Figure 1: Superior view of the interior of the skull. The skull floor consists of a series of anterior-to-posterior depressions or fossae: anterior, middle, and posterior. The anterior fossa is formed by the orbital plates of the frontal bone, cribriform plate of the ethmoid, and lesser wings of the sphenoid. It features the crista galli for attachment of the falx cerebri and the cribriform plate containing multiple foramina for the neurofilaments of the olfactory nerve. The middle fossa extends from the lesser wings of the sphenoid to the superior border of the petrous portion of temporal bone and includes the midline body of the sphenoid. Foramina of interest here include the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum, foramen lacerum, and intracranial opening of the carotid canal. The posterior fossa extends from the superior border of the petrous bone to the transverse groove of the occipital bone. These are the attachment sites of the tentorium cerebelli and contain, respectively, the superior petrosal sinus and transverse sinus. Foramina in the posterior fossa include the internal auditory meatus, jugular foramen, hypoglossal canal, and the foramen magnum. (Image courtesy of AL Rhoton, Jr.)

Figure 2: Endocranial view of skull base. The anterior cranial fossa is comprised of frontal and ethmoid bones and is bounded posteriorly by the lesser wings of sphenoid. The middle cranial fossa is comprised of the body and greater wings of the sphenoid and the squamous part of the temporal bone and is bounded posteriorly by the petrous portion of temporal bone. The middle cranial fossa contains the optic canal, superior orbital fissure, foramen rotundum, ovale, spinosum, and lacerum. The posterior cranial fossa is bounded anteriorly by the petrous bone and posteriorly by the occipital bone, the principal bone of this fossa. The posterior fossa contains the internal acoustic meatus, foramen magnum, jugular foramen, and hypoglossal canal. The dorsum sellae of the sphenoid and the basilar portion of the occipital combine to form the clivus, the slope of bone anterior to the foramen magnum. The sella turcica is bounded anteriorly by the tuberculum sellae, a ridge of bone posterior to the chiasmatic groove, and the dorsum sellae posteriorly, and contains the pituitary gland. The jugular foramen is partially divided by the intrajugular processes of temporal and occipital bones into two parts: pars nervosa and pars vascularis. The pars nervosa is the smaller anteromedial part of the jugular foramen and transmits the inferior petrosal sinus and the glossopharyngeal nerve. The pars vascularis is the larger posterolateral portion of the foramen for the internal jugular vein and vagus and spinal accessory nerves. (Image courtesy of AL Rhoton, Jr.)

Figure 3: Superior view of sphenoid bone. This view presents the midline body and paired greater and lesser wings of the sphenoid. The body and lesser wings form the anterior boundary of the middle cranial fossa, while the greater wings and squamous portions of the temporal bones form the lateral margins. The middle fossa is bounded posteriorly by the petrous portions of the temporal bones. The body consists anteriorly of the smooth surfaced planum sphenoidale that connects the right and left lesser wings (and roofs the anterior most part of the sphenoid sinus), the chiasmatic sulcus, the tuberculum sellae, sella turcica, and dorsum sellae. The optic nerves exit the orbit through the optic canals within the lesser wings and the anterior portion of the optic chiasm lies within the prechiasmatic sulcus. The central depression within the sella turcica is the hypophyseal fossa which is occupied by the hypophysis (pituitary gland). Pairs of clinioid processes project from the lesser wings (anterior), lateral edges of the tuberculum sellae (middle), and from lateral margins of the dorsum sellae (posterior). The greater wings contain a "crescent" of foramina beginning anteriorly as the superior orbital fissure between the lesser and greater wings (not labeled here), the foramen rotundum for the maxillary nerve,  the foramen ovale for the mandibular and lesser petrosal nerves, and the foramen spinosum for the middle meningeal artery. Medial to the foramen ovale in this specimen are inconstant bilateral sphenoidal emissary foramina (foramen of Vesalii) for passage of a vein from the pterygoid venous plexus to the cavernous sinus.  (Image courtesy of AL Rhoton, Jr.)

