Which layer of the meninges lines the surface of the brain and spinal cord?

The correct answer is the pia mater, which is the innermost layer of the meninges that directly adheres to the surface of the brain and spinal cord. This delicate membrane follows the contours of the brain, including all its grooves and folds, providing an essential barrier between the central nervous system and the surrounding tissues. The pia mater is permeable and allows for the exchange of substances between the blood and the cerebrospinal fluid (CSF), which is crucial for maintaining the environment of the nervous system.

The other layers, such as the dura mater and arachnoid membrane, do not lie directly on the surface of the brain or spinal cord. The dura mater is the outermost and toughest layer, providing a strong protective layer, while the arachnoid membrane lies between the dura mater and the pia mater. Cerebrospinal fluid, while essential for cushioning the brain and spinal cord, is not a layer of the meninges but rather a fluid that occupies the subarachnoid space between the arachnoid membrane and the pia mater. Thus, the pia mater is the only layer that directly lines the surfaces of the brain and spinal cord.