VERTEBRATED ANIMALS. 



having motion only at the points of connection. Lev- 

 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 admir- 

 able degree, being hollow cylinders, with the enlarged 

 ends porous. 



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 wonderful provision 

 in that particular class to ensure lightness, the bones be- 

 ing thin cylindrical tubes filled with air. 



In the corresponding bones of quadrupeds, the interi- 

 Fig. 61 and 62. or ^ s filled with an oily substance 

 called marrow, which is entire and 

 undivided along the shaft, or small- 

 est part of the cylinder, but is con- 

 tained in pores towards the two ex- 

 tremities, where there is a spongy 

 expansion of bony matter, for the 

 purpose of strengthening the enlar- 

 ged size of the bone at these parts. 

 Fig. 61 represents a longitudinal 

 section of the femur, or human 

 thigh bone, showing the dense, sol- 

 id substance of the external parts, 

 and the cavernous and spongy 

 structure of the interior. Fig. 62 

 shows a similar section of the hu- 

 merus, or bone of the arm which 

 joins the shoulder. It is said by 

 mechanical philosophers, who have 

 made experiments on this subject, 

 that it would have been impossi- 

 ble to have otherwise formed with 

 the same quantity of solid mat- 

 ter, 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 1 

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

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



