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Please pardon the note from an amateur but I find Patrick Norton's
question quite interesting: Why don't terrestrial vertebrates have a
unidirectional flow respiratory system?  I believe the key is
ventilation.  As noted, gills on land would not work because of drying
and thus would need to be internalized. Exposing your 90 square meters
of repiratory surface to air by externalizing your lungs would cause
enough evaporative fluid loss to kill you quicker than the time it takes
a beached fish to stop flopping about.  Internalizing the repiratory
structures (and having turbinates) would keep inspired air warm and
moist (warm air holds more moisture than cool air) and moisture allows
the oxygen to dissolve (biologic processes need to use dissolved oxygen
rather than directly using gaseous oxygen).  To get the air to these
internal structures requires a ventilatory process--fish swim with their
mouths open creating a respiratory current known as ram ventilation.
When stationary they rely on buccal pumping.  These would be difficult
for terrestial animals to do (respiration by running with your mouth
open), and would require a tremendous amount of energy.  Thus the
evolution of various varieties of pump systems.  Why does air exit back
through the trachea instead of a unidirectional tube evolutionarily
created someplace else on the body?  I suspect:  a)because it works,
b)another tube would interfer with creating the negative pressure needed
for inspiration--it would need some type of valve c)another tube would
be associated with more drying and be another portal for respiratory
infections d)evolutionarily it may not be possible (retinas would work
much better if the blood vessels in the eye were behind them rather than
in front--but that is our evolutionary legacy).

At the risk of being over-verbose please allow me to pose the following
question:  With regard to
the digit controversy and bird evolution, in humans the first metacarpal
is different from the others in that it has a growth plate at the end
closest to the wrist, as do all the phalanges.  The other metacarpals
have a growth plate at the opposite end.  Indeed it has been suggested
that the reason thumbs have only two phalanges is that the metacarpal is
actually a modified phalange (and perhaps the trapezium is actually a
modified metacarpal).  I expect this reflects an ancient vertebrate
condition.  I know that birds do not have mammalian growth plates but is
there any way of telling if bird embryos metacarpals have different
growth sites?  (The closest I've come to examining bird embryos is
egging my neighbor's house in junior high school).  Being a good amateur
I read my The Complete Dinosaur and found on page 408 that birds can
develop a disease called condodystrophy where their epiphyses will
calcify like mammals. Could this information be used to tell if bird
digits are I, II, III, or II, III, IV?  Does anybody know what the hell
I'm talking about?  Thanks for your time--Ken Clay. M.D.