Re: Ornithodira, breathing with long necks

["James Norton" <JNorton@une.edu>]

Again,  what would be the mechanism for pressurizing this type of air
sac?  Forceful expiratory efforts with mouth and nostrils closed might
be able to inflate such sacs, but without regular re-inflation the gas
would be absorbed and the sacs would collapse.  There would have to be
one-way valves at the entrances to the sacs, allowing forceful
expiratory efforts to fill them but trapping the air in them with
inspiration, when tracheal and air sac pressures fall below
atmospheric.

JMN



>>> "Jerry D. Harris" <dinogami@hotmail.com> 04/13/2001 2:37:41 p.m.
>>>
>However, I'm not familiar with the term 'passive
>stay devices', so I don't know if we're talking about the same thing. 

>Please
>elaborate a bit on 'passive stay devices'.

   To the extent of my knowledge, it means (broadly, and in my own
words) "a 
means of maintaining a supportive stiffness of a body part without (or
with 
absolute minimal) muscular effort."  The standard example is the horse

hindlimb: the patella can slide onto a special facet so that the
hindlimb 
cannot bend at the knee or the ankle -- a somewhat complex arrangement
of 
various ligaments and tendons ensure this.  Thus, the horse can rest
its 
weight on the hindlimbs without having to use muscles to prevent the
joints 
from bending (thus tiring the animal).

   In the avian wing, inflating the air sacs provides a means of
keeping the 
limb extended, again without muscular effort; my reading of what
Akersten 
proposes is that the parallel cervical air sac system of both birds and

sauropods acts the same way.  I tend to think of it in this (very 
simplistic!) way: imagine you have a series of models (made of
something 
like balsa wood) of articulated cervical vertebrae.  You thread one of
those 
kid's-party-style, long, hot-dog-shaped balloons through the transverse

foramina on either side of the column.  When you inflate each balloon,
it 
becomes rigid with air pressure and rises to a horizontal (or so)
position; 
when both are inflated simultaneously, they turgidly hold the cervical

vertebral column out horizontally with it.  Thus, simply inflating the

balloons holds the vertebrae horizontally; no muscles are needed -- the
neck 
is held out horizontally (as in, for example, flight) passively, not 
actively.

   Of course, it's really more complicated than this, especially in 
sauropods, because each vertebra, plus all the muscle, is really quite
heavy 
-- I am curious as to the nature of the proposed air sac membrane to
support 
all that weight (as opposed to, for example, balsa wood!), particularly

since the membrane in chickens is really, really thin, flimsy, and
easily 
punctured! (As an aside, this may be a problem for any sauropod
sustaining a 
bad neck injury -- if the air sacs are punctured, the entire neck
support 
system might collapse!)  That is to say, I don't know what the maximum

weight that an air sac system _alone_ could support, but it certainly
has 
something to do with the overall mass of the neck, the nature of the
air sac 
membrane, and the kinds of pressures normally maintained in said sacs,

which, in turn, speaks to the ability to get air in and out of the
system!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Dept of Earth & Environmental Science
University of Pennsylvania
240 S 33rd St
Philadelphia PA  19104-6316
Phone: (215) 573-8373
Fax: (215) 898-0964
E-mail: jdharris@sas.upenn.edu 
and     dinogami@hotmail.com 
http://www.sas.upenn.edu/~jdharris 

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