4 ANIMAL MECHANICS 



the source of weakness, and, perhaps, of de- 

 struction ; and safety is thus voluntarily sacrificed 

 in part to obtain another property of motion. 



So in the animal body : sometimes we see the 

 safety of parts provided for by strength calcu- 

 lated for inert resistance ; but when made for 

 motion, when light and easily influenced, they 

 become proportionally weak and exposed, unless 

 some other principle be admitted, and a different 

 kind of security substituted for that of weight 

 and solidity : still a certain insecurity arises from 

 this delicacy of structure. 



We shall afterwards have occasion to show that 

 there is always a balance between the power of 

 exertion and the capability of resistance in the 

 living body. A horse or a deer receives a shock 

 in alighting from a leap ; but still the inert power 

 of resisting that shock bears a relation to the 

 muscular power with which they spring. And 

 so it is in a man : the elasticity of his limbs is 

 always accommodated to his activity ; but it is 

 obvious, that in a fall, the shock, which the lower 

 extremities are calculated to resist, may come on 

 the upper extremity, which, from being adapted 

 for extensive and rapid motion, is incapable of 

 sustaining the impulse, and the bones are broken 

 or displaced. 



The analogy between the structure of the hu- 

 man body and the works of human contrivance, 



