The principal divisions of the nervous system are the brain, spinal marrow, and nerves. The tissue of this system is included in membranes or sheaths, and consists of two differently colored pulpy materials, one of which is white or medullary, and the other gray, cortical, or cineritious. The sheath of the nerves is called the neurilemma, and the internal material neurine. All ganglia and nervous centres consist of a mixture of white fibres and gray globules.

An anastomosis is the interchange of fascilculi between two trunks, each fasciculus remaining unaltered, although in contact with another. A combination of anastomoses into a network is called a plexus.