Final answer:
At the caudal medulla, 90% of axons in the CST cross to form the lateral corticospinal tract in the pyramidal decussation, which controls limbs' muscles. The remaining fibers form the anterior corticospinal tract for trunk muscle control.
Step-by-step explanation:
At the caudal medulla, 90% of the axons in the corticospinal tract (CST) cross over to form the pyramidal decussation. This crossing occurs at the medullary-spinal border, where the lateral corticospinal tract is subsequently formed. The lateral tract is composed of fibers that control appendicular muscles, which are the muscles of the limbs. The remaining fibers, which do not cross at the pyramidal decussation, form the anterior corticospinal tract and are responsible for controlling the muscles of the body trunk.