THE INTERNAL CAROTID ARTERY. 769 
it ascends beneath the posterior belly of the digastric and the stylo-hyoid, it gets 
to the inner side of that vessel. After entering the submaxillary triangle it ascends, 
in relation with the posterior surface of the parotid gland, to the carotid canal in 
the temporal bone, through which it passes to the apex of the petrous portion of 
the temporal bone, where it turns upwards, through the upper part of the foramen 
lacerum medium, into the middle cranial fossa. It then runs forwards in the outer 
wall of the cavernous sinus to the lower root of the small wing of the sphenoid, 
where it turns upwards and then backwards and outwards to its termination. 
Relations.—The relations of the various parts of the artery require separate 
consideration. 
In the neck.—Posterior.—The rectus capitis anticus major, the prevertebral fascia and 
the sympathetic cord, separate it from the transverse processes of the cervical vertebree, 
and somewhat to its outer side are the internal jugular vein and the vagus nerve. The 
spinal accessory and the glosso-pharyngeal nerves are also behind and to the outer side of 
the artery in the upper part of the neck, and they intervene between it and the internal 
jugular vein. nternal or deep to the internal carotid is the external carotid artery for 
a short distance below, and afterwards the wall of the pharynx, the ascending pharyngeal 
artery, the pharyngeal plexus of veins, and the external and internal laryngeal nerves. 
Just before it enters the temporal bone the levator palati muscle is to its inner side. 
External or superficial to it are the sterno-mastoid, skin, and fascia, and it is crossed 
beneath the sterno-mastoid from below upwards by the hypoglossal nerve, the occipital 
artery, and the posterior auricular artery. It is also crossed more superficially by the 
digastric and stylo-hyoid muscles, and in the upper part of its extent it is covered by the 
posterior part of the parotid gland. Passing obliquely across its anterior and outer sur- 
face, and separating it from the external carotid artery, are the following structures, viz. : 
the stylo-pharyngeus, the tip of the styloid process or the stylo-glossus muscle, the glosso- 
phary yngeal nerve, the pharyngeal branch of the vagus, and some sympathetic twigs. 
In the carotid canal.—The artery, as it passes upwards and inwards, is in front of and 
below the cochlea and the tympanum ; behind and internal to the canals for the Eustachian 
tube and the tensor tympani ; and below the Gasserian ganglion. The thin lamina of bone 
which separates it from the tympanum is frequently perforated, and that between it and 
the Gasserian ganglion is not infrequently absent. In its course through the canal it is 
accompanied by small veins and nerves. The veins are tributaries from the tympanum, 
which communicate above with the cavernous sinus and below with the internal jugular 
vein. The nerves are the upward continuations of the sympathetic cord ; they form two 
plexuses—one on the outer side of the artery, the carotid plexus, and one on the inner 
side, the cavernous plexus. 
As it enters the cavity of the cranium the internal carotid artery pierces the external 
layer of the dura mater and passes between the lingula and the sixth cranial nerve 
externally, and the posterior petrosal process of the body of the sphenoid internally. 
In the cranial cavity.—Vhe artery runs forwards in the outer wall of the cavernous sinus 
in relation with the third, fourth, the ophthalmic division of the fifth, and the sixth cranial 
nerves externally, and with the endothelial wall of the sinus internally. When it reaches 
the lower root of the small wing of the sphenoid it turns upwards to the inner side of 
the anterior clinoid process, pierces the inner layer of the dura mater, and comes into 
close relation with the under surface of the optic nerve immediately behind the optic 
foramen. It then turns abruptly backwards beneath the optic nerve, and on the inner side 
of the anterior clinoid process which it frequently grooves; inclining outwards, it runs 
between the second and third nerves, and beneath the anterior perforated space, to the 
inner end of the stem of the Sylvian fissure, where it turns upwards, at some distance 
from the outer side of the optic chiasma, and, after piercing the arachnoid, divides into 
its two terminal branches, the anterior and middle cerebral arteries. 
BRANCHES OF THE INTERNAL CAROTID ARTERY. 
Branches are given off from the internal carotid in the temporal bone and in 
the cranium, but, as a rule, no branches are given off in the neck. 
In the temporal bone.-—(1) A tympanic branch (ramus carotico- tympanicus), very 
small, perforates the posterior wall of the carotid canal, and anastomoses in the tympanum 
with the stylo-mastoid artery and with the tympanie branches of the internal maxillary 
and ascending pharyngeal arteries. 
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