Page 233 - My FlipBook

BLOOD-VESSEL SYSTEM OF THE HEAD, ETC. 243
and at times it is absent. When this is the case the two anterior cere-
brals are united into one in a similar manner to the two vertebrals
which form the basilar artery.
The upper portions of the two internal carotid arteries are situated
close to the anterior clinoid processes of the sphenoid bone, and in
calibre are much the largest of any of the arteries which form the circle
of Willis, though they constitute but a small part of its circumference.
Anteriorly they give off the anterior cerebral arteries, while posteriorly
they give origin to the posterior communicating arteries.
Tlie Lateral (Posterior) Commumcating Arteries are situated laterally
instead of posteriorly, as the generally-used name would imply. They
are seldom of equal size, the right being most frequently the larger.
They extend from the upper extremity of the internal carotid backward
and slightly inward, and pass beneath the optic tract and the crus cerebri
to the posterior cerebral arteries. At their most anterior portion in front
of the pons varolii they give off numerous branches to parts in close
proximity.
Variations.—The two lateral (posterior) communicating arteries occa-
sionally arise from the middle cerebral instexid of the internal carotid.
The Tu'o Posterior Cerebral Arteries form the posterior portion of the
circle of Willis. They are larger in calibre than any of the other ar-
teries which form the circle, except the terminal extremities of the inter-
nal carotids. They extend from the bifurcation of the basilar artery in
front of the pons varolii outward and slightly forward to the point of
junction of the lateral (posterior) communicating arteries.
Variations.—Occasionally this portion of the posterior cerebral artery
is quite small. When this is the case the corresponding artery of the
opposite side is proportionately large, thus equalizing the blood-supply.
The Basilar Artery is formed by the union of the right and left ver-
tebrals, which are branches of the subclavian arteries at the base of the
neck. This arrangement of vessels, together with the external carotids
and the circle of Willis, forms such a continuous communication that
if the common carotid be ligated or entirely obliterated on either side,
the blood may yet circulate to all parts of the brain, and also pass out
of the brain-case and supply the external parts of the head and face
through the anastomotic unions of the vessels of these parts.

Subclavian Arteries.
The Subcfarian Arteries are two in number, right and left, extending
from their origin, the right from the innominate, the left from the arch
of the aorta, to their terminations at the first ribs. Each forms an arch,
the concavity of which is directed downward, and the greater portion of
which is situated in the inferior posterior cervical triangle of the neck.
The proximal portion rests in the thoracic cavity. The artery passes
over the first rib and under the central portion of the clavicle into the
axilla. The summit of the arch is situated within the neck posterior to the
scalenus anticus muscle. The origin and relations of the proximal portion
of the arteries on either side are dissimilar, and will therefore be separately
described. The subclavian arteries are divided into three portions.