Blood Supply of the Brain

• Enters the carotid canal in the petrous portion of the temporal bone.
• Is separated from the tympanic cavity by a thin bony structure.
• Lies within the cavernous sinus and gives rise to small twigs to the wall of the cavernous sinus, to the hypophysis, and to the semilunar ganglion of the trigeminal nerve.
• Pierces the dural roof of the cavernous sinus between the anterior clinoid process and the middle clinoid process, which is a small projection posterolateral to the tuberculum sellae.
• Forms a carotid siphon (a bent tube with two arms of unequal length), which is the petrosal part just before it enters the cranial cavity.

1. Ophthalmic artery

• Enters the orbit via the optic canal with the optic nerve.

2. Posterior communicating artery

• Arises from the carotid siphon and joins the posterior cerebral artery.
• Runs backward below the optic tract and supplies the optic chiasma and tract and hypothalamus.

3. Anterior choroidal artery

• Supplies the choroid plexus of the lateral ventricles, optic tract and radiations, and lateral geniculate body.

4. Anterior cerebral artery

• Enters the longitudinal fissure of the cerebrum, supplies the optic chiasma and medial surface of the frontal and parietal lobes of the brain, and unites each by the short anterior communicating artery.

5. Middle cerebral artery

• Passes laterally in the lateral cerebral fissure and supplies the lateral convexity of the cerebral hemisphere.

• Arise from the first part of the subclavian artery and ascend through the transverse foramina of the vertebrae C1 to C6.
• Curve posteriorly behind the lateral mass of the atlas, pierce the dura mater into the vertebral canal, and then enter the cranial cavity through the foramen magnum.
• Join to form the basilar artery.
• Give rise to the following:

1. Anterior spinal artery

• Arises as two roots from the vertebral arteries shortly before the junction of the vertebral arteries.
• Descends in front of the medulla, and the two roots unite to form a single median trunk at the level of the foramen magnum.

2. Posterior spinal artery

• Arises from the vertebral artery or the posterior–inferior cerebellar artery.
• Descends on the side of the medulla, and the right and left roots unite at the lower cervical region.

3. Posterior inferior cerebellar artery

• Is the largest branch of the vertebral artery, distributes to the posterior inferior surface of the cerebellum, and gives rise to the posterior spinal artery.

• Is formed by the union of the two vertebral arteries at the lower border of the pons.
• Ends near the upper border of the pons by dividing into the right and left posterior cerebral arteries.

1. Pontine arteries

• Are several in number and supply the pons.

2. Labyrinthine artery

• Enters the internal auditory meatus and supplies the cochlea and vestibular apparatus.

3. Anterior inferior cerebellar artery

• Supplies the anterior part of the inferior surface of the cerebellum.
• Gives rise to the labyrinthine artery in 85% of the population.

4. Superior cerebellar artery

• Passes laterally just behind the oculomotor nerve and supplies the superior surface of the cerebellum.

5. Posterior cerebral artery

• Is formed by bifurcation of the basilar artery passes laterally in front of the oculomotor nerve, winds around the cerebral peduncle, and supplies the midbrain and the temporal and occipital lobes of the cerebrum.

• Is formed by the posterior cerebral, posterior communicating, internal carotid, anterior cerebral, and anterior communicating arteries.
• Forms an important means of collateral circulation in the event of obstruction.