84 THE HUMAN BODY. 



the power is applied at the insertion of the biceps muscle 

 into the radius ; the weight is that of the forearm and 

 hand and whatever may be held in the latter, and acts at 

 the centre of gravity of the whole, somewhere on the far 

 side of the point of application of the power. Usually 

 (as in this case), the power-arm is very short, so as to gain 

 speed and extent of movement, the muscles being strong 

 enough to work at a considerable mechanical disadvantage. 

 The limbs are thus also made much more shapely than 

 would be the case were the power applied near or beyond 

 the weight. 



Pulleys in the body. Fixed pulleys are used in the 

 body ; they give rise to no loss or gain of power, but 

 serve to change the direction in which certain muscles 

 pull. One of the muscles of the eye-ball, for example, 

 has its origin at the back of the eye-socket, from 

 there it passes to the front and ends, before it reaches the 

 eye-ball, in a long tendon. This tendon passes on to the 

 margin of the frontal bone, which arches over the front 

 of the eye-socket, and there passes through a ring 

 and turns back to the eye-ball. The direction in which 

 the muscle moves the eye is thus quite different from 

 what it would be if the tendon went directly to the eye- 

 ball. 



Standing. We only slowly learn to stand in the year 

 or two after birth, and though we finally come to do it 

 without conscious attention, standing always requires the 

 co-operation of many muscles, guided and controlled by 



Why is the power-arm in the body usually short? 

 What kind of pulley is used in the body? Is any mechanical ad 

 vantage gained from it? What is it used for? Give an example. 

 Is standing a simple process? 



