The internal carotid artery supplies the anterior part of the brain, the eye and its appendages, and sends branches to the forehead and nose.
Its size, in the adult, is equal to that of the external carotid, though, in the child, it is larger than that vessel. It is remarkable for the number of curvatures that it presents in different parts of its course. It occasionally has one or two flexures near the base of the skull, while in its passage through the carotid canal and along the side of the body of the sphenoid bone it describes a double curvature and resembles the italic letter S.
In considering the course and relations of this vessel it may be divided into four portions in Terminologia Anatomica: cervical, petrous, cavernous, and cerebral parts
Bouthillier et al described (in 1996) a seven segment internal carotid artery (ICA) classification system. It remains the most widely used system for describing ICA segments:
The length of the internal carotid varies according to the length of the neck, and also according to the point of bifurcation of the common carotid. It arises sometimes from the arch of the aorta; in such rare instances, this vessel has been found to be placed nearer the middle line of the neck than the external carotid, as far upward as the larynx, when the latter vessel crossed the internal carotid. The course of the artery, instead of being straight, may be very tortuous. A few instances are recorded in which this vessel was altogether absent; in one of these the common carotid passed up the neck, and gave off the usual branches of the external carotid; the cranial portion of the internal carotid was replaced by two branches of the internal maxillary, which entered the skull through the foramen rotundum and foramen ovale, and joined to form a single vessel.
The cervical portion of the internal carotid gives off no branches. Those from the other portions are:
From the Petrous Portion:
From the Cavernous Portion:
From the Cerebral Portion: