STRUCTURE OF BONE. 



333 



dynamics are more or less exemplified in the con- 

 struction of this part of the animal fabric. Levers 

 of various kinds are most artificially combined in 

 the formation of the fins of fishes, the wings of 

 birds, and the limbs of quadrupeds. The power of 

 the arch in resisting superincumbent pressure is 

 exhibited in various parts of the osseous systems of 

 vertebrated animals ; such as the liuman foot, the 

 spine, the pelvis, and more especially in the vaulted 

 roof of the skull, and in the carapace, or upper 

 shell, of the tortoise. 



The construction of these levers evinces that a 

 minute attention has been bestowed on every con- 

 dition by which mechanical advan- 

 tage could be gained. In the more 

 perfect developements of struc- 

 tures, such as those which obtain 

 in the higher orders of mammalia, 

 and also in the class of birds, all 

 the long bones are hollow cylin- 

 ders ; and their cavity is largest in 

 the middle of their length. This 

 is shown in Fig. 172, which repre- 

 sents a longitudinal section of a 

 human thigh bone, and in Fig. 1 73, 

 which is a similar section of the 

 humerus, or bone of the arm. The 

 v/allsof these bonesconsist of adense 

 and compact substance, formed by 

 the close cohesion of the osseous 

 plates. These walls are of greater 

 thickness in the middle of the 

 shank, or shaft of the column, and become thinner 

 as we follow them towards either end. This gra- 

 dual diminution in the thickness of the walls arises 



