miJller] THE AIR-SACS 0E THE PIGEON 403 



and consequently absorb oxygen and give off carbonic acid gas. 

 Even if there were a marked difference of air-pressure at the times 

 of inspiration and expiration in these spaces, the renewal of the air 

 in them could only be effected very gradually on account of the un- 

 yielding character of their walls. The carbonic acid gas has, there- 

 fore, much time to collect in these spaces, and yet but little such gas 

 can be demonstrated there. This shows how slight the blood oxida- 

 tion is in pneumatic bones, and, further, that the function of the air- 

 spaces in these bones cannot be a respiratory one. 



Sappey (1846) remarked in this connection that the penetration 

 of processes from air-sacs into bones enlarges their sectional area, 

 and thereby increases their power of resistance without increasing 

 their weight. Bergmann and Leuckart (1852, p. 235) ascribe a 

 great significance to the reduction of specific gravity imparted to the 

 skeleton by the pneumaticity of bones. Strasser (1877, p. 223) has 

 treated this subject most thoroughly, and has succeeded in showing 

 in the clearest manner that the utility of the pneumaticity of bones 

 is purely a mechanical one. Most recent authors have accepted this 

 interpretation; only P. Schulz (1896) holds a somewhat different 

 view. Taking into consideration those birds of good flight, which, 

 like the terns, possess no pneumatized bones, he thinks that the 

 mechanical advantages of the pneumaticity cannot be so great as is 

 assumed by many authors. 



Conclusions 



I do not consider the air-sacs, including the air-cavities of bones, 

 as organs having a positive and special function, but rather as a 

 system of empty interspaces. Their value lies in their emptiness — 

 that is, in their containing nothing that offers resistance or has an 

 appreciable weight. 



Flying is the highest form of locomotion, and as such only possible 

 to a body of high mechanical efficiency. Our most effective machines 

 are by no means compact and solid, but composed of parts as strong 

 as possible in themselves and arranged in the most appropriate man- 

 ner. The interspaces between the parts are left empty and taken 

 up by air. 



The Sauropsida, at the time they obtained the power of flight, 

 became adapted to its mechanical requirements, and thereby similar 

 to the efficient machines mentioned above ; they divested themselves 

 of all superfluous material, filling the body-spaces thus obtained with 

 air-sacs. While the body-wall, adapting itself to the mechanical 

 requirement, became a compact, hollow cylinder serving as a support 



