This barrier is a natural defence against potentially harmful chemicals floating around the body but has also stopped useful drugs reaching the brain as well.
Petrifaction may also involve a simultaneous exchange of the original substance of a dead plant or animal with mineral matter of a different composition.
Japan, India and China have taken on the material trappings of modernism: skyscrapers, rail systems, telecoms networks, parliaments, judiciaries and, in some cases, ballot boxes.
In view of this behavior it has been suggested that chemicals present in fresh buck rubs may help physiologically induce and synchronize fertility in females that visit these rubs.