Brain

The brain is the body's command and control centre and its main site of information processing, sitting in the cranial cavity and wrapped in three protective layers — the tough outer duramater, a thin middle arachnoid mater, and an innermost piamater that clings closely to the brain's surface, with the subdural and subarachnoid spaces separating these layers from each other. The brain itself is organised into three major regions: the forebrain, the midbrain, and the hindbrain.

The forebrain's largest part is the cerebrum, often called the 'seat of intelligence'. Its outer layer, the cerebral cortex, looks grey because it is packed with unmyelinated neuron cell bodies, dendrites and glial cells — hence the everyday term 'grey matter'. Its surface is thrown into folds (gyri) separated by shallow grooves (sulci) or deeper fissures, a wrinkling that dramatically increases the cortex's surface area within a fixed skull volume; these grooves divide the cerebrum into a matching pair of frontal, parietal, temporal and occipital lobes, each with a distinct job — frontal areas handle behaviour, intelligence, memory and movement; parietal areas handle language, reading and sensation; temporal areas handle speech, hearing and memory; and occipital areas handle visual processing. A deep median longitudinal fissure splits the cerebrum into left and right hemispheres, which stay connected by a thick band of nerve fibres called the corpus callosum; a separate transverse fissure separates the cerebrum from the cerebellum below it. Functionally, the cortex has three kinds of areas: sensory areas (in the parietal, temporal and occipital lobes) that receive and interpret sensory input; a motor area (in the posterior frontal lobe) that controls voluntary muscle movement; and association areas, which handle integrative work like memory, communication, learning and reasoning. Beneath the cortex lies the white medulla (a nerve-fibre tract, not to be confused with the medulla oblongata of the hindbrain), and beneath that sit the basal nuclei.

The forebrain's other component, the diencephalon, is built of three paired structures. The epithalamus forms its non-nervous roof; a folded vascular region there forms the anterior choroid plexus, and just behind it a short stalk ends in the pineal body, which secretes melatonin to regulate the sleep-wake cycle. The thalamus is grey matter that acts as the brain's major relay and coordinating centre for sensory and motor signalling between the spinal cord, brain stem and cerebrum, and also plays a role in learning and memory. The hypothalamus forms the diencephalon's floor; it maintains homeostasis by controlling body temperature and the urges to eat and drink, houses neurosecretory cells that release hypothalamic hormones, acts as a satiety centre, and connects to the pituitary gland via a downward extension called the infundibulum. It also carries a pair of small rounded mammillary bodies involved in olfactory reflexes and emotional responses to smell.

The limbic system — sometimes called the 'emotional brain' — is the inner part of the cerebral hemisphere, made up of the olfactory bulbs, cingulate gyrus, mammillary body, amygdala, hippocampus and hypothalamus. It plays the primary role in regulating pleasure, pain, anger, fear, sexual feeling and affection, and its hippocampus and amygdala also contribute to memory. The brain also contains four fluid-filled cavities called ventricles: a lateral ventricle inside each cerebral hemisphere (separated from each other by the septum pellucidum), connecting through the interventricular foramen (foramen of Monro) to a narrow third ventricle in the diencephalon, which in turn connects via the cerebral aqueduct (aqueduct of Sylvius) to a fourth ventricle in the hindbrain. A network of blood capillaries called the choroid plexus lines the roof of these ventricles and filters blood to produce cerebrospinal fluid (CSF), which cushions the CNS against shock, supplies nutrients and oxygen, clears metabolic wastes, and helps maintain steady pressure inside the cranial vessels. …