The brain is surrounded by mesodermal coverings, the meninges. The outer layer is the tough pachymeninx, or dura mater (A1). The inner layer is the soft leptomeninx which consists of two sheets, the arachnoidmater (A2) and the pia mater (A3).
The arachnoid mater (A2) adjoins closely the inner surface of the dura mater and is separated from it only by a capillary cleft, the subdural space (A12). It encloses the sub-arachnoid space (A13), which containscerebrospinal fluid and is connected with the pia mater by trabeculae (A14) and septae that form a dense meshwork and create a system of communicating cham-bers.Peduncled mushroomlike vegetations of the arachnoid protrude into the large sinuses, the arachnoid granulations (meningealgranules, or pacchionian bodies) (A15). Theyconsist of an arachnoid meshwork and are covered by mesothelium. The dura mater, which encloses them, is reduced to a mem-brane. These arachnoid villi are most abun-dant in the vicinity of the superior sagittal sinus (A16) and at the lateral lacunae (A17), and less frequent at the exits of spinal nerves. The CSF is absorbed into the venous blood in the area of the granulations. In older people, the granulations may also penetrate into the bone (formation of granular foveolae) (A18) and invaginate into the diploic veins.