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VENOUS DRAINAGE OF THE BRAIN

The brain is drained by three sets of blood vessels. These are: the superficial

veins, the deep veins and the dural venous sinuses. All these vessels do not have

valves. The superficial veins drain blood from the cerebral cortex (via corticalveins) and white matter (via medullary veins) and pass it directly into dural venous

sinuses. The deeper veins drain blood from the deeper parts of the white matter,

basal ganglia, corpus callosum and diencephalon into the great cerebral vein

which, in turn, enters the straight sinus.

Superficial cerebral veins

These are the cortical veins that run with the corresponding cortical arteries within

the subarachnoid space. The cortical veins on the surface of the cerebral

hemispheres are called superficial cerebral veins. They drain the lateral surface ofthe cerebral hemispheres and empty into the superior sagittal sinus. In addition,

there is a superficial middle cerebral vein that closely follows the lateral cerebral

fissure. It empties into the cavernous sinus. Two main anastomotic veins occur.

The superior anastomotic vein (vein of Trolard) connects the superficial middle

cerebral vein to the superior sagittal sinus. The inferior anastomotic vein (vein of

Labb) connects it to the transverse (dural venous) sinus.

Before ending in the dural venous sinuses, superficial cerebral veins leave the

subarachnoid space and run a short course between the dura and arachnoid. Theseshort subdural venous segments are called bridging veins. The latter are of great

clinical importance in that they may rupture due to head trauma, resulting in a

subdural hematoma.

Deep cerebral veins

These veins drain the internal structures of the brain. Some of the most important

are the thalamostriate vein and the choroidal vein. These drain the basal ganglia,

thalamus, internal capsule, choroid plexus and hippocampus. These veins merge to

form the internal cerebral vein. The internal cerebral veins from each side thenunite in the midline to form the great cerebral vein (of Galen). This lies beneath

the splenium of the corpus callosum. The great cerebral vein is a short vein that

becomes continuous with the straight (dural venous) sinus at the point where the

latter joins the inferior sagittal sinus. Therefore, both the superficial and deep

cerebral veins drain into the dural venous sinuses.

Venous sinuses of the dura mater (Dural Venous Sinuses)

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The venous sinuses of the cranial cavity are situated between the layers of the dura

mater. Some are found between the periosteal and meningeal layers of dura mater.

Others occur within foldings of the menigeal layer. The sinuses are lined by an

endothelium, and their walls lack a muscle layer. They contain no valves. Thesinuses are collecting channels of venous blood from various parts of the brain,

meninges and diplo. They also communicate with external veins through emissary

veins. They end directly or indirectly in the internal jugular vein.

Some sinuses are unpaired, being located in the midline. Others are paired,

occurring on either side of the midline.

The Superior Sagittal Sinus

This is situated in the attached border of the falx cerebri. Here it occupies thetriangular interval between the periosteal layer and the diverging meningeal sheets

which form the falx cerebri. It begins at the anterior end of the crista galli in the

anterior cranial fossa. It continues posteriorly in the median plane until it reaches

the internal occipital protuberance. The sinus becomes larger as it extends

posteriorly. At some sites, the sinus is expanded to form lateral lacunae. The

lacunae contain the arachnoid villi through which cerebrospinal fluid is re-

absorbed into the venous blood. The sinus receives the superficial cerebral, diploic

and meningeal veins. It also receives the parietal emissary vein. At the internal

occipital protuberance, the sinus dilates to form the confluence of sinuses.

The Inferior Sagittal Sinus

It is located in the free inferior margin of the falx cerebri. It runs posteriorly and

joins the great cerebral vein to form the straight sinus. The inferior sinus receives

tributaries from the falx cerebri and from the medial surfaces of the hemispheres.

The Straight Sinus

It occupies the line of junction of the falx cerebri with the tentorium cerebelli. It isthe continuation of the inferior sagittal sinus into the tentorium cerebelli. It runs

inferoposteriorly to join the confluence of sinuses.

The transverse sinuses

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These are large paired sinuses, and they commence at the internal occipital

protuberance. Each sinus occupies the attached margin of the tentorium cerebelli,

making grooves into the occipital bone. The sinuses may arise from the confluence

of sinuses. [However, the right transverse sinus is usually continuous with the

superior sagittal sinus, while the left one is continuous with the straight sinus].

The sigmoid sinuses

These are paired S-shaped continuations of the transverse sinuses. Each turns

downward and medially, creating grooves on the mastoid part of the temporal

bone. Here they lie behind the mastoid antrum. The sinuses turn forward and then

downward to enter the superior bulbs of the internal jugular vein.

