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BLOOD SUPPLY TO CEREBRAL HEMISPHERES
    AND BRAINSTEM

 
 The two vertebral arteries and the two internal carotid arteries  supply blood to the brain.

1. The VERTEBRAL ARTERIES  arise from the subclavian arteries in the neck, traverse the foramina transversaria of the upper six cervical vertebrae, hook over the posterior  arch of the atlas and enter the skull through the foramen magnum to pierce the dura mater and lie in the SUBARACHNOID SPACE on the anterior aspect of the medulla oblongata. On the ventral aspect of the medulla oblongata the two vertebral arteries unite at the lower border of the pons to form the BASILAR artery which lies in a groove on the ventral aspect of the pons.

Cerebral arteries.jpg (63469 bytes) (Click to enlarge)
Branches from the vertebral arteries include:

1.The posterior spinal artery.
2. The posterior inferior cerebellar artery.
3. The anterior spinal artery.

Branches from the basilar artery are the:

1.Anterior inferior cerebellar artery.
2.The labyrinthine artery.
3.The pontine branches.
4.The superior cerebellar arteries.

The basilar artery terminates by dividing into the POSTERIOR CEREBRAL ARTERIES at the upper border of the pons.

 

INTERNAL CAROTID ARTERIES

These are terminal branches of the common carotid arteries in the neck which enter the skull through the carotid canal to lie in the cavernous sinus on each side of the body of the sphenoid bone.
The INTERNAL CAROTID artery give off two branches:
1.The POSTERIOR COMMUNICATING artery.
2.The ANTERIOR CHOROIDAL artery.
It then divides into its terminal divisions, the ANTERIOR CEREBRAL artery and the MIDDLE CEREBRAL artery.

THE CIRCLE OF WILLIS (CIRCULUS ARTERIOSUS)
The arteries just described, the posterior cerebral, posterior communicating, anterior cerebral and anterior communicating arteries form an arterial circle on the base of the brain.
Branches from this circle are of two types:
1.CENTRAL branches, which are slender and grouped together in piercing the deeper areas of the cerebral hemispheres.
2.CORTICAL branches spread over the surface of the hemisphere in the pia mater and anastomose freely as they give rise to other branches which penetrate deeper into the brain tissue.
The areas of cerebral cortex supplied by the cortical branches are:
The MIDDLE CEREBRAL ARTERY supplies most of the superolateral surface of the hemisphere and a small area on the medial surface below the hypothalamus. The major supply of the medial surface of the hemisphere is from the anterior and posterior cerebral arteries with a small contribution from both these vessels to the superolateral surface. The inferior surface receives supply from all three of these vessels with the posterior artery the most involved. (The distribution of these vessels to the cerebral hemispheres is well illustrated in 'Neuropics' or any standard neuroanatomy text).

 

 VENOUS DRAINAGE OF THE BRAIN
 
 The veins lie chiefly on the surface of the hemispheres in the subarachnoid space. They cross the subarachnoid and subdural spaces to drain into the venous sinuses of the dura mater.
 The superolateral surface of the hemisphere is drained by the SUPERIOR CEREBRAL VEINS which course up to the superior saggital sinus. The INFERIOR CEREBRAL VEINS drain to the SUPERFICIAL MIDDLE CEREBRAL VEIN on the line of the lateral sulcus. The inferior cerebral veins on the occipital lobe drain inferiorly to the transverse sinus. The SUPERFICIAL MIDDLE CEREBRAL VEIN terminates in the cavernous sinus. There are often superior and inferior anastomotic veins connecting the middle superficial vein to the superior saggital and transverse sinuses respectively. There are also connections between these superficial veins and veins deeper within the tissue of the hemispheres.
 The veins of inferior surface of the brain  drain to the ANTERIOR and SUPERFICIAL MIDDLE CEREBRAL VEINS. Posteriorly they drain to the BASAL VEIN, and also into the superior petrosal, straight and transverse sinuses.
 On the medial surface of the hemisphere the GREAT CEREBRAL VEIN  (of Galen) emerges from under the splenium of the corpus callosum, curving superiorly on the splenium to join the inferior saggital sinus and forming the straight sinus.
 Tributaries from the midbrain and cerebellum, and the basal vein itself drain into the great cerebral vein. The BASAL VEIN is formed deep in the lateral sulcus by the confluence of the ANTERIOR CEREBRAL VEIN, the DEEP MIDDLE CEREBRAL VEIN in the depths of the lateral sulcus on the insula and the STRIATE vein which emerges on to the surface of the brain through the anterior perforated substance.
The veins of the medulla oblongata comunicate with those of the spinal medulla (see spinal cord).

