MULLEEJ THE AIR-SACS OF THE PIGEON 395 



Hunter (1774) and Owen (1886, p. 217) have advanced the view 

 that the air-sacs (when inflated) serve to fix the wings in the ex- 

 tended position. The latter says (1. c.) concerning this: "A fourth 

 use of the air-receptaclCs relates to the mechanical assistance which 

 they afford to the muscles of the wings. This was suggested by ob- 

 serving that an inflation of the air-cells in a gigantic crane (Ciconia 

 argala) was followed by an extension of the wings, as the air found 

 its way along the brachial and antibrachial cells. In large birds, 

 therefore, which, like the Argala, hover with a sailing motion for a 

 long-continued period in the upper regions of the air, the muscular 

 exertion of keeping the wings outstretched will be lessened by the 

 tendency of the distended air-cells to maintain that condition." To 

 this I must remark that a soaring bird holds its wings horizontally 

 without muscular effort, and hangs from them, so that a special 

 apparatus for their fixation in this position is unnecessary. 



Most recent authors consider the air-sac system as an apparatus 

 serving as a mechanical adjunct to the respiratory system. This 

 theory that they are bellows appears at first sight to have much in its 

 favor. Even Harvey (1651), who was the first to carefully describe 

 the air-sacs, entertained this opinion. This hypothesis was fur- 

 ther developed by Perrault (1666), who pointed out that the respira- 

 tory changes in the volume of the thorax must cause a change 

 of the air in the air-sacs. He thought these movements were of 

 such a nature as to cause an antagonism between the sacci inter- 

 medii and the sacci abdominales. According to him the air passes 

 from the sacci abdominales into the sacci intermedii during inspira- 

 tion ; and conversely, from the sacci intermedii to the sacci abdom- 

 inales during expiration. Girardi (1784), and especially Sappey 

 (1847) and Siefert (1896), have warmly supported this theory. 

 Campana (1875) was a ' so * this opinion, but he thought that extra- 

 thoracic sacs (portions of sacs) were compressed by the circumjacent 

 muscles. In 1816 Fuld disputed Perrault's theory of the alternate 

 inflation of the air-sacs, and his views were supported by Roche 

 (1891), Soum (1896), and others. Baer (1896, p. 477) concludes, 

 from anatomical considerations and physiological experiments, that 

 there is no antagonism of the kind suggested by Perrault in the 

 movements (expansions) of the various groups of air-sacs, and he 

 thinks it far more probable that all air-sacs are enlarged during in- 

 spiration, and, conversely, all contracted during expiration. I cannot 

 wholly agree to this, as I do not believe that the cervical sacs and 

 extra-thoracic diverticula of other sacs enlarge and contract during 

 respiration at all. Baer, indeed, paid no attention to the extra- 



