The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric and triploblastic animals—that is, all multicellular animals except sponges and diploblasts.

Only arthropods, cephalopods and vertebrates have a true brain, though precursor structures exist in onychophorans, gastropods and lancelets.

The rest of this article exclusively discusses the vertebrate central nervous system, which is radically distinct from all other animals.

[2] The meninges provide a barrier to chemicals dissolved in the blood, protecting the brain from most neurotoxins commonly found in food.

[2] Within the CNS, the interneuronal space is filled with a large amount of supporting non-nervous cells called neuroglia or glia from the Greek for "glue".

[8] The spinal cord reaches from the base of the skull, and continues through[8] or starting below[10] the foramen magnum,[8] and terminates roughly level with the first or second lumbar vertebra,[9][10] occupying the upper sections of the vertebral canal.

Different forms of glial cells have different functions, some acting almost as scaffolding for neuroblasts to climb during neurogenesis such as bergmann glia, while others such as microglia are a specialized form of macrophage, involved in the immune system of the brain as well as the clearance of various metabolites from the brain tissue.

[6] Astrocytes may be involved with both clearance of metabolites as well as transport of fuel and various beneficial substances to neurons from the capillaries of the brain.

Apart from the spinal cord, there are also peripheral nerves of the PNS that synapse through intermediaries or ganglia directly on the CNS.

The olfactory epithelium is significant in that it consists of CNS tissue expressed in direct contact to the environment, allowing for administration of certain pharmaceuticals and drugs.

Additionally, parts of the visual and auditory systems are located in the midbrain, including control of automatic eye movements.

[9] Despite its previous classification as a motor structure, the cerebellum also displays connections to areas of the cerebral cortex involved in language and cognition.

Previously it was considered only a "relay station", but it is engaged in the sorting of information that will reach cerebral hemispheres (neocortex).

[9] Apart from its function of sorting information from the periphery, the thalamus also connects the cerebellum and basal ganglia with the cerebrum.

[9] The hypothalamus engages in functions of a number of primitive emotions or feelings such as hunger, thirst and maternal bonding.

Various structures combine to form the cerebral hemispheres, among others: the cortex, basal ganglia, amygdala and hippocampus.

[9] One of the most important parts of the cerebral hemispheres is the cortex, made up of gray matter covering the surface of the brain.

Axons in the CNS are often very short, barely a few millimeters, and do not need the same degree of isolation as peripheral nerves.

The metencephalon becomes, among other things, the pons and the cerebellum, the myelencephalon forms the medulla oblongata, and their cavities develop into the fourth ventricle.

[9] Rhinencephalon, amygdala, hippocampus, neocortex, basal ganglia, lateral ventricles Epithalamus, thalamus, hypothalamus, subthalamus, pituitary gland, pineal gland, third ventricle Tectum, cerebral peduncle, pretectum, mesencephalic duct Pons, cerebellum Planarians, members of the phylum Platyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a CNS and a PNS.

[16][17] Their primitive brains, consisting of two fused anterior ganglia, and longitudinal nerve cords form the CNS.

The major trend that can be observed is towards a progressive telencephalisation: the telencephalon of reptiles is only an appendix to the large olfactory bulb, while in mammals it makes up most of the volume of the CNS.

Indeed, the allometric study of brain size among different species shows a striking continuity from rats to whales, and allows us to complete the knowledge about the evolution of the CNS obtained through cranial endocasts.

Lastly, cancers of the central nervous system can cause severe illness and, when malignant, can have very high mortality rates.

Symptoms depend on the size, growth rate, location and malignancy of tumors and can include alterations in motor control, hearing loss, headaches and changes in cognitive ability and autonomic functioning.

Specialty professional organizations recommend that neurological imaging of the brain be done only to answer a specific clinical question and not as routine screening.