The part of the central nervous system below the medulla oblongata is called the spinal cord. It originates from the medulla oblongata and extends through the foramen magnum to the lumbar vertebrae. Its lower narrow part is called conus medullaris. Its inner layer is made up of gray matter and outer layer is made up of white matter. Gray matter looks like the English letter H or a butterfly. The cavity of the ventricles is called the terminal ventricle. It is filled with cerebrospinal fluid. 31 pairs of afferent nerves arise from the axon. Each root has two parts. dorsal root and digital root. Human hymen is about 45 cm long, 1.25 cm wide and weighs about 40 grams.