The arterial blood supply to the brain comes from two distinct pairs of vessels - the vertebral arteries and the internal carotid arteries. These arteries are interconnected by communicating arteries, therefore forming a cerebral arterial circle called the Circle of Willis.

Vertebral artery

Each vertebral artery arises from the first part of each subclavian artery in the lower part of the neck, and passes superiorly through the transverse foramina of the upper six cervical vertebrae. On entering the cranial cavity through the foramen magnum, each vertebral artery gives off a small meningeal branch.

Continuing forward, the vertebral artery gives rise to three additional branches before joining with its companion vessel to form the basilar artery. Its side branches include the:

• Posterior inferior cerebellar artery (PICA)
• Anterior spinal artery - formed by two branches arising from each vertebral artery, this artery travels down the spinal cord within the anterior median fissure of spinal cord
• Posterior spinal artery - wraps around the medulla posteriorly and then descends on the posterior surface of the spinal cord; usually, there are two posterior spinal arteries that typically travel along the posterior spinal roots on each side

The basilar artery travels in a rostral direction along the anterior aspect of the pons. It gives off several branches on its Its way:

• Anterior inferior cerebellar arteries
• Pontine arteries
• Superior cerebellar arteries

The basilar artery eventually bifurcates into two posterior cerebral arteries.

Internal Carotid Artery

The two internal carotid arteries arise as one of the two terminal branches of the common carotid arteries. They proceed superiorly to the base of the skull where they enter the carotid canal.

Entering the cranial cavity, each internal carotid artery gives off the:

• Ophthalmic artery
• Posterior communicating artery
• Middle cerebral artery
• Anterior cerebral artery