Brief Anatomy of the Skull and Cranial Cavity


I. Skull
 Is the skeleton of the head and may be divided into two types of...
•   Consists of the squamous part, which is external to the lateral surface of
       the temporal lobe of the brain; the ...
•   Pterion: a craniometric point at the junction of the frontal, parietal, and
       temporal bones and the great wing o...
B. Middle cranial fossa

   •   Optic canal: optic nerve, ophthalmic artery, and central artery and vein of
       the ret...
•   Infraorbital foramen: infraorbital nerve and vessels.
   •   Mental foramen: mental nerve and vessels.

E. Foramina in...
•   Is perforated by 15 to 20 foramina, supports the olfactory bulb, and
       transmits olfactory nerves from the olfact...
•   Has a deep central depression known as the hypophyseal fossa, which
       accommodates the pituitary gland or the hyp...
•   May contain blood after hemorrhage of a cerebral artery.
   •   Projects into the venous sinuses to form arachnoid vil...
Subdural hematoma: is due to rupture of bridging cerebral veins as they pass from
the brain surface into one of the venous...
o    Is a crescentic fold of dura mater that supports the occipital lobes of
                the cerebral hemispheres and ...
E. Sigmoid sinus

   •   Is a continuation of the transverse sinus; arches downward and medially in
       an S-shaped gro...
high-dose antibiotics, and sometimes surgery is needed to drain the infected
sinuses. Corticosteroids may reduce edema and...
•   Are small veins connecting the venous sinuses of the dura with the diploic
       veins and the veins of the scalp.

V...
•   Enters the longitudinal fissure of the cerebrum, supplies the optic chiasma
       and medial surface of the frontal a...
•   Is the largest branch of the vertebral artery, distributes to the posterior
       inferior surface of the cerebellum,...
Is formed by the posterior cerebral, posterior communicating, internal carotid,
anterior cerebral, and anterior communicat...
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Review Of Skull And Cranial Cavity

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Review Of Skull And Cranial Cavity

  1. 1. 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
  2. 2. • 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.
  3. 3. • 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.
  4. 4. 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.
  5. 5. • 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
  6. 6. • 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.
  7. 7. • 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.
  8. 8. • 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.
  9. 9. 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
  10. 10. 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.
  11. 11. 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
  12. 12. 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
  13. 13. • 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
  14. 14. • 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
  15. 15. • 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)
  16. 16. 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.

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