11-13. CEREBROSPINAL FLUID (CSF)
A clear fluid called cerebrospinal fluid (CSF) is found in the cavities of the CNS.
CSF is found in the ventricles of the brain (para 11-9d), the subarachnoid space (para
11-11b(2)), and the central canal of the spinal cord (para 11-10b(1)). CSF 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 CSF Flow. Blood flows through the arterial capillaries of the
choroid plexuses. As CSF is produced by the choroid plexuses, it flows into all four
ventricles. CSF from the lateral ventricles flows into the third ventricle and then through
the cerebral aqueduct 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.
Section IV. THE PERIPHERAL NERVOUS SYSTEM (PNS)
(1) The peripheral nervous system (PNS) is that portion of the nervous
system generally concerned with commands for skeletal muscles and other muscles
made up of striated muscle tissue, as well as sensory information from the periphery of
the body. The sensory information is carried to the CNS where it is processed. The
PNS carries commands from the CNS to musculature.
(2) A nerve is a collection of neuron processes, together and outside the
CNS. (A fiber tract is a collection of neuron processes, together and inside the CNS.)
b. General Characteristics of the Peripheral Nerves. The PNS is made up
of a large number of individual nerves. These nerves are arranged in pairs. Each pair
includes one nerve on the left side of the brainstem or spinal cord and one nerve on the
right side. The nerve pairs are in a series, each pair resembling the preceding, from top
c. Categories of PNS Nerves. PNS nerves include cranial nerves and spinal