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Re: breathing



I'd have to go back and look at comparative embryology, but I think I
remember the development of lung sacs after gill development.  I don't
remember lung sacs ever communicating with the outside of the body other
than via the mouth.  If the airways were to communicate via another orifice
other than the trachea/hypopharnyx/mouth, there would have to be another
glottis, etc to protect the airway and allow positive and negative
pressures.  Again this is off the top of my head so someone correct me if
I'm wrong.  Animals may have remnants of gill slits, being in the neck in
humans .  Various types of tracheoesophogeal fistulae occur.

Best,

Michael Teuton
----- Original Message -----
From: Patrick Norton <ptnorton@email.msn.com>
To: Dinolist(message) <dinosaur@usc.edu>
Sent: Sunday, April 11, 1999 7:21 PM
Subject: Re: breathing


> >Why does air exit back through the trachea instead of a unidirectional
tube
> evolutionarily created someplace else on the body?<
>
> That's the nub of my question.   Except that I might phrase it as why such
a
> unidirectional tube wasn't 'preserved' among terrestrial vertebrates
rather
> than why it wasn't 'created' within that group, since such a
unidirectional
> counter-current gas exchange system was the primitive condition in that
> group.