Chapter 57
Section 57 of Grey's Anatomy, Part 4. For more information or to volunteer, please visit LibriVox.org. Recording by Anna Simon. Anatomy of the Human Body, Part 4 by Henry Grey. 1D.2
The Middle Ear or tympanic cavity. Cavan tympani. Drum. Tympanum. The Middle Ear or tympanic cavity is an irregular, laterally compressed space within the temporal bone. It is filled with air, which is conveyed to it from the nasal part of the pharynx through the auditory tube. It contains a chain of movable bones which connect its lateral to its medial wall
and serve to convey the vibrations communicated to the tympanic membrane across the cavity to the internal ear. The tympanic cavity consists of two parts. The tympanic cavity proper, opposite the tympanic membrane, and the attic or epitympanic recess, above the level of the membrane. The latter contains the upper half of the malleus and the greater part of the incus.
Including the attic, the vertical and antroposterior diameters of the cavity are each about 15 mm. The transverse diameter measures about 6 mm above and 4 mm below. Opposite the center of the tympanic membrane it is only about 2 mm. The tympanic cavity is bounded laterally by the tympanic membrane, medially by the lateral wall of the internal ear.
It communicates behind with the tympanic antrum and through it with the mastoid air cells and in front with the auditory tube. The tegmental wall or roof, paeus tegmentalis, is formed by a thin plate of bone, the tegment tympani, which separates the cranial and tympanic cavities. It is situated on the interior surface of the petrous portion of the temporal bone, close to its angle of junction with the squama temporalis.
It is prolonged backward so as to roof in the tympanic antrum, and forward to cover in the semicanal for the tensor tympani muscle. Its lateral edge corresponds with the remains of the petrous squamous suture. The jugular wall or floor, paeus jugularis, is narrow and consists of a thin plate of bone, fundus tympani, which separates the tympanic cavity from the jugular fossa.
It presents, near the labyrinthic wall, a small aperture for the passage of the tympanic branch of the glossoverential nerve. The membranous or lateral wall, paeus membranaceae, outer wall, is formed mainly by the tympanic membrane, partly by the ring of bone into which this membrane is inserted. This ring of bone is incomplete at its upper part, forming a notch, notch of rivinous, close to which are three small apertures, the aetacordy posterioris, the petrotimpanic fissure, and the aetacordy anterioris.
The aetacordy posterioris, aperture tympanica canaliculi cordi, is situated in the angle of junction between the mastoid and membranous wall of the tympanic cavity, immediately behind the tympanic membrane and on a level with the upper end of the manubrium of the malleus. It leads into a minute canal which descends in front of the canal for the facial nerve, and ends in that canal near the stylomastoid foramen.
Through it the cordatimpeni nerve enters the tympanic cavity. The petrotimpanic fissure, fissura petrotimpanica, glacerian fissure, opens just above and in front of the ring of bone into which the tympanic membrane is inserted. In this situation it is a mere slit about two millimetres in length. It lodges the anterior process and anterior ligament of the malleus,
and gives passage to the anterior tympanic branch of the internal maxillary artery. The aeteracordi anterioris, canal of Hugier, is placed at the medial end of the petrotimpanic fissure. Through it the cordatimpeni nerve leaves the tympanic cavity. The tympanic membrane, membrana tympani, separates the tympanic cavity from the bottom of the external acoustic meatus. It is a thin, semi-transparent membrane, nearly oval in form,
somewhat broader above than below, and directed very obliquely downward and inward, so as to form an angle of about 55 degrees with the floor of the meatus. Its longest diameter is downward and forward, and measures from 9 to 10 millimetres. Its shortest diameter measures from 8 to 9 millimetres. The greater part of its circumference is thickened, and forms a fibrocartilaginous ring,
which is fixed in the tympanic sulcus at the inner end of the meatus. This sulcus is deficient superiorly at the notch of rivinus, and from the ends of this notch, two bands, the anterior and posterior malleolar folds, are prolonged to the lateral process of the malleus. The small, somewhat triangular part of the membrane, situated above these folds, is lax and thin, and is named the pars flaccidae.
