Unlike the equator and the Tropic of Cancer and the Tropic of Capricorn, the Prime Meridian and all lines of longitude are completely imaginary lines and have no significance with regard to the Earth or to its relationship with the sun.
Black hairy tongue typically results when projections on the tongue called papillae grow longer and don't shed like normal. This makes the tongue look hairy. Debris, bacteria or other organisms can collect on the papillae and result in discoloration. The cause of black hairy tongue can't always be determined.
Symptoms & Signs. Hairy tongue, medically known as lingua villosa, is generally a harmless condition that alters the appearance of the tongue, making it appear discolored and furry. Hairy tongue is also commonly referred to as black hairy tongue (lingua villosa nigra), but the tongue discoloration may appear white, brown, pink, or green. The color of the tongue depends both on the underlying condition plus the presence of any additional factors, such as the type of food consumed.