MULLER] THE AIR-SACS OF THE PIGEON 399 



schielung der Muskeleinheiten na'ch der Seite der giinstigeren Wirk- 

 ung bin." I not only share these views of Strasser concerning the 

 extra-thoracic air-sacs, but helieve that the intra-thoracic air-sacs also 

 serve such a mechanical purpose. The mechanical advantages which 

 the intra-thoracic air-sacs render are: ist, an increase in the size of 

 the thorax without an increase in weight ; and, 2d, an increase in the 

 freedom of movement of the organs contained in it, especially of the 

 heart. 



The advantage of an increase in size of the thorax without in- 

 crease of weight is clear enough. The strength of hollow cylinders 

 differing from each other in diameter only is proportional to their 

 transverse diameter. The same is the case with other similarly 

 shaped bodies not exactly cylindrical in shape. By the coalescence of 

 the dorsal vertebras, the paratangential extension and coalescence of 

 the bones of the pelvis, the clenching of the ribs by means of the un- 

 cinate processes, the development of the sterno-costal bones, and 

 finally by the special structure of the sternum and coracoid, the sup- 

 port of the wall of the thorax of the bird has attained the shape of 

 such a hollow cylinder. The above-mentioned mechanical law there- 

 fore also applies to it. The same laws of structure which, as Schwen- 

 dener found everywhere governing the structure of the elastic por- 

 tions of plants, and which likewise invariably regulate the structure 

 of the bones in animals, control the configuration of the avian thorax, 

 where they find an expression in the development of the intra- 

 thoracic air-spaces. The same law also affects the organization of 

 flying insects, especially those which are good fliers, for such in- 

 sects also possess air-sacs. It even applies to the flying fish, which 

 have very much larger swimming bladders than other fishes. 

 That this extension of the thorax, without increase of weight, by 

 means of the interpolation of air-spaces, which occurs in such widely 

 separated animal groups, essentially increases the mechanical apti- 

 tude for flight cannot be doubted by any one acquainted with the laws 

 of mechanics. 



There is yet another circumstance greatly in favor of the view that 

 the function and raison d'etre of the air-sacs are mechanical and not 

 respiratory. This circumstance is their asymmetry, which, strangely 

 enough, has not been noticed in this connection by any previous 

 author. Baer, whom we must regard as the principal exponent of 

 the respiratory bellows-theory of the air-sacs, has worked out a 

 scheme, based on exhaustive studies and experiments, on the enlarge- 

 ment of the body-cavity of birds during respiration, which gives a 

 clear and accurate account of the respiratory mechanism. This 



