Just as binoculars are an extension of human sight, books are an extension of human memory, and just as pair of pliers is an extension of the human hand, the computer is an extension of the human mind. It is fast becoming evident that in the near future a computer will only be limited in how small it can be made by the size of its keyboards and screens - while we continue to use such archaic items. Developments like Windows, the mouse, the virtual glove and voice recognition, have moved us farther along the road to greater efficiency. What we are doing here is moving closer - getting into the machine. The next stage will be when we plug in.
Most direct extensions are at present the province of the medical world.
Prosthetics have been around since before Captain Hook and in the last century most of us have seen moveable plastic limbs. Prosthetics are, like the rest of our technologies, extensions of ourselves. Now consider where they are going. These most important and most remunerative developments of medical technology are going to reach the point where the replacement is better than the part replaced. And then people are going to want the replacement - needed or otherwise.