128 THE CONTBOL OF LIFE 



rigid piston. Whereas the flesh engines or muscles 

 work by pulling, and as they work they change their 

 shape ; and their piston-rods are flexible, we call them 

 sinews or tendons. A muscle is a ' pull ' engine ; a 

 motor-cycle is a ' push ' engine. 



Walking, when you come to think of it, is a very 

 wonderful performance, and we cannot be surprised 

 that it took us a good long time to learn. Nor do we 

 all walk as well as we should. When we are walking 

 at the rate of four miles an hour, only half a second 

 elapses from the moment the heel of the foot is raised 

 until the limb completes its swing forward and the 

 foot is planted firmly on the ground again. Yet in 

 that half-second fifty-four muscles or flesh-engines 

 have been started and stopped, speeded up and slowed 

 down, and that not once or twice but many times. 

 Another very interesting thing is this, that when cur 

 right leg swings forward, the whole weight of our body 

 is balanced on the slippery ball-shaped head of the left 

 thigh bone which works in the socket of the hip-joint. 

 It is difficult to balance a weight on a slippery ball-and- 

 socket joint ; it would not be possible were it not that 

 about fifteen muscles surrounding the hip- joint are set 

 in action and work together, acting against each other, 

 and yet in harmony with each other. But the balanc- 

 ing feat is helped by many other muscles in our momen- 

 tarily stationary left leg, some at the knee, some at the 

 ankle, and some at the arch of the foot. And next 

 half-second all this will be happening in our right leg. 

 Nor is this all, for as we walk we keep our body erect ; 



