Arachnoidea

ARACHNOIDEA, the arachnoid membrane of the brain, the outer surface of which is covered with endothelium and faces the dura mater, closing together with it the subdural space; the inner surface is provided with processes in the form of trabeculae and septa, covered with endothelium, connecting the Arachnoidea with the pia mater. The spaces thus formed between the two membranes are called subarachnoid cavities. From the outer surface of the Arachnoidea, in some places, protrusions rise, which extend into the dura mater and into the venous sinuses—Larianov's granulations (see). Arachnoiditis, inflammation of the avascular, or arachnoid, membrane of the brain (meningitis arachnoideae). Over a large extent, it is connected with meninx vasculosa (forming leptomeninx-pia mater) and together with it reacts to various harmful agents with inflammation (leptomeningitides). More independent significance, both clinical and morphological, is possessed by traumatic arachnoiditides (see Meningitides).