The Brain

The brain contains more than 90 percent of the body's neurons. Physically, the brain has three more or less distinct areas: the hindbrain, the midbrain, and the forebrain.

The hindbrain is found in even the most primitive vertebrates. It is made up of the cerebellum, the pons, and the medulla. The medulla is a narrow structure nearest the spinal cord; it is the point at which many of the nerves from the left part of the body cross to the right side of the brain and vice versa. The medulla controls such functions as breathing, heart rate, and blood pressure. The pons, located just above the medulla, connects the top of the brain to the cerebellum. Chemicals produced in the pons help maintain our sleep-wake cycle. The cerebellum is divided into two hemispheres and handles certain reflexes, especially those that have to do with balance. It also coordinates the body's actions.

The midbrain lies between the hindbrain and forebrain and is crucial for hearing and sight.

The forebrain is supported by the brain stem and buds out above it, drooping somewhat to fit inside the skull. It consists of the thalamus, the hypothalamus, and the cerebral cortex. The thalamus relays and translates incoming messages from the sense receptors—except those for smell. The hypothalamus governs motivation and emotion and appears to play a role in coordinating the responses of the nervous system in times of stress.

Hindbrain Medulla Sensory and motor nerves crossover
  Pons Regulation of sleep-wake cycle
  Cerebellum Reflexes (e.g., balance)
Coordinates movement
Midbrain   Hearing, vision relay point
Pain registered
Forebrain Thalamus Major message relay center
Regulates higher brain centers and peripheral nervous system
  Hypothalamus Motivation
Stress reactions
  Cerebral Hemispheres  
     Occipital lobe Receives and processes visual information
     Temporal lobe Complex vision
Balance and equilibrium
Emotions and motivations
Some language comprehension
     Parietal lobe Sensory projection and association areas
Visual/spatial abilities
     Frontal lobe Goal-directed behavior, concentration
Emotional control and temperament
Motor projection and association areas
Coordinate messages from other lobes

The cerebral hemispheres, located above the thalamus and hypothalamus, take up most of the room inside the skull. The outer covering of the cerebral hemispheres is known as the cerebral cortex. The cerebral hemispheres are what most people think of when they think of the brain. They are the most recently evolved portion of the brain, and they regulate the most complex behavior. Each cerebral hemisphere is divided into four lobes, delineated by deep fissures on the surface of the brain. The occipital lobe of the cortex, located at the back of the head, receives and processes visual information. The temporal lobe, located roughly behind the temples, is important to the sense of smell; it also helps us perform complex visual tasks, such as recognizing faces. The parietal lobe, which sits on top of the temporal and occipital lobes, receives sensory information, in the sensory projection areas, from all over the body and figures in spatial abilities. The ability to comprehend language is concentrated in two areas in the parietal and temporal lobes. The frontal lobe is the part of the cerebral cortex responsible for voluntary movement and attention as well as goal-directed behavior. The brain starts response messages in the motor projection areas, from which they proceed to the muscles and glands. The frontal lobe may also be linked to emotional temperament.

These four lobes are both physically and functionally distinct. Each lobe contains areas for specific motor sensory function as well as association areas. The association areas—areas that are free to process all kinds of information—make up most of the cerebral cortex and enable the brain to produce behaviors requiring the coordination of many brain areas.

The four lobes of the cerebral cortex.
Deep fissures in the cortex separate these areas or lobes. Also shown are the primary sensory and motor areas.

The Four Lobes
The Four Lobes
The Four Lobes