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Transverse View of Brainstem at Tentorial Incisura

Surgical Correlation

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Transverse view of brainstem at tentorial incisura (tentorial notch). The tentorium cerebelli is a dural fold that separates the occipital lobes of the cerebrum from the cerebellum. It attaches to the clinoid processes of the sphenoid anteriorly and along the superior border of the petrous temporal bone. It attaches posteriorly to the transverse groove of the occipital bone. A midline aperture anteriorly, the tentorial notch, permits passage of the brainstem into the posterior fossa. In this view, the optic nerves have emerged from the optic canals and join at the optic chiasm. From here, the right and left optic tracts course posterolaterally to end largely in the lateral geniculate nuclei within the thalami. The internal carotid arteries, located just lateral to the optic nerves, divide into their terminal anterior and middle cerebral arteries. The anterior cerebral arteries course medially toward the interhemispheric fissure (A1 segments). The short anterior communicating artery unites right and left anterior cerebral arteries, which continue as the A2 segments. The middle cerebral arteries curve laterally to enter the Sylvian fissures. Prior to their termination, the internal carotid arteries give rise to posterior communicating arteries that connect to the posterior cerebral arteries as part of the anastomotic cerebral arterial circle (of Willis). The posterior cerebral arteries are terminal branches of the basilar artery. A P2 segment is seen on the left coursing posteriorly in a supratentorial direction toward the occipital lobe. The oculomotor nerves emerge from the anterior midbrain between the posterior cerebral and superior cerebellar arteries. The trochlear nerve, on the other hand, emerges from the dorsal (tectal) surface of the midbrain. Both nerves course forward to enter the lateral wall of the cavernous sinus. The great cerebral vein (great vein of Galen) lies within the quadrigeminal cistern posterior to the colliculi of the midbrain and is formed by union of the two internal cerebral veins and basal veins of Rosenthal. It curves backward and upward to empty at the confluence of the inferior sagittal sinus and straight sinus at the junction of the falx cerebri and the tentorium.  (Image courtesy of AL Rhoton, Jr.)

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