<P> The shape of the arachnoid does not follow the convolutions of the surface of the brain and so looks like a loosely fitting sac . In particular, in the region of the brain a large number of fine filaments called arachnoid trabeculae pass from the arachnoid through the subarachnoid space to blend with the tissue of the pia mater . The arachnoid is composed of an outermost portion (arachnoid barrier cell layer) with tightly packed cells and no extracellular collagen; that is why it is considered to represent an effective morphological and physiological meningeal barrier between the cerebrospinal fluid and subarachnoid space and the blood circulation in the dura . </P> <P> The arachnoid barrier layer is characterized by a distinct continuous basal lamina on its inner surface toward the innermost collagenous portion of the arachnoid reticular layer . </P> <P> The pia mater (Latin: tender mother) is a very delicate membrane . It is the meningeal envelope that firmly adheres to the surface of the brain and spinal cord, following all of the brain's contours (the gyri and sulci). It is a very thin membrane composed of fibrous tissue covered on its outer surface by a sheet of flat cells thought to be impermeable to fluid . The pia mater is pierced by blood vessels to the brain and spinal cord, and its capillaries nourish the brain . </P> <P> The arachnoid and pia mater together are sometimes called the leptomeninges, literally "thin meninges" (Greek: λεπτός leptos - thin). Acute meningococcal meningitis can lead to an exudate within the leptomeninges along the surface of the brain . Because the arachnoid is connected to the pia by cob - web like strands, it is structurally continuous with the pia, hence the name pia - arachnoid or leptomeninges . They are responsible for the production of beta - trace protein (prostaglandin D synthase) a major cerebrospinal fluid protein . </P>

The meningeal layer that adheres to the surface contour of the brain