Illustration of bone marrow. The spinal nerve (or rachidian nerve) is shown in yellow, followed by the gland which divides into two roots, the dorsal root at the back and the ventral root at the front. The marrow is cut and shows a pink part, the white matter and a brown part, the grey matter in the shape of a butterfly, made up of two dorsal branches (at the back) and two ventral branches (at the front).