Figure 4: Inferior view of skull base. The maxilla is the central bone of the face and contains the largest paranasal sinus, the maxillary sinus. It also forms the principal part of the hard palate (the horizontal part of the palatine bone forms the posterior border of the hard palate). Behind the central incisors is the incisive foramen. Medial to the maxillary molar teeth is the greater palatine foramen. These transmit the neurovascular supply to the hard palate.  The pterygoid processes of the sphenoid are located posterior to the dental arch and form the medial boundary of the infratemporal fossa. The undersurface of the greater wing of the sphenoid forms the roof of this fossa and contains the foramen ovale and spinosum. The infratemporal fossa is continuous with the temporal fossa superiorly through the space between the zygomatic arch and lateral skull. The vomer is a midline bone that attaches from the inferior surface of the body of the sphenoid to the superior surface of the hard palate. It is the posterior border of the nasal septum. Lateral to it are the choanae; passageways connecting the nasal cavity with the nasopharynx. The foramen lacerum is a space bounded by the apex of the petrous temporal bone, the greater wing of the sphenoid, and the clivus. Its floor is filled in life by fibrocartilage. The petrous bone contains the opening of the carotid canal and posterior to its external opening is the jugular foramen, a space between the petrous bone and occipital bone. Behind the root of the zygomatic arch is the mandibular fossa for articulation with the mandibular condyles. Posterior to this fossa are the styloid and mastoid processes with the stylomastoid foramen between them for exit of the facial nerve. The mastoid notch is located on the medial surface of the mastoid for attachment of the posterior digastric muscle. Medial to this notch is the occipital groove for the occipital artery. The occipital bone forms the posterior aspect of the skull. It contains the large foramen magnum, the occipital condyles for articulation with the atlas, and nuchal lines (superior and inferior) and the external occipital protuberance for muscle and nuchal ligament attachments. (Image courtesy of AL Rhoton, Jr.)

Figure 5: Superior view of disarticulated sphenoid, temporal, and occipital bones. The sphenoid and occipital bones are unpaired unlike the temporal bones, which are paired. In this perspective, the body and greater and lesser wings of the sphenoid can be seen. The optic canals are openings in the lesser wings transmitting the optic nerve and ophthalmic artery. Within the greater wing is the foramen ovale for the mandibular and lesser petrosal nerves, and the foramen spinosum for the middle meningeal artery. The foramen lacerum is a space between the body and greater wing of the sphenoid and the apex of the petrous temporal bone. Its floor is filled with fibrocartilage superior to which courses the internal carotid artery (its lacerum segment). The occipital bone contains the foramen magnum, the largest of the cranial foramina, which transmits the vertebral arteries, spinal accessory nerves, spinal cord and meningeal coverings. Anterior to this foramen is the basilar part of the occipital bone that unites with the dorsum sellae of the sphenoid to form the clivus. The temporal bones articulate with both the sphenoid and occipital bones. Its petrous portion houses and protects middle and inner ear structures and serves as the boundary between the middle and posterior cranial fossae. On its posteromedial surface is the internal acoustic meatus for the facial and vestibulocochlear nerves and the labyrinthine artery. The junction between the petrous bone and clivus, the petroclival fissure, is occupied by the inferior petrosal sinus, which drains the cavernous sinus to the internal jugular vein via the jugular foramen. (Image courtesy of AL Rhoton, Jr.)

Figure 6: Anterior inferior view of skull base. In this specimen, the clivus and body of the sphenoid occupy the midline. The sphenoid sinus is exposed. The lesser and greater wings of the sphenoid can be observed as well as the superior orbital fissure, the space between them. Inferolateral to the sphenoid body descend the pterygoid processes, which give rise to lateral and medial pterygoid plates. The foramen rotundum and pterygoid canal open on the anterior superior surface of each pterygoid process which serves as the posterior boundary of the pterygopalatine fossa. The lateral pterygoid plate serves as the medial boundary of the infratemporal fossa and these two fossae communicate via the pterygomaxillary fissure. Posteriorly, the jugular foramen can be seen as a space between the occipital bone and the petrous portion of the temporal. (Image courtesy of AL Rhoton, Jr.)

