The central nervous system (CNS) consists of the brain and the spinal cord. It is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except radially symmetric animals such as sponges and jellyfish.
Some classifications of the CNS also include the retina and the cranial nerves. Together with the peripheral nervous system, it has a fundamental role in the control of behavior. The CNS is contained within the dorsal cavity, with the brain in 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 vertebrae, and both are enclosed in the meninges (Figure 2).
During early development of the vertebrate embryo, a longitudinal groove on the neural plate gradually deepens and the ridges on either side of it (the neural folds) become elevated and ultimately meet, transforming the groove into a closed tube, the ectodermal wall of which forms the rudiment of the nervous system. This tube initially differentiates into three vesicles (pockets): the prosencephalon at the front, the mesencephalon, and between the mesencephalon and the spinal cord, the rhombencephalon. At six weeks in the human embryo's development, the prosencephalon divides further into the telencephalon and diencephalon; and the rhombencephalon divides into the metencephalon and myelencephalon.
As the vertebrate grows, these vesicles differentiate further still. The telencephalon differentiates into, among other things, the striatum, the hippocampus, and the neocortex, and its cavity becomes the first and second ventricles. Diencephalon elaborations include the subthalamus, hypothalamus, thalamus, and epithalamus, and its cavity forms the third ventricle. The tectum, pretectum, cerebral peduncle, and other structures develop out of the mesencephalon, and its cavity grows into the mesencephalic duct (cerebral aqueduct). The metencephalon becomes, among other things, the pons and the cerebellum; the myelencephalon forms the medulla oblongata; and their cavities develop into the fourth ventricle.