The tongue is a strong muscle located in the mouth and is the primary organ of the special sense called taste. In humans the tongue is important in the formation of speech, chewing and swallowing food, as well as social behavior such as kissing. Covered in a mucous membrane, this strong appendage is comprised of very small nodules termed papillae that project along the top surface and provide a rough, unsmooth texture. Tiny bulb-like taste organs, often referred to as taste buds, are scattered over the top surface and sides of the tongue. These buds react to only four fundamental tastes that include sweet, sour, salty, and bitter sensations. Once detected, information about a flavor is sent through a vast network of nerves to the brain. Taste buds sensitive to salty flavors are typically found evenly distributed across the tongue. Sensations for sweet substances are captured by buds that lie primarily at the tip of the organ. Receptors for sour tasting foods aggregate along the sides of the tongue, while bitter flavors are predominately detected at the rear. The sensation of smell is very closely linked with taste and, when combined, produce the impression that a greater range of flavors is available. For this reason, when smell senses are impaired due to a cold, the sense of taste is also often diminished.