Figure 7: Posterior view of an isolated sphenoid bone. It is the principal bone of the middle cranial fossa. It consists of a midline body, a pair of lesser wings that contain the optic canals, a pair of greater wings that contain a "crescent of foramina" including the superior orbital fissure, foramen rotundum, foramen ovale, and foramen spinosum, and a pair of pterygoid processes that project inferiorly. The pterygoid canals are located within the base of the pterygoid processes inferomedial to the foramen rotundum. The lesser wing articulates anteriorly with the frontal bone and cribriform plate of the ethmoid. The occipitosphenoid suture/synchondrosis marks the articulation between the dorsum sella and basilar portion of the occipital bone. Together, these form the clivus or the slope of bone anterior to the foramen magnum. (Image courtesy of AL Rhoton, Jr.)

Figure 8: Right lateral view of articulated temporal, sphenoid, frontal, and zygomatic bones. The zygomatic arch is formed by the union of the temporal and zygomatic processes of the zygomatic and temporal bones, respectively. Deep to the zygomatic bone and arch is the temporal fossa which is occupied by the temporalis muscle and its neurovasculature. The pterion is a craniometric point deep within the temporal fossa that is formed by the union of the greater wing of the sphenoid, squamous portion of the temporal, frontal, and parietal bones. The pterion is a thin region of bone that overlies the anterior division of the middle meningeal artery. (Image courtesy of AL Rhoton, Jr.)

Figure 9: Right anterior view of the bony orbit. Portions of six bones form its pyramidal walls: frontal, ethmoid, palatine, lacrimal, maxilla, zygomatic, and sphenoid. A “V”-shaped space (with the V laying on its side) consisting of the superior and inferior orbital fissures communicate structures from the middle fossa and infratemporal/pterygopalatine fossa, respectively. Two small foramina appear near or in the frontoethmoidal suture; the anterior and posterior ethmoidal foramina. A few millimeters posterior to the posterior ethmoidal foramen is the optic canal. The infraorbital groove begins proximally along the floor of the orbit, near the inferior orbital fissure, and becomes the infraorbital canal before opening at the infraorbital foramen a few millimeters below the inferior margin of the orbit. Nerves and blood vessels traverse these foramina and fissures. (Image courtesy of AL Rhoton, Jr.)

Figure 10: Posterior surface of right petrous bone. The petrous bone forms the anterior boundary of the posterior cranial fossa. On its posterior surface are openings for the internal auditory meatus and extraosseous portion of the endolymphatic sac. Along its inferior border is the jugular foramen, a space between the petrous bone and occipital bone. Between the petrous bone and clivus is the petroclival fissure for the inferior petrosal sinus. This drains the cavernous sinus into the internal jugular vein through the jugular foramen. Posteriorly, the sigmoid groove for the sigmoid sinus leads into this foramen. 

Figure 11: Fundic view of bony right internal auditory canal. The internal auditory canal has been drilled to visualize the domains or areas the facial and separate vestibulocochlear nerves enter. The facial canal for the facial nerve is separated from the superior vestibular nerve area by the vertical crest or Bill's bar. The facial nerve lies anterior to the superior vestibular nerve here. The transverse crest separates a superior compartment containing the facial and superior vestibular nerves from an inferior compartment containing the cochlear and inferior vestibular nerves. The cochlear area lies anterior to the inferior vestibular area. Posterior to the inferior vestibular foramen is the foramen singulare for the singular branch of the inferior vestibular nerve. 

Figure 12: Lateral view of the surface of the right cerebral hemisphere. The frontal and parietal lobes are separated by the Rolandic sulcus (central sulcus), which divides the precentral gyrus of the frontal lobe from the postcentral gyrus of the parietal lobe. The frontal lobe has several prominent gyri; the superior, middle and inferior frontal gyri lie anterior to the precentral gyrus. The supramarginal gyrus along with the more posterior angular gyrus form the inferior parietal lobule. The Sylvian fissure divides the frontoparietal lobes from the temporal lobe, and from the Sylvian fissure emerge branches of the middle cerebral arteries. The parietooccipital sulcus separates the occipital and parietal lobes, and the pre-occipital notch separates the occipital and temporal lobes. (Image courtesy of AL Rhoton, Jr.)

Figure 13: A demonstration of the left lateral view of the cerebrum and its topographic features. (Image courtesy of AL Rhoton, Jr.)

