The Internal Carotid is much larger than the external in the young subject, but nearly of equal size in the adult: it arises opposite the superior margin of the thyroid cartilage, and its long and tortuous course may be divided into four stages: the first extends from its origin to the petrous portion of the temporal bone ; the second through the carotid canal in this portion of the bone; the third passes through the cavernous sinus; and the fourth is in immediate relation with the base of the brain.

In its first stage it constantly forms a curvature, the convexity of which looks outwards, and lies, for a short distance, to the outside of the external carotid artery. In the remainder of its ascent to the base of the skull it usually forms a number of other tortuosities seldom alike in any two subjects. Its posterior surface corresponds to the spine, rectus capitis anticus major muscle, and to the superior cervical ganglion, from which it is separated by the superior laryngeal and usually by the pharyngeal branch of the pneumogastric nerve. Near the base of the skull the internal jugular vein lies posterior and a little external to it, but separated from it by the hypo-glossal, glossopharyngeal, and pneumogastric nerves immediately after their exit from the interior of the cranium. Shortly after its first curvature, its anterior surface is covered inferiorly by the external carotid, from which it is separated a little higher up by the stylo-glossus and stylo-pharyngeus muscles, the styloid process, or by the stylo-hyoid ligament, a portion of the parotid gland, the glossopharyngeal nerve, and occasionally the pharyngeal branch of the pneumogastric nerve. Immediately before it pierces the base of the cranium, its anterior surface is related to the Eustachian tube and origin of the levator palati muscle. Its external surface corresponds to the glossopharyngeal nerve, to a portion of the styloid process, to the origin of the stylo-pharyngeus muscle, to an aponeurosis separating it from the parotid gland, and to the internal jugular vein. Its internal surface corresponds to the pharynx and the pharyngea ascendens artery, and higher up to the tonsil. In this locality the vessel is lodged in an angular space formed by the pterygoid muscles on the outside, and the superior constrictor of the pharynx on the inside. Near the termination of its first stage the superior cervical ganglion of the sympathetic nerve, which lies behind it, gives off a considerable branch which appears to be a prolongation of the upper extremity of the ganglion; this branch soon divides into two others, one at the inner and the other at the outer side of the vessel; they communicate in this situation with minute filaments from the glossopharyngeal nerve, and together with the artery they enter the carotid canal and there form the carotid plexus of nerves. The tonsil lies anterior and internal to the artery. The artery gives off no regular branches in the first stage.

In its second stage, we trace it forwards and inwards through the carotid canal, running in a curved direction, surrounded by the carotid plexus and also by a few small veins which terminate in the cavernous sinus. In this canal it is situated anterior and internal to the cavity of the tympanum, from which it is separated only by a thin partition of bone: it lies inferior to the cochlea, and, at the commencement of this stage, inferior also to the Eustachian tube; superior to which, however, it gradually passes as it enters upon its third stage. Having emerged from the carotid canal, it passes obliquely over the cartilaginous substance which fills the foramen lacerum anterius or spheno-temporal fissure; it then enters the cranium, and here its second stage terminates.

In its third stage the artery advances through the cavernous sinus, making two curvatures in the form of a Roman cd, being first convex superiorly, and more in front convex inferiorly: as it passes through the sinus, it is crossed from behind forwards by the sixth nerve, which is closely applied to its external surface: the carotid plexus of nerves surrounds the artery within the sinus, and a branch or two of the sympathetic nerve may be observed ascending on its outside and joining the sixth nerve, as the latter is passing the carotid artery. More externally, and in the outer wall of the cavernous sinus, are situated the third, fourth, and ophthalmic branch of the fifth nerve: these nerves are placed in their numerical order, from above downwards, and from within outwards. The lining membrane of the sinus is reflected on the artery and on the nerves in immediate connection with it, thus forming a sheath which separates them from the blood of the sinus.

On emerging from the cavernous sinus, the artery pierces the dura mater and enters its fourth stage: on reaching the under portion of the anterior clinoid process, it is here lodged in a deep notch, and makes a turn backwards and inwards, and terminates on the outside of the commissure of the optic nerves, and at the internal extremity of the fissure of Sylvius, by dividing into the posterior communicating and the anterior and middle arteries of the cerebrum. The arachnoid membrane gives a covering to the artery after it has entered into its fourth stage. Immediately after escaping from the cavernous sinus the internal carotid gives off the ophthalmic artery, and still later the choroid and posterior communicating arteries: it then terminates by dividing into the arteries already mentioned.

The internal carotid artery gives off the following branches:—

Tympanic. Ophthalmic.

Vidian. Choroid.

Receptacular. Meningeal.

Posterior communicating. Anterior Cerebral.

Middle Cerebral.

The Tympanic Branch

The Tympanic Branch is exceedingly slender: it arises from the artery in its second stage, and, passing through a portion of the bone, is distributed to the tympanum.

The Vidian Branch

The Vidian Branch is a very minute twig, given off also in the second stage: it anastomoses with the vidian artery, a branch of the internal maxillary.

The Receptacular Branches

The Receptacular Branches are small twigs given off by the artery in its third stage: they are distributed to the dura mater, to the walls of the inferior petrosal sinus, and to the pituitary body.

The Meningeal Branch

The Meningeal Branch is also distributed to the dura mater in the immediate vicinity, and anastomoses with the middle meningeal, a branch of the internal maxillary.