The occipital sinus

It is a small unpaired sinus occupying the attached margin of the falx cerebelli. It

starts near the foramen magnum and drains superiorly into the confluence of

sinuses. This sinus communicates with the internal vertebral venous plexus.

The confluence of sinuses

This a dilatation at one side of the internal occipital protuberance. It represents the

union of the superior sagittal, straight, and occipital sinuses with the right and left

transverse sinuses.

The cavernous sinuses

These are paired sinuses lying on either side of the body of the sphenoid bone.

Each sinus extends from the superior orbital fissure anteriorly to the apex of the

petrous part of the temporal bone posteriorly. They are formed between the

meningeal and periosteal layers of the dura. Numerous trabeculae from each layer

cross the space, giving it a spongy or reticular appearance, hence the name

cavernous. This sinus is remarkable in that it has nerves and a major blood vessel

passing through it. In its outer wall the following structures pass through it startingfrom superior to inferior: (i) oculomotor nerve (ii) trochlear nerve (iii)

ophthalmic division of trigeminal nerve and (iv) maxillary division of the

trigeminal nerve. The following structures run within the sinus: (i) internal

carotid artery (surrounded by its sympathetic nerve plexus)and(ii) abducent

nerve. The artery and nerves are separated from the blood in the sinus by an

endothelial lining.

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The sinus drains posteriorly into the superior petrosal sinus, and inferiorly into the

pterygoid venous plexus. The two sinuses communicate with each other via the

anterior and posterior intercavernous sinuses. Each sinus has an important

communication with thefacial vein through thesuperior ophthalmic vein.

The superior petrosal sinuses

These are paired sinuses lying rostrally in the margin of the tentorium cerebelli.

The sinus connects the posterior end of the cavernous sinus to the junction between

the transverse and sigmoid sinuses. Each superior petrosal sinus drains the

corresponding cavernous sinus.

The inferior petrosal sinuses

These are paired sinuses lying in the petro-occipital fissure of each side. Each side

drains the corresponding cavernous sinus directly into the jugular vein. The

inferior perusal sinus arises from the posteroinferior portion of the cavernous sinus

and ends in the bulb of the internal jugular vein.

The basilar sinus (plexus)

This is made up of interlacing venous channels in the dura of the basilar part of the

occipital bone. It connects the two inferior petrosal sinuses. It is also connected tothe internal vertebral venous plexus and contributes to the drainage of the

cavernous sinuses.

Emissary veins

These veins connect the dural venous sinuses within the cranium with veins

external to it. They are valveless and therefore may conduct blood in any direction.

However, the flow of blood is usually away from the cranial cavity. Some emissary

veins are constant. Others occur occasionally. The emissary veins include:

- Mastoid emissary vein this unites the posterior auricular vein with the

sigmoid sinus.

- Parietal emissary vein this occupies the parietal foramen and connects the

veins of the scalp of the vertex with the superior sagittal sinus.

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- Frontal emissary vein connects veins of the nasal cavity with the superior

sagittal sinus. It passes through the foramen cecum of the skull and may be

absent in some adults.

- Posterior condylar emissary vein when present, it connects the sigmoid

sinus with veins of the sub-occipital triangle of the neck.

Diploic veins

These are present in the frontal, parietal and occipital bones. When the sutures of

the adult skull obliterate, diploic veins anastomose and increase in size. Thus they

are no longer associated with only one bone. They drain the diplo and

communicate with veins of the meninges and scalp.

Some clinical aspects regarding dural venous sinuses

Connections between the dural sinuses and extra-cranial veins mainly serve to

equalize pressure and regulate temperature. However, because these veins have no

valves, blood flow can be either into, or away from, the cranial cavity. Hence these

anastomoses are of clinical importance.

Some veins of the face are tributaries of the superior ophthalmic vein. The latter

drains the eye and its accessory structures in the orbit and empties intra-cranially

into the cavernous sinus. Free passage of blood from these tributaries and from the

lips and face via the facial vein may lead to transmission infection into thecavernous sinus. This may result in a condition called cavernous sinus

thrombosis. Also, free communication of blood between the internal vertebral

plexus and basilar sinuses means that tumour cells and bacterial infections in the

vertebrae may spread to the brain and vice versa. The sigmoid sinus lies very close

to the mastoid air cells and the middle ear. Therefore, infections of the middle ear

(otitis media) may lead to infective thrombosis of the sigmoid and transverse

sinuses. Similarly, sagittal sinus thrombosis may follow infections of the skull

bones, nose, face or scalp through its diploic and emissary connections.