The brain and spinal cord are covered by the meninges, which are three layers or membranes of connective tissue that not only protect the brain and spinal cord, but also form a framework for vessels and venous sinuses. Just think of this as the brain needing three layers of blankets when going to bed at night to make sure it is extra cozy and secure! These three layers, from superficial to deep, are the dura mater, arachnoid mater, and pia mater.

The dura mater is a tough, thick, fibrous external meningeal layer. Deep to the dura mater is the thin arachnoid mater. The dura and arachnoid mater are separated from each other by a potential space called the subdural space. Deep to the arachnoid mater is the pia mater. The pia mater is a delicate vascular layer that is intimately adhered to the brain, covering the gyri and extending along the different sulci and fissures. Together, the arachnoid and pia mater are collectively known as the leptomeninges. Between the arachnoid mater and pia mater is the subarachnoid space, also known as the leptomeningeal space, which is a true space between the arachnoid and pia mater which contains cerebrospinal fluid or CSF for short, as well as major vessels, and cranial nerves.

Okay, let's dive in and take a closer look at the dura mater, which is the thickest, outermost meningeal layer. The dura mater itself is further divided into two layers.

The first, more superficial, layer is called the endosteal layer, or periosteal layer of the dura mater. It is located on the interior surface of the skull bones and ends at the foramen magnum. The endosteal layer does not continue with the dura mater of the spinal cord, but instead becomes continuous with the periosteum on the external aspect of the skull bones. Between the endosteal layer of the dura mater and the skull bones, there lies a potential space called the extradural, or epidural space, which is not a natural space, but may become a pathological space during bleeding.