Re: Ornithodira, breathing with long necks

["Jerry D. Harris" <dinogami@hotmail.com>]

> 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.

  I'm unaware of any detailed anatomical study of the cervical air sac 
diverticula in birds, so I have no idea if they have any sort of one-way 
valves (that isn't to say such a study doesn't exist; just that I'm unaware 
of it!).  The standard model of bird breathing is as follows a nice diagram 
of this can be found in Proctor & Lynch's _Manual of Ornithology_, p. 213):

FIRST INHALATION: air passes down the trachea and into the abdominal air 
sacs.

FIRST EXHALATION: air pases from abdominal air sacs into lungs.

SECOND INHALATION: air passes from lungs into cranially-located air sacs 
(including the clavicular and, I presume, cervical sacs)*.

SECOND EXHALATION: air passes from cranially-located air sacs back out the 
trachea.

There are problems with this model, although again, no paper to my knowledge 
has done much further testing.  King (1966) (q.v. McLelland, 1989) follows 
with this:

"[The cervical air sacs] make contact with the lung laterally...The 
diverticula arise from the main chambers. They consist of two series of 
longitudinal tubes...They reach as far cranially as the third cervical 
vertebra. The lateral and dorsal tubes are connected at each intervertebral 
space by a transverse tube. The dorsal tube may extend caudally through the 
thoracic nerual canal...in [the pigeon], the main chamber of the cervical 
sac communicates directly with the clavicular sac..."

  At the point denoted above with the * during the second inhalation is, I 
assume, when air is pushed into the cervical air sac and its series of 
diverticula along the cervical vertebrae. Avian inhalation/exhalation seems 
to be driven by costal and sternal muscle and bone expansion/contraction 
(compressing, as has been noted, the air sacs, and _not_ the lungs [e.g., 
McLelland, 1989]!) -- I would guess that if the diameter of the diverticula 
"tubes" is narrow enough, then air forced in may have a pretty high velocity 
and fill the sacs fairly quickly, keeping them nicely pressurized.  How air 
_stays_ in there during the negative-pressure exhalation cycle that follows 
the second inhalation is beyond me, however -- like you, I would assume that 
there is at least one series of one-way flaps somewhere to prevent air from 
rushing out until the bird wants to do so (consciously or unconsciously), 
but I cannot find this actually stated anywhere. If someone knows of a 
reference for this, please speak up!

King, A.S. 1966. Structural and functional aspects of the avian lungs and 
air sacs, pp. 171-267 in Felts, W.J.L. and Harrison, R.J. (eds.) 
International Review of General and Experimental Zoology Volume 2. New York: 
Academic Press.

McLelland, J. 1989. Anatomy of the lungs and air sacs, pp. 221-279 in King, 
A.S. and McLelland, J. (eds.) Form and Function in Birds, Volume 4. London: 
Academic Press.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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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