Final answer:
The pia mater is the meninx that is closely associated with the contours of the spinal cord, fitting snugly around the CNS and following its every detail.
Step-by-step explanation:
The meninx that is closely associated with the contours of the spinal cord is the pia mater. It is the innermost of the meninges, thin fibrous layers covering the brain and spinal cord. The pia mater directly adheres to the surface of the central nervous system (CNS), following every detail of the underlying brain and spinal cord structures, including the convolutions of gyri and sulci, and filling other grooves and indentations. Unlike the dura mater, which is a thicker, more durable layer providing overall protection, or the arachnoid mater, which forms a web-like coverage between the dura and pia, the pia mater acts almost like a 'plastic wrap' by fitting snugly around the CNS. The blood vessels that nourish the CNS tissue pass between the pia mater and the nervous tissue, further illustrating the pia mater's intimate relationship with the CNS structures.