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THE AIR-SACS OF THE PIGEON 



By BRUNO MUULER 



The present investigation of the air-sacs of the pigeon was carried 

 out at the Zoological Laboratory of the German University at 

 Prague with the assistance of the Hodgkins Fund of the Smith- 

 sonian Institution. It was under the supervision of Prof. R. von 

 Lendenfeld, the director of that laboratory. I express my warmest 

 thanks for his active interest in this work, his kindness in placing the 

 necessary facilities at my disposal, and his valuable advice. 



Introduction 



Although the air-sacs of birds have been carefully investigated 

 by many authors and our knowledge of them is quite extensive, 

 none of the theories advanced concerning their function has 

 found universal acceptance. In this essay an attempt is made 

 to throw more light on this subject. The most reliable basis for 

 a solution of the problem seemed to me to be an exact knowledge 

 of the morphology of the air-sac system in some species of birds 

 known as good fliers and abundantly provided with these organs. 

 Such a bird is the pigeon, and this species appeared particularly 

 suitable because numerous specimens of it are readily obtainable, 

 a consideration of special importance, since, as was soon found, 

 the air-sacs are subject to very considerable variation in differ- 

 ent individuals of the same species, which makes it impossible 

 to describe these structures in any species with scientific exactitude 

 without studying them in a large number of specimens. But it was 

 not only this consideration that led me to select the pigeon as the 

 subject of my investigations. I was also led to make this choice 

 by the fact that most of the physiological studies on air-sacs have 

 been carried out with this species. Although much has been written 

 on the morphology of the air-sacs of the pigeon, the data given in 

 the literature on the subject are by no means complete and in several 

 respects inexact ; and there exist no correct and comprehensive 

 graphic representations of the air-sac system of this species. 



I have not included an historical review of the development of our 

 knowledge of the air-sacs, because such reviews are given in the 



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