Brain and cord—plus retina—as one protected core
The central nervous system is mainly brain and spinal cord, with the retina counting as CNS tissue too. Vertebrates seal both inside meninges and bathe them in cerebrospinal fluid, while the skull and vertebral canal supply bony armour for the body’s command trunk.
The name “central” marks integration: incoming signals converge, and outgoing commands fan to every region. Vertebrate CNS architecture differs sharply from other animals’ arrangements, so comparisons need care. Inside the meninges, cerebrospinal fluid replaces what would otherwise be a vulnerable chemical bath, buffering the tissue from many blood-borne toxins in food.
The dorsal body cavity houses the system—brain in the cranial vault, cord in the spinal canal. Supporting neuroglia pack the spaces between neurons. Optic nerve (cranial II), olfactory nerves, and olfactory epithelium also count as CNS because they connect to brain neurons without intervening ganglia; olfactory epithelium is the rare central tissue sitting outside the meninges.
Under the microscope the system splits into white matter, made of axons and myelin-forming oligodendrocytes, and gray matter, holding neuron bodies and unmyelinated fibres. The brain wears its gray matter on the outside as cortex, with white-matter tracts inside and clusters of subcortical nuclei. The cord runs from the base of the skull through the foramen magnum, protected by the vertebrae, and 31 spinal nerves branch from it, some merging into plexuses such as the brachial and sacral, each carrying both sensory and motor traffic. Twelve cranial nerves serve the head and neck, though the olfactory and optic pairs count as central because they synapse straight onto brain neurons. The brain is the largest part, but the cord handles reflexes and some locomotion on its own. The brainstem stacks medulla, pons and midbrain; nuclei in the medulla regulate blood pressure and breathing, while others serve balance, taste, hearing and facial muscles.
Source: Central nervous system
