STRUCTURE OF BONE. 



373 



in various parts of the osseous systems of verte- 

 brated animals ; such as the human 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 

 condition by w hich mechanical advantage could 

 be gained. In the more perfect developements 

 of structures, such as those which obtain in the 

 higher orders of mammalia, and also in the class 

 of birds, all the long bones are 

 hollow cylinders ; and their ca- 

 vity is largest in the middle of 

 their length. This is shown in 

 Fig. 172, which represents a 

 longitudinal section of a human 

 thigh bone, and in Fig. 173, 

 which is a similar section of the 

 humerus, or bone of the arm. 

 The walls of these bones consist 

 of a dense and compact sub- 

 stance, formed by the close co- 

 hesion of the osseous plates. 

 These walls are of greater thick- 

 ness in the middle of the shank 

 or shaft of the column, and be- 

 come thinner as we follow them 

 towards either of the ends. This 

 gradual diminution in the thickness of the walls 



