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White matter includes all of the nerves, and much of the interior of the brain and spinal cord.

Grey matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces.

The nervous system derives its name from nerves, which are cylindrical bundles of fibers (the axons of neurons), that emanate from the brain and spinal cord, and branch repeatedly to innervate every part of the body.

Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope.

In vertebrate species it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.

Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent.

"It is difficult to believe that until approximately year 1900 it was not known that neurons are the basic units of the brain (Santiago Ram? Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around 1930 (Henry Hallett Dale) and (Otto Loewi).

There are, however, a few exceptions to this rule, notably including the part of the forebrain called the basal ganglia Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly.It controls the mouthparts, the salivary glands and certain muscles.Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation.The sensory information from these organs is processed by the brain.In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling.

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