The major organ of the nervous system, located in the cranium (skull). The brain receives, sorts, and interprets sensations from the nerves that extend from the central nervous system (brain and spinal cord) to the rest of the body; it initiates and coordinates nerve signals involved in activities such as speech, movement, thought, and emotion. An adult brain weighs about 1.4 kg and has 3 main structures: the largest part, the cerebrum, consisting of left and right hemispheres; the brainstem; and the cerebellum. Each hemisphere in the cerebrum has an outer layer called the cortex, consisting of grey matter, which is rich in nerve-cell bodies and is the main region for conscious thought, sensation, and movement. Beneath the cortex are tracts of nerve fibres called white matter, and, deeper within the hemispheres, the basal ganglia. The surface of each hemisphere is divided by fissures (sulci) and folds (gyri) into distinct lobes (occipital, frontal, parietal, and temporal lobes), named after the skull bones that overlie them. A thick band of nerve fibres called the corpus callosum connects the hemispheres. The cerebrum encloses a central group of structures that includes the thalami and the hypothalamus, which has close connections with the pituitary gland. Encircling the thalami is a complex of nerve centres called the limbic system. These structures act as links between parts of the cerebrum and the brainstem lying beneath the thalami. The brainstem is concerned mainly with the control of vital functions such as breathing and blood pressure. The cerebellum at the back of the brain controls balance, posture, and muscular coordination. Both of these regions operate at a subconscious level. The brain and spinal cord are encased in 3 layers of membranes, known as meninges. Cerebrospinal fluid circulates between the layers and within the 4 main brain cavities called ventricles. This fluid helps to nourish and cushion the brain. The brain receives about 20 per cent of the blood from the heart’s output.