Agyrus is an elevated ridge of cerebral tissue inward folds of cerebral tissue are called

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An elevated ridge of cerebral tissue inward folds of cerebral tissue is called a GYRUS.

Gyrus are usually surrounded by sulci( depressions).
They exist because it allows the human, and other mammalian brains, to have a larger surface area. Since the brain is confined to the skull and the that space is limited  this will allow more cortical surface area, and greater cognitive function to exist.

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