Central Nervous System
The largest part of the nervous system is the central nervous system. This portion is comprised of the brain and spinal cord. The brain is divided into two main sections: the prosencephalon and the brain stem.
The prosencephalon is the main portion of the brain, responsible for maintaining body temperature, reproduction, sleeping, eating and emotional displays. In humans and other advanced animals, it is also the portion of brain that is responsible for higher cognitive functions.
The brain stem is responsible for passing information from the prosencephalon to the spinal cord and the rest of the body. It also controls motor function, sensations such as pain and temperature and regulates the respiratory and cardiac systems. Along with the brain, this is protected by the skull and a number of membranes.
The spinal cord is a bundle of nerve tissues that runs along the spine of vertebrates. It is protected by a series of bones that form a long column. Its primary job is to act as a relay center for neurological signals, however it is capable of a variety of autonomic responses such as reflexes.
Above: Central Nervous System (Provided by Grm wnr at Wikimedia Commons; Public Domain; http://en.wikipedia.org/wiki/File:Central_nervous_system.svg)