Final answer:
The white matter in the spinal cord decreases in size caudally due to the reduction of nerve fibers. The spinal cord has cervical and lumbar enlargements resulting from larger populations of neurons that innervate the limbs.
Step-by-step explanation:
When considering the size of the white matter in the spinal cord in a caudal direction, the white matter tends to decrease in size. This decrease is due to the reduction of nerve fibers as they leave the cord at various levels along the spinal column to innervate the body. The white matter is arranged into columns: posterior, anterior, and lateral. The posterior columns contain ascending sensory tracts, while the anterior and lateral columns have both ascending sensory tracts and descending motor tracts carrying commands from the brain.
Additionally, it's notable that certain enlargements occur in the spinal cord at the cervical and lumbar regions. These enlargements are due to a larger population of neurons necessary for the sensory input and motor output to the limbs, as in the cervical and lumbar enlargements, which are responsible for the innervation of the arms and legs, respectively.