Figure 14: Superficial venous vasculature of right cerebral hemisphere. The cerebral veins are a group of superficial veins that drain into the superior sagittal and transverse sinuses or into the superior anastomotic vein (vein of Trolard) or inferior anastomotic vein (vein of Labbe). The vein of Trolard connects the superficial middle cerebral vein with the superior sagittal sinus located in the upper part of the falx cerebri. The superficial middle cerebral vein (of Sylvius) begins on the lateral hemispheric surface and follows along the lateral sulcus anteriorly where it curves around the temporal lobe to end in the cavernous or sphenoparietal sinus.

Figure 15: Midsagittal view of cerebrum. Brainstem and cerebellum have been removed. The cingulate gyrus is closely associated with the corpus callosum throughout its length and is separated from the frontal and parietal lobes by the cingulate sulcus. The paracentral lobule is the medial continuation of the precentral and postcentral gyri and includes portions of the frontal and parietal lobes. Posteriorly, the parietooccipital sulcus separates the superior parietal lobe from the cuneus and lingual gyrus of the occipital lobe. The uncus is located on the medial aspect of the temporal lobe anterior to the parahippocampal gyrus. (Image courtesy of AL Rhoton, Jr.)

Figure 16: Posterior dissection of the posterior cranial fossa and foramen magnum. A suboccipital craniectomy and an upper cervical laminectomy have been performed in order to expose the contents of the posterior cranial fossa and the foramen magnum.  Several lobules of the posterior lobe of the cerebellum are visible, as are the cerebellar tonsils.  The latter structures, parts of the posterior lobe, are ovoid structures located on either side of the midline cerebellar vermis.  The tonsils are of clinical significance because with increased intracranial pressure (ICP) the tonsils can herniate into the foramen magnum (tonsillar herniation) and compress the caudal brainstem.  Immediately caudal to the inferior pole of the tonsils  the foramen magnum, a large, ovoid shaped opening in the occipital bone through which passes the C1 spinal cord, the vertebral arteries, and the spinal accessory nerves.  The dorsal rootlets of spinal nerves C2 and C3, and a cervical denticulate ligament, are visible near the bottom of the image attached to the upper cervical spinal cord. (Image courtesy of AL Rhoton, Jr.)

Figure 17: The fourth ventricle. The fourth ventricle is a midline, CSF-filled cavity located posterior to the pons and rostral medulla, and anterior to the cerebellum.  In this dissection, the cerebellar tonsils have been retracted laterally and the inferior medullary velum (a thin membrane that forms the posterior roof of the IV ventricle) has been incised to expose the walls and floor of the ventricle.  The view is looking rostrally into the fourth ventricle, in the direction of the cerebral aqueduct.  The floor of the IV ventricle is diamond-shaped; it is widest in its central region, where it lies dorsal to the pons-medulla junction, and tapers gradually in both the rostral and caudal directions.  Rostrally, it communicates with the cerebral aqueduct whereas caudally it continues as the central canal.  Inferiorly, the caudal walls of the IV ventricle fuse at the midline, wedge-shaped obex.   A prominent tuft of choroid plexus is suspended from the undersurface of the inferior medullary velum and extends anteriorly and laterally on both sides, through the lateral recesses, to enter the foramina of Luschke.  The latter are paired, tunnel-like openings that curve anteriorly around the brainstem to connect the midline IV ventricle with the cerebellomedullary cistern (cisterna magna).  A third opening, the midline, funnel-shaped foramen of Magendie, has been removed by the dissection; it would be located in the inferior medullary velum immediately superior to the obex. (Image courtesy of AL Rhoton, Jr.)

Figure 18: Posterior view of the Foramen of Magendie and fourth ventricle.  In this dissection, the cerebellar tonsils and vermis have been retracted to expose the inferior medullary velum, which forms the posterior roof of the fourth ventricle.  The velum consists largely of tela choroidea, a thin membrane lined by pia mater on its outer surface and ependyma on its inner surface.   A tuft of choroid plexus develops from the tela choroidea and suspends into the roof into the IV ventricle. The Foramen of Magendie, a midline opening in the inferior medullary velum, is one of three openings (the other two are the paired lateral foramina of Luschke) that drain CSF from the IV ventricle into the foramen magnum.  The posterior inferior cerebellar artery (PICA), seen prominently on the left side of the image, is a branch of the vertebral artery. PICA supplies the dorsolateral medulla and then reflects onto the posterior and inferior surface of the cerebellum. (Image courtesy of AL Rhoton, Jr.)

