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.