The trigeminal nerve, or cranial nerve V, is a somatic sensory nerve, primarily sensitive to mechanical and thermal stimulation, but the branches innervating the nasal and oral cavities and the cornea also include chemosensitive fibers. Those of the nasal cavity and cornea are sensitive to airborne chemicals. They have an important function as the sensory input triggering protective reflexes—respiratory, secretory, and cardiovascular—in response to high concentrations of irritating vapors. However, they also respond to low, nonirritating concentrations of the same chemicals and to other chemicals lacking any irritant qualities. Trigeminal input can thus contribute to the sense of smell, either directly or by influencing input from olfactory (cranial nerve I) receptors.