Anterior surface The anterior surface forms the posterior part of the
middle cranial fossa of the base of the skull, and is continuous with the inner surface of the squamous portion, to which it is united by the
petrosquamous suture, remains of which are distinct even at a late period of life. It is marked by depressions for the convolutions of the brain, and presents six notable points: • near the center, the
arcuate eminence (eminentia arcuata), which indicates the location of the
superior semicircular canal. • in front of and a little lateral to this eminence, a depression indicating the position of the
tympanic cavity: Here, the layer of bone that separates the tympanic from the cranial cavity is extremely thin, and is known as the
tegmen tympani • a shallow groove, sometimes double, leading lateralward and backward to an oblique opening, the
hiatus for greater petrosal nerve, for the passage of the greater petrosal nerve and for the
petrosal branch of the middle meningeal artery • lateral to the hiatus, a smaller opening, occasionally seen, for the passage of the
lesser superficial petrosal nerve • near the apex of the bone, the termination of the
carotid canal, the wall of which in this situation is deficient in front • above this canal the shallow trigeminal impression for the reception of the
trigeminal ganglion.
Posterior surface shown at (2) in blue The
posterior surface forms the anterior part of the
posterior cranial fossa of the base of the skull, and is continuous with the inner surface of the mastoid portion. Near the center is a large orifice, the
internal acoustic opening, the size of which varies considerably; its margins are smooth and rounded, and it leads into the
internal auditory meatus a short canal, about 1 cm. in length, which runs lateralward. It transmits the facial and acoustic nerves and the internal auditory branch of the
basilar artery. The lateral end of the canal is closed by a vertical plate, which is divided by a horizontal crest, the
falciform crest, into two unequal portions. Each portion is further subdivided by a vertical ridge into an anterior and a posterior part. • In the
portion beneath the falciform crest are three sets of foramina; these openings together with this central canal transmit the nerves to the
cochlea. • one group, just below the posterior part of the crest, situated in the
area cribrosa media, consists of several small openings for the nerves to the
saccule; • below and behind this area is the
foramen singulare, or opening for the nerve to the
posterior semicircular duct; • in front of and below the first is the
tractus spiralis foraminosus, consisting of a number of small spirally arranged openings, which encircle the
canalis centralis cochleæ. • The
portion above the crista falciformis presents behind, the area cribrosa superior, pierced by a series of small openings, for the passage of the nerves to the
utricle and the superior and lateral semicircular ducts, and, in front, the area facialis, with one large opening, the commencement of the canal for the facial nerve (aquæductus Fallopii). Behind the internal acoustic meatus is a small slit almost hidden by a thin plate of bone, leading to a canal, the aquæductus vestibuli, which transmits the ductus endolymphaticus together with a small artery and vein. Above and between these two openings is an irregular depression that lodges a process of the dura mater and transmits a small vein; in the infant, this depression is represented by a large fossa, the
subarcuate fossa, which extends backward as a blind tunnel under the
superior semicircular canal. • Falciform crest • Area facialis, with (2') internal opening of the facial canal • Ridge separating the area facialis from the area cribrosa superior • Area cribrosa superior, with (4') openings for nerve filaments • Anterior inferior cribriform area, with (5') the tractus spiralis foraminosus, and (5") the canalis centralis of the cochlea. • Ridge separating the tractus spiralis foraminosus from the area cribrosa media • Area cribrosa media, with (7') orifices for nerves to saccule • Foramen singulare.
Inferior surface The
inferior surface is rough and irregular, and forms part of the exterior of the base of the skull. It presents eleven points for examination: • near the apex is a rough surface, quadrilateral in form, which serves partly for the attachment of the
Levator veli palatini and the cartilaginous portion of the
auditory tube, and partly for connection with the
basilar part of the
occipital bone through the intervention of some dense fibrous tissue • behind this is the large circular aperture of the
carotid canal, which ascends at first vertically, and then, making a bend, runs horizontally forward and medially; it transmits into the cranium the internal carotid artery, and the carotid plexus of nerves • medial to the opening for the carotid canal and close to its posterior border, in front of the
jugular fossa, is a triangular depression; at the apex of this is a small opening, the
aquæductus cochleæ, which lodges a tubular prolongation of the dura mater establishing a communication between the perilymphatic space and the
subarachnoid space, and transmits a vein from the cochlea to join the
internal jugular • behind these openings is a deep depression, the
jugular fossa, of variable depth and size in different skulls; it lodges the bulb of the
internal jugular vein • in the bony ridge dividing the carotid canal from the jugular fossa is the small
inferior tympanic canaliculus for the passage of the
tympanic branch of the
glossopharyngeal nerve • in the lateral part of the jugular fossa is the
mastoid canaliculus for the entrance of the
auricular branch of the vagus nerve • behind the jugular fossa is a quadrilateral area, the
jugular surface, covered with cartilage in the fresh state, and articulating with the
jugular process of the
occipital bone • extending backward from the carotid canal is the
vaginal process, a sheath-like plate of bone, which divides behind into two laminæ; the lateral lamina is continuous with the tympanic part of the bone, the medial with the lateral margin of the jugular surface • between these laminæ is the
styloid process, a sharp spine, about 2.5 cm. in length • between the styloid and mastoid processes is the
stylomastoid foramen; it is the termination of the
facial canal, and transmits the
facial nerve and
stylomastoid artery • situated between the tympanic portion and the mastoid process is the
tympanomastoid fissure, for the exit of the
auricular branch of the
vagus nerve. == Angles ==