brain
(brān 
)
A large, highly organized complex of neuron cell bodies, axons, dendrites, and glia filling the cavity inside the skull; the brain is enveloped by fluid-filled protective membranes (meninges).
Anatomy: The brain is a soft, compact organ responsible for consciousness, planned neural programs, and cognition. The brain is continually active, and , although it is only 2% of the body’s mass, it receives 17% of the heart's output and consumes 20% of the body's oxygen supply. The brain receives its blood through four arteries: two large arteries, the right and left internal carotid arteries, run up from the chest in the front (anterior half) of the neck; and two smaller arteries, the right and left vertebral arteries, run in the back (posterior half) of the neck. The carotid arteries supply blood to about 80% of the brain, including most of the frontal, parietal, and temporal lobes of the cerebral hemispheres and the basal ganglia. The vertebral arteries supply blood to the remaining 20% of the brain, including the brainstem, cerebellum, and most of the posterior lobes of the cerebral hemispheres.
SEE: illus.
The most basic divisions of the brain are (from rostral to caudal) the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). Physically, the forebrain dominates: the rostral end of the forebrain comprises the two cerebral hemispheres, which grow over much of the remaining brain (the brainstem) and fill the skull with their wavy infolded cortices. Between the cerebral hemispheres lies the caudal portion of the forebrain, the diencephalon, which contains the thalami, collections of nuclei that are way-stations and gatekeepers for signals to and from the cerebral cortices, and the hypothalamus, a center of visceral signals and the site of the pituitary gland . Caudal to the diencephalon is the midbrain, marked by two pairs of bulges (the tectum or colliculi) on its dorsal surface. The final segment of the brainstem, the hindbrain, has a rostral division, the pontine region (metencephalon), from the dorsal side of which bulges the cerebellum. The most caudal portion of the hindbrain is the medulla oblongata (myelencephalon or, in older literature, the bulb), which smoothly grades into the spinal cord.
SEE: fasciculus; tract; illus.