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The internal carotid artery is a blood vessel that delivers oxygenated blood from the heart to the brain. A paired vessel with one on either side of the head, it is a branch of the larger common carotid artery, with the other branch being the external carotid artery, which supplies blood to various structures in the face, head, and neck. The internal carotid artery arises from the common carotid in the neck at approximately the height of the top border of the Adam’s apple. Upon penetrating the head, it branches into several smaller arteries, among them the ophthalmic artery, the anterior choroidal artery, the anterior cerebral artery, the middle cerebral artery, and the posterior communicating artery.
As a branch of the common carotid artery, the internal carotid artery can be traced back to its origins on the aorta. The largest blood vessel in the body, the aorta ascends from the heart and curves at the top of the chest to form the aortic arch. Here is where the left and right common carotid arteries originate, although the right common carotid branches not off the aorta directly but off the brachiocephalic artery. Each side of the common carotid ascends its respective side of the neck, and at the height of the fourth cervical vertebra each bifurcates into the external and internal carotid arteries, with the external carotid the most superficial of the pair, meaning that it lies closer to the skin.
Unlike the external carotid, which forms multiple branches in the neck immediately after diverging from the common carotid, the internal carotid artery continues up the neck and enters the head behind the angle of the jawbone, just deep to the ear, as an undivided tributary. This vessel can be classified into seven unique segments, with some segments branching into other vessels and others not branching at all. For instance, the portion in the neck is the cervical segment, also known as C1, though no branches are found here.
Upon entering the head, the petrous segment of the internal carotid artery, or C2, begins. It is named for its location inside of the petrous portion of the temporal bone, a pyramid-shaped section of the temporal bone in the skull, within which the entrance to the ear canal is found. A few minor branches of the internal carotid are found in C2, including the caroticotympanic and vidian arteries of the ear.
Above the petrous segment are the lacerum segment, C3, which forms no branches, and the cavernous segment, C4, which forms many small tributaries. Found within the cavernous sinus, a latticework of tiny blood vessels, C4 gives rise to the meningohypophyseal and inferolateral trunks. These smaller vessels supply such structures as the dura mater around the brain and the pituitary gland.
The final three segments are C5, C6, and C7, or the clinoid, ophthalmic, and communicating portions of the internal carotid artery. While the clinoid segment features no additional branches, the ophthalmic and communicating segments give rise to the major branches of the internal carotid: the ophthalmic and superior hypophyseal arteries in the ophthalmic segment; and the posterior communicating, anterior choroidal, anterior cerebral, and middle cerebral arteries in the communicating segment. The latter two arteries are terminal branches penetrating the brain, meaning that the internal carotid artery ends where it branches into the anterior and middle cerebral arteries.