The largest portion of the brain is called the cerebrum- see if you can find where it separates into 2 hemispheres. The cerebrum is largely made out of folded grey matter known as the cerebral cortex; these folds (or gyri) give the brain its wrinkled appearance and increase its surface area. This allows us to fit more neurons inside our skull and have higher brain function than animals with less folds in their brain. Not all of the cerebrum is made from grey matter however; beneath the grey matter we find white matter. White matter gets its name from the fatty myelin sheath covering its surface and is largely composed of nerves, connecting the 2 cerebral hemispheres via the corpus callosum.