The interface of the head and body to the CNS through the brainstem is the mark of complexity of the NS expression by cranial nerves. The PNS course of cranial nerves between their brainstem source and their exit of the cranium represents the subarachnoid space that must be crossed. Displacements of the brain and spinal cord in the cranium and vertebral column are limited by maximums that are incorporated into the flexible nature of these nerves as they cross the subarachnoid space. The long course of the abducen’s nerve is marked by it’s vulnerability to injury.

• The interface of the head and body to the CNS through the brainstem is the mark of complexity of the NS expression by cranial nerves. The PNS course of cranial nerves between their brainstem source and their exit of the cranium represents the subarachnoid space that must be crossed. Displacements of the brain and spinal cord in the cranium and vertebral column are limited by maximums that are incorporated into the flexible nature of these nerves as they cross the subarachnoid space. The long course of the abducen’s nerve is marked by it’s vulnerability to injury.
• Emergence from the brainstem surface to their exit through specific foramen in the scull represents two fixed points that define how far the brain can be displaced in the scull.
• The brainstem (except for the olfactory nerves) is virtually the source of all cranial nerves. Midbrain is the source of the third and fourth nerves. The pons gives rise to the fifth nerve.
• At the pons-medulla junction, the sixth, seventh and eighth nerves emerge. In the upper medulla (open medulla) the ninth tenth, and twelfth nerves emerge as small fascicles. These compare to those from the sixth, seventh and eighth nerves that form as bundles.