THE MUSCULAR SYSTEM 263 



4. That when the power-arm is longer than the weight-arm, the 

 weight is greater, but moves through a shorter distance than the power. 



5. That when the weight-arm is longer than the power-arm, the 

 power is greater and moves through a shorter distance than the weight. 



To show the Loss of Power in the Use of the Body Levers. 

 Construct a frame similar to, but larger than, that shown in Fig. 120, 

 (about 12 inches high), and hang a small spring balance (250 grams 

 capacity) at the place where the muscle is attached. Fasten the end 

 of a lever to the upright piece, at a point on a level with the end of the 

 balance hook. (The nail or screw used for this purpose must pass 

 loosely through the lever, and serve as a pivot upon which it can turn.) 

 The lever should consist of a light piece of wood, and should have a 

 length at least three times as great as the distance from the hook to 

 the turning point. Connect the balance hook with the lever by a 

 thread or string, and then hang upon it a small body of known weight. 

 Note the amount of force exerted at the balance in order to support the 

 weight at different places on the lever. At what point is the force just 

 equal to the weight? Where is it twice as great? Where three times? 

 Show that the force required to support the weight increases propor- 

 tionally as the weight-arm and as the distance through which the 

 weight may be moved by the lever. Apply to the action of the biceps 

 muscle in lifting weights on the forearm. 



A Study of the Action of the Biceps Muscle. Place the fingers 

 upon the tendon of the biceps where it connects with the radius of the 

 forearm. With the forearm resting upon the table, note that the ten- 

 don is somewhat loose and flaccid, but that with the slightest effort 

 to raise the forearm it quickly tightens. Now transfer the fingers to 

 the body of the muscle, and sweep the forearm through two or three 

 complete movements, noting the changes in the length and thickness 

 of the muscle. Lay the forearm again on the table, back of hand down, 

 and place a heavy weight (a flatiron or a hammer) upon the hand. 

 Note the effort required to raise the weight, and then shift it along the 

 arm. Observe that the nearer it approaches the elbow the lighter it 

 seems. Account for the difference in the effort required to raise the 

 weight at different places. Does the effort vary as the distance from 

 the tendon? 



