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.

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands