Final answer:
The vast majority of parasympathetic fibers are derived from cranial nerve X and are involved in the regulation of heart rate, breathing, and digestive system activity.
Step-by-step explanation:
The vast majority of parasympathetic fibers are indeed derived from cranial nerve X, also known as the vagus nerve. The vagus nerve originates from the brainstem and sends parasympathetic fibers to various organs in the thoracic and abdominal cavities.
These parasympathetic fibers play a significant role in the regulation of heart rate, breathing, and digestive system activity. For example, they slow down the heart rate and increase digestion in the stomach, liver, pancreas, gall bladder, and small intestine.