—develops within the mastoid process. Later, As a result of pneumatization, the other Cells of the process develop.

Fig. 225. Diploic Structure type of the mastoid process of the temporal bone. Mastoid antrum (right ear) with an area of 68 mm².

An axial computed tomography scan at the level of the lateral semicircular canal shows that the mastoid cells occupy the entire mastoid process and are also located in the walls of the tympanic cavity, squama et processus zygomaticus. In the diploic-type mastoid process, groups of cells are separated by thick bony septa (Fig. 225). The sclerotic-type mastoid process consists of compact Bone tissue. A combination of these three described forms produces a mixed-type process. These Anatomical Features must be kept in mind during mastoid surgeries.

The middle ear is vascularized by branches from the External Carotid Artery system: 1) the inferior tympanic artery (a branch of the ascending pharyngeal artery); 2) the posterior tympanic artery (a branch of the stylomastoid artery, which in turn is a branch of the posterior auricular artery); 3) the anterior tympanic artery (a branch of the maxillary artery); 4) the superior tympanic artery (a branch of the middle meningeal artery). From the Internal Carotid Artery system, the middle ear is supplied by the caroticotympanic Arteries, aa. caroticotympanici.

The Arteries of the middle ear are accompanied by Veins through which venous Blood drains into the pterygoid plexus, pharyngeal plexus, and superior petrosal sinus. Lymphatic drainage flows into the retropharyngeal and mastoid Lymph Nodes, nodi lymphatici retropharyngeales et mastoidei.

Sensory Innervation of the middle ear is provided by fibers of the tympanic nerve (a branch of the Glossopharyngeal nerve). The tensor tympani muscle is innervated by the Trigeminal nerve, and the stapedius muscle by the facial nerve.