The<unk>  is one of the three meninges, the membranes that cover the brain and spinal cord.  It is interposed between the two other meninges, the more superficial dura mater and the deeper pia mater, and is separated from the pia mater by the subarachnoid space.
The delicate, spiderweb-like (therefore the name)<unk>  layer, attached to the inside of the dura, surrounds the brain and spinal cord but does not line the brain down into its sulci (folds). Cerebrospinal fluid flows under this membrane in the subarachnoid space, which is full of the delicate fibres of the<unk>  extending down to attach to the pia mater.
The portions covering the brain and spinal cord are called<unk> ea encephali and<unk> ea spinalis, respectively.
The<unk>  and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version, leptomeninges. (Lepto- from the root meaning thin in Greek). Similarly, the dura in this situation is called the pachymeninx.
Arachnoid mater
Subarachnoid space
Dura mater
Subdural space
Epidural space
Cerebral aqueduct
Ependyma
Primitive ventricle