NATURAL THEOLOGY. 123 



The manner in which the muscles apply their 

 power to move the bones discovers a perfect acquain- 

 tance with the mechanical laws pertinent to such ca- 

 ses. Generally speaking, every muscle is fastened 

 firmly at one end to a bone it cannot move, or which 

 is sufficiently fast to pull by, and, at the other, to the 

 bone it is intended to move or to pull upon. A cord 

 tied between a door and the partition may give a very 

 good idea of the action of a muscle. The moving 

 bone is the door ; the joint, is the hinge ; while the 

 shortening of the muscle moves the bone, in the same 

 manner, as the shortening of the string moves the 

 door. Thus a muscle is fastened to the bone of the 

 upper arm which comes down over the elbow joint, 

 and is attached to the arm below. By the contrac- 

 tion of this muscle the lower arm is raised up. 



Fig. 19. 



In this figure b represents the bone of the upper arm ; a, the 

 muscle which bends the fore arm, and which is inserted below 

 the elbow into the radius at d. It will be seen that the short- 

 ening of this muscle must necessarily raise the arm, d, c. 



T. And what is observable is, that the place 

 where the muscle is fastened to the moving bone, is 



