Although the arachnoid mater of the spinal cord has a less adherent relationship with the dura mater than in the cranial cavity, the overall structure of the arachnoid mater is the same (eFig. 9.46A). The subarachnoid space created by the loose relationship of the arachnoid and underlying pia mater extends inferiorly to the level of the SII vertebra (eFig. 9.45). Given that the spinal cord terminates near the LI–LII vertebrae, this lower termination point of the subarachnoid space creates a safe and enlarged space for accessing CSF in the clinical setting (eFig. 9.45).