ANTERIOR CRANIAL FOSSA

The anterior cranial fossa is formed at the front and sides by the frontal bone. Its floor is composed of the orbital plate of the frontal bone, the cribriform plate and crista galli of the ethmoid bone, and the lesser wings and anterior part of the body of the sphenoid. Unlike the other cranial fossae, it does not directly communicate with the inferior surface of the cranium.

A perforated plate of bone, the cribriform plate of the ethmoid bone, spreads across the midline between the orbital plates of the frontal bone, and is depressed below them, forming part of the roof of the nasal cavity. Olfactory nerves pass from the nasal mucosa to the olfactory bulb of the brain through numerous small foramina in the cribriform plate. Anteriorly a spur of bone, the crista galli, projects upwards between the cerebral hemispheres. A depression between the crista galli and the crest of the frontal bone is crossed by the fronto-ethmoidal suture. It bears the foramen caecum, which is usually a small blind-ended depression, but which occasionally accommodates a vein draining from the nasal mucosa to the superior sagittal sinus. The anterior ethmoidal nerve enters the cranial cavity where the cribriform plate meets the orbital part of the frontal bone and then passes into the roof of the nose via a small foramen by the side of the crista galli: the nerve grooves the crista galli. The anterior ethmoidal vessels accompany the nerve. The posterior ethmoidal canal, which transmits the posterior ethmoidal nerve and vessels, opens at the posterolateral corner of the cribriform plate and is overhung by the sphenoid bone.

The convex cranial surface of the frontal bone separates the brain from the orbit and bears impressions of cerebral gyri and small grooves for meningeal vessels. Posteriorly, it articulates with the anterior border of the lesser wing of the sphenoid bone which forms the posterior boundary of the anterior cranial fossa. The medial end of the lesser wing constitutes the anterior clinoid process. The lesser wing joins the body of the sphenoid body by two roots which are separated by the optic canal. The anterior root, broad and flat, is continuous with the jugum sphenoidale, while the smaller and thicker posterior root joins the body of the sphenoid bone near the posterior bank of the sulcus chiasmatis. The frontosphenoid and sphenoethmoidal sutures divide the sphenoid from the adjacent bones.

The posterior border of each lesser wing fits the stem of the lateral cerebral sulcus and may be grooved by the sphenoparietal sinus. Above lies the inferior surface of the frontal lobe of the cerebral hemisphere and the anterior perforated substance lies medially. Inferiorly the lesser wing bounds the superior orbital fissure, and completes the orbital roof. Each anterior clinoid process gives attachment to the free margin of the tentorium cerebelli and is grooved medially by the internal carotid artery as it leaves the cavernous sinus. The anterior and middle clinoid processes may be connected by a thin osseous bar, formed by ossification either of the carotico-clinoid ligament or of a dural fold extending between the processes: the resulting carotico-clinoid foramen surrounds the artery.

MIDDLE CRANIAL FOSSA

The middle cranial fossa is deeper and more extensive than the anterior fossa, particularly laterally, where it supports the temporal lobes of the cerebral hemispheres. It is bounded in front by the lesser wings and part of the body of the sphenoid, behind by the superior borders of the petrous part of the temporal bone and the dorsum sellae of the sphenoid, and laterally by the squamous parts of the temporal bone, parietal bone and greater wings of the sphenoid. The region corresponds with the middle part of the cranial base.

The middle cranial fossa communicates with the orbits by the superior orbital fissures, each bounded above by a lesser wing, below by a greater wing, and medially by the body of the sphenoid bone. The fissures are wider medially, and each has a long axis sloping inferomedially and forwards. They are the major gateways into the orbit from the cranial cavity and contain the oculomotor, trochlear and abducens nerves, and the lacrimal, frontal and nasociliary branches of the ophthalmic division of the trigeminal nerve, together with filaments from the internal carotid plexus (sympathetic), the ophthalmic veins, the orbital branch of the middle meningeal artery, and the recurrent branch of the lacrimal artery (see Ch. 39).

