Brief Anatomy of the Skull and Cranial Cavity

I. Skull

Is the skeleton of the head and may be divided into two types of bones: 8 cranial

bones for enclosing the brain (unpaired frontal, occipital, ethmoid, and sphenoid

bones and paired parietal and temporal bones), which can be seen in the cranial

cavity; and 14 facial bones (paired

lacrimal, nasal, palatine, inferior turbinate, maxillary, and zygomatic bones

and unpaired vomer and mandible).

A. Cranium

Is sometimes restricted to the skull without the mandible.

B. Calvaria

Is the skullcap, which is the vault of the skull without the facial bones. It

consists of the superior portions of the frontal, parietal, and occipital

bones.

Its highest point on the sagittal suture is the vertex.

II. Bones of the Cranium

A. Frontal bone

Underlies the forehead and the superior margin and roof of the orbit and

has a smooth median prominence called the glabella.

B. Parietal bone

Forms part of the superior and lateral surface of the skull.

C. Temporal bone

Consists of the squamous part, which is external to the lateral surface of

the temporal lobe of the brain; the petrous part, which encloses the internal

and middle ears; the mastoid part, which contains mastoid air cells; and the

tympanic part, which houses the external auditory meatus and the

tympanic cavity.

D. Occipital bone

Consists of squamous, basilar, and two lateral condylar parts.

Encloses the foramen magnum and forms the cerebral and cerebellar

fossae.

E. Sphenoid bone

Consists of the body (which houses the sphenoid sinus), the greater and

lesser wings, and the pterygoid process.

F. Ethmoid bone

Is located between the orbits and consists of the cribriform plate,

perpendicular plate, and two lateral masses enclosing ethmoid air cells.

III. Sutures of the Skull

Are the immovable fibrous joints between the bones of the skull.

A. Coronal suture: lies between the frontal bone and the two parietal bones.

B. Sagittal suture: lies between the two parietal bones.

C. Squamous (squamoparietal) suture: lies between the parietal bone and the

squamous part of the temporal bone.

D. Lambdoid suture: lies between the two parietal bones and the occipital bone.

E. Junctions of the cranial sutures

Lambda: intersection of the lambdoid and sagittal sutures.

Bregma: intersection of the sagittal and coronal sutures.

Pterion: a craniometric point at the junction of the frontal, parietal, and

temporal bones and the great wing of the sphenoid bone.

Asterion: a craniometric point at the junction of the parietal, occipital, and

temporal (mastoid part) bones.

Nasion: a point on the middle of the nasofrontal suture (intersection of the

frontal and two nasal bones).

Inion: most prominent point of the external occipital protuberance, which

is used as a fixed point in craniometry.

Skull fracture: Fracture at the pterion may rupture of the middle meningeal artery,

and a depressed fracture may compress the underlying brain. A fracture of the

petrous portion of the temporal bone may cause blood or cerebrospinal fluid

(CSF) to escape from the ear, hearing loss, and facial nerve damage. Fracture of

the anterior cranial fossa causes anosmia, periorbital bruising (raccoon eyes), and

CSF leakage from the nose (rhinorrhea). A blow to the top of the head may

fracture the skull base with related cranial nerve injury, CSF leakage from a dura-

arachnoid tear, and dural sinus thrombosis. Tripod fracture is a facial fracture

involving the three supports of the malar (cheek or zygomatic) bone including the

zygomatic processes of the temporal, frontal, and maxillary bones.

IV. Foramina in the Skull

Include the following, which are presented here with the structures that pass

through them:

A. Anterior cranial fossa

Cribriform plate: olfactory nerves.

Foramen cecum: occasional small emissary vein from nasal mucosa to

superior sagittal sinus.

Anterior and posterior ethmoidal foramina: anterior and posterior

ethmoidal nerves, arteries, and veins.

B. Middle cranial fossa

Optic canal: optic nerve, ophthalmic artery, and central artery and vein of

the retina.

