The Central Nervous System (CNS) is of vital importance, meaning that any small trauma that it may suffer can represent a death risk for the individual. This lead natural selection to cause the organisms to develop a strong protection against those traumas. The Peripheral Nervous System (PNS) is of monumental importance, but the traumas suffered has local impact only. The PNS is also spreaded through the entire body, getting to the tip of our toes and fingers. Therefore, its evolutionary development didn't involve the need of having such a strong protection, and that would have been really difficult as the PNS is so widely spreaded throughout the body.