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The brain is surrounded and protected by the
rigid, bony skull and three membranes, called meninges.
The tough, fibrous outer membrane is the
dura mater. The archanoid, the intermediate membrane,
is web-like. The pia mater is the innermost
covering and is the most delicate. It is molded to the
shape of the brain. The cerebrospinal fluid (CSF)
surrounds the brain and spinal cord. It flows
through open chambers in the brain, called ventricles,
and out an opening to the spinal cord. The
brain actually floats in the shock-absorbing CSF,
and is thus protected from trauma. The CSF also
brings nutrients to the brain and removes wastes.
The brain has 3 main parts: forebrain, midbrain
and hindbrain, as shown in Fig. 1, (previous
page).
Forebrain (Prosencephalon)
Figure 2
nal nerve is attached to the spinal cord by a sensory
and a motor root. These exit between the vertebrae
and merge to form a large mixed nerve, whose
branches supply a defined area of the body. The
autonomic nervous system, sometimes considered
as another division, is part central and part peripheral.
The brain is a complex organ which receives
information from other parts of the body via the spinal
cord and the peripheral nervous system. It uses
this information to control basic life processes, like
breathing, body temperature and blood pressure, as
well as higher functions like creative thought and
emotions.
The brain is comprised of two types of cells:
nerve cells (neurons) and glial cells. Glial cells
have multiple functions, which include structurally
supporting neurons, repairing the CNS, and regulating
the biochemical balance of the brain. The bloodbrain-barrier
is composed of astrocytes, a special
type of glial cell. This barrier prevents many substances
in the blood from entering the brain.
The forebrain is the largest part of the brain,
most of which is the cerebrum, consisting of the
cerebral cortex, corpus striatum and olfactory
bulb. The other major division of the forebrain is
the diencephalon, which includes the hypothalamus,
thalamus, epithalamus (including the pineal
gland), and subthalamus.
(a) Cerebral Cortex
Cerebrum
The outermost and top layer of the brain is the
cerebral cortex. The cerebral cortex is composed
of two similar halves called hemispheres, having
different functions. The left side of the cerebral cortex
controls the right side of the body and speech.
The right side controls the left side of the body and
the perception of spatial relationships, such as
where one’s hand might be located in relation to the
ground, without looking. There are individual
variations, for example, the language center is on
the right in 2 percent of right-handed people and
also in about 50% of left-handed people
The hemispheres are separated by a deep
groove, but are linked by the corpus callosum, anterior
commissure, posterior commissure, and
hippocampal commissure. All of these transfer the
information between two hemispheres to coordinate
functions.
The outer layer of the hemispheres is composed
of gray matter, consisting of neuronal cell bodies.
September—October 2012 Yang-Sheng (Nurturing Life) 21