The veins of the brain possess no valves, and their walls, owing to the absence of muscular tissue, are extremely thin. They pierce the arachnoid membrane and the inner or meningeal layer of the dura mater, and open into the cranial venous sinuses. They may be divided into two sets, cerebral and cerebellar.
The cerebral veins (vv. cerebri) are divisible into external and internal groups according as they drain the outer surfaces or the inner parts of the hemispheres.
The external veins are the superior, inferior, and middle cerebral.
The Superior Cerebral Veins (vv. cerebri superiores), eight to twelve in number, drain the superior, lateral, and medial surfaces of the hemispheres, and are mainly lodged in the sulci between the gyri, but some run across the gyri. They open into the superior sagittal sinus; the anterior veins runs nearly at right angles to the sinus; the posterior and larger veins are directed obliquely forward and open into the sinus in a direction more or less opposed to the current of the blood contained within it.
The Middle Cerebral Vein (v. cerebri media; superficial Sylvian vein) begins on the lateral surface of the hemisphere, and, running along the lateral cerebral fissure, ends in the cavernous or the sphenoparietal sinus. It is connected
(a) with the superior sagittal sinus by the great anastomotic vein of Trolard, which opens into one of the superior cerebral veins;
(b) with the transverse sinus by the posterior anastomotic vein of Labbé, which courses over the temporal lobe.
The Inferior Cerebral Veins (vv. cerebri inferiores), of small size, drain the under surfaces of the hemispheres. Those on the orbital surface of the frontal lobe join the superior cerebral veins, and through these open into the superior sagittal sinus; those of the temporal lobe anastomose with the middle cerebral and basal veins, and join the cavernous, sphenoparietal, and superior petrosal sinuses.
The basal vein is formed at the anterior perforated substance by the union of
(a) a small anterior cerebral vein which accompanies the anterior cerebral artery
(b) the deep middle cerebral vein (deep Sylvian vein), which receives tributaries from the insula and neighboring gyri, and runs in the lower part of the lateral cerebral fissure
(c) the inferior striate veins, which leave the corpus striatum through the anterior perforated substance.
The basal vein passes backward around the cerebral peduncle, and ends in the internal cerebral vein (vein of Galen); it receives tributaries from the interpeduncular fossa, the inferior horn of the lateral ventricle, the hippocampal gyrus, and the mid-brain.
The Internal Cerebral Veins (vv. cerebri internæ; veins of Galen; deep cerebral veins) drain the deep parts of the hemisphere and are two in number; each is formed near the interventricular foramen by the union of the terminal and choroid veins. They run backward parallel with one another, between the layers of the tela chorioidea of the third ventricle, and beneath the splenium of the corpus callosum, where they unite to form a short trunk, the great cerebral vein; just before their union each receives the corresponding basal vein.
The terminal vein (v. terminalis; vena corporis striati) commences in the groove between the corpus striatum and thalamus, receives numerous veins from both of these parts, and unites behind the crus fornicis with the choroid vein, to form one of the internal cerebral veins. The choroid vein runs along the whole length of the choroid plexus, and receives veins from the hippocampus, the fornix, and the corpus callosum.
The Great Cerebral Vein (v. cerebri magna; great vein of Galen), formed by the union of the two internal cerebral veins, is a short median trunk which curves backward and upward around the splenium of the corpus callosum and ends in the anterior extremity of the straight sinus.
The Cerebellar Veins are placed on the surface of the cerebellum, and are disposed in two sets, superior and inferior. The superior cerebellar veins (vv. cerebelli superiores) pass partly forward and medialward, across the superior vermis, to end in the straight sinus and the internal cerebral veins, partly lateralward to the transverse and superior petrosal sinuses. The inferior cerebellar veins (vv. cerebelli inferiores) of large size, end in the transverse, superior petrosal, and occipital sinuses.