The central nervous system (CNS) consists of the brain and spinal cord, with the division between these two parts of the CNS placed at the level of the foramen magnum. The peripheral nervous system (PNS) consists of nerves and ganglia outside the CNS (see chapter 11). Nerves consist of axons, Schwann cells, and connective tissue sheaths. Ganglia are accumulations of cell bodies in the PNS. The PNS includes 12 pairs of cranial nerves and 31 pairs of spinal nerves.

The CNS receives sensory information, integrates and evaluates that information, stores some information, and initiates reactions. The PNS collects information from numerous sources both inside and outside the body and relays it through axons of sensory neurons to the CNS. Axons of motor neurons in the PNS relay information from the CNS to various parts of the body, primarily to muscles and glands, thereby regulating activity in those structures.

The spinal cord and spinal nerves are described in this chapter. The brain and cranial nerves are considered in chapter 13. The specific topics of chapter 12 are the spinal cord (p. 412), reflexes (p. 415), interactions with spinal cord reflexes (p. 420), structure of peripheral nerves (p. 420), and spinal nerves (p. 422).