Figure 19: Ventral view of brainstem and cerebellum and emergence of cranial nerves. The brain stem consists of the midbrain, pons, and medulla. The oculomotor nerve (CN III) exits at the midbrain and pontine junction while the trigeminal nerve (CN V) exits at the lateral pons. The trochlear nerve (CN IV) is not seen in this image as it exits from the dorsal midbrain (the only cranial nerve to do so). Abducens nerve (CN VI) exits medially at the pontomedullary junction while facial (CN VII) and vestibulocochlear (CN VIII) nerves exit at this junction more laterally. Glossopharyngeal (CN IX) and vagus (CN X) nerves exit the medulla lateral to the olives. The spinal  accessory nerve (CN XI) arises from ventral horn cells of the upper five cervical spinal segments and ascends through the foramen magnum to exit through the jugular foramen with glossopharyngeal and vagus nerves. The hypoglossal nerve exits the medulla between the pyramids and olives to pass through the hypoglossal canals. (Image courtesy of AL Rhoton, Jr.)

Figure 20: Lateral Perspective of the Right Brainstem and Cerebellum. At the top of the image, the middle cerebellar peduncle (MCP) connects the basis pons with the cerebellar white matter.  Note the attachment of the trigeminal nerve at the approximate transition between the basis pons and MCP, and the attachment of the abducens nerve on the ventral surface of the pontomedullary junction, in line with the preolivary sulcus.  The inferior cerebellar peduncle is located inferiomedially and masked from view in this lateral perspective. The foramen of Luschka is clearly visible, with a conspicuous tuft of choroid plexus extending through it, just inferior to the flocculus. Note the close association between the flocculus and the vestibulocochlear nerve complex. (Image courtesy of AL Rhoton, Jr.)

Figure 21: Ventral view of posterior brain vasculature. Paired vertebral arteries ascend through the foramen magnum where they give off posterior inferior cerebellar arteries (PICA). The PICA pass between the roots of the vagus (CN X) and spinal accessory (CN XI) nerves on their way to supply the cerebellum. The vertebral arteries unite to form the basilar artery which runs along the ventral pons. The anterior inferior cerebellar artery (AICA) branches from the basilar artery often between the roots of the facial (CN VII) and vestibulocochlear (CN VIII) nerves. At the distal end of the basilar artery, superior cerebellar arteries (SCA) branch and the basilar artery terminates at the posterior cerebral arteries (PCA). The oculomotor nerve (CN III) exits the midbrain-pontine junction between the SCA and PCA. (Image courtesy of AL Rhoton, Jr.)

Figure 22: Posterior superior view of the tentorium and underlying cerebellar hemispheres and neighboring vascular and cisternal elements. (Image courtesy of AL Rhoton, Jr.)

Figure 23: Posterior view of right dorsal cerebellum and posterior cerebrum. The quadrigeminal cistern is a subarachnoid cistern located between the collicular plate, splenium of corpus callosum, and dorsal surface of the cerebellum. It contains the posterior cerebral arteries, the superior cerebellar arteries, the great cerebral vein (of Galen), and the trochlear nerve (CN IV). The great cerebral vein merges with the inferior sagittal sinus (removed) to form the straight sinus located at the intersection of the falx cerebri and tentorium cerebelli. The trochlear nerve is the cranial nerve that arises from the dorsum of the midbrain. (Image courtesy of AL Rhoton, Jr.)

Figure 24: Posterior view of midbrain. The cerebellum has been retracted inferiorly to expose the superior and inferior colliculi. The tentorium cerebelli is visible on either side of the midbrain. The great cerebral vein of Galen lies in the quadrigeminal cistern and is formed by the union of the internal cerebral veins and the basal veins of Rosenthal just posterior to the pineal gland. The vein of Galen receives blood from deep cerebral veins as well as the vein of the cerebellomesencephalic fissure, which drains the superior aspect of the cerebellar vermis and adjacent portions of the cerebellar hemispheres. The vein of Galen travels posteriorly and superiorly to drain into the straight sinus, which is located in the junction between the falx cerebri and the tentorium cerebelli. (Image courtesy of AL Rhoton, Jr.)

