566 THE ARTERIES. 



blood, by increasing the extent of surface over which it moves and adding to the 

 amount of impediment produced from friction. 



In considering the course and relations of this vessel it may be conveniently 

 divided into four portions : a cervical, petrous, cavernous, and cerebral. 



Cervical Portion. This portion of the internal carotid commences at the bifur- 

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

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

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

 bone. It is superficial at its commencement, being contained in the superior 

 carotid triangle, and lying on the same level as the external carotid, but behind 

 that artery overlapped by the Sterno-mastoid and covered by the deep fascia, 

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

 by the hypoglossal nerve, the Digastric and Stylo-hyoid muscles, and the external 

 carotid and occipital arteries. Higher up, it is separated from the external carotid 

 by the Stylo-glossus and Stylo-pharyngeus muscles, the glosso-pharyngeal nerve, 

 and pharyngeal branch of the pneumogastric. It is in relation, behind, with the 

 Rectus capitis anticus major, the superior cervical ganglion of the sympathetic, 

 and superior laryngeal nerve ; externally, with the internal jugular vein and pneu- 

 mogastric nerve ; internally, with the pharynx, tonsil, the superior laryngeal nerve, 

 and ascending pharyngeal artery. 



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 inward, and again ascends as it leaves the canal to enter the cavity 

 of the skull. In this canal the artery lies at first anterior to the tympanum, from 

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

 subject, and often absorbed in old age. It is separated from the bony wall of the 

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

 the carotid plexus. 



Cavernous Portion. The internal carotid artery in this part of its course is 

 situated between the layers of the dura mater forming the cavernous sinus, but 

 covered by the lining membrane of the sinus. It at first ascends to the posterior 

 clinoid process, then passes forward by the side of the body of the sphenoid bone, 

 and again curves upward on the inner side of the anterior clinoid process, and 

 perforates the dura mater forming the roof of the sinus. In this part of its course 

 it is surrounded by filaments of the sympathetic nerve, and has in relation with it 

 externally the sixth nerve. 



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

 anterior clinoid process, the internal carotid enters the inner extremity of the 

 fissure of Sylvius, where it gives off its terminal or cerebral branches. This 

 portion of the artery has the optic nerve on its inner side, and the third nerve 

 externally. 



PLAN OF THE RELATIONS OF THE INTERNAL CAROTID ARTERY IN THE NECK. 



Externally. 



Skin, superficial and deep fasciae. 



Platysma. 



Sterno-mastoid. 



External carotid and occipital ar- /^ ~^\ Internally. 



teries. / \ Pharynx. 



Hypoglossal nerve. ( carot?d ] Superior laryngeal nerve. 



Parotid gland. I Artery. I Ascending pharyngeal artery. 



Stylo-glossus and Stylo-pharyngeus J Tonsil, 



muscles. 



Glosso-pharyngeal nerve. 



Pharyngeal branch of the pneumo- 

 gastric. 



Internal jugular vein. 



Pneumogastric nerve. 



