The brain: meninges and forebrain

The brain is the body's command and control system. It controls voluntary movement, balance, and the work of vital organs (lungs, heart, kidneys); body temperature, hunger and thirst; the body's daily 24-hour (circadian) rhythms; and the endocrine glands. It is also the seat of behaviour, vision, hearing, speech, memory, intelligence, emotions and thought.

The brain is protected by the skull and by three layers of membranes, the cranial meninges: the tough outer dura mater, a very thin middle layer, the arachnoid, and an inner layer in contact with the brain, the pia mater. The brain has three main parts: the forebrain, midbrain and hindbrain.

The forebrain. Its largest part is the cerebrum, divided by a deep groove into the left and right cerebral hemispheres, joined by a band of nerve fibres, the corpus callosum. The outer layer, the cerebral cortex, is deeply folded. It looks grey because it is made of neuron cell bodies (grey matter). It has motor areas (controlling movement), sensory areas, and large association areas, which are responsible for complex functions like linking information from different senses, memory and communication. Underneath, the cerebrum looks white: this white matter is made of myelinated nerve fibre tracts.

Wrapped inside the cerebrum is the thalamus, a major centre that coordinates sensory and motor signals. At its base is the hypothalamus, which controls body temperature and the urge to eat and drink. It also has special neurosecretory cells that make hypothalamic hormones.

The inner parts of the cerebral hemispheres, with deep structures like the amygdala and hippocampus, form the limbic system. Together with the hypothalamus, it regulates sexual behaviour, emotional reactions (excitement, pleasure, rage, fear) and motivation.