The Brain and Its Parts

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.

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.