The adult temporal bone (French: os temporal) consists of three parts, so firmly united as to afford little trace of its complex origin. At birth the three parts are easily separable as the squamosal, petrosal, and tympanic.
The squamosal resembles a large scale; it is attached at right angles to the petrosal, and forms part of the side wall of the skull. It is thin, and in places translucent. The outer surface is smooth and forms part of the temporal fossa; it presents one, and occasionally two nearly vertical grooves for the deep temporal arteries. A ridge of bone, the supra-mastoid crest, runs immediately above the external auditory meatus, and is continued onwards to the zygoma.
The zygoma is a narrow projecting bar of bone, jutting forwards and lying parallel with the squamosal. It has two surfaces and two borders. The outer surface is subcutaneous; the inner looks towards the temporal fossa. The inner surface and lower border give origin to the masseter muscle. The upper border receives the temporal fascia. The tip of the zygoma is serrated for articulation with the malar. Posteriorly the lower border ends in a tubercle, which is the meeting point of two ridges ; of these, the anterior passes inwards at right angles to the zygoma, and expands into the articular eminence which serves as an articular facet for the condyle of the mandible when the month is opened. The second ridge runs backwards and forms the upper boundary of the glenoid fossa, and curving downwards ends in a tubercle, the post-glenoid tubercle, immediately anterior to the Glaserian fissure. The oval deep depression between these ridges is the glenoid fossa, which receives the condyle of the mandible. This fossa is limited posteriorly by the Glaserian fissure.
The inner surface of the squamosal presents furrows for the convolutions of the brain and grooves for the middle meningeal arteries. The line of union between the squamosal and petrosal is sometimes indicated by a persistent petro-squamosal suture. Rarely the two portions remain permanently separate.
The superior border of the squamosal is thin, and beveled on the cerebral surface where it overlaps the parietal; anteriorly it is serrated for the posterior border of the greater wing of the sphenoid. Posteriorly it joins the rough serrated margin of the petrosal to form the parietal notch.
The petrosal element is a four- sided pyramid of very dense bone; its base is formed by the mastoid process; the apex is rough and forms part of the boundary of the foramen lacerum medium. Two sides of the pyramid project into the cranial cavity, of which one forms the posterior boundary of the middle fossa, and the other the anterior boundary of the posterior fossa of the cranium. Of the two remaining surfaces, one appears on the under surface of the skull, and the fourth constitutes the inner wall of the recess called the tympanum.
The posterior surface is bounded above by the superior border, which serves for the attachment of the tentorium cerebelli, and is grooved for the superior petrosal sinus; near the apex, this border presents the trigeminal notch (converted into a foramen by the tentorium) for the transmission of the trigeminal nerve. This border in old skulls sometimes terminates in a spiculum of bone - the petro-sphenoidal process- and extends to the dorsum ephippii, and completes a foramen (petro-sphenoidal) which transmits the sixth nerve. Near the middle of the posterior surface is an oblique inlet, the internal auditory meatus, which receives the auditory and facial nerves and the auditory artery. The meatus is about 10 mm. deep, and to be properly examined the surface of the bone should be cut away, or the parts studied in the petrosal of a fetus at or near the ninth month, for it is at this date relatively large and shallow.
The fundus of the meatus is divided by a transverse ridge of bone, the falciform crest, into a superior and inferior fossa. Of these, the superior is the smaller, and presents anteriorly the beginning of the aqueduct of Fallopius; this transmits the seventh nerve. The rest of the surface above the crest is dotted with small foramina (the superior cribriform area) which transmit nerve-twigs to the fovea hemielliptica and the ampullae of the superior and external semicircular canals. Below the crest there are two depressions and an opening. Of these, an anterior curled tract (the spiral cribriform tract) with a central foramen (foramen centrale cochleare) marks the base of the cochlea; the central foramen indicates the orifice of the canal of the modiolus, and the smaller foramina transmit the cochlear twigs of the auditory nerve. The posterior opening (foramen singulare) is for the nerve to the ampulla of the posterior semicircular canal. The middle depression (middle cribriform area) is dotted with minute foramina for the nerve-twigs to the saccule, which is lodged in the fovea hemispherica. The inferior fossa is subdivided by a low vertical crest. The fossa in front of the crest is the fossula cochlearis, and the recess behind it is the fossula vestibularis.
Behind the meatus is a small slit (large in the fetus) which formerly lodged the aqueductus vestibuli (ductus endolymphaticus); in the adult it is occupied by a small arteriole and venule and a process of dura mater. Occasionally a bristle can be passed along this passage into the vestibule. Above, and anterior to this, is a second slit also lodging a process of the dura mater. This is a remnant of the floccular fossa, so conspicuous in the fetus. Posteriorly, this surface has a deep groove for the lateral sinus.
