A method of navigating a spinal subarachnoid space in a living being includes percutaneously introducing a device into the spinal subarachnoid space at an entry location. The device has a first passageway that is sized to slidably receive, and work with, at least a guidewire. The device can be a catheter or a sheath. The method can also include advancing the device within the spinal subarachnoid space at least more than 10 centimeters from the entry location. Alternatively, the method can include advancing the device within the spinal subarachnoid space to facilitate intracranial access with a second device introduced through the first passageway. Also disclosed is a device suited for attachment to a patient's skin, such as a sheath, that includes an elongated member, a skin-attachment apparatus having a flexible skin-attachment flap, and a valve apparatus.