Cranial nerve X, also known as the vagus nerve, is one of the twelve pairs of cranial nerves. It is the longest cranial nerve, extending from the brainstem to the abdomen. The vagus nerve plays a crucial role in the autonomic nervous system, influencing functions such as heart rate, digestion, and respiratory rate. This nerve has both sensory and motor functions, providing information from the organs to the brain and sending signals to control muscle movements. It innervates various structures, including the heart, lungs, and gastrointestinal tract, making it essential for maintaining homeostasis in the body.