Brain slice. Gloved hand holding up a microscope slide of a section through a human brain. This is a coronal slice through the frontal region of the cerebrum. The two main types of brain tissue can be seen: the grey matter (pink) makes up the external layer of the brain, while the white matter (purple) is found at the brain's centre. The brain is made up almost exclusively of nerve cells (neurones). The grey matter contains most of the neurone cell bodies, and the white matter is mainly formed of nerve fibres connecting the cell bodies. By examining brain slices under a microsc- ope the health of the brain can be determined.