Current and resistance
You might find Conduction of Electrical Current to and Through the Human Body: A Review interesting. 

An excerpt from paragraph Skin resistance protects the body from electricity:


  The body has resistance to current flow. More than 99% of the body's
  resistance to electric current flow is at the skin. Resistance is
  measured in ohms. A calloused, dry hand may have more than 100,000 Ω
  because of a thick outer layer of dead cells in the stratum corneum.
  The internal body resistance is about 300 Ω, being related to the wet,
  relatively salty tissues beneath the skin.


This is also the reason why your tongue has a not so high resistance.