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bears some kind of proportion to the size of the 

 bone, or at least to the extent of the internal 

 cells, so that large birds, and large bones, have 

 much larger openings than small ones. With 

 respect to the internal air cells, great differences 

 exist. It is known that the air bones in young 

 birds are filled with marrow, which becomes gra- 

 dually absorbed, to make room for the admission 

 of air. This gradual expansion of the air cells 

 and absorption of the marrow, can no where be 

 observed so well as in young tame geese, when 

 killed at different periods of the autumn and 

 winter. The limits of the air cells may be clear- 

 ly seen from without, by the transparency of the 

 bones. From week to week the air cells in- 

 crease in size, till, towards the end of the season, 

 the bones become entirely transparent. In all 

 these bones, the marrow disappears first from 

 the vicinity of the opening which admits the air, 

 while it continues longest at the points farthest 

 removed from it. Towards the close of the sum- 

 mer and beginning of autumn, although in exter- 

 nal appearance the young goose resembles the 

 parent, no traces of air cells can be discovered 

 in the bones, the interior of them being then 

 filled with marrow., which does not begin to dis- 

 appear until the fifth or sixth month. Whether 

 birds possess the power of voluntarily letting out 

 the air, so as to render themselves specifically 

 lighter, has not yet been determined. * 



Let us now enquire how birds, endowed with 



* Lancet, 1828. 



