The central nervous system constitutes the largest part of the nervous system. It consists of the brain and spinal cord. In some classifications, retina and cranial nerves are also included in the CNS. There is no clear boundary separating the central nervous system, which has a fundamental role in behavioral control with the peripheral nervous system, from the peripheral nervous system. The CNS covers the brain in the body cavity, the cranial cavity, and the spinal cord in the spinal cavity. In vertebrates, the brain is protected by the skull, while the spinal cord is protected by the spine. Both of these are surrounded by meninges (brain and spinal cord). The ones shown in red in the figure are the main nerves of the CNS.