The nervous system is composed of two key areas: the central nervous system (consisting of the brain and spinal cord) and the peripheral nervous system (made up of the nerves and their branches).

Within the central nervous system, the brain itself has four major divisions: cerebrum, diencephalon, cerebellum and brain stem (Figure 46.1). It is one of the largest organs in the body, weighing about 1.4 kg. Owing to its delicacy and importance it requires careful protection, hence the three layers of the meninges guarding it. These consist of the tough, fibrous outer layer of the dura mater, with its inward extensions into other parts of the brain (falx cerebri, falx cerebelli and the tentorium cerebelli), the central web-like structure of the arachnoid mater and finally the innermost, transparent layer (closest to the brain and containing blood vessels) called the pia mater. The meningeal layers continue down the spinal cord, eventually terminating in the periosteum of the coccyx.