498 THE ARTERIES 



dividing oi)posite the anterior clinoid processes in the fissure of Sylvius, into the 

 anterior and middle cerebral arteries. 



In its course up the neck it often forms one or more curves, especially in old 

 people. Between the internal and the external carotids, at their angle of diverg- 

 ence, is situated the intercarotid body, or ganglion intercarotioum. 



The internal carotid is the continuation ui)wards of the ])rimitive dorsal aorta, 

 and supplies the greater part of the brain, the contents of tlie orl^it, and parts of 

 the internal ear, forehead, and nose. It is divided into three portions: — 1. a 

 cervical; 2. an intraosseous, or petrosal; and 3. an intracranial. 



1. The Cervical Portion 



Relations. — In the neck the artery is at first comparative!}' superficial, having 

 in front of it, as it lies in the superior carotid triangle, the skin, superficial fascia, 

 platysma and deep fascia, and the overlapping edge of the sterno-mastoid muscle. 

 Higher up, as it sinks beneath the parotid gland (fig. 829), it becomes deeply 

 placed, and is crossed by the posterior belly of the digastric and stylo-hyoid 

 muscles, the hypoglossal nerve, and the occipital and posterior auricular arteries; 

 whilst still higher it is separated from the external carotid artery, which here gets 

 in front of it, by the stylo-glossus and stylo-pharyngeus muscles, the glosso- 

 pharvngeal nerve, the pharyngeal branch of the pneumogastric nerve, and by the 

 stylo-hyoid ligament. 



Behind, it lies upon the rectus capitis anticus major, which separates it from the 

 transverse processes of the three upper cervical vertebrae, on the superior cervical 

 ganglion of the sympathetic nerve, and on the pneumogastric nerve. Xear the 

 base of the skull, the hypoglossal, pneumogastric, glosso-pharyngeal, and spinal 

 accessory nerves cross obliquely behind it, separating it at this spot from the 

 internal jugular vein, which, as the artery is about to enter the carotid canal, also 

 forms one of its posterior relations. 



On its outer side are the internal jugular vein and pneumogastric nerve. 



On its inner side it is in relation with the pharynx, the superior constrictor 

 muscle separating it from the tonsil. The ascending pharyngeal and ascending 

 palatine arteries, and at the base of the skull the Eustachian tube and levator 

 palati muscle are also internal to it. 



2. The Intraosseous or Petrosal Portion 



The intraosseous portion (fig. 331) is situated in the carotid canal in the 

 petrous portion of the temporal bone. It is here separated from the walls of the 

 canal V>y a prolongation downwards of the dura mater. In this part of its course 

 it first ascends in front of the tympanum and cochlea of the internal ear; it then 

 turns forwards and inwards, lying a little internal to and behind the Eustachian 

 tube, and enters the cranial cavity by turning upwards through the foramen 

 lacerum medium, lying upon the lingula of the si)henoid l)one. In this part of its 

 course it is accompanied by the ascending liranches from the superior cervical 

 ganglion of the sympathetic. These form a ])lexus about the artery, liut are situated 

 chiefly on its outer side. It is also surrounded by a number of small veins, Avhich 

 receive tributaries from the tympanum, and open into the cavernous sinus and 

 internal jugular vein. 



3. The Intracranial Portion 



On entering the cranium through the foramen lacerum medium, the internal 

 carotid first ascends towards the ])Osterior clinoid process, but soon changing its 

 direction, it curves forwards and slightly doAvnwards by the side of the l)ody of the 

 sphenoid bone on the inner wall of the cavernous sinus. Here it has the sixth nerve 

 innuediately external to it, and is covered by the lining membrane of the sinus. 

 Again turning upwards, it pierces the dura mater on the inner side of the anterior 

 clinoid process, and, passing l^etween the second and third nerves to the anterior 

 perforated spot at the inner end of the Sylvian fissure, divides into its two terminal 

 branches, the anterior and middle cerebral. After it has perforated the dura mater, 



