(3) Pia mater. The pia mater is a delicate membrane applied directly to the
surface of the brain and the spinal cord. It carries a network of blood vessels to supply
the nervous tissues of the CNS.
5-14. BLOOD SUPPLY TO THE CNS
a. Blood Supply of the Brain. The paired internal carotid arteries and the
paired vertebral arteries supply blood rich in oxygen to the brain. Branches of these
arteries join to form a circle under the base of the brain. This is called the cerebral
circle (of Willis). From this circle, numerous branches supply specific areas of the brain.
(1) A single branch is often the only supply to that particular part of the
brain. Such an artery is called an end artery. If it fails to supply blood to that specific
area, the area will die (as in a stroke).
(2) The veins and venous sinuses of the brain drain into the paired internal
jugular veins. These veins carry blood back toward the heart.
b. Blood Supply of the Spinal Cord. The blood supply of the spinal cord is by
way of combination of three longitudinal arteries running along its length and reinforced
by segmental arteries from the sides.
5-15. CEREBROSPINAL FLUID
A clear fluid called cerebrospinal fluid (CSF) is found in the cavities of the central
nervous system. Cerebrospinal fluid is found in the ventricles of the brain, the
subarachnoid space, and the central canal of the spinal cord. Cerebrospinal fluid and
its associated structures make up the circulatory system for the CNS.
a. Choroid Plexuses. Choroid plexuses are special collections of arterial
capillaries found in the roofs of the third and fourth ventricles of the brain. The choroid
plexuses continuously produce CSF from the plasma of the blood.
b. Path of the Cerebrospinal Fluid Flow. Blood flows through the arterial
capillaries of the choroid plexuses. As the choroid plexuses produce CSF, it flows into
all four ventricles. Cerebrospinal fluid from the lateral ventricles flows into the third
ventricle, through the cerebral aqueduct then into the fourth ventricle. By passing
through three small holes in the roof of the fourth ventricle, CSF enters the
subarachnoid space. From the subarachnoid space, the CSF is transported through the
arachnoid villi (granulations) into the venous sinuses. Thus, the CSF is formed from
arterial blood and returned to the venous blood.