This membrane has been compared to a spider's web from its extreme tenuity, and from this it derives its name. It is a serous membrane, and lines the dura mater throughout its whole extent. Like the other serous membranes, it is a sac without an opening, the walls of which, placed back to back, secrete a fluid. It adheres very strongly, by its external wall, to the dura mater, to which it moulds itself, and which it accompanies throughout its whole extent. Its internal wall is united to the pia mater, which separates it from the nervous substance at many points. This union is so intimate that many have thought the arachnoid has no existence except where it is detached from the pia mater, as at the level of the fissure of Sylvius, and in the cerebral sinuses, etc. In fact, the arachnoid does not enter those intervals where the dura mater does not penetrate, but is strictly confined to it. At these intervals there is a cavity between the serous membrane and the nervous centre which it surrounds, but does not touch, as is well illustrated by the spinal cord.
All the cavities formed by the arachnoid are filled with a serous fluid called the sub-arachnoid or cerebro-spinal fluid. The ventricles of the brain also contain, as has been stated, a quantity of serous fluid. The use of this fluid seems to be to protect the organs against the effect of blows and shocks. The brain and spinal cord being, as it were, suspended in the arachnoid, are held in place in the gentlest possible manner by the sub-arachnoid fluid, and by that in the ventricles which moistens the surfaces and prevents all friction.