the left hemisphere know what the right hemisphere is
thinking about, and the right hemisphere know what
the left hemisphere is thinking and talking about. A
brief example may be helpful. If you put your left hand
behind your back and someone places a pencil in your
hand (you are not looking at it) and asks you what it is,
would you be able to say? Yes, you would. You would
feel the shape and weight of the pencil, find the point
and the eraser. The sensory impulses from your left
hand are interpreted as “pencil” by the general sensory
area in your right parietal lobe. Your right hemisphere
probably cannot speak, but its thoughts can be con-
veyed by way of the corpus callosum to the left hemi-
sphere, which does have speech areas. Your left
hemisphere can say that you are holding a pencil.
Other aspects of the “division of labor” of our cerebral
hemispheres are beyond the scope of this book, but it
is a fascinating subject that you may wish to explore
further.
MENINGES AND
CEREBROSPINAL FLUIDThe connective tissue membranes that cover the brain
and spinal cord are called meninges; the three layers
are illustrated in Fig. 8–9. The thick outermost layer,
made of fibrous connective tissue, is the dura mater
(Latin for “tough mother”), which lines the skull and
vertebral canal. The middle arachnoid membrane
(arachnids are spiders) is made of web-like strands of
connective tissue. The innermost pia mater(Latin for
“gentle mother”) is a very thin membrane on the sur-
face of the spinal cord and brain. Between the arach-
noid and the pia mater is the subarachnoid space,
which contains cerebrospinal fluid (CSF), the tissue
fluid of the central nervous system.
Recall the ventricles (cavities) of the brain: two lat-
eral ventricles, the third ventricle, and the fourth ven-
tricle. Each contains a choroid plexus, a capillary
network that forms cerebrospinal fluid from blood
plasma. This is a continuous process, and the cere-
brospinal fluid then circulates in and around the cen-
tral nervous system (Fig. 8–10).
From the lateral and third ventricles, cerebrospinal
fluid flows through the fourth ventricle, then to the
central canal of the spinal cord, and to the cranial and
spinal subarachnoid spaces. As more cerebrospinal
fluid is formed, you might expect that some must be
reabsorbed, and that is just what happens. From the
cranial subarachnoid space, cerebrospinal fluid is reab-
sorbed through arachnoid villi into the blood in
cranial venous sinuses (large veins within the
double-layered cranial dura mater). The cerebrospinal
fluid becomes blood plasma again, and the rate of
reabsorption normally equals the rate of production.
Since cerebrospinal fluid is tissue fluid, one of its
functions is to bring nutrients to CNS neurons and to
remove waste products to the blood as the fluid is
reabsorbed. The other function of cerebrospinal fluid
is to act as a cushion for the central nervous system.
The brain and spinal cord are enclosed in fluid-filled
membranes that absorb shock. You can, for example,
shake your head vigorously without harming your
brain. Naturally, this protection has limits; very sharp
or heavy blows to the skull will indeed cause damage
to the brain.
Examination of cerebrospinal fluid may be used in
the diagnosis of certain diseases (see Box 8–8: Lumbar
Puncture).184 The Nervous System
BOX8–7 PARKINSON’S DISEASE
Parkinson’s diseaseis a disorder of the basal
ganglia whose cause is unknown, and though
there is a genetic component in some families, it
is probably not the only factor. The disease usu-
ally begins after the age of 60. Neurons in the
basal ganglia that produce the neurotransmitter
dopamine begin to degenerate and die, and the
deficiency of dopamine causes specific kinds
of muscular symptoms. Tremor, or involuntary
shaking, of the hands is probably the most com-
mon symptom. The accessory movements regu-
lated by the basal ganglia gradually diminish,
and the affected person walks slowly without
swinging the arms. A mask-like face is character-
istic of this disease, as the facial muscles become
rigid. Eventually all voluntary movements be-
come slower and much more difficult, and bal-
ance is seriously impaired.
Dopamine itself cannot be used to treat
Parkinson’s disease because it does not cross the
blood–brain barrier. A substance called L-dopa
does cross and can be converted to dopamine by
brain neurons. Unfortunately, L-dopa begins to
lose its therapeutic effectiveness within a few
years.
Other medications in use do not provide a
cure. Some researchers suggest that implants of
stem cells may offer the best hope of meaningful
therapy.