STRUCTURE OF BONES. 433 



strong and resisting material. The long bones in 

 man and animals, which form the limbs, are not 

 solid, but are filled with marrow. In birds, how- 

 ever, there is this remarkable difference their 

 bones are very rigid in consequence of their con- 

 taining a larger proportion than usual of earthy 

 matters, while they are also rendered extremely 

 light by the free admission of air into their cavities 

 from the lungs. Air cells thus penetrate the 

 hollow parts and divisions of the bones, extending 

 in different species through different proportions 

 of the bony frame, until in some birds, as in the 

 horn-bill, every bone of the skeleton is permeated 

 by air. Thus, it has been well said, there is no 

 class of animals which are so thoroughly pene- 

 trated by the medium in which they live and 

 move, as that of birds. Even the ends of the toes 

 are permeated by the air in the hornbills. The 

 bones are thus rendered both light and also strong, 

 for it is a familiar fact that hollow tubes, consist- 

 ing of a certain quantity of material, are stronger 

 than solid rods made of the same quantity of 

 materials. The immense extension of air-cells in 

 birds is not only important in consequence of its 

 diminishing their weight, but also because it 



