The hypothalamus is composed of several different areas and is located at the base of the brain. The hypothalamus is only 1/300 of the total brain weight. One function of the hypothalamus is the control of body temperature. The hypothalamus detects changes in body temperature and sends commands to adjust the temperature. For example, the hypothalamus can detect fever and respond by sending a command to expand capillaries in the skin. The expansion of the capillaries cools the blood and results in a drop in body temperature. The hypothalamus also controls the pituitary.
Like the brain, the spinal cord is covered by the meninges and contains both gray matter and white matter. The interior of the spinal cord consists of neurons contained within an H-shaped region of the spinal cord. This region is composed of gray matter. The gray matter region is surrounded by white matter containing axons insulated with a special covering called myelin . Myelin functions as an electrical insulator that helps axons to conduct nerve impulses more efficiently. Axons of the spinal cord carry signals both away from and toward the brain along descending and ascending tracts.
Planarians , members of the phylum Platyhelminthes (flatworms), have the simplest, clearly defined delineation of a nervous system into a central nervous system (CNS) and a peripheral nervous system (PNS).   Their primitive brains, consisting of two fused anterior ganglia, and longitudinal nerve cords form the CNS; the laterally projecting nerves form the PNS. A molecular study found that more than 95% of the 116 genes involved in the nervous system of planarians, which includes genes related to the CNS, also exist in humans.  Like planarians, vertebrates have a distinct CNS and PNS, though more complex than those of planarians.