 
 

VENOUS SINUSES
 
 These endothelial lined spaces are in the folds of dura mater or between the dura and the endocranium. They drain the nervous system and surrounding bone and communicate with external veins through various foramina. The system has no valves and blood can thus flow in either direction depending on the pressure gradient. The sinuses eventually drain to the internal jugular veins but also drains through emissary, diploic and meningeal veins. Cerebrospinal fluid drains from the arachnoid villi and granulations into the superior saggital sinus and its lacunae.

SUPERIOR SAGGITAL SINUS: This sinus extends from the CRISTA GALLI anteriorly and runs posteriorly, forming a median groove on the vault of the skull, to the internal occipital protuberance. Here, it usually drains into the right transverse sinus. There are lateral extensions of the sinus between the dura mater and the endocranium, termed LACUNAE. These lacunae receive the arachnoid villi and granulations through which the cerebrospinal fluid is drained back into the venous circulation. The lacunae and granulations may produce shallow pits and depressions in the vault of the skull on either side of the groove for the superior saggital sinus. The superior cerebral veins drain into the sinus and meningeal and diploic veins drain into the lacunae.

INFERIOR SAGGITAL SINUS: This sinus lies in the posterior 2/3rds of the free edge of the FALX CEREBRI. It drains the falx and the medial part of the cerebral hemisphere and drains into the straight sinus.

STRAIGHT SINUS:  The straight sinus is formed where the free edges of the falx cerebri and the tentorium cerebelli meet, and commences as a union of the GREAT CEREBRAL VEIN of Galen and the INFERIOR SAGGITAL SINUS. Draining the posterior and central parts of the cerebral hemispheres, parts of the cerebellum, the falx and tentorium, the straight sinus usually terminates by draining at the confluence of sinuses (at the internal occipital protuberance) into the left transverse sinus.

TRANSVERSE SINUS: Paired, the transverse sinuses course horizontally in the attached margin of the TENTORIUM CEREBELLI from the INTERNAL OCCIPITAL PROTUBERANCE to the base of the PETROUS TEMPORAL BONE and groove the occipital, parietal and temporal bones. The right transverse sinus which usually receives the superior saggital sinus is larger than the left, which usually drains the straight sinus. The transverse sinus receives veins from the occipital lobe, the cerebellum and the occipital diploic vein. Anteriorly, it receives the SUPERIOR PETROSAL sinus and drains finally into the SIGMOID sinus.

SUPERIOR PETROSAL SINUS: Lying in the margin of the tentorium cerebelli where it attaches to the petrous temporal bone this narrow sinus drains the posterior end of the cavernous sinus to the junction of the sigmoid and transverse sinuses.

INFERIOR PETROSAL SINUS: This sinus lies in the groove between the basilar part of the occipital bone and the petrous temporal bone. It also drains the cavernous sinus through the jugular foramen into the internal jugular vein.

SIGMOID SINUS: Behind the base of the petrous temporal bone the transverse sinus joined by the superior petrosal sinus continues into the S-shaped sigmoid sinus which cures down to groove the mastoid and petrous temporal bones, then at the base of the skull it curves forward on the occipital bone to the jugular foramen where it joins the inferior petrosal sinus to form the internal jugular vein.

CAVERNOUS SINUS: This important sinus lies between the superior orbital fisssure and the apex of the petrous temporal bone. Its importance derives from the structures which lie in and around it. It will be dealt with in more detail in the Head & Neck course.