Beneath the dome of the skull, behind the slope of the forehead, atop the pillar of nerves that descends down our spines, sits a three-pound mass of pink-gray tissue called the brain.

It is our most complicated organ, and its unseen flashings and signals are our single way of learning about the world and responding to it.

Yet it is merely a soft, almost fluid, Jello-like mass -- 85 percent water -- about the size of a smallish grapefruit.

It is gray on the outside, white on the inside, and split and fissured like an over-worried walnut.

Within this mass, somewhere between 10 billion and 100 billion neurons, or nerve cells, make up the brain. Its architecture is an evolutionary one -- most scientists believe the brain was built upward over the last 500 million years, from its most ancient elements to its newest. At the bottom is the brain stem, an upward ballooning of the spinal cord with controls for breathing and heartbeat. Here, too, is the reticular system, a nerve network that maintains awareness.

Attached to the brain stem at the relatively unprotected back of the head is the cerebellum. A ball-like "little brain" in appearance, it is a coordinating center that maintains posture and balance and permits smooth, rather than jerky, body movement.

Just atop the brain stem, about two inches back from the bridge of the nose, is the hypothalamus, a control area for emotions and drives: hunger, joy, anger, fear, pleasure, pain and sex.

It regulates many body organs, and controls the pituitary gland just beneath it, the producer of hormones that stimulate glands throughout the body. Perched just above the hypothalamus is the thalamus, a major relay station to transmit information to the brain from the limbs and sense organs.

Wrapped around the brain stem and hypothalamus, just beneath the brain's cap or cortex, is the limbic system, with structures affecting memory and emotions such as rage and libido. Among them are the amygdala, hippocampus, cingulate gyrus and mammillary bodies.

Together the brain's upper structures comprise the cerebrum, which has two-thirds of the brain's mass and is draped over the brain stem and other structures beneath.

The cerebrum has several lobes, or sections, and the entire cerebrum is divided into halves -- the "left brain" and "right brain" -- connected by a band called the corpus callosum. Beneath it, a smaller band, the fornix, connects different olfactory areas, or organs of smell.

At the top of both cerebral halves, finally, is the brain's fissured and wrinkled cortex, or "bark." It is is the brain's most recent addition, just 250,000 years old, and thus is often called the neocortex. Less than a quarter of an inch thick, this "thinking cap" has so many folds and convolutions -- evolution's way of packing much into a small space -- that it would cover about two square feet if spread out flat.

The cortex is a key site of motor control. If its motor or motion-directing area is damaged, paralysis results. It is also the prime site of thought, planning, data storage and decision-making, as well as worry. It is the site of the elements that make modern man homo sapiens: discerning, knowing man.