The meningeal vessels are classified, according to the fossa they supply, into anterior, middle, and posterior meningeal sets. The anterior are derived from the ophthalmic artery, the posterior from the occipital and ascending pharyngeal of the external carotid, and from the vertebral, while the middle fossa is supplied by the only large vessel of the series, the middle meningeal. The middle meningeal artery is derived from the internal maxillary, and enters the skull through the foramen spinosum, whence it runs up and forwards, often lodged in a groove or even a canal in the bone, and divides into anterior and posterior branches. Rupture of this vessel, from fracture of the skull or possibly severe concussion, is the main source of extensive extradural haemorrhage, the blood stripping the dura from the bone, and forming a large haematoma, which often causes compression, ending fatally. To arrest the haemorrhage it is necessary to trephine the skull, taking care not to cut the vessel embedded in the bone in doing so. The trunk is reached at a point ¾ inch above the zygoma, the anterior branch 1½ inches behind the external angular process of the frontal and 1½ inches above the zygoma, and the posterior branch 1½ inches above the external auditory meatus. Where the dura is torn, the haemorrhage may become subdural, and is then generally extensive, sometimes causing pressure on the medulla.
As already mentioned, the cerebral sinuses are formed by a splitting of the dura mater, and present a triangular section, with rounded base. The superior longitudinal sinus is connected, at least in early life, with the veins of the nose, through the foramen caecum, in front. As it runs backwards it lies slightly to the right of the middle line, and, gradually enlarging, receives tributaries which communicate with the scalp veins. It ends posteriorly by joining the lateral sinuses at the torcular Herophili. While septic infection does occasionally extend to this sinus through its communications, causing septic thrombosis, it is more frequently the seat of marasmic thrombosis, such as occurs in weakly emaciated children. The frequency of the marasmic type is accounted for by the slow circulation in this sinus, due to (a) tributaries entering at an angle opposed to the direction of the current ; (b) Pacchionian projections ; (c) rapid enlargement of the sinus ; (d) junction with the lateral sinus at right angles. While marasmic thrombosis occurs more frequently in azygos sinuses, such as this, septic thrombosis occurs most frequently in dual sinuses, such as the sigmoid. Naevi occurring along the middle line of the scalp not infrequently communicate with the sinus, and hence must be treated with caution.
The cavernous sinus is so called from the fibrous strands which divide its interior, and commences by receiving the ophthalmic vein (whence sepsis may spread from the orbit), and ends by dividing into the superior and inferior petrosal sinuses, while it communicates with its neighbour through the circular sinus, so that thrombosis generally extends from one side to the other. In its outer wall are embedded the internal carotid artery, and the third, fourth, first division of the fifth, and sixth nerves.
The inferior petrosal sinus receives large quantities of blood from the cavernous sinus, the blood being pumped into it from the communicated pulsations of the internal carotid artery, which practically lies in the cavernous sinus. The vessel pursues an almost vertical downward course to join the jugular bulb. When wounded, bleeding from this sinus is very difficult to arrest.
The lateral sinus runs first horizontally outwards from the torcular, and then, at its junction with the superior petrosal sinus, forms the sigmoid sinus. The right lateral sinus conveys most of the blood (brought by the longitudinal sinus) from the cortex, while the left lateral sinus conveys that brought by the straight sinus from the central ganglionic regions.
The sigmoid sinus pursues a tortuous course along the posterior wall of the petrous, and ends, after making an almost complete circle and forming the jugular bulb, in the internal jugular vein. The tortuous course, formation of the bulb which rests on a prominence of bone, and the comparatively narrow mouth of the jugular vein, are all factors in preventing the aspiration of this and the other communicating sinuses on inspiration. As the sinuses are rigid channels, they would otherwise be easily aspirated by the suction action of inspiration, anaemia of the brain, with consequent unconsciousness, resulting. This does occur in some individuals in whom the bony prominence under the jugular bulb is wanting, when a long breath is taken suddenly. Normally, however, the flow is arrested during inspiration by the flattening of the bulb upon the bony projection, while the inferior petrosal sinus, acting as an injector, rapidly restarts the flow once inspiration is at an end.
As the sigmoid sinus is in close relationship to the mastoid antrum, it frequently suffers from pyogenic thrombosis by extension of the septic process. Where the mastoid antrum is large, it sometimes extends under the sigmoid sinus, and comes directly into relationship with the cerebellum, cerebellar abscess thus occasionally arising from mastoid disease, without previous sigmoid sinus thrombosis.
The emissary mastoid vein leaves the sigmoid sinus just beyond its commencement, and not infrequently conveys septic mischief to it from the outside. The dome of the jugular bulb is situated directly under the middle ear.
The straight sinus runs along the junction of the falx with the tentorium, from the inferior longitudinal sinus to the left lateral sinus, near the torcular. It conveys blood from the inferior longitudinal sinus and also from the interior of the brain, brought to it by the veins of Galen, which emerge from under the corpus callosum. These veins are believed to take part in removing cerebro-spinal fluid from the ventricles of the brain, and hence obstruction of the veins may be a cause of hydrocephalus.