In neuroanatomy, a sulcus (Latin: "furrow", pl. sulci) is a depression or fissure in the surface of the brain.
It surrounds the gyri, creating the characteristic appearance of the brain in humans and other large mammals.
Large furrows (sulci) that divide the brain into lobes are often called fissures. The large furrow that divides the two hemispheres—the interhemispheric fissure—is very rarely called a "sulcus".
The sulcal pattern varies between human individuals, and the most elaborate overview on this variation is probably an atlas by Ono, Kubick and Abernathey: Atlas of the Cerebral Sulci.
Some of the larger sulci are, however, seen across individuals - and even species - so it is possible to establish a nomenclature.
The variation in the amount of fissures in the brain (gyrification) between species is related to the size of the animal and the size of the brain. Mammals that have smooth-surfaced or nonconvoluted brains are called lissencephalics and those that have folded or convoluted brains gyrencephalics. The division between the two groups occurs when cortical surface area is about 10 cm2 and the brain has a volume of 3–4 cm3. Large rodents such as beavers (40 pounds (Template:Convert/LoffAonSoff)Template:Convert/test/A) and capybaras (150 pounds (Template:Convert/LoffAonSoff)Template:Convert/test/A) are gyrencephalic and smaller rodents such as rats and mice lissencephalic.
In humans, cerebral convolutions appear at about 5 months and take at least into the first year after birth to fully develop. It has been found that the width of cortical sulci not only increases with age , but also with cognitive decline in the elderly. 
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Tao Liu, Wei Wen, Wanlin Zhu, Julian Trollor, Simone Reppermund, John Crawford, Jesse S Jin, Suhuai Luo, Henry Brodaty, Perminder Sachdev (2010) The effects of age and sex on cortical sulci in the elderly. Neuroimage 51:1. 19-27 May. PMID 20156569
↑ Tao Liu, Wei Wen, Wanlin Zhu, Nicole A Kochan, Julian N Trollor, Simone Reppermund, Jesse S Jin, Suhuai Luo, Henry Brodaty, Perminder S Sachdev (2011) The relationship between cortical sulcal variability and cognitive performance in the elderly. Neuroimage 56:3. 865-873 Jun. PMID 21397704
↑Gerhardt von Bonin, Percival Bailey, The Neocortex of Macaca Mulatta, The University of Illinois Press, Urbana, Illinois, 1947