General topography and cervical part
The initial segment of the internal carotid artery is located deeper and more medially than the external carotid artery, although at its very origin it lies posterior and lateral to it. In the neck, the vessel is always accompanied by the internal jugular vein (v. jugularis interna), which lies lateral to it.
The first segment is the cervical part (pars cervicalis). The artery ascends along the lateral wall of the pharynx, passing medial to the parotid salivary gland. It is physically separated from the glandular tissue by the styloglossus and stylopharyngeus muscles. The main anatomical feature of this segment is that the artery does not give off any branches here. At the very beginning of the cervical part (at the level of the bifurcation) lies the carotid sinus (sinus caroticus), a small fusiform dilation of the wall containing baroreceptors that control blood pressure. The cervical part terminates where the vessel enters the carotid canal of the petrous part of the temporal bone.
Petrous and cavernous parts
Inside the skull, the topography of the vessel becomes more complex as it passes through two specific segments:
- Petrous part (pars petrosa). The vessel runs inside the carotid canal of the temporal bone, precisely following all its natural S-shaped curves. The artery exits the canal into the cranial cavity through the foramen lacerum (foramen lacerum) and lies in a specialized carotid sulcus on the body of the sphenoid bone. In this segment, small caroticotympanic arteries (aa. caroticotympanicae) branch off to the mucosa of the tympanic cavity, along with the artery of the pterygoid canal (a. canalis pterygoidei), which supplies the pterygopalatine ganglion.
- Cavernous part (pars cavernosa). The artery penetrates directly into the cavernous venous sinus (sinus cavernosus), becoming literally surrounded by venous blood. Important cranial nerves run close to it: the oculomotor (III), trochlear (IV), abducens (VI), as well as the ophthalmic and maxillary nerves (V₁ and V₂). Here, the vessel makes an anterior and superior bend, giving off branches to the sinus walls, tentorium cerebelli, meninges, nerves, and the inferior hypophysial artery (a. hypophysialis inferior) to the posterior lobe of the pituitary gland.
Cerebral part and ophthalmic artery
The cerebral part (pars cerebralis) begins at the lesser wings of the sphenoid bone. Small vessels branch off here: the clival branch (r. clivi) to the dura mater and the superior hypophysial artery (a. hypophysialis superior) to the anterior lobe of the pituitary gland.
The largest branch of the cerebral segment is the paired ophthalmic artery (a. ophthalmica). It enters the orbit through the optic canal (canalis opticus) together with the optic nerve, lying lateral to it. This vessel has a rich network of branches:
- Lacrimal artery (a. lacrimalis) — to the lacrimal gland and eyelids;
- Central retinal artery (a. centralis retinae) — penetrates inside the optic nerve;
- Ciliary arteries (posterior and anterior) — to the choroid, ciliary body, and conjunctiva;
- Muscular branches — to supply the rectus and oblique muscles of the eye;
- Ethmoidal arteries (anterior and posterior) — supply the ethmoid air cells and nasal cavity;
- Supraorbital artery (a. supraorbitalis) — emerges into the frontal region.
At the medial angle of the eye, the ophthalmic artery divides into the supratrochlear artery and the dorsal nasal artery (a. dorsalis nasi). The latter forms a critically important clinical anastomosis with the angular artery (which belongs to the external carotid artery system).
Briefly: answers to topical questions
What branches does the cavernous segment of the internal carotid artery give off?
The cavernous segment of the internal carotid artery gives off branches to the cavernous sinus, dura mater, cranial nerves, and pituitary gland.
These include:
- Branch of the cavernous sinus (r. sinus cavernosi) — to the sinus wall.
- Meningeal branch (r. meningeus) to the dura mater.
- Basal and marginal tentorial branches (r. basalis tentorii, r. marginalis tentorii) — to the tentorium cerebelli.
- Branch to the trigeminal ganglion (r. ganglioni trigemini) — to the ganglion trigeminale.
- Branches to nerves (rr. nervorum) — to the trochlear, trigeminal, and abducens nerves.
- Inferior hypophysial artery (a. hypophysialis inferior) — to the posterior lobe of the pituitary gland.
Also, the meningohypophyseal and inferolateral trunks are distinguished in this segment.
What terminal branches does the cerebral part of the internal carotid artery give off?
The cerebral part of the internal carotid artery gives off two terminal branches.
Above the clinoid process, it divides into:
- Anterior cerebral artery (a. cerebri anterior) — located medially, supplies the superior sagittal margin of the hemisphere, as well as the medial sections of the frontal and parietal lobes.
- Middle cerebral artery (a. cerebri media) — lies laterally, enters the lateral sulcus, and is the largest branch providing blood supply to an extensive area on the convex surface of the hemispheres.
Which arteries participate in the formation of the Circle of Willis?
The Circle of Willis (circulus arteriosus cerebri) is formed by arteries of the carotid and vertebrobasilar systems.
The vascular ring at the base of the brain is formed by:
- Anterior communicating artery (a. communicans anterior) — anteriorly.
- Anterior cerebral artery (a. cerebri anterior) — proximal segment.
- Internal carotid artery (a. carotis interna) — distal segment.
- Posterior communicating artery (a. communicans posterior) — laterally.
- Posterior cerebral artery (a. cerebri posterior) — proximal segment.
- Basilar artery (a. basilaris) — its bifurcation posteriorly.
SheCaPeM — a mnemonic for the four parts of the internal carotid artery in the order of blood flow: Cervical, Petrous, Cavernous, Cerebral.
Class and exam questions
What is the main difference between the internal and external carotid arteries in the neck?
In the cervical region, the internal carotid artery gives off no collateral branches. This is a crucial landmark for surgeons.
What is the carotid sinus?
This is a small expansion of the wall of the internal carotid artery at the level of its origin from the common carotid. It contains numerous baroreceptors that help regulate blood pressure.
What is the clinical significance of the dorsal nasal artery?
The dorsal nasal artery forms a direct anastomosis with the angular artery (a branch of the facial artery). This is one of the main pathways for collateral blood flow between the internal and external carotid artery systems.
What else to study on this topic
- Topography and branches of the ophthalmic artery in the orbit
- Syntopy of the artery and cranial nerves within the cavernous sinus
- Blood supply to the pituitary gland (anastomoses of the superior and inferior hypophysial arteries)
- Course of the artery within the temporal bone pyramid and its connections with the tympanic cavity
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