The anterior face of the pyramid is separated from the squamosal by the petro-squamous suture, which may persist throughout life. It presents the following points of interest: near the apex it has a shallow depression for the Gasserian ganglion, and the recess of dura mater (Meckel's cave) in which it lies; below this is the termination of the carotid canal. Behind these are two small foramina, overshadowed by a thin osseous hp. Of these, the larger and more internal is the hiatus Fallopii, which transmits a small artery from the middle meningeal and the greater petrosal nerve. The smaller and external foramen is for the lesser petrosal nerve. Still more externally there is a thin translucent plate of bone, the tegmen tympani. Behind, and slightly internal to this, there is a ridge formed by the superior semicircular canal.
The inferior or basilar surface is very irregular, and has the following points of interest. At the apex is a quadrilateral smooth space for the tensor tymimni and levator imlati muscles. Behind this is the large circular orifice of the carotid canal, for the transmission of the carotid artery and a plexus of sympathetic nerves. Internal to this, near the inner border of the bone, is the orifice of the aqueductus cochleae (ductus perilymphaticus). In the adult it transmits a small vein from the cochlea to the internal jugular. Posteriorly is the elliptical jugular fossa with smooth walls for the ampulla which receives the lateral and in- ferior petrosal sinuses, and forms the commencement of the internal jugular vein.
In the ridge of bone between the fossa and the carotid canal there is a small foramen, the tympanic canaliculus, for the tympanic branch of the glosso-pharyngeal nerve. On the inner wall of the fossa a similar minute foramen, the auricular canaliculus, permits the passage inwards of the auricular branch of the vagus nerve. Behind the fossa is the rough jugular surface, which receives the jugular process of the occipital. Firmly ankylosed to the inner surface of the tympanic plate is the styloid process, varying in length from one to five cm. At its base is the stylo-mastoid foramen, from which issues the facial nerve; the stylo-mastoid artery enters the Fallopian canal through this opening. Kunning backwards from this foramen are two grooves; the outer is the digastric groove, from which the digastric muscle arises. The inner is narrower and shallower; it lodges the occipital artery.
Of the outer surface, the only part which appears externally is the mastoid process; the rest is occupied by a recess known as the tympanum. The mastoid process is a nipple- shaped prominence of bone, formed partly by the squamosal, but mainly by the petrosal. Its upper limit is the supra-mastoid crest. Below the crest an irregular furrow crosses the surface of the process from the parietal notch downwards, to the middle of the meatus. This furrow (squamo-mastoid) is often dotted with holes, and represents the line of union of squamosal and petrosal. The mastoid process gives attachment to the sterno-mastoid, splenius capitis trachelo-mastoid, occipito -frontalis, and retrahens aurem muscles.
The tympanum is hidden by the tympanic folate, which extends downwards from the Glaserian fissure to form the vaginal process. Anteriorly it extends forwards and ankyloses with the outer wall of the carotid canal. The Glaserian fissure separates it from the squamosal. This fissure transmits the tympanic branch of the internal maxillary artery, and lodges the slender process of the malleus. A narrow subdivision of this fissure, canal of Huguier, is traversed by the chorda tympani nerve. The tympanic plate forms the anterior, lower, and part of the posterior walls of the external auditory meatus. It is limited posteriorly by the auricular fissure, through which the auricular twig of the vagus nerve issues.
The external auditory meatus assumes the form of an elliptical bony tube. Its outer margin is rough and gives attachment to the cartilaginous portion of the pinna. Between the posterior edge of the meatus and the mastoid process is the auricular fissure. The tympanic orifice of the meatus is smooth, and presents a well-marked groove for the tympanic membrane. This is very conspicuous in young bones. The direction of the meatus is somewhat oblique. In children, and occasionally in adults, a circular opening exists in the anterior wall of the meatus.
The temporal bone articulates with the occipital, parietal, sphenoid, malar, and by a movable joint with the mandible. Occasionally the squamosal presents a process which articulates with the frontal. A fronto-squamosal suture is common in the skulls of the lower races of men, and is normal in the skulls of the chimpanzee, gorilla, and gibbon.
The muscles connected with the temporal bone are:
To the mastoid process:
- Splenius capitis.
- Attrahens aurem.
- Retrahens aurem.
- Attollens aurem.
To the styloid process :
- To the zygoma:
- Intrisic muscles
- Tensor tympani.
- To the petrosal
- Levator palati.
- Capsular of temporo-mandibular joint.
