286 THE BODY AT WORK 



which lift the forearm. Their tendons are inserted into the 

 radius and the ulna at a distance from the elbow-joint which 

 is about one-tenth as great as the distance from it of a weight 

 held in the hand. Their united cross-section is about 16 square 



16 x 10 

 centimetres : ^ =16. One cannot hold out in the hand, the 



elbow being pressed against the side, so that these muscles alone 

 are acting, a greater weight than 16 kilogrammes (34 pounds), 

 although the muscles are exerting a traction ten times as great 

 as this. The strength of muscle when pulling straight is well 

 illustrated by the thick white mass in the centre of an oyster. 

 It keeps the shell closed until a force equal to 1,300 times the 

 animal's weight has been applied. This muscle also affords a 



FIG. 17. BICEPS MUSCLE IN ACTION. 



good illustration of the part played by reflex contraction in 

 opposing stretching the reaction by which tone is maintained. 

 Anyone who inserts an instrument, such as the end of a screw- 

 driver, between the slightly open valves of an oyster lying 

 under water will find that he needs to give it an exceedingly 

 smart twist if he would catch the muscle asleep. Stretching 

 it causes a reaction proportional to the stretching force. 



The fact that the output of energy by muscle is proportional, 

 within certain limits, to the work to be done, is brought out 

 even in laboratory experiments. A nerve-muscle preparation 

 teaches that the amount of work is not a function of the 

 stimulus. Within certain limits a stronger stimulus evokes 

 a higher and stronger lift ; but the stimulus remaining the same, 

 the work done by muscle (i.e., the product of weight multiplied 

 by height) is, up to a certain optimum, increased by increasing 

 the weight. Often a very light load is not lifted as high by 



