The Brain

The scalp | Main Anatomy Index | Blood supply to the head & neck

The Main Parts of the Brain

The Cerebral Hemispheres (p. 693)

• The cerebral hemispheres form the largest part of the brain.
• They occupy the anterior and middle cranial fossae and extend posteriorly over the tentorium cerebelli and the cerebellum to the internal occipital protuberance.

• They comprise of the cerebral cortex, the basal nuclei, their fibre connections, and the lateral ventricles.
• These are the cavities in each cerebral hemisphere and are part of the ventricular system.

• There are four main lobes of the cerebral hemispheres: frontal, parietal, temporal, and occipital.

The Frontal Lobe (p. 693)

• These are the largest of all the lobes and they form the anterior part of the cerebral hemispheres.
• They are located anterior to the central sulci and superior to the lateral sulci.

The Parietal Lobes (p. 693)

• These lobes are related to the internal aspects of the posterior and anterior parts of the parietal bone.
• Each lobe is bound anteriorly by the central sulcus and posterior by the superior part of the line joining the parieto-occipital sulcus and the preoccipital notch.

The Temporal Lobes (p. 693)

• These lobes lie inferior to the lateral sulci.

The Occipital Lobes (p. 693)

• These lobes are relatively small and are located posterior to the parieto-occipital sulci.
• The rest on the tentorium cerebelli.

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The Midbrain (p. 696)

• This is the smallest part of the brain.
• It is located at the junction of the middle and posterior cranial fossae, lying partly in each.
• The cavity of the midbrain is represented by a narrow canal, called the cerebral aqueduct.
• It conducts CSF from the lateral and third ventricles to the fourth ventricle.

The Pons (p. 696)

• The pons lies in the anterior part of the posterior cranial fossa, posterior to the superior part of the clivus and posterior surface of the dorsum sellae.
• The part of it that is visible from the inferior surface is a wide, bridge-like transverse band of nerve fibres.

The Medulla Oblongata (p. 696)

• This is the most caudal part of the brainstem.
• It is located in the posterior cranial fossa with its ventral aspect facing the clivus.
• The medulla is continuous with the spinal cord at the foramen magnum, but the transition is gradual.
• The distinctive characteristics on its ventral surface are the elongated pyramids and the olives beside them.

The Cerebellum (pp. 696-7)

• This "little brain" (L. cerebellum) overlies the posterior aspect of the pons and medulla and extends laterally beneath the tentorium cerebelli.
• It occupies most of the posterior cranial fossa.

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Blood Supply of the Brain

• The brain is supplied through an extensive system of branches from two pairs of vessels, the internal carotid arteries and the vertebral arteries.

The Internal Carotid Artery (p. 701)

• Each artery arises in the neck from the common carotid artery opposite the superior border of the thyroid cartilage.
• The cervical part of the artery ascends almost vertically to the base of the skull, where it turns and enters the carotid canal in the petrous part of the temporal bone.

• The petrous part of the artery enters the middle cranial fossa through the superior part of the foramen lacerum and then runs anteriorly in the cavernous sinus.
• At the anterior end of the cavernous sinus, the artery makes a hairpin turn and leaves the sinus to enter the subarachnoid space.

• The cerebral part of the artery immediately gives off the ophthalmic artery that supplies the eye.
• The internal carotid artery passes superior to the optic nerve.

• Lateral to the optic nerve, it branches into the middle and anterior cerebral arteries.

• The sinuous course taken by the cavernous and cerebral parts of the internal carotid artery forms a U-shaped bend, often called the "carotid siphon".
• Within the cranial cavity, the internal carotid artery and its branches supply the hypophysis cerebri (pituitary gland), the orbit, and much of the supratentorial part of the brain.

