FLIGHT. 235 



transparency of the bony walls. From week to week 

 the air-cells increase in size, till, towards the close of 

 the season, the air-bones become transparent. In all 

 these bones the marrow first disappears from the 

 vicinity of the opening' which admits the air, and con- 

 tinues longest at the points farther removed from this 

 opening-. Towards the close of the summer and 

 beginning of autumn, although in external appear- 

 ance the young goose resembles the parent, no trace 

 of air-cells can be discovered in its bones, the interior 

 of the bones being then filled with marrow. About 

 the fifth or sixth month the marrow begins to disap- 

 pear. This circumstance, which applies also to other 

 birds, shows with what caution one should form an 

 opinion, from young birds only, on the size of the air- 

 cells. In many kinds of birds the air-cells of some 

 bones are nearly fully developed, although, they have 

 the openings of the bones which lead to the air- 

 cells *." 



Not only the bones, but the quills of the feathers 

 also make a part of this contrivance. These while 

 growing are filled with an organized pulp ; but as 

 soon as they arrive at their full growth, this pulp 

 being absorbed renders them light, and the lightness 

 is increased by air from the atmosphere being intro- 

 duced into their cavity through a small opening at 

 the termination of the furrow where the quill or barrel 

 ends and the plumelets of the feather begin. Air is 

 also introduced in a similar manner into the plumelets 

 themselves. 



The existence of these cells can be shown upon 

 any bird by simply blowing with a little force into the 

 windpipe, by which means the belly may be blown 

 up to a considerable size, a circumstance which would 

 not occur in other animals. Experiments of this 

 kind were performed by Colonel Montagu on the 

 * Blumenbach, Comp. Anat. 182. 



2 A 



