Cranium and Contents Anatomy
The part of the skull that contains the
brain and its immediate relations is called the neurocranium. Although the
detailed anatomy of the central nervous system is outside the scope of this
book, there are some important surface features of the brain (Figs 7.73 & 7.74) to which reference should be made when considering
the bony features of the interior of the cranium.
The vault of the skull consists of four
flat bones. Anteriorly is the frontal bone, posteriorly, the occipital bone and
on each side is a parietal bone (Fig. 7.72). The frontal and parietal bones meet along the coronal suture, and the
two parietal bones meet along the midline sagittal suture. Posteriorly, the
parietal bones meet the occipital bone at the lambdoid suture. The undersurface
of the vault bears a long shallow midline groove for the superior sagittal
venous sinus (see below), which terminates at a prominence, the internal
occipital protuberance (Fig. 7.75). Below the
level of each parietal bone, the greater wing of the sphenoid bone and the
squamous part of the temporal bone complete the lateral wall of the vault.
The cranial base is characterized by the anterior,
middle and posterior fossae, arranged in step-like sequence (Fig. 7.75), the anterior being superior. The anterior fossa, which accommodates
the frontal lobes of the brain, is formed by the frontal bone laterally, the
cribriform plates and crista galli of the ethmoid bone medially and is
completed posteriorly by the lesser wings of the sphenoid bone. The numerous
small foramina in the cribriform plates communicate with the nasal cavity (p.
356).
The middle fossa, occupied by the temporal lobes of
the brain, is formed by the greater wings and body of the sphenoid bone.
Posteriorly, the fossa is bounded by the petrous part of each temporal bone,
while anteriorly the fossa is completed by the lesser wings of the sphenoid.
Each of the lesser wings is perforated by the optic canal, which continues
forwards into the orbit. Between the lesser and greater wings is the superior
orbital fissure, which also provides access to the orbit. The greater wing is
traversed by the foramen rotundum (opening into the
pterygopalatine fossa; p. 353) and
the foramina ovale and spinosum (both opening into the infratemporal fossa;
Fig. 7.82). Lying in the angle between the greater wing and body of the
sphenoid and the apex of the petrous part of the temporal bone is the foramen
lacerum, which forms part of the roof of the infratemporal fossa. In the
midline the body of the sphenoid is hollowed, forming the sella turcica. This
is limited by the paired anterior and posterior clinoid processes and
accommodates the pituitary gland.
The posterior cranial fossa contains the cerebellum,
the pons and the medulla oblongata which is in continuity through the foramen
magnum with the spinal cord. The floor of this fossa is formed mostly by the
occipital bone, supplemented anteriorly by the body of the sphenoid and the
posterior surface of the petrous part of each temporal bone. Anterior to the
foramen magnum, the occipital and sphenoid bones fuse to form a smooth incline,
the clivus, on which the brainstem lies. The internal acoustic meatus opens
onto the posterior surface of the petrous part of the temporal bone while in
the suture between this bone and the occipital bone is the jugular foramen.
Running horizontally from the internal occipital protuberance is a groove for
the transverse venous sinus. An
S-shaped groove for the sigmoid sinus (Fig. 7.75) links the groove for the
transverse sinus with the jugular foramen. The hypoglossal canal traverses the
occipi- tal bone anterolateral to the foramen magnum (Fig. 7.82).
Surrounding the brain are three membranes, the
meninges, which comprise the dura mater, arachnoid mater and
pia mater.
The dura mater consists of outer periosteal and inner
meningeal layers. The periosteal dura attaches to and closely follows the bony
contours of the cranial cavity and is continuous through the sutures and
foramina with the periosteum (pericra- nium) on the outer surface of the skull.
Although generally bound to the periosteal dura, the meningeal dura is raised
in places as a double layer, forming the dural folds.
The largest of these folds is the midline falx cerebri
(Figs 7.76
& 7.77), which projects
downwards between the two cerebral hemispheres.
Anteriorly, the falx cerebri is attached to the crista
galli of the ethmoid bone. It arches over the corpus callosum and gains further
attachment to the frontal, parietal and occipital bones. Posteriorly, the falx ends by attaching to another dural fold, the tentorium cerebelli (Figs 7.76–7.78). The tentorium forms an incomplete roof over the posterior cranial
fossa and separates the occipital lobes of the cerebrum from the cerebellum.
