1.3. The middle ear
The middle ear is a cavity in the bone (the petrosal bone) containing the ossicular chain. This is a mechanical structure that transmits the vibration from the eardrum to the inner ear. The middle ear contains several parts that will be discussed in turn:
- The middle ear cavity
- The Eustachian tube
- The ossicles
- The facial nerve
- The chorda tympani.
The middle ear cavity, also called the ‘tympanic cavity’, is located in the petrosal bone. The cavity has the shape of a vertically standing biconvex disc with a height of about 15 mm, and a transverse dimension between the eardrum and the inner ear wall of only about 7 mm. The wall to the inner ear is called the ‘promontorium’ and can be seen as the ‘bump’ at the front of the oval and round windows.
The Eustachian tube forms the connection between the middle ear cavity and the nasopharynx. The skeleton consists of bone on the middle ear side and elastic cartilage on the nose-throat side. The length is about 35 mm, of which two-thirds is cartilaginous. In the adult, the tube runs obliquely downwards from the ear, such that the exit into the nasopharynx is 1 to 2.5 cm lower than the middle ear portion. In small children, the tube is shorter and the course is more horizontal. The opening into the nasopharynx is relatively low.
Normally, the mucous membrane-lined tube is kept closed by the elastic cartilage, but when swallowing or yawning, the tube opens briefly. This is done by two swallowing muscles attached to the outer wall, the ‘tensor veli palatini muscle’ and the ‘levator veli palatini muscle’. Mucous cells and cilia cells are present in the mucous membrane as they also occur in the epithelium of the nose and airways. The stroke of the cilia, intended for the discharge of substances, is directed towards the mouth in the nasopharynx.
In the mesotympanum, the ossicle chain hangs between the eardrum and the oval window. The three bones, the malleus (hammer), incus (anvil), and stapes (stirrup), are held in place in different ways. First of all, the malleus and stapes are attached to the eardrum and oval window, respectively. Furthermore, between the ossicles and the walls of the middle ear cavity are bundles of fibers (ligaments), two for the malleus, one for the incus, and an annular ligament connecting the stapes footplate to the oval window. Finally, there are two middle ear muscles. The ‘tensor tympani muscle’ is connected to the malleus stem (‘manubrium’) and runs parallel to the wall of the Eustachian tube.
- The malleus (hammer): The stem of the malleus is connected to the collagen fibers in the eardrum. The bone hangs from two ligaments attached to the malleus head, the ‘lateral malleus ligament’ and the ‘superior malleus ligament’. The suspension is such that the pivot point and the centre of gravity coincide, with the head acting as a counterweight. When the eardrum, and thus the malleus handle, move inwards, the malleus head moves out with the head of the incus, which rests against the malleus head. The pivot point of the incus is at the same height as that of the malleus. The malleus and incus rotate together approximately parallel to the eardrum. Attached to the handle of the malleus is a muscle that passes horizontally through the middle ear cavity through an opening in the bone of the inner wall (as if via a pulley) into the actual muscle, the ‘tensor tympani’. This tensor tympani muscle runs obliquely down to and then into the upper wall of the Eustachian tube.
- The incus (anvil): the incus has a shorter and a longer projection from a spherical body, the ‘corpus incudis’, which lies against the malleus head. The incus body is suspended by a ligament (the ‘superior incus ligament’) from the roof of the middle ear cavity. The projections are mutually perpendicular to each other. The upper short protrusion points horizontally backward (posterior) and is connected to the back wall of the middle ear cavity by a ligament, the ‘posterior incus ligament’. The long projection points more vertically, almost parallel to the malleus shank, and is connected to the top of the stapes. The connections with the malleus and the stapes consist of joints.
- The stapes (stirrup): the stapes footplate is movably attached to the oval window by a circular ligament. This ligament does not go all around. It is slacker at the front (anterior) than at the back (posterior). A tendon attaches to the neck of the stapes and becomes the actual muscle (the stapedius muscle), which lies in a bony canal in the back wall of the middle ear cavity. The stapedius muscle produces a tilting of the stapes.
The facial nerve (N.VII) deserves extra attention in the context of the middle ear. The facial nerve runs into the inner wall (medial wall) of the middle ear cavity in a bony canal, just above the oval window. It then bends about 90 degrees (the ‘bend in the canal’) and continues just behind the stapedius muscle. It is therefore a three-step process. The main function of the facial nerve is to take care of the motor skills of the face. With regard to hearing, the nerve is especially important as the origin of the motor innervation of the stapedius muscle. Furthermore, the ‘chorda tympani’, which will be discussed next, branches off from the facial nerve.
The ‘chorda tympani’ is a bundle of nerves that contains taste fibers originating in the anterior two-thirds of the tongue, and also fibers that innervate the salivary glands. Furthermore, there are autonomic (parasympathetic) fibers that influence the movement of the intestines (visceromotor fibers). This influence is achieved by activating the production of salivary glands medial to the lower jaw and under the tongue, ‘submandibular glands’ and ‘sublingual glands’, respectively.
The chorda tympani branches off the facial nerve where this nerve is located in the second tract (the facial canal). It enters the middle ear cavity through a narrow passage and arcs through this cavity, passing just behind the malleus shaft. The chorda then enters through a slit in the floor of the middle ear cavity into a descending canal (the ‘infratemporal fossa’) and then joins the fifth cranial nerve, the lingual nerve (Nerve V).
Next pages in section 1
1.4 The inner ear
Previous pages in section 1
1.2 The outer ear