The Vertebral Arteries (p. 701)

• They arise from the superior aspect of the first part of the subclavian artery.
• The vertebral arteries ascend through the transverse foramina of the cervical vertebrae, except for C7 vertebrae.
• After winding around the lateral mass of the atlas, the vertebral artery enters the skull though the foramen magnum.
• At the inferior border of the pons, the vertebral arteries join to form a median vessel called the basilar artery, which is an important arterial supplier to the brain.

The Basilar Artery (p. 701)

• This artery is formed by the union of the two vertebral arteries.
• It runs through the pontine cistern to the superior border of the pons, where it ends by dividing into the two posterior cerebral arteries.

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The Cerebral Arterial Circle of Willis (pp. 701-2)

Click here for a schematic of the arterial circle.

• This circle is an important anastomosis between the four arteries that supply the brain.
• It is formed by the posterior cerebral, posterior communicating, internal carotid, anterior cerebral, and anterior communicating arteries.

• The circle is located at the base of the brain, principally in the interpeduncular fossa.
• The cerebral arterial circle encircles the optic chiasma, the infundibulum, and the mamillary bodies.

• In general, each of the cerebral arteries supplies a surface and a pole of the brain.

The Anterior Cerebral Artery (p. 701)

• It supplies most of the medial and superior surfaces of the frontal pole.

The Middle Cerebral Artery (pp. 701-2)

• It supplies the lateral surface and temporal pole.

The Posterior Cerebral Artery (p. 702)

• It supplies the inferior surface and the occipital pole.

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Dura Septa or Reflections

• The dura mater is the outermost and toughest of the membranes covering the central nervous system.
• These septa divide the cranial cavity into three intercommunicating compartments, one subtentorial and two supratentorial.
• The dura septa provide support for parts of the brain, particularly the cerebral hemispheres.

The Falx Cerebri (p. 686)

• The falx cerebri (L. falx, a sickle) is a large sickle-shaped vertical partition in the longitudinal fissure between the two cerebral hemispheres.
• This thick, tough dural fold is attached in the medial plane to the internal surfaces of the cranium from the frontal crest of the frontal bone and the crista galli of the ethmoid bone anteriorly to the internal occipital protuberance posteriorly.
• The falx cerebri is also attached to the midline of the tentorium cerebelli.

• At the superior coven border of the falx cerebri, its two layers separate to enclose the superior sagittal sinus.
• The inferior sagittal sinus is enclosed within the free edge of the falx cerebri.

• The falx cerebri forms a rigid partition between the cerebral hemispheres that reduces side-to-side movement.

The Tentorium Cerebelli (p. 686)

• The tentorium cerebelli (L. tentorium, tent) is a wide crescentic, arched fold of dura that separates the occipital lobes of the cerebral hemispheres from the cerebellum.

• The attachment of the falx cerebri to the medial portion of the tentorium cerebelli holds the latter fold up.
• The tentorium cerebelli is attached anterolaterally to the superior edges of the petromastoid parts of the temporal bones and the anterior and posterior clinoid processes.
• Posteriorly, the tentorium is attached to the occipital bone along the groove for the transverse sinuses, which it encloses.
• Its concave anteromedial border is free.

• The oval opening surrounds the midbrain as it passes from the middle to the posterior cranial fossa.

The Falx Cerebelli (p. 687)

• This is a small, sickle-shaped, median dural fold in the posterior part of the posterior cranial fossa.
• It extends almost vertically, inferior to the inferior surface of the tentorium cerebelli.
• Its free edge projects slightly between the cerebellar hemispheres.

The Diaphragma Sellae (p. 687)

• This is a small, circular, horizontal sheet of dura that forms a roof for the hypophyseal fossa in the sella turcica.
• This fold is formed by the dura surrounding the hypophysis cerebri (pituitary gland) and encircling the stalk of this gland.
• The diaphragma sellae has a central aperture for the hypophyseal stalk (or infundibulum).

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Venous Sinuses of Dura Mater

• These sinuses are venous channels located between the dura and the internal periosteum lining the cranium.
• The venous sinuses drain all the blood from the brain.
• These sinuses have no valves or muscles in their walls.