Superior orbital fissure: oculomotor, trochlear, and abducens nerves;

ophthalmic division of trigeminal nerve; and ophthalmic veins.

Foramen rotundum: maxillary division of trigeminal nerve.

Foramen ovale: mandibular division of trigeminal nerve, accessory

meningeal artery, and occasionally lesser petrosal nerve.

Foramen spinosum: middle meningeal artery.

Foramen lacerum: nothing passes through this foramen, but the upper part

is traversed by the internal carotid artery and greater and deep petrosal

nerves en route to the pterygoid canal.

Carotid canal: internal carotid artery and sympathetic nerves (carotid

plexus).

Hiatus of facial canal: greater petrosal nerve.

C. Posterior cranial fossa

Internal auditory meatus: facial and vestibulocochlear nerves and

labyrinthine artery.

Jugular foramen: glossopharyngeal, vagus, and spinal accessory nerves and

beginning of internal jugular vein.

Hypoglossal canal: hypoglossal nerve and meningeal artery.

Foramen magnum: spinal cord, spinal accessory nerve, vertebral arteries,

venous plexus of vertebral canal, and anterior and posterior spinal arteries.

Condyloid foramen: condyloid emissary vein.

Mastoid foramen: branch of occipital artery to dura mater and mastoid

emissary vein.

D. Foramina in the front of the skull

Zygomaticofacial foramen: zygomaticofacial nerve.

Supraorbital notch or foramen: supraorbital nerve and vessels.

Infraorbital foramen: infraorbital nerve and vessels.

Mental foramen: mental nerve and vessels.

E. Foramina in the base of the skull

Petrotympanic fissure: chorda tympani and often anterior tympanic artery.

Stylomastoid foramen: facial nerve.

Incisive canal: nasopalatine nerve and terminal part of the sphenopalatine

or greater palatine vessels.

Greater palatine foramen: greater palatine nerve and vessels.

Lesser palatine foramen: lesser palatine nerve and vessels.

Palatine canal: descending palatine vessels and the greater and lesser

palatine nerves.

Pterygoid canal: runs from the anterior wall of the foramen lacerum to the

pterygopalatine fossa and transmits the nerve of the pterygoid canal (vidian

nerve).

Sphenopalatine foramen: sphenopalatine vessels and nasopalatine nerve.

V. Structures in the Cranial Fossae

A. Foramen cecum

Is a small pit in front of the crista galli between the ethmoid and frontal

bones.

May transmit an emissary vein from the nasal mucosa and the frontal sinus

to the superior sagittal sinus.

B. Crista galli

Is the triangular midline process of the ethmoid bone extending upward

from the cribriform plate.

Provides attachment for the falx cerebri.

C. Cribriform plate of the ethmoid bone

Is perforated by 15 to 20 foramina, supports the olfactory bulb, and

transmits olfactory nerves from the olfactory mucosa to the olfactory bulb.

D. Anterior clinoid processes

Are two anterior processes of the lesser wing of the sphenoid bone, which

are located in the middle cranial fossa.

Provide attachment for the free border of the tentorium cerebelli.

E. Middle clinoid process

Is a small inconstant eminence on the body of the sphenoid, posterolateral

to the tuberculum sellae.

F. Posterior clinoid processes

Are two tubercles from each side of the dorsum sellae.

Provide attachment for the attached border of the tentorium cerebelli.

G. Lesser wing of the sphenoid bone

Forms the anterior boundary of the middle cranial fossa.

Forms the sphenoidal ridge separating the anterior from the middle cranial

fossa.

Forms the boundary of the superior orbital fissure (the space between the

lesser and greater wings).

H. Greater wing of the sphenoid bone

Forms the anterior wall and the floor of the middle cranial fossa.

Presents several openings: the foramen rotundum, foramen ovale, and

foramen spinosum.

I. Sella turcica (Turk's saddle) of the sphenoid bone

Is bounded anteriorly by the tuberculum sellae and posteriorly by the

dorsum sellae.

Has a deep central depression known as the hypophyseal fossa, which

accommodates the pituitary gland or the hypophysis.

