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A sulcus is a sharply creased surface fold induced by compression in a soft material. It begins and grows like a crack, but is a continuous and reversible deformation. Sulci are found in primate brains and can be seen directly in a slice of bread (a) or in the arm of an infant (b), or indirectly as a disturbance pattern of glass dust on a folded and unfolded rubber slab (c) or as a pattern in refracted laser light passing through a folded rubber slab (d).
Reporters may freely use this image. Credit format: Image courtesy of Evan Hohlfeld, Lawrence Berkeley National Lab and Harvard University (2011).