372 THE MECHANICAL FUNCTIONS. 



object is either extensive protection, or the pro- 

 vision of broad surfaces for the attachment of 

 muscles, we find the osseous structure expanded 

 into flat plates ; as is exemplified in the bones 

 of the skull, in the shoulder blade, and still 

 more remarkably in the bony shield which sur- 

 rounds 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 bones are modelled into length- 

 ened cylinders, generally somewhat expanded at 

 the extremities, for greater convenience of mu- 

 tual connexion. 



In the form, the structure, and the arrange- 

 ment 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 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 