Lies directly above the sphenoid sinus located within the body of the

sphenoid bone; its dural roof is formed by the diaphragma sellae.

J. Jugum sphenoidale

Is a portion of the body of the sphenoid bone connecting the two lesser

wings, and forms the roof for the sphenoidal air sinus.

K. Clivus

Is the downward sloping surface from the dorsum sellae to the foramen

magnum.

Is formed by a part of the body of the sphenoid and a portion of the basilar

part of the occipital bone.

VI. Meninges of the Brain

A. Pia mater

Is a delicate investment that is closely applied to the brain and dips into

fissures and sulci.

Enmeshes blood vessels on the surfaces of the brain.

Pial hemorrhage: is due to damage to the small vessels of the pia and brain tissue.

Cerebral hemorrhage: is caused by rupture of the thin–walled lenticulostriate

artery, a branch of the middle cerebral artery, producing hemiplegia (paralysis of

one side of the body).

B. Arachnoid layer

Is a filmy, transparent, spidery layer that is connected to the pia mater by

web-like trabeculations.

Is separated from the pia mater by the subarachnoid space, which is filled

with CSF.

May contain blood after hemorrhage of a cerebral artery.

Projects into the venous sinuses to form arachnoid villi, which serve as

sites where CSF diffuses into the venous blood.

1. Cerebrospinal fluid (CSF)

Is formed by vascular choroid plexuses in the ventricles of the brain and is

contained in the subarachnoid space.

Circulates through the ventricles, enters the subarachnoid space, and

eventually filters into the venous system.

2. Arachnoid granulations

Are tuft-like collections of highly folded arachnoid (aggregations of

arachnoid villi) that project into the superior sagittal sinus and the lateral

lacunae, which are lateral extensions of the superior sagittal sinus.

Absorb the CSF into the dural sinuses and often produce erosion or pitting

of the inner surface of the calvaria, forming the granular pit.

Subarachnoid hemorrhage: is due to rupture of cerebral arteries and veins that

cross the subarachnoid space. It may be caused by rupture of an aneurysm on the

circle of Willis or, less commonly, by a hemangioma (proliferation of blood

vessels leads to a mass that resembles a neoplasm).

C. Dura mater

Is the tough, fibrous, outermost layer of the meninges external to the

subdural space, the space between the arachnoid and the dura.

Lies internal to the epidural space, a potential space that contains the

middle meningeal arteries in the cranial cavity.

Forms the dural venous sinuses, spaces between the periosteal and

meningeal layers or between duplications of the meningeal layers.

Subdural hematoma: is due to rupture of bridging cerebral veins as they pass from

the brain surface into one of the venous sinuses that results from a blow on the

front or the back of the head, causing displacement of the brain.

Epidural hematoma: is due to rupture of the middle meningeal arteries or veins

caused by trauma near the pterion, fracture of the greater wing of the sphenoid, or

a torn dural venous sinus. An epidural hematoma may put pressure on the brain

and form a bioconvex pattern on computed tomography (CT) scan or magnetic

resonance imaging (MRI).

1. Innervation of the dura mater

Anterior and posterior ethmoidal branches of the ophthalmic division of

the trigeminal nerve in the anterior cranial fossa.

Meningeal branches of the maxillary and mandibular divisions of the

trigeminal nerve in the middle cranial fossa.

Meningeal branches of the vagus and hypoglossal (originate from C1)

nerves in the posterior cranial fossa.

2. Projections of the dura mater

Falx cerebri

o Is the sickle shaped double layer of the dura mater, lying between

the cerebral hemispheres.

o Is attached anteriorly to the crista galli and posteriorly to the

tentorium cerebelli.

o Has a free inferior concave border that contains the inferior sagittal

sinus, and its upper convex margin encloses the superior sagittal

sinus.

Falx cerebelli

o Is a small sickle shaped projection between the cerebellar

hemispheres.

o Is attached to the posterior and inferior parts of the tentorium.

o Contains the occipital sinus in its posterior border.

