brain The structural and functional hub of the
NERVOUS SYSTEM. The brain regulates the body’s
functions, voluntary and involuntary. The adult
brain is a mass of soft, spongy tissue that weighs
about three pounds. It receives 20 percent of the
body’s BLOODflow and consumes 20 percent of the
body’s oxygen supply. The brain and the SPINAL
CORD collectively make up the CENTRAL NERVOUS
SYSTEM.
The brain resides within the enclosure of the
cranium (skull). The cranium’s fused bones limit
the portals of direct access to the brain. The largest
such portal is the foramen magnum, the opening
in the occipital bones through which the spinal
cord passes. Other smaller passages provide path-
ways for the CRANIAL NERVES, which terminate in
the structures of the brainstem and the underside
of the brain. Three layers of membranes, the
MENINGES, wrap around the brain for further physi-
cal protection. CEREBROSPINAL FLUID circulates
between the meninges, cushioning the brain as
well as maintaining its biochemical balance. The
BLOOD–BRAIN BARRIER, a specialized layer of cells
lining the blood vessels that serve the brain,
adds a final level of security by limiting the sub-
stances that can pass between the blood and the
brain.
The Brain’s Structure
Neurons (NERVEcells) and glial cells (support cells)
form the tissue of the brain. Neurons transmit
electrical impulses (nerve signals). Glial cells (also
called neuroglia) make myelin, a fatty protein
coating that nourishes and insulates neurons.
Though the brain contains its full complement of
neurons by about age three, glial cells grow and
divide throughout life. Though neurons do not
grow or divide, they do continue to form new
connections (synapses or synaptic circuits)
throughout life by extending and branching their
axons, the fibers that carry nerve impulses from
the NEURONto a synapse (a microscopic channel
that separates one neuron from another).
Areas of the brain that contain high concentra-
tions of neuron bodies, such as the cerebral cortex
and the basal ganglia, are the gray matter, so-
named because these areas are dark in color.
Areas of the brain such as the inner cerebrum and
the brainstem that are primarily concentrations of
axons, the pathways by which neurons communi-
cate, are the white matter. Myelin, which encases
and insulates the axons to contain the electrical
impulses they transmit, gives the axons their
white appearance. The fully developed adult brain
contains about 100 billion neurons and up to 50
times as many glial cells.
The brain’s three main structural components
are the forebrain (prosencephalon), midbrain
(mesencephalon), and hindbrain (rhomben-
cephalon). Each of these divisions has further
structural as well as functional subdivisions, mak-
ing the brain the most complex organ of the body.
For the most part the brain is a paired organ, with
its two halves, the right hemisphere and the left
hemisphere, roughly symmetrical in physical
structure though not in function. A connecting
bridge of neuronal tissue, the corpus callosum,
allows the hemispheres to communicate with each
other.
Within the structure of the brain are four con-
nected spaces, called ventricles, that produce and
contain cerebrospinal fluid. The lateral ventricles,
also called the first and second ventricles, are the
largest and contact the frontal, temporal, and occip-
ital lobes. The third ventricle is a small space that
joins with the lateral ventricles along the midline of
the brain and the fourth ventricle, also small, is at
the back of the brainstem and joins with the sub-
arachnoid mater (the middle of the meninges).The Forebrain
The forebrain is the largest of the brain’s structural
divisions, making up about 85 percent of the
brain’s mass and weight. Its composition is prima-
rily gray matter—neuron bodies. The forebrain’s
subdivisions are the telencephalon and the dien-
cephalon. The first cranial nerve, the olfactory
nerve, arises from the telencephalon. The second
cranial nerve, the OPTIC NERVE, originates in the
diencephalon. The forebrain also contains the lat-
eral ventricles and the third ventricle.
The telencephalon, which makes up the bulk of
the forebrain, contains the:- cerebral cortex, also called the cerebrum, which
handles all of the body’s functions related to
conscious activity, from thought and behavior
to movement and balance
230 The Nervous System