Would be of course the brain. Our five senses (maybe even the "sixth sense" included) function as antennae; they receive signals from the physical world and convert the input data to electronic signals to be processed by the brain. The brain recieves, computes, releases the appropriate chemical messages to the nervous system, and thereby interacts accordingly with the physical realm (through the hands, feet, wings, mouth, etc.). Input begets output.
As the physical world responds in a manner required by the laws of physics, that same output becomes feedback. Output begets input.
So goes the story of consciousness. It's a feedback loop.
Now somewhere in all this machinery is a chemical unlike any other; it completely shatters one's accepted notion of reality. Although it would be unfair to call it a foreign toxin (afterall it's as much a part of the brain as adrenaline, dopamine, testosterone, electricity or DNA molecules), it does have uncommon effects on one's brain, hence data gathering and response mechanisms, hence conscious reality.
So I believe it becomes a question of how much clout you would like to give the brain in its relationship to "ultimate reality" (whatever that may be). Is our life all there is to reality? Or is there a greater reality, the brain inherently not being capable of comprehension? Is the brain a clumsy organ in the skulls of beasts, which has no more significant function than keeping these beasts alive long enough to reproduce; reality being somewhere "out there" and not for us to know who, how or why?
Whatever the function of the brain, DMT is an organic molecule just like DNA, or RNA, or..., and it has as much a functional relationship with life as any other emotion inducing chemical messenger found in the brain-If reality can be "found in the brain", DMT plays as much a role as the other chemicals. It is as much a piece of our reality as sight, smell, happiness, or fear. So must be the spirits we see right before we die. Life is ultimately what a consciousness sees, feels, hears, thinks, tastes, and touches. Any experience is as real as any other... (???)
What do you think, Mike?
P.S. Animal brains produce and secrete DMT.
That means they too have an NDE (near death experience) and go to their own perception of heaven or hell upon the rapid random firing of dying neurons and cessation of brain activity. Recollected images and memories are recalled as all the gathered data throughout the animals lifetime is effectively "dumped". Pretty interesting... Do all dogs go to heaven? (just kidding)
P.P.S. Some people have described seeing relatives in heaven who have not died yet. Does this deal a serious blow to the notion of there being a "real" heaven or hell? Perhaps "heaven" and "hell" are just dying memory cells...
In that case, the brain may ultimately be all there is to life and afterlife after all, in which case consciousness (a.k.a. soul) ceases to exist after the decomposition of brain tissue. Some people refer to this as eternal darkness or blackness. Funny, I don't recall darkness before I was born. In fact, I don't recall anything ... NOTHING! It's not like you have to live with the miserable thought of, "damn, I'm going to be dead and in this shady blackness for the rest of eternity." Without the brain, there is no thought, no regrets, no misery, no peace and quiet, eternal rest, blackness...
Whatever year you were born, go back to the year before that and tell me what it felt like...