Although the extracranial veins of the scalp and face are not discussed in detail, they do communicate with the intracranial venous system and can have pathological importance. Scalp and facial veins may drain to the dural sinuses via emissary veins that pass through the skull and orbit. These extracranial veins also anastomose with both the cavernous sinus and the basilar, or clival, plexus. They play a relatively minor role in the normal circulatory pattern of the brain but can be important clinically as a nidus for fistula formation, as a path for the spread of infection, or as a collateral drainage system.
The principal route of venous drainage of the brain is through a system of cerebral veins that empty into the dural venous sinuses. The majority of dural sinuses join at the torcula of Herophili (confluence of sinuses) and then split into the left and right transverse sinuses, which flow into the sigmoid sinuses and ultimately into the internal jugular veins. The cerebral veins are conventionally divided into superficial and deep groups.
The major dural venous sinuses include the superior and inferior sagittal sinuses, the straight sinus, the superior and inferior petrosal sinuses, the occipital sinus, the transverse sinus, and the sigmoid sinuses (Fig. 5). The superior and inferior sagittal sinuses run in the midline along the superior and inferior edges of the falx cerebri (Fig. 6). The inferior sagittal sinus joins the vein of Galen to form the straight sinus, which then courses along the superior edge of the tentorium cerebelli. The occipital sinus originates at the posterior edge of the foramen magnum and meets the straight sinus and superior sagittal sinus at the torcula of Herophili. The superior petrosal sinus joins the cavernous to the sigmoid sinus and runs along the
superior ridge of the petrous bone. The inferior petrosal sinus has variable tributaries but drains into the internal jugular vein at the jugular bulb.
The superficial veins are quite variable and most are unnamed. Only three of these veins are reasonably constant (Fig. 7). The superficial middle cerebral vein runs along the lateral sulcus draining most of the temporal lobe into the cavernous sinus or into the nearby sphenoparietal sinus. The superior anastomo-tic vein, or vein of Trolard, typically travels across the parietal lobe and drains the superficial middle cerebral vein into the superior sagittal sinus. The inferior anastomotic vein, or vein of Labbe, travels posteriorly and inferiorly across the temporal lobe and connects the superficial middle cerebral vein with the transverse sinus.
The deep veins are more constant in configuration than are the superficial veins. The major deep vein is the internal cerebral vein, which is formed at the intraven-tricular foramen by the confluence of the septal vein and the thalamostriate veins (Fig. 5). The septal vein runs posteriorly across the septum pellucidum, and the thalamostriate vein travels in the groove between the thalamus and the caudate nucleus.
Immediately after forming, the internal cerebral vein bends sharply in a posterior direction. This bend is called the venous angle and is used in imaging studies as an indication of the location of the interventricular foramen. The paired internal cerebral veins proceed posteriorly through the transverse cerebral fissure and fuse in the superior cistern to form the unpaired great cerebral vein (or vein of Galen). The great cerebral vein turns superiorly and joins the inferior sagittal sinus to form the straight sinus. Along its short course, the great vein receives the basal veins of Rosenthal. The basal vein is formed near the optic chiasm by the deep middle cerebral vein, which drains the insula, and several other tributaries that drain inferior portions of the basal ganglia and the orbital surface of the frontal lobe.
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