VENOUS DRAINAGE OF THE BRAIN
The veins of the brain stem form a superficial venous plexus deep to the arteries.
Veins of the medulla oblongata drain into the veins of the spinal cord or the adjacent dural venous sinuses, or into variable radicular veins which accompany the last four cranial nerves to either the inferior petrosal or occipital sinuses, or to the superior bulb of the jugular vein. Anterior and posterior median medullary veins may run along the anterior median fissure and posterior median sulcus, to become continuous with the spinal veins in corresponding positions. Pontine veins, which may include a median vein and a lateral vein on each side, drain into the basal vein, cerebellar veins, the petrosal sinuses, transverse sinus or the venous plexus of the foramen ovale. Veins of the midbrain join the great cerebral vein or basal vein.
VEINS OF THE CEREBELLUMThe veins of the cerebellum drain mainly into sinuses adjacent to them or, from the superior surface, into the great cerebral vein. The cerebellar veins course on the cerebellar surface, and comprise superior and inferior groups. Superior cerebellar veins either run anteromedially across the superior vermis to the straight sinus or great cerebral vein, or they run laterally to the transverse and superior petrosal sinuses. Inferior cerebellar veins include a small median vessel running backwards on the inferior vermis to enter the straight or sigmoid sinus. Laterally coursing vessels join the inferior petrosal and occipital sinuses.
VEINS OF THE CEREBRAL HEMISPHEREExternal and internal cerebral veins (Figs 17.14-17.18) drain the surfaces and the interior of the cerebral hemisphere.
External cerebral veins may be divided into three groups, namely superior, middle and inferior.
Figure 17.15 The external (superficial) cerebral veins of the left hemisphere and their relationship to the dural venous sinuses.
Figure 17.16 The internal (deep) cerebral veins, viewed from above after removal of the central portion of the corpus callosum.
Figure 17.17 Lateral projection from a magnetic resonance venogram using time-of-flight methods. (By kind permission from Professor PD Griffiths, Academic Unit of Radiology, The University of Sheffield.)
Eight to twelve superior cerebral veins drain the superolateral and medial surfaces of each hemisphere. They mainly follow the sulci, although some do pass across gyri. They ascend to the superomedial border of the hemisphere, where they receive small veins from the medial surface, and then open into the superior sagittal sinus. Superior cerebral veins in the anterior part of the hemisphere join the sinus almost at right angles. The larger posterior veins are directed obliquely forwards, against the direction of flow in the sinus, an arrangement which may resist their collapse when intracranial pressure is raised.
The superficial middle cerebral vein drains most of the lateral surface of the hemisphere, and follows the lateral fissure to end in the cavernous sinus. A superior anastomotic vein runs posterosuperiorly between the superficial middle cerebral vein and the superior sagittal sinus, thus connecting the superior sagittal and cavernous sinuses. An inferior anastomotic vein courses over the temporal lobe and connects the superficial middle cerebral vein to the transverse sinus. The deep middle cerebral vein drains the insular region and joins the anterior cerebral and striate veins to form a basal vein. Regions drained by the anterior cerebral and striate veins correspond approximately to those supplied by the anterior cerebral artery and the central branches which enter the anterior perforated substance. The basal veins pass back alongside the interpeduncular fossa and midbrain, receive tributaries from this vicinity and join the great cerebral vein.
Inferior cerebral veins on the orbital surface of the frontal lobe join the superior cerebral veins and thus drain to the superior sagittal sinus. Those on the temporal lobe anastomose with basal veins and middle cerebral veins, and drain to the cavernous, superior petrosal and transverse sinuses.
Figure 17.18 Frontal projection from a magnetic resonance venogram using time-of-flight methods. (By kind permission from Professor PD Griffiths, Academic Unit of Radiology, The University of Sheffield.)
The basal vein begins at the anterior perforated substance by the union of a small anterior cerebral vein, which accompanies the anterior cerebral artery, a deep middle cerebral vein, which receives tributaries from the insula and neighbouring gyri and runs in the lateral cerebral fissure, and striate veins, which emerge from the anterior perforated substance. The basal vein passes back round the cerebral peduncle to the great cerebral vein (Fig. 17.14), and receives tributaries from the interpeduncular fossa, inferior horn of the lateral ventricle, parahippocampal gyrus and midbrain.
The internal cerebral vein drains the deep parts of the hemisphere and the choroid plexuses of the third and lateral ventricles. It is formed near the interventricular foramen, behind the column of the fornix, primarily by union of the thalamostriate and choroidal (choroid) veins, although numerous smaller veins from surrounding structures also converge here. The thalamostriate vein runs anteriorly, between the caudate nucleus and thalamus, and receives many tributaries from both. The choroidal vein runs a convoluted course along the whole choroid plexus, and receives veins from the hippocampus, fornix, corpus callosum and adjacent structures. After their formation, the two internal cerebral veins travel back parallel to one another, beneath the splenium of the corpus callosum, where they unite to form the great cerebral vein. The great cerebral vein is a short median vessel, which curves sharply up around the splenium of the corpus callosum and opens into the anterior end of the straight sinus after receiving the right and left basal veins.