The simple answer is that the connection between the brain and the body is unique to every individual. We cannot replicate or hope to understand all of the connections coming out of the brain into the rest of the body. Longerish: The brain is made up of millions, if not billions, of neurons. Each of these neurons serves some purpose in some aspect of our physical and mental lives. The main connection between the brain and the rest of the body is the central nervous system. The central nervous system is itself made up of hundreds of thousands of neurons, each of which goes to a specific part of the body to perform specific tasks. Some of these neurons originate deep inside of the brain and travel the entire length of your body (all the way down to your toes) to facilitate responsive movement and sensation. Transplanting the brain would require transplanting these neurons as well as the brain. We are no longer talking about simply cracking open one skull and putting a brain in it, now we need to perform surgery over the entire body. Technicalities: Even if we were to discover a way to categorize every neuron in the central nervous system of the host body, and now supposing that we knew all of the corresponding neurons for the donor's brain (if they even existed) and were able to match them up accordingly in some fashion; it still wouldn't work. The host body would have developed its CNS differently than the donor body had, and the brain's neurons would fire (activate) at incorrect times and with the incorrect magnitude(power) to result in the correct function of the host body. Theory: All of the above is supposing that we don't have to worry about connecting blood vessels and all we have to worry about is the neurons themselves. Supposing that we could connect the blood vessels (and such) appropriately to allow the donor brain to live, it is, in theory, possible to maintain the brain in a "living" state. This however would be a brain without any connection to the rest of the world. There would be no sensory input, and there would be no output. Given these conditions, I don't know how long the brain would survive without deteriorating. It would also be incredibly difficult to tell if the brain still had any form of consciousness. It might be possible to use some brain imaging techniques to prove cognition, but this is really stretching the frontier of technology and science... Providing we could do it in the first place.