JUULLER] THE AIR-SACS OF THE PIGEON 36/ 



abdominal cavities ; the crop is pushed forward and the internal 

 organs, especially the intestines, are compressed. The principal 

 expansible parts of the air-sac system appear in such preparations 

 much too large, the secondary diverticula which are less expansible 

 much too small. The high pressure usually bursts the thin walls of 

 the air-sacs in one or two places, causing effusions of the injected 

 material, which may lead to erroneous deductions. I am inclined to 

 suspect that the saccus subpectoralis described by Baer (1896, p. 

 437) owes its origin, or at least its great size, to a rupture of this 

 sort. This structure is also seen in my injected specimens, but it is 

 an entirely artificial product, arising as an effusion from a saccula- 

 tion of the diverticulum axillare. Similar effusions are often ob- 

 served arising from the diverticula that surround the heart. 



A better method than the direct injection of a setting mass is the 

 following: A pigeon killed with chloroform or ether is placed in 

 formol, and for a period varying from some days up to two weeks 

 strong formol is also injected with gentle pressure through a canula 

 into the trachea, and thence into the air-sacs. The apparatus I used 

 for this purpose is very simple. I connected a water reservoir with 

 a wash bottle 1.5 m. below it. This produced in the wash bottle a 

 constant and gentle pressure, which was conveyed to a second bottle 

 containing formol, which was thus forced through rubber tubing and 

 a canula into the trachea, and thence into the air-sacs. Bv this 

 method the air-sacs are hardened in a moderately distended condi- 

 tion. As soon as the hardening is complete, suitable openings can be 

 cut without causing any change in the shape of the air-sacs through 

 collapse, and they can then be satisfactorily injected. 



For the injections I used paraffine and gelatine (ten per cent with 

 Berlin blue or carmine). The large air-sacs could easily be com- 

 pletely filled with either material ; the smaller diverticula, however, 

 could only be filled with the gelatine and by changing the position of 

 the pigeon many times and using massage during the injection. 

 Even in this way I could not inject all parts of the air-sac system at 

 once, but I succeeded in filling all the parts of the system in different 

 specimens, so that it was possible, by combining the results from a 

 number of pigeons treated in this manner, to obtain a clear concep- 

 tion of their distribution, shape, and size. 



The air cavities in the bones can easily be made visible bv an in- 

 jection of glycerine and water with much Berlin blue. The animal 

 was distended with the injected fluid for about 12 hours, during 

 which time it was kept warm, often changed in position, shaken and 

 massaged, and after that it was hardened in alcohol. The pneumatic 



