CRANIAL NERVE

in vertebrates, any of the paired nerves that connect the muscles and sense organs of the head and thoracic region directly to the brain. In the higher vertebrates (reptiles, birds, mammals) there are 12 pairs of cranial nerves: olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), acoustic (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII). The lower vertebrates (fishes, amphibians) have 10 pairs. These nerves are made up of motor nerve cells (neurons), sensory neurons, or both. The 10th cranial nerve, the vagus, is one of the most important; it extends to many of the organs in the chest and upper abdomen. The cranial nerves are part of the parasympathetic nervous system.