The veins of the central nervous system lack tunica media and tunica adventitia and do not have valves. The veins of the brain are: the dorsal cerebral veins (drain to the dorsal sagittal sinus and diploic veins), the ventral cerebral veins (drain to the dorsal petrosal sinus), the dorsal cerebellar veins (drain to the transverse sinuses), the ventral cerebellar veins (drain to the sigmoid and basilar sinuses), the vein of the corpus callosum (is an unpaired vein located dorsally to the corpus callosum), the internal cerebral veins (run parallel to the plexus of the third ventricle), the great cerebral vein (is an unpaired vein formed by the confluence of the vein of the corpus callosum and internal cerebral veins), the thalamostriated vein (formed by the confluence of thalamic and striated veins that merges with the caudal part of the vein of the corpus callosum), the pontine and medullary veins (located on the ventral surface of the pons and medulla oblongata).
The veins of the encephalon and the cranial cavity bones drain into venous passages located within the dura mater (between the outer an inner layer of the dura mater) and osseous canals. They are called sinuses of the dura mater. They drain blood into the maxillary, internal jugular and vertebral veins, and into the internal vertebral venous plexus.
The dorsal sagittal sinus is located at the upper edge of the falx cerebri. It collects the blood from the dorsal cerebral veins and diploic veins. Caudally, it joins with the straight sinus.
The straight sinus is short and formed by caudal continuation of the great cerebral vein. This vein, in turn, is formed by the merging of the vein of the corpus callosum and the internal cerebral veins.
The left and right transverse sinuses run in the transverse canal to meet at the midline in the occipital bone. At this location, they receive blood from the dorsal sagittal and straight sinuses. The site of confluence is known as the confluence of sinuses. From this point, each transverse sinus runs laterally in the transverse canal and, then, in the transverse sulcus to divide into a temporal sinus, a dorsal temporal sinus, and a sigmoid sinus. The temporal sinus extends into the retroarticular foramen where it becomes the retroarticular vein that joins the maxillary vein. The sigmoid sinus bends to reach the condyloid canal where it forms the basilar sinus. This sinus extends into the vertebral canal and becomes the ventral internal vertebral venous plexus. After giving off the basilar sinus, the sigmoid sinus continues to the jugular foramen where it joins the caudal part of the ventral petrosal sinus. The dorsal petrosal sinus runs dorsal to the petrosal part of the temporal bone to receives the ventral cerebral veins.
The cavernous sinuses run in the paraselar region. They extend from the orbital fissure and optic canal to the petrobasilar canal where they become the ventral petrosal sinus. This sinus joins the sigmoid sinus at the jugular foramen to leave the cranial cavity. They become the internal jugular vein and the vertebral vein.
The meningeal veins drain the dura mater. With the meningeal arteries, they course between the two layers of dura. Running in parallel with the meningeal arteries, they serve as a cooling system for the brain allowing heat interchange. Also, the communication of the dural veins with the diploic veins trough the emissary veins, has a very important role in maintaining the temperature of cortical areas.
The rostral meningeal vein covers de frontal cortex and joins the frontal diploic vein at the level of the cribiform plate. The middle meningeal vein is the more extensive. At the foramen ovale it joins the emissary vein of the foramen ovale that connects the cavernous sinus with the maxillary vein. The caudal meningeal vein drains to the sigmoid sinus.
As the sinuses lack of valves, they allow blood flow in both directions depending on the pressure. This, in turn, may facilitate the spread of infection, metastasis or emboli. Any material that enters the veins of the face and head may reach the sinuses and diploic veins. The increase of pressure in the thorax or abdomen may cause venous distension overcoming valve resistance of the intervertebral veins allowing blood to enter the venous plexuses in the vertebral canal and reaching the sinuses of the cranial cavity.
 King, A.S. In page 29 of Physiological and clinical anatomy of the domestic animals. Volume 1. Central nervous system. Oxford Science Publications. 1987