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Re: Unidirectional [...] Science



 In addition to those observations previously mentioned: as any
 free-diver knows, skeletal movements are not constrained by the
 pressures encountered while diving (up to 19000 kg/m2, in personal
 experience).

Try moving your ribs under such pressure. That, you see, is the point.

_Of course_ you can move your arms around! The pressure on each side is the same, so drag is all you need to worry about! That doesn't tell you anything about ribcage expansion against the water pressure! <headshake>

 Were it anatomically possible for me to aerate my lungs to any degree
 by manipulating my skeleton, as opposed to being completely dependant
 on the diaphragm system, an increase in my current ability to
 overcome external pressure while "inhaling" would be a natural
 consequence. It is important to understand that in mammals, the lungs
 must be actively expanded by muscle power alone, and contraction
 occurs by relaxation (i.e., deflation is the relaxed state).

Wrong and wrong, respectively.

- You don't exclusively use the diaphragm system, you additionally expand the chest for inhaling. Have you never noticed!?! In fact, when I just sit, i can keep up using _only_ the ribcage for about half a minute. - Both expansion and contraction can be powered in mammals. During strenuous activity you do both. When I just sit here, I can exhale more deeply than normal by contracting the belly or even the ribcage (or of course both); the subsequent expansion occurs by elastic recoil alone. Try it yourself.

 In birds, [...] the relaxed state is _between_ full deflation and
 full inflation.

In me, too. I just don't deflate (or inflate) fully when I don't need to.

 In other words, the elasticity of the skeleton itself can play a role
 in inspiration under pressure.

(Not the skeleton, but the muscles, and maybe various ligaments. If you count the ligaments as part of the skeleton...)

 When the ribs are in a neutral
 position relative to ambient pressure, (further) expiration can be
 actively caused by 'contraction' of the sternum and ribs. When the
 driving muscles are relaxed, skeletal elasticity can return the ribs
 to neutral position, which in itself creates some inspiration, even
 in the extreme case where external pressure cannot be overcome by the
 muscles driving the skeleton.

And what makes you think this recoil is necessarily stronger than the pressure exerted by 2 m of water?

 Further, it is not possible to aerate "dead-end" or "tidal" lungs by
 small steps. You either can fill your lungs adequately w/ one breath,
 or you can't. The same is apparently NOT true of the uni-directional
 system, the lungs of which do not deflate, and can be aerated by
 small, sequential additions of air.

The lungs proper don't deflate much in a unidirectional system, but the air sacs that ventilate the lungs do. And very drastically so. Birds take much deeper breaths, and much fewer of them, than mammals of the same size.

 These additions can be accomplished in part by manipulation of the
 legs,

Only in turtles, where the ribcage and the belly cannot expand and the lungs are ventilated by modified leg muscles.

 and in the case of a quadruped, quite likely the back plays a
 role.

That's what I do when I lie on my belly: for inhalation, the ribs move dorsalwards, and the vertebral column appears do arch dorsalwards in the thoracic region and consequently ventralwards in the lumbar region. (Hm... we should reintroduce "dorsad" and "ventrad".)

I don't seem to ever breathe by diaphragm action alone. Perhaps the diaphragm cannot ventilate the cranialmost parts of the lungs. I just tried; I can keep it up for perhaps a minute while sitting, but not longer.

More drastically, when I sit, I can breathe by using _neither_ the diaphragm mechanism _nor_ the ribcage, but by straightening up and letting myself fall back into the usual crouched sitting position. This is, however, exhausting enough that I can't quite keep it up for more than about half a minute.

 In hindsight, I now see that ridiculous
 neck in itself as prime evidence of avian-style respiration.

It has long been argued that breathing through the neck of *Mamenchisaurus* is only possible with an air sac system, the one already indicated by the pneumatic vertebrae.

 Long post, chop likely, will post repair for those that care...

Strangely, that didn't happen.