The largest and deepest fissure in the cerebellum is named the horizontal fissure (or horizontal sulcus).

It commences in front of the pons, and passes horizontally around the free margin of the hemisphere to the middle line behind, and divides the cerebellum into an upper and a lower portion.

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  Animation. Horizontal fissure lined in red.
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  Upper surface of the cerebellum.
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  Sagittal section of the cerebellum, near the junction of the vermis with the hemisphere.

This article incorporates text in the public domain from page 789 of the 20th edition of Gray's Anatomy (1918)

• NIF Search - Horizontal Fissure via the Neuroscience Information Framework