610 THE VASCULAR SYSTEMS 



adult is equal to that of the external carotid, though in the child it is larger than 

 that vessel. It is remarkable for the number of curvatures that it presents in 

 different parts of its course. It occasionally has one or two flexures near the base 

 of the skull, while in its passage through the carotid canal and along the side of 

 the body of the sphenoid bone it describes a double curve which resembles some- 

 what the letter S. 



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

 divided into four portions the cervical, petrous, cavernous, and cerebral portions. 



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

 bifurcation 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 superior carotid triangle, and lies behind and4o the_ 

 outer side of the external carotid, overlapped by the Sternomastoid 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 Stylohyoid muscles, 

 and the occipital and posterior auricular arteries. Higher up, it is separated from 

 the external carotid by the Styloglossus and Stylopharyngeus muscles, the tip of 

 the styloid process and the stylohyoid ligament, the glossopharyngeal nerve, and 

 pharyngeal branch of the vagus. 



Relations. 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 vagus nerve, the nerve lying on a plane posterior to the artery; internally, 

 with the pharynx, tonsil, the superior laryngeal nerve, and ascending pharyngeal artery. At 

 the base of the skull the glossopharyngeal, vagus, spinal accessory, and hypoglossal nerves lie 

 between the artery and the internal jugular vein. 



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



In front. 



Skin, superficial and deep fascia. 



Platysma. 



Sternomastoid. 



Occipital and posterior auricular arteries. 



Hypoglossal nerve. 



Parotid gland. 



Styloglossus and Stylopharyngeus muscles. 



Glossopharyngeal nerve. 



Pharyngeal branch of the vagus. 



Externally. /^~ ^\ Internally. 



Internal jugular vein. / internal \ Pharynx. 



, 7 Carotid c i i 



Vagus nerve. \ Artery. / Superior laryngeal nerve. 



V / Ascending pharyngeal artery. 



V. ^ Tonsil. 



Behind. 



Rectus capitis anticus major. 



Sympathetic. 



Superior laryngeal nerve. 



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 



