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.