The human brain is well-protected from injury. It is firmly surrounded by three layers of membranes, encased in a rigid skull (the cranium) and covered by a muscular scalp. Each of these barriers to the brain is important, because brain tissue is fragile and unforgiving if injured.
Three membrane layers, the meninges, protect the brain from injury and infection. The dura mater, tough and fibrous, lines the skull. The thinner pia mater, highly vascular (containing many blood vessels), covers the brain's surface. Between these two is another layer called the arachnoid.
The brain floats in a protective cushion of cerebrospinal fluid (CSF), which flows within the subarachnoid space beneath the arachnoid membrane, on top of the pia mater. It also surrounds the spinal cord and fills open spaces (ventricles) inside the brain. Although the amount of CSF that circulates around the brain normally stays the same, it is replenished by the body three times each day and helps to maintain a constant pressure inside the skull, known as intracranial pressure (ICP).
The largest part of the brain is divided into two major areas, the left and right cerebral hemispheres, which control most of the body's motor and sensory processes. Some sections of each hemisphere can be "mapped" to correspond with the body functions for which they seem to be responsible: vision, speech, hearing, personality, memory, movement, touch, smell and taste.
The brain stem controls such vital automatic functions as breathing, heartbeat and eye movement. It cements the brain to the spinal cord and acts as the main circuit for all brain activity.
Twelve pairs of cranial nerves, emerging from the base of the brain and the brain stem, transmit nerve impulses for vision, hearing, smell and many other important body functions.