- Interarticular of temporo-mandibular joint.
- Internal lateral of temporo-mandibular joint.
- External lateral of temporo-mandibular joint.
- Ligaments connected with the ear-bones:
- Anterior ligament of malleus (laxator tympani).
- External ligament of malleus.
- Superior ligament of malleus.
- Ligament of incus.
Arteries supplying the temporal bone are derived from various sources.
The chief are:
- Stylo-mastoid from posterior auricular: it enters the stylo-mastoid foramen.
- Tympanic from internal maxillary: it passes through the Glaserian fissure.
- Petrosal from middle meningeal: transmitted by the hiatus Fallopii.
- Tympanic from internal carotid whilst in the carotid canal.
- Auditory from the basilar: it enters the internal auditory meatus, and is distributed to the cochlea, vestibule, &c.
Other less important twigs are furnished by the middle meningeal, the meningeal branches of the occipital, and by the ascending pharyngeal artery. The squamosal is supplied on its internal face by the middle meningeal, and externally by the branches of the deep temporal from the internal maxillary.
The tympanum is an irregular cavity in the temporal bone. At birth it is a recess in the outer wall of the petrosal, partially closed externally by the squamosal. When the various elements of the temporal bone coalesce, and the tympanic plate becomes fully developed, then the cavity is completely surrounded by bony walls, except where it communicates with the external auditory meatus.
The roof, or tegmen tympani, is a translucent plate of bone belonging to the petrosal; it separates the tympanum from the middle fossa of the skull. The floor is the plate of bone which forms the roof of the jugular fossa.
The inner wall is formed by the external surface of the petrosal bone, and presents the following points for study: in the angle between it and the roof is a horizontal ridge which extends backwards as far as the posterior wall, and then turns down- wards in the angle between the inner and posterior wall. This is the fallopian canal; it is occupied by the facial nerve (seventh). Near the roof, but below the fallopian canal, is the fenestra ovalis, which leads into the vestibule: this fenestra receives the base of the stapes. Below the fenestra ovalis is the promontory, which contains the commencement of the first turn of the cochlea. In the lower and posterior part of the promontory is the fenestra rotunda; this, in the recent state, is closed by the secondary membrane of the tympanum. In the macerated bone it leads into the spiral canal of the cochlea. The promontory is also furrowed by some delicate channels (sometimes canals) for the tympanic branch of the glosso-pharyngeal nerve, which enters the tympanum through the tympanic canaliculus. The posterior Avail of the tympanum is formed by the mastoid process. At the superior and internal angle of this wall an opening leads into the mastoid antrum. Immediately below this opening there is a small hollow cone, the posterior pyramid; its cavity is continuous with the descending limb of the fallopian canal. One or more bony spiculae often connect the apex of the pyramid with the promontory. The cavity of this cone is occupied by the stapedius and the tendon of the muscle emerges at the apex.
The roof and floor converge towards the anterior extremity of the tympanum, which is, in consequence, very narrow, and occupied by two canals: the lower for the Eustachian tube, the upper for the tensor tympani muscle. These grooves are sometimes described together as the canalis musculo-tubarius. In carefully prepared bones the upper canal is a small horizontal hollow cone (anterior pyramid), 12 mm. in length, which lodges the tensor tympani muscle ; the apex is just in front of the fenestra ovalis, and is perforated to permit the passage of the tendon of the muscle. As a rule the thin walls of the canal are damaged, and represented merely by a thin ridge of bone. The posterior portion of this ridge projects into the tympanum, and is known as the processus cochleariformis. The thin septum between the canal for the tensor tympani and the tube is pierced by a narrow canal which is traversed by the small deep petrosal nerve. The outer wall is occupied mainly by the external auditory meatus. This opening is closed in the recent state by the tympanic membrane. The rim of bone to which the membrane is attached is incomplete above; the defect is known as the notch of Rivinus. Anterior to this notch, in the angle between the squamosal and the tympanic plate, is the Glaserian fissure, and the small passage which transmits the chorda tympani nerve sometimes called the canal of Huguier.
The tympanic cavity may be divided into three parts. The part below the level of the superior margin of the external auditory meatus is the tympanum proper; the portion above this level is the attic of the tympanum; it receives the head of the malleus, the body of the incus, and leads posteriorly into a recess known as the mastoid antrum.
The mastoid antrum
This is quite distinct from the mastoid cells. It is an air-chamber communicating with the attic of the tympanum. It is separated from the middle cranial fossa by the posterior portion of the tegmen tympani; the floor is formed by the mastoid portion of the petrosal; it communicates with the mastoid cells. The outer wall is formed by the squamosal below the supra-mastoid crest. In children the outer wall is exceedingly thin, but in the adult it is of considerable thickness. The external semicircular canal projects into the antrum on its inner wall, and is very conspicuous in the fetus.
