NATURAL THEOLOfiY. 95 



to invent a more simple and beautiful contrivance. 

 The first spine of the fin, the first stick of the fan, has 

 a moveable bone in the shape of a ring attached to 

 the lower extremity, which plays upon another bone 

 that is immoveable. The ring bone has a hook, and 

 it is in the power of the animal, by turning the bone 

 round, to fasten the hook into a particular hole in the 

 immoveable bone. In this manner the fin becomes 

 so securely fixed that it cannot be moved except by a 

 motion directly contrary to that which hooked it to 

 the other. Any attempt to brush down the fin with- 

 out regularly unlocking it is ineffectual. It is thus 

 the siluri and the gasterostei give firmness to their fins 

 when they wish to employ them as weapons of de- 

 fence or assault. 



T. You may describe some of the principal bones, 

 besides those which have been mentioned. 

 THE ARM. 



A. The arm is a remarkable piece of mechanism. 



The situation of the arm upon the body is best ac- 

 commodated to the uses of the limb, as every one 

 will realize by imagining a different position. 



By being jointed it is capable of a much greater 

 variety of motions, than though it had been a single 

 bone. Without a hinge in the middle, it would have 

 been unmanageable ; with a multitude .of joints it 

 would have wanted strength. It consists of three 

 bones ; one above the elbow, railed the humerus ; the 

 other two between the elbow and the wrist. Of the 

 last, the one which is in a line with the thumb is 

 termed the radius — the other, the ulna. 



