"They are formed by pia mater of the spinal cord coursing in-between the dorsal and ventral nerve roots bilaterally. They function to provide stability to the spinal cord within the vertebral canal 1-3. There are usually 21 pairs evenly spaced along the length of the spinal cord but their number can range from 18 to 24 pairs. The first (most superior) pair inserts into the lateral borders of the foramen magnum, with the vertebral artery anterior to them and the hypoglossal nerve (CN XII) posterior. The last (most inferior) pair is at the level of the conus medullaris, namely, between the T12 and L1 segmental nerve roots, and its pia mater fuses with the filum terminale."