From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: bionet.general,sci.med,alt.med.veterinary
Subject: Re: What happens when an animal is born with two heads?
Date: 8 Jun 1999 07:33:13 GMT

In <375C9B84.9FBC1965@cs.uoregon.edu> Bret Wood
<bretwood@cs.uoregon.edu> writes:

>Hillary Gorman wrote:
>
>> Well, like I said - it's not one body with two heads. It is two fused
>> bodies with two heads. Please do not misinterpret this, it's an
>> important biological difference.
>
>I understand the distinction you are making here, but I don't understand
>how you can justify it.  If there is one heart, one liver, two arms, two
>legs, two lungs, etc, then I only count one body there. Obviously, things
>have to start being duplicated _somewhere_ below the neck, but there have
>definitely been cases where from approximately the sternum down, it was
>just one body, AFAIK.  And in the case of twins which don't completely
>separate, were they ever _completely_ separated, and then rejoined?  If
>not, then I fail to see how you can call them two fused bodies, rather
>than one body with duplicate organs.
>
>> In the case of the human twins I mentioned, they
>> have separate brains but closely integrated nervous systems.
>
>Do their two spinal cords eventually merge into a single one at a
>certain point, or are there two seperate, yet interrelated nervous
>systems?
>
>> They can engage in highly coordinated activities like running, riding a
>> bicycle, that kind of thing- even though there are two separate brains
>> in charge of the four limbs! But they have to cooperate with respect to
>> deciding what to do, when - BOTH must agree that it's time to run,
>> rather than to sleep, or whatever.
>
>If one is asleep, and the other is awake, can the one control all four
>limbs, or just two?


  One head has control of, and feeling in, one half the body, and the
other head has control and feeling in the other.  Simple as that.
Right head cannot feel or move left hand, and vice versa.  Any stuff
like walking is simply learned coordinated behavior.  If you took two
children and tired them together by the middle legs, as in a three
legged race, and then tied those legs UP so they couldn't contact the
ground, the children would eventually learn to walk, after a fashion.
After a lifetime of it they'd get so good you could hardly tell it from
anybody else's.  That's what's happened with the conjoined twins who
each are missing half a body from about the neck down (there are
separate spines and spinal cords).