374 ARTERIES OF THE HEAD AND NECK. 



of them anastomose with the ascending cei-vical branch of the subclavian 

 artery. 



(r) The mcniiit/riil branches are terminal twigs, -which pass through th& 

 foramen lacerum posticum and anterior condylar foramen, to end in the dura 

 mater. 



Varieties.- — This artery varies greatly in its place of origin from the carotid. 

 It occasionally springs from the occipital or internal carotid, and, in a few 

 instances, it has been seen double. 



INTERNAL CAEOTID AETERY. 



The iutcrnal carotid artery is distributed to tlie brain, to the eye 

 with its appendages, and in part to the forehead. It extends directly 

 upwards from the termination of the common carotid artery, opposite 

 the upper border of the thyroid cartilage, to the carotid foramen of the 

 temporal bone. Entering the cranial cavity through the carotid canal, 

 it crosses the foramen lacerum medium, and turning upwards on the 

 side of the sphenoid bone, it passes forward on the carotid groove of 

 that bone. Thence it turns abruptly upwards on the inner side of the 

 anterior clinoid process, and divides opposite the inner end of the 

 Sylvian fissure of the brain, into the anterior and middle cerebral: 

 arteries. 



I/i the neclc^ the internal carotid artery varies in length according to 

 the height of the division of the connnon carotid. It rests on the 

 rectus anticus major muscle, and has the pharynx and tonsil on its 

 inner side. The internal jugular vein is in contact with it as far as 

 the base of the skull, lying on its superficial and posterior aspect : and 

 placed more deeply behind it are the vagus nerve and main trunk of 

 the sympathetic. At its commencement the artery is covered only by 

 the sterno-mastoid muscle, by the platysma myoides, and by fascia, and 

 lies to the outer side of the external carotid. It soon, however, becouies; 

 concealed by the parotid gland, and lies internal and posterior to the 

 external carotid trunk, and is crossed first by the occipital artery, and 

 by the hypoglossal nerve and the digastric and stylo-hyoid muscles,, 

 three structures which lie superficial to both carotid arteries ; and 

 higher up, by the stylo-glossus and stylo-pharyngeus muscles, and the 

 glosso-pharyngeal nerve, which, together with, in some cases, the piha- 

 ryngeal branch of the vagus nerve, pass forwards between the external 

 and internal carotid arteries. 



Within the cranium, the internal carotid artery has a very tortuous 

 course, curving forwards and inwards within the carotid canal, then 

 turning upwards to reach the sphenoid bone, on which it is at first 

 directed horizontally forwards, and afterwards resumes the vertical 

 position on the inner side of the anterior clinoid process. In this 

 part of its course the artery is accompanied by the carotid and caver- 

 nous plexuses of the sympathetic nerve. After leaving the carotid 

 canal, it lies in the floor of the cavernous sinus, and in contact with it 

 externally are the nerves which pass through the sphenoidal fissure. 

 Opposite the anterior clinoid process it pierces the layer of dura mater 

 which forms the roof of the sinus, and becomes invested with arachnoid 

 membrane. 



Varieties. — In very rare cases of abnormal arrangement of the arch of 

 the aorta, the internal carotid artery has arisen as a primary trunk. A few 

 examples of its entire absence are recorded. 



