70 THE BOOK OF MIGRATORY BIRDS 



occupy a considerable space of the chest and belly. These 

 cells are much divided by partitions, furnished, as has been 

 observed in large birds, with muscular fibres, supposed to 

 be employed in sending the air back to the lungs, as is 

 done by the diaphragm in other animals, which is wanting 

 in birds. This is no doubt the reason why birds appear 

 to pant so much in breathing, a much greater portion of 

 the body being always put in motion than in quadrupeds. 

 Besides these air-cells there are others situated in the bones 

 themselves, particularly the larger bones, both those 

 which are cylindrical and those which are broad and 

 angular. It is not a little remarkable that all these bones 

 in birds are destitute of marrow at least, in the middle. 



"The air-bones in young birds" are described as "filled 

 with marrow, which becomes gradually absorbed to make 

 room for the admission of air. This gradual expansion of 

 the air-cells and absorption of the marrow can nowhere be 

 observed so well as in the young tame geese when killed 

 in different periods of autumn and winter. The limits to 

 the air-cells may be clearly seen from without by the trans- 

 parency 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. Towards the close of the summer and be- 

 ginning of autumn, although in external appearance the 

 young goose resembles the parent, no trace of air-cells can 

 be discovered in the bones, the interior of the bones being 

 then filled with marrow. About the fifth or sixth month 

 the marrow begins to disappear. Not only the bones but 

 the quills of the feathers also make a part of this con- 

 trivance. These, while growing, are filled with an 

 organised 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 

 introduced 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. The existence 

 of these cells can be shown upon any bird by simply 



