[3][4] In clinical settings, however, usually the classification system of the internal carotid artery follows the 1996 recommendations by Bouthillier,[5] describing seven anatomical segments of the internal carotid artery, each with a corresponding alphanumeric identifier: C1 cervical; C2 petrous; C3 lacerum; C4 cavernous; C5 clinoid; C6 ophthalmic; and C7 communicating.

The segments are subdivided based on anatomical and microsurgical landmarks and surrounding anatomy, more than angiographic appearance of the artery.

An alternative embryologic classification system proposed by Pierre Lasjaunias[6] and colleagues is invaluable when it comes to explanation of many internal carotid artery variants.

At the base of the skull the glossopharyngeal, vagus, accessory, and hypoglossal nerves lie between the artery and the internal jugular vein.

It is erroneously stated in several anatomy textbooks that the internal carotid artery passes through the foramen lacerum.

[8] The cavernous segment, or C4, of the internal carotid artery begins at the petrolingual ligament and extends to the proximal dural ring, which is formed by the medial and inferior periosteum of the anterior clinoid process.

The clinoid segment, or C5, is another short segment of the internal carotid that begins after the artery exits the cavernous sinus at the proximal dural ring and extends distally to the distal dural ring, after which the carotid artery is considered "intra-dural" and has entered the subarachnoid space.

The ophthalmic segment, or C6, extends from the distal dural ring, which is continuous with the falx cerebri, to the origin of the posterior communicating artery.

The ophthalmic segment courses roughly horizontally, parallel to the optic nerve, which runs superomedially to the carotid at this point.

The state and health of internal carotid arteries is usually evaluated using doppler ultrasound, CT angiogram or phase contrast magnetic resonance imaging (PC-MRI).

[11] This article incorporates text in the public domain from page 566 of the 20th edition of Gray's Anatomy (1918) ocular group: central retinal