The internal carotid artery (carotis interna, v. cerebralis) is that branch of the common carotid which is distributed to the brain, and to the eye with its appendages. It extends from the place of bifurcation of the common carotid, usually in a straight direction, to the base of the skull, where it ascends in a winding course through the temporal bone, and after entering the cranial cavity, ends by the side of the anterior clinoid process of the sphenoid bone.
The internal carotid artery may thus be conveniently studied in three parts of its course, viz., while in the neck; in passing through the carotid canal in the base of the skull; and within the cavity of the cranium.
Cervical part. -In the neck, the artery, commencing at the bifurcation of the common carotid, opposite the upper border of the thyroid cartilage, ascends nearly vertically to the base of the skull to reach the carotid foramen in the temporal bone. Like the common carotid, it is accompanied in this course by the internal jugular vein, which lies external to the artery.
This portion of the artery varies much in length. The length in different persons is in general dependent on the stature; but it is also materially influenced by the extent to which the common carotid reaches upwards in the neck ; and the length of the internal carotid may be said to be inversely as the length of that vessel. At first the internal carotid is easily accessible in the neck, inasmuch as it is placed in the intermuscular space in which the division of the common carotid artery occurs, and is covered only by the platysma and fascia, besides its sheath. Placed here to the outer side of the external carotid artery, it soon sinks beneath the parotid gland and becomes deep-seated, as it turns from behind under the external carotid. In its course upwards the internal carotid is crossed by the hypoglossal nerve, and the digastric and stylo-hyoid muscles, as well as by the external carotid and occipital arteries. Higher up, and under cover of the parotid gland, the vessel is likewise covered by thestylo-glossus and stylo-pharyngeus muscles, together with the glossopharyngeal nerve, — these structures being interposed between it and the external carotid artery.
Behind, this artery rests against the rectus anticus major, which muscle, with the sympathetic and vagus nerves, is placed between it and the transverse process of the upper cervical vertebrae : the pharynx and the tonsil, with the pharyngeal artery, are on its inner side.
On reaching the base of the skull, the internal carotid, following the direction of its proper canal in the petrous portion of the temporal bone, ascends perpendicularly a little way, then inclines forwards and inwards, near the inner side of the Eustachian tube, and again ascends as it escapes from the carotid canal. Whilst within the canal, the artery has in contact with it the carotid plexus of nerves.
On passing out of its canal in the temporal bone, the artery, now within the skull, ascends a short distance to gain the body of the sphenoid bone, where it enters the cavernous sinus, having perforated the layer of dura mater, which forms the outer boundary of that cavity. Then, still contained within the sinus, and, according to some anatomists, invested by the lining membrane, it runs horizontally forwards along the side of the sphenoid bone, which is grooved for its reception, and again curves upwards beneath the anterior clinoid process, where, piercing the upper or cerebral wall of the cavernous sinus, and becoming invested by the arachnoid membrane, it reaches the inner end of the fissure of Sylvius, and divides into its terminal branches. Whilst the internal carotid artery is within the cavernous sinus, it is crossed on its outer side by the following nerves, viz., the third, fourth, ophthalmic division of the fifth, and sixth.
Peculiarities of the internal carotid artery
The variations which occur in the length of this artery have been already noticed. Independently of the position at which it is given off in the neck from the bifurcation of the common carotid artery, the only change which has been observed in the place of origin of the internal carotid artery is that met with in those very rare cases already referred to; in which there was no common carotid artery — the external and internal carotids being both derived directly from the arch of the aorta. In these cases the internal carotid artery was nearer to the trachea than the external carotid, which curved forwards over the internal carotid opposite the larynx.
Instead of following a straight direction upwards in the neck, the internal carotid is often curved; and in one case it was observed, after having reached nearly to the base of the skull, to turn downwards for more than an inch, and then abruptly to ascend again, being thus twice closely curved or folded on itself.
A very few examples are recorded of entire absence of the internal carotid. In one of these cases the common carotid ascended in the neck, gave off all the usual branches of the external carotid and divided into the temporal and internal maxillary arteries. Supplying the place of the internal carotid, were two tortuous trunks from the internal maxillary, which entered the skull respectively through the oval and the round foramina of the sphenoid bone, and became united into a single vessel. The vessel thus formed was smaller than the ordinary internal carotid, but the internal carotid of the opposite side was larger than usual.