What Separates the Cerebellum from the Cerebrum?

The cerebellum and cerebrum are separated by a thick sheet of tissue called the tentorium cerebelli. This tough, tent-shaped membrane stretches horizontally across the inside of your skull, creating a physical floor beneath the cerebrum and a roof over the cerebellum. It divides the cranial cavity into two distinct compartments: the supratentorial space (above, housing the cerebrum) and the infratentorial space (below, housing the cerebellum and brainstem).

What the Tentorium Cerebelli Is Made Of

The tentorium cerebelli is made of dura mater, the outermost and toughest of the three membranes that surround your brain. Specifically, it is a rigid fold of collagenous connective tissue, meaning it has real structural strength. Think of it less like a thin curtain and more like a firm, slightly flexible shelf anchored to the inside of your skull. It is the second largest of four such dural folds inside the cranial cavity, and its tent-like shape gives the cerebellum a protected pocket in the lower back portion of the skull.

How It Attaches to the Skull

The tentorium is anchored at multiple points along the inner surface of the skull. Its outer, fixed edge attaches to the upper borders of the petrous bone (a dense ridge of the temporal bone near each ear) and runs along grooves on the occipital bone at the back of the skull. Its inner, free edge sweeps forward on each side and attaches to bony projections near the front of the skull called the anterior clinoid processes. These two edges, one fixed and one free, create a crescent-shaped opening at the front of the membrane.

The Tentorial Notch

The gap formed by the free inner edge of the tentorium is called the tentorial notch (or tentorial incisura). This opening is essential because it allows the brainstem to pass through the barrier, connecting the cerebrum above to the cerebellum and spinal cord below. Without this gap, the two compartments would be completely sealed off from each other, and the brainstem would have no pathway between them.

Venous Sinuses Within the Tentorium

The tentorium does more than just partition the brain. It contains important drainage channels for blood leaving the brain. The transverse sinuses run bilaterally along the tentorium where it attaches to the occipital bone, carrying used blood from the back of the brain toward veins in the neck. A smaller channel called the occipital sinus also runs along the posterior aspect of the tentorium. These venous sinuses are essentially tunnels enclosed within the layers of the dural fold itself, making the tentorium both a structural divider and a vascular highway.

The Transverse Fissure

You may also encounter the term “transverse fissure” when reading about this area. The transverse fissure is the actual groove or space between the upper surface of the cerebellum and the underside of the cerebrum’s occipital and temporal lobes. The tentorium cerebelli sits within this fissure, filling the gap. So while the transverse fissure is the natural space between the two brain regions, the tentorium is the tissue that occupies and reinforces that space.

Why This Barrier Matters Clinically

The tentorium’s rigid structure normally protects the cerebellum and brainstem from the weight and movement of the much larger cerebrum above. But this same rigidity becomes dangerous when pressure inside the skull rises abnormally, such as from a tumor, bleeding, or severe swelling. When pressure in one compartment exceeds the other, brain tissue can be forced through the tentorial notch. This is called transtentorial herniation, and it is a life-threatening emergency.

There are several patterns of herniation depending on where the pressure originates. In uncal herniation, the innermost part of the temporal lobe gets pushed downward through the notch, compressing the nerve that controls pupil size. The hallmark sign is a fixed, dilated pupil on the same side as the pressure source, along with loss of consciousness. The downward pressure can also compress a major artery against the tentorium, cutting off blood flow to the visual processing areas at the back of the brain and potentially causing blindness.

In central herniation, the brain is pushed straight down through the notch symmetrically. Early signs include progressive drowsiness, confusion, difficulty concentrating, and small pupils that barely react to light. As the pressure worsens, the brainstem itself can lose its blood supply, leading to small hemorrhages visible on brain scans known as Duret hemorrhages.

Herniation can also move upward. When pressure builds in the compartment below the tentorium, cerebellar tissue can be forced up through the notch. This compresses the brainstem from below, causing loss of consciousness and difficulty looking upward. If the narrow channel that drains fluid through the center of the brainstem gets pinched shut, fluid backs up rapidly inside the brain, a condition called obstructive hydrocephalus.

The concept underlying all of these scenarios is straightforward: the skull is a closed box with a fixed amount of space. Brain tissue, blood, and cerebrospinal fluid compete for that space. Once the natural cushioning mechanisms are exhausted, even a small increase in volume can force tissue across the tentorium. The tentorial notch, normally a vital passageway for the brainstem, becomes the weak point where damage concentrates.