Arachnoid Membrane

Arachnoid Membrane, one of the three coverings of the brain and spinal cord, is situate between the dura-mater and the pia-mater. It is non-vascular, transparent, and remarkably thin. Its outer surface, next the dura-mater, is free, smooth, and glistening; its inner surface is connected to the pia-mater by numerous delicate threads, which traverse the space (sub-arachnoid) between the two membranes. Some anatomists regard the space between arachnoid membrane and dura-mater (sub-dural) as a serous cavity bounded by a serous membrane, of which the arachnoid membrane is its visceral layer, and the inner free shining surface of the dura-mater its parietal layer. The sub-arachnoid space contains a fluid named cerebro-spinal, which in health varies in amount from two drachms to two ounces. See BRAIN, CEREBRO-SPINAL FLUID, PIA-MATER.