

THE INTERNAL CAROTID ARTERY 567 



Course and Relations. In considering the course and relations of this vessel 

 it may be divided into four portions: cervical, petrous, cavernous, and cerebral. 



Cervical Portion. -This portion of the internal carotid begins at the bifurca- 

 tion of the common carotid, opposite the upper border of the thyroid cartilage, 

 and runs perpendicularly upward, in front of the transverse processes of the upper 

 three cervical vertebrae, to the carotid canal in the petrous portion of the temporal 

 bone. It is comparatively superficial at its commencement, where it is contained 

 in the carotid triangle, and lies behind and lateral to the external carotid, over- 

 lapped by the Sternocleidomastoideus, and covered by the deep fascia, Platysma, 

 and integument: it then passes beneath the parotid gland, being crossed by the 

 hypoglossal nerve, the Digastricus and Stylohyoideus, and the occipital and pos- 

 terior auricular arteries. Higher up, it is separated from the external carotid by 

 the Styloglossus and Stylopharyngeus, the tip of the styloid process and the stylo- 

 hyoid ligament, the glossopharyngeal nerve and the pharyngeal branch of the vagus. 

 It is in relation, behind, with the Longus capitis, the superior cervical ganglion of 

 the sympathetic trunk, and the superior laryngeal nerve; laterally, with the internal 

 jugular vein and vagus nerve, the nerve lying on a plane posterior to the artery; 

 medially, with the pharynx, superior laryngeal nerve, and ascending pharyngeal 

 artery. At the base of the skull the glossopharyngeal, vagus, accessory, and hypo- 

 glossal nerves lie between the artery and the internal jugular vein. 



Petrous Portion. When the internal carotid artery enters the canal in the 

 petrous portion of the temporal bone, it first ascends a short distance, then curves 

 forward and medialward, and again ascends as it leaves the canal to enter the 

 cavity of the skull between the lingula and petrosal process of the sphenoid. The 

 artery lies at first in front of the cochlea and tympanic cavity; from the latter 

 cavity it is separated by a thin, bony lamella, which is cribriform in the young 

 subject, and often partly absorbed in old age. Farther forward it is separated 

 from the semilunar ganglion by a thin plate of bone, which forms the floor of the 

 fossa for the ganglion and the roof of the horizontal portion of the canal. Fre- 

 quently this bony plate is more or less deficient, and then the ganglion is separated 

 from the artery by fibrous membrane. The artery is separated from the bony wall 

 of the carotid canal by a prolongation of dura mater, and is surrounded by a number 

 of small veins and by filaments of the carotid plexus, derived from the ascending 

 branch of the superior cervical ganglion of the sympathetic trunk. 



Cavernous Portion. In this part of its course, the artery is situated between 

 the layers of the dura friater forming the cavernous sinus, but covered by the lining 

 membrane of the sinus. It at first ascends toward the posterior clinoid process, 

 then passes forward by the side of the body of the sphenoid bone, and again curves 

 upward on the medial side of the anterior clinoid process, and perforates the dura 

 mater forming the roof of the sinus. This portion of the artery is surrounded by 

 filaments of the sympathetic nerve, and on its lateral side is the abducent nerve. 



Cerebral Portion. Having perforated the dura mater on the medial side of 

 the anterior clinoid process, the internal carotid passes between the optic and oculo- 

 motor nerves to the anterior perforated substance at the medial extremity of the 

 lateral cerebral fissure, where it gives off its terminal or cerebral branches. 



Peculiarities. 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. 



