MULLER] XHE AIR-SACS OF THE PIGEON 4OI 



The increase of mobility of the internal organs, especially the 

 heart, clue to the air-sacs, is of especial importance on account of 

 the rigidity of the wall of the thorax. Since in birds the pericardium 

 and the amount of pericardial fluid contained in it are small, the 

 heart, if there were no air-sacs around it, could hardly move at all. 

 In birds the heart is relatively much larger, and beats with greater 

 energy and rapidity than in mammals. This energetic heart- 

 movement is necessary for the production of the power required for 

 flight, and it is made possible by the diverticula of the saccus inter- 

 clavicularis which surround it. The movements of the gizzard, the 

 peristaltic action of the intestine and the oviduct are also made easier 

 by the portions of air-sacs that surround these parts. The pneu- 

 maticity reduces the friction to a minimum, whereby a considerable 

 economy of labor is effected. 



Merrem and Perrault suggested that the alternation of positive 

 and negative pressure in the air-sacs may help to mix the contents 

 of the intestines, and so assist digestion. I cannot share this view. 



Sappey (1846) considered the air-sac system as an apparatus for 

 equilibration. I do not. think, however, that they can act as such, 

 for Lendenfeld has shown (1897) that the shifting of the center of 

 gravity occasioned by the alternate filling and emptying of the sacs 

 cannot be great. A balancing of the body during flight by the filling 

 of single air-sacs seems to me neither useful nor possible. Slight 

 alterations in the position of the head or the extremities are in this 

 respect much more effective than a shifting of the internal organs, 

 the former parts being so much farther away from the center of 

 gravity than the latter. 



Hunter (1774) thought that the air-spaces in the bones acted as 

 air-reservoirs, and recently Grober (1899) and Madarasz (1899) 

 have propounded the view that the storing up of air that may be 

 required for respiration during flight forms a special function of the 

 air-sacs. Baer entertains a similar opinion. He believes that the bird 

 breathes very differently during flight than when at rest, and that 

 the whole air-sac system, the complete aeration of which, during 

 repose, he has sought to prove both theoretically and experimentally, 

 acts during the most strenuous corporeal efforts of the bird when 

 flying merely as an air-reservoir. He says (1896, p. 487) : "Nach 

 alien diesen Erwagungen komme ich zu dem Schlusse, dass die 

 Durchluftung des Atemapparates wahrend des Fluges in ganz 

 anderer Weise erfolgen muss, als in der Ruhe oder bei der Bewegung 

 auf festem Boden und es darf fuglich angenommen werden, dass 

 besondere Atembewegungen neben den Flugelbewegungen nicht 



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