In it, a small orifice is sometimes seen. The manubrium of the malleus is firmly attached to the medial surface of the membrane as far as its centre, which it draws toward the tympanic cavity. The lateral surface of the membrane is thus concave, and the most depressed part of this concavity is named the umbo.
Structure The tympanic membrane is composed of three strata, a lateral, cutaneous, an intermediate, fibrous, and a medial, mucus. The cutaneous stratum is derived from the integument lining the meatus. The fibrous stratum consists of two layers, a radiate stratum,
the fibres of which diverge in the manubrium of the malleus, and a circular stratum, the fibres of which are plentiful around the circumference, but sparse and scattered near the centre of the membrane. Branched or dendritic fibres, as pointed out by Grube, are also present, especially in the posterior half of the membrane.
Vessels and nerves The arteries of the tympanic membrane are derived from the deep auricular branch of the internal maxillary, which ramifies beneath the cutaneous stratum, and from the stylomaster branch of the posterior auricular, and tympanic branch of the internal maxillary, which are distributed on the mucous surface.
The superficial veins open into the external jugular, those on the deep surface drain partly into the transverse sinus and veins of the duromator, and partly into a plexus on the auditory tube. The membrane receives its chief nerve supply from the auriculotemporal branch of the mandibula, the auricular branch of the vagus, and the tympanic branch of the glossopharyngeal also supplied.
The labyrinthic or medial wall, Peris labyrinthica, inner wall, is vertical in direction, and presents for examination the fenestri vestibuli and cochleae, the promontory and the prominence of the facial canal. The fenestri vestibuli, fenestra ovalis, is a reniform opening leading from the tympanic cavity into the vestibule of the internal ear.
Its long diameter is horizontal, and its convex border is upward. In the recent state it is occupied by the base of the stapes, the circumference of which is fixed by the annual ligament to the margin of the foramen. The fenestra cochleae, fenestra rotunda, is situated below and a little behind the fenestra vestibuli, from which it is separated by a rounded elevation, the promontory.
It is placed at the bottom of a funnel-shaped depression, and, in the mesurated bone, leads into the cochlea of the internal ear. In the fresh state it is closed by a membrane, the secondary tympanic membrane, which is concave towards the tympanic cavity, convex towards the cochlea. This membrane consists of three layers, an external, or mucus,
derived from the mucus lining of the tympanic cavity, an internal from the lining membrane of the cochlea, and an intermediate, or fibrous layer. The promontory, promontorium, is a rounded hollow prominence formed by the projection outward of the first tone of the cochlea. It is placed between the fenestra, and it is furrowed on its surface by small grooves
for the lodgment of branches of the tympanic plexus. A minute spicule of bone frequently connects the promontory to the pyramidal eminence. The prominence of the facial canal, prominencia canalis fascialis, prominence of aqueduct of fallopius, indicates the position of the bony canal in which the facial nerve is contained.
This canal traverses the labyrinthic wall of the tympanic cavity above the fenestra vestibuli, and behind that opening curves nearly vertically downward along the mastoid wall. The mastoid, or posterior wall, perius mastoidea, is wider above than below, and presents for examination the entrance to the tympanic antrum, the pyramidal eminence, and the fossa incudis.
The entrance to the antrum is a large irregular aperture, which leads backward from the epitympanic recess into a considerable airspace, named the tympanic or mastoid antrum. The antrum communicates behind and below with the mastoid air cells, which vary considerably in number, size, and form.
The antrum and mastoid air cells are lined by mucous membrane, continuous with that lining the tympanic cavity. On the medial wall of the entrance to the antrum is a rounded eminence, situated above and behind the prominence of the facial canal. It corresponds with the position of the ampullated ends of the superior and lateral semicircular canals.
