# Internal Carotid Artery Segments

> Internal carotid artery — cervical, petrous, cavernous and communicating segments, carotid siphon, branches for MBBS Anatomy.

- Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/internal-carotid-artery
- Exam / course: MBBS · Subject: Anatomy
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Internal Carotid Artery Segments", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/internal-carotid-artery

## Direct answer

The internal carotid artery gives no branch in the neck — the single fact that separates it from the external carotid at the bifurcation — then climbs the carotid canal of the petrous temporal bone, runs a winding course through the cavernous sinus, and pierces the dura to enter the subarachnoid space and divide into the anterior and middle cerebral arteries. The widely used Bouthillier numbering divides it into seven segments: C1 cervical, C2 petrous, C3 lacerum, C4 cavernous, C5 clinoid, C6 ophthalmic and C7 communicating. Its clinically essential branches emerge only from the last two segments — the ophthalmic artery, posterior communicating artery, anterior choroidal artery and the terminal anterior and middle cerebral arteries — so disease anywhere along this long silent trunk declares itself at the eye or the hemisphere.

## What you must remember

- **Origin:** from the common carotid at about the level of the upper border of the thyroid cartilage (C4 vertebral level), where the carotid sinus (baroreceptor) and carotid body (chemoreceptor) sit at the bifurcation, supplied by the glossopharyngeal nerve.
- **C1 cervical:** ascends in the carotid sheath with the internal jugular vein and vagus nerve, gives no branches, and enters the carotid canal after a short dilation — the carotid canal's entrance lies in front of the jugular foramen.
- **C2 petrous:** an S-curve within the petrous bone surrounded by a sympathetic plexus from the superior cervical ganglion; minute caroticotympanic branches may reach the middle ear — otherwise silent.
- **C4 cavernous:** the artery grooves the lateral wall relation of the cavernous sinus, forming the carotid siphon; related to cranial nerves III, IV, V1, V2 in the lateral wall and VI lying free beside the artery — the meningohypophyseal trunk and artery to the cavernous sinus arise here.
- **C6 ophthalmic segment:** after emerging from the dura, the ophthalmic artery arises and enters the orbit through the optic canal with the optic nerve; the superior hypophyseal artery also arises here.
- **C7 communicating segment:** posterior communicating artery (joining the posterior cerebral, and the aneurysm site that paralyses the third nerve), anterior choroidal artery (hippocampus, optic tract, posterior limb of internal capsule, choroid plexus), and the terminal anterior and middle cerebral arteries at the medial end of the lateral fissure.
- **Amaurosis fugax:** transient retinal ischaemia from emboli at the carotid bifurcation travelling up the silent cervical segment and out through the ophthalmic artery — a curtain over one eye, the classic carotid warning.
- **Cavernous-segment pathology:** aneurysm or fistula (post-traumatic carotid-cavernous fistula) presents with pulsatile proptosis, chemosis, orbital bruit and ophthalmoplegia, distinguishing it from cavernous sinus thrombosis.

## Following an embolus from bifurcation to field loss

A 62-year-old smoker describes a curtain descending over the left eye for five minutes, with complete recovery — follow the anatomy and the work-up orders itself. A plaque at the carotid bifurcation (where the carotid sinus wall is uniquely shear-stressed) liberates platelet emboli into the internal carotid, a vessel with no branches to intercept them in the neck. The embolus rides C1 into the petrous canal, through the cavernous siphon, and at the C6 segment meets the first branch — the ophthalmic artery — which carries it through the optic canal to the retinal arteries, producing the transient monocular blindness. Had the same embolus drifted past the ophthalmic origin, the middle cerebral territory would follow: contralateral face and arm weakness with aphasia if dominant. The clinical sequence closes the loop: carotid duplex on the bifurcation, antiplatelet therapy and statin, and endarterectomy for significant symptomatic stenosis — all aimed at the origin, not the silent trunk. The same map read backwards explains the surgical emergency of the painful third nerve palsy: a PCom aneurysm at the C7 segment presses the nerve as it runs beside the artery, and its rupture risk makes this the one dilated pupil that cannot wait.

## Where students slip

Asked "name the branches of the internal carotid in the neck", students invent the superior thyroid — that artery is external carotid territory; the internal carotid is silent below the skull base, which is precisely why ligation was historically tolerated. The second slip is the cavernous relations: the abducent nerve lies free in the sinus lumen beside the artery, not in the lateral wall with III, IV, V1 and V2. Third, the ophthalmic artery's origin — intradural, from the C6 segment under the optic nerve — is mislabelled as arising within the cavernous sinus; knowing it emerges just above the dural ring explains why orbital supply survives cavernous disease.

## Frequently asked questions

### Which branches does the internal carotid artery give in the neck?

None. This distinguishes it from the external carotid artery, which gives eight branches in the neck. The first branch of the internal carotid is the ophthalmic artery, after the artery has become intradural.

### Name the segments of the internal carotid artery.

C1 cervical, C2 petrous, C3 lacerum, C4 cavernous, C5 clinoid, C6 ophthalmic and C7 communicating, per the Bouthillier classification. The older shorthand groups them into cervical, petrous, cavernous and intracranial (supraclinoid) parts.

### What is the carotid siphon?

The S-shaped bend of the internal carotid artery within the cavernous sinus, seen as a loop on lateral angiography. It is surrounded by the sympathetic plexus and related to the abducent nerve, and its anatomy must be navigated in cavernous sinus and skull base surgery.

### Which branches arise from the terminal (communicating) segment?

The posterior communicating artery, the anterior choroidal artery, and the terminal division into anterior and middle cerebral arteries. The posterior communicating artery aneurysm is the classic cause of a painful third nerve palsy.

### Why does carotid disease present as transient monocular blindness?

Emboli from a bifurcation plaque travel the branchless cervical and intracranial internal carotid to the ophthalmic artery, its first major branch, and on to the retinal circulation. Interruption of retinal flow causes a curtain-like transient visual loss — amaurosis fugax.