Arachnoids are a membrane located between pia mater and dura mater, the other two protective membranes that cover brain and spinal cord tissues. They are thin, weblike tissues that resemble a spider web, hence the name arachnoid. Arachnoids contain cerebrospinal fluid (CSF) and trabeculae, which are delicate connective tissue projections. They act as a barrier against infections and regulate the flow of CSF. Arachnoiditis is a condition where arachnoids become inflamed.