The common carotid artery is found bilaterally, with one on each side of the anterior neck. Each common carotid artery is divided into an external and internal carotid artery. These arteries transfer blood to the structures inside and outside of the skull.
The external carotid artery brings blood to structures outside the skull, primarily the face, and the internal carotid to structures inside the skull, including the brain.
The internal carotid is responsible for supplying blood to the nearest brain hemisphere with the help of the basilar artery. The vertebra-basilar system supplies blood when the carotid artery cannot do so (due to a blockage, for example). The internal cartoid artery moves upward, along the neck, passing behind the ear into the subarachnoid space near the temporal lobe. It divides into two branches, the middle cerebral artery and anterior cerebral artery.
The external carotid artery is relatively straight, but the internal carotid artery twists and turns, increasing the likelihood of blockages. A severe blockage could cause a stroke because it would reduce blood flow to the brain.