LifeArt image © 2006 Lippincott Williams & Wilkins. All rights reserved The Brain and Its Parts |
What weighs
about three pounds, is the size of a large grapefruit, and is the most complex
organ in the human body? The answer is in your head: the brain. The human brain
is an incredibly important piece of anatomical machinery. Your body would be
completely useless without it. You couldn’t read this book. Writing would be
impossible. You’d have no memory, no thoughts, no emotions, and no way to
breathe. You couldn’t see, feel, sleep, eat, walk, talk, or log on to the
Internet. You need your brain for absolutely everything you do.
Since it is
such a vital organ, the brain is guarded from harm by at least five protective
layers. The first and outermost layer, and the one mentioned previously, is the
skull. The skull surrounds the brain like a permanent and perfectly fitted
bicycle helmet. It’s hard, sturdy, and a great first defense against everyday
bangs and bumps. The skull is the brain’s brick wall—its main coat of armor, so
to speak.
The brain’s
next three layers of protection are known collectively as meninges. The
meninges are separate sheets of body tissue that stack up one on top of the
other. The outer strip, a tough membrane attached to the inside of the skull,
is known as the dura mater. Beneath that is the middle meningeal layer, called
the arachnoid. Below the arachnoid—and separated from it by a narrow gap known
as the subarachnoid space—is the third meningeal layer, the pia mater, which
clings to the brain and all its numerous pits (sulci) and folds (gyri) like
plastic wrap on a chunk of raw hamburger.
The skull
surrounds the brain like a permanent and perfectly fitted bicycle helmet.
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Last but
certainly not least, among the brain’s physical protectors, is a clear,
waterlike substance known as cerebrospinal fluid. It is produced by the brain’s
vascular system and circulates within the subarachnoid space. It acts like a
liquid cushion between the brain and the skull.
The Three Parts of the Brain
Beneath the
meningeal layers is the real meat of the brain. There are three main parts: the
cerebrum, the cerebellum, and the brain stem.
The Cerebrum
The cerebrum is the brain’s largest
component, accounting for most of its weight and nearly three-fourths of its
volume. It forms the top of the brain and is the control center for thoughts,
feelings, sensations, and voluntary actions. The hills and valleys of the
cerebrum are covered by a layer of tissue called the cerebral cortex, and the
cerebrum is physically divided into two halves by a deep, canyonlike groove
called the longitudinal fissure. The left side of the split is known as the
left cerebral hemisphere. The right half is called the right cerebral
hemisphere.
Each hemisphere
consists of four rounded cerebral lobes, or regions. The lobes are named after
the particular skull bones that protect them and, like the cerebral
hemispheres, are separated by fissures. The frontal lobes are located in the
front, or ventral, portion of each hemisphere. Parietal lobes are medial—that
is, they’re near the middle. Occipital lobes are dorsal, or in the back.
Temporal lobes are lateral and inferior, or along the bottom sides. The central
sulcus (a deep fissure) divides the frontal and parietal lobes, while the
lateral sulcus separates the temporal lobe from the parietal and frontal lobes.
In order for
the two hemispheres to function efficiently with one another, they must be
connected, and that job goes to the corpus callosum. The corpus callosum is an
arching network of fibers that bridges the hemispheres from its location just
above the brain stem. By linking the hemispheres together, it allows them to
communicate and cooperate with each other. So when information is received by
or sent from one hemisphere, the other hemisphere knows all about it.
The Cerebellum
A second major
part of the brain, the cerebellum, lies inferior and dorsal to the cerebrum’s
occipital lobe. “Cerebellum” comes from the Latin word for “little brain,” and
that’s exactly what it is—a miniature version of the cerebrum, which most
people think of as “the brain.” The cerebellum is responsible for unconscious
movements—such as breathing, blinking, and coordination. By interpreting
information gathered from the eyes and the ears, it allows us to keep our
balance and control our movements. Like the cerebrum, the cerebellum is divided
into left and right hemispheres and has an irregularly shaped surface.
The Brain Stem
The last of the three main brain divisions is the brain
stem, which connects the cerebrum to the spinal cord. About three inches long,
the width of a carrot, and shaped like a funnel, it sticks out from the
inferior end of the cerebrum much like the stalk of a plant might protrude from
a flower. The brain stem has four major parts: the medulla oblongata, the pons,
the midbrain, and the diencephalon.
The medulla
oblongata is at the most inferior end of the brain stem and is continuous with
the spinal cord. It houses nerve centers that control the body’s breathing,
heart rate, blood pressure, swallowing, and other important functions. Above
the medulla oblongata is the pons. The bulbous, rounded pons has millions of
microscopic, threadlike nerve fibers. The smallest part of the brain stem is
the midbrain. The midbrain rests just above the pons and helps control eye
movement and hearing. Finally, at the top of the brain stem, sandwiched between
the midbrain and the cerebrum, is the diencephalon. The various parts of the
diencephalon, like the thalamus, hypothalamus, and epithalamus, regulate
internal body conditions like temperature and hunger. They also receive sensory
nerve impulses, or sensations, from the rest of the body and relay them to the
cerebrum.
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