Centrally the floor of the fossa is narrower and formed by the body of the sphenoid bone which contains the sphenoidal sinuses (see Ch. 32). The roof of the sinus is deeply concave and houses the pituitary gland (hypophysis): it is therefore termed the pituitary (hypophysial) fossa, also known as the sella turcica because it resembles the shape of a Turkish saddle. The anterior edge of the pituitary fossa is completed laterally by a middle clinoid process, the floor forms the roof of the sphenoidal air sinuses, and the posterior boundary presents a vertical pillar of bone, the dorsum sellae. The superolateral angles of the dorsum are expanded as the posterior clinoid processes. A fold of dura, the diaphragma sella, is attached to the anterior and posterior clinoid processes and roofs over the pituitary fossa. The smooth upper part of the anterior wall of the fossa, the jugum sphenoidale, is bounded behind by the anterior border of the grooved sulcus chiasmatis which leads laterally into the optic canals. Each canal transmits the optic nerve and ophthalmic artery. The optic chiasma usually lies posterosuperior to the sulcus chiasmatis, the tuberculum sellae lies below the sulcus. The cavernous sinus lies lateral to the pituitary fossa. The lateral wall of the body of the sphenoid contains a shallow carotid groove related to the internal carotid artery as it ascends from the carotid canal and runs through the cavernous sinus. Posterolaterally the groove may be deepened by a small projecting lingula.

The greater wing of the sphenoid bone contains three consistent foramina and other small variable foramina. The foramen rotundum, situated just below and behind the medial end of the superior orbital fissure, leads forwards into the pterygopalatine fossa, and contains the maxillary nerve. Behind the foramen rotundum is the foramen ovale which transmits the mandibular nerve: it is occasionally divided into two or three components. Accessory named foramina, the foramen of Vesalius and the cavernous foramen, may occur close to the foramen ovale. The small foramen of Vesalius occurs in some 20% of skulls: it is consistently symmetrical and lies anteromedial to the foramen ovale and lateral to the foramen rotundum and vidian canal. It transmits an emissary vein through which the cavernous sinus and pterygoid plexus communicate. The foramen spinosum lies posterolateral to the foramen ovale and transmits the middle meningeal artery and veins. The vessels groove the floor and lateral wall of the middle cranial fossa. The foramen ovale and foramen spinosum open into the underlying infratemporal fossa.

The foramen lacerum is situated at the posterior end of the carotid groove, posteromedial to the foramen ovale. It is bounded in front by the body and adjoining roots of the pterygoid process and greater wing of the sphenoid bone, posterolaterally by the apex of the petrous part of the temporal bone, and medially by the basilar part of the occipital bone. A small emissary sphenoidal foramen may occur at the root of the greater wing of the sphenoid medial to the foramen lacerum; when present it transmits a vein from the cavernous sinus.

The trigeminal impression, which accommodates the trigeminal ganglion, is situated posterior to the foramen lacerum on the anterior surface of the petrous part of the temporal bone near its apex. Its depth is variable. Posterolateral to the impression is a small, shallow, pit, limited posteriorly by a rounded arcuate eminence (produced by the underlying anterior semicircular canal). Lateral to the impression a narrow groove passes posterolaterally into the hiatus for the greater petrosal nerve, and even further laterally is the hiatus for the lesser petrosal nerve. A smooth trigeminal notch leads into the impression. It lies on the upper border of the petrous temporal, anteromedial to the groove for the superior petrosal sinus; at this point, the trigeminal nerve separates the sinus from bone. At the anterior end of the trigeminal notch is the petrosphenoidal ligament (of Gruber), attached to a minute bony spicule, directed anteromedially. The abducens nerve bends sharply across the upper petrous border, and passes between the ligament and the dorsum sellae anterior to the petrosphenoidal ligament (for a detailed analysis of the course of the abducens nerve in the petroclival region see Ozveren et al 2002).

