C1: Cervical segment

Level of 6th cervical vertebrae - still at level of common carotid but relationships are similar to those of cervical segment of internal carotid

The cervical segment, or C1, or cervical part of the internal carotid, extends from the carotid bifurcation until it enters the carotid canal in the skull anterior to the jugular foramen.

Internal carotid artery - dissection

At its origin, the internal carotid artery is somewhat dilated. This part of the artery is known as the carotid sinus or the carotid bulb. The ascending portion of the cervical segment occurs distal to the bulb when the vessel walls are again parallel.

The internal carotid runs vertically upward in the carotid sheath and enters the skull through the carotid canal. During this part of its course, it lies in front of the transverse processes of the upper three cervical vertebrae.

It is relatively superficial at its start, where it is contained in the carotid triangle of the neck, and lies behind and medial to the external carotid, overlapped by the sternocleidomastoid muscle, and covered by the deep fascia, the platysma, and integument: it then passes beneath the parotid gland, being crossed by the hypoglossal nerve, the digastric muscle and the stylohyoid muscle, the occipital artery and the posterior auricular artery. Higher up, it is separated from the external carotid by the styloglossus and stylopharyngeus muscles, the tip of the styloid process and the stylohyoid ligament, the glossopharyngeal nerve and the pharyngeal branch of the vagus nerve. It is in relation, behind, with the longus capitis, the superior cervical ganglion of the sympathetic trunk, and the superior laryngeal nerve; laterally, with the internal jugular vein and vagus nerve, the nerve lying on a plane posterior to the artery; medially, with the pharynx, superior laryngeal nerve, and ascending pharyngeal artery. At the base of the skull the glossopharyngeal, vagus, accessory, and hypoglossal nerves lie between the artery and the internal jugular vein.

Unlike the external carotid artery, the internal carotid normally has no branches in the neck.

C2: Petrous segment

The petrous segment, or C2, of the internal carotid, is that which is inside the petrous part of the temporal bone. This segment extends until the foramen lacerum. The petrous portion classically has three sections: an ascending, or vertical, portion; the genu, or bend; and the horizontal portion.

When the internal carotid artery enters the canal in the petrous portion of the temporal bone, it first ascends a short distance and then curves anteriorly and medially. The artery lies at first in front of the cochlea and tympanic cavity; from the latter cavity it is separated by a thin, bony lamella, which is cribriform in the young subject, and often partly absorbed in old age. Farther forward, it is separated from the trigeminal ganglion by a thin plate of bone, which forms the floor of the fossa for the ganglion and the roof of the horizontal portion of the canal. Frequently this bony plate is more or less deficient, and then the ganglion is separated from the artery by fibrous membrane. The artery is separated from the bony wall of the carotid canal by a prolongation of dura mater and is surrounded by a number of small veins and by filaments of the carotid plexus, derived from the ascending branch of the superior cervical ganglion of the sympathetic trunk.

The named branches of the petrous segment of the internal carotid artery are:

• the vidian artery or artery of the pterygoid canal
• the caroticotympanic artery

C3: Lacerum segment

The lacerum segment, or C3, is a short segment that begins above the foramen lacerum and ends at the petrolingual ligament, a reflection of periosteum between the lingula and petrous apex (or petrosal process) of the sphenoid bone. The lacerum portion is still considered "extradural" since it is surrounded by periosteum and fibrocartilage along its course. It is erroneously stated in several anatomy textbooks that the internal carotid artery passes through the foramen lacerum. This at best has only ever been a partial truth in that it passes through the superior part of the foramen on its way to the cavernous sinus. As such it does not traverse the skull through it. The inferior part of the foramen is actually filled with fibrocartilage. The broad consensus is that the internal carotid artery should not be described as travelling through the foramen lacerum.^[8]

C4: Cavernous segment

Oblique section through the cavernous sinus.

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 cavernous segment is surrounded by the cavernous sinus.

In this part of its course, the artery is situated between the layers of the dura mater forming the cavernous sinus, but covered by the lining membrane of the sinus. It at first ascends toward the posterior clinoid process, then passes forward by the side of the body of the sphenoid bone, again curves upward on the medial side of the anterior clinoid process, and perforates the dura mater forming the roof of the sinus. The curve in the cavernous segment is called the carotid siphon. This portion of the artery is surrounded by filaments of the sympathetic trunk, and on its lateral side is the abducent nerve, or cranial nerve VI.

The named branches of the cavernous segment are:

• the meningohypophyseal artery
• the inferolateral trunk

The cavernous segment also gives rise to small capsular arteries that supply the wall of the cavernous sinus.

C5: Clinoid segment

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 clinoid segment normally has no named branches, though the ophthalmic artery may arise from the clinoid segment.

C6: Ophthalmic segment

The ophthalmic artery and its branches

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 named branches of the ophthalmic segment are:

• the ophthalmic artery
• the superior hypophyseal artery

C7: Communicating segment

The communicating segment, or terminal segment, or C7, of the internal carotid artery passes between the optic and oculomotor nerves to the anterior perforated substance at the medial extremity of the lateral cerebral fissure. Angiographically, this segment extends from the origin of the posterior communicating artery to the bifurcation of the internal carotid artery.

The named branches of the communicating segment are:

• the posterior communicating artery
• the anterior choroidal artery

The internal carotid then divides to form the anterior cerebral artery and middle cerebral artery. The circle of Willis provides a collateral pathway for blood supply to the brain.