The central nervous system is surrounded by three layers or 'membranes' called meninges and is completely encased within the protective bones of the skull and vertebral column. The outer membrane forms the tough dun mater which is attached to the skull and vertebrae, and the inner membrane forms the thin Pia mater which lies next to the nervous tissues. Between the two is the arachnid 'membrane'. This includes a space, the arachnophobia space, strands of connective tissue, blood vessels and cerebrovascular fluid (CS). Most of this fluid is contained in the central canal of the spinal cord and continues forward to occupy four expanded cavities within the brain called the ventricles. The fluid therefore comes into contact with the outside and inside of the brain, and blood vessels lie within it for the supply of nutrients and oxygen to the nervous tissues and the removal of wastes. It also contains lymphocytes to protect against infection. Meningitis is caused by an infection of the meninges. About 100 cm3 of fluid is present in the CNS and, apart from its nutritive, excretory and defensive functions, it supports the nervous tissues and protects them against mechanical shock. A continual circulation of fluid is maintained by cilia ted cells lining the ventricles and central canal.
The Spinal Cord
The spinal cord is a cylinder of nervous tissue running from the base of the brain down the back. It is protected by the vertebrae of the backbone (vertebral column) and the meninges. It has a shaped central area of grey matter, composed of nerve cell bodies, dendrites and synapses surrounding a central canal which contains cerebrovascular fluid. Around the grey matter is an outer layer of white matter, containing nerve fibers whose fatty myelitis sheaths give it its characteristic color. There are 31 pairs of spinal nerves and these divide dose to the spinal cord to form two branches called the dorsal root and ventral root.
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