STRUCTURE OF BONE. 2G1 



pjtndcd into flat plates; as is exemplified in the hones of the 

 skull, in the shoulder hlade, and still more remarkahly in the 

 bony shield which surrounds the body of the tortoise. On 

 the other hand, where a system of levers is wanted, as in 

 the limbs, which have to sustain the weight of the trunk, 

 and to confer extensive powers of locomotion, the honcS arc 

 modelled into lengthened cylinders, generally somewhat ex- 

 panded at the extremities, for greater convenience of mutual 

 connexion. 



In the form, the structure, and the arrangement of these 

 levers, which allow of the regular and accurate application 

 of the moving power, and are calculated, in circumstances 

 so various, to give effectual support to the fabric, and also 

 to execute a great diversity of movements, we discern most 

 palpable manifestations of profound design, and the most 

 exquisite refinements of mechanic skill. All the scientific 

 principles of architecture and of dynamics are more or less 

 exemplified in the construction of this part of the animal 

 fabric. Levers of various kinds are most artificially com- 

 bined in the formation of the fins of fishes, the wines 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 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 at- 

 tention has been bestowed on every condition by whicli me- 

 chanical advantage could be gained. In the more perfect de- 

 velopments of structures, such as those which oi)tain in the 

 higher orders of mammalia, and also in the class of birds, all 

 the long bones are hollow cylinders, and their cavity 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 substance, formed by the 

 close coliesion of the osseous plates. These walls arc of 



