ri2 THE STRUCTURE AND LIFE OF BIRDS chap. 



Thus, if in the Sea Eagle's humerus length were 

 proportioned to girth, the bone would be more than 

 half an inch longer ; on the same principle the aggre- 

 gate length of the wing bones would be greater by more 

 than one and a half inch ; the Hornbill's wing would 

 be lengthened by more than eight inches, its humerus 

 by more than three ! If now we take a fine saw and 

 cut the humeri of the Skua and the Sea Eagle from 

 end to end, we shall find that the walls of the latter 

 arc not thicker in proportion to the greater girth of 

 the bone. The larger bone, compared with the small 

 one, has a girth two thirds as great again, a thickness of 

 wall only one third as great again. 1 We can now see 

 why small birds have so little aeration. In their case, 

 there would be no great reduction of weight since the 

 exterior shell of the bones forms a great part of their 

 bulk. In the case of a larger bird, with bones many 

 times multiplied in size, but the thickness of the walls 

 increased comparatively little, the removal of the 



1 The girths are in the ratio of 25 : 42 ; while 3 14 represents 

 the ratio of the thickness of the walls, the measurements being 

 i§q and j4 s of an inch. 



