The insula is the region of the cerebral cortex located at the lateral aspect of the hemisphere that lags behind during development and becomes covered by a fold formed by the fissure of Sylvius (lateral cerebral sulcus), which are adjacent regions of the hemisphere that grow more rapidly during fetal development. The parts of the hemisphere overlapping the insula are called opercula. They are named according to the cerebral lobe they belong to: the frontal operculum, the parietal operculum, and the temporal operculum. The insula can be seen by removing the opercula apart to expose it. They normally leave only a cleft, the lateral cerebral sulcus (fissure of Sylvius), which widens over the insula into the lateral fossa. The insula has roughly the shape of a triangle and is bordered at its three sides by the circular sulcus of the insula. The central sulcus of the insula divides the insula into a rostral and a caudal part. At its lower pole, the limen of insula, the insular region merges into the olfactory area, the paleocortex.
The insular cortex represents a transitional region between paleocortex and neocortex. The lower pole of the insula is occupied by the prepiriform area which belongs to the paleocortex. The upper part of the insula is covered by the isocortex with the familiar six layers. Between both parts lies a transitional region, the mesocortex. Unlike the paleocortex, it has six layers; however, these are only poorly developed as compared to the neocortex.