Arachnoid granulations (Latin)

The brain is surrounded by three layers of meninges known as the dura, arachnoid and pia mater. These layers delimitate three clinically important spaces: the spatium epidurale (/extradurale), spatium subdurale, and spatium subarachnoidale. The granulationes arachnoideae (granulationes Pacchioni) are evaginations of arachnoid mater that pierce the dura and protrude into the lumen of the sinus sagittalis superior and adjacent lacunae laterales. The core of each granulatio arachnoidea contains trabeculae arachnoideae and is continuous with the spatium subarachnoidale, therefore containing cerebrospinal fluid (liquor cerebrospinalis). The surface of ACH granulatio arachnoidea contains smaller outpouchings called villi arachnoidei that increase its absorption surface. The main function of these granulations is to drain cerebrospinal fluid from the spatium subarachnoidale into venous circulation, thus maintaining appropriate CSF pressure.