Arachnoid mater is one of the three meninges, the protective membranes that cover the brain and spinal cord. It lies between the more superficial dura mater and deep pia mater.
Structurally, arachnoid mater is delicate and impermeable. It is separated from the dura mater by a potential space called the subdural space and from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid.
It is important to understand that structures passing to and from brain to the skull or its foramina must pass through the subarachnoid space. All the cerebral arteries, veins and nerves pass through this space. The arachnoid forms a cover around these structures as they pass through the subarachnoid space. When the nerves exit from the skull, the arachnoid fuses with the epineurium
In case of the optic nerve, the arachnoid forms a sheath that extends into the orbital cavity through the optic canal and fuses with the sclera of the eyeball. Thus the subarachnoid space extends around the optic nerve as far as the eyeball.
The arachnoid mater bridges over the sulci on the surface of the brain, while the meninx beneath (the pia mater) closely adheres to it. Thus, in certain situations, the arachnoid and pia are widely separated to form the subarachnoid cisternae.
Arachnoid villi and granulations:
In certain areas, the arachnoid projects into the venous sinuses to form the arachnoid villi. They are most numerous along the superior sagittal sinus. Arachnoid granulations are aggregations of arachnoid villi.
Arachnoid villi and granulations are important because they serve as sites where the cerebrospinal fluid diffuses into the blood stream.