To pack more grey matter into the limited space inside the skull, animals with big brains, especially humans, the cerebral cortex is wrinkled. The grooves that make these wrinkles are called sulci and the ridges between them are called gyri. The human cerebral cortex is very deeply wrinkled, so it has a lot of grey matter, which has the large majority of all the neurons in the CNS. In Figure 5-2d, sulci (singular = sulcus) are shown by the curving lines on the cerebral hemispheres. The ridges between the sulci are the gyri (singular = gyrus). Figure 6-2d shows the "typical" pattern of gyri and sulci, though the pattern varies among individual brains.
Figure 5-2d. Left
cerebral
hemisphere, showing the sulci (black lines) with the gyri between them and the major primary
areas