394 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



most birds, including the pigeon, this hypothesis certainly cannot be 

 considered as satisfactory. 



Many authors advocate the view that the air-sacs are resonatory 

 organs for the purpose of increasing the strength of the voice. The 

 similarity in the structure of the air-sacs in the sound-producing 

 males and in the silent females shows the untenability of this view. 



The hypothesis advanced by Cuvier (1810), Meckel (1821), 

 Jacquemin (1842), Milne Edwards (1857), Owen (1866), and 

 Magnus (1869), tnat the air-sacs are directly respiratory in function, 

 has been refuted first by Sappey (1847, p. 48), and since by so many 

 others, that we only need to refer to what we have mentioned above, 

 viz., the slight development of the capillary system in the walls of the 

 air-sacs, in order to demonstrate the incorrectness of this idea. 



The statement made by Sappey (1846), Lendenfeld (1897), and 

 other authors that the specific gravity of birds is reduced by the air- 

 sacs contained in their bodies although opposed by Ficalbi (1884), 

 P. Schulz (1896), Madarasz (1899), anc l to some degree likewise by 

 Baer (1897), is of course incontrovertible. It is, however, question- 

 able whether such a reduction of specific gravity, accompanied as it 

 is by an increase of volume, can be of any use to the bird. It seems 

 to me that this could not be of any advantage to sailing birds like the 

 albatross, and that it would be directly disadvantageous to birds 

 like the pigeon that move their wings rapidly. A reduction of the 

 specific gravity obtained by inflating the body could only, it seems to 

 me, be of use to those ancestors of birds that were accustomed to 

 employ their wings as parachutes for effecting long jumps. 



Campanas (1875), Pagenstecher (1878), Bignous (1889), and 

 Soums (1896) consider the evaporation of water on their walls as 

 the most important function of the air-sacs, and Madarasz (1899) 

 has suggested that the moist walls of the air-sacs may functionally 

 replace the sweat-glands of mammals. Also Vescovi (1894) 

 holds this opinion and considers the air-sacs as organs assisting in 

 the regulation of the body temperature. I, for my part, do not be- 

 lieve that the air-sacs are a temperature-regulating apparatus, and 

 agree with Baer (1894) in the doubts he expresses concerning this 

 hypothesis. Considering the high body-temperature of birds, an 

 extensive cooling apparatus is surely unnecessary. 



Madarasz (1899) thinks that the air-sacs of birds are analogous 

 to the swimming bladders of fishes, and, like them, manometric 

 sense-organs comparable to aneroids. That the air-sacs should be 

 there to inform a bird of his height above sea-level seems to me, in 

 view of their irregularity and great extent, highly improbable. 



