The auditory ossicles

The three auditory ossicles in the tympanic cavity are called malleus, incus, stepes (hammer, anvil, stirrus), which they resemble in shape.

The malleus has a rounded head (caput mallei) which by means of the neck (collum mallei) is joined to the handle (manubrium mallei).

The incus has a body (corpus incudis) and 2 diverging processes, a short (crus breve) and a long process (crus longum). The long process projects ownwards just behind and parallel with the handle of the malleus. Its tip bears the lenticular process (processus lenticularis) which articulate with the stapes.

The stapes has a small head (caput stapedis), carring articulating surface for the lenticular process of the incus and 2 limbs: anterior limb (crus anterius) and posterior limb (crus posterius). The limbs are attached to an oval base (basis stapedis) fitted into the fenestra vestibuli.

In places where the auditory ossicles articulate with one another, 2 true joints of limited mobility are formed:

the incudomalleolar joint (articulatio incudomalleolaris)

the incudostapedial joint (articulation incudostapedia).

The base of stepes is joined with the edges of fenestra vestibule by tympanostapedial syndesmosis (syndesmosis tympanostapedia).

On the whole, all 3 ossicles form a more or less mobile chain running across the tympanic cavity from the tympanic membrane to the labyrinth.

Functions of the chain of ossicles:

1. The conduction of sound through the bones;

2. The mechanical transmission of ound vibrations to the fenestra cochlea.

Muscles, which regulate the movement of the chain of ossicles:

1). The tensor tympani muscle lies in the canal for the tensor tympany (semicanalis m.tensoris). Its tendon is fastened to the handle of the malleus near the neck. This muscle pulls the handle of the malleus medially, thus tensing the tympanic membrane. At the same time all the system of ossicles moves medially and the stapes presses into the fenestra cochlea.

2). The stapedius muscle is lodges in the pyramid of thee tympanum and fastened to the posterior limb of the stapes at the head. In function this muscle is an antagonist of the preceding one and accomplishes a reverse movement of the ossicle in the middle ear in the direction of the fenestra conhlea.

Functions of the muscles of the middle ear:

-maintain the normal tonus of the tympanic membrane and the chain of auditory assicles;

-protect the internal ear from excessive sound stimuli;

-accommodate the sound-conducting apparatus to sounds of different intensity and pitch.

-The auditory, or Eustachian, or pharyngotympanic tube (tuba auditiva (Eustachii)) lets the air pass from the pharynx into the tympanic cavity, thus equalizing the pressure in this cavity with the atmospheric pressure, which is essential for the proper conduction to the labyrinth of the vibrations of the tympanic membrane.

-The auditory tube consists of osseous and cartilaginous parts.

The bony part of the tube, beginning with its tympanic opening (ostium tympanicun tubae auditiva), occupies the largest inferior portion of the muscular-tube canal (semicanalis tubae auditivae) of the temporal bone.

The cartilaginous part, which is a continuation of the bony part, is formed of the elastic cartilage.

-The tube terminates on the lateral wall of the nasopharynx as the pharyngeal opening (ostium pharyngeum tubae auditivae).

THE INTERNAL EAR

The internal ear, or the labyrinth, lies in the petrous part of the temporal bone. It consists of the bony labyrinth within which there is a membranous labyrinth. The membranous labyrinth is filled with a fluid called endolymph. It is separated from the bony labyrinth by another fluid called the perilymph.

1. The bony labyrinth (labyrinthus osseus) comprises a number of very small intercommunicating cavities, whose walls of compact bone. It consists of three parts: the cochlea, anteriorly, the vestibule, in the middle, the semicircular canals, posteriorly.

Cochlea

Cochlea is a spiral bony canal (canalis spiralis cochleae).

It has a conical central axis (known as the modiolus) around which the cochlear canal makes 2 or 3 guarter turns.

The modius is directed forwards and laterally.

An osseous spiral lamina (lamina spiralis ossea) projects from the modiolus into the cavity of the canal along the entire length of its coils. The lamina divides the cavity of the cochlea into two sections: the scala vestibule which communicates with the vestibule, and the scala tympani which opens on the skeletized bone the cavity through the fenestra cochlea.

The scala vestibule communicates with the scala tympany at the apex of the cochlea by a small opening, called helicotrema.

Vestibute

This is the central part of the bony labyrinth.

Oval-shaped cavity of the labyrinth communicates in back through 5 openings with the semicircular canals.

In front it it communicates through a wider opening with the canal of the cochlea.

On the lateral vestibular wall facing the tympanic cavity is the mentioned above, the fenestra vestibulli, which is occupied by the base of the stapes.

Another opening is located at the beginning of the cochlea.