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242 ANATOiMY.

The Anterior Cerebral Artery is about 3^ mm. (i inch) in calibre.
It is one of the terminal divisions of the internal carotid, and arises at
the inner extremity of the fissure of Sylvius, close to the anterior clinoid
process. It passes inward and forward nearly at right angles with the
internal carotid, and at an obtuse angle with the middle cerebral, to a
point in close proximity to its junction with its fellow of the opposite
side, w^iich occurs at the rostrum of the corpus callosum, anterior to the
lamina cinerea, where it gives off the anterior communicating artery
which forms a part of the circle of Willis. From this point it passes
forward a short distance from and nearly parallel with its fellow of the
opposite side until it reaches the anterior portion of the corpus callosum,
around which it curves, and breaks up into several branches to supply
the structures in the anterior portion of the brain-case.
The Iliddle Cerebral Artery is about 5 mm. {^ inch) in calibre, and
is one of the largest of the terminal divisions of the internal carotid.
It arises at the inner extremity of the fissure of Sylvius, and passes
obliquely upward and outward within the fissure to the superior surface
of the island of Reil. Here it subdivides into several branches, which
are distributed to the brain. It also, on the anterior portion, gives
oif the lateral (posterior) communicating artery of the circle of Willis.
The Posterior Cerebral Artery is about 3| mm. (| inch) in calibre,
and is one of the t^vo terminal branches of the basilar, hereafter to be
described. It arises with the corresponding artery of the opposite side
at a point just anterior to the pons varolii, close to the posterior clinoid
process of the sphenoid bone. It extends outward, and then curves back-
ward around the crus cerebri, and passes outward and upward between
the occipital lobe of the cerebrum and the cerebellum. It gives off
numerous branches which supply the different structures of the brain,
as well as the lateral (posterior) communicating artery of the circle of
Willis.
The Circle of Willis is a system composed of several short arteries
which communicate with each other and form a vascular circle which
surrounds the following structiu^es at the base of the brain : the lamina
cinerea, optic commissure, infundibulum and tuber cinereum, corpora
albicantia, and posterior perforated space.
The circle of Willis is composed of the following arteries : the two
anterior cerebral, the anterior communicating, the upper portion of the
two internal carotids, the two lateral (posterior) communicating, and
the two posterior cerebrals.
The Two Anterior Cerebral Arteries form that portion of the circle of
Willis which extends forward and inward from their origin, which is
at the termination of the internal carotid, to the rostrum of the corpus
callosum just anterior to the lamina cinerea.
The Anterior Conwinnicating Artery is about two lines in length, and
passes from one anterior cerebral artery to the other across the rostrum
of the corpus callosum anterior to the lamina cinerea. It forms the
anterior connnunicating branch of the circle of Willis between the two
anterior cerebral arteries. It also gives oif branches which supply some
of the structures in close proximity to it.
Variations.—This artery is occasionally represented by two branches,