Nerve (cross section) (Latin)

A nerve is formed by a bundle of axons (or nerve fibers) outside the systema nervosum centrale. These nerve fibers can be myelinated (surrounded by a myelin sheath) or nonmyelinated. Nerves are also composed of connective tissue coverings, as well as nourishing blood vessels (vasa nervorum). The three layers of connective tissue are the epineurium, surrounding the outer surface of a nerve, the perineurium, surrounding each nerve fascicle (which is a group of axons), and the endoneurium, which surrounds each individual axon. Nervi spinales arise from the spinal cord (medulla spinalis) as anterior and posterior roots whereas nervi craniales arise from the brain (encephalon).