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Re: Unidirectional [...] Science



--- On Fri, 1/15/10, Mike Taylor <mike@indexdata.com> wrote:

> > J Exp Biol. 2001 Oct;204(Pt 20):3581-6.
> > The influence of locomotion on air-sac pressures in
> little penguins.
> > Boggs DF, Baudinette RV, Frappell PB, Butler PJ.
> >
> > "... There was no abdominal muscle activity associated
> with swimming or exhalation, but the abdominal muscles were
> active with the step cycle in walking penguins, and
> oscillations in posterior air-sac pressure (0.08+/-0.038
> kPa) occurred with steps."
> 
> This is an interesting finding.  But given that one
> atmosphere is 101
> KPa, the penguins' pressure fluctuations amount to less
> than one
> thousandth of an atmosphere -- which is not going to put a
> big dent in
> the problems of breathing two meters under water.

Sure it will -- The penguins in the study are relatively small, and larger 
fluctuations seem logical as body size increases drastically.

More to the point -- a balloon that is mechanically and cyclically altered in 
volume by a lever at ambient pressure will continue to be altered in volume 
when under pressure _unless_ the mechanical advantage of the lever is 
overwhelmed. 

I suppose one might claim that the abdomen of a sauropod could not return to 
the beginning of the avian-style breath cycle when under 1 atm pressure. I note 
however, that the active portion of the breath-cycle is apparently EXHALATION 
(muscle contraction), not inhalation, and exhalation is obviously not impeded 
by external pressure. Inhalation in birds is apparently largely accomplished by 
'elastic rebound' of the abdominal cavity. As external pressure increases, the 
abdomen compresses, but is 1 atm pressure enough to eliminate all elastic 
rebound in a sauropod abdomen? That seems unlikely in itself to me.

But then to further claim that the legs themselves would overwhelmed by the 
task of expanding the (e.g.) posterior air sac in the presence of 1 atm 
pressure leaves the realm of common sense, in my opinion. Given that the 
attachment of the posterior airsac to the actuating levers (
t the "balloon" would be ruptured by the "lever"...