96 MECHANISM OF THE BONES. 



T. What is the wisdom of this mechanism ? Why 

 one bone for the upper, and two for the lower arm ? 

 Why is there not the same number of bones in both 

 parts of the arm ? 



A.. It would be difficult to imagine a more striking 

 instance of mechanical ingenuity than we have in the 

 double bone of the lower arm. One of the bones, 

 (the ulna), makes~a hinge at the elbow, and permits 

 the arm to bend. The other has no concern in the 

 hinge : it barely touches at the elbow, and may be 

 said to be loose. But this arrangement is exactly the 

 opposite at the wrist. Here the bone which is hinged 

 at the elbow is only permitted to touch ; and that, on 

 the contrary, which touches at the elbow is united to 

 the hand by a hinge. When we want to bend the 

 arm, we use one hinge ; and when to perform the 

 same motion at the wrist, we employ the other. Had 

 these two motions been performed upon one and the 

 same bone, with a hinge at each extremity, the hand 

 could have swung backward and forward, but it 

 could not have revolved ; we could not have turned 

 the palm of the hand upward or downward. For 

 there can be no turning round, it is evident, upon a 

 hinge joint. If the hand must turn, the bone must 

 turn also to which it is hinged on ; and must therefore 

 be loose at the elbow, aud the elbow hinge must be 

 assigned to another bone. Such is the contrivance 

 actually adopted ! 



B. Any one might think how happy it is that we 

 are furnished with a double bone in a part we use so 

 much, so as to keep it more firm and steady under the 



