The Power of Flight Possessed by Birds 79 



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 remark- 

 able 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 ex- 

 pansion 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 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. 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 exist- 

 ence 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." 

 Some remarks by Sir Charles Bell on the subject of 

 buoyancy are well worth reproducing. " First," he says, 

 1 ' it is necessary that birds, as they are buoyed in the air, 



