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.
Also Read: How to get rid of sharp pain in tongue . Causes of blister on tongue. Blisters on the tongue are as a result of wide range of causes. They are named depending on the area of the tongue they affect and the time they may take to clear. The following are some of the many causes of blisters on the tongue: 1. Sexually Transmitted Diseases [STD]