Tentorium cerebelli

o Is a crescentic fold of dura mater that supports the occipital lobes of

the cerebral hemispheres and covers the cerebellum.

o Has a free internal concave border, that bounds the tentorial notch,

whereas its external convex border encloses the transverse sinus

posteriorly and the superior petrosal sinus anteriorly. The free

border is anchored to the anterior coronoid process, whereas the

attached border is attached to the posterior clinoid process.

Diaphragma sellae

o Is a circular, horizontal fold of dura that forms the roof of the sella

turcica, covering the pituitary gland or the hypophysis.

o Has a central aperture for the hypophyseal stalk or infundibulum.

VII. Cranial Venous Channels

A. Superior sagittal sinus

Lies in the midline along the convex border of the falx cerebri.

Begins at the crista galli and receives the cerebral, diploic meningeal, and

parietal emissary veins.

B. Inferior sagittal sinus

Lies in the free edge of the falx cerebri and is joined by the great cerebral

vein of Galen to form the straight sinus.

C. Straight sinus

Runs along the line of attachment of the falx cerebri to the tentorium

cerebelli.

Is formed by union of the inferior sagittal sinus and the great vein of

Galen.

D. Transverse sinus

Runs laterally from the confluence of sinuses along the edge of the

tentorium cerebelli.

E. Sigmoid sinus

Is a continuation of the transverse sinus; arches downward and medially in

an S-shaped groove on the mastoid part of the temporal bone.

Enters the superior bulb of the internal jugular vein.

F. Cavernous sinuses

Are located on each side of the sella turcica and the body of the sphenoid

bone and lie between the meningeal and periosteal layers of the dura

mater.

The internal carotid artery and the abducens nerve pass through these

sinuses. In addition, the oculomotor, trochlear, ophthalmic, and maxillary

nerves pass forward in the lateral wall of these sinuses.

Communicate with the pterygoid venous plexus by emissary veins and

receive the superior ophthalmic vein.

Cavernous sinus thrombosis: is the formation of thrombus in the cavernous sinus

and includes cases of thrombophlebitis (infectious inflammation secondary to

thrombus formation), phlebitis, phlebothrombosis, and septic thrombosis. The

most important cause of septic type is a Staphylococcus, and the most common

cause of spread of infection to the cavernous sinus is squeezing a pimple or boil

on the face above the upper lip near the nose. Cavernous sinus thrombosis may

produce papilledema (edema of the optic disk or nerve probably resulting from

increased intracranial pressure), exophthalmos or proptosis (protrusion of the

eyeball), diplopia (double vision), loss of vision (resulting from damage of the

optic nerve or central artery and vein of the retina), ophthalmoplegia (paralysis of

the eye movement muscles), edema of the eyelids, chemosis (swelling of the

conjunctivae), sluggish pupillary responses (resulting from damage of

sympathetic and parasympathetic nerves), and ptosis of upper eyelids (resulting

from damage of oculomotor nerve and sympathetic plexus on the internal carotid

artery). It is associated with significant morbidity and mortality because of the

formation of meningitis (inflammation of the meninges). It can be treated with

high-dose antibiotics, and sometimes surgery is needed to drain the infected

sinuses. Corticosteroids may reduce edema and inflammation as adjunctive

therapy.

G. Superior petrosal sinus

Lies in the margin of the tentorium cerebelli, running from the posterior

end of the cavernous sinus to the transverse sinus.

H. Inferior petrosal sinus

Drains the cavernous sinus into the bulb of the internal jugular vein.

Runs in a groove between the petrous part of the temporal bone and the

basilar part of the occipital bone.

I. Sphenoparietal sinus

Lies along the posterior edge of the lesser wing of the sphenoid bone and

drains into the cavernous sinus.

J. Occipital sinus

Lies in the falx cerebelli and drains into the confluence of sinuses.

K. Basilar plexus

Consists of interconnecting venous channels on the basilar part of the

occipital bone and connects the two inferior petrosal sinuses.

