56 ANIMAL MECHANICS 



ing or masticating, and they would, by this action, 

 have been driven deeper in the jaw, and rendered 

 useless, had there not been a provision against 

 this mechanical effect. This provision is a dispo- 

 sition to grow, or rather to shoot out of their 

 sockets; and this disposition to project balances 

 the pressure which they sustain ; and when one 

 tooth is lost, its opposite rises, and is in danger 

 of being lost also, for want of that very opposi- 

 tion. 



The most obvious proof of contrivance is the 

 junction of the foot to the bones of the leg at 

 the ankle-joint. The two bones of the leg, called 

 the tibia and the fibula, receive the great articu- 

 lating bone of the foot (the astragalus) betwixt 

 them. And the extremities of these bones of the 

 leg project so as to form the outer and inner 

 ankle. Now, when we step forward, and whilst 

 the foot is raised, it rolls easily upon the ends of 

 these bones, so that the toe may be directed ac- 

 cording to the inequalities of the ground we are 

 to tread upon ; but when the foot is planted, and 

 the body is carried forward perpendicularly over 

 the foot, the joint of the leg and foot becomes 

 fixed, and we have a steady base to rest upon. 

 We next observe, that, in walking, the heel first 

 touches the ground. If the bones of the leg were 

 perpendicular over the part which first touches 

 the ground, we should come down with a sudden 



