BONES, MUSCLES, EXERCISE, AND REST. 277 



shades of difference of sound. To produce the precision 

 of motion in the ends of the fingers, the muscles that move 

 them must contract with still greater precision, without 

 varying a fraction from the required amount of motion. 



639. Another illustration will present itself to any 

 one when he is writing. He moves his pen through 

 the forms of his letters by means of the muscles of the 

 thumb and fingers, and of the arm and shoulder. These 

 combine their actions, and carry the pen now up, now 

 down, in straight or curved lines, forward and backward. 

 They make the exact contractions that are needed. The 

 slightest variations of these contractions make an e in- 

 stead of an t, a d instead of an a ; yet these mistakes are 

 not made, but the letters are formed in the exact shape that 

 is required. 



640 This control over muscular action enables the 

 painter, the draughtsman, and the engraver, to produce the 

 very pictures which they desire, and the mechanic to use 

 his tools, not only with exactness, but with safety to himself. 

 When a carpenter is cutting with a broad-axe, upon a small 

 block, which he holds with his left hand, he strikes boldly 

 and accurately. But a very slight difference of the con- 

 traction of the muscles of the shoulder that bring the upper 

 arm downward and forward would carry the axe upon the 

 fingers, and not upon his block. When driving nails, he 

 holds the nail between the thumb and finger of the left 

 hand, while he lifts the hammer with the right. He hits 

 the nail upon the head ; and yet how exceedingly small a 

 variation of the muscular contraction would carry the ham- 

 mer upon the thumb or the finger, and not upon the nail ! 

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