Cerebral circulation

# Overview
Cerebral circulation refers to the movement of blood through the network of blood vessels supplying the brain. The arteries deliver oxygenated blood, glucose and other nutrients to the brain and the veins carry deoxygenated blood back to the heart, removing carbon dioxide, lactic acid, and other metabolic products. Since the brain is very vulnerable to compromises in its blood supply, the cerebral circulatory system has many safeguards. Failure of these safeguards results in cerebrovascular accidents, commonly known as strokes. The amount of blood that the cerebral circulation carries is known as cerebral blood flow.
The following description is based on idealized human cerebral circulation. The pattern of circulation and its nomenclature vary between organisms.

# Cerebral arteries
There are four cerebral arteries. The largest are the two internal carotid arteries, the left and right branches of the common carotid arteries in the neck which enter the skull, as opposed to the external carotid branches which supply the facial tissues. The two smaller arteries are the vertebral arteries, which branch from the subclavian arteries which primarily supply the shoulders, lateral chest and arms.
Within the cranium, which houses the brain, the two vertebral arteries fuse into the basilar artery, which is located underneath, and primarily supplies, the brainstem.
Both internal carotid arteries, within and along the floor of the cerebral vault, are interconnected via the anterior communicating artery. Additionally, both internal carotid arteries are interconnected with the basilar artery via bilateral posterior communicating arteries.
The Circle of Willis, long considered to be an important anatomic vascular formation, provides backup circulation to the brain. In case one of the supply arteries is occluded, the Circle of Willis provides interconnections between the internal carotid arteries and basilar artery along the floor of the cerebral vault, providing blood to tissues that would otherwise become ischemic.

# Cerebral venous drainage
The venous drainage of the cerebrum can be separated into two subdivisions:  superficial and deep.  The superficial system is composed of dural venous sinuses, which have wall composed of dura mater as opposed to a traditional vein.  The dural sinuses are, therefore located on the surface of the cerebrum.  The most prominent of these sinuses is the Superior sagittal sinus which flows in the sagittal plane under the midline of the cerebral vault, posteriorly and inferiorly to the torcula, forming the Confluence of sinuses, where the superficial drainage joins with the sinus the primarily drains the deep venous system.  From here, two transverse sinuses bifurcate and travel laterally and inferiorly in an S-shaped curve that form the sigmoid sinuses which go on to form the two jugular veins. In the neck, the jugular veins parallel the upward course of the carotid arteries and drain blood into the vena cava.  The deep venous drainage is primarily composed of traditional veins inside the deep structures of the brain, which join behind the midbrain to form the Vein of Galen.  This vein merges with the Inferior sagittal sinus to form the Straight sinus which then joins the superficial venous system mentioned above at the Confluence of sinuses.