INTERNAL CAROTID. 463 



tures that it presents in different parts of its course. In its cervical portion it 

 occasionally presents one or two flexures near the base of the skull, whilst 

 through the rest of its extent it describes a double curvature which resembles 

 the italic letter / placed horizontally X- These curvatures most probably 

 diminish the velocity of the current of blood, by increasing the extent of sur- 

 face over which it moves, and adding to the amount of impediment produced 

 from friction. In considering the courses 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 is superficial at its com- 

 mencement, 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 Platysma, deep fascia, 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-glos- 

 sus and Stylo-pharyngeus muscles, the glosso-pharyngeal nerve, and pharyngeal 

 branch of the vagus. It is in relation, behind, with the Rectus Anticus Major, 

 the superior cervical ganglion of the sympathetic, and superior laryngeal nerve ; 

 externally, with the internal jugular vein, and pneumogastric nerve; internally, 

 with the pharynx, tonsil, 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 forwards and inwards, 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 cribri- 

 form 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 sur- 

 rounded by filaments of the carotid plexus. 



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

 first ascends to the posterior clinoid process, then passes forwards by the side 

 of the body of the sphenoid bone, being situated on the inner wall of the 

 cavernous sinus, in relation, externally, with the sixth nerve, and covered by 

 the lining membrane of the sinus. The third, fourth, and ophthalmic nerves 

 are placed on the outer wall of the sinus, being separated from its cavity by the 

 lining membrane. 



Cerebral Portion. On the inner side of the anterior clinoid process the inter- 

 nal carotid curves upwards, perforates the dura mater bounding the sinus, and 

 is received into a sheath of the arachnoid. This portion of the artery is on 

 the outer side of the optic nerve ; it lies at the inner extremity of the fissure 

 of Sylvius, having the third nerve externally. 



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



In front. 



Skin, superficial and deep fasciae. 

 Parotid gland. 



Stylo-glossus and Stylo-phuryngeus muscles. 

 Glosso-pharyngeal nerve. 



Externally. /" \ Internally. 



Internal jugular vein. ( "* \ 



Pneumogattric nerve. \ Ky d J Ascend.ng pharyngeal artery. 



Behind. 



Rectus Anticus Major. 

 Sympathetic. 

 Superior laryngeal nerve. 



