2.  Arterial occlusion by an embolus or a
thrombus is usually followed by infarction of
a portion of the region supplied.
 Anastomotic channels are present between
branches of the major arteries on the surface
of the brain.
 There are also communications at the
arteriolar level, and the capillary bed is
continuous throughout the brain.

3.  These anastomoses, however, are usually
insufficient to sustain the circulation in the
region normally supplied by a major artery.
 The size of an infarction depends on the
caliber of the occluded artery, existing
anastomoses, and the time elapsing before
complete obstruction.

4.  In addition to intracranial occlusions,
impairment of the cerebral circulation is
often caused by stenosis of a carotid or
vertebral artery in the neck.
 The slender, thin-walled arteries that
penetrate the ventral surface of the brain to
supply the internal capsule and adjacent
gray masses are especially prone to rupture

6.  Hypertension and degenerative changes in
these arteries are major factors that lead to
cerebral hemorrhage.
 An aneurysm usually occurs at the site of
branching of one of the larger arteries at the
base of the brain.

10.  An aneurysm may leak or rupture, and there
is bleeding into the subarachnoid space.
 In some cases, adhesion of the aneurysmal
sac to adjacent structures can give rise to
hemorrhage that is intracerebral or into a
cranial nerve.

11.  Aneurysms often develop at sites of
branching of arteries in and near the arterial
circle, and they can rupture or leak, causing
subarachnoid hemorrhage.
 The most common sites for such aneurysms
are the terminal part of the internal carotid
artery, the anterior communicating artery,
the proximal part of the middle cerebral
artery, and the posterior communicating
artery.

12. A subarachnoid hemorrhage causes a severe
headache of sudden onset, with a stiff neck
and other signs of meningeal irritation