Each side of the tentorium slopes upwards towards its midline attach- ment to
the falx cerebri. Anteriorly, there is a large aperture in the tentorium
through which the brainstem passes. The thickened free edge of the tentorium
surrounding this aperture continues forwards to reach the anterior clinoid process.
The attached border of the tentorium runs laterally from the internal occipital
protuberance along a horizontal
groove on the inner surface of the occipital bone, then continues medially
along the superior border of the petrous part of the temporal bone to reach the
posterior clinoid process. Posteriorly, the small falx cerebelli descends
vertically from the tentorium and partially separates the two cerebellar
hemispheres. The sella turcica in the middle fossa has a roof of dura called
the diaphragma sellae, which is attached to the four clinoid processes and has
a central aperture for the pituitary stalk.
Dural venous sinuses
Lying between the two layers of dura are
endothelium-lined venous channels (Fig. 7.77). These dural venous sinuses,
which often groove the adjacent bones, collect blood from the brain and
meninges. They also drain cerebrospinal fluid that has been secreted into the
subarachnoid space by the choroid plexuses of the brain. A characteristic of
these sinuses is the absence of valves.
The superior sagittal sinus lies in the attached
margin of the falx cerebri (Figs 7.76 & 7.77). The sinus drains posteriorly and has along its length several
dilatations called lacunae. These lacunae possess arachnoid granulations
through which the reabsorption of cerebrospinal fluid takes place. The sinus
also receives numerous cerebral veins. At the internal occipital protuberance the superior sagittal sinus turns laterally, usually to the right, and
continues as the transverse sinus in the attached margin of the tentorium
cerebelli (Fig. 7.78). Just before reaching the petrous part of the temporal
bone, the sinus turns inferiorly to continue as the sigmoid sinus (Fig. 7.79).
This follows an S-shaped course to reach the jugular foramen, through which it
is continuous with the internal jugular vein.
The smaller inferior sagittal sinus lies in the free
border of the falx cerebri (Fig. 7.76). The sinus runs posteriorly and at the
tentorium cerebelli is joined by the great cerebral vein, which drains the
deeper structures of the cerebral hemispheres. The union of these vessels forms
the straight sinus (Figs 7.76–7.78), which continues posteriorly in the
attachment of the falx cerebri to the tentorium as far as the internal occipital
protuberance. Here, the straight sinus
usually turns to the left to form the transverse sinus, whose course mirrors
that on the opposite side. This region is known as the confluence of sinuses.
The cavernous venous sinuses (Figs 7.79 & 7.80)
lie on either side of the pituitary gland and the body of the sphenoid bone.
They contain numerous interconnected venous spaces, producing a spongy
appearance. The two sinuses communicate with each other and
receive blood from vessels that pass through the superior and inferior orbital
fissures from the ophthalmic veins and pterygoid venous plexuses. Posteriorly
each cavernous sinus drains via the superior and inferior petrosal sinuses. The
superior petrosal sinus runs along the superior border of the petrous part of
the temporal bone to terminate in the junction of the transverse and sigmoid
sinuses. The inferior petrosal sinus descends into the posterior cranial fossa
and unites with the sigmoid sinus in the jugular foramen to form the internal
jugular vein.
The arachnoid mater, the middle of the meningeal
layers, is loosely attached to the dura mater, generally following its folds.
The arachnoid is separated from the deeper pia mater by the subarachnoid space,
which contains cerebrospinal fluid and is traversed by the arteries of the
brain and the cranial nerves. Delicate
fibres from the arachnoid mater cross the subarachnoid space and attach to the
pia mater.
The pia mater is the innermost of the meninges and
clings to the surface of the brain, dipping into its numerous grooves or sulci.
Of the many arteries entering the cranium to supply
the meninges, one of particular importance is the middle meningeal artery,
which arises from the maxillary artery (p. 346) and enters through the foramen spinosum. This vessel runs laterally across the floor of the middle cranial fossa, grooving the bone, and divides on the
squamous part of the temporal bone into frontal (anterior) and parietal
(posterior) branches (Fig. 7.79). These
branches arch superiorly on the inner surface of the lateral part of the
skull and supply the meninges lining most of the vault. Meningeal veins follow
the arteries and communicate with the dural venous sinuses and with veins lying
outside the skull.
