The
central nervous system (
CNS) is the part of the
nervous system consisting of the
brain and
spinal cord. The central nervous system is so named because it integrates information it receives from, and coordinates and influences the activity of, all parts of the bodies of
bilaterally symmetric animals — that is, all multicellular animals except
sponges and
radially symmetric animals such as
jellyfish — and it contains the majority of the nervous system. Arguably, many consider the
retina and the
optic nerve (2nd
cranial nerve), as well as the
olfactory nerves (1st) and
olfactory epithelium as parts of the CNS, synapsing directly on brain tissue without intermediate
ganglia. Following this classification the
olfactory epithelium is the only central nervous tissue in direct contact with the environment, which opens up for
therapeutic treatments. The CNS is contained within the
dorsal body cavity, with the
brain housed in the
cranial cavity and the spinal cord in the
spinal canal. In
vertebrates, the brain is protected by the skull, while the spinal cord is protected by the vertebrae, both enclosed in the
meninges.
