There was a theory floating some time back that you could add more limbs or sensory inputs with minor consequences. The bad thing is that your synapses in your brain have to connect the dots to get the new limbs to work. Young children are prime candidates for prosthetics, since the common belief is that their brain is much more adept at these sort of things - learning how to walk, ride a bike, etc. The older you get, the harder it would be for you to gain additional senses/limbs because your brain has already culled much of the excess synapses to make it as efficient as possible (e.g. flexing your fore-finger. even though there's at several muscles per finger, you can't flex them individually... why would you do so, anyway?)
