Anatomy, Head and Neck, Subarachnoid Space

Three layers called the meninges encase the brain and spinal cord. From superficial to deep, these layers are the dura mater, arachnoid mater, and pia mater. The dura mater is a dense connective tissue layer that is adherent to the inner surface of the skull. Next is the arachnoid mater that is a thin impermeable layer, and the innermost is the pia mater, which is a vascular layer that closely invests over the brain and spinal cord. These membranes define three potential clinically significant spaces: the epidural space, which exists between the skull and the dura mater; the subdural space, found between the dura mater and arachnoid mater; and the subarachnoid space, which is between the arachnoid mater and pia mater. The epidural space in the skull is a potential space, while it is actually present in the spinal cord. The subarachnoid space consists of the cerebrospinal fluid (CSF), major blood vessels, and cisterns. The cisterns are enlarged pockets of CSF created due to the separation of the arachnoid mater from the pia mater based on the anatomy of the brain and spinal cord surface. The cisterns are created due to the close and firm adherence of the pia mater to the brain and spinal cord surface while rather loosely to the arachnoid mater.