Now we'll move on to look at the layers of tissue that give the brain a protective covering, and maintain its special fluid environment. These three layers, the dura, the arachnoid and the pia, are collectively called the meninges. We've already taken an inside look at the outer layer, the dura.
To see the two inner layers, the pia and the arachnoid, we need to add the brain itself to the picture. We're looking at the right cerebral hemisphere. The blood vessels on its surface have been filled with red latex. The surface of the brain is richly folded. An outward fold is called a gyrus, an inward fold is a sulcus.
The pia is almost invisibly thin. It's just the glossy surface that we see here. To see the extent of the pia we'll look at a frontal section. Here's a typical sulcus. The pia extends down into each sulcus, and back up onto the next gyrus. Each sulcus contains blood vessels which lie just outside the pia. Each vessel that enters the brain carries a sleeve of pia with it.
Now we'll add the arachnoid to the picture. This is the arachnoid. It's a delicate transparent membrane. Here's the arachnoid again. Unlike the pia, the arachnoid doesn't extend into the sulci. It bridges over, from one gyrus to the next. In this specimen the subarachnoid space is empty. Here we're injecting water to fill it.
Over most of the brain the subarachnoid space is narrow, but in a few places it's quite wide, notably here below the cerebellum, here above the cerebellum, and here in front of the top of the brainstem. These spaces are called cisterns.
Outside the arachnoid is the dura. We'll add it to the picture. The dura is a much tougher layer of tissue than either the pia or the arachnoid. The dura has almost no attachment to the arachnoid. The dura can be separated from the overlying bone, but is normally quite closely attached to it.
Now we'll add the rest of the dura to the picture. Here's the intact dura. These are branches of the middle meningeal artery, which runs in the thickness of the dura.
To look at the openings in the dura, we'll again look at it from the inside in an empty skull. The vessels and nerves that enter and leave the cranial cavity pass though openings in the dura. At each opening the dura forms a tunnel around the nerve or vessel for a short distance.
Typically a nerve or blood vessel runs beneath the dura for a distance between its opening in the dura and its opening in the bone, so the openings in the dura often don't match the openings in the bone. The difference between dural and bony openings is specially marked here in the middle cranial fossa.
As we saw in the previous tape, the bone here has many openings. By contrast the dura here has no openings. The corresponding openings in the dura are either up here, or back here.