Communicates with the internal vertebral venous plexus.

L. Diploic veins

Lie in the diploe of the skull and are connected with the cranial dura

sinuses by the emissary veins.

M. Emissary veins

Are small veins connecting the venous sinuses of the dura with the diploic

veins and the veins of the scalp.

VIII. Blood Supply of the Brain

A. Internal carotid artery

Enters the carotid canal in the petrous portion of the temporal bone.

Is separated from the tympanic cavity by a thin bony structure.

Lies within the cavernous sinus and gives rise to small twigs to the wall of

the cavernous sinus, to the hypophysis, and to the semilunar ganglion of

the trigeminal nerve.

Pierces the dural roof of the cavernous sinus between the anterior clinoid

process and the middle clinoid process, which is a small projection

posterolateral to the tuberculum sellae.

Forms a carotid siphon (a bent tube with two arms of unequal length),

which is the petrosal part just before it enters the cranial cavity.

1. Ophthalmic artery

Enters the orbit via the optic canal with the optic nerve.

2. Posterior communicating artery

Arises from the carotid siphon and joins the posterior cerebral artery.

Runs backward below the optic tract and supplies the optic chiasma and

tract and hypothalamus.

3. Anterior choroidal artery

Supplies the choroid plexus of the lateral ventricles, optic tract and

radiations, and lateral geniculate body.

4. Anterior cerebral artery

Enters the longitudinal fissure of the cerebrum, supplies the optic chiasma

and medial surface of the frontal and parietal lobes of the brain, and unites

each by the short anterior communicating artery.

5. Middle cerebral artery

Passes laterally in the lateral cerebral fissure and supplies the lateral

convexity of the cerebral hemisphere.

B. Vertebral arteries

Arise from the first part of the subclavian artery and ascend through the

transverse foramina of the vertebrae C1 to C6.

Curve posteriorly behind the lateral mass of the atlas, pierce the dura mater

into the vertebral canal, and then enter the cranial cavity through the

foramen magnum.

Join to form the basilar artery.

Give rise to the following:

1. Anterior spinal artery

Arises as two roots from the vertebral arteries shortly before the junction

of the vertebral arteries.

Descends in front of the medulla, and the two roots unite to form a single

median trunk at the level of the foramen magnum.

2. Posterior spinal artery

Arises from the vertebral artery or the posterior–inferior cerebellar

artery.

Descends on the side of the medulla, and the right and left roots unite at

the lower cervical region.

3. Posterior inferior cerebellar artery

Is the largest branch of the vertebral artery, distributes to the posterior

inferior surface of the cerebellum, and gives rise to the posterior spinal

artery.

C. Basilar artery

Is formed by the union of the two vertebral arteries at the lower border of

the pons.

Ends near the upper border of the pons by dividing into the right and left

posterior cerebral arteries.

1. Pontine arteries

Are several in number and supply the pons.

2. Labyrinthine artery

Enters the internal auditory meatus and supplies the cochlea and vestibular

apparatus.

3. Anterior and inferior cerebellar artery

Supplies the anterior part of the inferior surface of the cerebellum.

Gives rise to the labyrinthine artery in 85% of the population.

4. Superior cerebellar artery

Passes laterally just behind the oculomotor nerve and supplies the superior

surface of the cerebellum.

5. Posterior cerebral artery

Is formed by bifurcation of the basilar artery passes laterally in front of the

oculomotor nerve, winds around the cerebral peduncle, and supplies the

midbrain and the temporal and occipital lobes of the cerebrum.

D. Circle of Willis (circulus arteriosus)

Is formed by the posterior cerebral, posterior communicating, internal carotid,

anterior cerebral, and anterior communicating arteries.

Forms an important means of collateral circulation in the event of

obstruction.

IX. Development of the Skull

A. Cranial base develops mainly by endochondral ossification.

B. Cranial vault and facial skeleton develop by intramembranous ossification.

C. Sutures are important sites of growth and allow bones to overlap (molding)

during birth.