If you were to join a team of surgeons performing a brain operation, you would see them first drill a tiny hole into the skull. Then, with a powerful saw, they would remove a portion of bone.
Through the opening, you might expect to get your first glimpse of the brain. In fact, your initial view would be of what is called the dura mater, the tough leathery outer layer of the menningers, the three strong membranes that cover and protect the brain and the spinal cord.
Then, when the surgeon cuts into the dura, cauterises the bleeding vessels and peels back the dural flap, you would see a glistening grey-white substance pulsating gently. This outer layer of the brain is known as the cortex, a term applied also to other layer of other organs, such as the kidneys and adrenal glands. That of the brain is just 3 mm (1/8 in) thick and holds thousands of millions of nerve cells arranged in six layers. The cortex is the site of brain’s higher functions, the area concerned with thought, sensations and movement.
Amid a variety of technical terms used in detailed diagrams of the brain’s tructure – corpus callosum, sulcus, gyrus, ganglia and others – you will find the cortex described, even is specialist literature, as grey matter. Beneath the grey matter of the cortex lies white matter tracts of thin nerve-cell fibres, which project like tails. These tracts link various areas of the grey matter to one another and to nerve centres elsewhere in the brain and the brain stem.