i893 



THE AIR-SACS OF BIRDS. 



351 



We shall find that as the wing lengthened, so as to make a longer 

 and stronger stroke possible, the bones became larger in girth, larger 

 not only absolutely, but in proportion to their length ; and that a 

 decrease of weight accompanied the increase of strength, mainly 

 through the drying up of the marrow, but partly through a reduction, 

 if we allow for the increased size of the bones, in the thickness of the 

 hard, osseous shell. I shall give, first, a few measurements to show 

 that in the case of birds whose bones have little or no aeration, the 

 girth of the bones is, relatively to the bulk and weight of the body, 

 considerably less — 



Bones highly 

 pneumatic. 



Girth of Humerus. 



inch. 

 Screamer .. .. i§ 



Rhinoceros Hornbill i| 

 Golden Eagle . . i| 

 Vultur monachus . . 2\ 

 1 Marabou Stork .. 24-5 



Bones very 



little or not 



at all 



aerated. 



Girth of Humerus. 



inch. 



/Logger-headed Duck i^j 



Scoter Duck.. .. ig 



Nestor Parrot . . j 



Red-throated Diver 1 



\Spur-\vinged Goose. . i^ c 



These measurements speak for themselves, even without any exact 

 statement of the weight of the birds ; but the following illustration 

 and the accompanying tables will do more to explain the problem of 

 hollow bones. The shoulder bones of a skua gull, which has scarcely 

 any aeration, of an eagle and a hornbill, both of which are highly 

 pneumatic, are placed side by side. The greater girth of the hollow 

 bones in proportion to their length is at once clear. But to bring 

 this out more clearly, I have taken the wing-bones of the skua as a 

 model and calculated what would have been the length of the same 

 bones and of the whole wing in the eagle and the hornbill, if they 

 had been built on the same lines : — 



Girth of Humerus. 



Pomatorhine Skua 

 Vociferous Sea Eagle 

 Rhinoceros Hornbill 



3§ inch 



Humerus. 



Skua 

 Eagle 

 Hornbill 



Actual 

 Length. 



4l 

 6| 



4^ 



Length pro- 

 portionate to 

 girth. 



7& 

 7A 



Actual 

 Length. 



4A 



7H 



7 



Ulna. 



Length pro- 

 portionate to 

 girth. 



Girth of Ulna. 

 I inch 



1 



•• ii„ 



Aggregate length of Wing 

 bones. 



Length pro- 

 Actual portionate to 

 Length. girth of 



Humerus. 



7& 



i3i 



15$ 



22^r 



23*S 



Thus, if in the eagle's humerus length were proportioned to girth, as 

 in the skua's, the bone would be more than ^-inch longer; on the same 

 principle the aggregate length of the wing-bones would be greater by 

 more than i^-inch. The increase of length in the case of the hornbill 

 is far more startling. 



If now we take a saw and cut the humeri of the skua and 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 girths are 

 in the ratio of 25 : 42 ; while 3 : 4 represents the ratio of the thickness 

 of the walls, the measurements being j-g^ and y^ inch. We can now 

 see why small birds have so little aeration. In their case there would 



