The brain’s complex anatomy, with
its nested structures whose boundaries blur into one another,
can be somewhat bewildering at first. This complexity is the
result of the long process by which the brain develops as humans
grow from embryos into adults. And this process in turn has
resulted from the evolution of all animal species.
THE BRAIN
Unlike the brain’s
four external lobes, the limbic lobes, also called the cingulate
gyri, are visible only when a sagittal section of the brain is made.
Cross-section of
meninges
VENTRICLES AND MENINGES
The brain is the best protected
organ in the body. The first layer of protection
is the skull, which acts as armour shielding the
brain from blows. Next come the meninges, three membranes
that surround the brain to keep it from being damaged
by contact with the inside of the skull. It is these
membranes that become infected when someone gets
meningitis, and it is because the meninges are in
direct contact with the brain that meningitis is
so dangerous.
For even more protection, the
brain (and the spinal cord) are bathed in cerebro-spinal
fluid. This fluid circulates through a series of communicating
cavities called ventricles. Cerebro-spinal fluid also circulates
between the pia mater and the arachnoid mater of the meninges.
In addition to cushioning blows, this fluid reduces the pressure
at the base of the brain by causing the nerve tissue to “float”.
Cerebro-spinal fluid is secreted by the choroid plexus in
the upper ventricles and absorbed by the venous system at
the base of the brain. As this fluid flows downward, it carries
away toxic wastes and moves hormones between widely separated
regions of the brain.
The parts of the central nervous system
that contain grey matter (composed of neuron cell bodies)
are often called nuclei or ganglia. Certain
groups of axons found in the brain’s white matter are
called pathways or bundles.
These nerves participate in the
organism’s relationship with its external environment.
They send information to the brain from the body’s
various sensory detectors. These nerves also enable us
to respond to these stimuli by moving through our environment.
THE AUTONOMIC NERVOUS SYSTEM
These nerves are more involved
in regulating vital internal functions. They help to
maintain internal equilibrium by coordinating such activities
as digestion, respiration, blood circulation, excretion,
and the secretion of hormones. The autonomic nervous
system in turn is divided into two
categories.
In the central nervous system, the “grey
matter” is composed of the neurons’ cell
bodies and their dense network of dendrites. The grey
matter includes the centre of the spinal cord and the thin
outer layer of the cerebral hemispheres, commonly known as
the cortex. The white matter consists of the myelin sheathing
that covers the axons of these same neurons to enable them
to conduct nerve impulses more rapidly. These myelinated
axons are grouped into bundles (the equivalent of nerves)
that make connections with other groups of neurons.
