THE STRUCTURE AND LIFE OF BIRDS CHAP. 
II2 
Aggregate Length of 
Humerus. ie prerrs 
Girth of Le ih 
Humerus. | ! Length pro- 
Actual HeLa penal i Lahore 
Length. ‘ Length. ; to Girth o 
7 to Girth. ss ; Humerus. 
siesta ina pects = : | 
Skua. . $$ inches | 42 inches | 4§ inches 138 | 138 
SeaEagle .  . [ts 5, f 05 |7e0 2075 | 225 
Rhinoceros Hornbill] 1$ =, 548 035) | 7x5» 156, 2388 
| t 
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 
are 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.| 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 3:4 represents 
the ratio of the thickness of the walls, the measurements being 
téo and jé5 of an inch, 
