78 



VEilTEBRATED ANIMALS. 



ers of various kinds, most artificially and beautifully com- 

 bined are found in the limbs of quadrupeds, the wings of 

 birds, and the fins of fishes. The construction of these 

 bones combine strength and lightness to an admirable 

 degree, being hollow cylinders, with the enlarged end 

 porous. 



218. All the long bones of quadrupeds, birds, and man, 

 are made on this principle. When we come to the physi- 

 ology of the birds, we shall find a w r onderful provision 

 in that particular class to ensure lightness, the bones being 

 thin cylindrical tubes filled with air. 



219. In the corresponding bones of quadrupeds, the interi- 

 or is filled with an oily substance Figs. 61 and 62. 

 called marrow, which is entire and 



undivided along the shaft, or small- 

 est part of the cylinder, but is con- 

 tained in pores toward the two ex- 

 tremities, w^here there is a spongy 

 expansion of bony matter, for the 

 purpose of strengthening the enlarg- 

 ed size of the bone at these pails. 

 Fig. 61 represents a longitudinal 

 section of the femur, or human 

 thigh bone, showing the dense, solid 

 substance of the external parts, and 

 the cavernous and spongy structure 

 of the interior. Fig. 62 shows a" 

 similar section of the humerus, or 

 bone of the arm which joins the 

 shoulder. It is said by mechanical 

 philosophers, who have made ex- 

 periments on this subject, that it 

 would have been impossible to 

 have otherwise formed with the 

 same quantity of solid matter, a 

 lever so strong, and yet so light, 

 as that presented by the long 

 bones of the quadrupeds and man. 



What is said of the construction of bones which are used as levers ' 

 Give a description of the structure of the two bones shown by Figs. 61 

 What is said of the mechanical strength of the long bones ? 



