(3.02)

The brain is contained within the cranial cavity. Here's the cranial cavity in a dry skull. It's almost the same shape as the brain. As we saw in the last tape, two big steps divide the floor of the cavity into three parts.

The sphenoid ridges separate the anterior cranial fossa from the middle cranial fossa. This part of the cerebrum, the frontal lobe, occupies the anterior cranial fossa, this part, the temporal lobe occupies the middle cranial fossa.

The petrous temporal bones separate the middle cranial fossa from the posterior cranial fossa. The posterior cranial fossa contains the cerebellum and the brainstem. Here's the foramen magnum.

Now let's see how the cranial cavity looks in the living body. The cranial cavity is lined throughout by this layer of tough, shiny fibrous tissue, the dura.

Below, the layer of dura passes through the foramen magnum, becoming continuous with the dura that lines the vertebral canal. Two important extensions of the dura create partitions within the cranial cavity. They're the falx and the tentorium.

Here's the tentorium. Its full name is tentorium cerebelli. It separates the posterior cranial fossa from the rest of the cranial cavity, and separates two major parts of the brain, the cerebrum above from the cerebellum below.

This opening in the tentorium is called the tentorial incisure. The brain stem passes through it. The tentorium is attached along this lie on the occipital bone, and along the edge of the petrous temporal bone. Its attachment ends at the posterior clinoid process.

The upper surface of the tentorium is continuous with the dura of the floor of the middle cranial fossa. In the midline, the tentorium is attached to the other major partition, the falx, which we'll add to the picture.

This is the falx. Its full name is falx cerebri. The falx forms a mid-line partition between the two cerebral hemispheres. Here's its attachment to the tentorium.

Along its length it's attached to the occipital, parietal and frontal bones. Here in front the falx is attached to the crista galli. To see the falx in cross section we'll divide it along this line.

Near its attachment the falx splits into two layers, leaving a triangular space for the superior sagittal sinus, an important part of the brain's venous drainage system, as we'll see later in this tape.