Figure 25: Inferior view of the pineal region posterior to the midbrain. The superior colliculus and pineal gland are visible with retraction of the precentral cerebellar vein (also known as the vein of the cerebellomesencephalic fissure). Also visualized are the deep veins of the cerebrum: the basal veins of Rosenthal join the internal cerebral veins. Together, these veins form the vein of Galen. Medial posterior choroidal arteries are visualized here and usually arise from the proximal P2 segment of the posterior cerebral artery. (Image courtesy of AL Rhoton, Jr.)

Figure 26: Dorsal view of the midbrain and quadrigeminal (tectal) plate. Visualized are the superior and inferior colliculi of the midbrain. The trochlear nerve arises from the dorsal midbrain at the level of the cerebellomesencephalic fissure and courses laterally around the midbrain. The pineal gland and third ventricle are visualized after removal of the velum interpositum. (Image courtesy of AL Rhoton, Jr.)

Figure 27: Midsagittal section of the third ventricle and surrounding deep brain anatomy. This midline view demonstrates the roof of the third ventricle formed by the fornices, coursing with the internal cerebral veins. In the midline, the lamina terminalis, optic chiasm and pituitary infundibulum are visible and border the third ventricle anteriorly. The splenium, body, genu and rostrum of the corpus callosum are visible along with the pericallosal branch of the anterior cerebral artery. The floor of the third ventricle is composed of the tuber cinereum of the hypothalamus, the mammillary bodies, and the midbrain tegmen. The inferior sagittal sinus can be seen draining into the straight sinus along with the Vein of Galen. (Image courtesy of AL Rhoton, Jr.)

Figure 28: Floor of the fourth ventricle. The cerebellar peduncles have been sectioned and the cerebellum removed in order to expose the floor of the fourth ventricle.  The floor of the IV ventricle is formed by the dorsal surfaces of the pons and the rostral medulla.  As seen from above, the IV ventricle is diamond-shaped.  It is widest at the point where the pons and medulla merge, i.e., at the level of the lateral recess.  The lateral recess is a wedge-shaped, tapered region of the IV ventricle that merges laterally with the Foramen of Luschke.  The paired Foramina of Luschke are two of three openings (the third opening is the unpaired, midline Foramen of Magendie) through which CSF in the IV ventricle drains into the cisterna magnum.  Caudally, the ventrolateral walls of the ventricle fuse at midline as the obex.  At the obex, the IV ventricle continues caudally as the central canal (not visible in this dissection).  Rostrally, the IV ventricle tapers and continues into the midbrain as the tunnel-like cerebral aqueduct, also not visible in this image.  The floor of the IV ventricle is divided into symmetrical halves by the midline median sulcus.  Located immediately lateral to the median sulcus in the caudal pons are paired prominent bulges, the facial colliculi.  These subtle surface elevations are formed by the underlying abducens nucleus and the arching fibers (interal genu) of the facial nerve.  Further caudally, the columnar-shaped hypoglossal nucleus and dorsal motor nucleus of the vagus form subtle surface elevations on the dorsal surface of the medulla, the hypoglossal and vagal trigones, respectively. (Image courtesy of AL Rhoton, Jr.)

Figure 29: Superior view of the body of lateral ventricles with anterior horns of corpus callosum reflected. The corpus callosum forms the roof of the lateral ventricles. The lateral ventricles are separated medially by the midline septum pellucidum and bounded inferolaterally by the caudate nucleus. The thalamus is posteroinferior to the body of the lateral ventricles. The choroid plexus consists of ependymal cells that produce cerebrospinal fluid (CSF) and lie at the floor of the lateral ventricles. The interventricular foramen of Monro conveys CSF from the lateral ventricles to the midline third ventricle. (Image courtesy of AL Rhoton, Jr.)

Figure 30: Superior view of the lateral and third ventricles through the velum interpositum after partial resection of the fornices. The choroid plexus of both lateral ventricles is visible entering the foramina of Monro. At this level, the paired septal veins merge with the thalamostriate veins to form both internal cerebral veins, which course posteriorly along the roof of the third ventricle. Choroidal veins and arteries are visible. (Image courtesy of AL Rhoton, Jr.)