Human Central Nervous System and Peripheral Nervous System Connections
The central nervous system (CNS) of the human consists of the brain, spinal cord, and cranial nerve II (the optic nerve which connects to the eyeball).
When compared to the peripheral nervous system (PNS), the CNS differs in several key ways. It’s largely autonomic (requires no input for it to work) instead of voluntary, is much more protected (by bone and the blood-brain barrier), and interprets input, rather than integrating it.
As the PNS is much less protected, it’s vulnerable to damage by toxins, disease, mechanical injury, and autoimmune disorders. The degenerative conditions of the CNS are almost always hereditary.
Dictionnaire Universel d'Histoire Naturelle. Charles d'Orbigny, 1849.