Bleeding from veins or arteries between the meningeal
layers can raise intracranial pressure. An extradural (epidural) haematoma
results from extravasation between the dura and the skull. A subdural haematoma
is produced by bleeding between the dura and arachnoid layers, where normally
no space exists. Blood leaking from the vessels that cross the subarachnoid
space will give rise to a subarachnoid haemorrhage, the blood intermin- gling
with cerebrospinal fluid.
Arterial supply to the brain
The brain receives arterial blood from the vertebral
and internal carotid arteries (Fig. 7.80). The vertebral arteries (p. 330)
enter the posterior cranial fossa through the foramen magnum. Passing upwards
and forwards they unite in the midline on the clivus to form the basilar
artery. Branches to the brainstem and cerebellum arise from the vertebral and
basilar arteries before the latter divides at the upper border of the pons to
form the left and right posterior cerebral arteries. Before supplying the
posterior part of the cerebral hemisphere, each of these vessels gives rise to
a
posterior communicating artery, which passes forwards
to form part of the cerebral arterial circle (circle of Willis) by anastomosing
with the internal carotid artery.
The internal carotid artery traverses the carotid
canal (Fig. 7.82) to enter the middle cranial fossa, emerging from the upper
part of the foramen lacerum. The artery turns anteriorly to enter the cavernous sinus, then continues superiorly to leave the sinus through
its roof. Here, near the anterior clinoid process, the ophthalmic artery arises
and accompanies the optic nerve through the optic canal into the orbit. The
internal carotid artery terminates as the anterior and middle cerebral
arteries. The middle cerebral artery supplies the lateral portion of the
cerebral hemisphere, while the
anterior cerebral artery ascends between the frontal lobes to supply the medial
surface of the hemisphere.
The cerebral arterial circle is formed between the
branches of the internal carotid and vertebral arteries (Fig. 7.80). The two
anterior cerebral arteries are joined by the anterior communicating artery.
On each side, an anastomosis, via the posterior communicating artery, between
the posterior cerebral branch of the basilar artery and the internal carotid
artery completes the cerebral arterial circle. Aneurisms of the cerebral
arterial circle are subject to rupture and subsequent subarachnoid haemorrhage.
The 12 pairs of cranial nerves enter or leave the
skull through various foramina (Figs 7.79 & 7.81). The olfactory (I) nerves
emerge from the nasal cavity as a number of short branches, which traverse the
cribriform plates and terminate in the olfactory bulbs.
The optic (II) nerve leaves the orbit via the optic
canal and joins the optic chiasma immediately anterior to the pituitary stalk.
From the chiasma the optic tracts pass backwards to enter the brain.
Three cranial nerves enter the orbit through the
superior orbital fissure. To reach the fissure the oculomotor (III) and
trochlear (IV) nerves run forwards in the lateral wall of the cavernous sinus,
while the abducens (VI) nerve passes through the cavity of the sinus. Infection from the face can spread through veins to the cavernous
sinus, which may thrombose, causing damage to the abducens nerve and double
vision.
The ganglion of the sensory part of the trigeminal (V)
nerve lies covered in dura in a small depression on the apex of the petrous
part of the temporal bone. The three divisions of the nerve converge on the
anterior surface of the ganglion. From the orbit the branches of the ophthalmic
(V1) division traverse the superior orbital fissure, coalesce and continues
backwards, embedded in the lateral wall of the cavernous sinus, to reach the ganglion.
The maxillary (V2) division leaves the pterygopalatine fossa via the foramen
rotundum and passes backwards along the lower edge of the sinus to the
ganglion. The sensory part of the mandibular (V3) division, accompanied by the
motor root of the trigeminal nerve, ascends from the infratemporal fossa
through the foramen ovale. The motor root passes beneath and not through the
ganglion to traverse the foramen ovale.
The facial (VII) nerve enters, and the
vestibulocochlear (VIII) nerve emerges from, the internal acoustic meatus in
the petrous part of the temporal bone.
Three nerves leave via the jugular foramen to enter
the carotid sheath, namely the glossopharyngeal (IX), vagus (X) and acces- sory
(XI) nerves.
Finally, the hypoglossal (XII) nerve traverses the
hypoglossal canal.
