Final answer:
The vast majority of parasympathetic fibers are derived from cranial nerve X, the vagus nerve. This nerve controls various 'rest and digest' functions in the thoracic and abdominal cavities.
Step-by-step explanation:
The vast majority of all parasympathetic fibers are derived from cranial nerve number X, which is also known as the vagus nerve. The vagus nerve contains preganglionic parasympathetic fibers that influence a wide range of functions in the body. These fibers primarily affect the thoracic cavity, including the heart, bronchi, and esophagus, as well as the abdominal cavity, influencing the stomach, liver, pancreas, gall bladder, and small intestine. The parasympathetic nervous system is responsible for 'rest and digest' activities, and the vagus nerve plays a crucial role in this system. In addition to the vagus nerve, other cranial nerves that contain parasympathetic fibers include the oculomotor nerve (CN III), the facial nerve (CN VII), and the glossopharyngeal nerve (CN IX).