(3.32)

As we'll see in a minute, there are also venous sinuses that drain the base of the skull. Before we see them, we need to go back to the superior sagittal sinus, and look at the structures on each side of it that absorb cerebrospinal fluid.

These structures, the arachnoid granulations, were left out of the picture in the earlier section on the brain. To see them we'll return to this view of the surface of the brain. This central strip of dura contains the superior saggital sinus. We'll remove the dura that forms the roof of the sinus.

These small projections in the floor of the sinus, and on its sides, are arachnoid granulations. They're upward protrusions of the arachnoid membrane. At their surface, cerebrospinal fluid from the subarachnoid space is transferred back into the bloodstream.

Now we'll complete our picture of the venous sinuses by looking at the ones that drain the base of the skull. The most important of these are the two cavernous sinuses, one on each side.

We saw this view of the cavernous sinus when we looked at the internal carotid artery. The cavernous sinus is the space around the artery. It extends forwards to the superior orbital fissure, and backwards almost to the dorsum sellae. It's bounded medially by the dura that lines the pituitary fossa. As we've seen, the lateral wall of the cavernous sinus contains these three cranial nerves, the third, fourth and sixth.

Outside these lies the trigeminal ganglion, and outside that, the dura of the middle cranial fossa. To get a cross-sectional view of the cavernous sinus we'll go to a different specimen and divide it in the frontal plane along this line. This is the cavernous sinus. The big cavity in the midline is a sinus of a different order: it's the sphenoid sinus. Here's the divided internal carotid artery passing forwards.

Here are cranial nerves three, four and six. Here's the trigeminal ganglion, here's the dura. Here's the pituitary gland, contained within the dura that creates the pituitary fossa.

The two cavernous sinuses are connected to each other behind the pituitary gland. The cavernous sinus receives blood from several sources, including the superior orbital vein, a major vein from the orbit that connects the cavernous sinus to veins in the upper part of the face.

The cavernous sinus drains into the two petrosal sinuses, superior and inferior, which have been exposed on the right side. The petrosal sinuses also receive veins from the cerebellum. They empty into the sigmoid sinus up here, and under here. Now we've finished looking at the intracranial blood vessels. We'll follow the internal jugular vein in a few minutes.