"the arachnoid granulation"
by cm

Creative Commons Attribution Share AlikeCraniotomy - Credit: Dake, Wikimedia
The arachnoid mater is a delicate membrane covering the brain and spinal cord, resembling a spider's web.

Arachnoid granulations are small protrusions of the arachnoid mater through the upper dura mater, which allow cerebrospinal fluid to pass from the brain into the bloodstream.

The image shows the arachnoid mater covering the brain, rimmed by the dura mater, the skull and lastly the skin.