(anatomy, medicine, usually) Synonym of central nervous system.
(anatomy, embryology, medicine, sometimes) A subset of the central nervous system: i.e., the spinal cord, rhombencephalon, mesencephalon and diencephalon.
The picture revealed by spinal tap was one of an acute polyneuritis, but a polyneuritis of a most exceptional type: not like Guillian–Barré syndrome, with its overwhelming motor involvement, but a purely (or almost purely) sensory neuritis, affecting the sensory roots of spinal and cranial nerves throughout the neuraxis.