Anatomy Surrounding the spinal cord in the bony vertebral column are three membranes. From the immediate overlay of the cord to the periphery, there are the pia mater, the arachnoid mater, and the dura mater [11]. Between the pia and the arachnoid is the space of interest in spinal anesthesia, the subarachnoid space. In this space are the cerebrospinal fluid (CSF), spinal nerves, a trabecular n...