Subarachnoid Space

The subarachnoid space is the interval between the arachnoid mater and pia mater and therefore is present where these meninges envelop the brain and spinal cord. The space is filled with cerebrospinal fluid and contains the large blood vessels of the brain. It is transversed by a network of fine trabeculae formed of delicate connective tissue. The subarachnoid space completely surrounds the brain and extends along the olfactory nerves to the mucoperiosteum of the nose. The subarachnoid space also extends along the cerebral blood vessels as they enter and leave the substance of the brain and stops where the vessels become an arteriole or a venule.

In certain situations around the base of the brain, the arachnoid does not closely follow the surface of the brain so that the subarachnoid space expands to form subarachnoid cisterns. The largest cisterns are the cerebellomedullary cistern, the pontine cistern, and the interpeduncular cistern.

Inferirly, the subarachnoid space extends beyond the lower end of the spinal cord and invests the cauda equina. The subarachnoid space ends below at the level of the interval between the second and third sacral vertebrae.

The subarachnoid space surrounds the cranial and spinal nerves and follows them to the point where they leave the skull and vertebral canal. Here the arachnoid mater and pia mater fuse with the perineurium of each nerve.