In the chapter in Sacred Monsters entitled "Two-Headed Men and other Mutants," I quoted the following Gemara (which Daf Yomi studied recently):
Pelemo asked of Rebbi: A person with two heads, on which of them should he place tefillin? Rebbi replied: Either go into exile, or accept excommunication! (Talmud, Menachos 37a)
While I discussed this passage at length in the book, there's an important observation that I missed until now. But first, some background.
One of the most important new articles in the brain-death debate is that by Dr. Noam Stadlan (freely available here). In brief, the argument runs as follows: The presence of a human being can be logically and empirically demonstrated to be located in the brain, and in the brain alone. Any other view results in contradictions and impossibilities. All other organs can be transplanted, and maintained outside of a body, without any notion of their being “alive.” Respiration and circulation of blood can be artificially produced in a corpse! It is only the neurological activity of the brain which creates the presence of a person. And when babies are born with extra appendages or organs, it is only when they have two heads that they are considered to be two persons.
(As I see it, the logical consequences of this for brain death are as follows. We see that the presence of a person thus equates to the presence of a brain. For those who wish to speak about the soul, while it is a nebulous concept, it is certainly housed in the mind, and thus the brain. Accordingly, the irreversible loss of the brain’s fundamental functions means that the person has died.)
Now, previously I have noted that while in general Chazal's rulings hold authority even if based upon mistaken scientific beliefs (as with lice), this is not the case in matters relating to the preservation of life. Analyzing scientific errors in the Gemara in such cases is not merely of academic interest, but is also relevant to those who would seek to base contemporary halachah on such sugyas. And so, while Dr. Stadlan is absolutely correct to state that when babies are born with extra appendages or organs, they are considered to be two persons when they have two heads - it should be noted that Chazal did not view it that way!
In my monograph The Question of the Kidneys' Counsel, I showed how Chazal believed that significant components of the mind are located in the chest cavity - in the heart, kidneys, and other innards - rather than in the brain. Accordingly, dicephalus twins — conjoined twins with a single trunk and one head — were regarded as a single person with two heads. We see this in the way that the Gemara presents the question mentioned above. The Gemara discusses the question of upon which head such a person (described in the singular!) should place tefillin. We today, on the other hand, would say that these are two people, both of whom are obligated to wear tefillin. It's not a two-headed person - they are single-bodied people. If you have any doubts about that, just watch this documentary about the amazing Hensel twins (note: I saw the first few minutes, and the girls do not conform to Orthodox standards of tzniut!):
There is another example of how Midrashic statements about dicephalus twins reflect the ancient belief that the mind and personhood is housed in the body. In Sacred Monsters, I cited a (presumably non-historical) Midrash which describes King Solomon as performing a test to determine if such twins are one person or two; when he discovers that one blindfolded head can sense heat applied to the skin of the other head, he concludes that they are a single person. But in fact, in all cases of dicephalus twins (and there are many), there are two brains and thus two separate nervous systems; thus, one head would never be able to sense what is happening to the other head. Dicephalus twins are two people sharing a single body, not one person with two heads.
Thus, those who attempt to determine the status of brain death from the Gemara - something which I have previously explained to be in any case inherently impossible, since the Gemara could never have addressed such a scenario - are also hampered by the fact that the Gemara sees personhood as being embodied in the functioning trunk, rather than in the brain. But we today can clearly assert that personhood is exclusively limited to the brain.