White Matter changes in the spinal cord
Notice in the spinal cord that the amount of gray and white matter fluctuates as the nerves "jump on" or "jump off." Where in the cord is there more white matter? Remember what is held in the white matter? So why is there more white matter as you ascend the spinal cord? See the figure below.
Figure 8. From superior to inferior, the spinal cord becomes smaller. There is less and less white matter as it descends because there are fewer afferent tracts going up (they haven't "jumped on" yet), and there are fewer efferent tracts going down (they've "jumped off" already.) The sacral nerves are actually in the area close to the lumbar enlargement and the conus medullaris, since they will leave at the end of the spinal cord.