Which of the following structures is not part of the central nervous system?
a.
the brain
b.
a nerve
c.
the spinal cord
d.
a tract

The central nervous system consists of the two major structures: the brain and spinal cord. The brain is encased in the skull, and protected by the cranium.[7] The spinal cord is continuous with the brain and lies caudally to the brain and is protected by the vertebra. These are connected by nerve to send signals. So a tract is not part of the central nervous system.

a. to encourage apoptosis