Tentorium cerebelli

The tentorium cerebelli is a horizontal crescent-shaped fold of dura mater that forms the roof of the posterior cranial fossa. It separates the back part of the brain (occipital lobes) from the cerebellum and brainstem underneath.

It's worth noting that the covering over the brain, made up of the dura and arachnoid mater, makes it difficult to see the brain's folds and grooves (sulci and gyri). However, in certain areas, the inner layer of the dura mater extends into the brain and forms folds that connect to specific parts of the skull. These folds not only divide different areas of the brain but also provide support for its soft structure.

One of these dural folds is called the tentorium cerebelli, named for its tent-like shape. It covers structures in the posterior cranial fossa, namely the cerebellum and brainstem. Its side edges attach to the occipital bone and contains the right and left transverse venous sinuses. At the front, the edges attach to the upper border of the petrous temporal bone, where you can find the superior petrosal venous sinuses.

The tentorium cerebelli also serves as a horizontal base for the attachment of two other dural folds called the falx cerebri above it, and the falx cerebelli below it. There’s a small gap at the front of the tentorium cerebelli, known as the tentorial notch. This gap allows the midbrain to pass through as it descends as the pons and medulla oblongata into the posterior cranial fossa. If there is any bulging through this notch, such as the temporal lobes pushing into it (i.e. uncal herniation), it can put pressure on the midbrain and compress the oculomotor nerve, as well as certain nerve fibers related to movement, including the corticospinal and corticonuclear descending fibers.

Besides the tentorium cerebelli, there are other important dural folds in the brain. These include the falx cerebri, falx cerebelli, and diaphragma sellae.