A canal occasionally leads from the mastoid antrmii through the petrous bone to open in the recess which indicates the position of the floccular fossa; it is termed the petro- mastoid canal. (Gruber.)
The Fallopian canal
This canal begins at the anterior angle of the superior fossa of the internal auditory meatus, and passes directly outwards to the hiatus Fallopii; it then turns abruptly backwards and forms a horizontal ridge on the inner wall of the tympanum lying in the angle between it and the tegmen tympani. It passes immediately above the fenestra ovalis, and extends as far backwards as the entrance to the mastoid antrum; here it comes into contact with the inferior aspect of the projection formed by the external semicircular canal. It then tm-ns vertically downwards, running in the angle between the internal and posterior wall of the tympanum to terminate at the stylo-mastoid foramen.
The canal is traversed by the facial (seventh) nerve. Numerous openings exist in the walls of this passage. At the hiatus the greater and smaller superficial petrosal nerves escape from, and a branch from the middle meningeal artery enters, the canal. In the vertical part of its course the cavity of the posterior pyramid opens into it. There is also a small orifice by which the auricular branch of the vagus joins the facial, and near its termination the iter chordae posterius for the chorda tympani nerve leads from it into the tympanum.
This is an oval chamber situated between the base of the internal auditory meatus and the inner wall of the tympanum, with which it communicates by way of the fenestra ovalis. Anteriorly the vestibule leads into the cochlea, and posteriorly it receives the extremities of the semicircular canals. It measures about 3 mm. transversely, and is somewhat longer antero-posteriorly.
Its inner wall presents at the anterior part a circular depression, the fovea hemispherica, which is finely perforated for the passage of nerve-twigs. This fovea is separated by a vertical ridge (the crista vestibuli) from the vestibular orifice of the aqueductus vestibuli (ductus endolymphaticus), which passes obliquely backwards to open on the posterior surface of the petrosal bone.
The roof contains an oval depression - the fovea hemielliptica. Anteriorly the vestibule leads into the cochlea. Posteriorly it receives the five openings of the semicircular canals.
The semicircular canals are three in number. Each forms about two-thirds of a circle; they lie in different planes. One extremity of each canal is dilated to form an ampulla.
The superior canal lies transversely to the long axis of the petrosal, and is nearly vertical; its highest limb makes a projection on the anterior surface of the bone. The ampulla is at the outer end; the inner end opens into the vestibule conjointly with the superior limb of the posterior canal.
The posterior canal is nearly vertical and lies antero-posteriorly. It is the longest of the three; its upper extremity joins the inner limb of the superior canal, and opens in common with it into the vestibule. The lower is the ampullated end.
The external canal is placed horizontally and arches outwards; its external limb forms a prominence in the mastoid antrum. This canal is the shortest; its ampulla is at the outer end near the fenestra ovalis.
This is a cone-shaped cavity lying with its base upon the internal auditory meatus, and the apex directed outwards. It measures about five millimeters in length, and the diameter of its base is about the same. The center of this cavity is occupied by a column of bone - the modiolus - around which a delicate bony lamella appears to be wound. This lamella is the osseous spiral lamina, which gives attachment to the structures which form collectively the membranous cochlea. The lamina makes two and a half turns in all. The first turn is the largest, and forms a bulging, the promontory, on the inner wall of the tympanum . The lamina terminates at the apex of the cochlea in a hooldike process - the hamulus.
The modiolus is traversed by a central canal, and presents many canaliculi for the transmission of the twigs of the cochlear division of the auditory nerve. There is also a canal which winds round the modiolus at the base of the spiral lamina, known as the spiral canal of the modiolus.
The portion of the cochlea above the lamina is the scala vestibuli ; the part below, that is, on the basal aspect of the lamina, is the scala tympani ; it opens into the tympanum by way of the fenestra rotunda. Near the commencement of the scala tympani, and close to the fenestra rotunda, is the cochlear orifice of the aqueductus cochleae (ductus perilymphaticus). In the adult this opens on the inferior surface of the petrosal near the apex, and transmits a small vein from the cochlea to the jugular fossa.
Measurements of the principal parts connected with the auditory organs
Internal auditory meatus:
Length of anterior wall, 13-14 mm.
Length of posterior wall, 6.7 mm.
External auditory meatus . . 14-16 mm. (Gruber.)
Length, 13 mm.
Height in center of cavity, 15 mm.
Width opposite the membrana tympani, 2 mm.