The Superior Sagittal Sinus (p. 689)

• This lies in the median plane, along the attached border of the falx cerebri.
• It begins at the crista galli, runs the entire length of the superior attached border of the falx cerebri, and ends at the internal occipital protuberance.
• In 60% of cases the superior sagittal sinus ends by becoming the right transverse sinus.

• At the termination of the superior sagittal since is a dilation, known as the confluence of sinuses.
• Occasionally, this dilation is referred to as the sinus confluens or the torcular Herophili (L. winepress of Herophilus, an early anatomist and surgeon).

• This sinus is triangular in cross-section.

The Inferior Sagittal Sinus (p. 690)

• It occupies the posterior 2/3 of the free inferior edge of the falx cerebri.
• It ends by joining the great cerebral vein (of Galen) to form the straight sinus.

The Straight Sinus (p. 690)

• This is formed by the union of the inferior sagittal sinus with the great cerebral vein.
• It runs inferoposteriorly along the line of attachment of the falx cerebri to the tentorium cerebelli.
• It ends by becoming continuous with one of the transverse sinuses, usually the left.

The Transverse Sinuses (p. 690)

• These pass laterally from the confluence of the sinuses in the attached border of the tentorium cerebelli.
• They groove the occipital bone and the posteroinferior angle of the parietal bones.
• They then leave the tentorium and become the sigmoid sinuses.

The Sigmoid Sinuses (p. 690)

• These are shaped like the Greek letter sigma.
• They follow S-shaped courses in the posterior cranial fossa, forming deep grooves in the inner surface of the posterior part of the mastoid parts of the temporal bones, and in the lateral surfaces of the jugular tubercles of the occipital bone.

• The sigmoid sinuses then turn anteriorly and enter venous enlargements called the superior bulbs of the internal jugular veins, which occupy the jugular foramina.
• These large bulbs receive the inferior petrosal sinuses and continue as the internal jugular veins.

The Cavernous Sinuses (p. 690)

• The cavernous sinuses (L. caverna, cave) are located on each side of the sella turcica and the body of the sphenoid bone.
• Each cavernous sinus extends from the superior orbital fissure anteriorly, to the apex of the petrous part of the temporal bone posteriorly.

• Each cavernous sinus receives blood from the superior and inferior ophthalmic veins, the superficial middle cerebral vein and the sphenoparietal sinus.

• The cavernous sinuses communicate with each other through the intercavernous sinuses, which pass anterior and posterior to the hypophyseal stalk.

• The cavernous sinuses drain posteriorly and inferiorly through the superior and inferior petrosal sinuses and the pterygoid plexuses.

• The lateral wall of each cavernous sinuses contains, from superior to inferior: (1) oculomotor nerve (CN III); the trochlear nerve (CN IV); and the ophthalmic and maxillary divisions of the trigeminal nerve (CN V¹ and CN V²).
• Inside each cavernous sinus is the internal carotid artery and its sympathetic plexus and the abducent nerve (CN VI).

The Superior Petrosal Sinuses (p. 609)

• These are small channels that drain the cavernous sinuses.
• They run from the posterior ends of the cavernous sinuses to the transverse sinuses at the points where they curve inferiorly to form the sigmoid sinuses.
• Each superior petrosal sinus lies in the attached margin of the tentorium cerebelli, running in a small groove on the superior margin of the petrous part of the temporal bone.

The Inferior Petrosal Sinuses (p. 609)

• These are small venous sinuses that drain the cavernous sinuses directly into the inferior jugular vein.
• From the posterior end of the cavernous sinus, each inferior petrosal sinus runs posteriorly, laterally, and inferiorly in a groove between the petrous part of the temporal bone and the basilar part of the occipital bone.
• This sinus enters the jugular foramen and joints the superior bulb of the internal jugular vein, usually just inferior to the base of the skull.

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