The anterior surface of the petrous part of the temporal bone is formed by the tegmen tympani, a thin osseous lamina in the roof of the tympanic cavity, which extends anteromedially above the pharyngotympanic tube, anterolateral to the arcuate eminence. The posterior part of the tegmen tympani roofs the mastoid antrum, lateral to the eminence. The superior border of the petrous part of the temporal bone separates the middle and the posterior cranial fossae, and is grooved by the superior petrosal sinus. In young skulls, a petrosquamous suture may be visible at the lateral limit of the tegmen tympani but it is obliterated in adults. The tegmen tympani then turns down as the lateral wall of the osseous portion of the pharyngotympanic tube and its lower border may appear in the squamotympanic fissure. Lateral to the anterior part of the tegmen tympani, the squamous part of the temporal bone is thinned over a small area that coincides with the deepest part of the mandibular fossa.

POSTERIOR CRANIAL FOSSA

The posterior cranial fossa, the largest and deepest of the cranial fossae, contains the cerebellum, pons and medulla oblongata. It is bounded in front by the dorsum sellae, posterior aspects of the body of the sphenoid and the basilar part of occipital bone; behind by the squamous part of the occipital bone; laterally by the petrous and mastoid parts of the temporal bone and by the lateral parts of the occipital bone; and above and behind by the mastoid angles of the parietal bones. The region corresponds extracranially with the posterior part of the cranial base.

The most prominent feature in the floor of the posterior cranial fossa is the foramen magnum in the occipital bone. A sloping surface, the clivus, formed successively by the basilar part of the occipital bone, the posterior part of the body and then the dorsum sellae of the sphenoid bone, lies anterior to the foramen magnum. The clivus is gently concave from side to side. On each side it is separated from the petrous part of the temporal bone by a petro-occipital fissure, which is filled by a thin plate of cartilage and limited behind by the jugular foramen. Its margins are grooved by the inferior petrosal sinus. The spheno-occipital synchondrosis is evident on the clivus of a growing child.

The jugular foramen is a large opening sited at the posterior end of the petro-occipital fissure, above and lateral to the foramen magnum. Its upper border is sharp and irregular, and contains a notch for the glossopharyngeal nerve. The cochlear canaliculus, which contains the perilymphatic ‘duct’, is sited in the deepest part of the notch. The lower border of the jugular foramen is smooth. Posteriorly it is grooved by the sigmoid sinus which continues into the foramen as the internal jugular vein. The accessory, vagus and glossopharyngeal nerves pass through the anterior part of the jugular foramen from behind forwards, and may groove the jugular tubercle as they enter the foramen. They lie between the inferior petrosal sinus (accompanied by a meningeal branch of the ascending pharyngeal artery) and the sigmoid sinus (accompanied by a meningeal branch of the occipital artery).

The hypoglossal (anterior condylar) canal lies medial to and below the lower border of the jugular foramen, at the junction of the basilar and lateral parts of the occipital bone. It transmits the hypoglossal nerve and its recurrent branch together with the meningeal branch of the ascending pharyngeal artery and an emissary vein which links the (intracranial) basilar plexus with the (extracranial) internal jugular vein. When a posterior condylar canal is present behind the occipital condyle, its internal orifice is posterolateral to that of the hypoglossal canal and it contains a sigmoid emissary vein (associated with the occipital veins) and a meningeal branch of the occipital artery. The occipital condyles lie within the anterior aspect of the foramen magnum: their medial aspects are roughened for the attachments of the alar ligaments associated with the atlanto-axial joints.

The posterior surface of the petrous part of the temporal bone forms much of the anterolateral wall of the posterior cranial fossa. It contains the internal acoustic meatus, which lies anterosuperior to the jugular foramen, and transmits the facial and vestibulocochlear nerves, the nervus intermedius, and labyrinthine vessels.

