INTERNAL CAROTID ARTERY. 359 



(b) The external branches, small and irregular, are distributed to the rectus anticu- 

 muscle, the first cervical ganglion of the sympathetic nerve, some of the cerebra- 

 nerves as they issue from the skull, and to lymphatic glands. Some of them anasto- 

 mose with the ascending cervical branch of the subclavian artery, 



(c) The menintjeal branches are terminal twigs, which pass through the foramen 

 lacerum posticum to end in the dura mater. 



PECULIARITIES. This artery varies greatly in its place of origin from the carotid. 

 It occasionally springs from another source, as from the occipital or internal carotid, 

 and, in a few instances, it has been seen double. 



INTERNAL CAROTID ARTERY. 



The internal carotid artery is distributed to the 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 forwards 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 Sylviaii fissure of the brain, into 

 the anterior and middle cerebral arteries. 



In the neck, the internal carotid artery varies in length according to the 

 height of the division of the common 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 com- 

 mencement the artery is covered only by the sterno-mastoid muscle, by the 

 plcitysina myoides, and by fascia, and lies to the outer side of the external 

 carotid. It soon, however, becomes 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 styloid process, the stylo-pharyngeus 

 muscle and the glos?o-pharyngeal nerve, which, together with, in some 

 cases, the pharyngeal 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 hori- 

 zontally 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 cavernous plexuses of the sympathetic 

 n-jrve. After leaving the carotid canal, it lies iu 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 sinns, and becomes invested with 

 arachnoid membrane. 



By the winding course of the internal carotid artery in the skull, the brain is 

 probably in some degree protected from the force of the pulsations with which the 

 blood is propelled from the heart. Occasionally the artery presents considerable 

 tortuosity before entering the carotid canal, especially in apoplectic subjects, the 

 trunk having probably been elongated by the force of the pulsations. 



