The brain contains more than 100 billion neurons and weighs about 1.4 kg (3 lb). The brain and spinal cord contain two main types of tissue: grey matter, which originates and processes nerve impulses; and white matter, which transmits them. The largest structure in the brain is the cerebrum, which is divided into two halves or hemispheres. Other structures include the cerebellum, the brain stem, and a central region that includes the thalamus and hypothalamus. The spinal cord is an extension of the brain stem and continues downwards from the base of the skull.
Twelve pairs of cranial nerves emerge directly from the underside of the brain. Most of these nerves supply the head, face, neck, and shoulders. Certain organs in the chest and abdomen, including the heart, lungs, and much of the digestive system, are supplied by the vagus nerve.
Thirty-one pairs of spinal nerves emerge from the spinal cord and extend through the protective, bony spinal column. These nerves divide to supply all parts of the trunk and the limbs. Before reaching the limbs, bundles of nerves converge to form braid-like plexuses, called the brachial and lumbar plexuses, which then branch further along.
The spinal cord has a core of grey matter containing nerve cell bodies, dendrites, and supporting cells. Surrounding the grey matter is white matter containing columns of nerve fibres that carry signals to and from the brain along the length of the spinal cord.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.