This artery (A. carotis interna) is somewhat smaller than the occipital.
It usually arises just behind that artery, crosses its deep face, and
runs upward and forward on the guttural pouch to the foramen lacerum. It
is closely related to the vagus nerve and the superior cervical ganglion
of the sympathetic nerve, fibers from which accompany it. It is crossed
externally by the ninth and twelfth cranial nerves and the pharyngeal
branch of the vagus. It passes through the inferior petrosal sinus and
enters the cavernous sinus, within which it forms an =S=-shaped curve.
It is connected with the opposite artery by a transverse branch (A.
intercarotica), which lies behind the pituitary body in the
intercavernous sinus. A branch (A. caroticobasilaris) sometimes connects
it with the basilar artery. It then perforates the dura mater, gives off
the posterior communicating branch, and passes forward and divides at
the side of the optic chiasma into anterior and middle cerebral
arteries.
The =posterior communicating artery= (A. communicans posterior) turns
backward and joins the posterior cerebral. It gives off the =deep
cerebral artery= (A. cerebri profunda) which winds around the cerebral
peduncle and is distributed chiefly to the mid-brain. A smaller
collateral branch is the anterior choroid artery (A. chorioidea nasalis)
which passes along the optic tract and is distributed in the choroid
plexus of the lateral ventricle.
The =anterior cerebral artery= (A. cerebri anterior) unites with the
corresponding branch of the opposite artery above the optic chiasma.
From this junction proceeds the =artery of the corpus callosum= (A.
corporis callosi), which turns around the genu of the corpus callosum,
enters the great longitudinal fissure, divides into two branches, and is
distributed to the inner aspect of the cerebral hemispheres. A small
=anterior meningeal= branch (A. meningea nasalis) of the anterior
cerebral is distributed to the anterior part of the dura, and assists in
forming a network in the ethmoidal fossa (Rete ethmoidale), anastomosing
with the ethmoidal branch of the ophthalmic artery.
The =middle cerebral artery= (A. cerebri media) passes outward in the
lateral fissure (of Sylvius) and divides into branches on the outer
surface of the hemisphere.
The =arterial circle of Willis= (Circulus arteriosus) (Fig. 434) is
formed at the interpeduncular space of the base of the brain by the
union of the anterior cerebral arteries in front, by the diverging
posterior cerebral arteries behind, and is completed laterally by the
junction of the latter with the posterior communicating arteries and by
the internal carotid. It is irregularly polygonal in outline.
The cerebral arteries are very variable in arrangement, and the
foregoing account is a brief statement of the more usual disposition
of the larger vessels. The internal carotid artery often arises with
the occipital by a common trunk of variable length.