The pyramidal eminence, eminencea pyramidalis, pyramid, is situated immediately behind the fenestervestibuli, and in front of the vertical portion of the facial canal. It is hollow and contains the stapedius muscle. Its summit projects forward toward the fenestervestibuli,
and is pierced by a small aperture which transmits the tendon of the muscle. The cavity in the pyramidal eminence is prolonged downward and backward in front of the facial canal, and communicates with it by a minute aperture which transmits a twig from the facial nerve to the stapedius muscle. The fossa incudis is a small depression in the lower and back part of the epitempanic recess. It lodges the short crust of the incus.
The carotid or anterior wall, perius carotica, is wider above than below. It corresponds with the carotid canal, from which it is separated by a thin plate of bone perforated by the tympanic branch of the internal carotid artery and by the deep petrosal nerve which connects the sympathetic plexus on the internal carotid artery with the tympanic plexus on the promontory.
At the upper part of the anterior wall are the orifice of the semicanal for the tensor tympani muscle and the tympanic orifice of the auditory tube, separated from each other by a thin horizontal plate of bone, the septum canalis musculatuburi. These canals run from the tympanic cavity forward and downward to the retiring angle between the squama and the petrous portion of the temporal bone.
The semicanal for the tensor tympani, semicanalis and tensoris tympani, is the superior and the smaller of the two. It is cylindrical and lies beneath the tecmon tympani. It extends on to the labyrinthic wall of the tympanic cavity and ends immediately above the fenestre vestibuli.
The septum canalis musculatuburi processes cochleariformis, passes backward below the semicanal, forming its lateral wall and floor. It expands above the anterior end of the fenestre vestibuli and terminates there by curving laterally so as to form a pulley over which the tendon of the muscle passes.
The auditory tube, tuba auditiva, eustagin tube, is a channel through which the tympanic cavity communicates with the nasal part of the pharynx. Its length is about 36 mm and its direction is downward, forward and medial wood, forming an angle of about 45 degrees with the sagittal plane and one from 30 to 40 degrees with the horizontal plane.
It is formed partly of bone, partly of cartilage and fibrous tissue. The osseous portion, pars osseotubi auditivi, is about 12 mm in length. It begins in the carotid wall of the tympanic cavity below the septum canalis musculatuburi and gradually narrowing ends at the angle of junction of the squama and the petrous portion of the temporal bone, its extremity presenting a jagged margin which serves for the attachment of the cartilaginous portion.
The cartilaginous portion, pars cartilaginia tubi auditivi, about 24 mm in length, is formed of a triangular plate of elastic fiber cartilage, the apex of which is attached to the margin of the medial end of the osseous portion of the tube, while its base lies directly under the mucous membrane of the nasal part of the pharynx where it forms an elevation, the torus tuberius, or cushion, behind the pharyngeal orifice of the tube.
The upper edge of the cartilage is curled upon itself, being bent laterally so as to present on transverse section the appearance of a hook. A groove or furrow is thus produced which is opened below and laterally, and this part of the canal is completed by a fibrous membrane. The cartilage lies in a groove between the petrous part of the temporal and the great wing of the suenoid. This groove ends opposite the middle of the medial pterygoid plate. The cartilaginous and bony portions of the tube are not in the same plane, the former inclining downward a little more than a letter.
The diameter of the tube is not uniform throughout, being greatest at the pharyngeal orifice, least at the junction of the bony and cartilaginous portions, and again increased towards the tympanic cavity. The narrowest part of the tube is termed the isthmus. The position and relation to the pharyngeal orifice are described with the nasal part of the pharynge. The mucous membrane of the tube is continuous in front with that of the nasal part of the pharynx and behind with that of the tympanic cavity. It is covered with ciliated epithelium and is thin in the osseous portion, while in the cartilaginous portion it contains many mucous glands,
and near the pharyngeal orifice a considerable amount of adenoid tissue, which has been named by Gerlach the tube tonsil. The tube is opened during declination by the salpingo pharyngeus and dilatator tubae. The latter arises from the hook of the cartilage and from the membranes part of the tube, and blends below with the tensor villi palatini.