The mastoid part of the temporal bone lies behind the petrous part of the temporal bone in the lateral wall of the posterior cranial fossa. Anteriorly it is grooved by a wide sigmoid sulcus which runs forwards and downwards, then downwards and medially, and finally forwards to the jugular foramen: it contains the sigmoid sinus. Superiorly, where the groove touches the mastoid angle of the parietal bone, it becomes continuous with a groove that transmits the transverse sinus, and then crosses the parietomastoid suture and descends behind the mastoid antrum. A mastoid foramen which transmits an emissary vein from the sigmoid sinus and a meningeal branch of the occipital artery, sometimes large enough to groove the squamous part of the occipital bone, may be sited here. The lowest part of the sigmoid sulcus crosses the occipitomastoid suture and grooves the jugular process of the occipital bone. The right sigmoid sulcus is usually larger than the left.

A thin plate with an irregularly curved margin projects back behind the internal acoustic meatus. It bounds a slit that contains the opening of the vestibular aqueduct (which houses the saccus and ductus endolymphaticus and a small artery and vein). A small subarcuate fossa lined with dura mater lies between the internal acoustic meatus and the aqueductal opening. Near the superior border of the petrous part of the temporal bone the fossa is pierced by a small vein. In infants the fossa is a relatively large blind tunnel under the anterior semicircular canal.

The squamous part of the occipital bone displays a median internal occipital crest. This runs posteriorly from the foramen magnum to an internal occipital protuberance, gives attachment to the falx cerebelli, and may be grooved by the occipital sinus. The internal occipital protuberance is close to the confluence of the sinuses. It is grooved bilaterally by the transverse sinuses which curve laterally with an upward convexity to the mastoid angles of the parietal bones. The groove for the transverse sinus is usually deeper on the right, where it is generally a continuation of the superior sagittal sinus, while on the left it is frequently a continuation of the straight sinus. On both sides the transverse sulcus is continuous with the sigmoid sulcus. Below the transverse sulcus, the internal occipital crest separates two shallow fossae which house the cerebellar hemispheres. The posterior clinoid process and the margins of the grooves for the transverse and superior petrosal sinuses all provide anchorage for the attached margin of the tentorium cerebelli.

DURA MATER

Dura mater is thick, dense and fibrous. It is predominantly acellular, and consists mainly of densely packed fascicles of collagen fibres arranged in laminae. The fascicles run in different directions in adjacent laminae, producing a lattice-like appearance which is particularly obvious in the tentorium cerebelli and around the defects or perforations that sometimes occur in the anterior portion of the falx cerebri.

The cranial dura, which lines the cranial cavity, differs from the spinal dura mainly in its relationship to the surrounding bones. It has an inner, or meningeal, layer and an outer, or endosteal, layer. These layers are united except where they separate to enclose the venous sinuses that drain blood from the brain. There is little histological difference between the endosteal and meningeal layers. Both contain fibroblasts, and the endosteal layer also contains osteoblasts. Focal calcification may occur in the falx cerebri.

The dura mater adheres to the internal surfaces of the cranial bones, particularly at the sutures, the cranial base and around the foramen magnum. Fibrous bands pass from the dura into the bones, and it is difficult to remove the dura from the suture lines in young skulls. However, as the suture lines fuse the dura becomes separated from them. It also becomes thicker, less pliable, and more firmly adherent to the inner surface of the skull, particularly that of the calvaria, with increasing age. The endosteal layer of the dura is continuous with the pericranium through the cranial sutures and foramina and with the orbital periosteum through the superior orbital fissure. The meningeal layer provides tubular sheaths for the cranial nerves as they pass out through the cranial foramina, and these sheaths fuse with the epineurium as the nerves emerge from the skull. The dural sheath of the optic nerve is continuous with the ocular sclera. The dura fuses with the adventitia of major vessels, such as the internal carotid and vertebral arteries, at sites where they pierce it to enter the cranial cavity.

The inner aspect of the dura mater is closely applied to the arachnoid mater over the surface of the brain. However the two membranes are physically joined only at sites where either veins pass from the brain into venous sinuses, e.g. the superior sagittal sinus, or where they connect the brain to the dura, e.g. at the anterior pole of the temporal lobe.