The spinal marrow is the medullary column included within the bones or vertebrae of the spinal column. It has three coverings: 1st, The dura mater, which is a white fibrous membrane, and forms the external; 2d, The arachnoid, a serous membrane, forming the middle covering. It is extremely thin and transparent; 3d, The pia mater, a cellular membrane, forming the immediate covering. It is very vascular, consisting almost entirely of vessels.