Width opposite the tubal orifice, 3-4 mm.
Superior semicircular canal measures along its convexity 20 mm.
The posterior semicircular canal measures along its convexity 22 mm.
The external semicircular canal measures along its convexity 15 mm.
The canal is in diameter 1.5 mm. (Buschke.)
The ampulla of the canal 2.5 mm.
The Ossification of the Temporal Bone
At birth the temporal bone consists of three parts easily separable in the macerated j skull : they are the petrosal, squamosal, and the tympanic. The styloid process is cartilaginous with the exception of its basal element, the tympano-hyal, which, with the ear-bones, will be described with the appendicular elements of the cranium. I The squamosal and tympanic bones develop in membrane. The squamosal is formed from one center, which appears as early as the eighth week. Ossification extends into the zygoma, which grows concurrently with the squamosal. At first the tympanic border is nearly straight, but soon assumes its characteristic horseshoe shape. At birth the post- glenoid tubercle is conspicuous, and at the hinder end of the squamosal there is a recess where it conies into relation with the mastoid antrum. The center appears for the tympanic bone about the twelfth week. At birth it is a horseshoe-shaped ossicle slightly ankylosed to the lower border of the squamosal; the open arms being directed upwards. The tip of the anterior arm terminates in a small irregular process, and the inner aspect presents, in the lower half of its circumference, a groove for the reception of the tympanic membrane.
Up to the middle of the fifth month the periotic capsule is cartilaginous; it then ossifies so rapidly that by the end of the sixth month its chief portion is converted into porous bone. The ossific material is deposited in four centers, or groups of centers, named according to their relation to the ear-capsule in its embryonic position.
The nuclei are deposited in the following order:
- The opisthotic appears at the end of the fifth month. The osseous material is seen first on the promontory, and it quickly surrounds the fenestra rotunda from above down- wards, and forms the floor of the vestibule, the lower part of the fenestra ovalis, and the internal auditory meatus, it also invests the cochlea. Subsequently a plate of bone arises from it to surround the internal carotid artery and form the floor of the tympanum.
- The pro-otic nucleus is deposited behind the internal auditory meatus near the inner limb of the superior semicircular canal. It covers in a part of the cochlea, the vestibule, and the internal auditory meatus, completes the fenestra ovalis, and invests the superior semicircular canal.
- The pterotic nucleus ossifies the tegmen tympani and covers in the external semi- circular canal; the ossific matter is first deposited over the outer limb of this canal.
- The epiotic is the last to appear, and is first seen at the most posterior part of the posterior semicircular canal; often it is double. This center gives rise to the mastoid process.
At birth the bone is of loose and open texture, resembling biscuit or unglazed porcelain, thus offering a striking contrast to the dense and ivory-like petrosal of the adult. It also differs from the adult bone in several other particulars. The floccular fossa is widely open and conspicuous. Voltolini has pointed out that a small canal leads from the floor of the floccular fossa and opens posteriorly on the mastoid surface of the bone: it may open in the mastoid antrum. The hiatus Fallopii is unclosed, and the tympanic recess is filled with gelatinous connective tissue. The mastoid process is not developed, and the jugular fossa is a shallow depression.
After birth the parts grow rapidly. The tympanum becomes permeated with air; the various elements fuse ; and the tympanic annnlus grows rapidly and forms the tympanic plate. Growth in the tympanic bone takes place most rapidly from the tubercles at its upper extremities, and in consequence of the slow growth of the lower segment a deep notch is formed; gradually the tubercles coalesce, leaving a foramen in the anterior part of the bony meatus which persists until puberty, and even in the adult. In most skulls a cleft capable of receiving the nail remains between the tympanic element and the mastoid process; this is the auricular fissure. The anterior portion of the tympanic plate forms with the inferior border of the squamosal a cleft known as the Glaserian fissure, which is subsequently encroached upon by the growth of the petrosal. As the tympanic plate increases in size it joins the outer wall of the carotid canal and presents a prominent lower edge, known as the vaginal process.
The mastoid process becomes distinct about the first year, coincident with the obliteration of the petro- squamous suture. It increases in thickness by deposit from the periosteum. Towards puberty, rarely earlier, the process becomes pneumatic, the air-cells being lined by delicate mucous membrane. In old skulls the air-cells may extend into the jugular process of the occipital bone.
At birth the mastoid antrum is relatively large and bounded externally by a thin plate of bone belonging to the squamosal. As the mastoid increases in thickness the antrum comes to lie at a greater depth from the surface and becomes relatively smaller.
In 20 per cent, of skulls there are no air-cells in the mastoid process.
Internal view of the right temporal bone