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.
Diagnosis of black hairy tongue includes eliminating other conditions that may cause a similar appearance to the tongue, such as: 1  Normal variations in tongue color (pigment) 2  Poor oral hygiene. 3  Foods or medications that have stained the tongue.  Fungal or viral 1  infections. Inflammation of the lining of the mouth.